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Carbon Electrode Material with High Densities of Energy and Power
ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 1, January 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(1): 13−19.
ARTICLE
Carbon Electrode Material with High Densities of Energy and Power Jing Yang,
Yafei Liu,
Xiaomei Chen,
Zhonghua Hu*,
Guohua Zhao
Department of Chemistry, Tongji University, Shanghai 200092, P. R. China
Abstract:
Activated carbon (AC) samples as electrode materials were prepared by means of simultaneous physical-chemical
activation using walnut shells as precursors. The porosity and surface chemistry of the resultant AC samples were studied by the nitrogen adsorption at 77 K, and FTIR spectrum. The testing supercapacitors were assembled with resultant carbon electrode and electrolyte of 6 mol·L−1 KOH solution. Their electrochemical properties were investigated by charge-discharge of constant current, cyclic voltammogram, impedance spectrum and so on. The results showed that the capacitor had low inner resistance, low leakage current, high stability, and capacitance retainability. The specific capacitance of AC increased with increasing BET specific surface area. The specific capacitance of the AC sample with a specific area of 1197 m2·g−1 could be as high as 292 F·g−1. At a discharge current of 80 mA, the corresponding specific energy density, power density, and maximum power of the supercapacitor are 7.3 Wh·kg−1, 770 W·kg−1, and 5.1 W·g−1, respectively. Key Words:
Activated carbon; Activation; Functional group; Electrode material; Supercapacitor
The electrochemical capacitor, also called supercapacitor, is a new device for energy storage, in which the electric energy is stored through the electric double layer or reversible redox on the electrode surface. As it possesses many advantages, such as, high power density comparable with the conventional capacitor, high energy density comparable with rechargeable battery, output of large current, short time for recharge, high coulombic efficiency, long cycling life, no memory-effect, and free of maintenance. Therefore, it has received great attention in many fields, especially in information and technology, digital product, electric vehicle, aerospace and military etc [1−3]. The first patent on the supercapacitor was obtained by Becker in 1957, in which a capacitor based on carbon electrodes of high surface area carbon was described. Since then, researches on high-powered electrode materials have been a hot topic in this field. Three kinds of materials, carbon[4], metal oxides[5,6], and polymers[7], feature as the most important materials for electrodes of a supercapacitor. In the family of carbon materials, there are activated carbon, carbon black, ac-
tivated carbon fiber, glassy carbon, carbon foam, carbon nanotube and so on[8]. In general, the higher the surface area, the larger the capacitance. Owing to its abundant resources, low cost, and good properties, activated carbon has received the most attention. According to the principle of the electric double layer, the average capacitance value of a double electric layer of carbon is 25 µF·cm−2. If the specific surface area is 1000 m2·g−1, the specific capacitance will be 250 F·g−1, accordingly its specific energy will be 8.7 Wh·kg−1 at a working potential of 1 V in an aqueous electrolyte system. It is possible to increase the specific capacitance and the energy and power densities of carbon by increasing the surface area, optimizing the pore size and pore size distribution, and modifying of the carbon surface[9]. Liu and coworkers[10] reported that activated carbon of 1500 2 −1 m ·g had a specific capacitance of 114 F·g−1 and a maximum specific energy of 7.1 Wh·kg−1 in an aqueous electrolyte system. Pietrzak et al.[11] used a high specific surface (1255−2011 m2·g−1) microporous AC as electrode material, and 4 mol·L−1 H2SO4 solution and 7 mol·L−1 KOH solution as the electrolyte.
Received: July 5, 2007; Revised: September 27, 2007. * Corresponding author. Email: [email protected]; Tel: +8621-65982594. The project was supported by the National Natural Science Foundation of China (50472089). Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
The maximum specific capacitances were 191 and 200 F·g−1 and the corresponding specific energies were 6.6 and 6.9 Wh·kg−1, respectively. However, high surface area usually meant low mass-density; therefore, it was not favorable to increase volumetric capacitance. Vix-Guterl et al.[12] reported that the ordered mesoporous carbon of 2000 m2·g−1 prepared by the template procedure had a specific capacitance of 202 F·g−1 and energy density of 7.0 Wh·kg−1 in 1 mol·L−1 H2SO4 electrolyte. However, the template procedure was complicated, with a high-cost and therefore difficult to commercialize. Zhang et al.[13] reported in a review article that commercial supercapacitors usually had a specific energy of 4.5−4.9 Wh·kg−1; and high-performance, low-cost capacitor-carbon activated by KOH was also used as electrode material, the specific energy of the corresponding supercapacitor was 5.7 Wh·kg−1. The objective of this article is to prepare high-performance, low-cost carbon electrode materials and study the relation between the carbon property and performance of the supercapacitor with carbon-based electrodes. The carbon samples were prepared by simultaneous physical-chemical activation, using walnut shells as precursors. They were characterized by nitrogen adsorption and FTIR spectroscopy. The influences of surface area, pore structure, and surface function group of carbon electrode materials on the supercapacitor were investigated by electrochemical measurements.
