Effect of conductive filler on the impedance behaviors of activated carbon based electric double layer capacitors

Journal of Electroanalytical Chemistry 642 (2010) 75–81

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Effect of conductive filler on the impedance behaviors of activated carbon based electric double layer capacitors Ximiao Liu, Li Juan, Liang Zhan *, Li Tang, Yanli Wang, Wenming Qiao, Xiaoyi Liang, Licheng Ling State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China

a r t i c l e

i n f o

Article history: Received 22 August 2009 Received in revised form 2 February 2010 Accepted 4 February 2010 Available online 8 February 2010 Keywords: Electric double layer capacitors Electrode Diffusion Simulation Microstructure Activated carbon

a b s t r a c t Carbon aerogel, carbon black and graphite were used to analyze the influence of conductive filler on the impedance behaviors of activated carbon based electric double layer capacitors (EDLCs). According to the electrochemical impedance spectrum (EIS) data, a new equivalent circuit model was proposed involving the kinetic characteristics of ion diffusion, furthermore, Marquardt fit procedure was applied to the EIS data to obtain the model parameter values. The results indicated that carbon aerogel could significantly decrease the resistance of EDLCs by increasing the diffusion coefficient of ions within electrodes and decreasing the interface resistance because of its suitable particle size, mesoporous structures and finely-branched particle performance. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Electric double layer capacitors (EDLCs) are new energy storage devices with great application prospect based on the formation of the electric double layer at electrode/electrolyte interface. Compared with rechargeable batteries, EDLCs have many advantages, such as high power density, remarkable cycling performance, high safety, high-temperature stability, friendliness to environment and so on [1–3]. During the last decades, EDLCs have been extensively developed [4,5] to meet the increasing demand in hybrid power sources for electrical vehicles, digital telecommunication systems, pulse laser techniques and other energy fields [6,7]. High surface area activated carbon (HSAC) has been widely researched as electrode material of EDLCs because of its high specific surface area, excellent electric conductivity and low cost [8–11]. Recently, considerable efforts have been paid to decrease equivalent series resistance (ESR). The time constant for intra-particle pores is invariant with the electrode thickness, which is the result of an increase in the capacitance, that is, counterbalanced by a decrease in the intra-particle pore resistance. However, the time constant for inter-particle pores became larger, which results in increase of both the capacitance and inter-particle pore resistance. The poor rate performance in thick electrodes can thus be attributed to the increase in the time constant. When the resistance of ion transport in inter-particle pores within electrode layer was * Corresponding author. Tel.: +86 21 64252924; fax: +86 21 64252914. E-mail address: [email protected] (L. Zhan).

considered as the ESR component in addition to the intra-particle pore resistance, the theoretical derivation suggested a poor ion-diffusion rate in thick electrode [12,13]. Although the mechanical strength of electrode increased with the content of binder increasing, ESR of electrodes also increased [14,15]. Moreover, binders plug some of the pores of active materials, which lead to a significant decrease in the specific capacitor [16]. When porous carbon was used as electrode material of EDLCs, ion diffusion within the porous structure was an important ratedetermining step, which significantly affects the power performance of cell. Researchers have found that activated carbon with proper larger pores was more suitable for EDLCs applied in high power fields [17,18], and equivalent circuit models have also been proposed to interpret the experimental data [19–21]. In most of the equivalent circuit models, basic electric elements such as resistance, capacitance and constant phase element (CPE) were applied, and the simulated results may reflect the static characters of the process, however, no kinetic characteristics of ion diffusion was considered. In this paper, carbon aerogel (CAG), carbon black (CB) and graphite were used as conductive filler to improve the conductivity of HSAC, and it was important to research the impedance behavior of electrode materials. Based on the series of fundamental electrochemical impedance spectrum (EIS) data with different conductive filler and thickness of electrode, a new equivalent circuit model was proposed considering the kinetic of ion diffusion, which is beneficial to analyze the impedance behaviors for further investigation of electric performance of EDLCs with high power density.

