Energy Fuels 2009, 23, 4668–4677 Published on Web 08/05/2009
: DOI:10.1021/ef900560u
Various Treated Conditions to Prepare Porous Activated Carbon Fiber for Application in Supercapacitor Electrodes Jui-Hsiang Lin,*,† Tse-Hao Ko,† Yu-Hsin Lin,‡ and Chung-Kai Pan† †
Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan, and Department of Biological Science and Technology, China Medical University, Taichung, Taiwan
‡
Received March 22, 2009. Revised Manuscript Received July 22, 2009
Porous activated carbon fiber (ACF) is prepared from oxidized polyacrylonitrile (PAN) fiber through three activation methods, which are traditional chemical activation, traditional physical activation, and ameliorative chemical activation. The samples under various high temperature heat treatments are used as electrode for the supercapacitor. The structure and electrochemical properties of these samples are then characterized by nitrogen adsorption at 77 K, X-ray diffraction (XRD), Raman spectrum, electron spectroscopy for chemical analysis (ECSA), Fourier transform infrared spectrometer (FTIR), cyclic voltammetry (CV), and galvanotactic charge/discharge, respectively. Once formed by the ameliorative chemical activation method, which is used for the electrodes of supercapacitors, the samples exhibited excellent capacitance characteristics in the 1 M H2SO4 electrolyte and showed a high specific capacitance of 158 F/g, which is higher than the two traditional activation methods. HP20-1/3-900 presents a high specific capacitance of 173 F/g in 1 M H2SO4 electrolyte and a high discharge capacitance of 180 F/g. Moreover, HP20-1/3-900 has near triangular shapes, reflecting their excellent charge/discharge capacitive performance. Results of this study demonstrate that the ameliorative activation process for preparing makes this activated carbon fiber a highly promising electrode material for industrial applications of supercapacitors. nature of electrolytes such as ion species, ion sizes, and solvent molecules,6,7 but also by the essence of carbon materials such as an exposed surface area, pore size distribution, functional group on surfaces, and electronic conductivity.8 Activated carbon (AC) has been extensively studied for its preparation and characteristics in decades. As is well-known, the structural properties, that is, pores size, pores volume, size distribution, and surface area depend on its precursors to some extent, and on the carbonization and activation processes. Therefore, tailoring porous structure and surface chemistry of AC has a great importance to improving the electrochemical performance of supercapacitors. Various methods to prepare porous carbon materials, including methods, involve chemical activation, physical activation, and a combination of chemical and physical activation processes.9-11 Ruiz et al. studied how thermal treatment affects AC at 600 and 1000 °C, in which capacitance due to redox reactions occurred for samples with high oxygen content.12 In addition to focusing on various porous AC powders based on polymers with excellent capacitive performance13-15
1. Introduction Supercapacitors can be classified as electric double layer capacitors (EDLC) and pseudocapacitors. Supercapacitors can improve the performance of electrolytic capacitors in a specific energy by rechargeable batteries of a specific power. Additionally, supercapacitors have a much longer cycle life than batteries because they are no or negligibly small chemical charge transfer reactions involved. Among the many applications that porous materials can be found, energy storage, catalysis, and separation are included.1-5 Porous carbon materials can be promising candidates for supercapacitor applications owing to their high surface area, excellent chemical stability, and fine conductivity, which have great importance for high specific capacitance. In carbon supercapacitors, the electric charge is stored between the carbon electrode and electrolyte interfaces. Porous carbon is attractive electrode material for supercapacitors owing to its excellent electrical conductivity, high surface area, many pores distribution, and electrochemical stability. The electrochemical behaviors of supercapacitors are influenced not only by the
(7) Yang, C. M.; Kim, Y. J.; Endo, M.; Kanoh, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Am. Chem. Soc. 2007, 129, 20–21. (8) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1999, 146, 3639–3643. (9) Hyeon, T.; Lee, J.; Kim, J. Adv. Matter. 2006, 18, 2073–2094. (10) Walawender, W. P.; Zhang, T.; Fan, L. T.; Fan, M.; Daugaard, D.; Brown, R. C. Chem. Eng. J. 2004, 105, 53–59. (11) Lua, A. C.; Yang, T. J. Colloid Interf. Sci. 2003, 267, 408–417. (12) Ruiz, V.; Blanco, C.; Raymundo-Pinero, E.; Khomenko, V.; Beguin, F.; Santamaria, R. Electrochim. Acta 2007, 52, 4967–4973. (13) Frackowiak, E. Phys. Chem. Chem. Phys. 2007, 9, 1774–1785. (14) Zhang, C. X.; Long, D. H.; Xing, B. L.; Qiao, W. M.; Zhang, R.; Zhan, L.; Liang, X. Y.; Ling, L. C. Electrochem. Commun. 2008, 10, 1809–1811. (15) Beguin, F.; Szostak, K.; Lota, G.; Frackowiak, E. Adv. Funct. Mater. 2005, 17, 2380–2384.
