Experiment 3
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Experiment 3
Direct storage of electrical energy in capacitors
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Experiment 3
1.
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Motivation
Electrochemical capacitors, which are also known as supercapacitors or ultracapacitors are very
promising devices for the storage of electrical energy. Besides their high specific power, they also offer a
long cycle life regarding charging/discharging processes, which is moreover carried out within seconds.[1]
Compared to batteries, supercapacitors are of higher power density, but of lower energy density,[2]
which makes it necessary to combine supercapacitors with batteries to guarantee a steady energy supply
also for a longer time and to further research ways to enhance the energy density of these devices to
obtain values comparable to those of batteries.[1]
Supercapacitors however bring along a very important advantage to batteries, as they can be
constructed of a plenitude of different materials, for example metal oxides, conducting polymers, or also
environmental friendly substances like carbon or a combination of different compounds. Hereby the
materials used to construct the capacitor electrodes also determine the process how energy is stored
(Figure 1, left).[2]
Double-layer capacitance – Energy is stored through the accumulation of charge at the border
between the electrode material (different carbon types) and the electrolyte to form an electrical
double layer.
Pseudocapacitance – Energy is stored by redox processes of electroactive surface materials
(electroactive polymers, metal oxides) on the electrodes.[1,3]
Figure 1: Overview of supercapacitor types (left),[4] commercially available supercapacitors (right).[4,5]
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Experiment 3
2.
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
What will students learn?
Students will construct their own capacitors using different active materials and will recognise from the
shape of cyclic voltammogram the type of capacitor. They will understand the charge-discharge process
and how it can be used for the calculation of specific capacitance, energy and power of the device.
3.
Theoretical background
The capacitance of a capacitor can be calculated following Equation 1:
C=
Aε0 εr
d
(1)
C is the capacitance of the capacitor, ε0 and εr is the electric permittivity of the dielectric material resp.
free space, d is the distance between the capacitor plates, and A is their surface area. The equation
makes clear, that it is possible to increase the capacitance by the decreasing of the distance d and by
increasing the area A.[1] This is exactly targeted with the construction of supercapacitors and represents
besides the different energy storage mechanism (electrical field in case of capacitors) the difference
between the two energy storage devices.[1,2]
The performance of a supercapacitor can be described by three main quantities: its specific capacitance
Cs (Equation 3 and 4), its energy (E) and power density (P) (Equations 5 and 6) as well as its cycleability,
charge-discharge processes, etc.[1,2,6] Hereby it is possible to examine the capacitance of a supercapacitor
by three different methods: by cyclic voltammetry (CV, Figure 2 left, Equation 3), galvanostatic chargingdischarging (CC, Figure 2 right, Equation 4), and electrochemical impedance spectroscopy (EIS).[1]
Figure 2: Cyclic voltammograms of different capacitor types,[4] charge/discharge curve with IR drop.[1]
3
Experiment 3
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Calculation of capacitance, energy and power density (Equations 3 – 6):
From CV:
Cs =
From CC:
E=
a
2 ∙ m ∙ ∆U ∙ sr
Cs =
i
dV
( )·m
dt
Cs U2
2
U2
P = 4R
(3)
(4)
(5)
(6)
Here Cs [F/g] is again the specific capacitance of the capacitor, a [V·A] is the area of the CV cycle, m [g]
the mass of active material on the electrode surface, ΔU [V] represents the difference of applied voltage
in the CV measurements and sr [V/s] the applied scan-rate.[7]
i [A] is the constant discharge current, dV/dt [V/s] is the slope obtained from the charge-discharge curve
(Figure 1). U [V] represents the applied voltage and R [Ω] is the internal resistance. [1,2,6]
The internal resistance can be obtained from Equation 7:
R=
∆ViR
i
(7)
ΔV [V] is the voltage drop (iR drop) between the first two points of the discharge curve (Figure 2), i [A] is
iR
the applied current.[8]
4.
Materials and apparatus:
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1 M KCl (aqueous)
Isopropanol (IPA)
poly-vinylidene fluoride (PVDF)
MnO2 powder
High surface area carbon for commercial supercapacitors (SC)
Carbon powder Norit GSX (N-GSX)
Carbon felt
Cellulose separator paper
Stainless steel sandwich-type 2-electrode coin cells (CR2032 size)
Stainless steel spacer discs
Stainless steel wave springs
Cell holder
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Experiment 3
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Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Potentiostat/galvanostat
Ultrasonic bath (lab 216)
Balance (lab 216)
Laboratory lamp (lab 216)
Hole maker (16 and 20 mm)
Eppendorf pipette and tips, spatula, weighing boats, glass vials, 2 beakers, 4 Petri dishes, scissors
5.
Cell construction
5.1
Preparation of electrode discs and separators
First, cut with the hole maker of smaller diameter (16 mm) 4 discs from the carbon felt and weigh two
discs. Then put the two not weighed discs in a petri dish with ethanol, so that the material can be
activated. Leave in the ethanol until discs are needed. Also cut 6 discs of the cellulose separator paper
with the hole maker of larger diameter (20 mm). Put all of them in another petri dish with electrolyte to
let them soak.
5.2
Preparation of electrode inks
Prepare two kinds of ink for the positive electrodes of the two supercaps:
Cell 1: Dissolve 5 mg PVDF in 0.5 mL IPA and ultrasonicate the suspension for 15 min in a beaker filled
with water. Then add 50 mg of the supercapacitor carbon (SC) and treat the suspension in the ultrasonic
bath for another 15 min.
Cell 2: Also dissolve 5 mg PVDF in 0.5 mL IPA and ultrasonicate the suspension for 15 min in a beaker
filled with water. Then add 65 mg of MnO2 and 30 mg of the Norit carbon (N-GSX) and treat the
suspension in the ultrasonic bath for another 15 min.
