Design of high voltage AC/AC electrochemical capacitors in

Poznan University of Technology

Faculty of Chemical Technology

Institute of Chemistry and Technical Electrochemistry

Field of study: Chemical Technology

Paula Ratajczak

DESIGN OF HIGH VOLT A GE

AC/AC ELE CT ROCHEM I CA L CAPACIT ORS

IN AQUE OUS ELE CTROLY TE

P r o je k t o w a n ie w yso k o na p ię c io w yc h k o nd e n sa t o r ó w e l e k t r o c h e m i c z n y c h , p r a c u ją c yc h w e le k t r o lit a c h w o d n yc h

DOCTORAL DISSERTATION

P ro mo t e r : prof. François Béguin

Poznań 2015

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e

Badania do niniejszej pracy prowadzone były przy wsparciu przez projekt ECOLCAP realizowany w ramach Programu Welcome, finansowanego przez Fundację Nauki Polskiej

(FNP)zgodnie z Działaniem 1.2. „Wzmocnienie potencjału kadrowego nauki”, Programu

Operacyjnego Innowacyjna Gospodarka wspieranego przez Unię Europejską

Kierownik projektu: Profesor François Béguin

This thesis’ research was supported by ECOLCAP project funded in the frame of the

Welcome Programme implemented by the Foundation for Polish Science (FNP) within the Measure 1.2. ‘Strengthening the human resources potential of science’, of the

Innovative Economy Operational Programme supported by European Union.

Project leader: Professor François Béguin

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Część praca badawczej została wsparta przez projekt LIDER finansowany przez

Narodowe Centrum Badań i Rozwoju LIDER/018/513/L-4/12/NCBR/201„Kondensator elektrochemiczny o wysokiej gęstości energii i mocy operujący w roztworach sprzężonych par redoks:

Kierownik projektu: dr inż. Krzysztof Fic

A port of the research work was supported by the LIDER project funded by the National

Centre for Research and Development (NCBiR) LIDER/018/513/L-4/12/NCBR/201

"Electrochemical capacitor with high energy density and power operating in coupled redox couples solutions”.

Project leader: Dr Eng. Krzysztof Fic

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I am sincerely grateful to my supervisor,

Prof. François Béguin, for his guidance and all the efforts he put in my PhD work

I am also greatly thankful to Dr hab Eng Krzysztof Jurewicz, for our collaborative work on carbon materials and supercapacitors

It is also a great pleasure to thank

Prof. Dr hab Elżbieta Frąckowiak, and Dr Eng Krzysztof Fic for helping me to develop the skills and knowledge in electrochemistry and carbon materials

My sincere gratitude is also dedicated to all the ECOLCAP group members, especially,

Dr Qamar Abbas,

M.Sc Eng Piotr Skowron and M.Sc Eng. Paweł Jeżowski for their experimental support

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TABLE OF CONTENTS

INTRODUCTION _____________________________________________________ 9

CHAPTER I

LITERATURE REVIEW ON ELECTROCHEMICAL CAPACITORS __________ 16

I.1. General properties of electrochemical capacitors______________________ 17

1.1.

The electrical double-layer models ______________________________________________ 17

1.2.

1.3.

1.4.

O peration principle of an EDLC _______________________________________________ 19

Energy and power of electrochemical capacitors ___________________________________ 21

Pseudo-capacitive contribution _________________________________________________ 23

I.2. Electrode materials for electrochemical capacitors ____________________ 25

2.1.

2.2.

Commonly used carbon materials_______________________________________________ 25

Redox-active electrode materials _______________________________________________ 31

I.3. Structural and textural properties of activated carbons__________________ 31

3.1.

Manufacturing of porous carbons _______________________________________________ 31

3.2.

3.3.

Surface functional groups on carbons ____________________________________________ 33

Effect of porous texture of activated carbons on the capacitive performance______________35

I.4. Electrolytes for electrochemical capacitors___________________________ 39

4.1.

4.2.

4.3.

Aqueous electrolytes_________________________________________________________ 40

Organic electrolytes _________________________________________________________ 48

Ionic liquids _______________________________________________________________ 49

I.5. Conclusion ___________________________________________________ 51

CHAPTER II

ELECTROCHEMICAL TECHNIQUES

FOR ELECTROCHEMICAL CAPACITORS INVESTIGATION _______________ 53

II.1. Cyclic voltammetry ____________________________________________ 54

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II.2. Constant current charging/discharging ______________________________ 56

II.3. Impedance spectroscopy _________________________________________ 58

II.4. Accelerated ageing test __________________________________________ 60

CHAPTER III

STATE OF HEALTH OF AQUEOUS ELECTROCHEMICAL CAPACITORS

WITH STAINLESS STEEL CURRENT COLLECTORS

UNDER ACCELERATED AGEING _____________________________________ 63

III.1. High voltage ageing assessment of AC/AC electrochemical capacitors in lithium sulfate electrolyte ______________________________________ 65

1.1.

1.2.

Exploring the high operating voltage of AC/AC electrochemical capacitors in lithium sulfate electrolyte ___________________________________________________ 65

Degradation of ECs electrochemical performance under accelerated ageing ______________ 67

III.2. Factors contributing to ageing in aqueous electrolyte __________________ 74

2. 1.

Oxidation of carbon electrodes and corrosion of stainless steel current collectors __________ 74

2.1.1. Post-floating analysis of ECs by electrochemical techniques __________________________ 74

2.1.2. Post-floating analyses on carbon electrodes _______________________________________ 78

2.1.3. Effect of temperature on ageing ________________________________________________ 82

2.2.

Gas evolution during floating __________________________________________________ 83

III.3. Conclusion ___________________________________________________ 87

CHAPTER IV

STRATEGIES FOR IMPROVING THE LONG TIME PERFORMANCE

OF HIGH VOLTAGE CAPACITORS IN AQUEOUS ELECTROLYTES ________ 89

IV.1. Corrosion reduction of positive current collector ______________________ 90

1.1.

Alternative nickel current collectors _____________________________________________ 91

1.2.

Improvement of the current collector/electrode interface _____________________________ 95

1.2.1.

Carbon electrodes glued to stainless steel current collectors __________________________ 95

1.2.2. Nickel foil substrate _________________________________________________________ 97

1.2.3. Carbon conductive sub-layer _________________________________________________ 100

1.3.

Addition of corrosion inhibitor ________________________________________________ 103

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IV.2. Shifting of electrodes operating potentials __________________________ 109

2.1.

2.2.

Asymmetric configuration ___________________________________________________ 109

Current collectors coupling___________________________________________________ 117

IV.3. Conclusion __________________________________________________ 122

CHAPTER V

TOWARDS A NEW CONCEPT

OF HIGH VOLTAGE AC/AC CAPACITOR IN AQUEOUS ELECTROLYTES__ 124

III.1. The new concept of high voltage cell in aqueous electrolytes ___________ 125

III.2. Extension of voltage range by electrodes asymmetry _________________ 134

2.1 Adjustment of electrodes potential window by increasing m

+

/m

-

______________________ 134

2.2. Voltage extension by use of different carbon electrodes ____________________________ 136

III.3. Conclusion __________________________________________________ 138

GENERAL CONCLUSION ____________________________________________ 138

EXPERIMENTAL ANNEX____________________________________________ 142

A.1. Cell construction _________________________________________________ 143

1.1. Materials and chemicals _____________________________________________________ 143

1.2.

1.3.

Preparation of electrodes ____________________________________________________ 145

Cells configurations ________________________________________________________ 146

A.2. Electrochemical characterization ____________________________________ 147

A.3. Physico-chemical and surface characterization _________________________ 147

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REFERENCES ______________________________________________________ 149

SCIENTIFIC ACHIEVEMENTS________________________________________ 165

ABSTRACT ________________________________________________________ 172

STRESZCZENIE ____________________________________________________ 175

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INTRODUCTION

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Energy management has a deep influence in the humans’ everyday life, considering social, economic, ecological and political aspects. During the last 50 years, the world energy consumption, mainly based on petroleum-based fuels (oil, coal and

natural gas), has considerably increased (Figure 1), due to industrial development of the

western countries after the 2 nd

World War, accompanied by improving wealth in emerging markets and growth of the human population, especially in China and India.

Although renewable energy and nuclear power are the world fastest-growing energy sources in the recent years (each increasing around by 2.5% per year), fossil fuels still share more than 80% of the global energy consumption [1].

Figure 1 World energy consumption (based on [2] ).

Over the past decade, a general awareness appeared that fossil fuel consumption presents severe drawbacks, such as an important depletion of reserves and the emission of noxious gases leading in particular to the greenhouse effect and to associated temperature increase of the planet. The industry is partly able to handle with some of these problems, by introducing modern solutions, such as reducing emissions by placing catalysts in the exhaust systems of vehicles and in the chimneys of power plants.

Notwithstanding, if fossil fuels would remain the only power source for the future, the forthcoming crunch of their availability would lead to economic dislocations and serious political problems. Therefore, the incoming environmental and economic crisis predictions have suggested to develop strategies for improving energy efficiency (e.g.,

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e by improving buildings thermal insulation, by introducing hybridization in transportation systems, etc.) and for introducing renewables (sun and wind) in the energy mix. Due to the intermittent character of the ‘clean’ resources, to ensure a realtime balance of electricity supply to the demand over various time scales, the renewable technologies require energy-storage devices in order to adapt the energy delivery to the demand.

Figure 2 shows that energy can be stored via physical and chemical processes

and further delivered in the form of electricity. The main systems are based on gravity

(pumped hydro power storage), Compressed Air Energy Storage (CAES), kinetic energy (fly wheel), magnetic (Superconducting Magnetic Energy Storage (SMES), electric field (Electrical Double-Layer Capacitors (EDLCs)) and electrochemical reactions (batteries). “Pumped-hydro” is the most traditional way of storing energy on a large scale, by utilizing the excess of electric power to pump water from a lower to a higher-level reservoir. During the periods of high electricity demand, water is released to the lower elevation inducing the rotation of turbines and electricity generation.

Notwithstanding, this technology is geographically constrained and requires specific locations with a sufficient elevation difference between the two reservoirs, which makes the pumped-hydro plants non-transferable. A second interesting technology for largescale storage uses underground air compression (CAES) and requires specific geologic characteristics. However, the required equipment to store and extract the energy, including compressors and turbine-generators, generates high cost of the CAES plants.

Moreover, CAES generates heat in excess during compression, which reduces the yield of the process.

A technology which tends to be well-suited to ensure a real-time balance of electricity supply to the demand over various time scales is based on flywheels, which feature in a rapid response time. However, due to the high rotation speed of the rotor, for long-term performance, they require maintenance, and for this reason, are still considered to be not completely safe.

Since capital cost and environmental impact are a major barrier to deployment of energy storage, magnetic energy storage (SMES) seems to be a more economic technology than, e.g., pumped hydro and CAES. However, a typical SMES system includes a coil of superconducting material, a power conditioning system and

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e cryogenically cooled refrigerators, determining the final price of the equipment.

Moreover, SMES is not yet available on a large scale, but only for power application on a micro scale.

Figure 2 Energy storage systems which rely on physical and chemical processes.

At present, electrochemical systems (secondary batteries, electrochemical capacitors) appear as the most suited and flexible devices to adapt the electricity delivery to the demand, provided that the amount of energy involved is not extremely high. The storage batteries can convert the electrical work generated by, e.g., solar cells, into chemical free energy needed to force the reaction in a non-spontaneous direction.

Since rechargeable batteries (lead–acid, Ni-Cd, Ni-MH, Li-ion) appear in many different shapes and sizes, besides the grid energy storage applications, they are also

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e designed for individual customers to be used in automobile starters, portable devices, light vehicles and power supplies. Due to the chemical character of the operation, the discharge rate of batteries is limited and energy is lost due to the internal resistance of the cell components. Moreover, the concentration of a relatively large amount of chemical energy into a small package may result in hazardous events, such as numerous cases of fire and explosion in case of Li-ion batteries.

Electrochemical capacitors, due to their simple construction and the electrostatic

character of energy storage (Figure 2), are characterized by a fast response time, as

compared to the other available devices. As they apply high surface area porous carbon electrodes immersed in an electrolytic solution, they store several orders of magnitude higher energy than conventional dielectric capacitors. The most commonly developed systems at the industrial scale are electrical-double layer capacitors (EDLCs), which store the electrical charge in the Helmholtz double-layer. Due to the specific principle of operation, where a nanoscale layer of ions from the electrolyte is attracted to the surface of a polarized electrode material, ECs display high power density of 15 kW kg

-1

when compared to 2 kW kg

-1

offered by, e.g., Li-ion batteries which store the charge through electrochemical redox reactions. Therefore, ECs are adapted for high power applications in automotive industry, opening emergency doors of aircrafts, regenerative braking and stop-start technology in vehicles or power buffer in electric drive train. Moreover, they have a high cycle life of more than 1,000,000 charge/discharge cycles. However, due to the electrostatic charge storage mechanism, ECs store lower amounts of energy (5–8

Wh kg

-1

) than, e.g., Li-ion batteries (up to 180 Wh kg

-1

). Therefore, an important research attention is focused on enhancing their energy density, while realizing safe, environmentally friendly and cheap systems.

Since the energy density of ECs strongly depends on the applied maximum voltage, most of the industrial devices are based on organic electrolytes, although environment unfriendly and unsafe, which allow reaching 2.7 – 2.8 V. Aqueous electrolytes such as H

2

SO

4 and KOH have been also investigated for high power systems, but unfortunately voltage must be limited to less than 1 V in order to avoid electrolyte decomposition. Lately, it has been demonstrated by our research team that, by employing aqueous alkali sulfate and gold current collectors, voltage up to 2 V can be reached, due to a high over-potential of hydrogen evolution at the negative electrode.

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Taking into account the numerous advantages of water-based media over the organic ones, such as high conductivity, low cost, safety in operation and environmental friendliness, the ultimate aim of this doctoral dissertation is to develop a carbon-based, environmentally friendly and low-cost electrochemical capacitor (EC) operating in an aqueous electrolyte with cheap current collectors. To pursue this objective, the undertaken research requires considering and facing some obstacles which cause the cell performance to fade and reliability of the EC to decline. The perturbation phenomena occurring during long time operation of the capacitor are essentially related to aqueous electrolyte decomposition under high voltage operation, which can lead to oxidation of AC electrodes and/or internal pressure evolution and corrosion of metallic current collectors. Overall, the dissertation is divided into five chapters.

Chapter I is a literature review presenting the state-of-art on AC-based electrochemical capacitors. The operation principle and general properties of electrical double-layer capacitors (EDLCs) are described, and the common electrode materials employed for these devices are briefly introduced. The influence of structural and textural properties of carbons on the performance of electrochemical capacitors is summarized, with a special attention to the effect of porous texture on the capacitive.

ECs based on organic electrolytes, ionic liquids and aqueous media are critically compared, with a special emphasis placed on neutral aqueous solutions. Finally, in order to outline the pathway for the performed investigations, the drawn conclusions contain issues which still require to be resolved for improving high-voltage operation of carbon based electrochemical capacitors, while utilizing cheap stainless steel or nickel collectors and aqueous electrolytes.

To attain information about the performance of electrochemical capacitors, chapter II presents a survey of the electrochemical techniques used in our investigations.

In order to accelerate ageing of the analyzed devices, a test (so-called ‘floating’), initially developed by industry for systems with organic electrolyte, has been implemented and validated during our research on ECs in aqueous media.

The further parts of the dissertation are dedicated to attempts for extending the operating voltage of carbon-based ECs. The properties and performance of environmentally friendly AC/AC electrochemical capacitors using neutral salt aqueous electrolytes, e.g., essentially lithium sulfate, with cheap current collectors are presented

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e in chapter III. Since all the previous works with promising neutral sulfate electrolytes were conducted with expensive gold collectors, chapter III identifies the possible perturbation phenomena which occur during long-term operation in aqueous solution.

The actual effect of operating voltage on the state-of-health (SOH) of the device, evaluated by measuring cell capacitance and resistance evolution together with internal pressure evolution, is presented. The changes of physicochemical and surface properties of the cells’ constituents after long time operation, such as modifications of surface functionality and porosity of the carbon-based electrodes and corrosion of stainless steel current collectors are disclosed.

The strategies proposed in chapter IV to improve the long time performance of

AC/AC electrochemical capacitors in the neutral salt aqueous electrolyte are particularly intended to reduce the corrosion of stainless steel collectors and decrease its destructive effect on ECs operation. The undertaken tactics involve the replacement of the corrodible steel current collectors, the protection of the active material/collector interface and the addition of sodium molybdate corrosion inhibitor to lithium sulfate electrolyte. Cells with asymmetric configuration of electrodes and coupled kinds of current collectors are presented in the second part of chapter IV to avoid the decomposition of aqueous electrolyte by shifting the operating electrodes potentials to lower values.

Chapter V introduces a new concept of AC / AC cell using potassium hydroxide and sodium sulfate as catholyte and anolyte, respectively, and a cationic exchange membrane. Due to the pH difference between the two electrolytes, the cell can operate at higher voltage than the thermodynamic stability limit of water, e.g., 1.23 V. The effect of cell asymmetry, either by electrodes mass balancing or by use of different ACs, is critically discussed with regard to fit the electrodes potential extrema within the thermodynamic limits of water oxidation and hydrogen evolution. Besides, the proof-ofconcept allows a better understanding of the over-potential origin at the negative electrode of AC/AC capacitors in neutral aqueous electrolytes.

Finally, the manuscript ends with a general conclusion and perspectives for future research in the directions investigated and presented in this dissertation.

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CHAPTER I

LITERATURE REVIEW

ON ELECTROCHEMICAL CAPACITORS

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This chapter presents an overview on electrochemical capacitors literature appeared during the last decades. After a short introduction about the operation principle and general properties of electrical double-layer capacitors (EDLCs), the review will be focused on the fundamental role played by porous carbons and electrolytes on the electrochemical performance of EDLCs. The effect of pore size on the electrical double-layer capacitance (C dl

) and the strategies to adjust the pore size to the size of electrolyte ions will be emphasized. Particular attention will be paid to the ways by which the researchers exploit the potentialities of electrolytic solutions and carbons to increase the energy density by capacitance and voltage enhancement.

Electrolytes with extended stability window which are designed and customized for ECs will be presented, with a special emphasis on aqueous media. The sources of capacitance enhancement through faradaic contributions arising from oxygenated functional groups on the surface of carbons, redox-active electrode materials, electrochemical hydrogen storage and finally redox-active electrolytes will be also discussed.

On the basis of this literature review, the chapter finishes with a conclusion introducing the consecutive parts of the thesis, and emphasizing issues required to be improved for designing a high voltage ecologically friendly capacitor in salt aqueous electrolyte.

I.1. General properties of electrochemical capacitors

Electrochemical capacitors store energy in an electrical double-layer by electrostatic interaction at the interface created between the conductive solid material and the electrolyte [3, 4]. Contrary to conventional capacitors (such as aluminum electrolytic capacitors) which contain a dielectric material sandwiched between two electrodes facing each other, EDLCs use the electrical double-layer in their function.

1.1.

The electrical double-layer models

Over the last two centuries, scientists have developed various models of the EDL defining how ions from the electrolyte aggregate at the surface of polarized electrodes and in their vicinity. Helmholtz was the first to describe the phenomena which occur at the solid conductor-electrolyte boundary, and suggested that the interface consists of

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e two electrical layers which are: (i) electrons at the surface of the electrode, (ii) and a monolayer of ions in the electrolytic solution [5].

One of the shortcomings of the Helmholtz model was the assumption of stationary conditions where ions accumulate on the electrode surface. It did not take into account that, due to their motion, ions are not only compacted at the surface of the electrode, but form a diffuse space charge. Therefore, in the 1900’s Gouy and Chapman formulated a model according to which the capacitance depends also on the applied potential and ions concentration n [6], and is expressed by the equation (1):

𝑪

𝑮𝑪

= 𝜺𝜿

𝟒𝝅 𝒄𝒐𝒔𝒉 𝒛

𝟐

(1) where 𝜅 is the Debye-Hückel length [m] described in equation (2):

𝜿 = √

𝟖𝝅𝒏𝒆

𝟐

𝒛

𝟐

𝜺𝒌𝑻

(2) z - the valency of ions, n - the number of ions per cm

3

, T- the absolute temperature [K], and k

– the Boltzmann constant (1.3806488 10 -23

J K

-1

).

More than twenty years later, Stern included in his model both a compact and a diffuse layer [7], while Grahame divided this combined Stern layer into two regions [8]:

(i) a layer of adsorbed ions at the surface of the electrode, referred to as the inner

Helmholtz plane (IHP) (ii) and an outer Helmholtz plane (OHP) formed by the diffuse ions in the vicinity of the electrode surface. From the Grahame model, the capacitance C of the double-layer is described by equation (3):

𝟏

𝑪

𝑮

=

𝟏

𝑪

𝑯

+

𝟏

𝑪

𝑮𝑪

(3) with

𝐶

𝐻

, which corresponds to the specific capacitance of the Helmholtz’ compact double-layer, and

𝐶

𝐺𝐶

which results from the diffuse layer described by Gouy and

Chapman.

The currently used model (BMD model) of the electrical double-layer was described by Bockris, Devanathan and Muller [9], who proposed that a water layer is

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e present at the surface of the electrode and some other water molecules are displaced by specifically adsorbed ions (e.g., redox ions) which contribute to the pseudocapacitance.

The BMD model may be extended to charge-transfer reactions occurring in organic electrolytes with polar solvents, e.g., acetonitrile (AN), contributing to the potential drop across the electrode/electrolyte plane. As presented on the example of a negatively

polarized electrode (Figure 3), the inner Helmholtz plane (IHP) passes through the

centers of the specifically adsorbed ions and solvent molecules, which are oriented parallel to the electric field. Then, the outer Helmholtz plane (OHP) passes through the solvated ions centers, which are outside the IHP. Behind the outer Helmholtz plane, there is a diffuse layer region.

Figure 3 Schematic representation of the BMD double-layer model on a negatively polarized electrode (based on [9] ).

1.2.

Operation principle of an EDLC

In general, EDLCs are made from two identical electrodes made from a porous material (the most commonly carbon) coated on a current collector and separated by a porous membrane soaked with the electrolyte. When a device is connected to a power

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e supply the ions from the electrolyte aggregate on the surface of positively and

negatively polarized electrodes (Figure 4). As energy accumulation proceeds during

charging, the device is equivalent to two capacitors in series of capacitance C

+

and C

- and resistance R f+

and R f-

. The electrical double-layer capacitance of each electrode C dl is given by formula (4) [3]:

𝑪 𝒅𝒍

= 𝜺

𝒓

𝜺

𝟎

𝒅

𝑺

(4) where S is the surface area of the electrode/electrolyte interface,

ε r

- the relative permittivity of the electrolyte, ε

0

- vacuum permittivity (ε

0

= 8.854·10 −12

F m

-1

), d - the

EDL thickness.

.

Figure 4 Schematic representation of the charged state of a symmetric electrical doublelayer capacitor using porous carbon electrodes and its simplified equivalent circuit [10] .

Even in a symmetric capacitor, due to the different size of cations and anions in the electrolyte, the two electrodes display different capacitance values. Due to the series equivalent circuit, the capacitance C of the total system is given by equation (5):

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𝟏

𝑪

=

𝟏

𝑪

+

+

𝟏

𝑪

−

(5)

According to this relationship, the electrode with the smallest capacitance determines the capacitance of the system.

1.3.

Energy and power of electrochemical capacitors

The stored energy is directly related to ECs’ capacitance C and operating voltage window U , according to equation (6):

𝑬 =

𝟏

𝟐

𝑪𝑼 𝟐

(6)

Likewise, the maximum power density also depends on the applied voltage and is given by formula (7):

𝑷 =

𝑼

𝟐

𝟒𝑹

𝒔

(7) with R s

which states for the equivalent series resistance (ESR) of the device. During the charging and discharging processes, as the charges pass, the EDL flows to and from the electrolyte/electrode interface, and electrical losses take place. The main contributions to ESR come from [11]:

• electrolyte resistance;

• electrode material resistance;

• electrode/current-collector interfacial resistance;

• ionic (diffusion) resistance of: (i) ions reaching small pores; (ii) ions moving through the separator.

In order to customize energy storage devices for a wide range of applications,

energy and power are plotted versus each other in a so-called Ragone plot. Figure 5

shows the significantly large area covered by the ECs, which can deliver more power

(up to 15 kW kg

-1

) than redox systems such as Li-ion batteries (up to 2 kW kg

-1

) [12].

However, the specific energy reached by ECs is much lower than for Li-ion batteries,

(5–8 Wh kg

-1

compared to up to 180 Wh kg

-1

, respectively) [13].

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Figure 5 Ragone plot of various electrochemical energy storage systems (adapted from

[14] ).

The diagonal dashed lines in Figure 5 are obtained by dividing the energy

density by power, and inform how fast the energy can be distributed. This time constant of the device τ reveals the electrical losses during the charge storage, and is related to the equivalent series resistance R s

and capacitance of the system C according to formula

(8): 𝝉 = 𝑹 𝒔

𝑪

(8)

As seen in Figure 5, the charging/discharging process of EDLCs is very fast; this is due

to the purely physical character of the storage mechanism in the electrical double-layer.

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Since EDLCs are able to deliver all the stored energy within few seconds, they are particularly adapted for applications which require energy pulses during short periods of time, e.g., electric and hybrid vehicles, cranking of diesel engines and renewable energy harvesting, tramways, buses, cranes, forklifts, wind turbines, electricity load leveling in stationary and transportation systems, in opening emergency doors of aircrafts, etc. [12,

15].

Notwithstanding, the charge/discharge mechanism in EDLCs is fully reversible, with efficiency close to 100%. Therefore, the commercially available devices display a high cycle life of more than 1,000,000 charge/discharge cycles [16].

1.4. Pseudo-capacitive contributions

Whilst the main mode of energy storage in EDLCs originates from electrostatic charging, there are also pseudo-capacitive contributions associated with fast faradic

reactions at the electrode-electrolyte interface (Figure 6). In this case, the relation

between the charge exchanged dq and the change of potential dE is given by the formula (9) [3, 17] as in a capacitors:

𝑪 =

𝒅𝒒 𝒅𝑬

(9)

Figure 6 Schemes of EDL and faradaic energy storage in electrochemical capacitors

[18] .

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The pseudo-capacitive contributions are mainly associated with, e.g., redox reactions of electroactive species and electrosorption of nascent hydrogen or metal atoms (underpotential deposition). The contribution to capacitance from redox reactions comes from faradaic electron transfer involving an electrochemically active material and/or electrolyte species at the surface of an electrode. In the equilibrium state, the value of potential E is described by the Nernst equation (10) [19]:

𝑬 = 𝑬

𝟎

−

𝑹𝑻 𝒛𝑭

𝒍𝒏

𝒂 𝒐𝒙 𝒂 𝒓𝒆𝒅

(10) where E

0 is the standard electrode potential, R - gas constant (8.314472 J K

-1

mol

-1

); T - absolute temperature, z – number of moles of electrons transferred in the half-reaction,

F - Faraday constant (9.648 533

.

10

4

C mol

-1

), a - chemical activity of reducer ( a red

) and oxidant ( a ox

). When an electric current is applied, the equilibrium is disrupted and the electrode potential is changed to a value which depends on the amount of charge transferred q, where q is the product of the moles number z and Faraday constant F . The change of potential value is influenced by several factors: (i) the ionic conductivity of the electrolyte, (ii) the transport of species which participate in the reaction; (iii) and phase transition phenomena.

Another source of pseudocapacitance includes the reversible adsorption of atomic species at the surface of an electrode, accompanied by a partial transfer of charge, depending on the charge of the adsorbed atomic species A and the charge density at the electrode surface area S , as described by equation (11) [20]:

𝑨

± 𝒄

+ 𝑺

𝟏−𝜽

𝑨

± 𝒆

−

𝑬

↔ 𝑺

𝜽

𝑨

𝑨

𝒂𝒅𝒔

(11) where, c - concentration of adsorbable ions, 1-θ

A

is the fractional free surface area available for adsorption, θ

A

- coverage, E - potential. This specific process occurs when the adsorption of, e.g., anions is not only electrostatic in origin but also depends on electronic interactions between the valence electrons of the adsorbed anions and the surface orbitals of the electrode.

Paula Ratajczak

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Since the dissertation is focused on aqueous electrolytes, the pseudocapacitive effects which are likely to appear in these electrolytic solutions are presented in paragraph 2.2.

