Experiment 3 Direct storage of electrical energy in · PDF fileDirect storage of electrical...

Experiment 3 Practicum: Energy Systems Instructor: M.Sc. Heike Mosch

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Experiment 3

Direct storage of electrical energy in capacitors


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1. 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]

http://en.wikipedia.org/wiki/Double-layer_capacitance

http://en.wikipedia.org/wiki/Pseudocapacitance


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2. 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 =Aε0εr

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 charging-

discharging (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]


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Calculation of capacitance, energy and power density (Equations 3 – 6):

From CV: Cs = a

2 ∙ m ∙ ∆U ∙ sr (3)

From CC: Cs =i

(dV

dt) · m

(4)

E =CsU2

2 (5)

P =U2

4R (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

iR [V] is the voltage drop (iR drop) between the first two points of the discharge curve (Figure 2), i [A] is

the applied current.[8]

4. Materials and apparatus:

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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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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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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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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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. Analysis of results

7.1 Plot CV voltammograms for cell 1 and 2 and compare shape of the spectra, estimate type of

capacitor

7.2 Calculate CS from the CV voltammograms (Eq. 3) for cell 1 and 2.

7.3 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.

7.4 Compare the capacitances calculated from CV and CC for both cells. If there are differences in

the determined values, explain reasons for this.

7.5 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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8. 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 [email protected] or stop by in my lab (ITUC

216) or office (ITUC 203).

mailto:[email protected]

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