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A new gravity flow assisted Flowable electrode based energy
storage system for electric vehicles and grid storage application.
Submitted by….
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Acknowledgments
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Table of contents
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List of tables
List of figures
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Abstract
In this study an alternative energy storage system for both renewable sector and electric vehicle
is investigated. In electric vehicles, in particular, it concentrates on the potential reduction of
the stresses of the battery when a flow assisted electrochemical capacitors, a.k.a. super
capacitors, are added as high power energy storage system. A highly conductive flowable
electrode was synthesized, using which a single cell flow assisted electrochemical capacitor
was designed. Different control strategies are evaluated and an estimation of the performance is
done. In order to verify the simulation results, a working model of 80 ml capacity has been
designed and the cell performance has successfully evaluated using characterization
instruments. The cell capacitance was experimentally measured and its energy and power
densities were compared with currently used batteries and capacitors. In the proposed system
the energy storage mechanism is completely physical where micro-porous carbon particles are
used as the electrode material. The study also predict that by using such a system along with
battery can significantly reduce battery stress during high power demand times in Electric
vehicles and in grid storage sector.
Key words: electrochemical capacitor, flowable electrode, super capacitors.
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Hypothesis
“Electrical energy can be stored on a flowable fluid media composed of microporous carbon
particles and electrolyte and can be used for renewable energy storage and also for electrical
vehicle applications”.
“The flow assisted storage system will exhibit have high energy density than normal static
capacitors and will have high power density and cycle life than batteries”.
“Gravity assisted flow can be utilized for charging and discharging actions”
Goals and Objectives
The main objective of this work is to practically demonstrate the possibility of energy storage
in flowable carbon electrolyte slurry. To perform a scientific analysis on the performance and
its energy storage mechanism of the cell using computational approach.
In line with these goals, the objectives are
i)
To develop a single cell working model with all the basic functional units to
experimentally prove the working of the cell.
ii)
Establish a scientific framework to assess the single cell performance and to
compare with other storage methods. For this a MATLAB simulation was
developed.
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1.
Introduction
Electrical energy and its storage has become one of the most important, that we are
concerned about in our day today life. From cell phones, laptops, home invertors to electric
vehicles, grid storage and renewable energy plants; we need an efficient energy storage
system. Currently batteries and capacitors are used in many fields to satisfy our needs.
Batteries have high energy density, but it lack power density. On the other hand capacitors
offers goof power density and fast charging but it has very low energy density. The world is
changing, new devises and technologies are emerging and in most fields the demand for a
storage system that can offer faster charging, good energy density, high power delivering
capability and longer cycle life is on the increase. All over the world scientists are working
in the area of energy storage systems to improve its performance and shelf life.
The following are the different kinds of energy storage systems used at present.
1. Batteries
Batteries are chemical devices which do not store electricity, but rather its stores a series
of chemicals, and through chemical process electricity is produced [1]. Batteries operate
by converting chemical energy into electrical energy through electrochemical discharge
reactions. A single cell generally consists of a positive electrode, negative electrode,
separator, and electrolyte. Batteries are classified in to two types; primary and
secondary. Once used primary cells cannot be reused, it employs chemical reaction
which is irreversible in nature. Secondary cells depend on a reversible reaction and it can
be used again and again. It can be recharged by applying an external DC source across
its terminals to reverse the reaction [2].
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Figure 1.1: Internal structure of a Lead acid battery.
Courtesy: http://www.autoshop101.com/forms/hweb3.pdf
Batteries have penetrated to almost every field. 80% of energy storage in renewable sector is mainly
done using batteries. Main types of conventional storage batteries that are used extensively today: the
lead–acid batteries, the nickel based batteries and the lithium-based batteries. (rated voltage: 2 V,
energy density: 30 Wh/kg, power density: 180 W/kg, energy efficiencies (between 85 and 90%),
lifetime 1200 and 1800 charge/discharge cycles), . The cycle life is negatively affected by the depth
of discharge and temperature [3].
For the nickel-based batteries (rated: 1.2 V (1.65 V for the NiZn type), energy densities 50 Wh/kg for
the NiCd, 80 Wh/kg for the NiMH and 60 Wh/kg for the NiZn, cycle life of NiCd: 1500 to 3000
cycles, energy efficiencies between 65 and 70%). However, the NiCd battery may cost up to 10
times more than the lead–acid battery and their application in the field of industrial and renewable
sector is limited[3].