I.2. Electrode materials for electrochemical capacitors

Since the electrodes are the key part of electrochemical capacitors (ECs), the kind of selected electrode materials is very essential to determine the properties of ECs.

In this section, the storage principles and characteristics of electrode materials, including carbonaceous materials for EDLCs and redox-active electrodes for ECs are briefly depicted. Since the objective of this dissertation is related to the design of a low cost and environment friendly capacitor operating in aqueous electrolyte, special attention will be paid in the next section (I.3.) to the influence of surface properties of activated carbons (AC) for achieving high power and energy density.

2.1.

Commonly used carbon materials

In order to obtain a system characterized by high energy and power and excellent cycle life, materials with good physical properties and chemical inertness should be applied. Therefore, porous carbons are the most widely used electrode materials for EDLCs, due to their [11]:

• high electrical conductivity,

• high specific surface-area (from around 1 to around 2600 m

2

g

−1

),

• good corrosion resistance,

• relatively easily controlled porous texture,

• processability and compatibility in composite materials,

• low cost of production

• various forms (powders, fibers, nanotubes, graphene, foams, fabrics, composites, etc.).

Figure 7 presents the most commonly used carbons as electrodes for EDLCs, which include: activated carbons (ACs) [4, 21], carbon nanotubes (CNTs) [22], onionlike carbons (OLCs) [23], graphene [24] and carbide-derived carbons (CDCs) [25].

Nonetheless, low cost and high specific capacitance are the essential criteria which determine the choice of activated carbon as material for EDLCs.

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Figure 7 Electron microscopy images of high surface area carbon materials: (a) scanning electron microscopy (SEM) of AC particles [26] ; (b) SEM of AC fabrics [27] ;

(c) SEM of AC fibers [27] ; (d) SEM of vertically aligned CNT forest [28] ; (e) SEM of

CNT fabric [28] ; (f) SEM of randomly oriented CNTs within CNT paper mats [29] ; (g) transmission electron microscopy (TEM) of carbon onions [30] ; (h) SEM of multilayer graphene flakes [31] ; (i) SEM of carbide derived carbons (CDC) [32] .

Activated carbon

Activated carbon (AC) is a very complex and highly disordered material made of nano-scale units. In the early model of non-graphitizable carbon proposed by

Franklin

(Figure 8a) [33], the units constituted of few graphene layers [34] are oriented

randomly and connected with each other. The cross-links are sufficiently strong to impede the movement of the layers to a more parallel arrangement. However, after the model proposed by Stoeckli [35], it is believed that ACs sometimes involve single

fragments of graphene curved layers connected with each other, as presented in Figure

8b. It was found by high-resolution electron microscopy that high temperature treatment

of non-graphitizable carbon entails the production of faceted particles made of misoriented stacks of parallel graphene layers [36].

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Compared to some other forms of carbons (e.g., CNTs, OLCs), ACs are characterized by a lower conductivity, which for supercapacitor electrodes is usually compensated by using a percolator (carbon black or CNTs addition) and by appropriate electrodes manufacturing process [37, 38, 39].

Figure 8 (a) 2D model of a non-graphitizable carbonaceous material [33] ; (b) 3D model of carbonaceous material [40] .

Carbon nanotubes

Carbon nanotubes (CNTs) form a cylindrical 1D structure which contains either one rolled-up graphene layer (single-wall CNT - SWCNT) or several ones (multiwalled

CNT - MWCNT) (Figure 9). Generally, they are produced either by catalyst assisted

chemical vapor deposition (CCVD) using a hydrocarbon feedstock, such as methane, acetylene and propylene [41] or by CVD deposition in the nano-channels of an anodic alumina template [42].

In contrast to ACs and CDCs, CNTs have relatively low SSA and low density, which limit the volumetric capacitance and energy density of CNT-based EDLCs.

However, high electrical conductivity and open porosity of CNTs allow fast transport of ions, and thus the system to reach high power.

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Figure 9 (a) Structure of a single-wall carbon nanotube (SWCNT) and (b) multi-walled carbon nanotube (MWCNT) [43] .

Carbon onions

Carbon onions, also called carbon nano-onions (CNOs) or onion-like carbons

(OLCs) owe their name to the layered structure reminiscent to an onion, which contains

spherical closed carbon shells of fullerene or polyhedral nanostructure (Figure 10). They

offer a specific surface area up to 500-600 m

2

g

-1

which is fully accessible to ions [30].

They are produced via several techniques, such as electron beam irradiation, condensation of carbon vapor and vacuum precursor. Due to their 0D structure, small diameter (<10 nm), high electrical conductivity, relatively easy dispersion as compared to 1D nanotubes and 2D graphene, OLCs appear as a promising electrode material [44].

However, due to their high cost and low capacitance (about 30 F g

-1

), they are more preferably used as conductive agent to carbon based electrodes for high-power EDLCs.

Figure 10 3D structure of onion-like carbon (OLC) [45] .

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Carbide-derived carbons (CDCs)

Carbide-derived carbons (CDCs), also known as tunable nanoporous carbons, are a class of highly porous carbon materials derived from binary (e.g. SiC, TiC) or ternary carbides (e.g., Ti

2

AlC, Ti

3

SiC

2

), polymer-derived ceramics (e.g., Si-O-C or Ti-

C) or carbonitrides (Si-N-C) by selective etching of the metal atoms [46]. The most commonly used preparation method of CDCs is a reactive extraction of the metal from carbides with chlorine, where carbon grows from the outside to the core of particles

(Figure 11). To avoid sintering and aggregation of the material, generally, the synthesis

temperature does not exceed 1200 °C. In the last few years, CDCs attracted a lot of attention as electrode materials for ECs and hydrogen storage applications, due to their high specific surface area (up to 3100 m

2

g

−1

for CDCs synthesized by electrospinning of polycarbosilane with subsequent pyrolysis and chlorination) and broad range of pore sizes (0.3 – 30 nm) [47]. Owing to the highly tunable porosity, SiC-CDC enables to reach gravimetric capacitance of 75 F g

-1

in 1.5 mol L

-1

TEABF

4

/AN [48]. For the further developments of this manuscript, structural/textural properties of CDCs and activated carbons (ACs) will be considered as comparable.

Figure 11 Scheme of the carbide conversion to carbide-derived carbon (CDC) depending on the reaction time [49].

Graphene

Graphene is a 2D structured carbon material with fully accessible surface area

(reaching in theory 2670 m

2

g

-1 ) and high conductivity. However, due to the strong π-π

interactions, the graphene sheets tend to restack (Figure 12), which is a critical issue

entailing a decrease of accessible surface area and reduction of ions diffusion rates.

Therefore, techniques such as exfoliation and reduction of graphene oxide (GO), e.g.,

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e via microwave irradiation or heating of GO in propylene carbonate (PC), are applied to increase the gravimetric capacitance of graphene-based electrodes (190 F g

-1

in aqueous and 120 F g

-1

in organic electrolytes) [50]. Recently free-standing holey graphene frameworks (HGF) with efficient ion transport pathways were reported [51]. The HGF were prepared through hydrothermal reduction graphite oxide (GO) with simultaneous low temperature etching of graphene, owing to the presence of H

2

O

2

molecules. Due to the formation of nanopores in the graphene sheets, this 3D self-assembled structure enables to reach high and stable capacitance values (298 F g

-1

) in 1-ethyl-3methylimidazolium tetra-fluoroborate/acetonitrile (EMIMBF

4

/AN) during 25,000 galvanostatic cycles with current density of 25 A g

-1

.

Figure 12 Model of a layered microscopic segment of graphene sheets. [52]

2.2.

Redox-active electrode materials

In the past decades, many redox-active materials have been studied to gain additional charge from electrochemical reactions, such as conducting polymers [53] or transition metal oxides (RuO

2

, MnO

2

, Fe

3

O

4

) [54, 55, 56].

However, due to the faradaic charge storage mechanism, ECs with redox active electrodes do not exhibit long time operation with a high efficiency.

Over the years, one of the most studied materials with pseudocapacitive behavior has been conductive ruthenium oxide (RuO

2

) in acidic electrolytes. During the transitions from the Ru

+II

oxidation state to Ru

+IV

, a fast and reversible electron transfer with simultaneous electrosorption of protons on the surface of RuO

2

particles takes place, according to reaction (12) [14]:

RuO

2

 

H

  

e

 

RuO

2

 

(

OH

)

 (12)

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e where 0 ≤



≤ 2. The three distinct oxidation states of ruthenium during insertion or deinsertion of protons (Ru

+II

, Ru

+III and Ru

+II

) occur within 1.2 V, and allow ECs with amorphous RuO

2

reaching specific capacitance values of more than 600 F g

-1

[57].

Although, capacitance enhancement in Ru-based aqueous electrochemical capacitors is very attractive, their applications are limited due to the very high price and voltage window of only 1 V.

Therefore, less expensive oxides have been studied, such as iron, vanadium, and cobalt oxides, with particular emphasis on manganese oxide. In capacitors with

MnO

2

electrodes, the charge storage mechanism is based on the adsorption of cations from the electrolyte (C

+

= K

+

, Na

+ …) and incorporation of protons. Therefore, these reversible surface redox reactions are fast and close to those in pure EDLC, according to the reaction (13):

MnO

2

 

C

 

zH

 

(

 

z

)

e

 

MnOOC



H

z

(13)

In neutral aqueous electrolytes, MnO

2

micro-powders or micrometer-thick films exhibit specific capacitance of ~150 F g

–1

within a voltage window of less than 1 V. Therefore,

MnO

2

electrodes are frequently used in asymmetric configuration with an AC negative electrode, as an attractive alternative to conventional pseudocapacitors or EDLCs.

I.3. Structural and textural properties of activated carbons

To improve the performance of electrodes, researchers try to optimize the properties of carbons, focusing essentially on conductivity and specific surface area.

However, to better understand the role of carbon materials in ECs, it is also important to consider their structural/nanotextural diversity and surface functionality in more details.

3.1.

Manufacturing of porous carbons

The vast majority of carbon based electrode materials is derived from organic precursors by so-called carbonization process which involves heat treatment of a sample in inert atmosphere. Therefore, the structural and textural properties of carbons are dependent on the precursor, its state (e.g., solid material, gel) and conditions of processing [58]. The common natural organic precursors for activated carbon synthesis

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e include: coal, peat, fruit stones, nut shells, wood, petroleum coke, pitch, lignite, starch, sucrose, corn grain, leaves, coffee grounds, straw etc. [59, 60, 61, 62, 63, 64, 65, 66]. In general, carbonized samples from natural organic precursors have a relatively low porosity with a large number of interstices which block the pore entrances. Therefore, the pre-carbonized product must be further physically or chemically activated in order to open the porosity and to create new pores. The physical activation is conducted by gasification of the pre-carbonized char at temperatures ranging from 700 to 1000 °C, in the presence of an oxidizing agent (such as CO

2

, steam, air or mixture of these gases), which increases the pore volume and surface area of the material by a controlled carbon burn-off, according to equations (14) to (17) [67, 68]:

C



H

2

O



CO



H

2

C



CO

2



2 CO

C



O

2



CO

2

2 C



O

2



2 CO

(14)

(15)

(16)

(17)

The production of ACs by chemical activation is carried out at slightly lower temperatures (

∼

400–700 °C) and generally results in smaller pores and more uniform pore size distribution [11]. The process involves the reaction of a precursor or a char with a chemical reagent (such as KOH [69, 70], ZnCl

2

[71, 72] or H

3

PO

4

[73, 74]). As reported, by activation with potassium hydroxide, it is possible to obtain ACs with specific surface area above 2500 m

2

g

−1

[75, 76]. Nonetheless, to remove residual reactants as well as any inorganic residues (e.g., ash) which originate from the carbon precursor or are introduced during preparation, post-activation washing is always required.

Although it is generally believed that the activation process is required to open the pores of carbonized precursors, carbons with well-developed porosity and good capacitance values, as well as reproducible properties can be obtained by simple onestep carbonization of synthetic polymers, e.g., through a rapid microwave heating of polypyrrole (PPy) [77]. Recently, it has been also presented that self-activation proceeds during carbonization of appropriate biomass precursors, e.g., tobacco [78] or seaweeds

Paula Ratajczak

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[79, 80], where the second stage of chemical or physical activation is unnecessary. Due to the presence of naturally embedded group I and II elements (such as potassium, calcium, magnesium, sodium), during the thermal treatment, carbonization and selfactivation of the precursor occur simultaneously. For the Burley tobacco, the optimal self-activation temperature is considered as 800 °C. At higher temperatures, annealing of the materials dominates and provokes a decrease of specific surface area and average pore size [78].

3.2.

Surface functional groups on carbons

As presented in Figure 8b, carbon materials are constituted of fragments of

graphene layers connected with each other, each fragment containing edges and defect like vacancies, leading to the development of surface functional groups [68]. As a result of incomplete carbonization of the porous material, a part of the chemical structure is associated with heteroatoms which are in the vast majority oxygen and hydrogen, and in

a lesser degree nitrogen and sulfur (Figure 13). Therefore, in addition to electrical

double-layer charging, faradic electron transfer reactions involving the surface functional groups may be involved in energy storage [81, 82, 83]. In order to enhance this contribution, the surface functionality of ACs is generally developed through: (i) electrochemical polarization [84], (ii) chemical treatment [85], (iii) and plasma treatment [86].

There are three types of surface oxides present on the carbon material, namely,

acidic, basic and neutral (Figure 13) [11]. Surface oxides with acidic nature are formed

when carbons are exposed to di-oxygen at 200-750 °C or by reactions with oxidizing agents at room temperature. These surface groups include carboxylic, lactonic and phenolic functionalities. The basic and neutral groups are formed after heat treatment of

AC to eliminate the surface functionalities, and further exposition of AC to di-oxygen at low temperature. The basic oxygen-containing groups include ethers, carbonyls and pyrone structures. Although, the acidic or basic nature of quinone/hydroquinone functionalities is not strongly marked, their contribution to capacitance and creation of catalytic active sites for, e.g., oxidative dehydrogenation reactions cannot be neglected

[87]. The contribution of quinone/hydroquinone pairs to capacitance can be observed in cyclic voltammograms by cathodic and anodic waves at ~0 V vs Hg/Hg

2

SO

4

.

Paula Ratajczak

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Nonetheless, the recent trend is to introduce the quinone/hydroquinone redox pair into the electrolyte, which is simpler than, e.g., grafting of quinone derivatives on the surface of carbon [88, 89].

Figure 13 Possible functional groups on the surface of carbons related to the presence of heteroatoms: (a) oxygen, (b) nitrogen, and (c) sulfur. Acidic and basic functionalities are indicated in red and blue, respectively (adapted from [90] ).

Different techniques are available to analyze the surface functional groups on carbons, such as Temperature-Programmed Desorption (TPD), X-ray Photoelectron

Spectroscopy (XPS), Fourier Transform Infrared spectroscopy (FTIR), and chemical or electrochemical titration methods (i.e., Boëhm titration) [91]. Nowadays, the most popular method for characterization of surface oxides starts to be TPD. In this

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e technique, the functionalities present on the carbon surface are thermally decomposed releasing primarily CO

2

, CO and/or H

2

O at different temperatures [92]. The nature of the groups is evaluated from the type of released gas and the decomposition temperature

[93]. The TPD patterns of CO and CO

2

evolution are a sum of peaks, therefore, to estimate the amount of each type of oxygenated surface group, the spectra can be

deconvoluted by using, e.g. a multiple Gaussian function (Figure 14) [92].

Figure 14 Deconvolution of TPD patterns for a carbon sample oxidized with 5 mol L

-1 nitric acid for 6 hours at boiling temperature: (a) CO

2

pattern; (b) CO pattern; TPD experimental data ■ / ■ ; individual peaks ---; sum of the individual peaks ) (adapted from

[92]).

Apart from the capacitive contribution, the presence of functional groups on the surface of AC influence the double-layer properties of carbon, such as wettability, rest potential, ESR, leakage current and self-discharge characteristics [3, 11]. As the amount of oxygen associated with the carbon surface increases, the hydrophilicity of carbon increases. Therefore, ACs with high oxygen content can be easier wetted by water than pure carbons without oxygenated surface functionalities.

3.3.

Effect of porous texture of activated carbons on the capacitive performance

The nature of the organic precursor and the conditions of AC synthesis, such as carbonization/activation temperatures and kind of used activating agent, influence the pore size distribution of carbon materials. Due to the complex interconnected network of internal pores, the BET specific surface area of AC ranges between 500-3000 m

2

g

-1

.

Paula Ratajczak

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The parameters closely connected with the specific surface area (pore volume of carbons, size and shape of pores, tortuosity) also play an important role in charge storage. According to the IUPAC classification, there are three main kinds of pores: (i) micropores (with diameters <2 nm), mesopores (diameters from 2 to 50 nm) and macropores (diameters >50 nm) [94]. Since the macropores do not take part in the actual adsorption processes, their contribution to the total surface area is negligible. Ions are the most efficiently adsorbed in the micropores providing the high surface area, while the mesopores are intended to allow the ions to be transported to the micropores

[95, 96, 97]. To enhance capacitance and to lower the ESR values, it is important to keep an appropriate volume ratio of meso/micropores, while selecting carbons for

EDLCs [98, 99]. For AC/AC electrochemical capacitors in sulfuric acid, the optimum mesopore volume ratio is in the range of 20 to 50% [100]. The role of micropores is seen during slow charging (2 mVs

−1

scan rate), whilst the beneficial effect of mesoporous transportation channels on capacitance is pronounced at higher rates [100].

The adsorption of a gaseous medium at a fixed temperature (generally nitrogen at -192°C) is the most common method used to investigate the porosity of carbons. The characteristics of activated carbons are estimated by commercial sorption equipment, generally using in-built software based on the adsorption isotherm of a given adsorbate/adsorbent system and a model of the adsorption process [101, 102, 103].

Nevertheless, in highly porous materials, the adsorption may occur via a pore filling mechanism, rather than by surface coverage only (as it is assumed by the Langmuir and

Brunauer–Emmett–Teller theory (BET) [104]). Therefore, in the narrow pores, the application of the BET equation can lead to unrealistic surface-area (S

BET

) estimations

[105, 106]. More and more often, the regularized density functional theory (DFT) is taken into consideration as a more accurate way to correlate capacitance with SSA. In the model, slit-shaped pore geometry is assumed, and it concerns the adsorption and capillary condensation in pores of different geometry and surface chemistry [107].

Figure 15a shows that the gravimetric capacitance of ACs and carbon blacks

increases almost linearly with SSA up to S

BET

≈ 1500 m

2

g

-1

, and then for carbons with higher activation degree a plateau is visible [108]. For the same carbons, the proportionality region of capacitance with S

DFT

is more extended than when using S

BET

, but still for S

DFT

higher than 1200 m

2

g

-1

a capacitance saturation phenomenon can be

observed (Figure 15b). For carbons materials with S

DFT

around 1200 m

2

g

-1

, due to the

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e increase in pore volume, the carbon pore walls become too thin to accommodate additional charges, which results in capacitance saturation [108]

.

Figure 15 Gravimetric capacitance vs (a) BET specific surface area; (b) DFT specific surface area (adapted from [108] ).

To overcome the over- or under-estimation of SSA derived from the BET equation, it is more accurate to combine gas adsorption and immersion calorimetry for porous carbons of different origins, as proposed by Stoeckli et al [109, 110]. Contrary to the anomalous increase of C/S

BET

(F m

-2

) for TiC-based carbons in pores of less than 1 nm when using TEABF

4

in AN electrolyte [111], the C/S av

values are constant in pores between 0.7 and 1.8 nm [112, 113]. Furthermore, in this pore size range, the volumetric capacitance (C/W o

) increases with decreasing pore width (Figure 16). Interestingly, the

linearization of volumetric capacitance vs L

0

led to similar trend in 1 mol L

-1

TEABF

4 in AN and 6 mol L

-1

KOH electrolyte for two series of activated carbons, while assuming slit-shaped pores [114].

According to equation (4), capacitance might be also overestimated when L o decreases, if assuming constant electrolyte dielectric permittivity ε r

. In fact, since slitshape micropores contain a constant amount of ions which are surrounded by a variable amount of solvent molecules, the relative electrolyte permittivity in micropores decreases with the solvent to ion ratio, i.e. with the decrease of L

0

. Therefore, the Feng model [115] which suggests a gradual decrease of relative permittivity of TEABF

4

/AN, explains the almost constant value of C/S in pores below 1 nm. However, the studies on microporous carbons cannot longer rely on models, which still assume that solvated

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e ions occupy a central position in micropores, which in turn feature in well-defined shape and rigidity, and are not interconnected.

Figure 16 Volumetric capacitance of various microporous carbons in TEABF

4

/AN electrolyte vs average pore width (L o

) accessible to CCl

4

; W o

represents the volume of micropores deduced from the carbon tetrachloride (CCl

4

) isotherm, assuming that the diameters of TEA

+

(0.68 nm) and CCl

4

(0.63 nm) are comparable [113] .

From the foregoing, and considering the diameter of solvated TEA

+

(1.3 nm) and BF

4

-

(1.16 nm) and desolvated TEA

+

(0.67 nm) and BF

4

-

(0.48 nm) [116], it suggests that ions need to be at least partly desolvated to penetrate into the micropores

[117]. Desolvation of TEA

+

and BF

4

-

was confirmed by nuclear magnetic resonance

(NMR) on AC electrodes extracted from capacitors charged up to different voltage values in the TEABF

4

/AN

electrolyte. Figure 17 shows the molar proportions of TEA

+ and BF

4

-

and the relative amount of AN vs the total amount of electrolyte species after polarization at various voltages [118]. Predictably, due to charging, large TEA

+ cations in the positive electrode are replaced by smaller BF

4

- anions, leaving the place for solvent molecules, which amount remains nearly constant up to 4.0 V. Simultaneously, in the negative electrode, small anions are replaced by larger cations, and consequently the AN concentration decreases rapidly and becomes negligible at 2.7 V (no AN molecules are left in the micropores of the AC-based electrode). The AN solvent is expelled by incoming TEA

+ and is further stored in the mesopores.

Paula Ratajczak

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Figure 17 Molar proportions of TEA

+

and BF

4

- in the positive and negative electrodes of an AC/AC electrochemical capacitor calculated from NMR spectra, and relative amount of AN versus the total amount of electrolyte species, after polarization at various cell potentials for 30 min (adapted from [118] ).

I.4. Electrolytes for electrochemical capacitors

In order to extend the range of ECs applications, the current researches seek for strategies which improve their energy density. According to equation (6), the value of stored energy can be enhanced either by increasing the capacitance C or by extending the operating voltage U . Since the latter is closely determined by the stability window of the applied electrolyte, this paragraph is focused on pros and cons of electrolytes which are designed and customized for different ECs applications. Beside the electrochemical stability window, which is a key factor affecting the electrolyte selection, the physical properties of the electrolytic solution, such as, mobility and molar conductivity of ions, are found to be also important in terms of energy storage efficiency. It is commonly known that the charge storage capacitance and resistance of the electrode material are affected by the nature of the electrolyte, i.e. the ionic radii of unsolvated and solvated ions, the molar conductivity of ions and their mobility in the pores of electrodes [119].

Calvo et al.

showed that it is possible to predict the capacitance for each electrolyte based on the information about molar conductivity of ions and surface functionality of the electrode material [120].

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The most commonly used electrolytes for ECs are aqueous media (sulfuric acid and potassium hydroxide), organic electrolytes and ionic liquids (ILs) [121]. According to formula (4), the capacitance values of ECs with carbon electrodes of same SSA are significantly higher in aqueous electrolytes than in non-aqueous solutions, due to high dielectric constant of the aqueous media [122, 123]. The aqueous electrolytes display values of ionic conductivity up to

∼

1 S cm

−1

for 30% H

2

SO

4

[11], while for the commonly used organic electrolytes (e.g., TEABF

4 in propylene carbonate) it is only

∼

0.02 S cm

-1

[124], and ~0.01 S cm

-1

for typical room temperature ionic liquids

(RTILs) [125]. The electrolytic solution should be also thermally stable, have low viscosity, low toxicity and low cost [126]. But yet, none of the available electrolytes fully meet all the mentioned desires.

4.1.

Aqueous electrolytes

On the point of view of production, the main motive for the choice of aqueous electrolyte is the low cost. While implementing non-aqueous media, all components

(carbon material, separator, electrolyte itself) need to be well-dried in order to ensure a long cycle-life of the system, whereas drying is not required in case of aqueous electrolytes, which dramatically decreases the production cost of the final device.

Moreover, water-based solvents provide strong solvation and tendency for complete dissociation or minimum ion pairing, feature in large dipolar moments (through hydrogen bonded structures) and high dielectric constants, leading to lower ESR values than organic solvents [83].

When comparing the most commonly used aqueous electrolytes (sulfuric acid, potassium hydroxide) in electrochemical power sources, the highest capacitance values and best electrochemical performance are achieved with H

2

SO

4

due to its greater ionic conductivity, faster mobility of H

+

than K

+

and greater activity of the basic oxygenated groups on the surface of the electrode material.

Unfortunately, a major disadvantage of water-based electrolytes, when considering formulae (6) and (7), is their low thermodynamic stability and consequently the low reachable voltage of 1.23 V [120]. Practically, in symmetric AC/AC electrochemical capacitors with H

2

SO

4 and KOH aqueous electrolytes it is even less than 1 V [127, 128, 129, 130, 131], whereas 2.7 V-2.8 V can be reached with ECs in

Paula Ratajczak

P a g e 40

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e organic medium [132]. Therefore, researchers still seek for media which assure high energy storage by extended operating voltage range, while lowering the costs of the

EC’s assembling process and enabling to apply various current collectors due to less corrosive properties than, i.e., sulfuric acid.

Promising neutral salt aqueous electrolytes

Lately, voltage values as high as 1.6 V were found for AC/AC electrochemical capacitors in 0.5 mol L

−1

Na

2

SO

4

[131, 79] and even 2 V when using 1 mol L

−1

Li

2

SO

4

[133]. As presented on the Ragone plots of AC/AC electrochemical capacitors in

Li

2

SO

4

and KOH aqueous electrolytes (Figure 18), the energy density in Li

2

SO

4

is enhanced by 80% as compared with KOH [128]. The energy and power density reached at the time constant of 25 s are 12.3 Wh kg

−1 and 1.6 kW g

−1

in Li

2

SO

4

against 7.2 Wh kg

−1

and 1.0 kW g

−1 in KOH, respectively. Furthermore, due to much less corrosive properties than sulfuric acid, and possibility to extend the operating voltage by appropriate combination of electrode materials, these neutral electrolytes are by far much preferable for further scaling-up to an industrial production [130, 134, 135].

Figure 18 Ragone plots of AC/AC capacitors in 1 mol L

-1

Li

2

SO

4

and 6 mol L

-1

KOH aqueous solutions with cell operating potential windows 0−1.6 V and 0−1.0 V, respectively. Values calculated for the total mass of active materials [128] .

Paula Ratajczak

P a g e 41


The different performance of AC in neutral, acidic and basic electrolytes is

presented by the three-electrode cyclic voltammograms in Figure 19. The potential

window in Na

2

SO

4

is roughly twice larger than in the traditional KOH and H

2

SO

4 electrolytes [127, 128, 129, 131]. Such enhancement of the operating potential window has been attributed either to the strong solvation of cations and anions [133] or to the high over-potential for di-hydrogen evolution at the negative electrode [136].

SO

4

2-

is one of the biggest and strongest solvated inorganic anions, having up to 40 water molecules in the solvation shell, with desolvation energy of about 108 kJ mol

-1

per one bond between SO

4

2- and water [133].

Figure 19 Potential stability window of activated carbon in 6 mol L

-1

KOH, 1 mol L

-1

H

2

SO

4

and 0.5 mol L

-1

Na

2

SO

4

determined by three-electrode cyclic voltammograms

(2 mV s

-1

) [131] .

Due to the full reversibility of the chemisorption process, hydrogen storage is an interesting option enabling a potential faradaic contribution in addition to the EDL capacitance and extension of the electrochemical stability window. Since ACs are characterized by highly developed porosity and easily tunable ultramicroporosity, they

Paula Ratajczak

P a g e 42

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e appear as the most interesting materials for this purpose. Activated carbons can store up to 2 wt% of hydrogen formed by electrochemical reduction of water under ambient pressure and temperature conditions [137, 138, 139, 140, 141].