1 1.1
Experimental Materials
Walnut shells were used as precursors in the preparation of activated carbon. Graphite powder (0.056 mm), polypropylene films, and polytetrafluoroethylene (PTFE) were of reagent grade. Nickel foam was from Changsha Liyuan Company, China. CO2 with a purity of 99.99% was used as a physical activating agent. Other chemicals ZnCl2, KOH, HNO3, hydrochloric acid, and ethanol were all of analytical grade. 1.2
Experiment method
1.2.1 Preparation and characterization of activated carbons Activated carbon samples were prepared by simultaneous physical-chemical activation, a new method of one-step carbonization-activation, using CO2 and ZnCl2 as the physical and chemical active agents, respectively[14,15]. Walnut shells as precursor were ground to the particle size of 2.5−3.2 mm, washed and dried at 120 °C for over 12 h. Ten grams dried shell particles were impregnated with a concentrated solution of ZnCl2 with chemical/precursor ratios of 0.2, 0.4, 0.6, and 0.8 (m/m), respectively. After dehydration, the mixture was put into a quartz tube reactor in a temperature-programmed horizontal tube-furnace (CARBOLITE CTF12/75/700) and
activated at 700−800 °C with a rising rate of 10 °C·min−1. Nitrogen was used as protecting gas in the stage of temperature rising and cooling. CO2 was used in the activation period. The cooling products were washed with 10% HNO3 solution, followed with boiling water until the pH value of washed water was about 6−7. The details of the simultaneous physicalchemical activation method are described elsewhere[14,15]. Table 1 lists the five samples used in the present study and their preparation conditions. The carbon samples were characterized by nitrogen adsorption at 77 K (TRISTAR 3000, MICROMERITICS). The specific surface area, pore volume, pore size, and pore size distribution were estimated by the BET model, t-plot, and BJH method. FTIR spectroscopy (NEXUS912A, Nicolet) was used to investigate the function groups on the carbon surface. 1.2.2 Preparation of carbon electrode and assemblage of supercapacitor The carbon samples were ground to powder particles under a 180 mesh. Activated carbon, graphite, and suspension solution of polytetrafluoroethylene (PTEF) were mixed in a ratio of 8:1:0.6. The mixture paste was extruded and cut to form a disc-type carbon electrode with a thickness of 0.3 mm and diameter of 15 mm. Each electrode contained about 0.04 g activated carbon. The electrode was attached to a current collector of Ni foam by combined extrusion under a pressure of 8 MPa. Subsequently, sandwich-type capacitors were assembled with two carbon electrodes separated by polypropylene film in a test capacitor cell. KOH solution of 6 mol·L−1 was used as the electrolyte. 1.2.3
Electrochemical measurement of supercapacitor
The performance of supercapacitors was studied by galvanostatic charge-discharge using a battery-test apparatus (PCBT-100-8D, LISUN, China) and cyclic voltammetry, impedence spectrum, and leakage current using an Electrochemical Study Station (CHI660) at room temperature. The gravimetric specific capacitance (Cp) was calculated from the linear part of the discharge branch. Cp of carbon material, energy density (E), power density (P), and maximum specific power density (Pmax) were calculated by the following formula[16,17]: (1) Cp= 4i∆t ma∆U Table 1
Activation conditions of ZCH series activated carbons
Carbon sample m(ZnCl2)/m(shell) T/°C
t/min CO2 flow rate (L·h−1)
ZCH23
0.4
800
150
50
ZCH42
0.8
750
90
60
ZCH41
0.8
700
60
50
ZCH21
0.4
700
90
30
ZCH11
0.2
700
30
20
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.1
Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) of activated carbons
(■) ZCH23; (□) ZCH42; (▲) ZCH41; () ZCH21; (●) ZCH11; (○) SHAC; (dashed line) desorption isotherm
Pmax=U2/4Rm (2) 2 E=CU /2 (3) (4) P=E/td where C is the capacitance of supercapacitor; i, ∆t, and ∆U are the discharge current (A), the discharge time (s), and the difference of voltage (V) in the discharge during the period of ∆t, respectively; m is the total mass of the two electrodes, and a is the mass fraction of activated carbon in the electrode. U is the working voltage, td is the time spent in discharge, R is the equivalent series resistance (ESR) of supercapacitor (Ω). Besides, the area specific capacitance was also calculated according to Cs=Cp/SBET. SBET is the specific surface area of activated carbon.
2 2.1
Results and discussion The porosity and surface area of activated carbons
The N2 adsorption/desorption isotherms and pore size distribution of activated carbons are presented in Fig.1. It is found that the activated carbons prepared with low ZnCl2/shell ratio and the commercial carbon (SHAC) exhibit a typical type I isotherm[18] according to the IUPAC classification. The major uptake occurs at relative pressures less than 0.1. An almost horizontal plateau at higher relative pressures indicates that these carbon samples are highly microporous materials. The sample ZCH42 has a relatively clear desorption hysteresis loop (H3 type), suggesting the existence of mesopores, which can be further proved by the pore size distribution as shown in Fig.1B, where a large peak is found at about 4 nm. However, Table 2 2
−1
the micropores are predominant in all samples. For the most part, all samples possess supermicropores and some small mesopores (the definition of IUPAC: micropore<2 nm, mesopore: 2−50 nm, macropore>50 nm). The specific surface area and porous parameters of carbon samples have been calculated by the BET equation, t-plot, and BJH method, according to the N2 adsorption-desorption isotherms as listed in Table 2. The BET surface area is in the range of 500−1200 m2·g−1. The ratio of micropore volume/ total pore volume is above 90%, indicating that all samples are highly microporous material. The average pore diameter is about 2 nm. Sample ZCH42 has the largest average diameter. The result is in accordance with that from Fig.1B. KOH solution is one of the most used electrolytes in the supercapacitor. The diameter of hydrated ions of K+ and OH− are less than 0.4 nm. A diameter of about 2 nm is a suitable pore size for transfer and electrochemical adsorption[19]. 2.2 Relation between porous structure of electrode material and properties of capacitor Galvanostatic charge-discharge is a routine measurement for supercapacitors. From charge-discharge curves, we can obtain important information, such as, cell capacitance, specific capacitance of electrode material, and recycle life. A perfect capacitor must exhibit an isoceles triangle shaped chargedischarge curve. All capacitors with carbon samples, studied here as electrode materials, have a charge-discharge curve of similar shape. Fig.2 is a representative charge-discharge curve
Specific surface area and porosity parameters of activated carbons
Sample
SBET/(m ·g )
Smi/(m2·g−1)
Sext/(m2·g−1)
Vtot/(cm3·g−1)
Vmi/(cm3·g−1)
(Vmi/Vtot) (%)
D/nm
ZCH23
1197
1183
14
0.599
0.577
96
2.00
ZCH42
1126
1087
39
0.608
0.548
90
2.16
ZCH41
914
902
11
0.464
0.444
96
2.03
ZCH21
740
732
7
0.371
0.359
97
2.01
ZCH11
533
532
1
0.258
0.253
98
1.93
SHAC
943
932
11
0.468
0.444
95
1.99
SBET: BET surface area; Smi: micropore area; Sext: external surface area; Vtot: total volume; Vmi: micropore volume; D: average pore diameter
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.2
Galvanostatic charge-discharge curve of the ZCH23 supercapacitor at a constant current of 5 mA
of the ZCH23-capacitor at 5 mA. The almost isoceles triangleshaped curves indicate the excellent capacitance behavior, good reversible and recycling charge-discharge, and high coulombic efficiency. The potential drop (IR drop) at the very beginning of the discharge is very small. Thus, the equivalent series resistance (ESR) can be estimated to be 0.57 Ω. Accordingly, the maximum specific power of the ZCH23 capacitor is about 5.1 W·g−1 using Eq.(3), which is close to the 6.5 W·g−1 glassy carbon[20] and 6.8 W·g−1 carbon aerogel[21] electrode-based capacitors. From the point of view of costeffectiveness, the activated carbons prepared here have a great potential in commercial application. The variation in specific capacitance of carbon electrode materials as a function of discharge current is shown in Fig.3. As the charge-discharge current increases from 5 mA to 80 mA, the specific capacitance of the ZCH-series carbons decrease slowly; however, that of the commercial carbon (SHAC) drops dramatically as the discharge current increases from 5 mA to 10 mA. Beyond 20 mA, it keeps almost constant, which is smaller than that of the ZCH-series carbons. To investigate the influence of the carbon surface area on the specific capacitance, the correlations of specific capacitances at 5, 10, 20, 50, and 80 mA versus BET surface area or micropore surface area were plotted. It was found that the linear correlation coefficients (R2) were 0.96, 0.99, 0.97, 0.99,
