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Supercapacitor electrodes using nanoscale activated carbon from graphite by ball milling
Materials Letters 87 (2012) 165–168
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Supercapacitor electrodes using nanoscale activated carbon from graphite by ball milling$ R Nandhini, P.A. Mini, B. Avinash, S.V. Nair n, K.R.V. Subramanian n Amrita Centre for Nanosciences and Molecular Medicine, Ponekkara P.O., Kochi 682041, Kerala, India
a r t i c l e i n f o
abstract
Article history: Received 24 December 2011 Accepted 22 July 2012 Available online 31 July 2012
In this letter, we report on the process of preparation of a high performance supercapacitor electrode using activated carbon of nanoscale size (o 100 nm). The activated carbon was processed by highenergy ball milling of graphite followed by activation treatments with nitric acid and sulphuric acid. The activated carbon was coated on titanium substrates using electrophoretic deposition and subjected to a post-deposition annealing treatment at 100 1C. The electrophoretically deposited thin film layer ˚ that has adequate porosity with ultrafine pores (pores with diameter ranging from 20 A˚ to 100 A) constitute majority of number of pores (495%) and hence maximally contribute to the surface area of the carbon for charge storage purposes. The designed supercapacitor electrode with nanoscale activated carbon not only has excellent storage capacity (specific capacitance of 1071 F g 1 and area capacitance of 0.48 F cm 2) but also good control of discharge when used as a power source. The above process used by us is a cost-effective and novel technique, which expands the application of activated carbon for high performance supercapacitor electrodes by achieving the desired performance. & 2012 Elsevier B.V. All rights reserved.
Keywords: Activated carbon Ball milling Supercapacitor Nanoscale Energy storage and conversion Electrodeposition
1. Introduction Carbon in various modifications is the electrode material most frequently used in electrochemical capacitors. Carbon has advantages of low cost, high surface area, availability and established electrode production technologies. Activated carbon with surface area ranging from 1000 m2/g to 2500 m2/g has been used in electrochemical double layer capacitors with energy density of 18 Wh/kg [1]. Electrical double layer capacitors (EDLC) employ carbon-based materials for electrode purposes. Since carbon does not react with the electrolyte, it makes EDLC a non-faradaic type of super capacitor. The purpose of carbon-based materials is to increase the surface area of the electrode so as to allow more sites for charge storage. Energy is stored in the double-layer capacitor as charge separation in the double-layer formed at the interface between the solid electrode material surface and the liquid electrolyte in the meso/micro-pores of the electrodes. The ions displaced in forming the double-layers in the pores are transferred between the electrodes by diffusion through the
$ The information contained in this paper is patent pending (Indian Patent Application no. 3456/CHE/2011) and is protected by Indian Law. This work is sponsored by the Ministry of New and Renewable Energy, Government of India vide Grant no: 31/05/2009-10/PVSE. n Corresponding authors. Tel.: þ 91 484 285 8750; fax: þ 91 484 280 2030. E-mail addresses: [email protected] (S.V. Nair), [email protected] (K.R.V. Subramanian).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.092
electrolyte. The specific and area capacitance is dependent primarily on the characteristics of the electrode material that is, surface area, pore size distribution, i.e. active area in the pores on which the double-layer is formed. Thus, providing highly porous carbon as an electrode material can offer high stability and cyclability in EDLCs. The main problem in the production of the activated carbon for supercapacitor application is acquirement of high area capacitance, which is pore-size dependent. This limits the application of activated carbon for supercapacitor electrodes. One of the other problems is the complexity of the processes involved in producing activated carbons, which increases its cost. Specific capacitance of up to as high as 300 F/g in an aqueous electrolyte has been achieved for activated carbon electrodes [2]. The specific capacitance of carbon materials used in ultracapacitors is in the range of 75–175 F/g for aqueous electrolytes and 40–100 F/g using organic electrolytes. This is because a relatively large fraction of surface area is in pores that cannot be accessed by ions in the electrolyte [3]. Activated carbon fibres modified by ruthenium chloride showed up to 180 F/g offering good characteristics for use in supercapacitors [4]. Activated carbon fibres with high surface area and highly mesoporous structure have been prepared from polyacrylonitrile with NaOH activation. These yielded upto 371 F/g making them suitable for high performance EDLCs [5]. Very high surface area activated carbons (3000 m2/g) have yielded up to 300 F/g for use in supercapacitors [6]. There have been reports of ball milled graphite and its use as a supercapacitor electrode. Natural graphite was subjected to high