5.3
Casting the electrode inks on the discs
Take the two dry carbon discs and cast with the Eppendorf pipette the first resp. the second ink on a disc
generating a thin uniform layer on the whole disc area (you do not necessarily have to use all of the
respective ink, just as much that a perfect layer is generated! Around 0.25 mL is sufficient.). Let the ink
dry for 30 min under the laboratory lamp and weigh the two coated carbon discs to evaluate the amount
of active material on the discs.
5.4
Assembly of the cells
For cell 1, take one spacer disc and put it in the positive capacitor shell, then add the coated positive
electrode carbon disc with coated side upwards. Then carefully add the three separators one by one, so
that no powder from the coated carbon disc is removed. Then add the uncoated carbon disc from the
ethanol (drying not necessary) and one wave spring, see Figure 3. Before closing the cell with the
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Experiment 3
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
negative capacitor shell, add around 200 µL of electrolyte (1 M KCl). Place the coin cell in the holder.
Make sure there is no leakage.
Figure 3: Assembly of the capacitor cells. From left to right: Positive shell, spacer disc, coated
carbon felt (positive electrode), three separators, uncoated carbon felt (negative electrode),
wave spring, negative shell.
Repeat the same for cell 2 with the positive electrode carbon disc coated with the MnO2/carbon
suspension.
6.
Conducting the experiment
Switch on the potentiostat and computer. Link the cell in the cell holder with the potentiostat via the +/crocodile clips.
6.1
Cyclic voltammetry
Open the software VoltaScope and measure open circuit voltage (OCV) for 15 min until stable potential
is obtained (plot V = f(time) should reach plateau), see Figure 4.
Figure 4: OCV settings in VoltaScope.
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Experiment 3
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Record cyclic voltammograms at 50 mV/sec (5 scans).
Use the following setting in VoltaScope software as shown in Figure 5.
Figure 5: Settings for cyclic voltammetry in VoltaScope.
Save your data immediately after the end of the measurements on a USB-stick! Therefore, click on File 
Export all the DC data  choose your USB  save as .txt to process in Origin, SciDAVis, etc. Also save the
data on the laptop under File  Export all the DC data  System C  Benutzer  Öffentlich 
!Praktikum.
6.2
Galvanostatic charge-discharge measurements
Open the software EnergyScope and measure open circuit voltage (OCV) for 5 min until stable potential
is obtained (plot V = f(time) should reach plateau).
Perform charge-discharge characteristics at 1E-7 A for 120 s for each charge-discharge cycle. Therefore
load the measuring setup by choosing the file Charge-Discharge loop.stp available under File 
Open/Extract Measure Setup  System C  Benutzer  Öffentlich  !Praktikum
Check if the following settings have appeared in the EnergyScope software as shown in Figure 6.
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Experiment 3
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Figure 6: Settings for charge/discharge measurements in EnergyScope.
Start measurement and also save this data immediately after finishing on your USB and the laptop!
Now repeat the electrochemical tests 6.1 and 6.2 for cell 2.
After you finished the experiment, please switch off the computer and the potentiostat and clean the
workplace.
7.
7.1
7.2
7.3
7.4
7.5
Analysis of results
Plot CV voltammograms for cell 1 and 2 and compare shape of the spectra, estimate type of
capacitor
Calculate CS from the CV voltammograms (Eq. 3) for cell 1 and 2.
Plot charge-discharge curves, estimate R (internal resistance, Eq. 7) and dV/dt and calculate the
specific capacitance CS (Eq. 4), E (Eq. 5) and P (Eq. 6) for cell 1 and 2. Use hereby the voltage
measured at the peak of the charge-discharge curve you used to obtain dV/dt.
Compare the capacitances calculated from CV and CC for both cells. If there are differences in
the determined values, explain reasons for this.
Discuss the performance of the two capacitors with regards to the different cathode active
materials. Write the redox reaction during charge-discharge on the MnO2 electrode.
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Experiment 3
8.
Practicum: Energy Systems
Instructor: M.Sc. Heike Mosch
Literature
[1] P. Pieta et al., ECS J. Solid State Sci. Tech. 2013, 2, M3120-M3134.
[2] M. S. Halper, J. C. Ellenbogen, Supercapacitors: A brief overview, MITRE Nanosystems Group, 2006.
[3] P. Simon, Y. Gogotsi, Nat. Mater. 2008, 7, 845 - 854.
[4] http://en.wikipedia.org/wiki/Supercapacitor (24.10.2014)
[5] http://www.alibaba.com/product-detail/button-5-5v-105-gold-coins_ 1560427533.html (24.10.2014)
[6] M. Winter, R. J. Brodd, Chem. Rev. 2004, 104, 4245 - 4269.
[7] P. Liu et al., Synt. Met. 2013, 181, 72 - 78.
[8] N. Omar et al., Electrochim. Acta 2012, 86, 305 – 315.
9.
General remarks
Please bring a lab coat and safety goggles with you!
The antestat before the experiments starts will be held in German.
The report can however be written in German or English. It should not only consist of the analysis of
results, to say an introduction correlating with the topic of the experiment could be nice before analysing
the results. Also describe what the use of this experiment is and what you have learned, but do not
repeat the introduction of this instruction! Feel free to describe the background of the experiment and
your results as detailed as possible, but do not exceed 20 pages! You have 4 weeks to write your report
after the day of conducting the experiment. Please hand it in in paper form in a folder and as .pdf. Take
care about the appearance of your report.
If you have questions, just send me an e-mail to heike.mosch@uni-jena.de or stop by in my lab (ITUC
216) or office (ITUC 203).
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