Fig.3
Specific capacitance of carbon electrode materials as a function of discharge current
(■) ZCH23; (□) ZCH42; (▲) ZCH41; () ZCH21; (●) ZCH11; (○) SHAC
and 0.97 for Cp versus SBET and 0.97, 0.99, 0.96, 0.98, and 0.97 for Cp versus Smi, respectively. The average coefficient was above 0.97, suggesting that the specific capacitance increased linearly with an increase in both BET surface area and microporous surface area. Although ZCH41 and SHAC have similar BET surface area and pore volume, the specific capacitance of the former is much larger than that of the latter. Herein, the pore size and pore size distribution may act as important roles that affect the capacitive behavior of carbon materials. ZCH41 has more pores of 1.5 to 4 nm; but SHAC has more pores of less than 1.5 nm as shown in Fig.1(B). SHAC has a larger BET surface area, but smaller average pore diameter. It is well known that the carbon surface of the pores that ranges from 1.5 to 4 nm is the main location for the formation of an electric double layer in the inorganic electrolyte, such as KOH or H2SO4. In addition, mesopores are important channels for the diffusion of electrolyte ions to micropores. Thus, ZCH41 possesses a more effective surface area than SHAC, for the formation of a double electric layer. The simultaneous physical-chemical activation of ZnCl2-CO2 has been used to prepare AC samples in this study, because it can produce ACs with more pores between supermicropores (1.5 to 2 nm) and small mesopores[14]. It is also found that the specific capacitance of SHAC is even smaller than that of ZCH11, except discharge at 5 mA, as shown in Fig.3, suggesting that besides pore size, other properties, such as surface chemistry and pore structure of carbon, have an influence on the diffusion of electrolyte ions, hence, some pore surface of SHAC cannot be utilized to form a double electric layer, especially at a discharge current larger than 10 mA. ZCH23 has the highest specific capacitance of 292 F·g−1 at a current of 5 mA as shown in Fig.3. The corresponding area specific capacitance is 24.4 µF·cm−2, which is much larger than 10.1 µF·cm−2 of a high surface area AC (3310 m2·g), 14.0, 14.2, 12.1 µF·cm−2 of template-mesoporous carbon (1347, 1490, 1704 m2·g−1)[22], and 16.6 µF·cm−2 of nano-activated carbon fiber (1220 m2·g−1)[23]. It implies that the carbon samples prepared by the simultaneous physical-chemical activation of ZnCl2-CO2 have much more effective surface area than other carbons, for the formation of electric double layer capacitance. Fig.4 shows the Ragone plots of energy density (E) versus power density (P) for supercapacitors. It is evident that the power density (650−1070 W·kg−1) and energy density (4.6− 10.2 Wh·kg−1) of the ZCH-series carbons are higher than that of SHAC. All carbon samples show a similar characteristic of E−P. They have relatively high E at low P; however, E decreases remarkably in the range of P less than 200 W·kg−1; beyond that, it drops slowly with increasing P. This suggests that these carbon electrode-based capacitors can keep high energy density when the output of high power density is needed, that is, discharge at high current. For example, at a current of
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.4
Ragone plots of energy density versus power density for supercapacitors
(■) ZCH23; (□) ZCH42; (▲) ZCH41; () ZCH21; (●) ZCH11; (○) SHAC
80 mA, ZCH23 has a specific capacitance of 210 F·g−1, and the corresponding energy density of the capacitor is 7.3 Wh·kg−1, which is higher than that of an ordered-mesoporous carbon capacitor (4−6 Wh·kg−1); and its power density (770 W·kg−1) is close to that of the latter (800−1050 W·kg−1)[22]. From the Ragone plots, it is expected that the power can be further increased, whereas, the energy density decreases a little, if any. The FTIR spectra of carbon samples ZCH23, ZCH41, and SHAC are shown in Fig.5. They have similar curves of spectrum, suggesting that they have almost the same functional groups on the carbon surface, especially for ZCH23 and ZCH41. The broad band at 3500 cm−1 is assigned to −OH stretching vibration, attributed to the adsorbed water, hydroxyl and phenolic groups on the carbon surface. The absorption bands at 2800 and 1400 cm−1 belong to −CH3 symmetrical stretching vibration and symmetrical flexural vibration. The bands at 1630, 1300, and 1000 cm−1 are assigned to the stretching vibration of C=O, C−O and symmetrical stretching vibration of −NO2, suggesting the presence of oxygen containing functional groups. These functional groups can improve the wettability of carbon material on one hand; and can also generate pseudocapacitance on the other hand; therefore, they can significantly improve the capacitive behavior of the
Fig.5
FTIR spectra of activated carbons ZCH23 (a), ZCH41 (b), and SHAC (c)
carbon materials. The presence of pseudocapacitance is proved by the voltammogram as shown in Fig.6. The oxidation peaks, because of the reactions on the electrode between 0.7 V and 0.8 V, are obvious for all three samples. All voltammogram curves are very close to the rectangle shape, indicating good capacitive characteristics. ZCH41 has a higher responding current than SHAC, indicating that the former has a higher specific capacitance than the latter. However, SHAC exhibits a better rectangle-like shape, that is, the angles are close to 0 V and 1 V in the positive and negative scans, respectively, and are closer to 90°. This means the SHAC capacitor has a smaller RC (response current) time constant (quick current response). The difference in the RC time constant of different capacitors may be caused by pore structure or conductivity of the electrode material. From Fig.6, it is also obvious that ZCH23 has the highest responding current, the biggest area of currentvoltage window, and nice rectangular curve, suggesting that ZCH23 has the best capacitive characteristics among the three samples. 2.3 2.3.1
Other electrochemical properties of supercapacitors Alternating current impedance
Alternating current impedance spectrum is one of the important methods to study the electrochemical property of supercapacitors. Fig.7 shows the Nyquist plots of the capacitor with the ZCH23-based electrodes. The plot is characterized by a semicircle, a linear part with an angle of 45°, and an almost vertical line to the Z′ axis in the region of high, middle, and low (below 120 mHz) frequencies, respectively, indicating the typical capacitive behavior of the porous electrode. The semicircle represented the reaction resistance (Rr) of the electrode with the electrolyte ions, which was estimated to be about 0.1 Ω from the diameter of the semicircle. The equivalent series resistance (Rs) of the capacitor was estimated to be about 0.18 Ω from the cross point of the highest frequency with the real part of the impedance. The linear part with an angle of 45° indicated the diffusion of ions in the po-
Fig.6
Cyclic voltammograms of different samples
(a) ZCH23, (b) ZCH41, (c) SHAC; sweep rate: 5 mV·s−1
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.7
Nyquist plot of the supercapacitor with
Fig.9
Cycle life of the supercapacitor with
ZCH23-based electrodes
rous electrode. The almost vertical line to the Z′ axis reflected the phenomenon of charge saturation on the electrodes. The results of the impedance spectrum demonstrated that ZCH23 had a good electrochemical performance and a small inner resistance[24]. 2.3.2
Leakage current and cycle life of supercapacitors
Leakage current and cyclic life are the two very important factors of supercapacitors. On one hand the leakage current blocks the potential rise in the charge and on the other hand the speed potential declines in the discharge. This has a negative influence on the performance of supercapacitors. The leakage current is usually caused by the combined effects of electrolyte resistance, impurity, impedance of the electrode materials and so on. Fig.8 shows the current decrease as a function of time, that is, the I−t curve for the capacitor with ZCH23-based electrodes. At the start, the current decreases sharply with increasing time; after 30 min, the current tends to be stable and is kept at a constant, which is recognized as the leakage current. It is a maintaining current and consumed by the capacitor itself, because of the inherent property of the capacitor. As seen from the figure, the leakage current is small, 0.19 mA only, suggesting that the ZCH23 capacitor has a low ESR. The result coincides with that of the alternating current impedance study.