Under negative polarization, the electrons are supplied to carbon and, depending on pH, they lead to the formation of nascent hydrogen, accordingly to equations (18) or (19) [142]: in acidic solution:

H

3

O

+

+ e

-

→ H + H

2

O

(18) in alkaline solution:

H

2

O + e

-

→ H + OH

-

(19) then, the in statu nascendi hydrogen is rapidly chemisorbed onto the carbon surface

[143, 144]:

C + H → CH ad

(20)

The increase of negative current below -0.8 V vs NHE, in case of 1 mol L

-1

Li

2

SO

4

, indicates the plausible limit for negative polarization beyond which evolution of gaseous di-hydrogen takes place (as observed by the oscillations due to bubbling on the

CVs (Figure 20)), according to equation (21):

2H → H

2

(21)

Di-hydrogen is also partly formed from the chemisorbed hydrogen, accordingly to equations (22) and (23) [142]:

CH ad

+ H

2

O + e

-

→ H

2

+ OH

-

+ C

(22)

CH ad

+ CH ad

→ H

2

+ 2C

(23)

The reversible hydrogen chemisorption is further evidenced in the cyclic

voltammograms (Figure 20) by an anodic desorption peak at around 0.4 V

vs NHE

[136].

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P a g e 43


Figure 20 Three-electrode cyclic voltammograms of activated carbon in 1 mol L

-1

Li

2

SO

4

obtained by stepwise shifting of the negative potential limit. The vertical dashed line at -0.383 V vs.

NHE corresponds to the thermodynamic potential for water reduction [145] .

The high over-potential for di-hydrogen evolution can be explained by a higher pH in the porosity of the AC electrode than on its outer surface. With highly porous electrodes, where the adsorbed species are unable to leave the pores rapidly via diffusion or electro-migration, the estimation of local pH changes is a difficult issue.

The in-situ pH variations on the carbon electrode surface, when cathodic charging at -

500 mA g

-1

was applied, are presented in Figure 21 for electrolytes of different initial

pH [62]. The initial pH values were adjusted by addition of 1 mol L

-1

H

2

SO

4

or 1 mol L

-

1

NaOH to 0.5 mol L

-1

Na

2

SO

4

. After 12 hours of charging, the pH value reached approximately 11 for all the electrolytic solutions, except for the one with starting pH =

2, for which the value remained unchanged. The pH increase in the medium with initial pH = 4, is associated with either formation of OH

-

or reduced amount of H

3

O

+

.

Considering the electrolytic solution with pH = 2, the reduction of H

3

O

+ results in a negligible increase of pH, due to the excessive amount of hydronium ions. The presented research highlights again the importance of electrolyte pH on the high voltage performance of AC-based ECs in aqueous media.

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Figure 21 Variations of pH values on the surface of AC electrodes after cathodic charging for 12 hours (-500 mA g

-1

) in 0.5 mol L

-1

Na

2

SO

4

(adapted from [62] ).

Redox-active aqueous electrolytes

Besides hydrogen storage, redox-active materials and oxygen-rich carbons, additional charge can also originate from redox reactions involving electrolyte species at the surface of an electrode. A large part of reports about the redox reactions concern the carbon/iodine interface formed in aqueous KI electrolyte [146, 147, 148]. The electrochemical activity of this electrolyte is based on reactions appearing on the positive electrode (24) to (27):

2𝐼

−1

↔ 𝐼

2

+ 2𝑒

−

3𝐼

−1

↔ 𝐼

3

−1

+ 2𝑒

−

2𝐼

3

−1

↔ 3𝐼

2

+ 2𝑒

−

𝐼

2

+ 6𝐻

2

𝑂 ↔ 2𝐼𝑂

3

−1

+ 12𝐻

+

+ 10𝑒

−

(24)

(25)

(26)

(27)

However, the presented transitions occur in a very narrow potential window. Figure 22

shows galvanostatic charge/discharge at 500 mA g

-1

and cyclic voltammetry performed on an AC/AC capacitor with gold current collectors in 1 mol L

-1

KI, using a reference

Paula Ratajczak

P a g e 45

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e electrode to monitor the behavior of individual electrodes. Reversible redox peaks are

observed at the positive electrode (Figure 22b) between 0 to 0.14 V

vs Hg/Hg

2

Cl

2

, whereas the negative electrode has an EDL behavior with a typical rectangular shape of

CV [146]. Due to the series dependence of the electrochemical circuit, the gravimetric capacitance of the cell (260 F g

-1

) is limited by the electrode with the smallest capacitance, i.e. the negative electrode [146]. The AC/AC cell with this electrolyte and gold collectors has a good cycle life with more than 80% of the initial capacitance value after 10,000 galvanostatic cycles at a current density of 1 A g

-1

. It is worth to mention that iodide salts allow using cheap stainless steel current collectors, which broadens the possibilities of their applications as electrolytes for ECs. After 15,000 charge/discharge cycles at 2 A g

-1

as well as after 150 hours of floating at 1.2 V on AC/AC cell in 2 mol

L

-1

NaI, no traces of corrosion of stainless steel collectors were observed [148].

Figure 22 (a) Two-electrode AC/AC cell in 1 mol L

-1

KI solution with SCE reference electrode: galvanostatic charge/discharge (500 mA g

-1

); (b) cyclic voltammograms (5 mV s

-1

) of the electrodes and of the cell [146] .

As previously mentioned, the capacitance of the AC/AC capacitor is limited by the low capacitance of the EDL electrode (see equation (5)). Therefore, to enhance the capacitance of the negative electrode, an AC/AC capacitor using 1 mol L

-1

KI as anolyte and 1 mol L

-1

VOSO

4

as catholyte, and a Nafion membrane as separator has been developed [149]. With the selected redox-active electrolytes, the galvanostatic (0.5 A g

-

1

) discharge capacitance of the system reaches 500 F g

-1

. The capacitance of the negative electrode is enhanced by multi-electron reactions as in equations (28) to (32):

𝑉𝑂𝐻 2+ + 𝐻 + + 𝑒 − ↔ 𝑉 2+ + 𝐻

2

𝑂

[𝐻

2

𝑉

10

𝑂

28

] 4− + 54𝐻 + + 30𝑒 − ↔ 10𝑉 2+ + 28𝐻

2

𝑂

(28)

(29)

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P a g e 46


[𝐻

2

𝑉

10

𝑂

28

] 4− + 44𝐻 + + 20𝑒 − ↔ 10𝑉𝑂𝐻 2+ + 18𝐻

2

𝑂

𝐻𝑉

2

𝑂

7

3−

+ 9𝐻 + + 6𝑒 − ↔ 2𝑉𝑂 + 5𝐻

2

𝑂

𝐻𝑉

2

𝑂

7

3− + 13𝐻 + + 10𝑒 − ↔ 2𝑉 + 7𝐻

2

𝑂

(30)

(31)

(32)

Similarly, hydroquinone has been added to 1 mol L

-1

H

2

SO

4

electrolyte, transforming a symmetric AC/AC electrochemical capacitor into a hybrid redox system

[150, 151]. The development of the quinone/hydroquinone redox reaction on the carbon

surface (Figure 23) [152] led the positive electrode to behave as a battery one, whereas

the negative electrode remains of the EDL type.

Figure 23 Redox reaction of the quinone/hydroquinone redox pair.

However, the battery-like behavior of the positive electrode limits the long-term stability of the cell in HQ/H

2

SO

4

electrolyte, which demonstrates a decrease in capacitance to 65% of its initial value after 4,000 cycles up to 1 V at current density of

4.4 mA cm

-2

. The initial capacitance decay after 1,000 cycles is probably attributed to the non-completed quinone/hydroquinone redox reactions within the voltage window from 0 V to 1 V [151]. Although the hybrid systems demonstrate potentialities for gaining in capacitance, such cells operate almost always at the expense of the cycle-life.

Besides naturally occurring surface functionalities, active molecules can be chemically/electrochemically grafted onto the carbon surface to enhance the capacitance of AC/AC ECs in aqueous media through faradaic contribution. Grafting of quinone derivatives is usually performed by electrochemical or chemical reduction of diazonium cations [153, 154, 155]. Since, the possible redox mechanisms involve proton and electron transfers, the pH of the applied electrolyte has a significant influence on the capacitance properties of quinone-modified carbons. The attachment of anthraquinone

Paula Ratajczak

P a g e 47

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e groups to the surface of mesoporous carbon black (BP2000) enhances the capacitance from 100 F g

-1

for the unmodified carbon to 195 F g

-1

for the grafted carbon in 0.1 mol

L

-1

H

2

SO

4

[88] However, the highly corrosive character of H

2

SO

4

would impose to use, e.g., neutral electrolytes, especially in the presence of stainless steel current collectors.

4.2.

Organic electrolytes

Most of the industrial EDLCs available in the market are based on organic electrolytes, due to their high thermodynamic stability window. However, the practical operating voltage of organic media in symmetric AC/AC electrochemical capacitors depends strongly on the impurities of the components, such as water and functional groups on the surface of carbons [156, 157].

The most commonly used organic salt is tetraethylammonium tetrafluoroborate

(TEABF

4

), due to its moderately good conductivity and good solubility in non-aqueous solvents. Currently, the most widely used solvent is acetonitrile (AN); however, propylene carbonate (PC) has also many adherents, especially in Japan. It was reported that PC exhibits a slightly stronger polarity, a higher density, viscosity and dielectric constant than AN [158]. However, solutions in AN exhibit lower electric resistance, and the increase of power density is accompanied by nearly constant energy density values

[159]. The electrolytes based on AN are usually characterized by about four times higher conductivity than the PC-based ones [160]. Regarding the safety issues, AN has very low flash point (5

°

C) and emits toxic combustion products [161, 162]. Therefore, researchers have used different types of solvents for organic electrolytes, such as sulfone, dimethylsulfone, and ethyl methyl carbonate [163]. Moreover, it has been found recently, that nitrile- and dinitrile-based electrolytes, e.g., adiponitrile (ADN) and sebaconitrile, due to their high electrochemical stability, are appropriate for i.e., highvoltage Li-ion batteries [164, 165]. Therefore, ADN started to be also used for EDLCs

[166], and it was revealed that an EDLC in 0.7 mol L

-1

TEABF

4

/ADN is stable up to

3.5-3.6 V with capacitance loss of less than 20% after 50,000 cycles [167]. Adiponitrile has a very low vapor pressure and a moderated viscosity; however, poor solubility of

TEABF

4 in ADN (the maximum concentration of TEABF

4 in ADN at 25 °C is 0.8 mol L

-1

) limits the physical properties of ADN-based electrolytes [157]. For instance, it affects their ionic conductivity, which is about 11 times smaller than that of AN-based

electrolytes [166]. Figure 24 shows that an EDLC in TEABF

4

/ADN does not

Paula Ratajczak

P a g e 48

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e demonstrate a rectangular CV, which is attributed to different contributions from parallel resistances, due to diffusion of solvated ions in the pores of the electrode material [168]. Thus, the studies are now focused on solutions of ionic liquids in ADN which exhibit better electrochemical performance, such as those containing 2 mol L

-1

1ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C

2 mIM][TFSI] roomtemperature molten salt) in ADN [168] . However, more detailed and comparative electrochemical studies need to be conducted.

Figure 24 CVs (5 mV s

−1

) of EDLCs using different electrolytes: 1 mol L

-1

TEABF

4

in

AN, 0.7 mol L

-1

TEABF

4

in ADN and 2 mol L

-1

[C

2 mIM][TFSI] in ADN. The gravimetric current is expressed by total mass of electrodes [168] .

4.3.

Ionic liquids

The recent trend focused on electrolytes for EDLCs with large stability window, concerns AC/AC capacitors in ionic liquids (ILs). ILs are molten salts at room temperature, entirely composed of cations and anions, which enable to operate at temperatures as high as 300 °C with very low vapor pressure, featured in nonflammability and electrochemical stability [126, 121, 169, 170]. Besides, solvent-free

ILs do not possess any solvation shell, and thus can offer a well-defined ion size, enabling better understanding of the behavior of ions in the porosity of carbons and the design of proper electrode materials. The most commonly used ILs for EDLCs are

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e imidazolium, pyrrolidinium, and ammonium salts with anions such as tetrafluoroborate, trifluoromethanesulfonate, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide and hexafluorophosphate [126]. Galvanostatic cycling of an AC/AC electrochemical capacitor in N-methyl-N-butyl-pyrrolidinium bis(trifluoromethylsulfonyl) imide

(PYR

14

TFSI) demonstrated 95% efficiency at 3.5 V and 60°C after 65,000 cycles [169].

A constant voltage hold (floating) at 3.4 V revealed a long time operation (500 hours) of

EDLCs based on 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF

4

) with mesoporous carbon black (BP 2000) [171].

Since high viscosity, and thus high resistance and low conductivity at room temperature (typically ~14 mS cm

-1

[172]) influence the efficiency of the charging and discharging processes, the conductivity of EDLC with ILs phosphonium salts could be

improved by adding 25 wt% of acetonitrile (Figure 25) [173]. For capacitors with the

same mass of KOH activated carbon in the electrodes, the operating voltage is significantly increased to 3.4 V in the case of the ILs/AN 25% electrolyte in comparison with the conventional organic one (TEABF

4

/AN) and the aqueous acidic solution (1 mol

L

-1

H

2

SO

4

). The irregular shape of CV which is determined by different size of cation and anion of IL, suggests a need of suitable matching of pores size of positive and negative AC electrode with the ions size of the ILs.

Figure 25 Cyclic voltammograms (5mV s

-1

) for AC-based ECs in IL phosphonium salt

/AN 25%, organic and acidic electrolytic solutions [173] .

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P a g e 50


Notwithstanding, despite the wide voltage window of ILs, low conductivity at room temperature, high price and complex purification processes restrict further scaling-up.

Therefore, recently, the Azepane compound, which is a cheap by-product of polyamide industry, was used for the synthesis of Azp

14

TFSI and Azp

16

TFSI [174]. However, the large cation size of this electrolyte results in greater viscosity and lower conductivity as compared to, e.g., Pyr

14

TFSI. A further study on potentialities of this new class of electrolytes for EDLCs is still required [175].

I.5. Conclusion

In this chapter, the state-of-the-art on electrochemical capacitors (ECs) has been presented. The charge storage mechanism of electrical double-layer capacitors EDLCs is based on electrostatic interactions between electrolyte ions and the charged surface of carbon electrodes. Since the pure charging of the electrical double-layer does not involve any electron exchange, the power in EDLCs is much higher than in lithium batteries; however, in turn, the energy density is lower than in batteries. Therefore, most research efforts are focused on the energy density enhancement. It can be done by controlling voltage and capacitance.

The voltage range is essentially limited by the electrochemical stability of the electrolyte. Organic electrolytes allow high potential window – around 2.7 – 2.8 V to be reached, against around 1 V for conventional aqueous electrolytes applied in battery systems (sulfuric acid and potassium hydroxide). Hence, organic solutions are preferred in many industrial capacitors, despite their high cost, environment unfriendliness and low conductivity, while compared to aqueous media. A lot of attention is recently paid to ionic liquids, however, considering economical, safety and ecological aspects, aqueous electrolytes exhibit numerous advantages, and excel in high power densities as well. Furthermore, while considering the operating voltage window of ECs based on aqueous electrolytes, neutral aqueous sulfates, due to high over-potential of hydrogen evolution and strong solvation of ions, offer to reach 1.6 V in Na

2

SO

4

and even 2 V in

Li

2

SO

4

.

Since all the previous works with promising neutral sulfate electrolytes were conducted with expensive gold collectors, this thesis research will be focused on design and development of an environmentally friendly AC/AC electrochemical capacitor

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e using 1 mol L

-1

Li

2

SO

4

, with cheap stainless steel current collectors. Furthermore, the analysis on possible perturbation phenomena which occur during long-term operation of

ECs has been never conducted in the domain of high voltage AC/AC capacitors using aqueous electrolytes. Therefore, the actual effect of operating voltage on the state-ofhealth (SOH) of the device under accelerated ageing needs to be evaluated. The identification of factors contributing to ageing of ECs with cheap stainless steel or nickel current collectors in aqueous electrolytes is a preliminary step to apply any further improvement for the long time performance of these systems.

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CHAPTER II

ELECTROCHEMICAL TECHNIQUES

FOR ELECTROCHEMICAL CAPACITORS

INVESTIGATION

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This chapter presents the most frequently used electrochemical techniques for supercapacitor investigations, which include Cyclic Voltammetry (CV), Galvanostatic

Cycling with Potential Limitation (GCPL) and Electrochemical Impedance

Spectroscopy (EIS), as well as a technique adapted from industry to evaluate their cycle life (floating). EIS is a stationary technique which does not involve current and potential variations, whereas the transient ones (CV and GC) enable to investigate the whole supercapacitor and also each electrode separately, by measuring current or potential response. These basic techniques have served to establish a model of ideal EC comprising a capacitor with equivalent series resistance (ESR) and parallel leakage resistance (R f

) which determines the charge loss also referred to as self-discharge [176].

Among the years, the scientists have worked on specifying equivalent models which describe the influence of frequency, voltage and temperature on the entire cycle life of an EC [177].

II.1. Cyclic voltammetry

Cyclic voltammetry (CV) is a widely used technique in electrochemistry to acquire qualitative and pseudo-quantitative information about the interactions between the electrolyte ions and the surface of an electrode, as well as about possible redox reactions. Consecutively to a constant rate potential sweep, the current resulting from the flow of ions to charge and discharge the double-layer is measured. CV offers rapid information about the redox reactions and adsorption processes and, thanks to the ability to use a large range of scan rates, allows a quantitative kinetic analysis to be carried out.

A CV test consists of repetitive potential sweeps between two limits while measuring the resulting current. Therefore, CV is also an accurate technique to estimate the potential window of a supercapacitor (or an electrode in 3-electrode cell configuration) by the current leap which appears when irreversible faradaic reactions

(i.e., electrolyte decomposition, oxidation of electrode material) take place. The capacitance C in farad (F) can be calculated from the voltammogram using equation

(33), where I is the current (A), U the voltage (V), and U

1

and U

2

the limits of the voltage window [178]:

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𝑪 =

∫

∫ 𝒕𝑼𝟐 𝒕𝑼𝟏 𝒕𝑼𝟐 𝒕𝑼𝟏 𝒊 𝒅𝒕

𝑼 𝒅𝒕

(33)

The calculation is usually made from the backward scan, while a cell (or an electrode) is discharged.

For an ideal electrical double-layer capacitor, where the charge separation takes place between the surface of the electrode material and the solvated and non-solvated ions of the electrolyte, the cyclic voltammogram is represented by a rectangular profile

(Figure 26a). In the EDLC concept, during the potential sweep, the charges flow from

the external circuit and through the solution only to charge and discharge the doublelayer [8]. The influence of resistive components of the system (electrode, current collectors and separator material and its thickness) on the charging and discharging

processes is presented in Figure 26b by a parallelogram. The presence of this equivalent

series resistance (R s

) in series with the double-layer capacitance (C dl

) in the electrical circuit affects the power and energy and contributes to internal heating of the system

[21].

The other key factor affecting the supercapacitor performance is a leakage resistance (R f

), in parallel with the capacitance, which determines the charge loss, also referred to as self-discharge, causing the voltammetry characteristics to deviate from the parallelogram due to a delay while reversing the potential, ultimately coming from

kinetic processes during charging (Figure 26c). A deviation from the perfect rectangular

or parallelogram shape takes place when some charge passes across the double-layer interface through, e.g., Faradaic reactions from redox active species, giving a pseudocapacitive increase of current ( C p

) and a parallel resistance associated to the leakage reaction ( R p

) (Figure 26d). In summary, Figure 26c shows the classical RC

model of an EDL capacitor which includes the most important parameters affecting the shape of the experimental CV curves.

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Figure 26 Typical charge/discharge voltammetry characteristics and respective equivalent circuits of (a) an ideal EDL capacitor, (b) EDL capacitor with series resistance, (c) real EDL capacitor, (d) pseudo-capacitor.

II.2. Constant current charging/discharging

Galvanostatic Cycling with Potential Limitation (GCPL), also called chronopotentiometry, is based on measuring the voltage as function of time at imposed current. This transient technique is found as the most representative to determine parameters as capacitance and resistance, and also to test the cycle life of a supercapacitor.

The capacitance of an EC is calculated from the slope of the discharge curve, while the resistance is usually deduced from the potential drop when the current sign changes from charge to discharge. However, a better estimation of resistance is obtained

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e by measuring the IR drop when initiating a constant current discharge after a potentiostatic period [179]. The method is specified by the IEC (International

Electrotechnical Commission) to determine the ESR (Equivalent Series Resistance) and

EDR (Equivalent Distributed Resistance) values [180]. The ESR corresponds to all the resistive components within the supercapacitor, and the EDR includes the ESR and also a contribution from the charge redistribution process in the pores, due to a nonhomogeneous electrode structure. Hence, the resistance values are calculated by using the expressions (34) and (35):

𝑬𝑺𝑹 =

∆𝑼

𝟐

|𝑰 𝒅𝒊𝒔𝒄𝒉

|

(34)

𝑬𝑫𝑹 =

∆𝑼

𝟏

|𝑰 𝒅𝒊𝒔𝒄𝒉

|

(35) where, I disch

is the galvanostatic discharge current;

ΔU

2

- the voltage drop when the discharge current is switched on, and

ΔU

1

is obtained from the intersection of the vertical line at the time of starting discharge and the auxiliary line extended from the

linear discharge (see Figure 27) [181].

Figure 27 Galvanostatic charge and discharge of a supercapacitor with a constant voltage period [adapted from [181] ].

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The capacitance of the supercapacitor is determined from the galvanostatic discharge current ( I disch

) and the time of discharge (Δ t ) for a selected voltage window

(Δ U ), according to formula (36):

𝑪 =

𝑰

𝒅𝒊𝒔𝒄𝒉

𝜟𝑼

𝜟𝒕

(36)

It is also possible to monitor the cycle life of a cell by repeating many times the inversion of the charging and discharging processes for a given maximum voltage limit; in this case, C, ESR and EDR are plotted vs the number of cycles.

Whereas cyclic voltammetry yields basic information about capacitors (stability window, capacitance, etc.), the galvanostatic charge/discharge technique is needed to compute the energetic response [182]. Therefore, among the available techniques, CV and GCPL are considered to give qualitative and quantitative information on supercapacitor performance, respectively.

II.3. Impedance spectroscopy

Electrochemical Impedance Spectroscopy (EIS) allows determining the

EC’s real and imaginary components of the impedance response as a function of frequency. It requires special equipment for applying a small alternating current (AC).

The ESR and frequency-response behavior of a capacitor are dependent on the electrode characteristics:

 nature of substrate

 pore-size distribution

 engineering preparation parameters (e.g., thickness, quality of contact between particles).

The EIS technique can be implemented by measuring either the current or voltage response of the system, while the potential or current is controlled. However, the most widely used method is to set a sinusoidal signal of required potential with small amplitude at several frequencies ( f ). As shown in equation (37), the impedance ( Z ) is a complex quantity of magnitude (| Z |) which represents the ratio of the voltage difference amplitude, and the exponential function of the phase angle ( ф

) and the imaginary unit (j ) [183].

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𝒁 =

∆𝑼

∆𝑰

= |𝒁|𝒆

−𝒋 ф

= 𝑹𝒆(𝒁) + 𝒋𝑰𝒎(𝒁)

(37)

The most widely used representation of ECs impedance data is the so-called

Nyquist plot representing the imaginary part of impedance versus the real part.

However, the major shortcoming of this kind of representation is the lack of information about frequency on the plot. Therefore, another popular representation is the Bode plot, where the phase angle ( ф

) is represented vs frequency, usually in conjunction with the magnitude plot (| Z | vs log f ), to evaluate how much a signal is phase-shifted.

Electrochemical systems are often very complex, needing to be modeled with a combination of many elements. The most often used components are the double-layer capacitance ( C dl

) and the equivalent series resistance ( R s

). For an ideal electrochemical capacitor, where the total amount of charge comes from ions of the electrolyte, the

Nyquist plot is represented by a vertical line starting from the origin (Figure 28a). The

presence of an equivalent series resistance ( R s

), representing the electrical losses

(caused mainly by the electrolytic solution, but also by the separator and the electrodes during the charging and discharging processes), provokes a shift of the first point of the plot by R s

(recorded at the highest frequency) towards higher values on the Re(Z) axis

(Figure 28b). Figure 28c presents the creation of the semicircle with two intersect points

on the real axis: the R s

(equivalent series resistance) point and the R s

+R f

point which contains the equivalent series resistance and the charge transfer resistance which is developed by the charge-complexes close to the Helmholtz plane. The contribution of diffusion in impedance is represented by the so-called “Warburg impedance element”

( W

) which is presented in the Randles circuit (Figure 28d), which consists of the

equivalent series resistance ( R s

) in series with the parallel combination of the double layer capacitance ( C dl

) and the charge transfer resistance (R f

) in series with the Warburg element ( W ) [3]. The Warburg element represents the impedance of semi-infinite diffusion, and can be observed as a transition from the semicircular Im(Z) vs Re(Z) plot

to a 45° tilted line (Figure 28d).

The capacitance value at each applied frequency is calculated from equation

(38):

𝑪 = −

𝟏

𝟐𝝅𝒇 𝑰𝒎(𝒁)

(38) where C is capacitance, f - frequency and

–Im(Z)

- imaginary component.

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Figure 28 Nyquist plots of the respective equivalent circuits of (a) an ideal EDL capacitor, (b) capacitor with series resistance, (c) series resistance and capacitor in parallel with leakage resistance, (d) Randles circuit with Warburg impedance.

EIS is also found a useful method to distinguish various electrode degradation processes, e.g., current collector corrosion, increase of contact resistance, increase of electrode resistance, appearance of some inhomogeneity, adsorption processes etc., which give a resistive response. For instance, a non-homogeneous electrode structure, results in increase of interfacial charge transfer resistance ( Rf ), whereas inhomogeneities or adsorption processes can be disclosed by the presence of a constant phase element

(CPE) visible by a deviation from the pure capacitive vertical impedance response at low frequencies. Since EIS enables to propose an equivalent circuit for the studied systems, combined with other physical analyses (Electrical Quartz Crystal Microbalance for example), it helps to understand the kinetics of the occurring processes.

II.4. Accelerated ageing test

From the above techniques it is possible to get information about the performance of a capacitor (including capacitance, resistance, columbic efficiency, energy and power density) and to distinguish a pseudo-capacitive contribution from the pure EDL charge storage mechanism. In most of the scientific literature, the determination of operating limit conditions, possible perturbation phenomena, and

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e stability of an electrode material or EC system is based on galvanostatic charge/discharge cycling. In order to prove accurately the life-time stability of a device, manufacturers and end-users generally expect an accumulation of over several hundred thousand cycles. Taking into account that such tests are highly time-consuming, the length of investigation period is generally limited to few ten thousands galvanostatic cycles. Therefore, a voltage hold test (so-called ‘floating’), and in some cases using temperature higher than the ambient (to accelerate possible ageing), is considered as a relevant method of cells’ state-of-health (SOH) examination within shorter investigation time.

So far, these accelerated ageing tests have been generally applied on commercially available ECs in organic electrolytes [184, 185, 186]. However, since the presented research aims in particular to optimize the design of high voltage capacitors with salt aqueous electrolytes, the accelerated ageing protocol by floating has been applied to examine these systems, and to serve as groundwork for researchers analyzing new materials for ECs operating in aqueous electrolytes.

The validated accelerated ageing protocol is based on a combination of five galvanostatic charge/discharge cycles followed by high voltage 2-hours floating periods

(Figure 29). The few galvanostatic charges/discharges, in an amount of five cycles, are

employed for two reasons: (i) to evaluate the discharge capacitance and ESR values needed for the SOH assessment; (ii) to restore the system to its initial state after a high voltage floating period, which promotes packing of ions in hardly reachable pores (see part III.1). Hereof, the capacitance C is computed from the galvanostatic discharge in the range (Δ

U

2

) and the time (Δ t ) taken for this process, whereas the ESR is calculated from the voltage drop



U

1

, when the current changes from I (charge) to -I (discharge)

(ESR=



U

1

/2I ). Then, C and ESR are plotted versus the cumulated floating time. An EC is usually considered by manufacturers as out of service when the ESR is increased by

100% or the initial capacitance is reduced by 20% [187]. The floating and galvanostatic sequences are repeated, until reaching at least one of the mentioned end-of-life criteria.

It is usually sufficient to perform 60 series for a total cumulated floating time of 120 hours to distinguish the main failures which can appear during operation of carbon based ECs in aqueous electrolyte, such as: increase of equivalent series resistance, capacitance loss, corrosion of the positive current collector, oxidation of carbon and electrolyte decomposition.