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energy ball milling and gave capacitance of only up to 205 F g 1 [7]. In another study [8], activated carbon was prepared from petroleum coke with KOH activation and ball-milled for varying times. It was found that at long hours of milling (8 h), the specific surface area and total pore volume decreased. When assembled into EDLCs, this decreases the capacitance by 6%. Mixing activated carbons (commercially purchased) with CNTs and ionic liquids yielded composite electrodes with capacitance of 188 F g 1 [9]. Ball milling graphite under H2 atmosphere yielded nanostructured graphite that showed 12 F g 1 specific capacitance [10]. When juxtaposed with ball-milling methods, literature reports exist for simple methods of production of porous, activated carbon nanofibers. In [11], carbon nanofibers with mesopores and large surface areas were obtained by electrospinning of PAN/SiO2. Carbonization and HF etching produced the nanofibers, used as anodes in lithium-ion batteries. Porous carbon nanofibers for use in lithium-ion batteries can also be produced by simple electrospinning of polymer blends (PAN/PLLA) followed by thermal treatments [12], or from PAN/ZnCl2 mixtures carbonized and activated [13]. All these literature reports on ball-milled/other processes have demonstrated electrodes with an upper limit on the specific capacitance of up to 370–380 F g 1. In our letter, we have developed a novel, but slightly elaborate process of producing nanoscale activated carbon of good quality and purity, having controlled pore size and process reproducibility which can be suitably deposited as a porous thin film in a supercapacitor device. The activated carbon was processed by high-energy ball milling of graphite followed by activation treatments with nitric acid and sulphuric acid, electrophoretic deposition and post-deposition annealing treatments. The designed supercapacitor electrode with nanoscale activated carbon not only has excellent storage capacity (far greater than literature reports), but also good control of discharge when used as a power source.
2. Materials and methods Preparation of nanoscale activated carbon: Graphite powder is taken and subjected to rotary ball milling for 10–30 h with the usage of balls made up of tungsten carbide. Speed of rotation was in the range 250–500 rpm. Ball milling is carried out in the presence of a surfactant (commonly used anionic surfactants were used such as SDBS or SDS). The sample is washed with distilled water 3–5 times. 1 g of the ball-milled graphite is mixed with 20 ml of HNO3 and ultrasonicated for 1–5 h. A given volume of the supernatant is taken and further mixed with equal parts of H2SO4 and ultrasonicated for 3 h. The resulting solution containing the carbon particles is washed with de-ionized water sufficiently (5 times) to remove traces of acid and air-dried. Electrophoretic deposition of nanoscale activated carbon and post-deposition annealing: Electrophoretic deposition of nanoscale activated carbon was carried out in a two-electrode setup with titanium substrate being the cathode, platinum wire as the anode and solution consisting of nanoscale ball-milled activated carbon, isopropyl alchohol and o5 mM nickel nitrate. Electrophoretic deposition was carried out at 20 V. The electrophoretically deposited film was then subjected to post-deposition annealing in air at 100 1C. Material and supercapacitor electrode characterization: Ballmilled and activated nanoscale carbon was characterized by SEM (Make: JEOL-JSM-6490). Raman spectrum of ball-milled nanoscale activated carbon was taken using Raman spectrometer (UHTF 300) with excitation wavelength of 488 nm. Supercapacitance studies were carried out using cyclic voltammetry in a 3-electrode setup (Autolab potentiostat/galvanostat).
The activated carbon electrode was taken as the anode, platinum wire as cathode and Ag/AgCl as the reference electrode. Scans were taken at various scan speeds in 0.1 M KOH electrolyte. Porosity and pore fraction measurements were taken with the help of BET (Make: Micromeritics, TriStar 3000 V6.07 A).
