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Electrical capacitance of fibrous carbon composites in supercapacitors
Fuel Processing Technology 77 – 78 (2002) 181 – 189 www.elsevier.com/locate/fuproc
Electrical capacitance of fibrous carbon composites in supercapacitors Krzysztof Babel a,1, Krzysztof Jurewicz b,* a
Institute of Chemical Wood Technology, Agricultural Academy in Poznan´, ul. Wojska Polskiego 38/42, 60-637 Poznan´, Poland b Institute of Chemistry and Technical Electrochemistry, Poznan´ University of Technology, ul. Piotrowo 3, 60-965 Poznan´, Poland Received 4 February 2002; received in revised form 20 March 2002; accepted 25 March 2002
Abstract The aim of this work was the application of active carbon composites as electrode material for supercapacitors. We have produced and investigated composites from viscose cellulose fibers impregnated with novolak and resolic resins. Composition and porous structure of the composites were described and electrochemical properties determined by galvanostatic and potentiodynamic methods. Dependence of electrical capacitance on treatment procedure and some of the structural parameters was confirmed. The use of novolak resin for activation with carbon dioxide was more advantageous. Positive electrode revealed better performance in acidic conditions (185 F/g) while negative electrode in alkaline conditions (160 F/g). D 2002 Elsevier Science B.V. All rights reserved. Keywords: C – C composite; Supercapacitor; Active carbon
1. Introduction Active carbons have been recognized as the main candidate for electrode material in electrochemical capacitors due to their highly developed surface, sometimes exceeding 2000 m2/g, at relatively low production costs [1]. Degree of utilization of their large surface area during charging and discharging of electrical double layer depends on their wettability, which results mainly from structural and chemical composition of carbon
*
Corresponding author. E-mail addresses: [email protected] (K. Babe»), [email protected] (K. Jurewicz). 1 Tel.: +48-061-848-7435; fax: +48-061-848-7452. 0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 0 7 0 - X
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material. The same parameters are important for charge exchange dynamics, which is relevant for high energy capacitors. The existence of given quantities and types of surface functional groups on carbon surface allows for multiplication of purely electrostatic capacitance by enhancing the pseudocapacitive effects [2]. Active carbon as electrode material must be additionally characterized by good electrical conductivity and high resistance to electrolyte attack. Composite materials might be expected to meet these miscellaneous requirements. By combination of individual features of the precursor components it is possible to extend the range of the parameters modification and, hence, capacity characteristics of electrode materials. Following this assumption we have produced and investigated cellulosephenolic fiber C –C composites. Viscose cellulose, as the main carbon composite component, reveals in active fibers a structure with high degree of ordering [3,4] with narrow slit pores [5,6] between graphene planes. Such pore distribution, which is characteristic for microporous materials, evolves in the direction of pore size increase and development of considerable number of mesopores when the conditions of thermal treatment become more severe. In the case of formaldehyde-phenolic resins used in the same conditions, the number of uniform pores with rather microporous structure is increasing [5– 9]. Additionally, each of the composite precursors shows a tendency to generate different chemical structure. In the case of resins, acidic surface groups of phenol type are predominating. During charging and discharging of electrode material they are more stable than typical for cellulose precursor, carboxyl groups [10].