Lithium-based battery storage system is the young technology but it has not yet been used for energy
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storage in the context of an uninterrupted power supply (UPS) system although such applications are
being developed. Currently, lithium battery technology is typically used in mobile or laptop systems
and in the near future it is envisaged to be used in hybrid or electric vehicles. Lithiumion cells are
having (nominal voltage: 3.7 V, energy densities 80 to 150 Wh/kg (for lithium-polymer cells it
ranges from 100 to 150 Wh/kg), energy efficiencies: 90 to 100%, power density: 500 to 2000 W/kg
(for lithium-polymer it ranges from 50 to 250 W/kg), lifetime: 1500 cycles. Laos the life and
performance of the cell is greatly affected by temperature.
2. Capacitors
Capacitor is an electronic component that stores electric charge. Capacitor consists of two closely
spaced conducting plates placed parallel to each other that are separated by a dielectric material.
When an external electric field is applied across its plates, the plate’s starts accumulate electric
charge. Capacitors are widely used in electronic circuits for filtering and regulating electrical energy.
Capacitors are widely used for storing electric charges, conducting alternating current and blocking
direct current [4].
The capacitance is the amount of electric charge that is stored in the capacitor at voltage of 1 Volt.
The capacitance is measured in units of Farad (F).
1 Farad = 1 coulomb charge/ 1 volt
1 coulomb = 6 x 1019 electrons
V= Q/ C
where C is capacitance in farads (F).
Q is the charge on the capacitor in coulombs (C).
V is the voltage difference between the capacitor plates in volts (V).
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A capacitor whose capacitance value is 1 Farads can store a lot amount of electrical energy [5].
Capacitors that are used in most electronic devices are measured in microfarads ( F) or even
picofarads (pF). In practice, picofarads are often called “micromicrofarads” ( F) or just “puffs.”
The two primary attributes of a capacitor are its energy density and power density.
The energy E stored in a capacitor is directly proportional to its capacitance:
E = 1/2 CV 2
In general, the power P is the energy expended per unit time. To determine P for a capacitor, though,
one must consider that capacitors are generally represented as a circuit in series with an external
“load” resistance R, as is shown in Figure 1.2.
Figure 1.2: Schematic of a capacitor
𝑽𝟐
𝑷𝒎𝒂𝒙 =
𝟒 × 𝑬𝑺𝑹
The internal components of the capacitor (e.g., current collectors, electrodes, and dielectric material)
also contribute to the resistance, which is measured in aggregate by a quantity known as where ESR
is the equivalent series resistance( which is the sum of resistances of individual parts (current
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collectors, electrodes, and dielectric material) . This relationship shows how the ESR can limit the
maximum power of a capacitor [6].
3. Electro chemical capacitors
The third generation evolution is the electric double layer capacitor, where the electrical charge
stored at a metal/electrolyte interface is exploited to construct a storage device. The interface can
store electrical charge in the order of ~10 6 Farad. The main component in the electrode construction
is activated carbon [7]. Just like the normal capacitors, electro chemical capacitors also known as
supercapacitors, or ultracapacitors, stores energy as charge on a pair of electrodes. These capacitors
utilize electrode material which has greater surface area and thin electrolytic dielectrics to achieve
capacitances several orders of magnitude larger than conventional capacitors. In doing so,
supercapacitors are able to attain greater energy densities while still maintaining the characteristic
high power density of conventional capacitors. Unlike conventional capacitor electrochemical
capacitors stores energy in the electric double layer which forms at the interface between the
electrode material and electrolyte. An electro chemical capacitor generally consists of two electrodes,
electrolyte and an ion exchange membrane. The electrolyte commonly used is potassium hydroxide,
sulfuric acid, acetonitrile, propylene carbonate and also ionic electrolytes [8].
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Figure 1.3: Electrical double layer distribution of ions.
Curtsey: http://www.mdpi.com/2073-4360/3/4/2039/htm
The charge that can be stored on an electrochemical capacitor is mainly depends on the area of
electrode used. Activated carbon whose surface is around 1000 m2/g is used commonly as both anode
and cathode for an electrochemical capacitor. Electrochemical capacitors offer capacitance value in the
range of 100 to 140 F/g and energy density in the order of 2 to 5 WH/ Kg. Electrochemical capacitors
can be used in places where there is demand for high power and also when there is energy available
for a limited amount of time and it needs to be stored as fast as possible. Electro chemical capacitor
offers faster charging and also facilitates efficient recovery with negligible losses. Presently
electrochemical capacitors are used in hybrid vehicles to store wasted heat energy during breaking,
energy management in cranes, forklifts, spot welding etc. [8]. Figure 1.4 is showing the Ragone plot
comparing the energy and power densities of energy storage devices.