3. Results and discussion The process of high-energy ball milling introduces high shear forces, which converts the bulk graphite into nanoscale carbon together with some exfoliation. The Raman spectra shows the typical G band intense peak at 1571 cm 1, a small defect band D peak at 1357 cm 1 and 2D band peak at 2719 cm 1. Absence of the shoulder on the 2D band peak and the small defect band D peak suggests that the ball milling process has introduced a certain degree of exfoliation towards graphene-like structures [14] and also the high mechanical forces have introduced some defects in the nanoscale carbon. Fig. 1 shows the Raman spectra of the nanoscale carbon. SEM images were taken of the nanoscale carbon (Fig. 2a) and also the carbon after the chemical activation process (Fig. 2b). The nanoscale carbon shows a densely packed structure with most of the crystallite sizes being sub-100 nm. Some degree of coarsening in crystallite size was observed after the chemical activation process, which suggests the formation of internal pores and consequent apparent expansion of effective volume of the activated carbon. The electrochemical behaviour of the supercapacitor electrodes prepared by electrophoretic deposition and subsequent annealing process of the nanoscale activated carbon was studied with the help of cyclic voltammetry (CV). CV scans (Fig. 3) were taken at different scan speeds and the specific and area capacitance values computed at 100 mV s 1. The values obtained are high and repeatable (specific capacitance of 1071 F g 1 and area capacitance of 0.48 F cm 2) when compared to literature. We have also synthesized the activated carbon with different particle sizes (500 nm, 250 nm, 150 nm, and 80 nm) and found that specific capacitance decreases with coarsening of particle size. Highest capacitance values are obtained when particle size is sub-100 nm. Further, nanoscale activated carbon was used as a conductive backbone for synthesizing activated carbon/doped PEDOT composite supercapacitor electrodes [15]. The nanoscale activated carbon-doped PEDOT composite electrodes showed high specific capacitance, which is more than five times the storage capacity previously reported for activated carbon-PEDOT composites.
Fig. 1. Raman spectra of ball mill processed nanometer sized activated carbon.
R Nandhini et al. / Materials Letters 87 (2012) 165–168
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Fig. 2. SEM image of ball mill processed nanometer sized carbon (a) nanometer sized particles after ball milling in presence of surfactant and (b) certain degree of agglomeration seen after chemical activation.
Fig. 3. Cyclic voltammogram of ball mill processed nanometer sized activated carbon in 0.1 M KOH electrolyte (mass specific capacitance: 1071 F g 1; area specific capacitance: 0.48 F cm 2).
Studies were undertaken to ascertain the high supercapacitance values and its correlation to the porosity configuration of the nanoscale activated carbon. BET measurements yielded pore fraction data (Fig. 4), which showed that the activated carbon layer has adequate porosity with fine pores (pores with diameter ranging from 20 A˚ to 100 A˚ constitute majority of number of pores ( 495%) and hence maximally contribute to surface area of carbon) for charge storage purposes. The specific and area capacitance of an activated carbon EDLC is dependent primarily on the characteristics of the electrode material that is, surface area, pore size distribution i.e., active area in the pores on which the double-layer is formed. Thus, providing highly porous nanoscale carbon as a supercapacitor electrode material can offer high stability and cyclability in EDLCs.
4. Conclusions In conclusion, we were able to successfully synthesize composite supercapacitor electrodes with high electrical storage capacity made from nanoscale activated carbon processed by high-energy ball milling of graphite followed by chemical activation treatments, electrophoretic deposition and post-deposition annealing treatments. The designed supercapacitor electrode with nanoscale activated carbon having adequate porosity has excellent storage capacity (specific capacitance of 1071 F g 1 and area
Fig. 4. Graph showing pore size distribution in the activated carbon.
capacitance of 0.48 F cm 2). The above process used by us is a cost-effective and novel technique expanding the application of activated carbon for storage purposes.
Acknowledgments The authors acknowledge the Ministry of New and Renewable Energy, the Government of India, for supporting this work. The authors are also very grateful to the Amrita Institute of Medical Sciences for infrastructure support to this programme. Technical assistance of Mr. Sajin P. Ravi, Mr. S. Gowd (both ACNS, Kochi) and Department of Physics, Cochin University of Science and Technology (for ball mill usage) is also acknowledged.
References [1] Kotz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochim Acta 2000;45:2483–98. [2] Zheng JP, Jow TR. A new charge storage mechanism for electrochemical capacitors. J Electrochem Soc 1995;142:L6–8. [3] Burke A. Ultracapacitors: why, how, and where is the technology? J Power Sources 2000;91:37–50. [4] Wang CC, Hu CC. Electrochemical catalytic modification of activated carbon fabrics by ruthenium chloride for supercapacitors. Carbon 2005;43:1926–35. [5] Xu B, Wu F, Chen R, Cao G, Chen S, Zhou Z, Yang Y. Highly mesoporous and high surface area carbon: a high capacitance electrode material for EDLCs with various electrolytes. Electrochem Commun 2008;10:795–7.