*To whom correspondence should be addressed. Telephone: þ886-424517250, ext. 5303. Fax: þ886-4-24518401. E-mail: gonvcat@yahoo. com.tw. (1) Yuan, Z. Y.; Su, B. L. J. Mater. Chem. 2006, 16, 663–667. (2) Wang, Y. Q.; Yang, W. C.; Schmidt, W.; Spliethoff, B.; Bill, E.; Schuth, F. Adv. Mater. 2005, 17, 53–56. (3) Lai, X. Y.; Li, X. T.; Geng, W. C.; Tu, J. C.; Li, J. X.; Qiu, S. L. Angew. Chem., Int. Ed. 2007, 46, 738–741. (4) Wang, D. W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H. M. Angew. Chem., Int. Ed. 2008, 47, 373–376. (5) Wang, D. W.; Li, F.; Cheng, H. M. J. Power Sources 2008, 185, 1563–1568. (6) Qu, Q. T.; Wang, B.; Yang, L. C.; Shi, Y.; Tian, S.; Wu, Y. P. Electrochem. Commun. 2008, 10, 1652–1655. r 2009 American Chemical Society
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Table 1. The Codes and Processes of Samples code
process
P 4-4-700 P 4-4-800 P 4-4-900 P 4-4-1000 HP 0-0-700 HP 0-0-800 HP 0-0-900 HP 0-0-1000 HP 4-4-700 HP 4-4-800 HP 4-4-900 HP 4-4-1000 HP4-1/3-900 COP4-1/3-900 HHP4-1/3-900 HP20-1/3-900 COP20-1/3-900 HHP20-1/3-900
dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated at 700 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated at 800 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated at 900 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated at 1000 °C coactivated with steam at 700 °C coactivated with steam at 800 °C coactivated with steam at 900 °C coactivated with steam at 1000 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated with steam at 700 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated with steam at 800 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated with steam at 900 °C dipping in 4 wt % H3PO4 at room temperature in 4 h and coactivated with steam at 1000 °C dipping in 4 wt % H3PO4 at room temperature in 1/3 h and coactivated with steam at 900 °C dipping in 4 wt % H3PO4 at room temperature in 1/3 h and coactivated with CO2 gas at 900 °C dipping in 4 wt % H3PO4 at 85 °C in 1/3 h and coactivated with steam at 900 °C dipping in 20 wt % H3PO4 at room temperature in 1/3 h and coactivated with steam at 900 °C dipping in 20 wt % H3PO4 at room temperature in 1/3 h and coactivated with CO2 gas at 900 °C dipping in 20 wt % H3PO4 at 85 °C in 1/3 h and coactivated with steam at 900 °C
and activation methods of AC powder precursors.16-19 There are many studies that have discussed AC fiber (ACF) used in supercapacitor electrodes.20-23 AC materials used in supercapacitors must have the following characteristics: (i) a high specific surface area (m2/g) to ensure high specific capacitance value, (ii) a high conductivity, and (iii) a microtexture welladapted to allow good electrolyte accessibility into the inner surface of electrode. More recent studies have developed supercapacitors electrodes using various porous materials, such as modified porous materials, nano ACF, and carbon cloths.24-26 In this study, porous ACF is prepared for in supercapacitor electrode applications by using oxidized polyacrylonitrile (PAN) fibers as a precursor, following treatment at various temperature and using various activation methods. Exactly how physical and electrochemical properties are related is also elucidated to select an appropriate treatment temperature and activation method for supercapacitors.