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Figure 1.4: Ragone plot for various energy storage and energy conversion devices.
Courtesy: https://commons.wikimedia.org/wiki/File:Supercapacitors-vs-batteries-chart.png
4. Advantages and dis advantages of super capacitors
4.1 The advantages:
1. Unlimited cycle life; as compared to the electrochemical battery, they are not subject to the wear
or aging.
2. On-hand charge methods; no full-charge circuit required.
3. Quick charging times
4. Low impedance; by paralleling it with a battery, it enhances the pulse current.
5. Cost effective storage; a very high cycle count compensates the lower density.
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4.2 The disadvantages:
1. Low energy density; usually holds 1/5 – 1/10 of a battery.
2. Cannot use the full energy spectrum for some applications.
3. Low voltage cells; to get higher voltages, serial connections are required.
4. Voltage balancing needed; when more than 3 supercapacitors are connected in series, the circuit
needs a voltage balancing element.
5. High self-discharge as compared to electrochemical batteries [9].
5. The Electrochemical Flow Capacitor
In 2011, Drexel University reported a novel technology, the electrochemical flow capacitor (EFC),
which can potentially overcome some of the major limitations of supercapacitors [10]. In this
particular cell a new concept was introduced making the electrode as a flowable medium. This
medium is prepared by mixing porous activated carbon powders (any derivatives of carbon),
electrolyte and additives that can enhance storage capacity. In this system charge storage is happens
on the surface of three dimensional electrode particles. The slurry is allowed to circulate in a cavity.
The cavity is divided in to two sections by an ion exchange membrane. The anode material flows
through one side and the cathode flows through another side, charging and discharging happens
while the slurry is flowing through the cavity.
Unlike conventional static capacitors the, the unique aspect of flow-assisted systems is the
decoupling of energy and power ratings and, thus, the system energy storage capacity. In a flow
capacitor, the energy storage capacity is determined by the choice of electrolyte and size of
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electrolyte reservoirs while the power handling ability is dependent on the size and quantity of
electrochemical cell stacks. Once charged the capacitive slurry can be stored in containers and can be
kept for long without any loses. At the time of discharge this slurry can be flowed back between the
membrane and energy can be discharged.
Figure 2.1: Showing the basic block diagram of a flow capacitor.
Courtesy: The Electrochemical Flow Capacitor: Capacitive Energy Storage
in Flowable Media/ Drexel university thesis paper
6. PROTOTYPE DESIGN
In this work we aims at developing a more advanced model of flow capacitor with better
performance and flow characteristics. By designing a prototype which uses of the unique aspect of
flowable electrodes, we intend to prove that, it possible to convert many conventional
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electrochemical systems (e.g. batteries, supercapacitors, etc.) into more scalable flow assisted
architecture.
Based on the information’s obtained from the published literatures and after understanding the
working of a EFC cell, a 3D model for the prototype was created in AUTOCAD.
6.1 Prototype 1
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Figure 3: Showing the 3D design and dimensions’ of the prototype.
6.1.1 Different parts of the prototype
Figure 4: Showing different parts of the working model.
1. Middle chamber
The design consists of a middle chamber which is the back bone of the design. This chamber is
of 200 ml capacity and is divided in to two sections namely positive half chamber and negative
half chamber. The separation is done using a non-conducting ion exchange membrane. The
chamber is made up of poly propylene plastic in cylindrical geometry.
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2. Ion transport membrane
It is a non-conducting micro porous membrane whose primary function is to transport ions
across the half chambers without direct mixing of the flowable electrode. For the prototype we
used 5 micron nano porous PTFE membrane.
3. Discharged slurry reservoir
There are two reservoirs discharged slurry reservoir 1 and discharged slurry reservoir 2 each 100
ml capacity which are dedicated to store discharged slurry. They are also made up of nonconducting material (plastic).
4. Charged slurry reservoir
There are two reservoirs charged slurry reservoir 1 and charged slurry reservoir 2 each 100 ml
capacity which are dedicated to store charged slurry. They are also made up of non-conducting
material (plastic).