2. Experimental As fibrous component for the production of carbon composite, standard cellulose fibers Argona (manufactured by ZWCH WISTONA, Poland) were used. Fibers were incrusted with phenolic resins (produced by ZTS ERG Pustko´w, Poland): resolic type in the form of aqueous solution or novolak resin in alcohol solution. Precursor composite was produced by vacuum impregnation of viscose fibers with phenolic resin solutions at concentration of 10 or 25 wt.%. Fibers saturated with resolic resin were acidified in hydrochloric acid to pH 4 and heated to the temperature of 170 jC. Novolak resin fibers were cured by hexamethylenetetraamine at 155 jC. Fiber pyrolysis was carried out in oxygen-free atmosphere to the final temperature of 600 or 900 jC. Carbonization products were activated by carbon dioxide or by steam at temperature of 800 or 850 jC. Porous structure of active carbon composites was investigated by the measurements of nitrogen adsorption at 196 jC using apparatus Micromeritics ASAP 2010. Electrochemical investigations were made using three-electrode vessel with mercury sulfate reference electrode for acidic conditions (4 M H2SO4) or mercury oxide electrode for alkaline conditions (7 M KOH). Capacity parameters were defined by galvanostatic measurements at current density from 100 to 500 mA/g and potentiodynamic measurements at sweep rate 2 mV/s in the full range of voltage change (50 –1050 mV) on capacitor. Charge exchange dynamics was described on the basis of voltammogram
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Table 1 Parameters of thermal treatment and capacitances of active carbon composites Sample Resin Activator Temperature (jC)
Capacitance (F/g) Positive electrode
Negative electrode
Carbonization Activation Galvanostatic
2 mV/s
DC
2 mV/s
DC
Acid electrolyte R-1 R-10 CO2 R-2 R-10 CO2 R-3 R-10 CO2 R-4 R-25 CO2 R-5 R-10 H2O N-1 N-10 CO2 N-2 N-10 CO2 N-3 N-10 CO2 N-4 N-25 CO2 N-5 N-10 H2O N-6 N-10 H2O
600 600 900 600 600 600 600 900 600 600 900
800 850 800 800 800 800 850 800 800 800 800
185 154 165 158 141 178 162 167 148 136 118
173 141 167 175 155 160 185 171 98 163 147
57 65 51 63 48 64 49 50 78 34 34
157 124 146 138 123 144 164 145 94 131 114
53 60 45 53 35 60 42 41 77 18 15
Alkaline electrolyte R-2 R-10 CO2 R-7 R-25 CO2 N-2 N-10 CO2
600 600 600
850 850 850
107 75 118
92 65 97
55 58 31
125 121 160
67 78 58
shape changes and drop of capacity (DC) at increased potential sweep to 10 mV/s (Table 1).
3. Results and discussion The use of two types of resins for the impregnation of cellulose fibers allowed to obtain composites with different morphology (Fig. 1). Novolak resin in alcohol solution is a good fiber wetting agent and, due to melting ability below cross-linking temperature, it shows a tendency to form laminar coatings. For resolic resins the situation is just the opposite— since it is precipitated from acidified water solutions, it locates in cellulose fiber rather in definite places. As an infusible material, during thermal treatment it generates globular structures, which do not shield the fibers. This has a direct impact on the thermal formation of porous structure, being a resultant of the components used. Novolak resin gives the composite its characteristic microporous structure (adsorption isotherm type I) and protects less resistant cellulose against excessive gasification, even at higher activation temperatures (Fig. 2a). In case of resolic resin, the composite is more reactive and at the same activation temperature, results in considerable increase of mesopores (Fig. 2b—isotherm type IV) with the contribution of both components. Incrustation of cellulose fibers by novolak resin alcohol solution was more efficient. With 25 wt.% solution it was possible to obtain 71% mass fraction of resin in carbon precursor composite, while with 10 wt.% solution, 25% mass fraction was obtained. For resolic resin, the resulting mass fraction was considerably lower, giving 27% and 11%, respectively.
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Fig. 1. SEM pictures of active carbon composites made of phenolic resin impregnated cellulose fibers. (a) novolak resin composite; (b) resolic resin composite.
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Fig. 2. Isotherms of nitrogen adsorption on carbon dioxide activated composite fibers; (a) with resolic resin, activation at 800 jC; (b) with resolic resin, activation at 850 jC. (c) with novolak resin, activation at 850 jC;
For acidic conditions, the behavior of composites obtained with CO2 activator, depending on the kind and quantity of resin, is given in Fig. 3. For sample with comparable, intermediate resin content (N-2 and R-3), potentiodynamic curves have the
Fig. 3. Influence of resin type and content on potentiodynamic curves for selected carbon composites in 4 M H2SO4 electrolyte (carbonization: 600 jC, CO2 activation: 800 jC). Gravimetric resin content in precursor material: resolic resin: R-1—11%, R-4 — 27%; novolak resin: N-1 — 25%, N-4—71%.