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Figure 29 (a) Scheme of the accelerated ageing protocol and (b) magnification of the fifth galvanostatic cycle. The fifth cycle of each series is considered to estimate the capacitance and ESR values.

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CHAPTER III

STATE OF HEALTH

OF AQUEOUS ELECTROCHEMICAL CAPACITORS

WITH STAINLESS STEEL CURRENT COLLECTORS

UNDER ACCELERATED AGEING

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As it was presented in the literature review, AC/AC electrochemical capacitors

(ECs) with gold current collectors in aqueous alkali sulfate electrolytes are able to operate up to 1.6 V in Na

2

SO

4

during 10,000 charge/discharge cycles [21, 14] and even

2 V in Li

2

SO

4

at room temperature [19]. Such high voltage is possible due to the overpotential for di-hydrogen evolution in these neutral electrolytes [136]. Since alkali sulfates are less corrosive than traditional battery electrolytes, e.g., H

2

SO

4

, they give an opportunity to realize high energy density ECs with non-noble metal current collectors, being environmental friendly, cheap and safe.

The objective of this chapter is to determine the performance limits of AC/AC electrochemical capacitors using stainless steel current collectors in 1 mol L

-1

Li

2

SO

4

.

Accelerated ageing by floating has been performed in order to determine the possible perturbation phenomena occurring in aqueous media, while using stainless steel current collectors. Since the main symptoms during ageing of ECs are a loss of capacitance and an increase of resistance, the SOH diagnosis of the ECs in Li

2

SO

4

was realized by monitoring these parameters at various periods of time during the operation of the system.

Beside electrochemical measurements, the chapter also presents the examination of gas evolution under galvanostatic cycling and floating, to disclose electrolyte decomposition as a failure which appears when an EC operates above its voltage stability limit. Post-floating measurements on carbon electrodes (specific surface area, porosity analysis, and quantification of oxygenated surface groups by

Temperature Programmed Desorption (TPD)) have been also realized to reveal the origins of performance decay during accelerated ageing of ECs in salt aqueous electrolyte with stainless steel current collectors.

The identification of factors contributing to ageing of ECs with cheap current collectors in aqueous electrolytes is the first step to allow proposing and verifying strategies for improving the long time performance of these systems, and thereby gaining the scope of the dissertation.

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III.1. High voltage ageing assessment of AC/AC electrochemical capacitors in lithium sulfate electrolyte

For the present study, a commercially available activated carbon powder (DLC

Super 30, Norit, further named S30), with a specific surface area of 1843 m

2

g

-1

, has been chosen as active electrode material (see experimental annex A.1.1). The pore size distribution (Figure A1b) indicates that the micropores of this carbon are essentially in the region of 0.8-0.9 nm; the carbon exhibits also some mesoporosity needed for enhanced charge propagation. The Temperature Programmed Desorption (TPD) analysis (Table A2) reveals a small amount of oxygenated functionalities on the surface of S30, with relatively low oxygen amount of 1.5 wt %.

1.1.

Exploring the high operating voltage of AC/AC electrochemical capacitors in lithium sulfate electrolyte

In order to estimate the maximum operating voltage of ECs with stainless steel current collectors in 1 mol L

-1

Li

2

SO

4

, the electrodes potential limits were determined by galvanostatic (200 mA g

-1

) cycling on a two-electrode assembly with reference

electrode, and were plotted vs voltage (Figure 30). The practical di-hydrogen evolution

potential represented by a horizontal line at around -0.8 V vs NHE on this figure was determined by the oscillations due to bubbling on three-electrode CVs, as shown in

Figure 20 in the literature part. This potential is much lower than the thermodynamic at

pH = 6.5 (E

-

= -0.384 V vs NHE). This is due to the reduction of water and production of OH

-

, which accordingly to the Nernst law results in an increase of local pH in the

pores of S30. As seen in Figure 30, whatever the value of voltage up to 1.6 V, the

lowest potential of the negative electrode is always higher than -0.8 V vs NHE, which means that di-hydrogen evolution at this electrode might be considered as negligible. By contrast, the positive S30 electrode operates below the thermodynamic water oxidation limit (marked by the upper dashed line) only up to a voltage of 1.4 V. Above the latter value, one might expect detrimental oxidation of the positive S30 electrode. In other words, these measurements suggest an approximate maximum operating voltage of 1.4

V for the S30/S30 cell in 1 mol L

-1

Li

2

SO

4

.

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Figure 30 Electrodes potential extrema vs voltage measured during galvanostatic (200 mA g

-1

) cycling on an S30/S30 capacitor with stainless steel collectors in 1 mol L

-1

Li

2

SO

4.

E

OCP

- open circuit potential.

Figure 31 shows the cyclic voltammograms of the individual electrodes recorded

in the potential ranges determined during galvanostatic cycling of the S30/S30 cell with reference electrode in 1 mol L

-1

Li

2

SO

4

(see Figure 30). The CVs in Figure 31 prove

that, even at voltage of 1.6 V, the lowest potential of the negative electrode is always higher than the practical di-hydrogen evolution potential (-0.8 vs. NHE), where oscillations on the curves would be visible. By contrast, an anodic current leap together with a corresponding cathodic wave appears for the positive electrode, as the potential for oxygen evolution (0.845 V vs. NHE) is exceeded. The anodic peak might be also related to the electrochemical oxidation of the carbon electrode [188], the redox reactions between the generated oxygenated surface groups and the electrolyte [131], and the corrosion of the positive stainless steel current collector.

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-1

) of the individual electrodes for a S30/ S30 cell in 1 mol L

-1

Li

2

SO

4

. Scans are realized up to voltages of 0.8 V, 1.0 V, 1.2 V, 1.5 V and 1.6 V.

Overvoltage is the most commonly suspected origin for capacitors ageing, which leads to decomposition of electrolyte and final starvation of ions, corrosion of positive current collector and damaging of the positive electrode material by surface oxidation and/or looseness of the electrode materials due to gas evolution. Therefore, taking into account the data presented in Figures 30 and 31, real operating limits, possible perturbation phenomena, and stability of S30/S30 cells with stainless steel collectors in

1 mol L

-1

Li

2

SO

4

have been determined by accelerated ageing tests and are presented in the section III.2.

1.2.

Degradation of ECs electrochemical performance under accelerated ageing

Taking into account that an EC can operate millions of cycles, potentiostatic floating is much more efficient than galvanostatic cycling to determine quickly the EC operation stability limit and the perturbation phenomena which affect its stability. In case of batteries, beside detrimental effects of applying high voltage, charge transfer reactions which occur in the bulk of the electrodes at intermediate voltages may be even harmful. This is why galvanostatic cycling is usually performed to demonstrate the stability of battery systems. By contrast, in the case of electrochemical capacitors, the electrochemical degradation reactions do occur only at high voltage [131]. Hence, when

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e an EC is investigated by galvanostatic charge/discharge, its voltage is most of the time

below the stability limit (Figure 32).

Figure 32 Schematic representations of voltage profile for a capacitor during floating and galvanostatic cycling with potential limitation.

The changes which appear during operation of capacitors, such as electrolyte decomposition, corrosion of positive current collector and damaging of the positive electrode material by oxidation of the carbon surface and/or looseness of the electrode material due to gas evolution entail a deterioration of the electrochemical performance generally revealed by a decrease of capacitance and an increase of resistance. Therefore,

S30/S30 cells with stainless steel collectors in 1 mol L

-1

Li

2

SO

4

have been floated at 1.6

V and 24°C (RT), and the capacitance and resistance values were determined after each floating sequence. Each two-hour period was preceded and followed by five galvanostatic (1 A g

−1

referred to the average active mass of both electrodes) charge/discharge cycles. If not mentioned otherwise, the capacitance and resistance were estimated from the 5 th

discharge (see chapter II.4). Each series consisting of galvanostatic cycling and floating period was repeated 60 times for a total floating time of 120 hours. To stabilize the wetting of fresh electrodes, cyclic voltammetry (100 cycles) up to 1 V at a scan rate of 10 mV s

−1

was applied to all systems before starting floating.

Figure 33 shows that, at any time of the test, the capacitance values calculated

from the 1 st

discharge (just after floating) are always higher than estimated from the 5 th discharge. This is explained by the fact that a prolonged high voltage period promotes a

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e better packing of ions, especially in the smallest pores, where the ions penetrate more slowly due to steric effects. These narrow pores filled with ions during floating are liberated after the few galvanostatic discharges to 0 V, and cannot be then reached again by the ionic species during galvanostatic charging; they are accessed only during the next voltage hold. Five galvanostatic cycles have been found sufficient to restore the system to its initial state after a high voltage floating period. Therefore, capacitance and resistance determined from the 5 th

discharge are further considered to be representative of the system’s performance.

Figure 33 Capacitance of an S30/ S30 capacitor in 1 mol L

-1

Li

2

SO

4

vs floating time at

1.6 V and 24

⁰

C measured from the 5 th

( ○ ) and 1 st

( ■ ) galvanostatic discharge.

As the floating voltage increases from 1.5 V to 1.7 V, the galvanostatic discharge capacitance increases from 80 F g

−1

to 123 F g

−1

. This capacitance enhancement with voltage is attributed to a better packing of ions leading to a decrease of EDL thickness ( d in equation (4)). Due to this effect, to better estimate the effect of voltage on the SOH of the S30 / S30 capacitor in Li

2

SO

4

electrolyte, in Figure 34,

capacitance and resistance are referred to their initial values, C/C

0

and R/R

0

, respectively. The evolution of the two parameters has been further analyzed by taking into account the end-of-life criteria generally accepted by manufacturers, e.g., an increase in resistance by 100% or a decrease in capacitance by 20%, as compared to the initial values [187]. During floating at 1.7 V, the capacitance decreases by 20% after 70

cumulated hours (Figure 34a) and the resistance increases by 100% after only 40 hours

(Figure 34b), while when the EC is aged at 1.6 V, the capacitance drops by 20% after

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120 h and the resistance is doubled after 50 h. Consequently, the cells aged at 1.6 V and

1.7 V are no longer in good SOH after 50 and 40 hours, respectively. It is interesting to notice that, in both cases, the lifetime is controlled essentially by the increase of resistance. It means that one can suspect parasitic phenomena altering the contacts between the active material and the current collectors or between the carbon particles themselves in the electrodes, due for example to corrosion of current collectors or gas evolution.

When the floating voltage is reduced to 1.5 V, the SOH of the cell is still excellent after a long period of 120 hours. The progressive increase of capacitance with floating time is attributed to the progressive penetration of ions in pores of small size.

As compared to 1.6 V or 1.7 V floating, the increase of resistance after floating at 1.5 V is much smaller. However, its slight increase still reveals some detrimental effects, which will be further presented in the next sections of this manuscript. Notwithstanding, the stability voltage determined by floating fits quite well with the conclusions driven from Figures 30 and 31, and regarding effect of oxidation on the positive electrode data.

Figure 34 Effect of the floating voltage at 24

⁰

C on (a) relative capacitance and (b) relative resistance of an S30/ S30 EC in 1 mol L

-1

Li

2

SO

4

.

To perceive dissimilarities between the impacts of galvanostatic cycling and floating on ageing, the capacitance and resistance of the S30 / S30 electrochemical capacitor (with stainless steel collectors in 1 mol L

-1

Li

2

SO

4

) has been plotted during

galvanostatic cycling up to 1.6 V or floating at 1.6 V (Figure 35). The voltage of 1.6 V

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has been selected in light of Figure 34 which revealed that detrimental reactions are

negligible during floating at lower voltage. Figure 35 shows that the capacitance

decreases by 6% after 285 hours (10,000 galvanostatic cycles) at a current density of 1

A g

-1

(Figure 35a), while for the cell aged by floating, the capacitance loss of 6% is

evidenced after only 124 hours (55 th

floating series). Despite the capacitance packing occurring at the beginning of floating and leading to a transient increase of capacitance, the time for reducing the capacitance by 6% is shorter than in galvanostatic cycling. The efficiency of floating for ageing the cell, as compared to galvanostatic cycling, would be even amplified if the investigation time would be extended. The poor effectiveness of

galvanostatic cycling is well-illustrated by Figure 32, showing that the time at high

voltage to provoke degradation phenomena in the cell is extremely short. This time is even shorter, because the imposed voltage is not reached due to the ohmic drop (in the present case, the ohmic drop at current density of 1 A g

-1

is 8 mV). The acceleration of cells’ ageing can be even easier seen by comparison of resistance evolution of the two

cells (Figure 35b). For the system aged by galvanostatic cycling, the resistance is very

stable until the end of the test, whilst R/R

0 increases just from the initial 2-hour floating sequence at 1.6 V.

Figure 35 Comparison of stability performance of S30/ S30 cells in 1 mol L

-1

Li

2

SO

4 during floating at 1.6 V and galvanostatic (1 A g

-1

) cycling up to 1.6 V: (a) capacitance and (b) resistance evolution.

Notwithstanding, Figure 35 reveals differences in the profiles of capacitance

and resistance evolution depending on the ageing procedure, either by GCPL or by

floating. As observed previously for the cells floated at 1.5, 1.6 and 1.7 V (Figure 34),

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e the capacitance of the cells initially increases, due to a better wetting of the electrodes by the electrolyte during prolonged polarization, allowing narrow pores to be accessed by ions. The initial capacitance enhancement could be also the result of electrostatic field modification on the electrode surface due to an increase in oxygen content, allowing the micropores to be more easily wetted by the ions from the aqueous electrolyte [189, 82, 83]. When all accessible pores are reached by floating at 1.6 V, the capacitance remains almost constant for a period of time, until it starts to decay

continuously after 50 hours of floating (Figure 35), indicating a progressive ageing of

the system, which is the most probably due to the reduction of accessible surface area of the S30 electrodes. Therefore, due to this kind of capacitance and resistance evolution profile, depending on the ageing procedure, it is not easy to establish a correlation between a 2-hour floating period and a number of corresponding galvanostatic cycles.

To appreciate the importance of side reactions contributing to ageing by floating, it is interesting to compare the evolution of leakage current (LC) during 60 repeated

sequences at 1.5 and 1.6 V (Figure 36) [3]. As shown in Figure 36a, during

potentiostatic floating, the leakage current rapidly decreases within a few minutes and then stabilizes at an equilibrium value. The LC drop is related to the structure of the double-layer formed at the electrode/electrolyte interface during charging. As mentioned before, according to the Grahame double-layer model, the EDL consists of:

(i) a diffusion layer, with ions weakly interacting with the carbon electrode, (ii) and a compact layer where ions strongly interact with the electrode [8]. The dramatic current decay originates from the loss of charge, when weakly interacting ions flow to the bulk of the electrolyte. As the floating time proceeds, the ions of the diffusion layer are pushed to the compact layer, and the structure of the EDL is ordered, until reaching equilibrium [190]. However, in an electrochemical capacitor, the electrodes are made of a porous network which hinders so simple charge exchange; moreover, in the confined volume of micropore, the traditional models of the EDL are not applicable.

The initial values of leakage current measured at the beginning of each floating sequence at 1.6 V increase with the number of floating periods, indicating a slower transition from galvanostatic charging current to leakage current. The equilibrium leakage current itself reveals the occurrence of side reactions: the higher its value, the higher amount of charge contributes to side reactions [3]. While the profile of equilibrium leakage current is almost constant during ageing at 1.5 V, it can be observed

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e that it increases from eight cumulated floating hours at 1.6 V, reaching its highest value

at ~50 hours. By comparing Figure 34a and Figure 36b, it is interesting to note that both

capacitance and the equilibrium leakage current increase up to fifty cumulated floating hours at 1.6 V, revealing a modification of electrostatic field on the oxidized surface of the carbon electrode, allowing the micropores to be more easily wetted by the ions from aqueous electrolyte [189, 82]. When considering the capacitance, from 8 to 50 hours, there is a balance between capacitance increase related to access to narrow pore and capacitance decrease related to side reactions, and after 50 hours capacitance decays because new pores are no longer accessed. The charge revealed by the leakage current is utilized for, e.g., decomposition of electrolyte, corrosion of current collectors, resulting in resistance increase by 100% after 50 cumulated floating hours at 1.6 V. The parallel decrease of equilibrium leakage current and capacitance is attributed to the blockage of pores entrances and hindrance of ions access, due to the formation of oxygenated surface groups and gases together with the deposition of corrosion products.

Figure 36 (a) Leakage current profile on an S30/ S30 capacitor in 1 mol L

-1

Li

2

SO

4 electrolyte during one two-hour floating period at 1.6 V; (b) Evolution of leakage current during 60 two-hour floating sequences at 1.5 V and 1.6 V.

The foregoing demonstrates that floating in potentiostatic conditions is an accurate method to simulate aging during the performance of S30 / S30 ECs in aqueous lithium sulfate electrolyte and to monitor their SOH. The obtained results also clearly

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e reveal that GCPL with a limited number of cycles (ca 10,000) is definitively not an accurate test to determine the maximum operating voltage of ECs. Notwithstanding, the differences in the profiles of capacitance and resistance evolution depending on the ageing procedure at 1.6 V, either by GCPL or by floating, disclose that it would be inaccurate to estimate the floating ageing time corresponding to a given number of galvanostatic cycles.

Nevertheless, the performed experiments allowed to assess that S30 / S30 ECs in

1 mol L

-1

lithium sulfate can operate with a very long cycle life at voltage as high as 1.5

V. At the same time, they indicate that most of the claims in the literature have to be considered with great care, meaning that the mentioned voltage values should be reduced by around 0.3–0.4 V, especially when gold current collectors were used.

Notwithstanding, the voltage value of 1.5 V is still remarkably high for an aqueous electrolyte compared to only 0.7–0.8 V generally possible for KOH or H

2

SO

4 electrolytes. However, to move towards further optimization of the high voltage AC/AC

ECs with stainless steel collectors in aqueous 1 mol L

-1

Li

2

SO

4

, the possible perturbation phenomena under long time operation must be identified.

III.2. Factors contributing to ageing in aqueous electrolyte

As presented in section III.1, the long term operation of an EC in 1 mol L

-1

Li

2

SO

4

at voltages higher than 1.5 V leads to ageing of the components, revealed by a

drop of capacitance and an increase of resistance (Figure 34). The plot of leakage

current evolution during repeated floating sequences at 1.6 V (Figure 36b) indicates the

occurrence of side reactions which contribute to ageing.

2. 1.

Oxidation of carbon electrodes and corrosion of stainless steel current collectors

2.1.1. Post-floating analysis of ECs by electrochemical techniques

We have used electrochemical impedance spectroscopy (EIS) to distinguish the origins of electrode degradation processes, such as electrolyte decomposition, and other possible perturbation phenomena (i.e., oxidation of S30 electrode and/or corrosion of stainless steel collectors, internal pressure evolution). EIS data at open circuit voltage

(OCV) have been compared for a freshly built cell with S30 electrodes in 1 mol L

−1

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Li

2

SO

4

and after 120 hours of ageing at 1.6 V (Figure 37). Due to floating, the ESR of

the system increases from 0.7 to 1.1 Ω, suggesting a more resistive path of ions in the active surface of S30 and probably worse contact between the grains of S30 due to gas evolution. The increase of equivalent distributed resistance (EDR) causes a decrease of the slope in the low frequency branch and is attributed to the limited penetration of ions in the pores of electrodes due to side reactions. Moreover, the presence of a constant phase element (CPE) visible by deviation from the pure capacitive vertical impedance

response at low frequencies (Figure 37a) indicates a non-uniform thickness of the

double-layer, inhomogeneity or adsorption processes [186]. The increase of interfacial charge transfer resistance R f

from 1.2 to 24.9 Ω results probably from an uneven distribution of current in the positive electrode, due to the appearance of corrosion products. The shift in transition from the high frequency semicircle to the midfrequency distributed charge storage impedance region suggests time dependence in the charging process, probably as a result of a low conducting layer formed by corrosion products.

The capacitance vs frequency dependence (Figure 37b) reveals almost constant

capacitance at low frequency for the fresh cell, which exhibits a typical EDLC behavior.

The almost ideal performance of the system in the initial state is confirmed in the Bode

plots by phase angle values very close to -90° at low frequency (Figure 37c). After 120

floating hours, the capacitance is higher than before ageing (107 F g

−1

compared to 73 F g

-1

) at the lowest frequencies, and it decreases more rapidly than of the fresh cell in the frequency range 0.1 Hz–1 Hz (phase close to 0

◦

). The higher capacitance values measured up to 0.1 Hz for the aged cell originate from faradaic contribution, probably due to a conductive layer of corrosion products and oxygenated surface functionalities on the S30 electrode. Since EIS does not involve current and potential variations, it allows distinguishing the phenomena occurring in the porosity of electrodes (at high frequencies) and at the electrode/current collector interface (at low frequencies).

Contrariwise, in the transient techniques by measuring current or potential response (CV and GCPL), the changes in the electric field of each component of the EC cannot be differentiated. Therefore, for the aged cell, the capacitance value of 107 F g

−1 determined by EIS at the electrode/current collector interface is not disclosed by CV or

GCPL, where the capacitance decrease related to blocked porosity of S30 is dominant.

Paula Ratajczak

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Figure 37 Impedance spectroscopy data on an AC/AC capacitor in 1 mol L

-1

Li

2

SO

4 before floating (

○

) and after 60 two-hour periods of floating at 1.6 V (

Δ

): (a) Nyquist plots; (b) Capacitance vs. frequency; (c) Bode phase angle.

Paula Ratajczak

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The time constant

τ

for the fresh and aged cells, calculated at the frequency

where reactance and resistance are equal (-45◦ phase angle) (Figure 37c) [119], is 4 s

and 100 s, respectively, showing an important decay after ageing and confirming deleterious changes of the S30 / S30 system in lithium sulfate electrolyte after ageing at

1.6 V at room temperature.

In order to analyze the energy storage abilities of ECs after ageing at 1.5 V and

1.7 V, cyclic voltammetry (10 mV s

-1

) was used to record the capacitive current

variations (Figure 38). The fresh cells display a typical rectangular shape of CV,

whereas after each 20 series of floating sequences at 1.5 and 1.7 V the ECs exhibit a more resistive character by an inclined voltammogram; the increase of capacitive current at low voltage values could be attributed to pseudo-capacitive contributions related with the creation of surface oxygenated groups on the positive carbon electrode.

Considering the CV curves recorded up to the ageing voltage of 1.5 V (Figure 38c) and

1.7 V (Figure 38d), the diminishing of capacitive current at voltage higher than 1 V

negatively affects the deliverable energy and power density. The capacitance decrease and resistance increase can be attributed to electrolyte starvation as a consequence of reduced ion availability [191]. Indeed, due to electrolyte decomposition, the electrolyte reservoir decreases, and is finally not sufficient to cover the working surface area of electrodes at higher voltages. The second possible explanation for the narrowing of

CVs, as voltage increases, is the saturation of the electrode material porosity by the stored ions. The deposition of decomposition/corrosion products in the porosity of the positive electrode and/or the formation of surface oxygenated groups reduces the S30 accessible pore volume for ions, thus leading to a fade of capacitive current at voltages higher than 1 V [192]. Furthermore, the formation of such products results in an increase of the leakage resistance ( R f

), due to worse charge propagation after 20 floating

series at 1.7 V (Figure 38b, d). The phenomenon is not pronounced for the cell aged at

1.5 V; however, the slight saturation of the carbon porosity by the stored ions during the prolonged high voltage period discloses a detrimental effect of 1.5 V for the deliverable energy and power density after even 20 series of accelerated ageing.

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Figure 38 Cyclic voltammograms (10 mV s

-1

scan rate) recorded after each 20 series of two-hour floating sequences (a and c) at 1.5 V and (b and d) at 1.7 V on a S30/ S30 capacitor in 1 mol L

-1

Li

2

SO

4

.

2.1.2. Post-floating analyses on carbon electrodes

To explain the above demonstrated changes in electrochemical performance of the aged ECs, the oxygenated surface functionality of aged electrodes has been characterized and quantified by Temperature Programmed Desorption (TPD). To avoid the interference of the electrode binder during the post-floating analysis of electrodes by

TPD, self-standing electrodes from activated carbon cloth (ACC 507-20, Kynol,

S

BET

=2231 m

2

g

-1

and L

0

=0.99 nm) were selected. The TPD analyses have been realized in helium atmosphere at 20 °C min -1

up to 950 °C, on the pristine ACC, the positive and

negative aged ACC electrodes treated. Figure 39 presents the mass loss, and CO

2

and

CO profiles obtained by TPD for the fresh ACC and the positive and negative electrodes extracted from the EC after 120 floating hours at 1.7 V. The important weight loss at

950 °C for the positive carbon electrode is related to an important surface oxidation and formation of functionalities evolving essentially as CO

2

together with a lesser amount of

Paula Ratajczak

P a g e 78


CO. The surface of the aged positive electrode is modified by new oxygenated groups, while some other functionality disappeared (disappearance of CO

2

peak at 850 °C) as compared to the pristine ACC. The aged negative electrode is also slightly oxidized, although one could expect that it should not occur under negative polarization of this electrode. In fact, di-oxygen which is formed at the positive electrode is dissolved in the electrolyte, being able to diffuse to the negative electrode to form peroxide ions which oxidize the carbon material [188]. The cumulated amount of CO

2

evolved from the positive and negative aged ACC electrodes is 3.6 and 2.2 mmol g

-1

, respectively, as compared to 0.9 mmol g

-1

for the as-received ACC. The cumulated released CO is 3.9,

1.1 and 0.9 mmol g

-1

, for the positive, negative and untreated ACC electrodes, respectively.

Figure 39 TPD on pristine ACC (full line) and on positive (dashed line) and negative

(dotted line) ACC electrodes after 120 hours of floating at 1.7 V in 1 mol L

-1

Li

2

SO

4

: (a)

CO

2

evolution; (b) CO evolution.

A multiple Gaussian function was used for the deconvolution of the CO

2

and CO patterns and to determine the types of oxygenated complexes formed on the surface of

the aged positive electrode [92, 93]. Figure 40a presents the CO

2

desorption peaks at

270 °C (peak 1), 500 °C (peak 2) and 620 °C (peak 3), which are attributed to carboxylic and two kinds of peroxide groups, respectively [193]. The quite stable

oxygenated complexes desorbed as CO at 710 °C (peak 1) and 920 °C (peak 2) (Figure

40b) are assigned as carbonyl/quinone groups and pyrone-type structures, respectively

[193, 194]. The deconvolution of the TPD patterns for the positive electrode (Figure 40a

and b) includes sharp peaks noticeable at around 700 °C which, together with a

Paula Ratajczak

P a g e 79


discontinuity in the TG curve (Figure 39), are probably associated to the catalytic

desorption of oxygenated functionalities, due to the presence of metallic impurities resulting from the corrosion of the positive stainless steel collector. The new oxygenated groups appearing on the surface of the negative electrode can be recognized as peroxide groups, desorbing as CO

2

at 550-600°C [193].

Figure 40 Deconvolution of TPD patterns for the positive ACC electrode after 120 hours ageing at 1.7 V: (a) CO

2

pattern; (b) CO pattern (

—

, TPD experimental data; ---, individual peaks;



, sum of the individual peaks).

Hence, the resistance increase which is observed during floating might be at least partly related to the formation of surface groups on the porous carbon electrodes.

Similarly, these groups also contribute to the decay of capacitance during floating

(Figure 34).

To better demonstrate the destructive effect of accelerated ageing in 1 mol L

−1

Li

2

SO

4

, nitrogen adsorption/desorption isotherms at -196 °C have been recorded on a fresh S30 electrode (pellet with 85 wt. % of DLC Super 30), and on positive and

negative electrodes aged by 120 floating hours at 1.7 V (Figure 41). The micro

V micro and mesopore volumes V meso

were obtained directly from the calculated cumulative pore size distribution (PSD) determined using the 2D non-local density functional theory

(2D-NLDFT) [107]. The porous texture data were referred to the total mass of one electrode. Table 1 shows, as expected, that S

BET of the positive aged electrode decreases after ageing.

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Electrode

BET

V micro

V meso surface

< 2 [nm] 2-50 [nm] area

[m

2

g

-1

] [cm

3

g

-1

] [cm

3

g

-1

]

Average micropore size

[nm]

1348 0.528 0.125 0.92 fresh electrode aged positive

895 0.355 0.074 0.89 aged negative

1345 0.525 0.128 0.92

Table 1 Porosity data obtained by nitrogen adsorption at -192°C on a fresh electrode and on positive and negative electrodes aged by floating a S30/ S30 capacitor during

120 hours at 1.7 V in 1 mol L

-1

Li

2

SO

4

.