5. DC Pumps
Four individual DC pumps are provided to transport slurry back and forth form reservoirs to
middle chamber and vice versa. For the prototype DC pump having specification 12 Volt, 300
rpm, 2 watt were used. These pumps are of special type whose having an internal turbine wheel
which does not have direct contact with the motor shaft. The transmission is done by magnetism
and the rotating turbine is completely isolated from the rest of the pump body. This feature of the
pump will avoid the chance of contact between slurry and metallic part of the pump there by
avoiding the risk of accidental discharge.
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6. Tubing
Flexible plastic tubing connects the reservoir and half chamber with the inlet and outlet of the
pump respectively. These tubing carry the slurry back and forth form reservoirs to middle
chamber and vice versa.
7. Current collector
Two current collector plates are provided inside each half chambers (terminal 1 and terminal 2).
Current collector plates are made of nickel metal having 1 mm thickness. The surface of which
are grinded using sand paper and washed thoroughly with sand paper to improve the surface area
and to remove all the oxide layer and contaminants from the surface. Current is supplied in and
taken out of the flowable electrode is done through this terminals.
Slurry preparation
The slurry consisting of activated carbon suspended in an aqueous electrolyte was used.
14 wt% activated carbon was mixed with 2 wt% carbon black (conductive additive; 100%
compressed, Alfa Aesar, USA) in a 1M KOH (anhydrous, Alfa Aesar, USA) aqueous
electrolyte solution. Deionized water was added to achieve complete wetting of the
carbon particles, and the mixture was stirred for a minimum of 45 minutes to disperse the
activated carbon and carbon black. The slurry was then gently heated until the additional
mass of excess water had evaporated away. At this point, the composition of the slurry
was assumed to have returned to the mass fractions specified during the initial mixing.
The final slurry was quite flowable, with a consistency similar to motor oil. Addition of
isopropyl alcohol is also recommended for good flowability. Experiment result showd
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that slurry containing 30 – 70 % alcohol has showed good flowablility.
(include slurry preparation images)
(Sem images ..particle size)
Steps of making slurry :The carbon powder is taken in a silica crucible and is allowed to dry in an oven set at 120
degrees for 12 hrs. This will remove all the moisture inside the carbon powder. The
powder is then weighed using a mass balance. Carbon black is also measured by the same
fashion and both are mixed together in 14:2 ratios in a ball milling machine runs at 300
rpm for 2 hours duration. After mixing a homogeneous mixture of activated carbon and
carbon black will be obtained. Carbon black is added as an additive in order to improve
the conductivity of the slurry. Now 40 ml of 1 molar potassium hydroxide solution is
prepared in deionized water and is mixed with the carbon powder. 90 ml of isopropyl
alcohol is added to the slurry and mixed thoroughly for about 8 hours in a slurry mixer.
This will make the slurry to achieve a good consistency and proper viscosity which has
good flowability.
Prototype assembly
Based on the 3 d design a prototype was constructed. Four reservoirs are fixed at the four
corners of a acrylic plate having dimension 300mm x 300 mm x 4 mm. the middle
chamber was place at a platform 100 mm higher to the reservoir base. Each reservoir and
middle chamber is having 100 ml capacity. Four DC pumps were fixed on the base plat
form and they are connected to respective half chamber and reservoir using flexible
plastic tubes. Electronics and control witches were mounted on the top plate and a
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voltmeter was also placed for monitoring the charge discharge profile of the cell.
Results obtained for prototype 1:

Prototype one demand the use of four individual DC motors and it was found that the
amount of electrical energy needed for pumping hold high.

The middle chamber volume is high and the terminal to slurry interface area is low.

The slurry was not charging at several instances due to high resistance and large ion
transport distances.

The charge discharge curve found difficult to obtain due to difference in velocity of slurry
through the half chambers.

To solve this problem the system might require an additional circuitry to run the pump at
same rpm and that will increase the cost.

0nly 2 milli amps of current were able to discharge by the system

As a conclusion the system was found to be inappropriate for large scale implementation.
Based on the observations made from the first test it was decided to design a new flow
mechanism which could use the benefit of gravity was designed.
Prototype 2
The aim was to develop a model that uses less electric drive systems (motors) and also that can
use gravity to assist in the flow. A mechanism was designed using single gear DC motor to drive
a mechanical liver mechanism which will help in up and down motion of the containers. The
design is shown below.