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same shape but reveal apparently higher capacity in case of resolic resin. Additionally, resolic resin in negative electrodes with similar capacities benefits in definitely better charge exchange dynamics. Advantageous capacity effect, and particularly the improvement of negative electrode dynamics, results from at least double decrease of resolic resin content (sample R-1). On the other hand, triple increase of novolak resin content (sample N-4) causes a sudden deterioration of capacity and dynamics of both electrodes, probably due to deposition of thick microporous carbon coating, which inhibits wetting of the surface and charge transportation. Comparably better capacity parameters in acidic conditions for both electrodes of the capacitor were obtained for sample N-2 produced from precursor material containing 25 wt.% of novolak resin, when the temperature of carbon dioxide activation was increased from 800 to 850 jC (Fig. 4). It can be explained by the evolution of beneficial porous structure with reasonably increased mesopore content. (Fig. 2c). This assumption is confirmed by good dynamic properties of both electrodes. If resolic resin was used in the same conditions, capacity parameters were apparently deteriorated. It was not possible to obtain the expected improvement of parameters at increased carbonization temperature from 600 to 900 jC, although galvanostatic measurements of such material confirmed almost a double decrease in ohmic drop. It becomes clear that the composites used as positive electrode materials show higher capacities. As negative electrode materials, they reveal better charge exchange dynamics,
Fig. 4. Influence of carbonization and activation temperature on potentiodynamic curves for selected carbon composites. Electrolyte 4 M H2SO4, potential sweep 2 mV/s. R-2, N-2 — carbonization: 600 jC, activation: 850 jC. R-3, N-3 — carbonization: 900 jC, activation: 800 jC.
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especially when steam was used for activation (Fig. 5 —sample N-5). Steam, being more aggressive activator, apparently improves the shape of potentiodynamic curves for positive electrodes, particularly in the initial step of discharge. It is probably due to the evolution of more advantageous porous structure dominated by larger micropores, and pores on the boundary of micro- and mesopores. This fact might be essential for the production of cellulose-phenolic composites used in high power superconductors. In alkaline conditions, the behavior of composites was opposite to that in acidic conditions. Capacitance characteristics were deteriorated. From a practical point of view, only negative electrodes could be interesting. For example, reasonable capacity of 160 F/g and good dynamic properties were obtained for novolak resin composites produced with CO2 activator (Fig. 6 — sample N-2). Novolak and resolic resins had modifying effect on porous structure and chemical properties of cellulose fibers and, in consequence, the electrochemical properties of composite material. If novolak resins are used, which apparently change the structure of cellulose fibers, positive electrode reveals higher capacities after activation with CO2 or better charge exchange dynamics after activation with H2O. Resolic resins, which sum up their properties with that of cellulose, allow to produce electrodes more similar to the cellulose active carbon fibers. The best solution seems to be the addition of reasonable amount of resins. If added in excess quantity, they can deteriorate charge exchange
Fig. 5. Influence of carbonization temperature and the kind of activator on potentiodynamic curves for novolak resin composites. Activation temperature 800 jC, electrolyte 4 M H2SO4, potential sweep 2 mV/s. N-1: 600 jC, CO2 activation; N-5: 600 jC, H2O activation; N-6: 900 jC, H2O activation.
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Fig. 6. Influence of resin type and content on potentiodynamic curves for selected carbon composites in 7 M KOH electrolyte, potential sweep 2 mV/s (carbonization: 600 jC; CO2 activation: 850 jC). Gravimetric resin content in precursor material: resolic resin: R-3 —11%, R-7—27%; novolak resin: N-2 — 25%.
dynamics of resolic resin composites or cause capacity drop of novolak resin composites.
Acknowledgements This work has been done under research project 7 TO8E 01019 of the Polish Committee for Scientific Research.
References [1] [2] [3] [4] [5]
R. Kon tz, M. Carlen, Electrochim. Acta 45 (2000) 2483. B.G. Conway, Electrochemical Supercapacitors, Kluwer Academic/Plenum, NY, 1999. F. Rodriguez-Reinoso, A.C. Pastor, H. Marsh, A. Huidobro, Carbon 38 (1999) 397. J.B. Parra, J.C. de Sousa, J.J. Pis, J.A. Pajares, R.C. Bansal, Carbon 33 (1995) 801. M.C. Roman Martinez, D. Cazorla-Amoros, A. Lineres-Solano, C. Salinas-Martinez de Lecea, F. Atamny, Carbon 34 (1996) 719. [6] J. Oya, S. Yoshida, J. Alcaniz-Monge, A. Linares-Solano, Carbon 33 (1995) 1085. [7] N. Yoshizawa, Y. Yamada, M. Shiraishi, XRD and TEM Analysis of Disordered Structure of Carbon
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189
Materials with Low Crystallinity, Carbon ’99—24th Biennial Conference on Carbon, American Carbon Society, Ext. Abstr., USA (July 1999) 240. [8] K.A. Trick, T.E. Saliba, S.S. Sandhu, Carbon 35 (1997) 393. [9] J.S. Hayes, Activated Carbon Fibre and Fabric, American Kynol, Pleasantville, NY, 1994. [10] A. Yoshida, I. Tanashashi, A. Nishino, Carbon 28 (1990) 610.