1572-6657/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.02.008

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2. Experimental 2.1. Materials and synthesis HSAC was prepared from petroleum coke with KOH as activation agent [22]. CAG was prepared by sol–gel polymerization of phenolic resin and furfural in 1-propanol followed by supercritical drying and pyrolysis [23]. CB and graphite were commercial products supplied by Carbot and Aldrich corporation, respectively.

-Zimag

Rs

2.2. EDLCs assembly Conductive filler (CAG, CB and graphite) was mixed with HSAC powder in a weight ratio of 5–30% by mechanical milling for 2 h in an agate ball grinder. Polytetrafluoroethylene (PTFE) emulsion with a concentration of 60% was added to the composite electrode materials as binder with a weight ratio of 5%, and then kneaded and pressed at 6 MPa to form a round electrode with a diameter of 13 mm and thickness of 0.4–1.2 mm. The cells were assembled by stacking two electrodes with a separator, in which nickel foam (100 ppi) and polypropylene porous membrane were used as current collector and separator, respectively. All electrodes were vacuum wetted in 3 M KOH electrolyte before being assembled the simulated EDLCs. 2.3. Sample characterization The pore characteristics of samples were analyzed by nitrogen adsorption at 77 K using an automated apparatus (ASAP 2020 M, Micromeritics). The BET surface area (SBET) was analyzed by the Brunauer–Emmett–Teller method. Micropore surface area (Smic) was obtained by the t-plot method using an adsorption branch of isotherm. Total pore size distribution of HSAC and mesopore size distribution of CAG were analyzed with the aid of Density-Functional-Theory (DFT) and Barrett–Johner–Halendar (BJH) model, respectively. The morphologies of composite electrode were observed under scanning electron microscopy (SEM, JEOL JSM6360LV). All the electrochemical experiments were carried out at 25 °C. EDLC cells were charge/discharged for 100 times between 0.05 and 0.9 V at different current densities using an Arbin electrochemical instrument. Specific capacitance was calculated from the slope of the discharge curve by:

Fig. 1. Experimental Nyquist plot of EDLCs.

CPE Rs

Ri Fig. 2. The equivalent circuit model for the circuit plot at high frequency region.

The intercept of the curve with the real axis gives an estimation of the solution resistance Rs. The impedance of a CPE can be determined

Z CPE ¼ 1=½Y 0 ðjxÞa 

ð1Þ

where C is the single electrode specific capacitance, I the discharge current, m the mass of carbon material in one electrode, and dV/dt the slope of the discharge curves. Cyclic voltammetry (CV) measurements were performed at various sweep rates from 5 to 40 mv/s. EIS was carried out in a frequency range from 10 mHz to 50 kHz with a Gamry electrochemical instrument.

3. Theory The Nyquist plot for a typical experimental double layer capacitor in the whole frequency range is shown in Fig. 1, which consists of a semicircle at high frequency followed by a 45° inclination and a vertical line at low frequency region. Porosity or roughness of the electrode surface displays a fractal character and is claimed to give rise to the constant phase element (CPE) at high frequency region. The equivalent circuit model to the circuit plot at high frequency region is shown in Fig. 2 where a capacitor is parallel with an interface resistance Ri.

ð2Þ

pffiffiffiffiffiffiffi where j is the imaginary unit (j ¼ 1) and x the angular frequency (- ¼ 2pf ; f being the frequency), a is a dimensionless parameter ranging between 0 and 1 at a solid electrode/solution interface. When Eq. (2) is used to describe an ideal capacitor, the constant Y 0 ¼ C i (the capacitance) and a ¼ 1 [4,15]. The impedance of the RC circuit can be given by

Z RC ¼ Rs þ Z i

ð3Þ

Zi is the impedance of the circuit, which can be simulated by

Zi ¼

1 Ri

0

þ Y 0 xa cosða2pÞ  jY xa sinða2pÞ

ð4Þ

ðR1 Þ2 þ ðR2 ÞY 0 xa cosða2pÞ þ ðY 0 xa Þ2 i

C ¼ 2I=ðm  dV=dtÞ

ZReal

i

00

since Z i ¼ Z 0i þ jZ i , where Z 0i ; Z 00i are the real part and imaginary part of Zi, respectively, and then Eq. (4) can be described as:

