- Email: [email protected]
carbon nanotubes composite electrodes for high energy density supercapacitors
Letters to the Editor / Carbon 42 (2004) 423–460
451
Electrochemical characterization on RuO2 Æ xH2O/carbon nanotubes composite electrodes for high energy density supercapacitors X. Qin a
a,*
, S. Durbach a, G.T. Wu
b
Department of Chemistry, Technikon Witwatersrand, Doorfontein, P.O. Box 17011, 2028 Johannesburg, South Africa b Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan Received 19 August 2003; accepted 11 November 2003
Keywords: A. Carbon nanotubes, electrodes; D. Electrochemical properties
Carbon nanotubes (CNTs) are among the most promising nanomaterials that have been discovered, with potential applications as nanodevices, field emission displays and energy storage devices [1–4]. Hollow, open-ended, well-ordered CNTs are, however, required for these applications. The anodic aluminum oxide (AAO) template method is used to produce such CNTs, which are more suitable for filling than normal twisted CNTs [5,6]. In our experiments, commercially available AAO templates (Whatman ) were used in the production of CNTs. Electrochemical capacitors are unique energy storage devices that exhibit high power density and long cycle life [7]. Since CNTs have high surface area polarizablity, low resistance and high stability, this suggests that they could be suitable for electrochemical double layer capacitors [7,8]. Similarly, RuO2 is the important material in such capacitors, and has been studied extensively [9–12]. In this paper, we report the preparation and electrochemical characterization of a new type of RuO2 Æ xH2 O/CNTs composite electrode. Highly ordered CNTs were grown by flowing a 1:10 volume ratio of acetylene to nitrogen (110 ml min1 ) for 15–45 min at 900 C over Whatman AAO templates (200 nm diameter; 60 lm thickness). The samples were then annealed in nitrogen for 10 h at this temperature. The resulting CNTs–AAO samples were divided into two quantities. The first, used as a standard, was immersed in an HF solution at room temperature to remove the AAO template. The second was used to produce the RuO2 Æ xH2 O/CNTs composite. Initially the CNTs–AAO samples were immersed in a 0.1 mol l1 solution of RuCl3 in 20% HCl and evacuated for 3 h. Thereafter, they were placed in an ultrasonic bath, dispersed in a 1 mol l1 solution of NaOH and evacuated for 3 h. Several batches of these composite materials *
Corresponding author. Tel.: +27-11-406-2329; fax: +27-11-4062761. E-mail address: [email protected] (X. Qin). 0008-6223/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2003.11.012
were prepared then annealed at 120 C in air. The RuO2 Æ xH2 O/CNTs–AAO samples were then immersed in an HF solution as before. About 20% of the weight filled in the CNTs was RuO2 Æ xH2 O. Both the CNTs and RuO2 Æ xH2 O/CNTs composite materials were characterized by a scanning electron microscope (SEM, JEOL JSM-6700F) and a transmission electron microscope (TEM, JEOL JEM-100S). Electrodes were then prepared by addition of either highly ordered CNTs or RuO2 Æ xH2 O/CNTs composite to 5 wt% Nafion solution, dispersed in isopropyl alcohol. The dry mass content of Nafion in the electrode active layer was 25 wt%. Electrodes were then immersed in a 1 mol l1 H2 SO4 solution and evacuated for 1 h before measurement of their electrochemical properties by cyclic voltammetry (CV) in the threeelectrode cell. CV experiments on these electrodes were performed using a Solartron SI1280B electrochemistry unit. Micrographs of the synthesized nanotubes are shown in Fig. 1. These correspond to samples obtained after removing the alumina template. From Fig. 1, openended nanotubes display a highly ordered array with very thin walls and have almost uniform size. The average outer diameters are 200 nm, which are similar to the pore diameters of the Whatman AAO template. Micrographs of the CNTs after being filled with RuO2 Æ xH2 O are shown in Fig. 2. It can be seen that RuO2 Æ xH2 O filled the inner pores of the CNTs when compared to Fig. 1(b). Also RuO2 Æ xH2 O was found to be uniformly distributed in the carbon tube, with only a small fraction of the inner walls of CNTs not having been covered. The outer walls of the composite CNTs were smooth with no RuO2 Æ xH2 O on them. Hence the composite CNTs had low contact resistance due to the good conductivity of the pure CNTs. CVs were performed in the potential range of )0.40 to 0.40 V vs. Hg/Hg2 SO4 to obtain a measurement of the capacitance of the electrodes. The total capacitance from Fig. 3 was calculated by: C ¼ mvi , where m is the
452
Letters to the Editor / Carbon 42 (2004) 423–460
Fig. 2. TEM images of the RuO2 Æ xH2 O/CNTs composite.