Electrical capacitance of fibrous carbon composites in supercapacitors Krzysztof Babel a,1, Krzysztof Jurewicz b,* a
Institute of Chemical Wood Technology, Agricultural Academy in Poznan´, ul. Wojska Polskiego 38/42, 60-637 Poznan´, Poland b Institute of Chemistry and Technical Electrochemistry, Poznan´ University of Technology, ul. Piotrowo 3, 60-965 Poznan´, Poland Received 4 February 2002; received in revised form 20 March 2002; accepted 25 March 2002
Abstract The aim of this work was the application of active carbon composites as electrode material for supercapacitors. We have produced and investigated composites from viscose cellulose fibers impregnated with novolak and resolic resins. Composition and porous structure of the composites were described and electrochemical properties determined by galvanostatic and potentiodynamic methods. Dependence of electrical capacitance on treatment procedure and some of the structural parameters was confirmed. The use of novolak resin for activation with carbon dioxide was more advantageous. Positive electrode revealed better performance in acidic conditions (185 F/g) while negative electrode in alkaline conditions (160 F/g). D 2002 Elsevier Science B.V. All rights reserved. Keywords: C – C composite; Supercapacitor; Active carbon
1. Introduction Active carbons have been recognized as the main candidate for electrode material in electrochemical capacitors due to their highly developed surface, sometimes exceeding 2000 m2/g, at relatively low production costs [1]. Degree of utilization of their large surface area during charging and discharging of electrical double layer depends on their wettability, which results mainly from structural and chemical composition of carbon
*
Corresponding author. E-mail addresses: [email protected] (K. Babe»), [email protected] (K. Jurewicz). 1 Tel.: +48-061-848-7435; fax: +48-061-848-7452. 0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 0 7 0 - X
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material. The same parameters are important for charge exchange dynamics, which is relevant for high energy capacitors. The existence of given quantities and types of surface functional groups on carbon surface allows for multiplication of purely electrostatic capacitance by enhancing the pseudocapacitive effects [2]. Active carbon as electrode material must be additionally characterized by good electrical conductivity and high resistance to electrolyte attack. Composite materials might be expected to meet these miscellaneous requirements. By combination of individual features of the precursor components it is possible to extend the range of the parameters modification and, hence, capacity characteristics of electrode materials. Following this assumption we have produced and investigated cellulosephenolic fiber C –C composites. Viscose cellulose, as the main carbon composite component, reveals in active fibers a structure with high degree of ordering [3,4] with narrow slit pores [5,6] between graphene planes. Such pore distribution, which is characteristic for microporous materials, evolves in the direction of pore size increase and development of considerable number of mesopores when the conditions of thermal treatment become more severe. In the case of formaldehyde-phenolic resins used in the same conditions, the number of uniform pores with rather microporous structure is increasing [5– 9]. Additionally, each of the composite precursors shows a tendency to generate different chemical structure. In the case of resins, acidic surface groups of phenol type are predominating. During charging and discharging of electrode material they are more stable than typical for cellulose precursor, carboxyl groups [10].