temperature. The traditional physical activation method is then coactivated with steam at high temperature. Next, the ameliorative chemical activation method is dipped in a chemical activation agent and coactivated with steam at a high temperature. The chemical activation agent concentrations are 4 wt % of H3PO4, and the activation agent temperatures are set at room temperature. The oxidized PAN fiber cloths are dipped in this chemical activation agent for 4 h. After dipping in chemical activation agent, these samples are dried at 120 °C for 12 h, in which they are coactivated with steam (0.2 mL/min) in 10 min at a high temperature. A precursor of oxidized PAN fiber cloths is then treated in phosphoric acid, which is used as a chemical activation agent, and coactivated with steam and carbon dioxide gas at 900 °C. The chemical activation agent concentrations are 4 and 20 wt % of H3PO4; the activation agent temperatures are set at room temperature and 85 °C. Next, the oxidized PAN fiber cloths are dipped in these chemical activation agents for 20 min, respectively. Additionally, these samples are dried at 120 °C for 12 h. Finally, these samples are coactivated with steam (0.2 mL/ min) and carbon dioxide gas (2 L/min) in 10 min at 900 °C, respectively. Table 1 lists all codes and processes. 2.2. Characterization. The porous texture of samples are characterized by nitrogen adsorption/desorption at 77 K and 760 mmHg using Micromeritics ASAP-2020 instrument. The samples are then degassed below 1.33 Pa at 90 °C for 1 h and 300 °C for 8 h before the measurements. Next, pore size distributions of samples are evaluated by using the density functional theory (DFT) model. Moreover, the specific surface areas (SBET) are calculated from adsorption data by using the Brunauer-Emmett-Teller (BET) model. The micropore volume is decided by the t-plot model, and the mesopore volume is calculated by the difference of Vtotal and Vmicro. Nevertheless, the Langmuir isotherm is nonetheless the first choice for most models of adsorption, and has many applications in surface kinetics (usually called Langmuir-Hinshelwood kinetics) and thermodynamics. Despite their slight differences, the Langmuir and BET methods differ mainly in that the former can only be used for surfaces covered by only one layer of gas; in addition, the BET surface area is calculated using the multilayer model. The X-ray diffraction (XRD) analysis is performed by using MXP3, MAC Science Instrument, which is analyzed by using Cu KR radiation of 5-60° as the light source with the tube potential of 40 kV and tube current of 30 mA. The Raman spectrum analysis is performed by using the Renishaw Raman imaging microscope system 2000, which is analyzed at 8002000 cm-1 using HeNe ion laser (632.8 nm) as the incident radiation. The scattered light is collected at a right angle to the incident beam, analyzed by a double-grating monochromator, and, then, detected by a cooled photomultiplier tube. XRD and
2. Experimental Section 2.1. Preparation of ACF. All the raw oxidized polyacrylonitrile (PAN) fiber cloths in this study are LOW6701 (0.73 mm; 310 g/m2) and come from CCTeks, Taiwan. A precursor of oxidized PAN fiber cloths is treated in phosphoric acid, which is used as traditional chemical activation, traditional physical activation, and ameliorative chemical activation methods. These samples are coactivated at 700, 800, 900, and 1000 °C, respectively. The traditional chemical activation method is then dipped in a chemical activation agent and coactivated at a high (16) Wen, Z. B.; Qu, Q. T.; Gao, Q.; X Zheng, W.; Hu, Z. H.; Wu, Y. P.; Liu, Y. F.; Wang, X. J. Electrochem. Commun. 2009, 11, 715–718. (17) Tian, Y. M.; Song, Y.; Tang, Z. H.; Guo, Q. G.; Liu, L. J. Power Source 2008, 184, 675–681. (18) Xue, Y.; Chen, Y.; Zhang, M. L.; Yan, Y. D. Mater. Lett. 2008, 62, 3884–3906. (19) Gamby, J.; Taberna, P. L.; Simon, P.; Fauvarque, J. F.; Chesneau, M. J. Power Source 2001, 101, 109–116. (20) Wang, K. P.; Teng, H. S. Carbon 2006, 44, 3218–3225. (21) Xu, B.; Wu, F.; Chen, R. J.; Cao, G. P.; Chen, S.; Zhou, Z. M.; Yang, Y. S. Electrochem. Commun. 2008, 10, 795–797. (22) Kim, C.; Choi, Y. O.; Lee, W. J.; Yang, K. S. Electrochim. Acta 2004, 50, 883–887. (23) Tanahashi, I. J. Appl. Electrochem. 2005, 35, 1067–1072. (24) Hulicova, D.; Yamashita, J.; Soneda, Y.; Hatori, H.; Kodama, M. Chem. Mater. 2005, 17, 1241–1247. (25) Wang, D. W.; Li, F.; Chen, Z. G.; Lu, G. Q.; Cheng, H. M. Chem. Mater. 2008, 20, 7195–7200. (26) Hulicova-Jurcakova, D.; Li, X.; Zhu, Z.; De Marco, Roland; Lu, G. Energy Fuels 2008, 22, 4139–4145.