#2  2 "  2  Ri cot a2p Ri Ri ap Z 0i  þ Z 00i  ¼ 2 2 2 sin 2

ð5Þ

Therefore, the impedance of the semicircle in Fig. 1 can be described by Eq. (2), and the diameter of the semicircle is determined 1 by Ri sin ðap=2Þ. As the electrochemical process only occurs at the exterior surface of the electrodes at high frequencies, the CPE and

CPE Rs

Ri

W

Fig. 3. The equivalent circuit model for the circuit plot at whole frequency range.

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X. Liu et al. / Journal of Electroanalytical Chemistry 642 (2010) 75–81 Table 1 Pore structure data of carbon electrode materials. Samples

SBET (m2/g)

Smic (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

Average particle size (lm)

HSAC CAG CB Graphite

2532 434 68 1.8

1980 117 0 –

1.38 0.91 0.19 –

2.76 14.2 10.5 –

4.8 0.5 0.5 –



SBET, the BET surface area; Smic, the surface area of micropores.

HSAC CB graphite

0.12 0.10 0.08 0.06 0.04 0.02 0.00

1

10

260 -1

a

Specific capacitance (F.g )

3

-1

Incremental Volume (cm g )

0.14

220 200 180 160 140

100

Pore Width (nm) 6

-1

Specific capcitance (F.g )

-1

dV/dlog(D) (cm g nm )

3 -1

3 2 1 1

10

15

20

25

30

HSAC+CB theoretical value

220 200 180 160 140

100

b 0

5

10

15

20

25

30

Content of carbon black (wt%)

Fig. 4. Pore size distributions of carbon materials.

-1

Specific capacitor(F.g )

260

1.0

Voltage(V)

10

240

Pore width (nm)

HSAC 10% CAG 10% CB 10% graphite

0.8

5

260

CAG

4

0

a 0

Content of CAG(wt%)

b

5

HSAC+CAG theoretical value

240

0.6 0.4 0.2

220 200 180 160 140

0.0

HSAC+graphite theoretical value

240

c 0

5

10

15

20

25

30

Content of graphite (wt%) 0

1000

2000

3000

4000

Time (s) Fig. 5. The charge/discharge curves of EDLCs with different conductive filler.

interface resistance Ri are attributed to the impedance at the interface between the current collector and carbon particles as well as that between the carbon particles. At low frequency region, semi-infinite diffusion can be considered as the rate-determining step for a porous electrode. The impedance created by ion diffusion is known as Warburg imped-

Fig. 6. The relationships between conductive filler and specific capacitance of composite electrode. Theoretical value = x  CHSAC + y  Cconductive filler, in which x, y, CHSAC, Cconductive filler was the content of HSAC (wt.%), the content of conductive filler (wt.%), the specific capacitance of HSAC and the specific capacitance of conductive filler, respectively.

ance, which appears as a straight line with a slope of 45° as shown in Fig. 1. The Warburg impedance can be ignored because ions cannot diffuse into the inter pores at high frequencies, whereas, ions can diffuse deeper within the porous structure at low frequencies. Therefore, the Nyquist plot becomes a vertical line indicating the pure double layer capacitive behavior.

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X. Liu et al. / Journal of Electroanalytical Chemistry 642 (2010) 75–81

Specific capacitance (F/g)

260

If the diffusion layer is totally bounded within a thin slice of solution or a thin slice of material, the Warburg impedance is called the Bounded Warburg (common example for this case is electric double layer capacitor) [4,5]. The impedance of the Bounded Warburg is given by

10% CAG 10% CB 10% graphite HSAC

240 220 200

qffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffi Z w ¼ Z 0 = ðjxÞ coth jB jxj

180

ð6Þ

160

in which

140

pffiffiffiffi Z 0 ¼ RT=ðn2 F 2 AC s DÞ pffiffiffiffi B ¼ d= D

120 100

0

2000

4000

6000

8000

10000 12000

Fig. 7. The relationship between current density and specific capacitance of composite electrode.