ZCH23-based electrodes
Fig.9 shows the cyclic life test of 1000-cycle charge-discharge at 20 mA for the capacitor with ZCH23-based electrodes. Although the specific capacitance has a small decrease in the beginning of the cycle, it is stable in the entire test. Above 90% of the initial capacitance can be maintained after 1000 cycles, indicating that ZCH23 is a stable electrode material.
3
Conclusions
Low-cost and high-performance electrode materials were prepared by simultaneous physical-chemical activation, using walnut shells as precursor. The results demonstrated that pore structure, especially pore size distribution of carbon electrode material, had a significant inference on the performance of the supercapacitor. For KOH electrolyte solution, the pore size distribution in the range of 1.5−4 nm was favorable for the formation of the electric double layer on the carbon surface. And the surface functional groups could improve the wettability and generate pseudocapacitance to increase the specific capacitance. The specific capacitance increased linearly with an increase in microporous or BET surface area for the AC materials with an average pore diameter of ca 2 nm. The specific capacitance of carbon material could be as high as 292 F·g−1 at 5 mA charge-discharge current; and it still kept a high value of 210 F·g−1 at 80 mA. The resistance and leakage currents of the testing supercapacitor were small, 0.57 Ω and 0.19 mA, respectively; and the energy density, power density, and maximum power density were 7.3 Wh·kg−1, 770 W·kg−1, and 5.1 W·g−1, respectively. The capacitance maintained above 90% after a 1000-cycle charge-discharge. Therefore, these AC materials have a great commercial applied potential because of their low cost, simple preparation procedure, and excellent electrochemical performance.
References 1 Kötz, R.; Carlen, M. Electrochimica Acta, 2000, 45: 2483 Fig.8
Leakage current of the supercapacitor with ZCH23-based electrodes
2 Burke, A. Journal of Power Sources, 2000, 91: 37 3 Yuan, G. H. Electrochemical capacitors. Beijing: Chemical In-
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
dustry Press, 2006: 1 4 Ruiz, V.; Blanco, C.; Raymundo-Piñero, E.; Khomenko, V.; Béguin, F.; Santamaría, R. Electrochimica Acta, 2007, 52: 4969 5 Zhang, B. H.; Zhang, N. Acta Phys. -Chim. Sin., 2003, 19(3): 286 6 Wang, X. F.; Wang, D. Z.; Liang, J.; Liu, Q. G. Acta Phys. -Chim. Sin., 2002, 18(8): 750 7 Wang, J.; Xu, Y. L.; Chen, X.; Du, X. F. Journal of Power Sources, 2007, 163: 1120 8 Pandoflo, A. G.; Hollenkamp, A. F. Journal of Power Sources, 2006, 157: 11 9 Zhang, W. B.; Wang, G. Q. Chinese Battery Industry, 2006, 11(1): 42 10 Liu, H. T.; Zhu, G. Y. Journal of Power Sources, 2007, 171: 1054 11 Pietrzak, R.; Jurewicz, K.; Nowicki, P.; Babel, K.; Wachowska, H. Fuel, 2007, 86: 1086 12 Vix-Guterl, C.; Frackowiak, E.; Jurewicz, K.; Friebe, M.; Parmentier, J.; Béguin, F. Carbon, 2005, 43: 1293 13 Zhang, H.; Cheng, J.; Cao, G. P.; Yang, Y. S. Battery Bimonthly, 2006, 36(2): 109 14 Hu, Z. H.; Srinavasan, M. P.; Ni, Y. M. Carbon, 2001, 39: 877
15 Hu, Z. H.; Srinavasan, M. P.; Ni, Y. M. Advanced Materials, 2000, 12: 62 16 Xing, W.; Qiao, S. Z.; Li, F.; Lu, G. Q.; Yan, Z. F.; Cheng, H. M. Carbon, 2002, 40: 216 17 Wang, X. F.; Wang, D. Z.; Liang, J. Acta Phys. -Chim. Sin., 2005, 21(2): 117 18 Zhao, Z. G. Applied principle of adsorption. Beijing: Chemical Industry Press, 2005: 71−72 19 Zhou, P. W.; Li, B. H.; Kang, F. Y.; Zeng, Y. Q. New Carbon Materials, 2006, 21(2): 125 20 Wen, Y. H.; Cao, G. P.; Yang, Y. S. Journal of Power Sources, 2005, 148: 121 21 Pöbstle, H.; Schmitt, C.; Fricke, J. Journal of Power Sources, 2002, 105: 189 22 Xing, W.; Qiao, S. Z.; Ding, R. G.; Li, F.; Lu, G. Q.; Yan, Z. F.; Cheng, H. M. Carbon, 2006, 44: 216 23 Chan, K. Journal of Power Sources, 2005, 142: 382 24 Liang, K.; Chen, A.; Feng, Z. S.; Ye, Z. X. Acta Phys. -Chim. Sin., 2002, 18(4): 38
ARTICLE
Carbon Electrode Material with High Densities of Energy and Power Jing Yang,
Yafei Liu,
Xiaomei Chen,
Zhonghua Hu*,
Guohua Zhao
Department of Chemistry, Tongji University, Shanghai 200092, P. R. China
Abstract:
Activated carbon (AC) samples as electrode materials were prepared by means of simultaneous physical-chemical
activation using walnut shells as precursors. The porosity and surface chemistry of the resultant AC samples were studied by the nitrogen adsorption at 77 K, and FTIR spectrum. The testing supercapacitors were assembled with resultant carbon electrode and electrolyte of 6 mol·L−1 KOH solution. Their electrochemical properties were investigated by charge-discharge of constant current, cyclic voltammogram, impedance spectrum and so on. The results showed that the capacitor had low inner resistance, low leakage current, high stability, and capacitance retainability. The specific capacitance of AC increased with increasing BET specific surface area. The specific capacitance of the AC sample with a specific area of 1197 m2·g−1 could be as high as 292 F·g−1. At a discharge current of 80 mA, the corresponding specific energy density, power density, and maximum power of the supercapacitor are 7.3 Wh·kg−1, 770 W·kg−1, and 5.1 W·g−1, respectively. Key Words:
Activated carbon; Activation; Functional group; Electrode material; Supercapacitor
The electrochemical capacitor, also called supercapacitor, is a new device for energy storage, in which the electric energy is stored through the electric double layer or reversible redox on the electrode surface. As it possesses many advantages, such as, high power density comparable with the conventional capacitor, high energy density comparable with rechargeable battery, output of large current, short time for recharge, high coulombic efficiency, long cycling life, no memory-effect, and free of maintenance. Therefore, it has received great attention in many fields, especially in information and technology, digital product, electric vehicle, aerospace and military etc [1−3]. The first patent on the supercapacitor was obtained by Becker in 1957, in which a capacitor based on carbon electrodes of high surface area carbon was described. Since then, researches on high-powered electrode materials have been a hot topic in this field. Three kinds of materials, carbon[4], metal oxides[5,6], and polymers[7], feature as the most important materials for electrodes of a supercapacitor. In the family of carbon materials, there are activated carbon, carbon black, ac-
tivated carbon fiber, glassy carbon, carbon foam, carbon nanotube and so on[8]. In general, the higher the surface area, the larger the capacitance. Owing to its abundant resources, low cost, and good properties, activated carbon has received the most attention. According to the principle of the electric double layer, the average capacitance value of a double electric layer of carbon is 25 µF·cm−2. If the specific surface area is 1000 m2·g−1, the specific capacitance will be 250 F·g−1, accordingly its specific energy will be 8.7 Wh·kg−1 at a working potential of 1 V in an aqueous electrolyte system. It is possible to increase the specific capacitance and the energy and power densities of carbon by increasing the surface area, optimizing the pore size and pore size distribution, and modifying of the carbon surface[9]. Liu and coworkers[10] reported that activated carbon of 1500 2 −1 m ·g had a specific capacitance of 114 F·g−1 and a maximum specific energy of 7.1 Wh·kg−1 in an aqueous electrolyte system. Pietrzak et al.[11] used a high specific surface (1255−2011 m2·g−1) microporous AC as electrode material, and 4 mol·L−1 H2SO4 solution and 7 mol·L−1 KOH solution as the electrolyte.
Received: July 5, 2007; Revised: September 27, 2007. * Corresponding author. Email: [email protected]; Tel: +8621-65982594. The project was supported by the National Natural Science Foundation of China (50472089). Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
The maximum specific capacitances were 191 and 200 F·g−1 and the corresponding specific energies were 6.6 and 6.9 Wh·kg−1, respectively. However, high surface area usually meant low mass-density; therefore, it was not favorable to increase volumetric capacitance. Vix-Guterl et al.[12] reported that the ordered mesoporous carbon of 2000 m2·g−1 prepared by the template procedure had a specific capacitance of 202 F·g−1 and energy density of 7.0 Wh·kg−1 in 1 mol·L−1 H2SO4 electrolyte. However, the template procedure was complicated, with a high-cost and therefore difficult to commercialize. Zhang et al.[13] reported in a review article that commercial supercapacitors usually had a specific energy of 4.5−4.9 Wh·kg−1; and high-performance, low-cost capacitor-carbon activated by KOH was also used as electrode material, the specific energy of the corresponding supercapacitor was 5.7 Wh·kg−1. The objective of this article is to prepare high-performance, low-cost carbon electrode materials and study the relation between the carbon property and performance of the supercapacitor with carbon-based electrodes. The carbon samples were prepared by simultaneous physical-chemical activation, using walnut shells as precursors. They were characterized by nitrogen adsorption and FTIR spectroscopy. The influences of surface area, pore structure, and surface function group of carbon electrode materials on the supercapacitor were investigated by electrochemical measurements.