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Figure 1. Curves of nitrogen adsorption isotherms of three activation methods and various activation conditions.
used as a separator. Cyclic voltammetry (CV) is performed in the potential range of 0-0.75 V at 5 mV/s by using CHI627B, CH Instruments. Notably, Ag/AgCl electrode is used as the reference electrode, and the counterelectrode is platinum. The gravimetric specific capacitance (F/g) of the electrodes is calculated based on the charge and discharge curve. The charge/ discharge capacitance of electrodes is then measured by using the BaSyTec battery test system. Charge and discharge voltages are ranged between 0 and 0.75 V at 2 mA by using a direct current (DC). Surface capacitance is defined as the gravimetric specific capacitance divided by the BET surface area.
Raman show the stacking height of layer planes (Lc) and extending length of layer planes (La), respectively. The Fourier transform infrared spectrometer (FTIR) is collected by using FTX3500 DIGILAB, which is analyzed in a range of 4004000 cm-1. The electron spectroscopy for chemical analysis (ECSA) is collected by using VG Scientific ESCALAB250, which is analyzed with a variation of functional groups on surface by C1S. 2.3. Electrochemical Measurements. Electrochemical properties are performed in the experimental cell with a three-electrode system, electrolyte of 1 M H2SO4 solution, and a glassy paper 4670
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Figure 2. Curves of pore size distribution of three activation methods and various activation conditions.
each of them. The International Union of Pure and Applied Chemistry (IUPAC) classification of adsorption/desorption is based on BDDT classification, which has classified these hysteresis loops. In this study, all hysteresis loops are similar to the H4 type. This hysteresis loop is attributed mainly due to that the factors are similar to the asymmetric slit-shape pore.27,28 With
3. Results and Discussion 3.1. Structural Analysis. The nitrogen adsorption/desorption isotherms at 77 K, the pore size distribution, BET surface area, and porosity of the samples are shown in Figures 1 and 2. The micropore volume is decided by the t-plot model, which is shown in Table 2. Figure 1 shows that the nitrogen adsorption/ desorption isotherms of samples are characteristic of porous ACFs. The BDDT (Brunauer-Deming-Deming-Teller) model is type I. According to Figure 1, the shapes of these isotherms are similar, where each one has a hysteresis loop for
(27) Moreira, R. F. P. M.; Jose, H. J.; Rodrigues, A. E. Carbon 2001, 39, 2269–2275. (28) Sing K. S. W. Gregg S. J. Adsorption, Surface Area and Porosity, 2 ed.; Academic Press: London, 1982.
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Table 2. Micropore Area and External Surface Area Measure by t-Plot Model specific surface area (m2/g) t-plot sample code
multipoint BET
Langmuir
micropore area
external surface area
P 4-4-700 P 4-4-800 P 4-4-900 P 4-4-1000 HP 0-0-700 HP 0-0-800 HP 0-0-900 HP 0-0-1000 HP 4-4-700 HP 4-4-800 HP 4-4-900 HP 4-4-1000 HP4-1/3-900 COP4-1/3-900 HHP4-1/3-900 HP20-1/3-900 COP20-1/3-900 HHP20-1/3-900
14 146 176 76 166 357 519 1099 100 255 764 1087 587 428 823 829 416 1011
19 195 234 101 220 473 685 1464 132 336 1019 1486 783 567 1097 1105 554 1368
14 104 139 61 150 316 457 758 90 223 575 427 378 366 607 596 302 473
0 42 37 15 16 41 62 341 10 32 189 660 209 62 216 233 114 538
Figure 3. Stacking height of carbon layer planes and extending length of carbon layer planes of three activation methods and various activation conditions.
tion of more pores to be produced during the coactivated stage. Notably, HHP4-1/3-900 and HHP20-1/3-900 have a higher value of adsorption than HP4-1/3-900 and HP20-1/3-900. According to the t-plot model, the micropore area and external surface area of HHP20-1/3-900 are 473 and 538 m2/g,
DFT software, pore size distribution and pore volume of these samples are determined from the nitrogen adsorption isotherm data. In Figure 1e, when the concentrations of H3PO4 concentrations are 4 and 20 wt %, the difference in values of adsorption are apparent. The high content of H3PO4 leads to the produc4672
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Figure 4. (a) ESCA to identify the functional groups changing. (b) FTIR spectra of three activation methods coactivated at 900 °C.
