experimental studies of asymmetric capacitors

ECHE789B Special Project – Lingyun Liu 05/28/2002

ECHE789B Special Project

Experimental Studies of Asymmetric

Capacitors

Instructor: Dr. Popov

By: LINGYUN LIU

May 5, 2002

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ABSTRACT

The performance of nickel hydroxide asymmetric capacitors is

studied and compared with that of carbon symmetric capacitors.

The asymmetric capacitor has better energy efficiency. Optimize

the design parameters of the asymmetric capacitors such as

thickness, state-of-charge. The behavior of the devices under

different discharging rates will also be studied. Ragone plots are

used to evaluate the power density and achievable energy density

for both symmetric and asymmetric capacitors. The results show

some unexpected tendency. The possible reasons are analyzed.

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INTRODUCTION

Electrical energy storage is needed in lots of area. The energy storage and conversion

devices include batteries, fuel cells, capacitors etc. Capacitor is one kind of the mostly

common used energy storage backup devices. Capacitors store energy by charge

separation. It can provide pulse electrical energy. Generally, there are three kinds of

capacitors, say, film capacitors, electrolytic capacitors, and electrochemical capacitors.

Electrochemical capacitor is also called supercapacitor or ultracapacitor. Electrochemical

capacitors may improve battery performance in terms of power density or may improve

capacitor performance in terms of energy density when combined with the respective

device. At the same time, electrochemical capacitors are expected to have much longer

cycle life. Figure 1 and figure 2 show the reason why electrochemical capacitors are

concerned. They fill in the gap between batteries and converntional capacitors such as

electrolytic capacitors or metallized film capacitors. In terms of specific energy as well as

in terms of specific power this gap covers several orders of magnitude [1].

Fig. 1. Sketch of Ragone plot for various energy storage and vonversion devices. The indicated

areas are rough guide line (by R. Kotz and M. Carlen)

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Fig. 2. Ragone plane: available energy of an energy storage device for fixed power. Different

types of energy storage devices are typically located in different regions. Characteristic times

correspond to lines with unity slope. Every energy storage device is represented by a curve E(P)

(inset). Internal dissipation and leakage losses lead to a drop of the energy for sufficiently high

and low power (by T. Christen, M.W. Carlen)

The electrochemical capacitor is constructed like a battery in that it has two electrodes

immersed in an electrolyte with a separator between the electrodes [2]. There are two

types of electrochemical capacitors are highly concerned nowadays, that is, double-layer

capacitors and hybrid capacitors using pseudocapacitance. In this paper, we call the

former one as symmetric capacitors since the properties are same in both positive

electrode and negative electrode, and the latter one as asymmetric capacitors since the

capacitor are fabricated with double-layer capacitance material ( i.e. carbon) as negative

electrode and pseudocapacitance material (i.e. metal oxides) as the positive electrode.

Energy is stored in the double-layer capacitor as charge separation in the double-layer

formed at the interface between the solid electrode material surface and the liquid

electrolyte in the micropores of the electrodes. The mechanism is shown in figure 3. The

capacitance depends on the characteristics of the electrode material (i.e. surface area and

pore size distribution) [2].

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Fig. 3. Schematic of a double-layer capacitor. (by A. Burke)

Unlike the double-layer capacitor which is nonfaradic, pseudocapacitor use faradic

process to store energy. The electrical energy is converted to chemical energy through

charge and the chemical energy release as electrical energy when discharge. Metal oxide

materials are usually used to obtain pseudocapacitance. Lots of researchers are working

on it and trying to find higher efficiency and lower cost electrode material. There are

great achievements on this. RuO2 is one of the most recommended materials since its

high capacitance and energy density [3], but the material is quite expensive which limits

the widely application. Other materials such as CoO, V2O5 have the same problem [4].

Compared with these materials, nickel oxides material has apparently advantages: 1. low

cost, 2. low toxicity, and 3. much knowledge of electrochemical characteristics of nickel

oxides (hydroxides) can be obtained from nickel batteries’ study.