0.6

a -Z'' imag (ohm)

-Z'' imag (ohm)

b

0.5

0.4 0.3 0.4mm thickness 10% CAG 10% CB 10% G Pure HSAC fitted curve

0.2 0.1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.4 0.3

0.6mm hickness 10% CAG 10% CB 10% G Pure HSAC simulated curve

0.2 0.1 0.0

1.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Z'real (ohm)

Z'real (ohm)

0.6

0.6

c

0.5 0.4 0.3

0.8mm thickness 10% CAG 10% CB 10% G Pure HSAC simulated curve

0.2 0.1 0.0 0.1

d

0.5

-Z'' imag (ohm)

-Z'' imag (ohm)

ð8Þ

where B and Z0 are the Warburg factors, D is the diffusion coefficient of the ions in electrolyte, d is Nernst diffuse layer thickness, n is the valency of the ion, F is the Faraday constant, A is the area of electrode and Cs is the concentration of the electrolyte on the sur-

Current density (mA/g)

0.5

ð7Þ

0.2

0.3

0.4

0.5 0.6

0.7

0.8

0.9

0.4 0.3 1.0mm thickness 10% CAG 10% CB 10% G Pure HSAC simulated curve

0.2 0.1 0.0

1.0

Z'real (ohm)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Z'real (ohm)

0.5

e

-Z'' imag (ohm)

0.4 0.3 0.2

1.2mm thickness 10% CAG 10% CB 10% G Pure HSAC simulated curve

0.1 0.0

0.1 0.2

0.3 0.4

0.5

0.6 0.7 0.8

0.9

1.0

Z'real (ohm) Fig. 8. The experimental and simulated Nyquist plots of EDLCs. The thickness of carbon electrode is 0.4 (a), 0.6 (b), 0.8 (c), 1.0 (d) and 1.2 mm (e). Solid and open symbols are the EIS data, and the line presents the simulated Nyquist plots.

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X. Liu et al. / Journal of Electroanalytical Chemistry 642 (2010) 75–81

face of electrode. Cs can be replaced by the bulk concentration of the electrolyte because of the reversible formation process of electric double layer. Above discussion suggests that three parts should be considered when modeling an equivalent circuit corresponding to the capacitor impedance: the electrolyte solution, the interface among carbon particles or/and between particle and current collector, and the ion diffusion within the porous structures. Therefore, considering the kinetic of ion diffusion, a new equivalent circuit model can be described as Fig. 3 and its Nyquist plot is shown in Fig. 1. The whole impedance of the equivalent circuit can be simulated by Eqs. (2) and (6), in detail, Eq. (2) simulates the impedance at high frequency region and Eq. (6) simulates the impedance at low frequency region.

4. Results and discussion 4.1. Physical properties of electrode materials Table 1 lists the fundamental pore structure data of HSAC, CAG, CB and graphite, and the pore size distributions are shown in Fig. 4. The results indicate that HSAC has a high specific surface area of 2532 m2/g and its pore size mainly distributes in the range of 1– 4 nm. CAG has a wide pore size distribution ranging from 2 to 30 nm with an average pore size of 14.2 nm. Compared with HSAC, the porosity of CB and graphite can be negligible.

4.2. Electrochemical performance of the composite electrodes Fig. 5 shows the charge/discharge curves of carbon electrode samples. It is evident that a symmetrical triangle is observable in the range of 0.05–0.9 V at a constant current density of 40 mA/g. The specific capacitances of the composite materials with different content of conductive filler were obtained from charge/discharge curves and the results are shown in Fig. 6. As can be seen, the specific capacitance of composite electrode decreased with the content of conductive filler increasing. At a same content of