respectively. This phenomenon is attributed to the chemical activation agent (H3PO4) during the coactivated stage, which develops volumes of mesopore. Figures 1e and 2e show similar results, in which a high concentration of H3PO4 produces greater external surface area and mesopore volume than a low concentration. Panels a and b of Figure 3, show the stacking height of layer planes (Lc) and extending length of layer planes (La), as measured by XRD and Raman analyses. Ko et al.32 demonstrated that the carbon basal planes packed together more and more, and formed a greater degree of order in the structure of ACF, caused by increasing the carbonization temperature. As is well-known, carbon with higher carbonization and better crystallite structure has a better conductivity and higher temperature treatment capability, which are an effective means of accelerating the formation of the crystallite structure of carbon and improved the degree of carbonization. Molleyre and Bastick33 indicated that oxygen gas attacks the surface of fiber and produces cracks, and slits and so on. Furthermore, the carbon layer stacking closes together and orderly on the surface of fiber, subsequently increasing Lc at the same time. Figure 3b shows the extending length of layer planes (La) of three activation methods samples. Carbon materials generally have two specific absorption bonds in Raman spectrum 1580 (G-band) and 1350 (D-band) cm-1, respectively. Intensity of D-band/G-band is the degree of structure order (R). The value of 44/R is La.34 Crystalline follows the variation of activation. The samples of coactivated with steam reveal a higher La than without steam. The coactivated with steam (small molecule) increases the opening pores and reduces the closing pores, which cause a high crystallinity. In this study, COP4-1/3-900 and
respectively. The H3PO4 and steam at a high temperature formed H4P2O7 and (HPO3)n and then formed ring-compounds in polymerization during the coactivated stage. HHP20-1/3900 is diped in a high concentration and temperature of H3PO4 and has a larger external surface area than others. COP4-1/3900 and COP20-1/3-900 have similar specific surface area and are smaller than the coactivated samples with steam at 900 °C. The similar surface area is caused by a larger molecule of carbon dioxide gas and slower rate of diffusion and activated rate of carbon materials.29 The external surface area of COP4-1/3900 and COP20-1/3-900 are 64 and 114 m2/g, respectively. The external surface area shows that the contribution of chemical activation agent (H3PO4) during the coactivated stage contributes to the development of volumes of mesopore. Figure 2 shows the pore size distribution. Samples of the traditional physical activation method produce micropores at 700, 800, and 900 °C. These samples produce mesopores by combining mircopores until coactivation occurs at 1000 °C. Samples obtained from the traditional chemical activation method produce the most of micropores. Samples obtained from the ameliorative chemical activation method produce similar numbers of mircopores and mesopores when coactivated at 900 and 1000 °C. Consequently, H3PO4 and steam at high temperature formed H4P2O7 and (HPO3)n and, then, formed ring compounds in polymerization during the coactivated stage. Recent studies30,31 have found that the samples dipped into a H3PO4 solution initiated the breaking of bonds and formed polyphosphates compounds. The best of the BET surface area in three activation methods are P44-900 (176 m2/g), HP0-0-1000 (1099 m2/g), and HP4-41000 (1087 m2/g). According to Table 2, the micropore area and external surface area of HP00-1000 are 758 and 341 m2/ g, respectively. Additionally, the micropore area and external surface area of HP4-4-1000 are 427 and 660 m2/g,
(32) Ko, T. H.; Chiranairadul, P.; Lu, C. K.; Lin, C. H. Carbon 1992, 30, 647–655. (33) Molleyre, F.; Bastick, M. Traitement de fibres de carbone pas xidation en phase gazeuse. In Proc Conf Carbon; Deutsche Keram Gesell: Baden-Baden, 1976. (34) Ko, T. H. J. Appl. Polym. Sci. 1996, 59, 577–580.
(29) Watt, W.; Johnson, W. Nature 1975, 257, 257. (30) Alonso, A. M.; Tascon, J. M. D. Carbon 2004, 42, 1419–1426. (31) Jagtoyen, M.; Derbyshire, F. Carbon 1998, 36, 1085–1097.