The asymmetric capacitors use Ni(OH)2/Co(OH)2 thin films as the positive electrode and

traditional porous carbon electrodes as the negative. The pseudocapacitance of the nickel

hydroxide comes from the reaction taken place when charge and discharge, as following:

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disch arge2 2ch arge

NiOOH H O e Ni(OH) OH− −→+ + +←

Figure 4 illustrates the capacitance mechanism of nickel hydroxides film.

Metal Substrate

NiOOH/Ni(OH)2

discharge charge

H+ H+

ElectrolyteH2O OH- H2O OH-

e- e-

Fig. 4. The mechanism of pseudocapacitance of Ni(OH)2/NiOOH

The former experimental data verify that the potential of Ni(OH)2/Co(OH)2 thin films

would maintain high level during a long time when discharge because of reaction. It is

hypothesized that the energy density of this device can be a factor of four times larger

than traditional electrochemical capacitor due to the extremely high capacity of nickel

hydroxide. The data indicates that the capacity of nickel hydroxide is about 10 times than

that of carbon even though the mass of the nickel hydroxide is 1% the mass of the carbon.

Therefore, it is hypothesized that the voltage of the positive electrode will remain

essentially constant during the discharge of a device yet contributes negligible weight.

This hypothesis will be fully tested.

The mathematical models of both asymmetric capacitors and symmetric capacitors

predict the performance and energy efficiency of the capacitors. Figure 5 shows the

potential profile through electrodes when discharge. The area covered by asymmetric

capacitors is almost twice as that of symmetric capacitors, which indicate the asymmetric

capacitor has much lager capacitance. Figure 6 is the prediction of the energy density and

power density. Both of them are several magnitudes large for asymmetric capacitors than

for symmetric capacitors.

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EXPERIMENTAL

A computer-controlled EG&G Princeton Applied Research M273 Potentiostat/

galvanostat using the M270 software is used in studies.

Thin films of Ni(OH)2/Co(OH)2 have been fabricated by depositing them on nickel foil

film with exposed area of 1cm2. The deposition is taken at 56°C (RM6 Lauda, Brikmann)

in the beaker containing 1.8M Ni(NO3)2, 0.18M Co(NO3)2, and 0.075M NaNO3 in the

solvent of 50 vol% ethanol. A cathodic current density of 5.0mA/cm2 was applied for

25min which according to these previous deposition studies should result in 350µg films

with a capacity of 277mC (i.e. 790 C/g). The expected capacity was confirmed by

performing cyclic voltammetry on nickel hydroxide in 3wt% KOH and integrating the

area under the reduction peak of a stable cyclic voltammogram [5]. Before study in KOH

solution, the nickel hydroxide film needs to be rinsed in DI water. Saturated Calomel

electrode (SCE) and platinum mesh are used as reference and counter electrode. The film

is constant current charged at first with the current density of 1mA, and then cycles in

solution for 10 cycles, and then steady state capacitance is measured. After that, the

constant current discharge process is taken with the current density of 0.02mA.

XC-72 is used as negative electrode material. The carbon has been dispersed in 5 wt%

Nafion solution, and small volume of isopropanol has been added to enhance the

dispersion. The mixture is stirred for more than 8 hours to get good ink. Spray the ink on

decals with the area of 10cm2 with the sprayer (P-163) Pssache Millennium Set). The

decals had been cleaned and weighted before use. Dry the sprayed decals at the

temperature of 105˚C for 10 min, then weight it after cool. The average carbon loading

can be calculated. Porous carbon electrode has been fabricated by press the decal into

Nafion 117 membrane (Dupon) at 150˚C. Pelt the decal off after cool. Hot press (Carver,

Inc.) has been used. Prepare several carbon electrodes with different average carbon

loadings for use.

To fabricate asymmetric capacitor, punch 1cm2 carbon electrode–membrane assembly as

negative electrode, and the nickel hydroxide film prepared before as positive electrode.