conductive filler, the specific capacitance of composite electrode follows the order of CAG > CB > graphite. To investigate the contribution of the specific capacitance of conductive filler to the specific capacitance of composite electrode, the theoretical specific capacitance of composite electrodes were calculated (theoretical value = x  CHSAC + y  Cconductive filler, in which x, y, CHSAC, Cconductive filler was the content of HSAC, the content of conductive filler, the specific capacitance of HSAC and the specific capacitance of conductive filler, respectively.) depending linearly on the conductive filler content as shown in Fig. 6, in which the specific capacitance of CAG, CB and graphite are 96, 8.7 and 0.5 F/g, respectively. It is very interesting that when HSAC was mixed with 5 and 10 wt.% CAG, the specific capacitance of composite electrode was higher than that of theoretical value. This should be attributed to the mesopores of CAG which is favorable for ions diffusing from mesopores into micropores of HSAC. When the content of CAG was higher than 10 wt.%, or CB and graphite were used as conductive filler, the specific capacitance of composite electrode was lower than that of theoretical value. Because the wettability of conductive filler with electrolyte may be lower than that of HSAC. In addition, the relationship between specific capacitance and current density was derived from the charge/discharge curves of capacitors at a current density varying from 40 mA/g to 12 A/g, whose electrode was prepared with 90% HSAC and 10% conductive filler. The results in Fig. 7 indicates that although the specific capacitance decreased with current density increasing, the capacitance of the electrode using CAG as conductive filler gave a high value of 172 F/g even if the current density was 12 A/g. When the current density was 9 A/g, its specific capacitance was 182 F/g, which still kept 74% of the value measured at 40 mA/g. However, once the current density was higher than 9 A/g, the specific capacitance could not be measured except CAG was used as conductive filler. Therefore, CAG can be the best candidate of conductive filler for EDLCs with high power density. 4.3. Application of the new equivalent circuit model When the electrode thickness was 0.4–1.2 mm with 10% conductive filler, the Nyquist plots of EDLCs presented in Fig. 8 include

Table 2 Parameter data simulated with Eqs. (2) and (6). Samples

Parameters

Electrode thickness (mm) 0.4

0.6

0.8

1.0

1.2

HSAC

Rs (mX) Ri (mX) Y0  103 (X1 sa)

248 86.0 1.32 0.919 2.60 1.90

255 96.0 0.776 0.948 2.50 1.91

262 111 0.992 0.918 2.96 2.56

263 138 1.73 0.853 2.98 3.22

281 145 3.17 0.782 2.98 3.79

222 40.6 1.76 0.998 3.83 1.89

240 54.7 3.04 0.888 3.99 2.19

248 60.6 8.38 0.805 4.10 2.79

260 64.2 2.58 0.905 4.29 2.95

272 84.0 4.82 0.830 4.40 4.11

224 43.5 0.82 0.996 2.45 2.21

240 60.6 2.24 0.909 2.67 2.23

253 66.5 2.97 0.980 2.88 2.46

260 70.9 4.62 0.852 2.96 3.09

271 87.3 4.04 0.783 3.08 3.80

245 57.6 0.737 0.989 2.32 1.84

258 67.3 8.73 0.770 2.60 1.97

269 83.9 1.27 0.914 2.93 2.60

271 97.8 9.69 0.716 2.96 2.84

283 112 3.08 0.816 3.21 3.72

a D  108 (cm2 s1) d  104 (cm) CAG

Rs (mX) Ri (mX) Y0  103 (X1 sa)

a D  108 (cm2 s1) d  104 (cm) CB

Rs (mX) Ri (mX) Y0  103 (X1 sa)

a D  108 (cm2 s1) d  104 (cm) Graphite

Rs (mX) Ri (mX) Y0  103 (X1 sa)

a D  108 (cm2 s1) d  104 (cm)

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Fig. 9. SEM images of carbon electrodes added with CAG (a), CB (b), G (c) and no conductive filler (d).