2. Experimental As fibrous component for the production of carbon composite, standard cellulose fibers Argona (manufactured by ZWCH WISTONA, Poland) were used. Fibers were incrusted with phenolic resins (produced by ZTS ERG Pustko´w, Poland): resolic type in the form of aqueous solution or novolak resin in alcohol solution. Precursor composite was produced by vacuum impregnation of viscose fibers with phenolic resin solutions at concentration of 10 or 25 wt.%. Fibers saturated with resolic resin were acidified in hydrochloric acid to pH 4 and heated to the temperature of 170 jC. Novolak resin fibers were cured by hexamethylenetetraamine at 155 jC. Fiber pyrolysis was carried out in oxygen-free atmosphere to the final temperature of 600 or 900 jC. Carbonization products were activated by carbon dioxide or by steam at temperature of 800 or 850 jC. Porous structure of active carbon composites was investigated by the measurements of nitrogen adsorption at 196 jC using apparatus Micromeritics ASAP 2010. Electrochemical investigations were made using three-electrode vessel with mercury sulfate reference electrode for acidic conditions (4 M H2SO4) or mercury oxide electrode for alkaline conditions (7 M KOH). Capacity parameters were defined by galvanostatic measurements at current density from 100 to 500 mA/g and potentiodynamic measurements at sweep rate 2 mV/s in the full range of voltage change (50 –1050 mV) on capacitor. Charge exchange dynamics was described on the basis of voltammogram
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Table 1 Parameters of thermal treatment and capacitances of active carbon composites Sample Resin Activator Temperature (jC)
Capacitance (F/g) Positive electrode
Negative electrode
Carbonization Activation Galvanostatic
2 mV/s
DC
2 mV/s
DC
Acid electrolyte R-1 R-10 CO2 R-2 R-10 CO2 R-3 R-10 CO2 R-4 R-25 CO2 R-5 R-10 H2O N-1 N-10 CO2 N-2 N-10 CO2 N-3 N-10 CO2 N-4 N-25 CO2 N-5 N-10 H2O N-6 N-10 H2O
600 600 900 600 600 600 600 900 600 600 900
800 850 800 800 800 800 850 800 800 800 800
185 154 165 158 141 178 162 167 148 136 118
173 141 167 175 155 160 185 171 98 163 147
57 65 51 63 48 64 49 50 78 34 34
157 124 146 138 123 144 164 145 94 131 114
53 60 45 53 35 60 42 41 77 18 15
Alkaline electrolyte R-2 R-10 CO2 R-7 R-25 CO2 N-2 N-10 CO2
600 600 600
850 850 850
107 75 118
92 65 97
55 58 31
125 121 160
67 78 58
shape changes and drop of capacity (DC) at increased potential sweep to 10 mV/s (Table 1).
3. Results and discussion The use of two types of resins for the impregnation of cellulose fibers allowed to obtain composites with different morphology (Fig. 1). Novolak resin in alcohol solution is a good fiber wetting agent and, due to melting ability below cross-linking temperature, it shows a tendency to form laminar coatings. For resolic resins the situation is just the opposite— since it is precipitated from acidified water solutions, it locates in cellulose fiber rather in definite places. As an infusible material, during thermal treatment it generates globular structures, which do not shield the fibers. This has a direct impact on the thermal formation of porous structure, being a resultant of the components used. Novolak resin gives the composite its characteristic microporous structure (adsorption isotherm type I) and protects less resistant cellulose against excessive gasification, even at higher activation temperatures (Fig. 2a). In case of resolic resin, the composite is more reactive and at the same activation temperature, results in considerable increase of mesopores (Fig. 2b—isotherm type IV) with the contribution of both components. Incrustation of cellulose fibers by novolak resin alcohol solution was more efficient. With 25 wt.% solution it was possible to obtain 71% mass fraction of resin in carbon precursor composite, while with 10 wt.% solution, 25% mass fraction was obtained. For resolic resin, the resulting mass fraction was considerably lower, giving 27% and 11%, respectively.
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Fig. 1. SEM pictures of active carbon composites made of phenolic resin impregnated cellulose fibers. (a) novolak resin composite; (b) resolic resin composite.
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Fig. 2. Isotherms of nitrogen adsorption on carbon dioxide activated composite fibers; (a) with resolic resin, activation at 800 jC; (b) with resolic resin, activation at 850 jC. (c) with novolak resin, activation at 850 jC;
For acidic conditions, the behavior of composites obtained with CO2 activator, depending on the kind and quantity of resin, is given in Fig. 3. For sample with comparable, intermediate resin content (N-2 and R-3), potentiodynamic curves have the
Fig. 3. Influence of resin type and content on potentiodynamic curves for selected carbon composites in 4 M H2SO4 electrolyte (carbonization: 600 jC, CO2 activation: 800 jC). Gravimetric resin content in precursor material: resolic resin: R-1—11%, R-4 — 27%; novolak resin: N-1 — 25%, N-4—71%.