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Figure 5. Curves of CV capacitances with three-electrode system of three activation methods and various activation conditions.
COP20-1/3-900 have lower La and Lc values than others (Figure 3, panels c and d). Crystallinity follows the variation of activation. The coactivated with steam (small molecule) increases pores opening and reduces pores closing, subsequently causing a high crystallinity. A situation in which carbon dioxide gas (huge molecule) is used during the coactivated stage, it causes a low degree of carbonization and lowly crystalline.
Figure 4a summarizes the analysis results of ESCA to identify the functional groups changing by the binding energy (eV) of C1S. There are C-C, C-H, C-O, -COOH, and -CdO groups in Figure 4a. The C-C functional group improves with increasing treatment temperature. Additionally, HP-4-4-900 has greater C-O and -COOH functional groups than others. Figure 4b describes the FTIR spectra of samples, indicating that the band intensities of the bands of 4674
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Figure 6. Curves of relationship between CV capacitances and BET surface area of three activation methods and various activation conditions.
C-H at 2950-2850 cm-1, CtC at 2350-2300 cm-1, CdO at 1750-1700 cm-1, and CdC at 1650-1600 cm-1. Alonso and Tascon30 indicated recently that band intensities of phosphate and polyphosphate are at 1220-1180 and 1080-1070 cm-1, respectively. This finding is caused by the asymmetric stretching of P-O-C and symmetrical vibration of P-O. In P4-4-900 and HP4-4-900 spectra, there are many phosphorus groups, which are formed in the activated stage. 3.2. Electrochemical Characterization. Figure 5 shows the electrochemical performance of the electrode materials, including I-V curves, cyclic voltammetry capacitances, and Galvanostatic charge/discharge curves of various samples. This figure also displays the capacitance versus voltage profiles for these samples. These profiles are derived by the scan rate (5 mV/s).35,36 The potential ranges from is 0 to 0.75 V versus Ag/AgCl reference electrode. The steep current changing at the switching potentials reflects the quick charge propagation in the corresponding electrodes. The deviation from the imaginary rectangle is due to the pseudocapacitive effects. Evidently, reversible redox transitions involving proton exchange occurred when the samples are polarized.37 According to Figure 5, panels c and d report that the CV curves exhibited nearly rectangular shapes with a slightly
distorted, which is characteristic of an electrochemical capacitor. Figure 5, panels a and b, shows the low specific capacitance in the test range, which is attributed to be caused by low volume of pore and BET surface area at a low coactivated temperature. Figure 5e shows the capacitance versus voltage profiles for various activation conditions. Under this condition all CV curves exhibited near slightly distorted rectangular shapes, which is a characteristic of the electrochemical capacitor. The deviation from the imaginary rectangle emerged, which is due to the pseudocapacitive effects. It is evident that reversible redox transitions involving proton exchange occurred when the samples were polarized.38 In Figure 6, panels a-c, HP4-4-900 shows the largest specific capacitance (158 F/g) in the test range, although it does not have the highest BET surface area (764 m2/g). HP0-0-1000 shows a lower specific capacitance (136 m2/g), but has the highest BET surface area (1087 m2/g). Notably, although the low BET surface area causes a low specific capacitance, the high BET surface area certainly does not cause a high specific capacitance. Compared with HP0-0-900 and HP4-4-900, the higher specific capacitance is caused by the volumes of pore size. The contribution of chemical activation agent (H3PO4) during the coactivated stage contributes to the development of the numbers of mesopore. HP4-4-900 exhibits the largest specific capacitance, although HP4-4-1000 and HP0-0-1000 have a
(35) Yoon, S.; Jang, J. H.; Ka, B. H.; Oh, S. M. Electrochim. Acta 2005, 50, 2255. (36) Wu, M. S.; Chiang, P. C. Electrochem. Solid St. 2004, 7, A123. (37) Beguin, F.; Ania, C. O.; Khomenko, V.; Raymundo-Pinero, E.; Parra, J. B. Adv. Funct. Mater. 2007, 17, 1828–1836.
(38) Beguin, F.; Ania, C. O.; Khomenko, V.; Raymundo-Pinero, E.; Parra, J. B. Adv. Funct. Mater. 2007, 17, 1828.