2 5
I/mA
1 0
-5
-0.4
-0.2
0.0
0.2
0.4
Potential vs Hg/Hg2SO4 Fig. 3. Cyclic voltammogram (sweep rate: 5 mV/s) of the electrodes in 1 mol l1 H2 SO4 . (1) Highly ordered CNTs electrode, (2) RuO2 Æ xH2 O/ CNTs composite electrode. Fig. 1. The images of the highly ordered CNT. (a) SEM of highly ordered CNTs and (b) TEM of highly ordered CNTs.
Thus, RuO2 /CNTs composite could be a promising candidate for use as high energy density electrochemical supercapacitors.
mass of active material, i is the even current response and v is the potential sweep rate. The specific capacitances of the RuO2 Æ xH2 O/CNTs composite electrode and the highly ordered CNTs electrode were 295 and 27 F g1 , respectively. The results indicated that the capacitance of RuO2 Æ xH2 O/CNTs composite electrode was up to 10 times higher than the unfilled CNTs electrode. It is most likely that there is a significant increase in the capacitance due to a pseudocapacitance of RuO2 Æ xH2 O, which makes the response of this type of electrode similar to that generated by the capacitor (RuO2 ). Both of these can be oxidized and reduced reversibly through the following electrochemical protonation:
References
RuO2 þ dHþ þ de ! RuO2d ðOHÞd
0 6 d 6 1½9
[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Collins PG, Arnold MS, Avouris P. Engineering carbon nanotubes and nanotube circuits using electrical breakdown. Science 2001;292:706–9. [3] Yuan ZH, Huang H, Dang HY, Cao JE, Hu BH, Fan SS. Field emission property of highly ordered monodispersed carbon nanotube arrays. Appl Phys Lett 2001;78(20):3127–9. [4] Wang GX, Ahn J, Yao J, Lindsay M, Liu HK, Dou SX. Preparation and characterization of carbon nanotubes for energy storage. J Power Sources 2003;119–121:16–23. [5] Li J, Papadopoulos C, Xu JM. Highly ordered carbon nanotube arrays for electronics applications. Appl Phys Lett 1999; 75(3):367–9. [6] Sui YC, Gonzalez-Le on JA, Berm udez A, Saniger JM. Synthesis of multi branched carbon nanotubes in porous anodic aluminum oxide template. Carbon 2001;39(11):1709–15.
Letters to the Editor / Carbon 42 (2004) 423–460 [7] Frackowiak E, Beguin F. Carbon materials for the electrochemical storage of energy in capacitors. Carbon 2001;39(6):937– 50. [8] Chen JH, Li WZ, Wang DZ, Yang SX, Wen JG, Ren ZF. Electrochemical characterization of carbon nanotubes as electrode in electrochemical double-layer capacitors. Carbon 2002;40(8): 1193–7. [9] Ramani M, Haran BS, White RE, Popov BN. Synthesis and characterization of hydrous ruthenium oxide–carbon supercapacitors. J Electrochem Soc 2001;148(4):A374–80.
453
[10] Park JH, Park OO. Morphology and electrochemical behaviour of ruthenium oxide thin film deposited on carbon paper. J Power Sources 2002;109(1):121–6. [11] Zhang J, Jiang D, Chen B, Zhu J, Jiang L, Fang H. Preparation and electrochemistry of hydrous ruthenium oxide/active carbon electrode materials for supercapacitor. J Electrochem Soc 2001;148(12):A1362–7. [12] Zhang JP, Cygan PJ, Jow TR. Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J Electrochem Soc 1995;142(8):2699–703.