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Figure showing the different parts of prototype 2
Figure showing the 3D image of prototype 2
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Figure showing the dimensions of prototype 2
Figure showing the flow mechanism of prototype 2
Working
In this system the flow is achieved by changing the angle of the platform with the normal to the
ground. The mechanical movement is governed by a 12 Volt DC gear motor. All the reservoirs and
middle chamber are held up on a pivot and is elevated at a height of 210mm from the ground. The
clockwise rotation of the motor will bring the right side of the platform to the ground and at the same
time this will lift the left side reservoirs up the normal plane. The anticlockwise rotation of the motor
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will bring the left side reservoirs below the normal plane and at the same time this will lift the right
side reservoirs up the normal plane. The difference in angular position of the reservoirs will facilitate
the flow due to the action of gravity on the slurry material. In the apparatus charging of the system
happens in the clockwise direction and discharging happens in the anticlockwise direction.
Figure showing up and down mechanism
Charging and discharging mechanism:
Initially the uncharged slurry is stored in the two left side containers each having a capacity of 200 ml.
During charging the motor runs in clockwise direction, the right side of the platform along with the
right side containers moves downwards and the left side moves upwards. The motion is completely
governed by the motor and the counter weight provided. At this point gravitational force acts on the
slurry material and it will flow to the right side containers through the middle chamber. At this time an
external voltage supply of 1.7 volt is applied across the two terminals of the middle chamber and this
will create an electric field inside the middle chamber. When slurry flows through this electric field the
ions in the slurry will get polarized and the positive ions will get adsorbed on to the surface of carbon
particles on negative side of the middle chamber and negative ions will get adsorbed on to the surface
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of porous carbon on the positive side of the middle chamber. That means charge is get stored on to the
carbon particle while flowing from one side of middle chamber to the other. This charged slurry then
continue its path to the reservoirs and get stored inside two separate containers. Positive charged slurry
in one and the negative slurry in the other. This charged slurry can be stored for a longer period of
time and the possibility of self-discharge becomes zero.
The flow velocity depends on the angle of inclination of the platform, larger the angle higher will be
the velocity. Velocity of flow also depends on the viscosity of the slurry. By suitably tuning this two,
we can regulate the flow.
During discharging the motor has to run anticlockwise so that the right side of the platform moves up
and the left side goes down. Now the charged slurry from both containers starts to flow backwards
through the middle chamber. At this time if we connect a load across the two terminals of the middle
chamber current will flow through the circuit and the slurry will get discharged.
The test results done using a digital oscilloscope are shown below. Charge discharge curve for the
prototype have been plotted and the power density and energy density calculations has been
performed.
4,5
4
flow rate (ml/sec)
3,5
3
2,5
2
1,5
1
0,5
0
0
20
40
angle (degrees)
60
80
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Testing of flow cell
The capacitance measurement of the cell was done using a Digital oscilloscope. The
charge discharge curves were analyzed and the specific capacitance of the cell was
calculated.
The cell was allowed to a cyclic charge discharge cycle and at every 100th cycle the
deformation was monitored and the total life cycle of the cell was estimated.
The leakage test was done by storing the charged slurry for five days and monitoring thee
discharge curve.
The power density energy density of the cell were experimentally determines from the
charge discharge curve obtained.
1.6
1.4
1.2
current (A)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
0
50
100
Time (seconds)
150
200
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Charging curve
0.2
0.0
-0.2
current (A)
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
0
50
100
Time (seconds)
Discharging curve
Charge discharge curve
150
200
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Also in the field of electric vehicles electric energy storage system is very important aspect
which is to be considered as critical. The motorized vehicle is more than a hundred years old
and has been continuously developed. Today, politicians and consumers are more and more
considering the environmental effects of vehicular traffic and accordingly there is an interest
in exchanging the conventional mechanical drive train with an electrical one and making the
vehicle to either an all-electric vehicle (EV), or a hybrid electric vehicle (HEV). An EV has
no internal combustion engine (ICE), but instead a large battery, charged from an external
source when the vehicle is at rest. The HEV has a smaller battery, charged from either a
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generator driven by an on board ICE, or a fuel cell (FC). Unfortunately, energy density
[Wh/kg] and power density [W/kg] of conventional batteries are often dependent on each
other. Batteries with high energy density have poor power density and vice versa. In
addition, large charge and discharge currents cause losses and heating of the battery, which
significantly decreases the battery lifetime. For these reasons, batteries must be oversized in
terms of energy capacity to meet the power requirements of an HEV.