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same shape but reveal apparently higher capacity in case of resolic resin. Additionally, resolic resin in negative electrodes with similar capacities benefits in definitely better charge exchange dynamics. Advantageous capacity effect, and particularly the improvement of negative electrode dynamics, results from at least double decrease of resolic resin content (sample R-1). On the other hand, triple increase of novolak resin content (sample N-4) causes a sudden deterioration of capacity and dynamics of both electrodes, probably due to deposition of thick microporous carbon coating, which inhibits wetting of the surface and charge transportation. Comparably better capacity parameters in acidic conditions for both electrodes of the capacitor were obtained for sample N-2 produced from precursor material containing 25 wt.% of novolak resin, when the temperature of carbon dioxide activation was increased from 800 to 850 jC (Fig. 4). It can be explained by the evolution of beneficial porous structure with reasonably increased mesopore content. (Fig. 2c). This assumption is confirmed by good dynamic properties of both electrodes. If resolic resin was used in the same conditions, capacity parameters were apparently deteriorated. It was not possible to obtain the expected improvement of parameters at increased carbonization temperature from 600 to 900 jC, although galvanostatic measurements of such material confirmed almost a double decrease in ohmic drop. It becomes clear that the composites used as positive electrode materials show higher capacities. As negative electrode materials, they reveal better charge exchange dynamics,
Fig. 4. Influence of carbonization and activation temperature on potentiodynamic curves for selected carbon composites. Electrolyte 4 M H2SO4, potential sweep 2 mV/s. R-2, N-2 — carbonization: 600 jC, activation: 850 jC. R-3, N-3 — carbonization: 900 jC, activation: 800 jC.
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especially when steam was used for activation (Fig. 5 —sample N-5). Steam, being more aggressive activator, apparently improves the shape of potentiodynamic curves for positive electrodes, particularly in the initial step of discharge. It is probably due to the evolution of more advantageous porous structure dominated by larger micropores, and pores on the boundary of micro- and mesopores. This fact might be essential for the production of cellulose-phenolic composites used in high power superconductors. In alkaline conditions, the behavior of composites was opposite to that in acidic conditions. Capacitance characteristics were deteriorated. From a practical point of view, only negative electrodes could be interesting. For example, reasonable capacity of 160 F/g and good dynamic properties were obtained for novolak resin composites produced with CO2 activator (Fig. 6 — sample N-2). Novolak and resolic resins had modifying effect on porous structure and chemical properties of cellulose fibers and, in consequence, the electrochemical properties of composite material. If novolak resins are used, which apparently change the structure of cellulose fibers, positive electrode reveals higher capacities after activation with CO2 or better charge exchange dynamics after activation with H2O. Resolic resins, which sum up their properties with that of cellulose, allow to produce electrodes more similar to the cellulose active carbon fibers. The best solution seems to be the addition of reasonable amount of resins. If added in excess quantity, they can deteriorate charge exchange
Fig. 5. Influence of carbonization temperature and the kind of activator on potentiodynamic curves for novolak resin composites. Activation temperature 800 jC, electrolyte 4 M H2SO4, potential sweep 2 mV/s. N-1: 600 jC, CO2 activation; N-5: 600 jC, H2O activation; N-6: 900 jC, H2O activation.
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Fig. 6. Influence of resin type and content on potentiodynamic curves for selected carbon composites in 7 M KOH electrolyte, potential sweep 2 mV/s (carbonization: 600 jC; CO2 activation: 850 jC). Gravimetric resin content in precursor material: resolic resin: R-3 —11%, R-7—27%; novolak resin: N-2 — 25%.
dynamics of resolic resin composites or cause capacity drop of novolak resin composites.
Acknowledgements This work has been done under research project 7 TO8E 01019 of the Polish Committee for Scientific Research.
References [1] [2] [3] [4] [5]
R. Kon tz, M. Carlen, Electrochim. Acta 45 (2000) 2483. B.G. Conway, Electrochemical Supercapacitors, Kluwer Academic/Plenum, NY, 1999. F. Rodriguez-Reinoso, A.C. Pastor, H. Marsh, A. Huidobro, Carbon 38 (1999) 397. J.B. Parra, J.C. de Sousa, J.J. Pis, J.A. Pajares, R.C. Bansal, Carbon 33 (1995) 801. M.C. Roman Martinez, D. Cazorla-Amoros, A. Lineres-Solano, C. Salinas-Martinez de Lecea, F. Atamny, Carbon 34 (1996) 719. [6] J. Oya, S. Yoshida, J. Alcaniz-Monge, A. Linares-Solano, Carbon 33 (1995) 1085. [7] N. Yoshizawa, Y. Yamada, M. Shiraishi, XRD and TEM Analysis of Disordered Structure of Carbon
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Materials with Low Crystallinity, Carbon ’99—24th Biennial Conference on Carbon, American Carbon Society, Ext. Abstr., USA (July 1999) 240. [8] K.A. Trick, T.E. Saliba, S.S. Sandhu, Carbon 35 (1997) 393. [9] J.S. Hayes, Activated Carbon Fibre and Fabric, American Kynol, Pleasantville, NY, 1994. [10] A. Yoshida, I. Tanashashi, A. Nishino, Carbon 28 (1990) 610.