Production of activated carbon from candlenut shell by CO2 activation M. Turmuzi a, W.R.W. Daud a, S.M. Tasirin a, M.S. Takriff a, S.E. Iyuke a
b,*
Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, DE, Malaysia Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
b
Received 21 January 2003; accepted 26 November 2003
Keywords: A. Activated carbon; B. Activation; C. Adsorption; D. Porosity
Almost any carbonaceous material with low organic volatile contents, high in elementary carbon and sufficient strength can be converted into activated carbon [1]. However, for carbonaceous materials that provide mass yields below 25%, the volatile losses are so high as to nearly preclude their use [2]. The price of carbonaceous material also influences the choice of precursor. Agricultural solid wastes that are available in large quantities, such as coconut shell [2], palm kernel shell [3], apricot stone [4] and walnut shell [5] have been used as raw materials for the preparation of activated carbons. The aim of this study therefore, is to investigate the feasibility of preparing activated carbon from agricultural solid, waste candlenut shell, and to study the influence of time and activation temperature as pore structures are produced. Candlenut shell from Sumatra, Indonesia, was used as the raw material for the activated carbon production. Candlenut shells were ground in a crusher and sieved to particle sizes of 1.7–2.35 mm. The ground candlenut shells were carbonized in a vertical furnace (ID 77 mm) equipped with automatic temperature control systems. 25 g candlenut shells were placed on a perforated crucible. The crucible was placed into the furnace and then heated at rate of 8 C/min to the final temperature of 700 C, for 1 h in nitrogen gas flow of 105 ml/min to ensure complete removal of the volatile matter and tarry materials. In the physical activation, nitrogen gas flow rate *
Corresponding author. Tel.: +60-389-466-294; fax: +60-386-567099. E-mail address: [email protected] (S.E. Iyuke). 0008-6223/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2003.11.015
was reduced to 60 ml/min and CO2 gas was supplied at a rate of 70 ml/min. Two different series of physical activation conditions were chosen: (a) Series 1: temperature was fixed at 800 C and hold times were varied at 0.5, 2, 3, 4, 5 and 6 h. (b) Series 2: hold time was fixed at 1 h and temperatures were varied at 700, 750, 800 and 900 C. Burn-off was calculated based on the weight difference of char which pyrolysed at 700 C and the weight of activated carbon after activation. The activated carbon pore structures were characterized by nitrogen adsorption at 77 K using an automatic equipment (Quantachrome Autosorb-1C) at an equilibrium time of 7 min. The narrow micropore volume was obtained from carbon dioxide isotherm measured at 273 K by using Dubinin–Raduskhevich (DR) equation [6]. Activated carbon yield in the study is defined as the ratio of sample weight after activation to raw material weight. The effects of activation temperature and hold time on the yield of activated carbon are shown in Fig. 1. In series 1, activated carbon yield decreased linearly as hold time was increased for a fixed activation temperature. Decreasing yield was due to oxidation by reaction between carbon and carbon dioxide. In series 2, activated carbon yield also decreased linearly as temperature was increased for a fixed hold time due to the increased oxidation rate at higher reaction temperatures. The adsorption isotherms of nitrogen (at 77 K) on activated carbons obtained by carbon dioxide activation and on the unactivated char are presented in Fig. 2. The adsorption isotherms exhibit type I in the BDDT classification [7]. These indicated that activated carbons and
451
Electrochemical characterization on RuO2 Æ xH2O/carbon nanotubes composite electrodes for high energy density supercapacitors X. Qin a
a,*
, S. Durbach a, G.T. Wu
b
Department of Chemistry, Technikon Witwatersrand, Doorfontein, P.O. Box 17011, 2028 Johannesburg, South Africa b Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan Received 19 August 2003; accepted 11 November 2003
Keywords: A. Carbon nanotubes, electrodes; D. Electrochemical properties
Carbon nanotubes (CNTs) are among the most promising nanomaterials that have been discovered, with potential applications as nanodevices, field emission displays and energy storage devices [1–4]. Hollow, open-ended, well-ordered CNTs are, however, required for these applications. The anodic aluminum oxide (AAO) template method is used to produce such CNTs, which are more suitable for filling than normal twisted CNTs [5,6]. In our experiments, commercially available AAO templates (Whatman ) were used in the production of CNTs. Electrochemical capacitors are unique energy storage devices that exhibit high power density and long cycle life [7]. Since CNTs have high surface area polarizablity, low resistance and high stability, this suggests that they could be suitable for electrochemical double layer capacitors [7,8]. Similarly, RuO2 is the important material in such capacitors, and has been studied extensively [9–12]. In this paper, we report the preparation