The 2D-NLDFT pore size distribution presented in Figure 41b does not reveal

any significant change in the porous texture of the negative electrode, whereas all pores of the positive electrode are affected by floating at 1.7 V (as shown in Table 1, the average micropore size L

0

remains unchanged). The volume of micropores (V micro

) and mesopores (V meso

) is around 1.5 times lower for the positive electrode compared to the fresh one. The reduction of SSA and pore volume for the aged positive electrode supports the assumption that capacitance decay and resistance increase of S30 / S30 cells is due to a partial blockage of pores by oxygenated functional groups and/or decomposition and corrosion products.

Figure 41 (a) Nitrogen adsorption/desorption isotherms at -196 °C and (b) 2D-NLDFT pore size distribution of a fresh S30 electrode and of aged positive and negative electrodes after 120 h of floating at 1.7 V in 1 mol L

-1

Li

2

SO

4

.

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2.1.3. Effect of temperature on ageing

Since the application of 1.7 V potentiostatic floating was found to be deleterious for the positive electrode, and consequently for the cell, and accelerated ageing at 1.5 V

indicates no important resistance increase after 120 hours at 24 °C (Figure 34b), we

have investigated the effect of raising the temperature to 35 °C and 40 °C, while floating at 1.5 V. After ageing a S30 / S30 capacitor in 1 mol L

-1

Li

2

SO

4

at 1.5 V and 40

°C and opening the cell, a russet colour attributed to corrosion has been perceived

essentially on the positive stainless steel current collector and on the separator (Figure

42).

Figure 42 Corroded components of a S30/ S30 capacitor in 1 mol L

-1

Li

2

SO

4 after 120 hours of floating at 1.5 V and 40°C.

Figure 42 shows the effect of floating at 1.5 V and different temperatures on

relative capacitance and resistance evolution of S30 / S30 cells in 1 mol L

-1

Li

2

SO

4

; for

comparison, the data of Figure 34 obtained at 24°C are also reported. During the first 50

hours of floating at 1.5 V and 40 °C, the capacitance increases while resistance slightly decreases. Such behaviour is attributed to a better mobility of ions at higher temperature, which enhances the electrolyte penetration in the porosity of S30.

However, as the floating time proceeds at 40 °C, the capacitance starts to decline and

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e the resistance increases more rapidly than at lower temperature, at which the parasitic reactions in the system are reduced. Nonetheless, it is important to notice, that even at

1.5 V, the perturbation phenomena are triggered when increasing the temperature by around 15 °C. These so-called perturbations are essentially related to electrolyte decomposition, which leads to carbon oxidation and decreases its conductivity, and the formation of corrosion products at the interface between the current collector and the carbon electrode. Therefore, gas evolution has been monitored during cycling and accelerated ageing at RT, 35 °C and 40 °C, and is presented in part 2.2.

Figure 43 Effect of temperature increase on the accelerated ageing of S30/ S30 capacitors in 1 mol L

-1

Li

2

SO

4

at 1.5 V: (a) relative capacitance; (b) relative resistance.

2.2.

Gas evolution during floating

At positive electrode potential higher than the value for electrolyte oxidation, gases such as di-oxygen, CO and CO

2

may evolve, and activated carbon be oxidized

[131]. Likewise, below the reduction potential of water at the negative electrode, hydrogen is produced. The generation of gases at the carbon electrodes results in a rise of cell internal pressure and may contribute to reducing its lifetime.

When a capacitor device is extremely overcharged, excessive gassing at the electrodes may cause leakage, cracks and permanent damages of cell constituents or even explosion. The installation of safety vents, which open if the overpressure limit is exceeded, generally solves the security issues [195]; however, it does not solve the loss of electrolyte accompanying its

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e decomposition. For this reason, to investigate the SOH of an EC, it is necessary to simultaneously monitor pressure inside the cell during long time performance at overcharged state.

A special cell was designed for in-situ monitoring pressure evolution during

GCPL or floating tests. It is made from a stainless steel 316L case with an outlet to

which a pressure sensor can be connected (Figure 44a). The cell was assembled by

sandwiching a 16 mm diameter separator (AGM, Bernard Dumas, thickness = 0.52 mm) between S30 pellet electrodes (16 mm in diameter), and then introducing the sandwich in a PTFE guide sleeve which is placed in the lower case of the cell. Then, the separator and the carbon electrodes were soaked with the electrolyte and pressed by a stainless steel plate and a spring, before screwing the upper cover together with the lower one. In order to improve the accuracy of pressure measurements, the system was completely filled with electrolyte (around 3 mL) through the upper outlet, such a way that the dead volume is minimized. A digital pressure sensor KELLER 35X Ei (pressure range 0–3

bars; total error band of 0.05 %) (Figure 44b) was then connected to the upper outlet. A

climatic chamber (Suszarka SML 25/250 ZALMED, Poland) was used to stabilize the cell temperature at ± 1 ºC. The pressure values were recorded using the READ30 software.

Figure 44 (a) Main components of the pressure test cell; (b) Test cell inside the climatic chamber connected to the KELLER 35X Ei pressure sensor and to the potentiostat.

Paula Ratajczak

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The internal overpressure was first measured during few GCPL cycles with a current density of 1 A g

-1

at room temperature (Figure 45). The plot clearly indicates a

higher rate of electrolyte decomposition and higher internal pressure evolution at 1.8 V than at 1.6 V and 1.5 V, which can finally lead to electrolyte depletion and/or lower cohesion of the electrode material.

Figure 45 Internal overpressure during galvanostatic (1 A g

-1

) cycling of a S30/ S30 cell in 1 mol L

-1

Li

2

SO

4 at 24°C up to 1.5 V, 1.6 V and 1.8 V.

The pressure variations at 24 °C were measured under ageing consisting of 4 2hour potentiostatic periods at 1.5 V, interspaced with five galvanostatic cycles at 1 A g

-

1

. Thereafter, the EC was maintained at open circuit voltage (OCV, self-discharge) for

24 hours (Figure 46a). A floating sequence at 1.5 V, preceded and followed by galvanostatic cycling up to 1.5 V, is shown versus time in Figure 46b. During the 2-

hour voltage hold period at 1.5 V, pressure increases linearly by ~207 mbar. During charging at constant current, the pressure is first stable and it starts to increase as

voltage is higher than ~1 V (Figure 46c); a steep pressure growth is observed above

around 1.25 V. This latter value is in agreement with the data obtained by galvanostatic

cycling on a two-electrode assembly with reference electrode (Figure 30), where the

positive S30 electrode reaches the thermodynamic oxygen evolution potential. During galvanostatic discharge, the pressure declines very slightly, indicating that even

Paula Ratajczak

P a g e 85

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e galvanostatic cycling at 1.5 V can reduce the lifetime on account of electrolyte depletion and/or loss of electrode cohesion. Moreover, when considering the self-discharge at

OCV from 1.5 V (Figure 46a), the pressure slightly decreases. Such profile can be

assigned to very slow gas recombination into water and/or partial dissolution of gases in the electrolyte [196, 197] .

Figure 46 a) Internal overpressure evolution at 24 °C during full ageing protocol at 1.5

V on a S30/ S30 cell in 1 mol L

-1

Li

2

SO

4

; (b) magnification of pressure evolution during one 2-hour sequence preceded and followed by galvanostatic cycling at 1.5 V (1 A g

-1

);

(c) magnification of pressure evolution during galvanostatic cycling (1 A g

-1

) up to 1.5

V.

Paula Ratajczak

P a g e 86


To observe the effect of temperature on gas evolution, potentiostatic floating has been performed at 1.5 V and different temperatures on S30 / S30 cells in 1 mol L

-1

Li

2

SO

4,

with simultaneous measurement of pressure evolution (Figure 47); for

comparison, the data of Figure 46b obtained at 24 °C are reported. For the three ECs

analyzed at 24 °C, 35 °C and 40 °C, the pressure increases linearly with floating time.

Considering the SOH of the S30/ S30 cells in 1 mol L

-1

Li

2

SO

4

, it can be easily seen that, even during the first floating period at 1.5 V, the gas evolution increases when increasing the temperature by around 15 °C.

Figure 47 Internal overpressure evolution at 24 °C, 35 °C and 40 °C during a 2-hour floating period at 1.5 V on a S30/S30 cell in 1 mol L

-1

Li

2

SO

4

.

These pressure evolution data disclose that the factors which lead to long time performance deterioration of S30/S30 cells in aqueous Li

2

SO

4 at different temperatures are related to electrolyte decomposition. This proved phenomenon entails carbon oxidation and its conductivity decrease, while corrosion products are formed at the interface between the current collector and the carbon electrode.

III.3. Conclusion

The performed experiments revealed that, to know perfectly the state of health

(SOH) of an electrochemical capacitor with stainless steel collectors in aqueous lithium sulfate electrolyte, it is necessary to monitor simultaneously the pressure inside the cell, capacitance and resistance at various lifetimes of the system and at various temperatures. The monitoring of these parameters under potentiostatic floating allowed

Paula Ratajczak

P a g e 87

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e to assess that ECs in 1 mol L

-1

lithium sulfate electrolyte can operate with a very long cycle life at voltage as high as 1.5 V at room temperature; such voltage is around 0.3–

0.4 V lower than the values mentioned in the literature when using gold current collectors. Notwithstanding, the value of 1.5 V for the system in 1 mol L

-1

Li

2

SO

4

is remarkably high when compared to 0.7–0.8 V for standard aqueous electrolytes (KOH or H

2

SO

4

).

At voltage higher than 1.5 V, the decrease of capacitance during floating is related to: (i) reduced accessible surface area due to oxidation of the carbon surface or pore blockage by electrolyte decomposition or corrosion products; (ii) electrolyte decomposition leading to ionic starvation in the electrode. The resistance increase under accelerated ageing is generally due to: (i) electrolyte decomposition which leads to deposition of corrosion products in the separator and on the positive electrode surface

(ionic contribution) and may be also caused by increased contact resistance between the electrodes and current collectors (electronic contribution); (ii) gas products evolution leading to weakening of the adhesion between the active mass and the current collector, and also to the electronic contribution to contact resistance at the electrode/current collectors interface.

Taking into account the results of this chapter, to improve the long time performance of carbon-based electrochemical capacitors in neutral salt aqueous electrolyte, strategies should be particularly intended to: (i) reduce the corrosion of stainless steel collectors and decrease its destructive effect on ECs operation; (ii) and avoid the decomposition of aqueous electrolyte through a shift of operating electrodes potentials towards lower values. The results of these strategies will be presented in the next chapter.

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CHAPTER IV

STRATEGIES FOR IMPROVING

THE LONG TIME PERFORMANCE

OF HIGH VOLTAGE CAPACITORS

IN AQUEOUS ELECTROLYTES

Paula Ratajczak

P a g e 89


The investigations performed in chapter III allowed disclosing the most important factors contributing to ageing during high voltage operation of ECs based on activated carbon electrodes and aqueous lithium sulfate electrolyte, which are: (i) carbon oxidation reducing the accessible surface area, and/or electrolyte decomposition leading to ionic starvation in the electrode [198], both causing a loss of capacitance; (ii) formation of resistive decomposition products on the current collectors or gas products evolution leading to weakening of the active mass adhesion on the current collector and to increasing of resistance. Therefore, this chapter focuses on approaches intending to cope with these possible perturbation phenomena which appear during high voltage operation of ECs based on aqueous electrolytes. To eliminate or reduce the formation of corrosion products at the interface between the AC electrode and the current collector, three tactics have been particularly introduced: i) replacement of stainless steel current collectors by nickel; ii) coating of the metallic foils with a conductive carbon layer; iii) addition of sodium molybdate to the electrolytic solution to inhibit the corrosion of steel. Finally, cells with asymmetric configuration of electrodes and coupled kinds of current collectors have been used to avoid the decomposition of aqueous electrolyte through down-shifting the operating electrodes potentials. The validity of the proposed strategies was verified by electrochemical techniques, such as cyclic voltammetry and impedance spectroscopy, as well as accelerated ageing by floating and monitoring of internal pressure evolution.

IV.1. Corrosion reduction of positive current collector

In analogy to experiments presented in chapter III, a commercially available carbon powder DLC Super 30 (Norit, S30) with a specific surface area of 1843 m

2

g

-1 has been chosen as electrode active material for manufacturing pellet electrodes. The electrodes were composed of 85 wt% S30, 10 wt% polyvinylidene fluoride as binder

(PVdF, Kynar HSV900, Arkema) and 5 wt% carbon black (C65, Timcal). To proceed in the optimization of the system, coated electrodes (see paragraph 1.2.) were realized by spreading the electrode material layer on the current collectors with an automatic applicator using a Doctor blade. In this case, the electrode composition was 83.5 wt% activated carbon YP80F (Kuraray Chemicals Co, YP80F), 8.5 wt% carbon black (C65,

Timcal) and 8 wt% polyvinylidene difluoride (PVdF, Kynar HSV 900, Arkema).

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1.1.

Alternative nickel current collectors

One of the strategies to improve the long-term stability of cheap electrochemical capacitors in aqueous electrolyte involves the replacement of stainless steel by less corrodible current collectors. In this study, nickel was used as alternative material to stainless steel, due to its availability and immunity to corrosion under negative

polarization, as shown by the Pourbaix diagram in Figure 48 [199]. Besides, due to the

pH shift to higher values (pH = 10) observed previously during operation of AC/AC capacitors in 1 mol L

-1

Li

2

SO

4

, the use of nickel, even under positive polarization, should not be a problem, since corrosion occurs only for pH lower than 8. For these reasons, at first, paragraph 1.1 will present the performance of ECs with (-) nickel/nickel (+) assembly, and then, paragraph 2.2. will show the examination of cells with (-) nickel/stainless steel (+) combination of collectors.

Figure 48 Pourbaix diagram of nickel; the dashed lines show the equilibrium potentials for (a) H

2

/H

2

O and (b) O

2

/H

2

O [199] .

To establish the performance differences due to the use of the two types of collectors, ECs were realized in Swagelok-type PTFE vessel, using 1 mol L

-1

Li

2

SO

4

as

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P a g e 91

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e electrolyte, and investigated by the electrochemical techniques. Cyclic voltammograms of S30/S30 capacitors in 1 mol L

-1

Li

2

SO

4

with stainless steel and nickel collectors

(Figure 49) demonstrate nearly square shaped curves with good charge propagation at

10 mV s

-1

scan rate.

Figure 49 Cyclic voltammograms (up to 1.7 V at scan rate of 10 mV s

-1

) of S30/S30 cells in 1 mol L

-1

Li

2

SO

4

with stainless steel and nickel collectors.

Since the dynamic behavior of both ECs is similarly good during charging/discharging, the S30/S30 cells in 1 mol L

-1

Li

2

SO

4

have been subjected to potentiostatic floating at 1.6 V over a total time of 120 hours. Contrary to the fresh cells

(dotted lines), the CVs recorded after 120 hours of potentiostatic floating at 1.6 V reveal a more resistive character of the systems with both types of collectors (full lines)

(Figure 50). After floating of the cell with nickel collectors, the capacitive current is not

diminished at voltages higher than 1 V, as it can be observed in the case of stainless steel. It suggests that the porosity saturation observed in chapter III due to the reduction of positive electrode pore volume (as a consequence of carbon oxidation or/and formation of corrosion products) is not demonstrated for the cell with nickel collectors.

Paula Ratajczak

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-1

) of S30/S30 capacitors in 1 mol L

-1

Li

2

SO

4

with stainless steel and nickel collectors: (a) fresh cells (dotted lines); (b) cells aged by floating at 1.6 V during 120 hours (full lines).

However, when opening the cell with electrodes coated on nickel after 120 hours of potentiostatic floating at 1.6 V, black and pale green deposits were noticed on the separator and at the edges of the positive current collector, and pale green on the negative one. These residues are probably associated with the observed pH variations during ageing: the pH increased to 7-8 and 10 on the surface of the positive and negative electrode, respectively. To discern the oxidation states of nickel in the discovered deposits, it is important to measure the electrode potential values during charging from 0 V to voltage of 1.6 V, which are 0.15 V-1.04 V and -0.15 V-0.56 V vs

NHE, for the positive and negative electrodes, respectively (Figure 51). Taking into

account the Pourbaix diagram of nickel (Figure 48) [199], these residues correspond to

Ni(III) (black) and Ni(II) (pale green) compounds on the positive electrode and Ni(II)

(pale green) compound on the negative one. The oxidation of Ni metal to Ni(OH)

2

,

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P a g e 93

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e appearing as a pale green residue on the edges of the positive electrode, is evidenced by an increase of the anodic current at potential of 1.04 V vs NHE [200]. Afterwards, part of the green Ni(II) can be oxidized to Ni(III) forming black Ni

2

O

3

or NiOOH deposits

[201, 202].


-1

) of individual electrodes of S30/S30 ECs in 1 mol L

-1

Li

2

SO

4 with nickel collectors, recorded up to voltage of 0.8 V, 1.0V, 1.2 V, 1.5 V, and 1.6 V; the vertical dashed lines corresponds to the thermodynamic limits for water decomposition.

Due to the good conductive properties of NiOOH, its presence will not much

impede the electrochemical performance of the EC. Figure 52 shows that, during

floating of the EC with nickel collectors at 1.6 V, the resistance remains stable till the end of the test, suggesting that the performance of the cell is not affected by the appearance of the residues. However, the exact nature of the deposits and their real effect on the cells constituents during long time operation has not been yet investigated.

Notwithstanding, while the maximum voltage for long term operation of S30/S30 ECs with stainless steel collectors under floating is 1.5 V (chapter III), the S30/S30 system with nickel collectors can operate up to 1.6 V without deterioration of the

electrochemical performance after 120 hours of accelerated ageing (Figure 52).

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Figure 52 Capacitance and resistance evolution during floating at 1.6 V of a S30/S30 capacitor with nickel collectors in 1 mol L

-1

Li

2

SO

4

.

1.2.

Improvement of the current collector/electrode interface

1.2.1.

Carbon electrodes glued to stainless steel current collectors

As it was noticed in Figure 43, the increase of resistance and decrease of

capacitance may be triggered above 35 °C, although floating at 1.5 V. After increasing the temperature to 40°C during floating at 1.5 V, corrosion products are clearly

observed on the positive stainless steel current collector (Figure 42). In order to

eliminate the deposition of these products at the interface between the carbon electrodes and the current collectors, pellet carbon electrodes were stick to the stainless steel collectors with a conductive glue (Carbon Conductive Adhesive 502,

Electron Microscopy Sciences, CG) consisting of carbon particles in a fluoroelastomer dissolved in methyl-ethyl-ketone (MEK) [203]. The capacitance and resistance of the obtained AC/AC capacitor in 1 mol L

−1

Li

2

SO

4

were measured during floating at 1.5 V

and 24 °C or 35 °C (Figure 53). At both temperatures, the profiles of capacitance and

resistance evolution remain identical to the case when pellets are in direct contact with the current collectors. However, at the end of floating, the resistance values are lower in presence of conductive glue as compared to the cell without CG at both temperatures.

Paula Ratajczak

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Figure 53 (a) Relative capacitance and (b) resistance of ECs with stick S30 electrodes in 1 mol L

-1

Li

2

SO

4

during floating at 1.5 V and 24°C or 35°C.

The resistance values calculated from the 5 th

galvanostatic discharge at 1 A g

-1 before and after 60 floating sequences at 1.5 V and 24 °C or 35 °C, with and without the

presence of conductive adhesive, are given in Table 2. The presented data reveal that

CG improves the contact between the electrodes and the current collectors, resulting in almost twice lower initial values of resistance at both temperatures. Moreover, the contact between the active mass and the current collectors is not weakened during floating by forming an insulating layer of any corrosion product at this interface.

However, some decomposition products created during prolonged floating at 35

°

C probably still block the pores of S30, which entails a capacitance decrease by 8% as

observed in Figure 53.

Resistance, Ω before floating

24 °C

0.8

with CG

35 °C

1.0

24 °C

1.6

without CG

35 °C

1.6

after floating 0.9

2.0

2.8

4.5

Table 2 ECs in 1 mol L

-1

Li

2

SO

4

with S30 electrodes placed on the stainless steel collectors with and without conducting glue (CG): resistance values determined before and after floating at 1.5 V and 24 °C or 35 °C.

Paula Ratajczak

P a g e 96


1.2.2. Nickel foil substrate

Since some improvement was given by the use of conducting glue, and to proceed further in the optimization of the system, we decided to follow the industrial way of electrodes realization, where the active material layer is spread on the current collectors with an applicator using a Doctor Blade. Moreover, since stable resistance and capacitance was observed during ageing the EC with nickel collectors at 1.6 V

(Figure 52), we decided to use nickel foil (200/201 grade, Schlenk, thickness = 20 μm)

as substrate to prepare the coated electrodes. For this study, the commercially available carbon YP80F (Kuraray Co.) with a high specific surface area of 2270 m

2

g

-1

and

L

0

=1.05 nm has been chosen as electrode active material (see experimental annex

A.1.1).

Unfortunately, accelerated ageing at 1.5 V and 24 °C revealed an insufficient contact between the nickel foil substrate and the electrode material, which peeled off from the foil during ageing. In the literature concerning EDLCs in aqueous electrolyte, it has been demonstrated that the performance, especially the contact resistance, is dramatically improved by etching the current collector in order to better anchor the coating layer [204].

Figure 54 Scanning electron microscopy (SEM) images of (a, c) nickel foil 200/201, and (b, d) soft-annealed nickel foil current collectors.

Paula Ratajczak

P a g e 97


In case of plain nickel (200/201) foil, the scanning electron microscopy (SEM) images show a relatively even surface with rows parallel to the extrusion direction

(Figure 54a and c) where the coating layer cannot be anchored. For this reason, we have

used soft annealed nickel (Schlenk, thickness = 25 μm) as substrate for the carbon

coating. The SEM images on this material (Figure 54b and d) clearly show a rough

surface, with homogeneously distributed sub-micrometric grains. The observed grains with a size of around 500-800 nm, formed from the recrystallized nickel structure during annealing, are expected to ensure well-anchored coatings when using this foil.

The differences in electrochemical properties of the two kinds of capacitors

made with plain and soft annealed nickel collectors are demonstrated in Figure 55 by

the Nyquist plots, obtained from impedance spectroscopy at open circuit voltage

(OCV). Since the separator, electrolyte, and carbon electrode material are the same in both cells, the ESR values are equal in the two systems. The slight decrease of equivalent distributed resistance (EDR) by 0.25 Ω in the case of heat-treated nickel is attributed to lower contact resistance between the coating electrode material and the current collector.

Figure 55 Nyquist plots at OCV of YP80F/YP80F cells in 1 mol L

-1

Li

2

SO

4

made with

(

o

) nickel 200/201 and (



) soft-annealed nickel foil.

Paula Ratajczak

P a g e 98


The most prominent difference between the two capacitors is visible by the decreased slope in the low frequency branch for the EC with soft annealed nickel

collectors (Figure 55). As mentioned in chapter III.2, the presence of a CPE indicates a

non-uniform thickness of EDL, inhomogeneity or adsorption processes [186]. The more developed surface area of the soft annealed nickel collector can lead to higher reactivity during electrochemical ageing in aqueous medium and to the formation of corrosion products at the interface between the active material and the collector, which is not protected by a CCI pre-coating layer.

Figure 56 presents the evolution of capacitance and resistance during floating at

1.5 V and 1.6 V and room temperature on ECs with AC electrodes coated on the soft annealed nickel. A significant increase of resistance by around 100% is observed after 6 cumulated floating hours, both at 1.5 V and at 1.6 V. The decrease of capacitance by

20% after 43 potentiostatic sequences at 1.6 V is attributed to the accumulation of products in the pores of the positive YP80F electrode. Due to the well-developed surface area of annealed nickel, the reactivity during electrochemical ageing in aqueous medium is certainly much higher than in case of plain nickel, leading to the formation of resistive corrosion products which can deposit in the bulk of the electrode material, causing capacitance decay.

Figure 56 (a) Capacitance and (b) resistance evolution during floating at 24°C and 1.5

V and 1.6 V on ECs in 1 mol L

-1

Li

2

SO

4

with YP80F electrodes coated on soft-annealed nickel collectors.

Paula Ratajczak

P a g e 99


1.2.3. Carbon conductive sub-layer

In order to both improve the adhesion of the carbon coating to the substrate and protect the electrode/nickel foil interface, a conductive carbon ink (CCI, Electrodag PF-

407A, specially designed for the production of low voltage circuitry) with finely divided carbon particles dispersed in a thermoplastic resin has been applied as a so-called ‘precoating’ layer. Apart from good electrical conductivity, CCI provides resistance to abrasion, scratching, flexing, and improves the contact of the electrode material to the substrate, as proved visually and mechanically by scratching and cross-cut testing. The

SEM image presented in Figure 57a reveals a rough surface of the CCI layer on nickel substrate, favourable to improve adhesion of the subsequently coated electrode material.

The magnified image of Figure 57b shows carbon agglomerates connected to each other by polymer fibres which ensure good mechanical features of the layer and provide good conductivity of the carbon ink.

Figure 57 Scanning electron microscopy (SEM) images of a conductive carbon ink

(CCI, Electrodag PF-407A) pre-coating of 15 μm thickness: (a) general view of the CCI surface; (b) polymer fibres connecting the carbon-based agglomerates.

A cell with electrodes made of YP80F coating on nickel foil 200/201 previously covered by a 15 μm thick CCI layer has been investigated by EIS at open circuit voltage. The low frequency line almost parallel to the imaginary part of the Nyquist plot

(Figure 58) at low frequency reveals a good penetration of ions in the pores of the

electrodes and a uniform thickness of the double-layer. The reduced CPE discloses that the conductive CCI sub-coating prevents from the adsorption of resistive deposits on the

Paula Ratajczak

P a g e 100

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e surface of nickel. As compared to the cell with the same nickel 200/201 substrate without CCI, the increase of low frequency branch slope results from the decrease of

equivalent distributed resistance (EDR) (Figure 58 and Table 3).

Figure 58 Nyquist plot at OCV of a YP80F/YP80F cell in 1 mol L

-1

Li

2

SO

4

made with nickel 200/201 collectors with (



) and without (

o

) CCI pre-coating.

Since the path of ions to the active surface area is the same in both cells with and without CCI pre-coating, they display almost identical ESR values (Table 3).

Notwithstanding, the current is distributed more evenly when the electrode coating is anchored to the substrate with help of the conductive ink. The charge transfer resistance value R f,

(responsible for the radius of the high-frequency semi-circle), is a bit lower for the cell with pre-coating as compared to the cell with plain nickel collectors.

In conclusion, anchoring of coating is improved in fresh cells either by CCI precoating of plain nickel or by use of annealed nickel. However, one should not forget that floating tests have revealed a high surface reactivity of annealed nickel, and demonstrated its incompatibility with lithium sulfate electrolyte. Therefore, potentiostatic floating is necessary to validate the improvement observed on the fresh cells using CCI pre-coated nickel collectors.

Paula Ratajczak

P a g e 101


ESR

EDR

R f

Nickel 200/201

0.77

1.78

0.41

Pre-coated nickel 200/201

0.76

1.27

0.38

Table 3 ESR, EDR and R f

values obtained from the Nyquist plots of YP80F/YP80F capacitors in 1 mol L

-1

Li

2

SO

4

made with nickel 200/201, with and without CCI precoating.

Figure 59 shows the evolution of relative capacitance and resistance during

potentiostatic floating at 1.5 V and 1.6 V on YP-80F/YP-80F cells in 1 mol L

-1

Li

2

SO

4 with electrodes coated on nickel pre-coated by CCI. Definitely, the comparison with

Figure 56 reveals a dramatic improvement in stability of electrochemical performance;

after 120 cumulated hours of floating at 1.5 V or 1.6 V, the values of resistance and capacitance are almost identical to the initial values. As for the previously examined

ECs without CCI, the more pronounced initial capacitance increase for the cell examined at 1.6 V is the most probably attributed to a better penetration of ions in the

porosity of electrodes (Figure 59a); nonetheless, further floating series do not influence

ageing of the cell.

Figure 59 (a) Capacitance and (b) resistance evolution of YP80F/YP80F cell in 1 mol

L

-1

Li

2

SO

4

made with pre-coated nickel foil with CCI during floating at 24°C and 1.5 V and 1.6 V.