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Figure 7. Charge/discharge curves of three activation methods and various activation conditions.
larger number of mesopores than HP4-4-900 (Table 2). Simultaneously, H3PO4 produces more oxygen functional groups such as C-O and -COOH on the sample surface. Notably, oxygen functional groups can predict the deficiency of electrons in attracting anions in electrolyte, which improves the higher specific capacitance. Figure 6d shows the relationship between CV capacitance and BET surface area, indicating that the area of CV capacitance for HP20-1/3-
900 is obviously larger than others. The highest capacitance is observed for HP20-1/3-900, whereas the capacitance of the COP4-1/3-900 and COP20-1/3-900 are the lowest. Notably, COP4-1/3-900 and COP20-1/3-900 have many narrow, thin micropores and fewer numbers of pores, which is a critical position that causes a low capacitance. Indeed, the high concentration and temperature of H3PO4 accelerates the speed of activated reaction, in which a violent 4676
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reaction produces a large amount of CO gas to increase the number of micropores. Although the BET surface area is raised, the ACF loses a substantial number of surface functional groups. Figure 7 indicates that the charge/discharge behavior of DC in 1 M H2SO4 solution is highly reversible. Restated, the discharge curves are approximately linear and symmetric to their corresponding charge curves. Galvanostatic charge/ discharge experiments are performed with various current densities ranging between 0 and 0.75 V at 2 mA in order to further investigate the performance of the samples, which are used to calculate the charge/discharge capacitive performance. The highest discharge capacitance is observed for HP4-4-900, which is attributed to a high BET surface area, wide pore size, and more oxygen functional groups. A situation in which the charge/discharge curve is approaches to the triangular shapes reflects their excellent charge/discharge capacitive performance. The IR drop against the discharge current is plotted in Figure 7b. This potential drop originates mainly from the bulk solution resistance, electrode resistance, and ion migration resistance in the electrode. Notably, the overall resistance can be estimated based on the slope of the linear relationship between the IR decline and discharge current; this resistance can be used to express the slope of straight and decreasing percentage after heattreatment. This phenomena may be attributed to the low surface utilization for its complex pore structure. In this structure, the hydrated ions have some difficulty in penetrating into the inner pores of samples. According to Figure 7d, the discharge capacitance of HP4-1/3-900, COP4-1/3900, HHP4-1/3-900, HP20-1/3-900, COP20-1/3-900, and HHP20-1/3-900 are 128, 112, 125, 180, 134, and 115 F/ g, respectively. The highest discharge capacitance is observed for HP20-1/3-900, which is attributed to the high BET surface area, wide pore size, and more oxygen functional group. The charge/discharge curves of both HHP4-1/3900 and HP20-1/3-900 have nearly triangular shapes, reflecting their excellent charge/discharge capacitive performance. Thus, HP20-1/3-900 may be more promising for
supercapaitor electrode applications owing to its ability to store the most energy. 4. Conclusion In this study, the oxidized PAN fiber as a precursor to prepare porous ACF is prepared for application in supercapaitor electrode applications by using the oxidized PAN fiber as a precursor. The electrode is formed by traditional chemical activation, traditional physical activation, and ameliorative chemical activation methods. The ameliorative chemical activation method causes more oxygen functional groups (e.g., C-O and -COOH) and an external surface area to increase the specific capacitance. Such porous ACF materials are highly promising for supercapacitors, especially for applications involving a high power output and high energy density. Additionally, the electrode materials of a supercapacitor exhibit an excellent capacitive performance and high specific capacitance of 158 F/g, which is higher than the two traditional activation methods. Under this circumstance, the ACFs were shown to be different in porous structure, stacking size of carbon layer, and capacitive performance. HHP20-1/ 3-900 has a higher BET surface area (1011 m2/g) and larger pore volume (0.47 cm3/g) than other ACF samples. However, HHP20-1/3-900 has a lower CV capacitance and DC discharge capacitance than HP20-1/3-900. Experimental results indicate that, in addition to BET surface area and pore volume, the surface oxygen functional group is also essential to capacitive performance. Comparative analysis performed under the same experimental reveals that the capacitors built with HP20-1/3-900 are superior to other supercapacitors in terms of the release of the store electric energy, compared with others for supercapacitors. Acknowledgment. We acknowledge the financial support from the Ministry of Economic Affairs, ROC (97 Program No: 97-EC-17-A-08-S1-099), National Science Council, and Feng Chia University. The FTIR analysis was supported by Dr. YuHsin Lin from the Department of Biological Science and Technology, China Medical University.
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