EC’s, also called supercapacitors or ultracapacitors, have extremely high capacitance
compared to conventional capacitors (kF compared to µF). In contrast to batteries, EC’s
have high power density and poor energy density. Furthermore, they have almost negligible
losses and a comparably long lifetime. Consequently, a combination of these two types of
energy storage will in theory yield an equivalent energy storage system with both high
energy density and power density, where energy is stored in the battery and peak power is
supplied by the EC’s.
Flowable electrochemical systems such as redox flow batteries (RFBs) are alternative
technologies primarily considered for grid-scale energy storage. Unlike static batteries, the
unique aspect of flow-assisted systems is the decoupling of energy and power ratings
and, thus, the system energy storage capacity. In these systems, the energy storage capacity
is determined by the choice of electrolyte and size of electrolyte reservoirs while the
power handling ability is dependent on the size and quantity of electrochemical cell stacks.
While the decoupled power/energy storage feature allows these systems to be scaled to
meet different applications.
In contrast to batteries, supercapacitors demonstrate the performance characteristics
necessary to address load-leveling, peak-shaving and grid stabilization issues as they
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exhibit rapid charging and discharging. The energy storage mechanism is based on physical
energy storage via ion electrosorption in the electric double layer (EDL) at the interface
between the electrolyte and a solid electrode, and is inherently faster than the Faradaic
(redox) processes used in batteries. When compared to Li-ion batteries, supercapacitors
provide ~10x higher power density, ~100x faster charge/discharge rates, and ~1000x
longer lifetimes at a potentially lower cost. Although conventional supercapacitors are
already considered for use in wind farms and solar farms in conjunction with batteries, their
high costs and moderate energy density (~20x lower than batteries) hinders their
widespread implementation as a stand-alone EES solution.
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6.1
The Electrochemical Flow Capacitor
The EFC is one particular technology within the broader class of ‘flowable electrode
systems’ which has emerged since 2011. These systems are distinguished by their use of
a flowable suspension of particles instead of the freestanding film electrodes found in
conventional systems. Utilizing a flowable electrode, as opposed to a conventional
electrode, provides the unique ability to store charge/energy within the suspension, and
then pump it outside of the electrochemical cell for storage or further processing. This
unique aspect of flowable electrodes makes it possible to convert many conventional
electrochemical systems (e.g. batteries, supercapacitors, etc.) into a more scalable flowassisted architecture, similar to flow batteries.
Figure 1: Showing the basic block diagram of a flow capacitor.
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Figure explanation: Operational schematic of the electrochemical flow capacitor. Uncharged
slurry flows through polarized plates and charged. At the pore level, electrode neutrality is
maintained at the interface between the electrolyte and active material. This slurry is then
pumped into external reservoirs for storage. The process is reversed during discharge.
It is a fundamentally new concept that has not been explored before and offers a significant
potential for grid-scale electrical energy storage. The EFC combines the scalable energy
capacity of flow batteries with the high power ratings of supercapacitors and provides
decoupled power/energy capacity.
The electrochemical flow capacitor utilizes a system architecture similar to redox flow
batteries. The system consists of an electrochemical flow cell for charging and/or
discharging which is connected to an external circulatory system of pumps and reservoirs
that contain carbon slurry (Figure 1). The system operation is similar to a redox flow
battery, molten salt battery or semi-solid flow battery. However, rather than using a
traditional liquid redox electrolyte as the electrochemically active material, the EFC
utilizes a flowable, capacitive slurry of porous carbon particles suspended in an electrolyte.
This ‘flowable electrode’ is analogous to the solid-state electrode found in conventional
supercapacitors.
Fundamentally, the EFC is based on the same charge storage mechanism as
supercapacitors, where reversible polarization leads to the formation of the EDL (electric
double layer) by counterbalancing the surface charges of porous electrodes. During
operation, uncharged slurry is pumped through a polarized flow cell. Within the flow cell,
interactions between the carbon particles and the current collectors create a network of
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conductive pathways, propagating the applied potential to each connected particle (Figure 2).