Paula Ratajczak

P a g e 102


Stable C/C

0

profile suggests that neither any decomposition products block the carbon active surface, nor the good contact between the electrodes and the current collectors is disturbed. This statement is supported by the resistance evolution presented

in Figure 59b, which values are maintained below 1 Ω during the whole 60 series both

at 1.5 V and at 1.6 V. Besides, after opening the cell, the AC electrodes were still well attached to the nickel foils pre-coated with the conductive ink.

Hence, the floating tests reveal the necessity of conductive pre-coatings in the manufacturing of carbon electrodes, in order to ensure satisfactory long-term performance of high voltage electrochemical capacitors in neutral aqueous electrolytes.

To sum up, when nickel collectors are used with self-standing S30 electrodes (part 1.1), as well as with YP-80F electrodes coated on CCI/nickel substrate (part 1.2), ECs in 1 mol L

-1

Li

2

SO

4 can operate up to 1.6 V without deterioration of electrochemical performance.

1.3.

Addition of corrosion inhibitor

According to the investigations performed by Q. Abbas in our research group, sodium molybdate (Na

2

MoO

4

) as additive to lithium sulfate electrolyte reduces the corrosion of current collectors in AC/AC capacitors, whereas the capacitance is enhanced through faradaic contributions [205]. Therefore, we have prolonged this research on lifetime improvement, by investigating the performance of S30/S30 cells with 0.1 mol L

-1

Na

2

MoO

4

+ 1 mol L

-1

Li

2

SO

4

electrolyte at 24 °C and 40 °C.

Cyclic voltammograms of the capacitors with S30 electrodes in the form of pellets in 1 mol L

-1

Li

2

SO

4

(pH = 6.5, conductivity = 64 mS cm

-1

) and 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

(pH = 6.7, conductivity = 72 mS cm

-1

) performed at 10 mV s

-1

scan rate up to 1.5 V are presented in Figure 60. Due to redox processes involving the

molybdate ions, the capacitance is higher in Li

2

SO

4

+ Na

2

MoO

4

than in Li

2

SO

4

. This enhancement of capacitive current, as well as more rectangular shape of the CV curve for EC with Li

2

SO

4

+ Na

2

MoO

4

, as compared to the one with Li

2

SO

4

, could be also partly attributed to the higher conductivity of the electrolyte with the molybdate additive.

Paula Ratajczak

P a g e 103



-1

) of S30/S30 capacitors in 1 mol L

-1

Li

2

SO

4

and 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

electrolytes up to 1.5 V.

To determine the impact of the molybdate additive on ageing, ECs in 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4 were submitted to floating at 1.5 V at 24 °C or 40 °C

with simultaneous monitoring of cell capacitance and resistance (Figure 61). As for the

previously presented capacitors in Li

2

SO

4

(Figure 43), capacitance of ECs with Li

2

SO

4

+ Na

2

MoO

4

increases during the first 20 floating hours. However, when the floating is prolonged, the capacitance remains almost constant, while it decreased for the capacitor in 1 mol L

-1

Li

2

SO

4

at 40°C.

Figure 61 Effect of floating at 1.5 V and 24°C or 40°C on the evolution of (a) specific capacitance and (b) relative resistance of a S30/S30 capacitor in 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

.

Paula Ratajczak

P a g e 104


The beneficial effect of sodium molybdate on the long-term electrochemical performance of a capacitor in the salt aqueous electrolyte is more quantitatively

presented in Table 4. The ECs in 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4 reveal stable capacitance, which is higher than in Li

2

SO

4

by 19 F g

-1

and 37 F g

-1

at 24 °C and 40 °C, respectively.

The impact of the electrolyte mixture is also revealed by the resistance evolution

presented in Figure 61, with almost constant value at 24 °C and only a slight increase by

40% at 40 °C. The resistance increase for the cell with the additive is much lower than for the EC in 1 mol L

-1

Li

2

SO

4

, by 43% and 60% at 24 °C and 40 °C, respectively (see

Figure 43b). As presented in literature, the corrosion provoked by aggressive anions,

such as chlorides or sulfates, can be inhibited by molybdate addition. The additive strengthens the hydrated iron oxide layer on the stainless steel surface in neutral aqueous solutions [206]. The interaction between MoO

4

2-

and Fe

2+

results in the formation of FeMoO

4

, which in the presence of dissolved di-oxygen is further transformed into insoluble complex preventing from corrosion and related resistance increase [207].

before floating after floating

Li

2

SO

4

+ Na

2

MoO

4

24 °C 40 °C 24 °C

Capacitance, F g -1

Li

2

SO

4

40 °C

98

105

101

106

82

86

74

69 before floating 1

Relative resistance, -

1 1 1 after floating 1.14 1.36 1.63 2.18

Table 4 Capacitance values and relative resistance for ECs in 1 mol L

-1

Li

2

SO

4

and 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

determined before and after floating at 1.5 V and 24°C or 40°C.

Paula Ratajczak

P a g e 105


In Figure 62 showing the aspect of the stainless steel current collectors after

floating at 1.5 V and 40°C in 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

, a grey deposit is apparent essentially on the positive current collector and separator. According to the

Pourbaix diagram of molybdenum [199], the reaction (39) occurs at neutral pH of the electrolyte:

MoO

2

+ 2H

2

O → MoO

4

2-

+ 4H

+

+ 2e

-

(39)

Therefore, the deposit on the positive electrode is probably assigned to a protective passive film, composed mainly of MoO

4

2−

, MoO

2

and some traces of MoO

3

. The transformation of molybdate ions into HMoO

4

-

and further into MoO

3

can originate from water oxidation at the positive electrode, which contributes to a locally decreased pH and shift of equilibrium potentials [199].

Figure 62 Collectors and separator of a S30/S30 cell after 120 hours of floating at 1.5 V and 40°C in 1 mol L -1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

.

Paula Ratajczak

P a g e 106


Beside the faradaic contribution and inhibition of corrosion, the presence of sodium molybdate in neutral aqueous electrolyte is supposed to reduce also another important factor contributing to ageing of the system at high voltage, namely electrolyte oxidation. Indeed, the investigations on two-electrode S30/S30 cell equipped with a reference electrode presented by Abbas et al.

[205] showed that the potential reached by the positive electrode (E

+

) in presence of Li

2

SO

4

+ Na

2

MoO

4

electrolyte is shifted by around -0.162 V as compared to Li

2

SO

4

. For this reason, whilst in the cell with lithium sulfate electrolyte, the potential of the positive electrode reaches the thermodynamic oxygen evolution limit of 0.84 V vs NHE at voltage of only 1.35 V, the system with sodium molybdate additive can operate at room temperature up to 1.6 V with positive electrode potential 0.042 V below the thermodynamic oxygen evolution potential. In conclusion from the referred study [205], the shift of potentials toward negative values should practically prevent from electrolyte decomposition and positive stainless steel current collector corrosion during ageing at 1.5 V.

To verify the above statement, the evolving rate of gases during one 2-hour floating period at 1.5 V at room temperature (24 °C) and at 40 °C was measured with a pressure sensor connected to the electrochemical cells with Li

2

SO

4 and Li

2

SO

4

+

Na

2

MoO

4

(see Figure 44 for the system construction), and the results are shown in

Figure 63. Considering the experiments performed at 24 °C, the addition of molybdate

to the electrolyte dramatically reduces the internal pressure increase by ~25 mbar. At higher temperature of 40°C, the impact of the additive is less significant, and the pressure increases in both cells (with and without molybdate) by 130 mbar and 140 mbar, respectively. This diminished effect of the corrosion inhibitor to reduce gases evolution can be attributed to the inactivity of MoO

4

2-

to form molybdenum complexes

at 40 °C. The curves presented in Figure 63 exhibit a linear pressure growth,

demonstrating the destructive effect of ageing at higher temperatures, which occurs just from the beginning of floating. Consequently, the increase of resistance displayed

during prolonged potentiostatic voltage hold at 40°C in Figure 61b seems to be

essentially related to the evolution of gases which might worsen the electrical contacts between electrodes and current collectors.

Paula Ratajczak

P a g e 107


Figure 63 Internal overpressure evolution in S30/S30 cells with and without sodium molybdate additive during two hours potentiostatic floating at 1.5 V and 24

⁰

C or 40

⁰

C.

As observed in chapter III.3, the increase of resistance during floating, as well as pressure evolution, due to electrolyte decomposition, suggest a possible oxidation of the

positive carbon electrode. Figure 64 presents the mass loss, and CO

2

and CO profiles obtained by TPD for the fresh ACC and the positive and negative ACC electrodes of an

EC in 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4 electrolyte aged by 120 floating hours at

1.7 V and RT. The data also reveal surface oxidation of carbon electrodes as compared

to the case of the cell without molybdate addition (Figure 39). The mass loss at 950 °C

is 44.8 % for the positive and 18.5 % for the negative electrode, as compared to 43.6% and 14.5%, respectively, for the EC in 1 mol L

-1

Li

2

SO

4

. However, the surface functionality of the positive and negative aged electrodes is different. It is rich in new oxygenated groups, releasing 4.8 mmol g

-1

and 1.2 mmol g

-1

of CO

2

, respectively. The cumulated released CO is 6.8 mmol g

-1

and 4.9 mmol g

-1

, for the positive and negative electrode, respectively, which is actually few times more than for the ACC electrodes operating in Li

2

SO

4

. During floating in presence of molybdate ions, the carbon surface is oxidized with formation of new CO-evolving groups identified as carbonyl/quinone groups and pyrone-type structures at 820 °C and 940 °C, respectively [193, 194].

Paula Ratajczak

P a g e 108


Figure 64 TPD on pristine ACC (black line) and on positive (red line) and negative

(blue line) aged carbon electrodes after 120 hours of floating at 1.7 V in 1 mol L

-1

Li

2

SO

4

+ 0.1 mol L

-1

Na

2

MoO

4

: (a) CO

2

evolution; (b) CO evolution.

Hence, the addition of 0.1 mol L

-1

Na

2

MoO

4

to 1 mol L

-1

Li

2

SO

4

inhibits the corrosion of the positive stainless steel collectors and enhances capacitance through faradaic contributions. Moreover, the internal pressure increase is reduced, even at 40

°C. However, to reduce the influence of electrolyte decomposition on the lifetime and performance of the AC/AC electrochemical capacitor, further improvement of the cell is needed.

IV.2.

Shifting of electrodes operating potentials

2.1. Asymmetric configuration

According to formula (40) [129]:

𝒎

+

𝑪

+

∆𝑬

+

= 𝒎

−

𝑪

−

∆𝑬

−

(40) expressing equality of the electric charge passed through each electrode (where m

+ and m

are the active carbon mass, C

+ and C

-

- the specific capacitance,

ΔE

+

and

ΔE

-

the potential range of the positive and negative electrodes, respectively), the electrodes potential range (and consequently the electrodes potential extrema) can be shifted by adjusting the mass ratio of electrodes or/and by using different materials of different capacitance. Therefore, to improve long-term performance of ECs, by avoiding

Paula Ratajczak

P a g e 109

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e decomposition of electrolyte, electrochemical capacitors with asymmetric configuration of electrodes with different carbons in 1 mol L

-1

Li

2

SO

4

were built. Such effect of shifting electrodes operating potential has been previously observed with (-) AC /MnO

2

(+) capacitors in 0.5 mol L

−1

Na

2

SO

4 using a mass ratio m

+

/m - = 2.5 and voltage of 2 V

[208]. Although the short time experiments in two-electrode cell with reference electrode showed the possibility to reach 2.2 V with the latter system, galvanostatic cycling revealed that the maximum voltage for this system needs to be reduced by 0.2 V for a good cycle life. Therefore, in the present study, after the basic electrochemical investigations on the realized asymmetric carbon/carbon cells, both the SOH of ECs and the actual values of potentials reached by the positive and negative electrode have been simultaneous monitored during accelerated ageing by floating at 1.5 V.

Taking into account equation (40), our objective has been to realize a capacitor with electrodes of same mass of two carbons with different capacitance. The cyclic voltammograms of two symmetric cells built with the S30 and Burley800 (S

BET

= 1651 m

2

g

-1

; L

0

= 0.86 nm) [78] (further named as B800) carbons in 1 mol L

-1

Li

2

SO

4

demonstrate higher capacitance for B800 (Figure 65); from galvanostatic

charge/discharge measurements, the capacitance values are 82 F g

-1

and 125 F g

-1

for

S30 and B800, respectively. Therefore, an asymmetric capacitor has been built with

B800 as positive electrode and S30 as negative one, in order to get ΔE

-

>ΔE

+

and consequently to shift the potential extrema of electrodes towards lower values.

Figure 65 Cyclic voltammograms at 10 mV s

-1

for symmetric cells based on the S30 and B800 carbons in 1 mol L

-1

Li

2

SO

4

.

Paula Ratajczak

P a g e 110


In order to prove the effectiveness of the S30(-)/B800(+) construction, Figure 66

compares the electrodes potential ranges vs cell voltage for three different couplings of the two carbons: S30 (-) /S30 (+), B800 (-)/B800 (+) and S30(-)/B800(+).

Figure 66 Electrodes potential range vs voltage during galvanostatic cycling at 1 A g

-1 on: (a) S30 (-) / S30 (+); (b) B800 (-) / B800 (+) and (c) S30 (-) / B800 (+) cells in 1 mol L

-1

Li

2

SO

4

. The measurements were realized in two-electrode cells with reference electrode.

Paula Ratajczak

P a g e 111


For the two symmetric systems in 1 mol L

-1

Li

2

SO

4

, lower value of positive

electrode potential is reached at voltage of 1.6 V with B800 (-) /B800 (+) (Figure 66b) as compared to S30 (-) /S30 (+) (Figure 66a) , 0.78 V and 0.96 V, respectively. Due to

the higher capacitance of the B800 carbon as compared to S30, the shift of positive electrode potential is more pronounced for the S30 (-) /B800 (+) asymmetric

configuration (Figure 66c). Taking into account the limit for water reduction and

noticeable H

2

evolution in 1 mol L

-1

Li

2

SO

4

(-0.8 V vs NHE estimated by three-

electrode cell measurements as presented in Figure 20), at voltage of 1.2 V, the potential

of the negative electrode is lower than the potential for di-hydrogen evolution (lower

horizontal line at -0.8 V vs NHE in Figure 66c). However, on the CVs of the negative

electrode of the S30 (-) /B800 (+) system, no oscillations due to bubbling were observed below the potential of -0.8 V vs NHE. Notwithstanding, the shift of potentials higher than eventually expected using this S30 (-)/B800 (+) construction may require slight adjustment of electrodes masses. A statement about that will be presented once floating experiments have been realized with this system.

In order to accurately state on the effects of electrodes potential shift, floating at

1.5 V has been realized on the three cell configurations at 24

⁰

C, with simultaneous

monitoring of the SOH of the systems (Figure 67). The symmetric EC based on the

B800 carbon exhibits the best performance, when considering the evolution of both capacitance and resistance. Due to the well-developed microporosity of the tobacco carbon, the B800 (-) / B800 (+) capacitor exhibits the highest initial capacitance value, which is maintained during 120 floating hours of the test. Overall, the B800 (-) / B800

(+) capacitor largely outperforms the S30 (-)/S30 (+) one [78]. The asymmetric cell, obtained by coupling the positive electrode from microporous B800 carbon with the negative one from S30, exhibits an intermediate capacitance value of 102 F g

-1

. When compared to the symmetric cells, the asymmetric system displays the worst long-term performance at 1.5 V, both for capacitance and resistance evolution. It suggests that either down-potential shifting resulting from the asymmetric construction is too high

(although, as noticed, detrimental effects of gas bubbling are not observed at the negative electrode) or that important changes occur in the electrodes potential range during floating.

Paula Ratajczak

P a g e 112


Figure 67 Effect of the floating voltage at 24

⁰

C on (a) capacitance and (b) resistance of the cells with three different configurations of carbon electrodes in 1 mol L

-1

Li

2

SO

4

.

In order to better understand what happened during accelerated ageing of the asymmetric EC, the evolution of electrodes potentials was monitored during the repeated floating sequences at 1.5 V. As it can be seen for a two-hour potentiostatic

period (Figure 68a), the potentials of positive and negative electrodes increase

remarkably at the beginning of the period and then the shift is less pronounced as the system tends to an equilibrium state. Due to the applied polarisation, the ions attracted to the active surface of the carbon electrodes reach the more highly confined porosity and are further pushed from the diffusion layer to the compact one, ordering the structure of the EDL. During the further floating sequences, the electrodes potentials

shift by around +0.3 to +0.4 V after 120 hours of floating (Figure 68b), which can

finally lead to subsequent possible effects: i) positive electrode oxidation; ii) accumulation of corrosion products. The potential of the positive electrode exceeds the thermodynamic limit for water oxidation after around 40-50 hours of floating at 1.5 V,

which is in agreement with Figure 67b, where the resistance of the asymmetric cell

begins to suddenly increase from this time.

Paula Ratajczak

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Figure 68 Electrodes potential profile of a S30 (-) / B800 (+) capacitor in 1 mol L

-1

Li

2

SO

4

electrolyte during (a) one 2-hour floating period at imposed voltage of 1.5 V during the third sequence of accelerated ageing; (b) 60 2-hour sequences at 1.5 V.

Figure 69 presents the capacitance evolution of the asymmetric S30 (-) / B800

(+) cell and of the individual positive B800 and negative S30 electrodes during floating at 1.5 V. It can be easily noticed, that the capacitance decay of the whole system is essentially related to the positive electrode degradation, while the negative electrode is not much influenced by the ageing.

Figure 69 Capacitance evolution of the S30 (-) / B800 (+) cell and individual electrodes during floating at 1.5 V in 1 mol L

-1

Li

2

SO

4

.

Paula Ratajczak

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The TPD data show that, as compared to S30, the B800 carbon has a higher

amount of surface oxygenated functional groups (Table 5). Hence, the higher reactivity

observed at the positive B800 electrode during floating must be attributed to the higher number of active sites of this carbon. It is likely that additional annealing treatment could reduce the oxygen content [78] and probably would improve the cyclability of the positive electrode.

Carbon material

CO

2

CO H

2

O O

µmol g -1

µmol g

-1

µmol g

-1

wt%

S30

B800

317

440

249

598

44 1.5

416 3.0

Table 5 TPD analysis data on carbons S30 and B800.

The previous interpretation suggesting decomposition reactions at the positive

B800 electrode are confirmed when comparing the shape of voltammograms recorded

after floating the S30 (-) / B800 (+) cell (Figure 70a) and the S30 (-) /S30 (+) one

(Figure 38c) at 1.5 V. In both cases, the CVs deviation from the rectangular shape

indicates worse charge propagation after floating. However, the narrowing of voltammogram at high voltage for the S30 (-) / B800 (+) cell is more pronounced than with S30 (-)/S30 (+), which indicates higher porosity saturation for the former cell, related with decomposition reactions at the positive electrode.

The CVs of the

individual electrodes for the S30 (-) / B800 (+) cell, before (Figure 70b) and after 120 hours of floating at 1.5 V (Figure 70c), clearly demonstrate the non-EDL behaviour of

the positive electrode after floating. Additionally, due to the shift of potentials towards higher values, the potential of the positive electrode exceeds the limit for water

oxidation (Figure 70c), which can lead to electrode oxidation and/or accumulation of

corrosion products in the porosity of carbon, finally causing a reduction of positive electrode active surface.

Paula Ratajczak

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-1

) recorded before and after 60 sequences of 2-hour floating at 1.5 V, using a S30 (-) / B800 (+) cell with reference electrode in 1 mol L

-1

Li

2

SO

4

: (a) full cell; (b) individual electrodes before floating and (c) individual electrodes after floating .

Paula Ratajczak

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The presented data have revealed that, when compared to symmetric configurations of cells, the investigated asymmetric system displays worse long time performance at 1.5 V. Even though, coupling the positive electrode from microporous

B800 carbon with the negative one from S30 results in satisfactory shift of electrodes operating potentials towards lower values, the whole cell exhibits an intermediate capacitance which is decreased by 20% after 110 hours of floating at 1.5 V.

Notwithstanding, it seems that the reactivity of the B800 carbon could be at the origin of the performance decay. Therefore, some further efforts should be dedicated to better stabilize this material by annealing. The too strong potential shift given by this construction cannot be rejected as additional cause of the poor cycle life. Better control of the potential shift by finely tuning the electrodes mass ratio should be also further investigated.

2.2.

Current collectors coupling

Considering the optimization of electrochemical capacitors in salt aqueous electrolyte by adapting the components, besides asymmetric configuration of the cells utilizing carbon electrodes with different mass and/or nature, or various kinds of electrolytes, coupling of different current collectors can be applied to shift the maximum potential of the positive electrode towards lower values. As presented in section IV.1, nickel was found as an alternative and promising material to stainless steel to improve the long-term stability of electrochemical capacitors in aqueous electrolyte.

To verify the difference between the implemented collectors configuration in 1 mol L

-1

Li

2

SO

4

, ECs were realized in PTFE Swagelok-type assembly with YP-80F coated electrodes either on stainless steel or nickel 200/201 foil (previously pre-coated by

CCI), using the corresponding cylindrical current collectors, either from stainless steel or nickel.

The investigations previously performed in symmetric cells (part 1.1.) served as

a basis to propose coupling of stainless steel and nickel collectors. Table 6 presents

capacitance values [F g

-1

] determined from galvanostatic discharge at -1 A g

−1

from 1.6

V to 0 V according to equation (43) in experimental annex. The data reveal the same discharge capacitance for the two cells with stainless steel (−) /stainless steel (+) and nickel (−) /nickel (+) collectors. However, when considering the individual electrodes, the positive electrode displays a high capacitance in the stainless steel (−) /stainless steel

Paula Ratajczak

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(+) cell, while it is the negative one in the nickel (−) /nickel (+) cell. Therefore, we decided to investigate if the performance could be enhanced by a nickel (−) /stainless steel (+) assembly, and to verify if the corrosion of the stainless steel positive collector corrosion could be reduced when nickel is applied as a negative collector. positive electrode negative electrode whole cell steel (-) /steel (+) nickel (-) /nickel (+)

125 72

63

87

100

85

Table 6 Capacitance [F g

-1

] of electrochemical capacitors and individual electrodes coated on stainless steel or nickel foils in 1 mol L

-1

Li

2

SO

4

determined by galvanostatic cycling (1 A g

-1

) up to 1.6 V of stainless steel (−) /stainless steel (+) and nickel (−)

/nickel (+)configuration of current collectors.

Figure 71 presents cyclic voltammograms of the three cells with different

configuration of collectors. Although capacitance of the whole cell (85 F g

-1

) is not enhanced by coupling the two kinds of collectors, the voltammogram of the EC with nickel (−) /steel (+) configuration exhibits a slightly improved shape with a diminished current leap at the charged state, when compared to the steel (−) /steel (+) one.


-1

) of electrochemical capacitors with carbon electrodes coated on stainless steel or nickel foils in 1 mol L

-1

Li

2

SO

4

.

Paula Ratajczak

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The size of the high-frequency semi-circle in the Nyquist plot of the three cells

(Figure 72a) reveals that the cell with the combined collectors presents the lowest

charge transfer resistance value R f

(0.94 Ω), when compared to 1.1 Ω and 1.24 Ω for nickel (−) /nickel (+) and stainless steel (−) /stainless steel (+) configurations, respectively. For high power, R f

and time constant are crucial parameters indicating the electrical losses taking place in all resistive components of the cell during charging and discharging. The time constants of 0.80 s for the steel (-) / steel (+) and 0.57 s for nickel

(-) / steel (+) configurations reveal a harmful effect of steel on the dynamics of charge exchange in the ECs, when compared to the nickel (-) / nickel (+) cell for which τ is

0.46 s. This impact is also demonstrated in the Bode plots (Figure 72b) where in the

frequency range 0.1 Hz – 1 Hz the phase angle increases more rapidly for the steel (-

) / steel (+) cell. The two configurations with nickel collectors reveal similar performance up to around 1 Hz, above which the impact of the stainless steel collector in the coupled cell is observed by the lower phase angle. Nevertheless, it is important to note that the three ECs exhibit almost ideal capacitive behaviour at low frequency represented by the value of phase angle very close to -90°.

Figure 72 (a) Nyquist and (b) Bode plots of the three YP-80F/YP-80F cells in 1 mol L

-1

Li

2

SO

4

with different configurations of collectors.

To analyse the SOH of the three cells with different combination of the current collectors during long time performance, floating at high voltage of 1.6 V has been

applied (Figure 73). The most stable capacitance and resistance values during floating

are revealed by the cell with the coupled nickel (-) / steel (+) collectors.

Paula Ratajczak

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Figure 73 (a) Capacitance and (b) resistance evolution during floating at 1.6 V of YP-

80F/YP-80F capacitors in 1 mol L

-1

Li

2

SO

4

with electrodes coated on stainless steel and nickel collectors.

Once opening the YP-80F_nickel (-) / YP-80F_steel (+) cell after 120 hours of floating at 1.6 V, no corrosion of the positive current collector was observed, although some green deposits appeared on the negative one. The mixed-conductive nickel compounds, formed on the surface of the negative current collector during floating, did not affect the cell performance. The decrease of capacitive current at voltage higher than

1 V is much less pronounced as in the case of the two other cells using stainless steel

collectors (Figure 74).


-1

) recorded after 120 hours of floating at 1.6 V on YP-

80F/YP-80F capacitors with different current collectors in 1 mol L

-1

Li

2

SO

4

.

Paula Ratajczak

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From CVs of individual electrodes recorded on two-electrode cells with reference electrode, the reduced corrosion of the positive stainless steel collector in the nickel (−) / stainless steel (+) system can be attributed to the −105 mV shift of the

electrode potentials at a voltage of 1.6 V (Figure 75a), as compared to the steel (−) / stainless steel (+) combination (Figure 75b).


-1

) of individual electrodes (coated on stainless steel or nickel foils) of YP-80F/YP-80F cells in 1 mol L

-1

Li

2

SO

4

at 1.6 V with different collectors combinations: (a) nickel (-) / nickel (+); (b) steel (-) / steel (+); (c) nickel (-) / steel (+).

Paula Ratajczak

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The CV of the positive electrode in the nickel (−) / stainless steel (+) system

(Figure 75c) discloses a smaller anodic current leap, as compared to the symmetric collector combinations (Figure 75a and b). Such characteristics traduce diminished

electrolyte decomposition, electrode oxidation and/or formation of corrosion products on the positive current collector. Moreover, the accumulation of Ni(II) and Ni(III) derivatives may contribute to polarize the surface of electrodes, and thus, cause a shift of the E

OCP

from 0.285 V (Figure 75b) to 0.224 V vs NHE (Figure 75c) [209].

The performance of the capacitors in 1 mol L

−1

Li

2

SO

4

electrolyte is improved by using both nickel (-) / nickel (+) and nickel (-) / steel (+) collectors configuration when high voltages are applied. However, it would be worth to characterise the nature of the nickel deposits formed after long-term operation, their actual effect on the cell’s performance and to investigate strategies for reducing their creation.

IV.3.

Conclusion

Strategies to improve the long time performance of AC/AC electrochemical capacitors in neutral salt aqueous electrolyte were presented in this chapter. The undertaken tactics intended to reduce the corrosion of stainless steel collectors and to avoid the decomposition of aqueous electrolyte by shifting the operating electrodes potentials to lower values.

The reduction of ECs lifetime due to collectors corrosion can be prevented by:

(i) using non-corrodible nickel collectors; (ii) avoiding deposition of the corrosion products on the electrode-collector plane by coating the electrode material on metallic foils; (iii) adding a corrosion inhibitor to lithium sulfate electrolyte.

As presented in chapter III, the maximum voltage for long term operation of

S30/S30 electrochemical capacitors with stainless steel collectors under floating is 1.5

V, while the S30/S30 system with nickel collectors can operate up to 1.6 V without deterioration of electrochemical performance after 120 hours of accelerated ageing.

Due to the well-developed surface area of annealed nickel, the reactivity during electrochemical ageing in aqueous medium is certainly much higher than in case of plain nickel, leading to corrosion of the collectors and deterioration of electrochemical performance

Paula Ratajczak

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A good adhesion of carbon coating to the substrate and protection of the electrode/substrate interface from accumulation of decomposition products is a key factor in manufacturing of carbon electrodes by coating. Therefore, the application of conductive carbon ink (CCI) pre-coating between the metallic substrate and the electrode material appears to be necessary, in order to ensure satisfactory long-term performance of electrochemical capacitors in neutral aqueous electrolytes at high floating voltage.