the experimental and fitted curves. As seen in the figures, all of the Nyquist plots are similar to the Nyquist plot shown in Fig. 1. Furthermore, the simulated curves fit with the experimental data very well. According to Eqs. (2) and (6), Marquardt fit procedure is applied to EIS data to obtain the values of parameter Rs, Ri, Y0, a, D, d and the results are listed in Table 2. Although the thickness of electrode verifies from 0.4 to 1.2 mm, the values of solution resistance Rs are almost same, because it is mainly determined by the electrolyte. Under the same electrode thickness, the interface resistance Ri of the cell with pure HSAC as electrode material is much higher than that of the CAG modified capacitors, and the value of Ri follows the order of CAG < CB < G < HSAC. Observed from Fig. 9a–d, the average particle sizes of CAG, CB and HSAC, were about 0.5, 0.5 and 4.8 lm, respectively. In addition, the morphologies showed that conductive filler of CAG and CB were closely packed in the gap formed among HSAC particles, while graphite particles could not fill the voids as shown in Fig. 9c. It should be noted that CAG particles has a finely-branched structure comparing with that of CB, which leads to multi-point contact at the interface between HSAC and CAG particles, resulting in a better contact. Therefore, when CAG was used as conductive filler, it could make the interface resistance Ri lower than that of CB and graphite. The parameter Y0 of CPE was too small to be taken into account when evaluating the interface capacitance Ci. This implies that the effect of interface capacitance can be ignored. When CAG and CB were used as conductive filler and the electrode thickness was 0.4 mm, the values of parameter a were close to 1 indicating that HSAC particles had a good inter-particle contact with CAG or CB, which agree well with the results as shown in Fig. 9a and b. When the thickness of electrode was same, the values of diffusion coefficient D were almost the same for the capacitors prepared with HSAC or with CB (or G) as conductive filler. When CAG was

used as conductive filler, the value of diffusion coefficient D was much larger than others, this should be attributed to the following reasons. On one hand, the framework of CAG is composed of aggregated primary particles. These aggregates formed interconnected pores, which has an open-celled structure with homogenously interconnected porosity. And its pore size mainly distributed in the range of 2–30 nm (shown in Fig. 4) which is favorable for ions diffusing from mesopores into micropores. On the other hand, there is a good contact between CAG and HSAC particles because of the suitable particle size and finely-branched structure of CAG. It should be noted that, the higher diffusion coefficient D is, the lower internal resistance RX is. The values of parameter d were very low and almost identical at the same electrode thickness, but it increased slightly with electrode thickness increasing. This indicates that the concentration polarization was not remarkable. 5. Conclusions In this work, CAG, CB and graphite were used to analyze the effects of conductive filler on the electrochemical performance of carbon electrode by using an impendence method and relating model. According to the series of experimental EIS data, an equivalent circuit model is suggested as follows considering the kinetic of ion diffusion,

Z CPE ¼ 1=½Y 0 ðjxÞa  qffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffi Z w ¼ Z 0 = ðjxÞ coth jB jxj

ð9aÞ ð9bÞ

in detail, Eqs. (9a) and (9b) simulate the impedance at high frequency region and low frequency region, respectively. Marquardt fit procedure was applied to the EIS data to obtain the model parameter values of Rs, Ri, Y0, a, D and d. The results indi-

X. Liu et al. / Journal of Electroanalytical Chemistry 642 (2010) 75–81

cated that CAG could significantly decrease the resistance of EDLC by increasing the diffusion coefficient D of ions within electrodes and decreasing the interface resistance Ri because of its suitable particle size, mesoporous structure and finely-branched particle performance. When CAG was used as conductive filler, its specific capacitance has a higher contribution to that of composite electrode, and the composite electrode has a higher capacitance retention of 74% even if the current density was 9 A/g. Acknowledgments This work was supported by the National Science Foundation of China (Nos. 50730003, 50672025, 20806024) and the Research Fund of China for the Doctoral Program of Higher Education (No. 20070251008). References [1] X.J. He, J.W. Lei, Y.J. Geng, X.Y. Zhang, M.B. Wu, M.D. Zheng, J. Phys. Chem. Solids 70 (2009) 738–744. [2] M.S. Balathanigaimani, W.G. Shim, M.J. Lee, C. Kim, J.W. Lee, H. Moon, Electrochem. Commun. 10 (2008) 868–871. [3] M. Itagaki, S. Suzuki, I. Shitanda, K. Watanabe, H. Nakazawa, J. Power Sources 164 (2007) 415–424. [4] B.E. Conway. Journal of Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, New York, USA, 1999.

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