1 1.1
Experimental Materials
Walnut shells were used as precursors in the preparation of activated carbon. Graphite powder (0.056 mm), polypropylene films, and polytetrafluoroethylene (PTFE) were of reagent grade. Nickel foam was from Changsha Liyuan Company, China. CO2 with a purity of 99.99% was used as a physical activating agent. Other chemicals ZnCl2, KOH, HNO3, hydrochloric acid, and ethanol were all of analytical grade. 1.2
Experiment method
1.2.1 Preparation and characterization of activated carbons Activated carbon samples were prepared by simultaneous physical-chemical activation, a new method of one-step carbonization-activation, using CO2 and ZnCl2 as the physical and chemical active agents, respectively[14,15]. Walnut shells as precursor were ground to the particle size of 2.5−3.2 mm, washed and dried at 120 °C for over 12 h. Ten grams dried shell particles were impregnated with a concentrated solution of ZnCl2 with chemical/precursor ratios of 0.2, 0.4, 0.6, and 0.8 (m/m), respectively. After dehydration, the mixture was put into a quartz tube reactor in a temperature-programmed horizontal tube-furnace (CARBOLITE CTF12/75/700) and
activated at 700−800 °C with a rising rate of 10 °C·min−1. Nitrogen was used as protecting gas in the stage of temperature rising and cooling. CO2 was used in the activation period. The cooling products were washed with 10% HNO3 solution, followed with boiling water until the pH value of washed water was about 6−7. The details of the simultaneous physicalchemical activation method are described elsewhere[14,15]. Table 1 lists the five samples used in the present study and their preparation conditions. The carbon samples were characterized by nitrogen adsorption at 77 K (TRISTAR 3000, MICROMERITICS). The specific surface area, pore volume, pore size, and pore size distribution were estimated by the BET model, t-plot, and BJH method. FTIR spectroscopy (NEXUS912A, Nicolet) was used to investigate the function groups on the carbon surface. 1.2.2 Preparation of carbon electrode and assemblage of supercapacitor The carbon samples were ground to powder particles under a 180 mesh. Activated carbon, graphite, and suspension solution of polytetrafluoroethylene (PTEF) were mixed in a ratio of 8:1:0.6. The mixture paste was extruded and cut to form a disc-type carbon electrode with a thickness of 0.3 mm and diameter of 15 mm. Each electrode contained about 0.04 g activated carbon. The electrode was attached to a current collector of Ni foam by combined extrusion under a pressure of 8 MPa. Subsequently, sandwich-type capacitors were assembled with two carbon electrodes separated by polypropylene film in a test capacitor cell. KOH solution of 6 mol·L−1 was used as the electrolyte. 1.2.3
Electrochemical measurement of supercapacitor
The performance of supercapacitors was studied by galvanostatic charge-discharge using a battery-test apparatus (PCBT-100-8D, LISUN, China) and cyclic voltammetry, impedence spectrum, and leakage current using an Electrochemical Study Station (CHI660) at room temperature. The gravimetric specific capacitance (Cp) was calculated from the linear part of the discharge branch. Cp of carbon material, energy density (E), power density (P), and maximum specific power density (Pmax) were calculated by the following formula[16,17]: (1) Cp= 4i∆t ma∆U Table 1
Activation conditions of ZCH series activated carbons
Carbon sample m(ZnCl2)/m(shell) T/°C
t/min CO2 flow rate (L·h−1)
ZCH23
0.4
800
150
50
ZCH42
0.8
750
90
60
ZCH41
0.8
700
60
50
ZCH21
0.4
700
90
30
ZCH11
0.2
700
30
20
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.1
Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) of activated carbons
(■) ZCH23; (□) ZCH42; (▲) ZCH41; () ZCH21; (●) ZCH11; (○) SHAC; (dashed line) desorption isotherm
Pmax=U2/4Rm (2) 2 E=CU /2 (3) (4) P=E/td where C is the capacitance of supercapacitor; i, ∆t, and ∆U are the discharge current (A), the discharge time (s), and the difference of voltage (V) in the discharge during the period of ∆t, respectively; m is the total mass of the two electrodes, and a is the mass fraction of activated carbon in the electrode. U is the working voltage, td is the time spent in discharge, R is the equivalent series resistance (ESR) of supercapacitor (Ω). Besides, the area specific capacitance was also calculated according to Cs=Cp/SBET. SBET is the specific surface area of activated carbon.
2 2.1
Results and discussion The porosity and surface area of activated carbons
The N2 adsorption/desorption isotherms and pore size distribution of activated carbons are presented in Fig.1. It is found that the activated carbons prepared with low ZnCl2/shell ratio and the commercial carbon (SHAC) exhibit a typical type I isotherm[18] according to the IUPAC classification. The major uptake occurs at relative pressures less than 0.1. An almost horizontal plateau at higher relative pressures indicates that these carbon samples are highly microporous materials. The sample ZCH42 has a relatively clear desorption hysteresis loop (H3 type), suggesting the existence of mesopores, which can be further proved by the pore size distribution as shown in Fig.1B, where a large peak is found at about 4 nm. However, Table 2 2
−1