The addition of 0.1 mol L

-1

Na

2

MoO

4

to 1 mol L

-1

Li

2

SO

4

inhibits the corrosion of the positive stainless steel collectors and enhances capacitance through faradaic contributions. However, in order to avoid oxidation of carbon electrodes after long time performance at high voltage, further improvement of the AC/AC electrochemical capacitor in neutral aqueous electrolytes with corrosion inhibitor is needed.

Coupling a highly microporous B800 carbon as positive electrode with industrial

S30 as negative one results in satisfactory shift of electrodes operating potentials towards lower values. However, when compared to symmetric configurations of cells, the investigated asymmetric system displays worse long time operation at 1.5 V. Since the reactivity of the B800 carbon seems to be at the origin of the performance decay, annealing of this material should be performed.

By using nickel (-) / nickel (+) and nickel (-) / steel (+) collectors configuration, the performance of the capacitors in 1 mol L

−1

Li

2

SO

4

electrolyte is improved.

Although, the appearance of the residues does not affect the performance of the cell during 120 hours of floating at 1.6 V, it would be worth to characterise the nature of the nickel deposits formed after long-term operation.

Paula Ratajczak

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CHAPTER V

TOWARDS A NEW CONCEPT

OF HIGH VOLTAGE AC/AC CAPACITOR

IN AQUEOUS ELECTROLYTES

Paula Ratajczak

P a g e 124


As presented in the literature review, aqueous electrolytes offer several important advantages for electrochemical capacitors (ECs) application compared to solutions in organic solvents. Nonetheless, pure water has a thermodynamic stability window of only 1.23 V, and it was observed that the practical cell potential in conventional aqueous electrolytes used in batteries (H

2

SO

4

, KOH) is even limited to lower values. Notwithstanding, for the electrochemical capacitor application, the overpotential of di-hydrogen evolution at the negative electrode may in certain conditions enhance the stability potential window when using an aqueous electrolyte. As it was lately observed, th is

over-potential on an AC electrode is higher in neutral aqueous electrolytes than in aqueous KOH or H

2

SO

4

; a voltage of 1.6 V during 10,000 charge/discharge cycles was claimed with a symmetric AC/AC capacitor in aqueous

Na

2

SO

4 with gold current collectors [131]. However, as it was presented in the previous chapters of this dissertation, the operating voltage of ECs with stainless steel current collectors in neutral aqueous electrolytes is essentially dictated by the positive electrode, due to oxidation of the electrode material and corrosion of current collectors, when the thermodynamic limit of water oxidation is exceeded. Therefore, in order to increase the overall voltage of the EC, asymmetric systems should be more extensively investigated to optimize the operating potential range of both electrodes.

For extending the operating voltage of carbon-based ECs, this chapter presents a new concept of AC/AC cell using KOH and Na

2

SO

4

as catholyte and anolyte, respectively. Besides, developing this new cell will help to validate our interpretations for the over-potential observed at the negative electrode of AC/AC capacitors in salt aqueous electrolytes.

III.1. The new concept of high voltage cell in aqueous electrolytes

The concept cell which will be presented in this chapter is based on the fact that the potentials of water oxidation and reduction are dependent on the electrolyte pH.

According to the Nernst law, one can imagine to extend the potential difference between water oxidation and reduction (i.e. the operating voltage of an EC cell) by using a catholyte with higher pH than the anolyte, both electrolytes being separated by a cation exchange membrane (CEM). In the study, a homogeneous electro-dialysis membrane used in standard demineralisation applications (FKS-PET-130, FuMA-Tech GmbH) has

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e been chosen as CEM, due to its high chemical stability (pH 5-14) and relatively good selectivity (>96%). The nature and concentration of the electrolytes were practically imposed by the characteristics of the selected CEM. They were selected for keeping chemical inertness vs the CEM and relatively constant pH in both anodic and cathodic compartments during the whole electrochemical experiment. After systematic investigations based on the latter criteria, our final choice was 0.5 mol L

-1

KOH as catholyte and 1.0 mol L

-1

Na

2

SO

4

as anolyte.

The electrolyte decomposition potentials E vs NHE of half-cells in 0.5 mol L

-1

KOH (pH= 13.2) and in 1.0 mol L

-1

Na

2

SO

4

(pH= 6.6) at equilibrium are given by

Nernst equations (41) and (42), respectively [210]:

𝑬

−

= −𝟎. 𝟎𝟓𝟗𝟏𝒑𝑯 = −𝟎. 𝟕𝟖𝟎 𝑽

(41)

𝑬

+

= 𝟏. 𝟐𝟑 + (−𝟎. 𝟎𝟓𝟗𝟏𝒑𝑯) = 𝟎. 𝟖𝟒𝟎 𝑽

(42)

When the negative electrode potential is below E

-

, di-hydrogen evolves from the catholyte. In turn, if the potential of the positive electrode is higher than E

+

, water from the anolyte is oxidized producing nascent oxygen which may: (i) provoke corrosion of the positive stainless steel current collector; (ii) oxidize the AC carbon electrode; (iii) or evolve as di-oxygen. It follows, that the full cell should be theoretically able to operate safely up to 0.840 – (-0.780) = 1.620 V, which is much more than the thermodynamic limit of water decomposition (1.23 V).

Accordingly to the theory of the extended stability window due to the existing pH difference between the cathodic and anodic compartment, one might ask to replace the neutral Na

2

SO

4

by, e.g., acidic solution. However, if sulfuric acid would be used as anolyte, during charging, protons would migrate through the membrane towards the negatively polarized electrode and cause neutralization of the initially basic catholyte.

Figure 76 shows the principle of cell operation, when it is charged with a

power generator. Due to the electrical potential difference between the electrodes, the

SO

4

2-

ions migrate towards the positive electrode, where they are stored in the pores of carbon. The sodium ions from the anolyte migrate through the CEM towards the catholyte and are adsorbed together with potassium ions on the active surface of the

Paula Ratajczak

P a g e 126

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e negative electrode. During, discharge of the cell, the ions may move in opposite directions. Since the CEM contains in its matrix negatively charged groups, it controls the transport of ionic species and prevents the OH

-

ions to flow from the catholyte to the anolyte.

Figure 76 Charging principle of the concept cell with potassium hydroxide and sodium sulfate as catholyte and anolyte, respectively, activated carbon electrodes, and a cation exchange membrane as separator.

The electrochemical investigations on the new concept cell were realized in

PTFE case, using stainless steel (316L grade) collectors and two reference electrodes, namely (Pt) Hg/Hg

2

SO

4

in 1 mol L

-1

H

2

SO

4 and Hg/HgO in 6 mol L

-1

KOH, for the 1.0 mol L

-1

Na

2

SO

4

anolyte compartment and 0.5 mol L

-1

KOH catholyte compartment,

respectively (Figure 77). By sweeping/monitoring the voltage between the two activated

carbon electrodes, the system can be investigated as a two-electrode cell. Moreover, in this two-electrode assembly, the data of either positive or negative carbon electrode can be monitored vs the corresponding reference electrode, using the other carbon electrode as counter one. If not referred otherwise, a commercially available activated carbon powder (YP 80F, Kuraray Chemicals Co, further named as YP80F), with S

BET

=2270 m

2 g

-1

and L

0

=1.05 nm, has been chosen as electrode active material for the study on the concept cell (see experimental annex A.1.1).

Paula Ratajczak

P a g e 127


Figure 77 Schematic representation of the new concept AC/AC electrochemical capacitor using two aqueous electrolytic solutions of different pH separated by a cation exchange membrane (CEM), stainless steel collectors and reference electrodes.

The CEM membrane (diameter of 2.7 cm) was then placed in a cave secured by silicone ring gaskets from both sides and retained by joining the two PTFE bodies of the device together by screws, in the way to prevent from any leak and movement of the membrane. The area of CEM exposed to electrolyte after assembling was 1.77 cm

2

. For optimal performance with minimal wrinkling and lowest electrical resistance, the cell was filled with demineralized water for 24 h at room temperature.

Figure 78 compares the cyclic voltammograms (CVs) of the new concept (-)

YP80F-KOH / YP80F-Na

2

SO

4

(+) capacitor and of the (-) YP80F-Na

2

SO

4

/ YP80F-

KOH (+) cell with reversed configuration of electrolytes. The (-) YP80F-KOH / YP80F-

Na

2

SO

4

(+) cell displays a near-ideal rectangular CV typical for an EDL capacitor up to

1.6 V (Figure 78a) contrary to the other cell where the sign of redox contributions is easily visible even for low values of voltage (Figure 78b). The charging CVs of the

concept (-) YP80F-KOH / YP80F-Na

2

SO

4

(+) cell are not featured by any current leap, related to catholyte reduction and/or electrochemical oxidation of the positive carbon

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e electrode, and thereof proves the possible extension of operating voltage by an adapted selection of the electrolytes.

Figure 78 CVs (0.4 mV s

−1

) of YP80F/YP80F electrochemical capacitors in: (a) (-) 0.5 mol L

-1

KOH / 1.0 mol L

-1

Na

2

SO

4

(+); (b) (-) 1.0 mol L

-1

Na

2

SO

4

/ 0.5 mol L

-1

KOH

(+) electrolytes for voltage windows from 0 V up to 0.8, 1.0, 1.2, 1.4, 1.5 and 1.6 V.

Galvanostatic cycling at 40 mA g

-1

with simultaneous monitoring of E

-

and E

+

( vs the respective reference electrodes introduced in each compartment) was performed

to verify the potential range of electrodes (Figure 79). According to formulae (41) and

(42), at a voltage of 1.6 V, the potential reached by the positive electrode (E

+

) should not theoretically exceed 0.84 V vs NHE. However, even if the electrodes have an equal mass, their capacitance values are uncontrolled. According to formula (40), the potential range of the positive electrode may be higher than expected [129], leading the maximum potential of this electrode to be higher than the value calculated from

equation (42) and represented by the upper horizontal dashed line on Figure 79.

From this figure, it is clearly seen that the maximum possible voltage of the

YP80F/YP80F capacitor with equal electrodes masses should be 1.5 V. Since the electrodes potentials are shifted towards higher values than indicated by the thermodynamic assumptions (equations (41) and (42)), the potential of the negative electrode is higher than -0.78 V vs. NHE at a voltage of 1.6 V. It means that, in the present configuration of the system, the maximum possible voltage range is not fully

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e exploited; further, we will show that a change of relative masses of electrodes, as well as ACs, can help in shifting the electrodes potentials.

Figure 79 Electrodes potential extrema vs voltage measured during galvanostatic cycling at 40 mA g

-1

on a (-) YP80F-KOH / YP80F-Na

2

SO

4

(+) cell with equal electrode masses. E

OCP


The cyclic voltammograms of individual activated carbon electrodes in of the

(-) YP80F-KOH/YP80F-Na

2

SO

4

(+) cell were recorded in the potential ranges

determined from galvanostatic cycling (see Figure 79) for voltages up to 1.4 V, 1.5 V

and 1.6 V and are presented in Figure 80. These CVs confirm that, in practice, the

potential of the negative electrode does not reach the di-hydrogen evolution potential in

KOH (-0.78 vs. NHE), where oscillations on the CVs would be visible. The CVs of the positive electrode are not featured by a significant current leap, even at the highest voltage (1.6 V). It correlates with the previous observation made in two-electrode

assembly (Figure 78), that the system is able to operate up to 1.6 V, with almost ideal

rectangular shape of CVs and without electrolyte decomposition to gaseous products in the form of O

2

and H

2

. The minor current increase during positive polarization in

Paula Ratajczak

P a g e 130


Na

2

SO

4

with increasing voltage can be also attributed to the redox reactions between the oxygenated surface groups and the electrolyte [131].

Figure 80 CVs of the individual electrodes in a (-) YP80F-0.5 mol L

-1

KOH / YP80F-

1.0 mol L

-1

Na

2

SO

4

(+) cell for maximum voltages up to 1.4, 1.5, and 1.6 V (scan rate for the cell 0.4 mV s

−1

).

Taking into account Figure 79, the maximum voltage of the cell with YP80F

electrodes of equal mass should not be higher than 1.5 V, to avoid water oxidation at the positive electrode. Therefore, galvanostatic (current density of 100 mA g

-1

) cycling has been performed up to 1.5 V, to verify the previous conclusion about high voltage

operation of the new concept cell. Figure 81 presents the evolution of capacitance and

resistance vs number of galvanostatic cycles, after initially conditioning the EC cell by

10 CV cycles at 0.4 mV s

−1 and 10 galvanostatic charge/discharges at 40 mA g

-1

. The increase of capacitance during the first 200 cycles is attributed to better wetting of the electrode material by the electrolyte, allowing narrow pores to be accessed by ions [211,

212]. Afterwards, capacitance very slowly decreases to reach 98% of the initial value

(C

0

) after 1,000 cycles. Likewise, the value of resistance at the end of cycling is

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e increased by only 20 % as compared to the ESR in the first cycle (R

0

). Overall, within this number of cycles, the SOH of the cell is still very good, with capacitance and resistance variations below the generally accepted end-of-life criteria [187].

Figure 81 Effect of galvanostatic (100 mA g

-1

) cycling up to 1.5 V on capacitance and resistance of the (-) YP80F-KOH /YP80F-Na

2

SO

4

(+) cell.

After cycling of the new concept cell at 1.5 V, no traces of corrosion on the positive stainless steel collector were noticed. During the long time performance, the anolyte pH increased up to 8.9 after 550 galvanostatic cycles, and then did not change anymore until the end of cycling; taking into account the initial pH value of 6.5, this pH increase traduces almost negligible OH

-

migration to the anolyte during cycling.

Notwithstanding, the process of steel corrosion remains inhibited in the anolyte pH range close to neutrality (6.5-8.9).

To summarize, the presented concept of carbon-based electrochemical capacitor using two aqueous electrolytic solutions (KOH (-) / Na

2

SO

4

(+)) separated by a CEM has been validated by the electrochemical investigations. Due to the pH difference between 0.5 mol L

-1

potassium hydroxide as catholyte and 1.0 mol L

-1

sodium sulfate as anolyte, the theoretical potential difference between water oxidation and reduction is increased to 1.62 V. However, when the system with two identical carbon electrodes is charged up to a voltage of 1.6 V, there is still a waste range of negative potential which

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e is not utilized and, at the same time, the positive electrode potential exceeds the thermodynamic limit of anolyte oxidation. The YP80F/YP80F capacitor with equal electrodes masses operates with a good stability only up to 1.5 V.

III.2. Extension of voltage range by electrodes asymmetry

Although a maximum voltage slightly higher than 1.6 V is theoretically predicted for the (-) YP80F-KOH/YP80F-Na

2

SO

4

(+) capacitor, the previous experiments have demonstrated that, in practice, this value cannot be reached due to the high potential of the positive electrode leading to carbon oxidation. Therefore, we now suggest balancing the electrodes in order to reduce the potential range ∆E

+

of the positive electrode, and consequently to lower its maximum potential below the oxidation limit of the anolyte; according to equation (40), this can be realized by increasing either m

+

/m

-

or C

+

/ C

-

. Several examples of electrodes potential window adjustment by this strategy are available in the literature, e.g., for the asymmetric carbon/MnO

2

capacitors [208], or for symmetric ECs in neutral [213] and organic electrolytes [214] using the same activated carbon in both electrodes.

2.1 Adjustment of electrodes potential window by increasing m

+

/m

-

In our attempts to reduce ΔE

+

, we have applied different values of mass ratio m

+

/m

-

. Although a small increase of m

+

/m

-

should theoretically be sufficient, it turned out that, in practice, m

+

/m

-

should be increased up to 2.25 in order to sufficiently shift the potential of the positive electrode. With this mass ratio, at a voltage of 1.6 V, the potential of the positive YP80F electrode is E

+

= 0.852 vs NHE (Figure 82), i.e., close

to the value of 0.840 V calculated from equation (42). While comparing with Figure 79,

it is clear that

ΔE

+

/

ΔE

-

is significantly reduced in Figure 82, but at the same time E

OCP

is shifted to higher values. Hence, it can be concluded that the difficulty to reduce the maximum potential of the positive electrode is related to this shift of E

OCP

. It can be anticipated that the important change of positive electrode thickness accompanying its mass increase can lead to reduced charge propagation in this electrode and correlatively perturbations in E

OCP

.

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Figure 82 Electrodes potential extrema vs voltage measured during galvanostatic (40 mA g

-1

) cycling of (-) YP80F-KOH / YP80F-Na

2

SO

4

(+) cell with unequal electrode masses (m

+

/m

-

=2.25). E

OCP


The consequence of applying a thicker positive electrode can be seen in the comparison of GCPL curves (40 mA g

-1

) for cells with equal and unequal electrodes

masses (Figure 83), where the voltage drop at 1.5 V is 28 mV and 32 mV, respectively.

Additionally, for the asymmetric cell, the discharge time is reduced as compared to the symmetric one, traducing a diminishing of gravimetric capacitance as consequence of increasing the total mass of carbon electrodes.

Figure 83 Galvanostatic (40 mA g

-1

) charge-discharge profiles of (-) YP80F-

KOH/YP80F-Na

2

SO

4

(+) cell and of the individual electrodes for cells with: (a) equal electrode masses (m

+

/m

-

=1), (b) unequal electrode masses (m

+

/m

-

=2.25).

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Likewise, when the cell with unequal electrode masses is charged, polarization distribution across the positive electrode differs from the negative one [215]. The hindrance in charge propagation during charging of the cell with thicker positive YP80F electrode is visible by a more rounded CV as compared to the cell with symmetric

electrodes (Figure 84).

Figure 84 CVs (0.4 mV s

−1

) of YP80F/YP80F electrochemical capacitors in (-) 0.5 mol

L

-1

KOH / 1.0 mol L

-1

Na

2

SO

4

(+) with equal (m

+

/m

-

=1) and unequal electrode masses

(m

+

/m

-

=2.25).

The results with the unequal electrodes masses reveal that it is possible to shift the operating potential range of the carbon electrodes; however, the impediments resulting from different thicknesses of carbon electrodes suggest performing further experiments with positive and negative electrodes made of different carbons.

2.2. Voltage extension by use of different carbon electrodes

Accordingly to equation (40), the

ΔE

+

/ΔE

ratio can be also reduced by increasing the ratio C

+

/ C

-

between the specific capacitances of the positive and negative electrodes. With this objective in mind, we have selected the YP 80F carbon (Kuraray

Chemicals Co, with S

BET

=2270 m

2

g

-1

and L

0

=1.05 nm) for the positive electrode, with

YP50F (Kuraray Chemicals Co, with S

BET

=1522 m

2

g

-1

and L

0

=0.86 nm) for the negative one. The lower SSA of YP50F, essentially due to a lower content of mesopores

(see experimental annex A.1.1), should result in lower capacitance and different kinetics in the pores of the negative electrode.

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Successfully, the potential of the positive YP80F electrode in the (-) YP50F-

KOH / YP80F-Na

2

SO

4

(+) cell is diminished to E

+

= 0.844 vs NHE at a voltage of 1.6

V (Figure 85a), value which is close to of the theoretical one of 0.840 V calculated from

equation (42). Moreover, contrary to the (-) YP80F/YP80F (+) cell with unequal electrode masses (m

+

/m

-

=2.25) (Figure 82), E

OCP

is not shifted to higher values when

the two different carbon electrodes are used (Figure 85a). Overall, as seen in Figure 10a,

the potential ranges of both electrodes for the (-) YP50F-KOH / YP80F-Na

2

SO

4

(+) cell perfectly fit within the thermodynamic stability limits represented by the dashed lines.

The near-rectangular shape of CV (0.4 mV s

−1

) up to 1.5 V for the (-) YP50F-

KOH / YP80F-Na

2

SO

4

(+) cell with different carbon electrodes in Figure 85b proves

good charge propagation. Contrarily, since the potential of the positive electrode in the

(-) YP80F-KOH / YP80F-Na

2

SO

4

(+) capacitor exceeds the thermodynamic limit for water oxidation in Na

2

SO

4

(Figure 79), the CV of the cell with the same YP80F

electrodes is featured by a current leap related to carbon electrochemical oxidation.

Since, such phenomenon is not revealed by the (-) YP50F-KOH / YP80F-Na

2

SO

4

(+) cell, which exhibits nearly constant capacitive current during the CV scan, the possible extension of operating voltage by an adapted selection of the electrode materials seems to be proved.

Figure 85 (a) Electrodes potential extrema vs. voltage measured during galvanostatic

(40 mA g

-1

) cycling of a (-) YP50F-KOH / YP80F-Na

2

SO

4

(+) cell; (b) CVs (0.4 mV s

−1

) of (-) YP50F-KOH / YP80F-Na

2

SO

4

(+) and (-) YP80F-KOH / YP80F-Na

2

SO

4

(+) cells.

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As discussed above, the margin before reaching the di-hydrogen evolution potential at the negative electrode can be utilized either by balancing the mass of the electrodes or by using different optimized carbons for the positive and negative electrodes. An adjustment of positive and negative electrodes mass ratio (m

+

/m

-

=2.25) enables to extend the operating cell potential to 1.6 V; however, the higher resistance of this system due to different thickness of the carbon electrodes suggests optimization of the cell through different AC electrodes. An application of carbon with different pore size distribution as electrode active material for positive and negative electrode enables to extend the operating voltage, and at the same time, to keep good electrochemical properties of the EC. Since the (-) YP50F/YP80F (+) configuration revealed promising results in the voltage extension, further experiments, i.e., cycling or accelerated ageing are obviously planned.

III.3. Conclusion

According to thermodynamic considerations, due to the pH difference between the basic catholyte and the neutral anolyte, an (-) AC-KOH / AC-Na

2

SO

4

(+) capacitor can theoretically operate up to 1.62 V. The effect of Galvani potential difference for AC electrodes in KOH and Na

2

SO

4

, as catholyte and anolyte, respectively, has been validated for a system with equal electrodes masses, which demonstrates good cycle life up to 1.5 V. The experiments on the (-) YP50F-KOH / YP80F-Na

2

SO

4

(+) cell confirmed that the operating voltage is essentially limited by the positive carbon electrode.

The exploration of the effects of electrode materials porosity on voltage expansion is found to be crucial for enhancing the voltage range in the new concept

KOH (-) / Na

2

SO

4

(+) capacitor. Obviously, there is still plenty of room for future experiments and for a subsequent design of the cation exchange membrane and current collectors which appear as ways for developing a new electrochemical capacitor generation. Notwithstanding, the performed experiments initiated a new direction for further studies based on this new concept cell, taking into account the advantages of both alkaline and neutral aqueous media (possibility to use stainless steel or nickel collectors) to develop high voltage and cheap AC/AC electrochemical capacitors.

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GENERAL CONCLUSION

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Improving the energy density, while keeping high power density and long cycle life, is the main objective in AC/AC electrochemical capacitor development. For being industrially implemented, such devices should be also cheap and easily manufactured, environmentally friendly and fulfill all the security requirements. To design and develop ECs in neutral aqueous electrolyte with stainless steel current collectors, three directions were particularly explored in this research work: i) the main factors contributing to ageing of AC-based electrochemical capacitors in lithium sulfate aqueous electrolyte with stainless steel collectors were disclosed by accelerated ageing based on potentiostatic floating; ii) strategies were proposed and verified to improve the long-term performance of the ECs at high voltage while implementing cheap constituents; iii) a new concept cell, based on two aqueous electrolytic solutions of different pH has been suggested and validated in order to extend the operating voltage window. The results obtained by the various physico-chemical and electrochemical investigations allow the following conclusions to be formulated.

Potentiostatic floating including two-hour floating periods is an accurate method for accelerated ageing of electrochemical capacitors based on carbon electrodes in aqueous electrolyte. Owing to the longer periods at high voltage, this test is more effective for determining ECs operation stability limits than galvanostatic cycling.

The failures which mainly appear during operation of the ECs are an increase of equivalent series resistance, capacitance loss and electrolyte decomposition. The postfloating investigations reveal carbon oxidation and accumulation of corrosion products on the positive electrode as subsequent factors causing ageing. The formed oxygenated surface groups block the pores, limiting the access of ions to the electrode active surface, and causing a drop of capacitance, whereas the accumulation of corrosion products at the electrode/collector interface causes a resistance increase. Moreover, the gases generated at the electrodes shorten the cell life due to electrolyte depletion and/or loss of electrode cohesion.

The reduction of ECs lifetime due to collectors’ corrosion has been, at least in part, prevented by: (i) using non-corrodible nickel collectors; (ii) coating the electrode material on the metallic collector in order to avoid the accumulation of corrosion products at the electrode-collector interface; (iii) adding a corrosion inhibitor to lithium sulfate electrolyte. The deposition of oxidized nickel compounds on the collectors and

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e separator during ageing at 1.6 V is probably associated with the observed pH variations.

Nevertheless, the appearance of these residues does not affect the performance of the cell which exhibits stable resistance values up to 120 hours at 1.6 V floating voltage.

The application of a conductive carbon ink (CCI) pre-coating on the metallic substrate improves the adhesion of the carbon coating and prevents from the accumulation of decomposition products at the electrode/substrate interface. The corrosion of stainless steel collectors was diminished by adding sodium molybdate inhibitor, which additionally enhances the cell capacitance.

The other strategy to improve the long-term operation of capacitors in aqueous media is a downshift of electrodes operating potential to avoid oxidation phenomena at the positive electrode (i) either by asymmetry of electrode materials; (ii) or by combining two kinds of cheap collectors. As expected, the maximum potential of the positive electrode could be reduced by coupling highly microporous Burley tobacco carbon (B800) as positive electrode with the industrial DLCS30 one as negative electrode. However, due to important surface functionality and correlated reactivity of

B800, the potential of the positive electrode shifted to higher values during floating, with subsequent deterioration of electrochemical performance. Finally, the corrosion of the positive stainless steel collector disappeared by combining (-) nickel and (+) stainless steel collectors. The potential shift towards lower values results in negligible electrolyte decomposition, electrode oxidation and/or formation of corrosion products on the positive current collector, allowing the cell to reach up to 1.6 V.

Due to the pH difference between potassium hydroxide and sodium sulfate separated by a proton exchange membrane, the new concept AC/AC capacitor is able to operate up to 1.5 V with a good stability while using stainless steel collectors. By asymmetry of electrodes, the operating voltage could be extended to 1.6 V.

As research perspectives on AC-based electrochemical capacitors in neutral salt aqueous electrolytes, in-situ analysis of evolved gases during accelerated ageing, by coupling gas chromatography (GC) with mass spectrometry (GC/MS), would be useful to elucidate the real decomposition mechanisms and to suggest adapted strategies (for example components enhancing recombination processes) to reduce internal pressure increase and oxidation of electrodes. Analysis by, e.g., X-ray photoelectron spectroscopy (XPS) of the nickel deposits formed after accelerated ageing on the

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e collectors and separator could provide information about their exact nature and allow strategies to be proposed for reducing their creation during long-term performance. On the basis of the aqueous lithium sulfate solution, another important objective would be to work out an electrolyte formulation for covering a wide temperature range, i.e. from -

40°C to +60°C. The ultimate work on cells implementing aqueous lithium sulfate would be to build pouch cells including all the optimized components previously identified and to investigate their electrochemical properties in various environments.

The performance of the new concept cell with two kinds of electrolytes could be improved by modifying the geometry and also by implementing more stable and highly conductive CEM with a good selectivity, in order to enhance cycle life and reduce the cell resistance. Applying gel electrolytes could be also a way to reduce some of the difficulties inherent to the use of a cationic exchange membrane.

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EXPERIMENTAL ANNEX

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A.1. Cell construction

1.1. Materials and chemicals

Electrode materials:

(i) The commercial activated carbons:



DLC Super 30 (Norit) was used for manufacturing pellet electrodes (named in the main text as S30)



YP-80F (Kuraray Chemicals Co) was used for manufacturing coated electrodes

(named in the main text as YP80F)



YP-80F and YP-50F (Kuraray Chemicals Co) were used as self-standing electrodes for the new concept cell (named in the main text as YP80F and

YP50F, respectively)

(ii) The tobacco carbon:

 Burley carbon (named as B800) was prepared from the leaves’ stems wastes of tobacco industry carbonized in a tubular furnace under nitrogen flow rate of 100 mL min

-1

and heated at 10 °C min

-1

up to 800 °C for one hour. The detailed process of sample preparation is given in the reference [78].



For the post-floating analysis of electrodes by thermoprogrammed desorption

(TPD), self-standing electrodes from activated carbon cloth (ACC 507-20,

Kynol) were selected to avoid the interference of the electrode binder.

Figure A1 P orous texture of carbons used in the study: (a) nitrogen adsorption/desorption isotherms recorded at 350 °C; (b) Pore size distribution determined using the 2D-NLDFT model.