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[6] Obreja VVN. On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated material—a review. Physica E 2008;40:2596–605. [7] Li HQ, Wang YG, Wang CX, Xia YY. A competitive candidate material for aqueous supercapacitors: high surface-area graphite. J Power Sources 2008;185:1557–62. [8] Zhang LM, Zhang R, Zhan L, Qiao WM, Liang XY, Ling LC. Effect of ball-milling technology on pore structure and electrochemical properties of activated carbon. J Shangai Univ 2008;12:372–6. [9] Lu W, Hartman R. Nanocomposite electrodes for high-performance supercapacitors. J Phys Chem Lett 2011;2:655–60. [10] Gomibuchi E, Ichikawa T, Kimura K, Isobe S, Nabeta K, Fujii H. Electrode properties of a double layer capacitor of nano-structured graphite produced by ball milling under a hydrogen atmosphere. Carbon 2006;44: 983–8.
[11] Liwen J, Zhan L, Andrew JM, Xiangwu Z. Porous carbon nanofibers from electrospun polyacrylonitrile/SiO2 composites as an energy storage material. Carbon 2009;47:3346–54. [12] Liwen J, Xiangwu Z. Fabrication of porous carbon nanofibers and their application as anode materials for rechargeable lithium-ion batteries. Nanotechnology 2009;20:155705–12. [13] Liwen J, Xiangwu Z. Generation of activated carbon nanofibers from electrospun polyacrylonitrile–zinc chloride composites for use as anodes in lithiumion batteries. Electrochem Commun 2009;11:684–7. [14] Weifeng Z, Ming F, Furong W, Hang W, Liwei W, Guohua C. Preparation of graphene by exfoliation of graphite using wet ball milling. J Mater Chem 2010;20:5817–9. [15] Sonia T.S., Mini P.A., Nandhini R., Sujith K., Avinash B., Nair S.V., Subramanian K.R.V. Composite supercapacitor electrodes made of activated carbon/PEDOT:PSS and activated carbon/doped PEDOT. Bull Mat Sci, accepted for publication.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Supercapacitor electrodes using nanoscale activated carbon from graphite by ball milling$ R Nandhini, P.A. Mini, B. Avinash, S.V. Nair n, K.R.V. Subramanian n Amrita Centre for Nanosciences and Molecular Medicine, Ponekkara P.O., Kochi 682041, Kerala, India
a r t i c l e i n f o
abstract
Article history: Received 24 December 2011 Accepted 22 July 2012 Available online 31 July 2012
In this letter, we report on the process of preparation of a high performance supercapacitor electrode using activated carbon of nanoscale size (o 100 nm). The activated carbon was processed by highenergy ball milling of graphite followed by activation treatments with nitric acid and sulphuric acid. The activated carbon was coated on titanium substrates using electrophoretic deposition and subjected to a post-deposition annealing treatment at 100 1C. The electrophoretically deposited thin film layer ˚ that has adequate porosity with ultrafine pores (pores with diameter ranging from 20 A˚ to 100 A) constitute majority of number of pores (495%) and hence maximally contribute to the surface area of the carbon for charge storage purposes. The designed supercapacitor electrode with nanoscale activated carbon not only has excellent storage capacity (specific capacitance of 1071 F g 1 and area capacitance of 0.48 F cm 2) but also good control of discharge when used as a power source. The above process used by us is a cost-effective and novel technique, which expands the application of activated carbon for high performance supercapacitor electrodes by achieving the desired performance. & 2012 Elsevier B.V. All rights reserved.