the micropores are predominant in all samples. For the most part, all samples possess supermicropores and some small mesopores (the definition of IUPAC: micropore<2 nm, mesopore: 2−50 nm, macropore>50 nm). The specific surface area and porous parameters of carbon samples have been calculated by the BET equation, t-plot, and BJH method, according to the N2 adsorption-desorption isotherms as listed in Table 2. The BET surface area is in the range of 500−1200 m2·g−1. The ratio of micropore volume/ total pore volume is above 90%, indicating that all samples are highly microporous material. The average pore diameter is about 2 nm. Sample ZCH42 has the largest average diameter. The result is in accordance with that from Fig.1B. KOH solution is one of the most used electrolytes in the supercapacitor. The diameter of hydrated ions of K+ and OH− are less than 0.4 nm. A diameter of about 2 nm is a suitable pore size for transfer and electrochemical adsorption[19]. 2.2 Relation between porous structure of electrode material and properties of capacitor Galvanostatic charge-discharge is a routine measurement for supercapacitors. From charge-discharge curves, we can obtain important information, such as, cell capacitance, specific capacitance of electrode material, and recycle life. A perfect capacitor must exhibit an isoceles triangle shaped chargedischarge curve. All capacitors with carbon samples, studied here as electrode materials, have a charge-discharge curve of similar shape. Fig.2 is a representative charge-discharge curve
Specific surface area and porosity parameters of activated carbons
Sample
SBET/(m ·g )
Smi/(m2·g−1)
Sext/(m2·g−1)
Vtot/(cm3·g−1)
Vmi/(cm3·g−1)
(Vmi/Vtot) (%)
D/nm
ZCH23
1197
1183
14
0.599
0.577
96
2.00
ZCH42
1126
1087
39
0.608
0.548
90
2.16
ZCH41
914
902
11
0.464
0.444
96
2.03
ZCH21
740
732
7
0.371
0.359
97
2.01
ZCH11
533
532
1
0.258
0.253
98
1.93
SHAC
943
932
11
0.468
0.444
95
1.99
SBET: BET surface area; Smi: micropore area; Sext: external surface area; Vtot: total volume; Vmi: micropore volume; D: average pore diameter
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.2
Galvanostatic charge-discharge curve of the ZCH23 supercapacitor at a constant current of 5 mA
of the ZCH23-capacitor at 5 mA. The almost isoceles triangleshaped curves indicate the excellent capacitance behavior, good reversible and recycling charge-discharge, and high coulombic efficiency. The potential drop (IR drop) at the very beginning of the discharge is very small. Thus, the equivalent series resistance (ESR) can be estimated to be 0.57 Ω. Accordingly, the maximum specific power of the ZCH23 capacitor is about 5.1 W·g−1 using Eq.(3), which is close to the 6.5 W·g−1 glassy carbon[20] and 6.8 W·g−1 carbon aerogel[21] electrode-based capacitors. From the point of view of costeffectiveness, the activated carbons prepared here have a great potential in commercial application. The variation in specific capacitance of carbon electrode materials as a function of discharge current is shown in Fig.3. As the charge-discharge current increases from 5 mA to 80 mA, the specific capacitance of the ZCH-series carbons decrease slowly; however, that of the commercial carbon (SHAC) drops dramatically as the discharge current increases from 5 mA to 10 mA. Beyond 20 mA, it keeps almost constant, which is smaller than that of the ZCH-series carbons. To investigate the influence of the carbon surface area on the specific capacitance, the correlations of specific capacitances at 5, 10, 20, 50, and 80 mA versus BET surface area or micropore surface area were plotted. It was found that the linear correlation coefficients (R2) were 0.96, 0.99, 0.97, 0.99,
Fig.3
Specific capacitance of carbon electrode materials as a function of discharge current
(■) ZCH23; (□) ZCH42; (▲) ZCH41; () ZCH21; (●) ZCH11; (○) SHAC
and 0.97 for Cp versus SBET and 0.97, 0.99, 0.96, 0.98, and 0.97 for Cp versus Smi, respectively. The average coefficient was above 0.97, suggesting that the specific capacitance increased linearly with an increase in both BET surface area and microporous surface area. Although ZCH41 and SHAC have similar BET surface area and pore volume, the specific capacitance of the former is much larger than that of the latter. Herein, the pore size and pore size distribution may act as important roles that affect the capacitive behavior of carbon materials. ZCH41 has more pores of 1.5 to 4 nm; but SHAC has more pores of less than 1.5 nm as shown in Fig.1(B). SHAC has a larger BET surface area, but smaller average pore diameter. It is well known that the carbon surface of the pores that ranges from 1.5 to 4 nm is the main location for the formation of an electric double layer in the inorganic electrolyte, such as KOH or H2SO4. In addition, mesopores are important channels for the diffusion of electrolyte ions to micropores. Thus, ZCH41 possesses a more effective surface area than SHAC, for the formation of a double electric layer. The simultaneous physical-chemical activation of ZnCl2-CO2 has been used to prepare AC samples in this study, because it can produce ACs with more pores between supermicropores (1.5 to 2 nm) and small mesopores[14]. It is also found that the specific capacitance of SHAC is even smaller than that of ZCH11, except discharge at 5 mA, as shown in Fig.3, suggesting that besides pore size, other properties, such as surface chemistry and pore structure of carbon, have an influence on the diffusion of electrolyte ions, hence, some pore surface of SHAC cannot be utilized to form a double electric layer, especially at a discharge current larger than 10 mA. ZCH23 has the highest specific capacitance of 292 F·g−1 at a current of 5 mA as shown in Fig.3. The corresponding area specific capacitance is 24.4 µF·cm−2, which is much larger than 10.1 µF·cm−2 of a high surface area AC (3310 m2·g), 14.0, 14.2, 12.1 µF·cm−2 of template-mesoporous carbon (1347, 1490, 1704 m2·g−1)[22], and 16.6 µF·cm−2 of nano-activated carbon fiber (1220 m2·g−1)[23]. It implies that the carbon samples prepared by the simultaneous physical-chemical activation of ZnCl2-CO2 have much more effective surface area than other carbons, for the formation of electric double layer capacitance. Fig.4 shows the Ragone plots of energy density (E) versus power density (P) for supercapacitors. It is evident that the power density (650−1070 W·kg−1) and energy density (4.6− 10.2 Wh·kg−1) of the ZCH-series carbons are higher than that of SHAC. All carbon samples show a similar characteristic of E−P. They have relatively high E at low P; however, E decreases remarkably in the range of P less than 200 W·kg−1; beyond that, it drops slowly with increasing P. This suggests that these carbon electrode-based capacitors can keep high energy density when the output of high power density is needed, that is, discharge at high current. For example, at a current of