and electrochemical characterization of a new type of RuO2 Æ xH2 O/CNTs composite electrode. Highly ordered CNTs were grown by flowing a 1:10 volume ratio of acetylene to nitrogen (110 ml min1 ) for 15–45 min at 900 C over Whatman AAO templates (200 nm diameter; 60 lm thickness). The samples were then annealed in nitrogen for 10 h at this temperature. The resulting CNTs–AAO samples were divided into two quantities. The first, used as a standard, was immersed in an HF solution at room temperature to remove the AAO template. The second was used to produce the RuO2 Æ xH2 O/CNTs composite. Initially the CNTs–AAO samples were immersed in a 0.1 mol l1 solution of RuCl3 in 20% HCl and evacuated for 3 h. Thereafter, they were placed in an ultrasonic bath, dispersed in a 1 mol l1 solution of NaOH and evacuated for 3 h. Several batches of these composite materials *
Corresponding author. Tel.: +27-11-406-2329; fax: +27-11-4062761. E-mail address: [email protected] (X. Qin). 0008-6223/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2003.11.012
were prepared then annealed at 120 C in air. The RuO2 Æ xH2 O/CNTs–AAO samples were then immersed in an HF solution as before. About 20% of the weight filled in the CNTs was RuO2 Æ xH2 O. Both the CNTs and RuO2 Æ xH2 O/CNTs composite materials were characterized by a scanning electron microscope (SEM, JEOL JSM-6700F) and a transmission electron microscope (TEM, JEOL JEM-100S). Electrodes were then prepared by addition of either highly ordered CNTs or RuO2 Æ xH2 O/CNTs composite to 5 wt% Nafion solution, dispersed in isopropyl alcohol. The dry mass content of Nafion in the electrode active layer was 25 wt%. Electrodes were then immersed in a 1 mol l1 H2 SO4 solution and evacuated for 1 h before measurement of their electrochemical properties by cyclic voltammetry (CV) in the threeelectrode cell. CV experiments on these electrodes were performed using a Solartron SI1280B electrochemistry unit. Micrographs of the synthesized nanotubes are shown in Fig. 1. These correspond to samples obtained after removing the alumina template. From Fig. 1, openended nanotubes display a highly ordered array with very thin walls and have almost uniform size. The average outer diameters are 200 nm, which are similar to the pore diameters of the Whatman AAO template. Micrographs of the CNTs after being filled with RuO2 Æ xH2 O are shown in Fig. 2. It can be seen that RuO2 Æ xH2 O filled the inner pores of the CNTs when compared to Fig. 1(b). Also RuO2 Æ xH2 O was found to be uniformly distributed in the carbon tube, with only a small fraction of the inner walls of CNTs not having been covered. The outer walls of the composite CNTs were smooth with no RuO2 Æ xH2 O on them. Hence the composite CNTs had low contact resistance due to the good conductivity of the pure CNTs. CVs were performed in the potential range of )0.40 to 0.40 V vs. Hg/Hg2 SO4 to obtain a measurement of the capacitance of the electrodes. The total capacitance from Fig. 3 was calculated by: C ¼ mvi , where m is the
452
Letters to the Editor / Carbon 42 (2004) 423–460
Fig. 2. TEM images of the RuO2 Æ xH2 O/CNTs composite.
2 5
I/mA
1 0
-5
-0.4
-0.2
0.0
0.2
0.4
Potential vs Hg/Hg2SO4 Fig. 3. Cyclic voltammogram (sweep rate: 5 mV/s) of the electrodes in 1 mol l1 H2 SO4 . (1) Highly ordered CNTs electrode, (2) RuO2 Æ xH2 O/ CNTs composite electrode. Fig. 1. The images of the highly ordered CNT. (a) SEM of highly ordered CNTs and (b) TEM of highly ordered CNTs.
Thus, RuO2 /CNTs composite could be a promising candidate for use as high energy density electrochemical supercapacitors.
mass of active material, i is the even current response and v is the potential sweep rate. The specific capacitances of the RuO2 Æ xH2 O/CNTs composite electrode and the highly ordered CNTs electrode were 295 and 27 F g1 , respectively. The results indicated that the capacitance of RuO2 Æ xH2 O/CNTs composite electrode was up to 10 times higher than the unfilled CNTs electrode. It is most likely that there is a significant increase in the capacitance due to a pseudocapacitance of RuO2 Æ xH2 O, which makes the response of this type of electrode similar to that generated by the capacitor (RuO2 ). Both of these can be oxidized and reduced reversibly through the following electrochemical protonation:
References
RuO2 þ dHþ þ de ! RuO2d ðOHÞd
0 6 d 6 1½9
[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Collins PG, Arnold MS, Avouris P. Engineering carbon nanotubes and nanotube circuits using electrical breakdown. Science 2001;292:706–9. [3] Yuan ZH, Huang H, Dang HY, Cao JE, Hu BH, Fan SS. Field emission property of highly ordered monodispersed carbon nanotube arrays. Appl Phys Lett 2001;78(20):3127–9. [4] Wang GX, Ahn J, Yao J, Lindsay M, Liu HK, Dou SX. Preparation and characterization of carbon nanotubes for energy storage. J Power Sources 2003;119–121:16–23. [5] Li J, Papadopoulos C, Xu JM. Highly ordered carbon nanotube arrays for electronics applications. Appl Phys Lett 1999; 75(3):367–9. [6] Sui YC, Gonzalez-Le on JA, Berm udez A, Saniger JM. Synthesis of multi branched carbon nanotubes in porous anodic aluminum oxide template. Carbon 2001;39(11):1709–15.