Keywords: Activated carbon Ball milling Supercapacitor Nanoscale Energy storage and conversion Electrodeposition
1. Introduction Carbon in various modifications is the electrode material most frequently used in electrochemical capacitors. Carbon has advantages of low cost, high surface area, availability and established electrode production technologies. Activated carbon with surface area ranging from 1000 m2/g to 2500 m2/g has been used in electrochemical double layer capacitors with energy density of 18 Wh/kg [1]. Electrical double layer capacitors (EDLC) employ carbon-based materials for electrode purposes. Since carbon does not react with the electrolyte, it makes EDLC a non-faradaic type of super capacitor. The purpose of carbon-based materials is to increase the surface area of the electrode so as to allow more sites for charge storage. Energy is stored in the double-layer capacitor as charge separation in the double-layer formed at the interface between the solid electrode material surface and the liquid electrolyte in the meso/micro-pores of the electrodes. The ions displaced in forming the double-layers in the pores are transferred between the electrodes by diffusion through the
$ The information contained in this paper is patent pending (Indian Patent Application no. 3456/CHE/2011) and is protected by Indian Law. This work is sponsored by the Ministry of New and Renewable Energy, Government of India vide Grant no: 31/05/2009-10/PVSE. n Corresponding authors. Tel.: þ 91 484 285 8750; fax: þ 91 484 280 2030. E-mail addresses: [email protected] (S.V. Nair), [email protected] (K.R.V. Subramanian).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.092
electrolyte. The specific and area capacitance is dependent primarily on the characteristics of the electrode material that is, surface area, pore size distribution, i.e. active area in the pores on which the double-layer is formed. Thus, providing highly porous carbon as an electrode material can offer high stability and cyclability in EDLCs. The main problem in the production of the activated carbon for supercapacitor application is acquirement of high area capacitance, which is pore-size dependent. This limits the application of activated carbon for supercapacitor electrodes. One of the other problems is the complexity of the processes involved in producing activated carbons, which increases its cost. Specific capacitance of up to as high as 300 F/g in an aqueous electrolyte has been achieved for activated carbon electrodes [2]. The specific capacitance of carbon materials used in ultracapacitors is in the range of 75–175 F/g for aqueous electrolytes and 40–100 F/g using organic electrolytes. This is because a relatively large fraction of surface area is in pores that cannot be accessed by ions in the electrolyte [3]. Activated carbon fibres modified by ruthenium chloride showed up to 180 F/g offering good characteristics for use in supercapacitors [4]. Activated carbon fibres with high surface area and highly mesoporous structure have been prepared from polyacrylonitrile with NaOH activation. These yielded upto 371 F/g making them suitable for high performance EDLCs [5]. Very high surface area activated carbons (3000 m2/g) have yielded up to 300 F/g for use in supercapacitors [6]. There have been reports of ball milled graphite and its use as a supercapacitor electrode. Natural graphite was subjected to high
166
R Nandhini et al. / Materials Letters 87 (2012) 165–168
energy ball milling and gave capacitance of only up to 205 F g 1 [7]. In another study [8], activated carbon was prepared from petroleum coke with KOH activation and ball-milled for varying times. It was found that at long hours of milling (8 h), the specific surface area and total pore volume decreased. When assembled into EDLCs, this decreases the capacitance by 6%. Mixing activated carbons (commercially purchased) with CNTs and ionic liquids yielded composite electrodes with capacitance of 188 F g 1 [9]. Ball milling graphite under H2 atmosphere yielded nanostructured graphite that showed 12 F g 1 specific capacitance [10]. When juxtaposed with ball-milling methods, literature reports exist for simple methods of production of porous, activated carbon nanofibers. In [11], carbon nanofibers with mesopores and large surface areas were obtained by electrospinning of PAN/SiO2. Carbonization and HF etching produced the nanofibers, used as anodes in lithium-ion batteries. Porous carbon nanofibers for use in lithium-ion batteries can also be produced by simple electrospinning of polymer blends (PAN/PLLA) followed by thermal treatments [12], or from PAN/ZnCl2 mixtures carbonized and activated [13]. All these literature reports on ball-milled/other processes have demonstrated electrodes with an upper limit on the specific capacitance of up to 370–380 F g 1. In our letter, we have developed a novel, but slightly elaborate process of producing nanoscale activated carbon of good quality and purity, having controlled pore size and process reproducibility which can be suitably deposited as a porous thin film in a supercapacitor device. The activated carbon was processed by high-energy ball milling of graphite followed by activation treatments with nitric acid and sulphuric acid, electrophoretic deposition and post-deposition annealing treatments. The designed supercapacitor electrode with nanoscale activated carbon not only has excellent storage capacity (far greater than literature reports), but also good control of discharge when used as a power source.