Accordingly, the particles develop a surface charge which is balanced by the electrosorption
of counter- ions from the surrounding electrolyte, forming an electrical double layer (EDL)
and storing energy. Positively charged solid particles in the slurry attract negatively charged
ions for charge balancing (and vice versa). This energy storage mechanism is purely
physical (e.g., it does not rely on redox reactions), and thus is faster and more efficient. Ion
diffusion between the electrodes occurs through an ion-permeable electrically insulating
membrane. Charged slurry is pumped into isolated, external reservoirs where it is stored until
the stored energy is required, at which time the process is reversed (charged slurry is
pumped back to the flow cell, where it is discharged before being stored uncharged slurry
reservoirs). Charging can occur very rapidly, yet power output and energy storage are
decoupled, overcoming the major limitation of supercapacitors: the moderate amount of
stored energy.
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1.1 Simple Capacitor
1.2 The simplest form of the capacitor is the parallel plate capacitor. It consists of two metallic plates
having surface area A and separated by a distance d. In between the plates will be a dielectric with
the dielectric constant. The capacitance is given by the equation
C= εA/d.
From the above equation the capacitance is indeed a geometrical quantity which is purely
determined by the sizes, shapes and separation of the two conductors. In SI units the 2 capacitance
is measured in Farads.
The total energy stored in the capacitor is
1/2CV2.
The total charge stored on the capacitor is given by the equation
Q=CV,
Q being the charge and V is the voltage. The following are the basic terminologies associated with
the capacitor.
Specific capacitance: The capacitance normalized to unit mass of the electrode material. It is
expressed in F/g.
Energy Density: The total energy stored in a capacitor normalized to volume (Wh/L) or mass
(Wh/kg).
Power Density: The power delivered by the capacitor normalized to volume (kW/L) or mass
(kW/kg).
Coulombic efficiency: The ratio of the total charge taken out from the capacitor to the total charge
given in to the capacitor.
Energy efficiency: The ratio of the total energy derived from the capacitor to the total energy
consumed on charging. The specific capacitance and energy density of the simple capacitors are so
low that their usage is very limited in high energy applications.
1.3 The Double layer
The boundary between two different phases will have the properties which differentiates one
extended phase from the other. In an electric double layer capacitor the interface is between the
metal and the electrolyte. Whenever there exists a charged surface, a balancing counter charge will
be formed near that surface. This charge distribution may not be uniformly distributed in the other
phase but concentrated near the charged surface [6]. There are various theories which explain the
double layer formation. The prominent ones are discussed below.
1.2.1 The Helmholtz Model
The first model of the double layer was proposed by Helmholtz [7]. He postulated that when a
charged electrode is immersed in an electrolyte it will repel the co-ions and counter ions will be
adsorbed on to the electrode surface. This voltage distribution within the double layer is linear
which is analogous to the voltage distribution in the dielectric capacitor. Hence the capacitance of
the double layer follows the same equation as that of the dielectric capacitor. The capacitance of the
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Helmholtz double layer is given by the following equations, H being the double layer thickness
which is approximated as the radius of the solvated ions and Cs being the specific capacitance ie the
capacitance per unit area [8-12].
1.2.2 The Gouy - Chapmen Model
The Helmholtz model had not taken in to account about the mobility of ions in the electrolytic
solutions. The ions are constantly under the influence of the diffusive and electrostatic forces. This
results in a diffuse layer at the interface as proposed by Gouy and Chapman [13,14]. The potential
drop in this diffused layer will be exponential in contrast to the Helmholtz model where the drop
was linear. The Poisson - Boltzmann equation determines the electric potential at a point. The
equation is stated as follows [15, 16].
ε0 - the permittivity of free space , εr - the relative permittivity, z - the valence molar concentration
e - the elementary charge, c∞ - the bulk molar concentration, ѱ the local electric potential, KB - the
Boltzmann constant, T - the absolute temperature, NA - the Avogadro number. The exact solution
of the above equation gives the specific capacitance of planar electrodes qs - The surface charge
density, λD is the Debye length 1.2.3 The Gouy - Chapmen - Stern Model This is a combination of
both the above models. As per the proposal of Stern the double layer consists of two layers - the
Stern layer and the diffuse layer. The Stern layer is exactly same as the Helmholtz layer where the
ions are strongly adsorbed on the electrode surface. The Stern layer is followed by the diffuse layer
which is explained by Guoy -Chapman model [17]. The Stern layer and the diffuse layer acts as
series capacitances. This model can be represented by the following differential equation [8]. The
following figure clearly differentiates the three different models of the double layer [8]. 5 Fig1: The
electric double layer- 3 different models, their ionic distribution and potential drop. 1.3 The
Electrochemical Double layer Capacitor An EDLC configuration consists of two porous electrodes
coated on suitable current collectors. The electrodes are electrically isolated and ionically connected
through a porous separator on which the electrolyte is socked. The electrolyte can be aqueous ionic
or organic electrolytes. The schematic of an EDLC is shown below. This capacitor behaves exactly
as the dielectric capacitor when majority of the counter ion concentration lies within the stern layer.