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.4
Ragone plots of energy density versus power density for supercapacitors
(■) ZCH23; (□) ZCH42; (▲) ZCH41; () ZCH21; (●) ZCH11; (○) SHAC
80 mA, ZCH23 has a specific capacitance of 210 F·g−1, and the corresponding energy density of the capacitor is 7.3 Wh·kg−1, which is higher than that of an ordered-mesoporous carbon capacitor (4−6 Wh·kg−1); and its power density (770 W·kg−1) is close to that of the latter (800−1050 W·kg−1)[22]. From the Ragone plots, it is expected that the power can be further increased, whereas, the energy density decreases a little, if any. The FTIR spectra of carbon samples ZCH23, ZCH41, and SHAC are shown in Fig.5. They have similar curves of spectrum, suggesting that they have almost the same functional groups on the carbon surface, especially for ZCH23 and ZCH41. The broad band at 3500 cm−1 is assigned to −OH stretching vibration, attributed to the adsorbed water, hydroxyl and phenolic groups on the carbon surface. The absorption bands at 2800 and 1400 cm−1 belong to −CH3 symmetrical stretching vibration and symmetrical flexural vibration. The bands at 1630, 1300, and 1000 cm−1 are assigned to the stretching vibration of C=O, C−O and symmetrical stretching vibration of −NO2, suggesting the presence of oxygen containing functional groups. These functional groups can improve the wettability of carbon material on one hand; and can also generate pseudocapacitance on the other hand; therefore, they can significantly improve the capacitive behavior of the
Fig.5
FTIR spectra of activated carbons ZCH23 (a), ZCH41 (b), and SHAC (c)
carbon materials. The presence of pseudocapacitance is proved by the voltammogram as shown in Fig.6. The oxidation peaks, because of the reactions on the electrode between 0.7 V and 0.8 V, are obvious for all three samples. All voltammogram curves are very close to the rectangle shape, indicating good capacitive characteristics. ZCH41 has a higher responding current than SHAC, indicating that the former has a higher specific capacitance than the latter. However, SHAC exhibits a better rectangle-like shape, that is, the angles are close to 0 V and 1 V in the positive and negative scans, respectively, and are closer to 90°. This means the SHAC capacitor has a smaller RC (response current) time constant (quick current response). The difference in the RC time constant of different capacitors may be caused by pore structure or conductivity of the electrode material. From Fig.6, it is also obvious that ZCH23 has the highest responding current, the biggest area of currentvoltage window, and nice rectangular curve, suggesting that ZCH23 has the best capacitive characteristics among the three samples. 2.3 2.3.1
Other electrochemical properties of supercapacitors Alternating current impedance
Alternating current impedance spectrum is one of the important methods to study the electrochemical property of supercapacitors. Fig.7 shows the Nyquist plots of the capacitor with the ZCH23-based electrodes. The plot is characterized by a semicircle, a linear part with an angle of 45°, and an almost vertical line to the Z′ axis in the region of high, middle, and low (below 120 mHz) frequencies, respectively, indicating the typical capacitive behavior of the porous electrode. The semicircle represented the reaction resistance (Rr) of the electrode with the electrolyte ions, which was estimated to be about 0.1 Ω from the diameter of the semicircle. The equivalent series resistance (Rs) of the capacitor was estimated to be about 0.18 Ω from the cross point of the highest frequency with the real part of the impedance. The linear part with an angle of 45° indicated the diffusion of ions in the po-
Fig.6
Cyclic voltammograms of different samples
(a) ZCH23, (b) ZCH41, (c) SHAC; sweep rate: 5 mV·s−1
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
Fig.7
Nyquist plot of the supercapacitor with
Fig.9
Cycle life of the supercapacitor with
ZCH23-based electrodes
rous electrode. The almost vertical line to the Z′ axis reflected the phenomenon of charge saturation on the electrodes. The results of the impedance spectrum demonstrated that ZCH23 had a good electrochemical performance and a small inner resistance[24]. 2.3.2
Leakage current and cycle life of supercapacitors
Leakage current and cyclic life are the two very important factors of supercapacitors. On one hand the leakage current blocks the potential rise in the charge and on the other hand the speed potential declines in the discharge. This has a negative influence on the performance of supercapacitors. The leakage current is usually caused by the combined effects of electrolyte resistance, impurity, impedance of the electrode materials and so on. Fig.8 shows the current decrease as a function of time, that is, the I−t curve for the capacitor with ZCH23-based electrodes. At the start, the current decreases sharply with increasing time; after 30 min, the current tends to be stable and is kept at a constant, which is recognized as the leakage current. It is a maintaining current and consumed by the capacitor itself, because of the inherent property of the capacitor. As seen from the figure, the leakage current is small, 0.19 mA only, suggesting that the ZCH23 capacitor has a low ESR. The result coincides with that of the alternating current impedance study.