2. Materials and methods Preparation of nanoscale activated carbon: Graphite powder is taken and subjected to rotary ball milling for 10–30 h with the usage of balls made up of tungsten carbide. Speed of rotation was in the range 250–500 rpm. Ball milling is carried out in the presence of a surfactant (commonly used anionic surfactants were used such as SDBS or SDS). The sample is washed with distilled water 3–5 times. 1 g of the ball-milled graphite is mixed with 20 ml of HNO3 and ultrasonicated for 1–5 h. A given volume of the supernatant is taken and further mixed with equal parts of H2SO4 and ultrasonicated for 3 h. The resulting solution containing the carbon particles is washed with de-ionized water sufficiently (5 times) to remove traces of acid and air-dried. Electrophoretic deposition of nanoscale activated carbon and post-deposition annealing: Electrophoretic deposition of nanoscale activated carbon was carried out in a two-electrode setup with titanium substrate being the cathode, platinum wire as the anode and solution consisting of nanoscale ball-milled activated carbon, isopropyl alchohol and o5 mM nickel nitrate. Electrophoretic deposition was carried out at 20 V. The electrophoretically deposited film was then subjected to post-deposition annealing in air at 100 1C. Material and supercapacitor electrode characterization: Ballmilled and activated nanoscale carbon was characterized by SEM (Make: JEOL-JSM-6490). Raman spectrum of ball-milled nanoscale activated carbon was taken using Raman spectrometer (UHTF 300) with excitation wavelength of 488 nm. Supercapacitance studies were carried out using cyclic voltammetry in a 3-electrode setup (Autolab potentiostat/galvanostat).
The activated carbon electrode was taken as the anode, platinum wire as cathode and Ag/AgCl as the reference electrode. Scans were taken at various scan speeds in 0.1 M KOH electrolyte. Porosity and pore fraction measurements were taken with the help of BET (Make: Micromeritics, TriStar 3000 V6.07 A).
3. Results and discussion The process of high-energy ball milling introduces high shear forces, which converts the bulk graphite into nanoscale carbon together with some exfoliation. The Raman spectra shows the typical G band intense peak at 1571 cm 1, a small defect band D peak at 1357 cm 1 and 2D band peak at 2719 cm 1. Absence of the shoulder on the 2D band peak and the small defect band D peak suggests that the ball milling process has introduced a certain degree of exfoliation towards graphene-like structures [14] and also the high mechanical forces have introduced some defects in the nanoscale carbon. Fig. 1 shows the Raman spectra of the nanoscale carbon. SEM images were taken of the nanoscale carbon (Fig. 2a) and also the carbon after the chemical activation process (Fig. 2b). The nanoscale carbon shows a densely packed structure with most of the crystallite sizes being sub-100 nm. Some degree of coarsening in crystallite size was observed after the chemical activation process, which suggests the formation of internal pores and consequent apparent expansion of effective volume of the activated carbon. The electrochemical behaviour of the supercapacitor electrodes prepared by electrophoretic deposition and subsequent annealing process of the nanoscale activated carbon was studied with the help of cyclic voltammetry (CV). CV scans (Fig. 3) were taken at different scan speeds and the specific and area capacitance values computed at 100 mV s 1. The values obtained are high and repeatable (specific capacitance of 1071 F g 1 and area capacitance of 0.48 F cm 2) when compared to literature. We have also synthesized the activated carbon with different particle sizes (500 nm, 250 nm, 150 nm, and 80 nm) and found that specific capacitance decreases with coarsening of particle size. Highest capacitance values are obtained when particle size is sub-100 nm. Further, nanoscale activated carbon was used as a conductive backbone for synthesizing activated carbon/doped PEDOT composite supercapacitor electrodes [15]. The nanoscale activated carbon-doped PEDOT composite electrodes showed high specific capacitance, which is more than five times the storage capacity previously reported for activated carbon-PEDOT composites.
Fig. 1. Raman spectra of ball mill processed nanometer sized activated carbon.
R Nandhini et al. / Materials Letters 87 (2012) 165–168
167
Fig. 2. SEM image of ball mill processed nanometer sized carbon (a) nanometer sized particles after ball milling in presence of surfactant and (b) certain degree of agglomeration seen after chemical activation.
Fig. 3. Cyclic voltammogram of ball mill processed nanometer sized activated carbon in 0.1 M KOH electrolyte (mass specific capacitance: 1071 F g 1; area specific capacitance: 0.48 F cm 2).