In that case the voltage distribution is linear inside the double layer. The capacitance of the EDLC is
mainly contributed by the electrode area and the double layer thickness. When nanostructures are
used as the electrode material it provides enormous area . Since the double layer thickness is as
small as the radius of the solvated ions, the net capacitance of the double layer will be very large. In
the case of the symmetrical system as shown in the figure 2 the potential is exactly distributed
between the two double layers of the electrodes. The three important components of the EDLC are
the electrode material, the separator and the electrolyte. 6 1.3.1 The electrode material The electrode
material is usually carbon based material such as graphene, graphite, CNT or activated carbon.
Usually the nano structured materials are used to obtain a higher surface area. The electrical
conductivity of the material also plays a significant role. If the material is highly conducting it adds
to decrease the net equivalent series resistance (ESR). The porosity of the material plays an
important role. If the material is having a high surface area but is less porous, then electrolyte is
denied access to majority of the electrode area. This results in the total charge storing capacity of
the capacitor and hence the net capacitance increases. The porosity of the material depends on
various factors such as deposition technique, the processing conditions such as the temperature,
pressure etc. 1.3.2 The separator The separator is usually accompanied by two filter papers at either
end. The filter papers are the sources if electrolyte. They are micro porous structures which when
soaked in the electrolyte absorb them. The filter paper is in immediate contact with the electrode.
The double layer is formed at the interface between the electrode and the filter paper. The separator
is used to electrically isolate the two electrodes to prevent short circuiting. But at the same time it is
permeable to the electrolyte. 1.3.3 The electrolyte The electrolyte predominantly determines the
operating conditions of the capacitor. 7 The electrolyte is the supplier of ions which forms the
double layer. The maximum operating voltage is determined by the electrolyte used in the system.
The electrolyte can be aqueous or non aqueous (organic or ionic). 1.3.3.1 Aqueous electrolyte The
33
aqueous electrolyte consists of an electrolyte dissolved in an aqueous solvent. The common
examples are potassium hydroxide, sodium hydroxide and sulphuric acid. The aqueous electrolytes
tend to generally corrode the system at high concentrations. The higher the concentration the lower
the ESR which improves the power densities. The cost is less compared to other electrolytes. The
solvated ionic radius of these electrolytes are small so that they can access minute pore in the
electrode material which in turn contribute to higher charge storage ie higher energy density[18].
But the main disadvantage of the system using these electrolytes is that the cell voltage is limited to
1.2V which is the voltage at which the electrolysis of water occurs. 1.3.3.2 Non aqueous electrolyte
The non aqueous electrolytes comprises of the electrolytic salts in non aqueous solvents. Common
salts
used
are
tetraethylammonium
hexaphosphate,
tetraethylmethylammonium
hexafluorophosphate. The common solvents include acetonitrile, propylene carbonate etc. Ionic
liquids can also be used as the electrolyte with or without solvent [19]. 8 Figure 2: The basic
structure and the potential distribution in a symmetrical EDLC 1.3.4 The electronic equivalent
model of EDLC. An EDLC can be electronically modeled as a capacitor with a series resistance and
a parallel resistance. Figure 3: the electronic model of EDLC For an ideal capacitor the ESR should
be as low as possible and the parallel resistance r should be as high as possible. The ESR is mainly
contributed by the solution resistance from the electrolyte to the electrode and resistance of the
electrode itself. The parallel resistance represents leakage resistance or the parasitic resistance
through which the charged capacitor discharges. The lower the value of r, the higher will be the
charge leakage. This self discharge limits the capacitor in the storage applications. In the above
model the capacitance was having a fixed value. This is the case when the charge storage
predominantly occurs in the Stern layer where the capacitance depends only on the electrode
geometry. When charge storage in the diffuse layer becomes significant the capacitance will not be
fixed but a 9 function of the applied voltage and temperature. Then the equivalent model will be as
follows. Figure 4: the electronic model of EDLC differentiating the stern and diffuse capacitances.
c1=f (V, T) V being the applied voltage and T the temperature