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Carbon

DLCS30

YP-80F

YP-50F

B800

ACC 507-20

Surface area

[m

2

g

-1

]

1843

2270

1522

1651

2231

V micro

[cm

3

g

-1

]

0.715

0.827

0.628

0.664

0.886

Table A1 Nitrogen adsorption data of the carbons used in the study.

V meso

[cm

3

g

-1

]

0.183

0.237

0.059

0.190

0.029

Weight loss

950 °C

CO

2

CO H

2

O O

Carbon material wt% µmol g -1

µmol g

-1

µmol g

-1

wt%

DLCS30

YP-80F

YP-50F

B800

2.9

2.4

5.3

317

245

731

249

331

186

44

246

43

1.5

1.7

2.7

10.9

7.2

440

1347

598

331

416

133

3.0

5.1

ACC 507-20

Table A2 TPD on the carbon samples used for the experiments: weight loss at 950 ° C, amount of desorbed CO

2

, CO and H

2

O, and oxygen content calculated from the desorbed gases.

L

0

< 2 [nm]

[nm]

0.92

1.05

0.86

0.86

0.99

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Electrolytes:

(i) The output electrolyte in the study was 1 mol L

−1

lithium sulfate (Li

2

SO

4

,

Sigma-Aldrich, >99%) (pH = 6.5, conductivity = 64 mS cm

-1

)

(ii) In order to reduce the corrosion of current collectors, in some experiments in chapter V.1, 0.1 mol L

−1

sodium molybdate (Sigma Aldrich, >99.5%) has been added to the Li

2

SO

4

solution, (pH = 6.7, conductivity = 72 mS cm

-1

)

(iii) For the new concept cell presented in chapter V, 1 mol L

-1

sodium sulfate

(Na

2

SO

4

, Sigma-Aldrich, >99%) (pH = 6.6, conductivity = 68 mS cm

-1

) and

0.5 mol L

-1

potassium hydroxide (KOH, POCh, min. 85%) (pH = 13.2, conductivity = 56 mS cm

-1

) were used as catholyte and anolyte, respectively.

1.2. Preparation of electrodes

Pellet electrodes:

Pellet electrodes were prepared by mixing activated carbon (85 wt. %) with 5 wt.

% polyvinylidene fluoride as binder (PVdF, Kynar HSV900, Arkema) and 5 wt. % carbon black (C65, Timcal) conductivity enhancer. The three components were mixed with acetone (Avantor, 99.5%), then rolled to get a film, and the pellets with a thickness of around 0.3 mm and mass 8–10 mg and 1 cm diameter were pressed under

4.870 kg cm

-2

. The prepared electrodes were dried under vacuum at 110°C for 12 hours.

Coated electrodes:

Unless otherwise noted, for realizing coated electrodes, the surface of grade

1.431 stainless steel (Interbelts, thickness = 15 μm) or nickel (Schlenk, thickness = 20

μm) foils was pre-coated with a thin layer (15 μm) of carbon conductive ink (CCI)

(Electrodag™ PF-407A™, Acheson) to provide a rougher surface of the substrate.

Activated carbon YP 80F (83.5 wt. %), carbon black (SUPER C65, Timcal, 8.5 wt. %) conductivity enhancer and polyvinylidene difluoride (PVdF, Kynar® HSV 900,

Arkema, 8 wt. %) binder dissolved in 1-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) were mixed with an homogenizer (IKA ULTRA-TURRAX® T 18 basic), and the obtained slurry was cast with a Doctor Blade applicator (Elcometer® 3600) on the previously prepared surface of stainless steel or nickel foil. Afterwards, the coating is dried overnight by slow evaporation in air, followed by heating under vacuum at 120°C

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e for 12 h. 10 mm diameter disk electrodes (mass of active material ~3.5 mg, coating thickness ~100 μm) were punched out from the coating.

1.3. Cell configurations

Cells with pellet electrodes

Cells with electrodes in the form of pellets were realized by sandwiching a porous glass microfiber membrane GF/A (Whatman™, thickness = 0.26 mm) between two pellet electrodes (DLC Super 30, Norit) and two current collectors either from stainless steel or nickel, using PTFE Swagelok-type vessels with or without inlet for a reference electrode (Hg/Hg

2

SO

4 in 0.5 mol L

-1

H

2

SO

4

) (Figure A2). Before being

closed, the assembled system was soaked under vacuum with 1 mol L

-1

Li

2

SO

4 electrolytic solution. The current collectors (diameter 1.2 cm) were made from a low carbon content stainless steel 316L alloy consisting of the following major elements: Fe,

C (0.02%), Cr (16%), Ni (10%) and Mo (2%) or commercially pure nickel 200/201.

Their surface was cleaned with emery paper (P1000) before the investigations.

Figure A2 Schematic representation of the capacitors in PTFE Swagelok-type assembly with reference electrode.

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Cells with coated electrodes

Electrochemical capacitors with electrodes coated on stainless steel or nickel were realized in PTFE Swagelok-type vessel with an additional inlet for Hg/Hg

2

SO

4 reference electrode for measurements of CVs of individual electrodes, by sandwiching an absorptive glass mat separator (AGM, Bernard Dumas, thickness = 0.52 mm) between two coated electrodes (Kuraray YP-80F) and cylindrical current collectors, either from stainless steel 316L or nickel 200/201 (in analogy to the cells with pellet electrodes). Before being closed, the assembled system was soaked under vacuum with

1 mol L

-1

Li

2

SO

4

electrolytic solution.

All the data with reference electrode presented in the manuscript were calculated vs normal hydrogen electrode (NHE).

A.2. Electrochemical characterizations

The electrochemical properties of the capacitors and the new concept cell were investigated by cyclic voltammetry (CV), galvanostatic cycling with potential limitation

(GCPL) and impedance spectroscopy (EIS) at open circuit voltage (OCV) in the frequency range 1 mHz to 100 kHz and amplitude of 5 mV, using a VMP3 multichannel potentiostat/galvanostat (Bio-Logic Instruments, France). Data were collected using EC-

Lab V10.34 software. Capacitance was calculated from the galvanostatic discharge and expressed per average active mass of electrodes [F g

-1

] according to formula (43):

C

= 2

I

/ [(

dV/dt

)

m

]

(43) where I is the current [A], dV/dt is the slope of the discharge curve [V s

-1

], m is the average mass of carbon active material [g] .

A.3. Physico-chemical and surface characterization

Temperature-programmed desorption (TPD) analysis

The surface oxygenated functionality of fresh and aged carbon electrodes

(Kynol, ACC 507-20) was characterized by temperature-programmed desorption (TPD), using TG equipment (TG209 F1 Iris, NETZSCH) coupled with a mass spectrometer

(QMS 403C Aëolos, NETZSCH). To investigate the evolution of surface chemistry

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e after accelerated ageing, ACC electrodes were taken out from ECs after 120 h of floating, washed carefully with distilled water to remove the electrolyte and dried under vacuum at 120 °C. During surface functionality determination by TPD, c.a. 10 mg of

ACC was heated up to 950 °C (heating rate 20 °C min

-1

) under helium flow rate of 50 mL min

-1

. The surface functional groups evolving as CO

2

and CO were quantified after calcium oxalate monohydrate calibration, taking into account CO disproportionation

[216]. To determine the types of oxygenated complexes formed on the surface of the aged positive electrode, the deconvolution of CO

2

and CO patterns have been made with a multiple Gaussian function using the Origin 9.0 software.

Porous texture characterisation

In courtesy of Mgr. Piotr Skowron help from our research group, the porous texture of carbons was determined from nitrogen adsorption/desorption isotherms recorded at -196 °C using an ASAP2020 (Micrometrics). Prior to the measurements, the fresh and aged electrodes (around 60 mg) were degassed under vacuum for 36 h at

100°C. The pore size distribution (PSD) was determined using the 2D non-local density functional theory (2D-NLDFT) [107], the micro V micro

and mesopore volumes V meso were obtained directly from the calculated cumulative PSDs. The average micropore size (L

0

) was determined from the integration of the PSD area for the pores below 2 nm.

Scanning electron microscopy (SEM)

In courtesy of Dr. Eng. Tomasz Rozmanowski , scanning electron microscopy

(SEM) images of nickel 200/201, with and without carbon conductive ink (CCI), and soft-annealed nickel foils were analysed in a high vacuum mode with the use of Hitachi

Model S-3400N Scanning Electron Microscope with secondary electron (SE) detector.

Magnifications of x50 to x5.00k were obtained with a voltage of 15.0 kV.

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SCIENTIFIC ACHIEVEMENTS

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1.

Chapters in scientific books

1.

E. Frąckowiak, P. Ratajczak , F. Béguin; Ed.: P.N. Bartlett, R.C. Alkire, J.

Lipkowski, „Electrochemistry of Carbon Electrodes: Electrochemical Capacitors

Based on Carbon Electrodes in Aqueous Electrolytes”, Wiley-VCH , Weinheim,

2015

2.

Publications

2.1.

Publications in international journals from the Philadelphia list

1.

Q. Abbas, P. Ratajczak

, P. Babuchowska, A. Le Comte, D. Bélanger, T.

Brousse, F. Béguin, Strategies to Improve the Performance of Carbon/Carbon

Capacitors in Salt Aqueous Electrolytes, J. Electrochem. Soc.

162 (2015)

A5148-A5157

2.

P. Kleszyk, P. Ratajczak

, P. Skowron, J. Jagiełło, Q. Abbas, E. Frąckowiak, F.

Béguin, Carbons with narrow pore size distribution prepared by simultaneous carbonization and self-activation of tobacco stems and their application to supercapacitors, Carbon , 81 (2015) 148–157

3.

Q. Abbas, P. Ratajczak

, F. Béguin, Sodium Molybdate - An additive of choice for enhancing the performance of AC/AC electrochemical capacitors in salt aqueous electrolyte, Faraday Discuss., 172 (2014) 199-214

4.

P. Ratajczak , K. Jurewicz, P. Skowron, Q. Abbas, F. Béguin, Effect of accelerated ageing on the performance of high voltage carbon/carbon electrochemical capacitors in salt aqueous electrolyte, Electrochim. Acta , 130

(2014) 344–350

5.

P. Ratajczak

, K. Jurewicz, F. Béguin, Factors contributing to ageing of high voltage carbon/carbon supercapacitors in salt aqueous electrolyte, J. Appl.

Electrochem ., 44 (2014) 475.

3.

Conferences

3.1.

Oral presentations

1.

F. Béguin, Q. Abbas, P. Babuchowska,

P. Ratajczak , Development of a high energy AC/AC capacitor in aqueous electrolyte, 16th Topical Meetingof the International Society of Electrochemistry, Brazil, Angra dos Reis 2015

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2.

F. Béguin, Q. Abbas, P. Jezowski,

P. Ratajczak , Development of a high-voltage capacitor prototype in environment friendly salt aqueous electrolyte, Advanced

Automotive Battery Conference AABC 2014, USA, Atlanta, 2014

3.

P. Ratajczak , P. Jeżowski, F. Béguin , Performance improvement of

AC/AC capacitors in aqueous medium through modiﬁcation of the current collector/active material interface, 65th Annual Meeting of the International

Society of Electrochemistry, Switzerland, Lausanne 2014

4.

P. Ratajczak

, P. Jeżowski, P. Skowron, K. Jurewicz, F. Béguin, Design and development of AC/AC supercapacitors in salt aqueous electrolyte, Winter seminar „Latest Developments in Electrochemical Capacitors“ , Estonia, Tartu

2013

5.

P.M. Kleszyk, P. Ratajczak

, P. Skowron, F. Béguin, Samo-aktywacja biomasy: nowa metodologia wytwarzania węgli aktywowanych z kontrolą rozkładu wielkości porów, Węgiel aktywny w ochronie środowiska i przemyśle, Poland,

Białowieża

2013

6.

P.M. Kleszyk, Q. Abbas, P. Ratajczak

, P. Skowron, F. Béguin, Manufacturing of nanoporous carbons by self-activation of tobacco and their application for energy storage in supercapacitors, 5th International Conference on Carbon for

Energy Storage/Conversion, Germany, Mülheim a.d. Ruhr 2013

7.

F. Béguin, Q. Abbas, P. Ratajczak , E. Frackowiak, A new generation of high voltage and environment friendly supercapacitor using salt-based aqueous electrolytes, The 64th Annual Meeting of the ISE, Mexico, Santiago de

Querétaro 2013

8.

F. Béguin, P.M. Kleszyk,

P. Ratajczak , P. Skowron, Novel nanoporous carbons prepared by self-activation of biomass and their properties in supercapacitors,

Annual World Conference on Carbon - Carbon 2013, Brazil, Rio de Janeiro

2013

9.

P. Ratajczak

, K. Jurewicz, F. Béguin, Performance limits of high voltage aqueous AC/AC supercapacitors under accelerated ageing, 3rd International

Symposium on Enhanced Electrochemical Capacitors (ISEECap2013), Italy,

Taormina 2013

10.

P. Ratajczak

, K. Jurewicz, F. Béguin, Monitoring the state of health (SOH) of high voltage aqueous AC/AC supercapacitors during the life of the system,

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International Conference on Advanced Capacitors (ICAC2013), Japan, Osaka

2013

11.

F. Béguin, Q. Abbas, P. Kleszyk, P. Ratajczak , Towards the prototyping of high voltage AC/AC capacitors in neutral aqueous electrolyte, International

Conference on Advanced Capacitors (ICAC2013), Japan, Osaka 2013

12.

Q. Abbas, P.M. Kleszyk, P. Ratajczak , F. Béguin, Performance of

Electrochemical Capacitors with Microporous Carbon Electrodes in New Types of Aqueous Electrolytes, 13th Topical Meeting of the International Society of

Electrochemistry - Advances in Electrochemical Materials Science and

Manufacturing, South Africa, Pretoria 2013

13.


, P. Skowron, F. Béguin, Novel

Nanoporous Carbons Based on Tobacco and Their Electrochemical Properties in

Supercapacitors, 13th Topical Meeting of the International Society of

Electrochemistry - Advances in Electrochemical Materials Science and

Manufacturing, South Africa, Pretoria 2013

14.


, P. Skowron, F. Béguin, Novel nanoporous carbons based on tobacco and their properties in supercapacitors,

VII International Scientific and Technical Conference – Carbon Materials &

Polymer Composites, Poland, Ustroń – Jaszowiec 2012

15.

P. Skowron, P. Ratajczak , M. Anouti, E. Frąckowiak and F. Béguin,

Supercapacitor application of activated carbons modified by electrografting with pyridine-4-diazonium chloride, VII International Scientific and Technical

Conference – Carbon Materials & Polymer Composites, Poland, Ustroń –

Jaszowiec 2012

16.

F. Béguin,

P. Ratajczak

, P. Kleszyk, P. Jeżowski, Q. Abbas, P. Skowron, K. Fic and E. Frąckowiak, Strategies for enhancing the performance of carbon-based supercapacitors, VII International Scientific and Technical Conference – Carbon

Materials & Polymer Composites, Poland, Ustroń – Jaszowiec 2012

17.

F. Béguin, K. Fic,

P. Ratajczak , K. Jurewicz, Q. Abbas, G. Lota, G. Gao, L.

Demarconnay, E. Raymundo, E. Frackowiak, Performance limits of 2 V C/C supercapacitors in alkali sulfate aqueous media, 222nd Meeting of ECS — The

Electrochemical Society (PRiME 2012), USA, Honolulu, Hawaii 2012

18.

P. Ratajczak , P. Jezowski, K. Jurewicz, G. Lota, E. Frackowiak and F. Béguin,

Influence of supercapacitors operating conditions on their performance in

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e aqueous electrolyte, COST Action MP1004 “Hybrid Energy Storage Devices and Systems for Mobile and Stationary Applications”, Turkey, Kayseri 2012

3.2. Poster presentations

1.

F. Béguin, Q. Abbas, A. Laheäär,

P. Ratajczak

, B. Górska, P. Skowron, P.

Jeżowski, P. Przygocki, P. Babuchowska, Development of high performance and ecologically friendly supercapacitors for energy management – ECOLCAP project, Interdisciplinary FNP conference Warsaw, Poland 2015

2.

P. Ratajczak

, K. Jurewicz, F. Béguin, Overcoming the electrochemical stability limits of aqueous electrolyte capacitors, 65th Annual Meeting of the

International Society of Electrochemistry Lausanne, Switzerland 2014

3.

P. Ratajczak

, A. Ślesiński, K. Jurewicz, P. Skowron, E. Frąckowiak, F. Béguin,

Gas evolution and accompanying reactions - main factors contributing to deterioration of electrochemical capacitors in salt aqueous electrolyte, 65 th

Annual Meeting of the International Society of Electrochemistry Lausanne,

Switzerland 2014

4.

P. Ratajczak


International Society of Electrochemistry Lausanne, Switzerland 2014

5.

L. Garcia-Cruz, P. Ratajczak , J. Iniesta, V. Montie, F. Béguin, Self-Discharge of Carbon/Carbon Supercapacitors in Salt Aqueous Electrolyte, The World

Conference on Carbon, Jeju, Korea 2014

6.

P. Ratajczak

, P.M. Kleszyk, K. Jurewicz, F. Béguin, Effect of ageing supercapacitors operating in aqueous medium on the surface chemistry of activated carbon, 5th International Conference on Carbon for Energy

Storage/Conversion, Germany, Mülheim a.d. Ruhr 2013

7.

P. Skowron, P. Ratajczak , K. Fic, M. Anouti, E. Frąckowiak, F. Béguin, Effect of diphenols addition to protic ionic liquid electrolytes on the performance of supercapacitors, 5th International Conference on Carbon for Energy


8.

Q. Abbas, P. M. Kleszyk, P. Ratajczak

, F. Béguin, Effect of pH on the performance of activated carbons based symmetric capacitors in aqueous

Paula Ratajczak

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e electrolytes, 3rd International Symposium on Enhanced Electrochemical

Capacitors (ISEECap2013), Italy, Taormina 2013

9.

K. Torchała, K. Kierzek, P. Ratajczak , F. Béguin, J. Machnikowski , Effect of surface functionalization on the performance of activated carbon as positive and negative electrode in asymmetric aqueous capacitor, 3rd International


Taormina 2013

10.

P. M. Kleszyk, Q. Abbas, P. Skowron, P. Ratajczak F, Béguin, Self-activated carbons based on biomass for application in supercapacitors, 3rd International


Taormina 2013

11.

P. Ratajczak

, P. M. Kleszyk, K. Jurewicz, F. Béguin, New insights on stability of high voltage supercapacitors utilizing tobacco-based carbons, 3rd

International Symposium on Enhanced Electrochemical Capacitors

(ISEECap2013), Italy, Taormina 2013

12.

P. Ratajczak, P. M. Kleszyk, K. Jurewicz, F. Béguin, Effect of carbons on the performance of aqueous electrochemical capacitors under accelerated ageing,

International Conference on Advanced Capacitors (ICAC2013), Japan, Osaka

2013

4.

Awards

4.1. Best poster awards

1.

P. Ratajczak


International Society of Electrochemistry, Lausanne, Switzerland 2014

2.

P. Ratajczak , P.M. Kleszyk, K. Jurewicz, F. Béguin, Effect of ageing supercapacitors operating in aqueous medium on the surface chemistry of activated carbon, 5th International Conference on Carbon for Energy


3.

P. Ratajczak , P. M. Kleszyk, K. Jurewicz, F. Béguin, New insights on stability of high voltage supercapacitors utilizing tobacco-based carbons, 3rd

International Symposium on Enhanced Electrochemical Capacitors

(ISEECap2013), Italy, Taormina 2013

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5.

Participation in research projects

5.1.

As a stipendee (PhD thesis):

ECOLCAP project funded in the frame of the Welcome Programme implemented by the Foundation for Polish Science (FNP) within the Measure

1.2. ‘Strengthening the human resources potential of science’, of the Innovative

Economy Operational Programme supported by European Union.

Project leader: Prof. François Béguin

5.2.

As a coordinator of the researches:

Statutory grant no 03/31/DSMK/0287

Statutory grant no 03/31/DSMK/0305

This thesis’ research was partially supported by statutory grants

5.3.

Employed as a scientific assistant:

LIDER project financed by National Centre for Research and Development

(NCBiR): „Kondensator elektrochemiczny o wysokiej gęstości energii i mocy operujący Procesy pseudopojemnościowe na granicy faz elektroda/elektrolit w elektrochemicznych systemach magazynowania energii w roztworach sprzężonych par redoks”.

Project leader: Dr. Eng. Krzysztof Fic

5.4.

As a stipendee (Master thesis):

ECOLCAP project funded in the frame of the Welcome Programme implemented by the Foundation for Polish Science (FNP) within the Measure

1.2. ‘Strengthening the human resources potential of science’, of the Innovative

Economy Operational Programme supported by European Union.

Project leader: Prof. François Béguin

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ABSTRACT

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Taking into account the numerous advantages of water-based media over organic solutions, the ultimate aim of this doctoral dissertation is to design and develop a carbon-based environmentally friendly and low-cost electrochemical capacitor (EC) operating in an aqueous electrolyte and using non-noble collectors. To pursue this objective, an accelerated ageing test has been adapted, factors contributing to failures during operation have been determined, and finally, a number of solutions allowing the cells performance to be optimized have been proposed. Overall, after a general introduction, the dissertation is divided into five chapters and ends by a general conclusion.

The first chapter presents the state-of-the-art of electrochemical capacitors

(ECs). At first, the operating principle and general properties of electrical double-layer capacitors (EDLCs) are briefly described. Then, the common electrode materials (in particular porous carbons) and electrolytes generally employed for ECs are introduced, with their advantages and disadvantages. A special emphasis is placed on neutral aqueous electrolytes exhibiting a high over-potential for di-hydrogen evolution, and thereof allowing high operating voltages to be obtained.

Chapter II presents the experimental techniques and procedures used in the development of the dissertation. The principles of galvanostatic cycling with potential limitation (GCPL), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), together with the parameters determined from these methods, are introduced. Taking into account the limited time allowed for the preparation of this dissertation, the advantages of an accelerated ageing protocol, by so-called potentiostatic floating, are presented to evaluate the end of life of ECs in a reduced research time.

The adaptation of the floating protocol and the determination of maximum operating voltage for a carbon-based capacitor in aqueous lithium sulfate electrolyte with stainless steel collectors are presented in chapter III. To evaluate the state-of-health

(SOH) of this system, capacitance, resistance and internal pressure of the cell are monitored during the test. The possible factors contributing to ageing of the ECs in aqueous solution with stainless steel current collectors are identified. The alterations in physicochemical properties of the cell constituents after long time operation, such as modifications of surface functionality and porosity of the AC electrodes, together with corrosion of the stainless steel current collectors, are revealed.

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In chapter IV, strategies are investigated to improve the cycle-life of ECs in aqueous lithium sulfate electrolyte, essentially for reducing the corrosion of stainless steel collectors and decreasing its detrimental effect on cells operation. The suggested solutions include the replacement of stainless steel by nickel collectors, the protection of the active material/current collector interface and the addition of sodium molybdate corrosion inhibitor to lithium sulfate electrolyte. Another tactics involves the application of an asymmetric configuration of electrodes, i.e., different current collectors and different mass or kind of carbon for the two electrodes, in order to shift the electrodes operating potentials toward lower values.

Chapter V is directed to new perspectives for the research on ECs in aqueous electrolytes. A new concept cell is proposed by implementing an anolyte and a catholyte of different pH, both separated by a cationic exchange membrane. The application of potassium hydroxide as catholyte and sodium sulfate as anolyte should result in a higher voltage of the cell than the thermodynamic limit of 1.23 V for water decomposition.

Practically, the cell is able to operate up to 1.5 V with an excellent cycle life. Although the proposed new concept cell still requires some optimizations, it opens new insights for the R&D on ECs in aqueous electrolytes.

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STRESZCZENIE

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Z uwagi na liczne zalety elektrolitów wodnych nad roztworami organicznymi, bezpośrednim celem prezentowanej pracy doktorskiej było opracowanie przyjaznych dla środowiska oraz tanich kondensatorów elektrochemicznych, działających w elektrolicie wodnym oraz przy użyciu kolektorów prądowych wytworzonych z metali nieszlachetnych. Pierwszym krokiem do zrealizowania powyższego założenia było dostosowanie do badanych układów testu przyspieszonego starzenia. Następnie określono czynniki, które przyczyniają się do pogorszenia pracy kondensatorów w elektrolitach wodnych. Ponadto przeprowadzono badania, weryfikujące zaproponowane rozwiązania, mające na celu zoptymalizowanie tych układów. Po ogólnym wprowadzeniu rozprawa podzielona jest na pięć rozdziałów i kończy się ogólnymi wnioskami.

Pierwszy rozdział przedstawia przegląd literatury nt. kondensatorów elektrochemicznych. Na początku krótko opisano zasadę działania oraz ogólne właściwości kondensatorów podwójnej warstwy elektrycznej. Następnie przedstawiono materiały elektrodowe (w szczególności porowate elektrody węglowe) oraz elektrolity zwykle stosowane w kondensatorach elektrochemicznych, wraz z ich zaletami i wadami. Szczególny nacisk został położony na neutralne elektrolity wodne, wykazujące znaczący nadpotencjał wydzielania wodoru, który pozwala na uzyskanie wysokiego napięcia pracy układu.

Rozdział II przedstawia techniki i procedury eksperymentalne użyte w badaniach do przedłożonej pracy doktorskiej. W pierwszej kolejności zaprezentowano techniki, które są wykorzystywane do rozpatrywania cykliczności kondensatorów elektrochemicznych poprzez galwanostatyczne ładowanie/wyładowanie, woltamperometrię cykliczną oraz spektroskopię impedancyjną. Ponadto, biorąc pod uwagę ograniczoną ilość czasu, pozwalającego na przygotowanie tej rozprawy, skupiono się również na teście przyspieszonego starzenia przez tzw. floating, dla oceny

‘końca życia’ kondensatora przy skróconym czasie badań.

W rozdziale III przedstawiono adaptację protokołu przyspieszonego starzenia oraz określono maksymalne napięcie pracy kondensatora na bazie węgla, działającego w wodnym roztworze siarczanu litu oraz z kolektorami ze stali nierdzewnej.

Monitorowanie takich parametrów jak: pojemność, opór oraz ciśnienie wewnętrzne, zostały uznane za niezbędne do oceny stanu analizowanych układów. Przeprowadzone badania pozwoliły na zidentyfikowanie przyczyn spadku żywotności kondensatora,

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D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e pracującego w roztworze wodnym z zastosowaniem kolektorów prądowych ze stali nierdzewnej. Ponadto, po długim czasie pracy zanalizowano zmiany we właściwościach fizyko-chemicznych poszczególnych elementów ogniwa, takich jak: redukcja powierzchni właściwej i porowatości elektrod węglowych oraz korozja stalowych kolektorów prądowych.

W celu poprawy długoterminowej pracy kondensatora elektrochemicznego w wodnym roztworze siarczanu litu, w rozdziale IV przedstawiono strategie, skupiające się głównie na zmniejszeniu korozji kolektorów prądowych ze stali nierdzewnej i zredukowaniu szkodliwego wpływu tych depozytów na działanie ogniwa. Proponowane rozwiązania obejmują zastąpienie stali nierdzewnej przez kolektory niklowe, ochronę granicy faz elektroda/kolektor oraz dodanie inhibitora korozji (molibdenianu sodu) do elektrolitycznego roztworu siarczanu litu. W celu przesunięcia potencjału elektrody dodatniej w kierunku niższych operacyjnych wartości, asymetryczne konfiguracje

(poprzez sparowanie dwóch różnych kolektorów prądowych lub użycie różnych elektrod węglowych dla dodatniej i ujemnej polaryzacji) zostały wykorzystane.

Rozdział V zorientowany jest na perspektywiczne badania nad kondensatorami w elektrolitach wodnych. Nowa koncepcja ogniwa elektrochemicznego polega na zastosowaniu anolitu i katolitu o różnym pH, oddzielonych od siebie przez membranę kationowymienną. Użycie wodorotlenku potasu i siarczan sodu (odpowiednio, jako katolitu i anolitu), powinno skutkować wyższym napięciem pracy układu od termodynamicznego limitu rozkładu wody (1,23 V). W praktyce zbudowany kondensator jest w stanie działać do napięcia 1,5 V z satysfakcjonującą cyklicznością.

Mimo że, proponowana koncepcja ogniwa wciąż wymaga pewnych optymalizacji, otwiera ona nowe perspektywy dla badań i rozwoju nad kondensatorami w elektrolitach wodnych.

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