Studies were undertaken to ascertain the high supercapacitance values and its correlation to the porosity configuration of the nanoscale activated carbon. BET measurements yielded pore fraction data (Fig. 4), which showed that the activated carbon layer has adequate porosity with fine pores (pores with diameter ranging from 20 A˚ to 100 A˚ constitute majority of number of pores ( 495%) and hence maximally contribute to surface area of carbon) for charge storage purposes. The specific and area capacitance of an activated carbon EDLC is dependent primarily on the characteristics of the electrode material that is, surface area, pore size distribution i.e., active area in the pores on which the double-layer is formed. Thus, providing highly porous nanoscale carbon as a supercapacitor electrode material can offer high stability and cyclability in EDLCs.
4. Conclusions In conclusion, we were able to successfully synthesize composite supercapacitor electrodes with high electrical storage capacity made from nanoscale activated carbon processed by high-energy ball milling of graphite followed by chemical activation treatments, electrophoretic deposition and post-deposition annealing treatments. The designed supercapacitor electrode with nanoscale activated carbon having adequate porosity has excellent storage capacity (specific capacitance of 1071 F g 1 and area
Fig. 4. Graph showing pore size distribution in the activated carbon.
capacitance of 0.48 F cm 2). The above process used by us is a cost-effective and novel technique expanding the application of activated carbon for storage purposes.
Acknowledgments The authors acknowledge the Ministry of New and Renewable Energy, the Government of India, for supporting this work. The authors are also very grateful to the Amrita Institute of Medical Sciences for infrastructure support to this programme. Technical assistance of Mr. Sajin P. Ravi, Mr. S. Gowd (both ACNS, Kochi) and Department of Physics, Cochin University of Science and Technology (for ball mill usage) is also acknowledged.
References [1] Kotz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochim Acta 2000;45:2483–98. [2] Zheng JP, Jow TR. A new charge storage mechanism for electrochemical capacitors. J Electrochem Soc 1995;142:L6–8. [3] Burke A. Ultracapacitors: why, how, and where is the technology? J Power Sources 2000;91:37–50. [4] Wang CC, Hu CC. Electrochemical catalytic modification of activated carbon fabrics by ruthenium chloride for supercapacitors. Carbon 2005;43:1926–35. [5] Xu B, Wu F, Chen R, Cao G, Chen S, Zhou Z, Yang Y. Highly mesoporous and high surface area carbon: a high capacitance electrode material for EDLCs with various electrolytes. Electrochem Commun 2008;10:795–7.
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R Nandhini et al. / Materials Letters 87 (2012) 165–168
[6] Obreja VVN. On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated material—a review. Physica E 2008;40:2596–605. [7] Li HQ, Wang YG, Wang CX, Xia YY. A competitive candidate material for aqueous supercapacitors: high surface-area graphite. J Power Sources 2008;185:1557–62. [8] Zhang LM, Zhang R, Zhan L, Qiao WM, Liang XY, Ling LC. Effect of ball-milling technology on pore structure and electrochemical properties of activated carbon. J Shangai Univ 2008;12:372–6. [9] Lu W, Hartman R. Nanocomposite electrodes for high-performance supercapacitors. J Phys Chem Lett 2011;2:655–60. [10] Gomibuchi E, Ichikawa T, Kimura K, Isobe S, Nabeta K, Fujii H. Electrode properties of a double layer capacitor of nano-structured graphite produced by ball milling under a hydrogen atmosphere. Carbon 2006;44: 983–8.
[11] Liwen J, Zhan L, Andrew JM, Xiangwu Z. Porous carbon nanofibers from electrospun polyacrylonitrile/SiO2 composites as an energy storage material. Carbon 2009;47:3346–54. [12] Liwen J, Xiangwu Z. Fabrication of porous carbon nanofibers and their application as anode materials for rechargeable lithium-ion batteries. Nanotechnology 2009;20:155705–12. [13] Liwen J, Xiangwu Z. Generation of activated carbon nanofibers from electrospun polyacrylonitrile–zinc chloride composites for use as anodes in lithiumion batteries. Electrochem Commun 2009;11:684–7. [14] Weifeng Z, Ming F, Furong W, Hang W, Liwei W, Guohua C. Preparation of graphene by exfoliation of graphite using wet ball milling. J Mater Chem 2010;20:5817–9. [15] Sonia T.S., Mini P.A., Nandhini R., Sujith K., Avinash B., Nair S.V., Subramanian K.R.V. Composite supercapacitor electrodes made of activated carbon/PEDOT:PSS and activated carbon/doped PEDOT. Bull Mat Sci, accepted for publication.