On the other hand, carbon electrode-membrane-carbon electrode assembly severs as

symmetric capacitor. The sketches of the capacitors are shown in Figure 6.

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Fig. 6. The rough sketches of the capacitors. (a) asymmetric capacitor, (b) symmetric capacitor.

T-cells have been use to handle the test with carbon electrode as working electrode and

SCE as reference electrode. Both of these capacitors have been cycled over potential

range from 0mV to -600mV for at least 20 cycles. 3 wt% KOH solution is used as

electrolyte. Different currents will be used to discharge the capacitors. The capacity will

be measured for various discharging rate.

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RESULTS AND DISCUSSION

Nickel hydroxide film study — Figure 7 shows the CV of nickel hydroxide film at a sweep

rate of 5mV/s. The redox peaks represent the oxidation/ reduction reaction of charge and

discharge. The ideally reversibility predict the long cycle life. Figure 8 is the constant

current charge and discharge curve of nickel hydroxide film. There are two plateaus in

charge curve. The first one is the oxidation of Ni(OH)2 with the formation of NiOOH,

and the second one is the water dissociation with evolution of oxygen. In discharge curve,

the potential drop sharply at the initial and end regions. Within the large region between

then, the potential change very slowly. If initial state-of-charge (SOC) is set around 50%,

the potential is almost stable when charge or discharge 10mV. That’s the potential

window of operation.

-8-6

-4-20

246

810

0 100 200 300 400 500 600

E, mV

I, m

A

Fig. 7. CV of nickel hydroxide film with the scan rate of 5mV/s

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0

100

200

300

400

500

0 500 1000 1500 2000 2500

time, s

Pote

ntia

l, m

V vs

. SC

E

Fig. 8. Constant charge and discharge curve of the nickel hydroxide film. Charge rate is 1mA and

discharge rate is 0.02mA

Asymmetric capacitors and symmetric capacitors — Cyclic voltammetry is used to

evaluate the capacitance. Figure 9 is the cyclic voltammograms for asymmetric capacitors

as well as symmetric capacitor. Boxed shape curve shows the characteristics of capacitor

behavior, that is, the current remain constant when cell potential changes. From the area

of the steady state cyclic voltammogram, the capacity of capacitors is obtain. Figure 10

illustration the difference between those capacitors. The capacity of asymmetric capacitor

is bigger than that of symmetric capacitor. For asymmetric capacitors with different

carbon loadings, the capacity increases with respect to carbon loading at first, then

decreases. The reason of this phenomenon maybe is because when carbon layer is too

thick only surface carbon contact thoroughly with electrolyte. The lower carbon cannot

provide double-layer capacitance, but extremely increases the resistance.

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-500-400-300-200-100

0100200300400

-600 -500 -400 -300 -200 -100 0

E, mV

i, m

A

1.8mgC/cm^2

2.8mgC/cm^2

5.1mgC/cm^2

6.2mgC/cm^2

C-C 0.5mg/cm^2

Fig. 9. CVs of symmetric capacitor and asymmetric capacitors with different average carbon

loadings.

0

5

10

15

20

25

30

C-C sy

mEC

#1 as

ymEC

#2 as

ymEC

#3 as

ymEC

#4 as

ymEC

#5 as

ymEC

Carbon Loading,mg/cm 2̂Capacity, mC

Fig. 10. Capacity comparison for different type and different carbon loading capacitors.

Constant current charge and discharge are used to study the capacitor performance and

energy behavior. Figure 11 is the discharge curve for different carbon loading capacitors.

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Its behavior shows the same trend as we got from CVs. Figure 12 is the discharge curve

of constant carbon loading with different current. With the current decreasing, the

discharge time increasing. This do make sense.

Constant discharge curve (1.8mg/cm^2 carbon loading)

0

100

200

300

400

0 500 1000 1500t. s

E, m

V

10mA/s

1mA/s

0.1mA/s

0.01mA/s

0.001mA/s

Fig. 11. Constant current (1mA) discharge for different carbon loading capacitors.