ZCH23-based electrodes
Fig.9 shows the cyclic life test of 1000-cycle charge-discharge at 20 mA for the capacitor with ZCH23-based electrodes. Although the specific capacitance has a small decrease in the beginning of the cycle, it is stable in the entire test. Above 90% of the initial capacitance can be maintained after 1000 cycles, indicating that ZCH23 is a stable electrode material.
3
Conclusions
Low-cost and high-performance electrode materials were prepared by simultaneous physical-chemical activation, using walnut shells as precursor. The results demonstrated that pore structure, especially pore size distribution of carbon electrode material, had a significant inference on the performance of the supercapacitor. For KOH electrolyte solution, the pore size distribution in the range of 1.5−4 nm was favorable for the formation of the electric double layer on the carbon surface. And the surface functional groups could improve the wettability and generate pseudocapacitance to increase the specific capacitance. The specific capacitance increased linearly with an increase in microporous or BET surface area for the AC materials with an average pore diameter of ca 2 nm. The specific capacitance of carbon material could be as high as 292 F·g−1 at 5 mA charge-discharge current; and it still kept a high value of 210 F·g−1 at 80 mA. The resistance and leakage currents of the testing supercapacitor were small, 0.57 Ω and 0.19 mA, respectively; and the energy density, power density, and maximum power density were 7.3 Wh·kg−1, 770 W·kg−1, and 5.1 W·g−1, respectively. The capacitance maintained above 90% after a 1000-cycle charge-discharge. Therefore, these AC materials have a great commercial applied potential because of their low cost, simple preparation procedure, and excellent electrochemical performance.
References 1 Kötz, R.; Carlen, M. Electrochimica Acta, 2000, 45: 2483 Fig.8
Leakage current of the supercapacitor with ZCH23-based electrodes
2 Burke, A. Journal of Power Sources, 2000, 91: 37 3 Yuan, G. H. Electrochemical capacitors. Beijing: Chemical In-
Jing Yang et al. / Acta Physico-Chimica Sinica, 2008, 24(1): 13−19
dustry Press, 2006: 1 4 Ruiz, V.; Blanco, C.; Raymundo-Piñero, E.; Khomenko, V.; Béguin, F.; Santamaría, R. Electrochimica Acta, 2007, 52: 4969 5 Zhang, B. H.; Zhang, N. Acta Phys. -Chim. Sin., 2003, 19(3): 286 6 Wang, X. F.; Wang, D. Z.; Liang, J.; Liu, Q. G. Acta Phys. -Chim. Sin., 2002, 18(8): 750 7 Wang, J.; Xu, Y. L.; Chen, X.; Du, X. F. Journal of Power Sources, 2007, 163: 1120 8 Pandoflo, A. G.; Hollenkamp, A. F. Journal of Power Sources, 2006, 157: 11 9 Zhang, W. B.; Wang, G. Q. Chinese Battery Industry, 2006, 11(1): 42 10 Liu, H. T.; Zhu, G. Y. Journal of Power Sources, 2007, 171: 1054 11 Pietrzak, R.; Jurewicz, K.; Nowicki, P.; Babel, K.; Wachowska, H. Fuel, 2007, 86: 1086 12 Vix-Guterl, C.; Frackowiak, E.; Jurewicz, K.; Friebe, M.; Parmentier, J.; Béguin, F. Carbon, 2005, 43: 1293 13 Zhang, H.; Cheng, J.; Cao, G. P.; Yang, Y. S. Battery Bimonthly, 2006, 36(2): 109 14 Hu, Z. H.; Srinavasan, M. P.; Ni, Y. M. Carbon, 2001, 39: 877
15 Hu, Z. H.; Srinavasan, M. P.; Ni, Y. M. Advanced Materials, 2000, 12: 62 16 Xing, W.; Qiao, S. Z.; Li, F.; Lu, G. Q.; Yan, Z. F.; Cheng, H. M. Carbon, 2002, 40: 216 17 Wang, X. F.; Wang, D. Z.; Liang, J. Acta Phys. -Chim. Sin., 2005, 21(2): 117 18 Zhao, Z. G. Applied principle of adsorption. Beijing: Chemical Industry Press, 2005: 71−72 19 Zhou, P. W.; Li, B. H.; Kang, F. Y.; Zeng, Y. Q. New Carbon Materials, 2006, 21(2): 125 20 Wen, Y. H.; Cao, G. P.; Yang, Y. S. Journal of Power Sources, 2005, 148: 121 21 Pöbstle, H.; Schmitt, C.; Fricke, J. Journal of Power Sources, 2002, 105: 189 22 Xing, W.; Qiao, S. Z.; Ding, R. G.; Li, F.; Lu, G. Q.; Yan, Z. F.; Cheng, H. M. Carbon, 2006, 44: 216 23 Chan, K. Journal of Power Sources, 2005, 142: 382 24 Liang, K.; Chen, A.; Feng, Z. S.; Ye, Z. X. Acta Phys. -Chim. Sin., 2002, 18(4): 38