Letters to the Editor / Carbon 42 (2004) 423–460 [7] Frackowiak E, Beguin F. Carbon materials for the electrochemical storage of energy in capacitors. Carbon 2001;39(6):937– 50. [8] Chen JH, Li WZ, Wang DZ, Yang SX, Wen JG, Ren ZF. Electrochemical characterization of carbon nanotubes as electrode in electrochemical double-layer capacitors. Carbon 2002;40(8): 1193–7. [9] Ramani M, Haran BS, White RE, Popov BN. Synthesis and characterization of hydrous ruthenium oxide–carbon supercapacitors. J Electrochem Soc 2001;148(4):A374–80.
453
[10] Park JH, Park OO. Morphology and electrochemical behaviour of ruthenium oxide thin film deposited on carbon paper. J Power Sources 2002;109(1):121–6. [11] Zhang J, Jiang D, Chen B, Zhu J, Jiang L, Fang H. Preparation and electrochemistry of hydrous ruthenium oxide/active carbon electrode materials for supercapacitor. J Electrochem Soc 2001;148(12):A1362–7. [12] Zhang JP, Cygan PJ, Jow TR. Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J Electrochem Soc 1995;142(8):2699–703.
Production of activated carbon from candlenut shell by CO2 activation M. Turmuzi a, W.R.W. Daud a, S.M. Tasirin a, M.S. Takriff a, S.E. Iyuke a
b,*
Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, DE, Malaysia Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
b
Received 21 January 2003; accepted 26 November 2003
Keywords: A. Activated carbon; B. Activation; C. Adsorption; D. Porosity
Almost any carbonaceous material with low organic volatile contents, high in elementary carbon and sufficient strength can be converted into activated carbon [1]. However, for carbonaceous materials that provide mass yields below 25%, the volatile losses are so high as to nearly preclude their use [2]. The price of carbonaceous material also influences the choice of precursor. Agricultural solid wastes that are available in large quantities, such as coconut shell [2], palm kernel shell [3], apricot stone [4] and walnut shell [5] have been used as raw materials for the preparation of activated carbons. The aim of this study therefore, is to investigate the feasibility of preparing activated carbon from agricultural solid, waste candlenut shell, and to study the influence of time and activation temperature as pore structures are produced. Candlenut shell from Sumatra, Indonesia, was used as the raw material for the activated carbon production. Candlenut shells were ground in a crusher and sieved to particle sizes of 1.7–2.35 mm. The ground candlenut shells were carbonized in a vertical furnace (ID 77 mm) equipped with automatic temperature control systems. 25 g candlenut shells were placed on a perforated crucible. The crucible was placed into the furnace and then heated at rate of 8 C/min to the final temperature of 700 C, for 1 h in nitrogen gas flow of 105 ml/min to ensure complete removal of the volatile matter and tarry materials. In the physical activation, nitrogen gas flow rate *
Corresponding author. Tel.: +60-389-466-294; fax: +60-386-567099. E-mail address: [email protected] (S.E. Iyuke). 0008-6223/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2003.11.015
was reduced to 60 ml/min and CO2 gas was supplied at a rate of 70 ml/min. Two different series of physical activation conditions were chosen: (a) Series 1: temperature was fixed at 800 C and hold times were varied at 0.5, 2, 3, 4, 5 and 6 h. (b) Series 2: hold time was fixed at 1 h and temperatures were varied at 700, 750, 800 and 900 C. Burn-off was calculated based on the weight difference of char which pyrolysed at 700 C and the weight of activated carbon after activation. The activated carbon pore structures were characterized by nitrogen adsorption at 77 K using an automatic equipment (Quantachrome Autosorb-1C) at an equilibrium time of 7 min. The narrow micropore volume was obtained from carbon dioxide isotherm measured at 273 K by using Dubinin–Raduskhevich (DR) equation [6]. Activated carbon yield in the study is defined as the ratio of sample weight after activation to raw material weight. The effects of activation temperature and hold time on the yield of activated carbon are shown in Fig. 1. In series 1, activated carbon yield decreased linearly as hold time was increased for a fixed activation temperature. Decreasing yield was due to oxidation by reaction between carbon and carbon dioxide. In series 2, activated carbon yield also decreased linearly as temperature was increased for a fixed hold time due to the increased oxidation rate at higher reaction temperatures. The adsorption isotherms of nitrogen (at 77 K) on activated carbons obtained by carbon dioxide activation and on the unactivated char are presented in Fig. 2. The adsorption isotherms exhibit type I in the BDDT classification [7]. These indicated that activated carbons and