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12

t, s

E, m

V

0.5mg/cm^2

1.8

2.8

3.16

5

6

Fig. 12. Constant current discharge for asymmetric capacitors with different current.

Specific discharge capacitance is calculated as:

C = (2 × I × t) / (w × ∆E)

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where C is the specific discharge capacitance (F/g), I the discharge current (A), t the

cutoff time (s), w the weight of the capacitor (g), and ∆E the potential difference (V).

Figure 13 shows the specific discharge capacitances for different capacitors.

00.2

0.40.60.8

11.21.4

1.61.8

0 1 2 3 4 5 6 7

Carbon loading, mg/cm^2

capa

cita

nce

F/g

1mA/s

0.1mA/s

0.01mA/s

0.001mA/s

Fig. 13. Specific discharge capacitance.

The energy density and power density are also studied based on the constant current

discharge curve. Ragone plots was developed to compare and contrast those capacitors.

The energy density (W/kg) calculate from:

0

ciE Vw

τ

dτ= ∫

and power density (W-h/kg) from:

0

c

c

iP Vw

τ

dττ

= ∫

where cτ is the cutoff time (s)[7]. The Ragone plots are illustrated in figure 14. At low

current region, the curves appear unusually push-back in stead of straight lines with

almost constant energy density. There are two possible reasons. First, maybe because the

limiting factor is not carbon loading as we expected. That means the nickel hydroxide

film is not enough. From the former study, it is sure that the nickel hydroxide film have

much bigger capacity than that of the carbon, but this not means this reason is not

possible. After a bunch of operations, some Ni(OH)2 pelt from the nickel foil. Perhaps the

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substance left is not enough to make the carbon as the limitation. This can be eliminated

through increasing nickel hydroxide film loading and enhance nickel foil surface

characteristics. Another probable reason is the self-discharge. The nickel hydroxide film

is prepared from nickel nitrate. The nitrate ion is known as extremely easy to self-

discharge. To avoid this possibility, the nickel hydroxide film should be rinse more

thoroughly or find some better way to get rid of nitrate ion.

0.001

0.01

0.1

1

10

100

0.0001 0.001 0.01 0.1

Energy Density, W-h/kg

Pow

er D

ensi

ty, W

/kg

1.8mgC/cm^22.8mgC/cm^20.5mgC/cm^25.1mgC/cm^26.2mgC/cm^2

Cut-off potential: 0V

Fig. 14. Ragone plots for asymmetric capacitors.

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CONCLUSIONS

Nikel hydroxide has big capacitance which can be used in hybrid capacitor. Asymmetric

capacitor is superior to symmetric capacitor. The average carbon of the negative

electrode in asymmetric capacitor heavily affects capacitor behavior. The critical carbon

loading may exist. Most experiments should be done in future work to confirm the results

and eliminate the factors discussed.

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REFERENCES

[1] R. Kotz, M. Carlen, Electrochimica Acta 45 (2000) 2484.

[2] Andrew Burke, J. Power Sources 91 (2000) 39.

[3] J.P. Zheng, T.R. Jow, J. Power Source 62(1996) 155.

[4] S. Chin, S. Pang et al., J. Electrochem. Soc. 149(200) A379.

[5] V. Srinivasan, J. Weidner, J. Electrochem. Soc. 147(2000) 880.

[6] R. Huggins, Solid State Ionics 134(2000) 179

[7] V. Srinivasan, J. Weidner, J. Electrochem. Soc. 146(1999) 1654.

[8] J. Weidner, P. Timmerman, J. Electrochem. Soc. 141(1994) 346.

[9] W.G. Pell, B.E. Conway, J. Power Source 63(1996) 258

[10] G. Amatucci, F. Badway et al.. J. Electrochem. Soc. 148(2001) A930

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experimental studies of asymmetric capacitors

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