180

Micro and Nanosystems, 2012, 4, 180-185

Double Layer Energy Storage in Graphene - a Study
C.K. Subramaniama* and T. ...
Double Layer Energy Storage in Graphene - a Study

electrolyte. The super capacitor had specific capacitances of
135 F/g a...
182 Micro and Nanosystems, 2012, Vol. 4, No. 3

Subramaniam and Maiyalagan

Table 1. Specific Capacitance of Various Graph...
Double Layer Energy Storage in Graphene - a Study

Micro and Nanosystems, 2012, Vol. 4, No. 3

183

Fig. (1). Variation of...
184 Micro and Nanosystems, 2012, Vol. 4, No. 3

Subramaniam and Maiyalagan

Fig. (3). Cyclic voltammograms at scan rate of...
Double Layer Energy Storage in Graphene - a Study
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Double layer energy storage in graphene a study

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Double layer energy storage in graphene a study

  1. 1. 180 Micro and Nanosystems, 2012, 4, 180-185 Double Layer Energy Storage in Graphene - a Study C.K. Subramaniama* and T. Maiyalaganb a Materials Physics Division, School of Advanced Sciences, VIT University, Vellore, TN, India b Division of Chemical and Biomolecular Engineering, Nanyang Technological University, Singapore 637722 Abstract: An alternate energy storage device for high power applications are supercapacitors. They store energy either by pure electrostatic charge accumulation in the electrochemical double layer or as pseudo capacitance from fast reversible oxidation reduction process. However, they have low energy density. The electrodes in the Electrochemical Double Layer Capacitors (EDLC) are made of high surface area carbon. The carbon that can be used range from activated carbon to Graphene, with varying particle size, surface area, pore size and pore distribution. The main emphasis in the development of EDLCs is fabrication of electrodes having high surface area which would enhance the storage density of the EDLC. The EDLCs are assembled with different electrolytes which determine the operational voltage. Solid electrolytes can also be used as electrolyte and have an advantage in that we can avoid electrolyte leaks and are easy to handle. This would improve the reliability. They can also be shaped and sized to suit the application. The perflurosulfonic acid polymer as electrolyte has been used by various groups for EDLC application. The perflurosulfonic acid polymer possesses high ionic conductivity, good thermal stability, adequate mechanical strength and excellent chemical stability. The EDLCs, which are based on high-surface area carbon materials, utilize the capacitance arising from a purely non-Faradaic charge separation at an electrode/electrolyte interface. Carbon is widely used for many practical applications, especially for the adsorption of ions and molecules, as catalyst supports and electrode materials. The chemical characteristics of carbon determine the performance in all these applications. It is now possible to synthesize one-, two-, or three-dimensional (1-, 2-, or 3-D) carbons. Thus, carbon materials are very suitable candidates for super capacitor electrodes. We can overcome some of the problems in activated carbon like varying micro or meso pores, poor ion mobility due to varying pore distribution, low electrical conductivity, by using Graphene. Many forms of Graphene have been used by various groups. Graphene nanoplates (GNP), with narrow mesopore distribution have been effectively used to enhance charge storage performance. It has been found that graphene shows smaller decrease in storage capacity with increasing scan rate. Keywords: EDLC, double layer, solid electrolyte, graphene nanoplates, energy storage. INTRODUCTION Electrochemical supercapacitors have important implications in energy storage [1]. Graphene is an interesting 2- dimensional material. Studies of graphene are not limited to mono layer of graphene but also to multilayer of graphene. Graphene layers can be grown on a wide variety of transition metal substrates, with a number of layers by simple decomposition of hydrocarbons. [2-5] Bi- layer and multilayer graphene can be prepared by thermal exfoliation of graphite oxide or reduction of graphene oxide (RGO) by using suitable reducing agents [6]. Super capacitors made using graphene as electrodes have been studied with specific capacitance of the order of 120 F/gm. [7]. Graphene materials as super capacitor electrode material have also been investigated by Y. Wang et al. [8]. The Graphene was prepared from graphene oxide sheets that subsequently undergo a gas based hydrazine reduction to restore the conducting carbon network. A maximum specific capacitance of 205 F/g with a measured power density of 10 kW/kg, at energy density of 28.5 Wh/kg, in an aqueous electrolyte solution was obtained. These super capacitor devices exhibit excellent cycle life with ˜ 90% specific *Address correspondence to this author at the Materials Physics Division, School of Advanced Sciences, VIT University, Vellore, TN, India; Tel: 91-416-2243091; Fax: 91-416-2243092; E-mail: cksubra@gmail.com 1876-4037/12 $58.00+.00 capacitance retained after 1200 cycle tests. This demonstrates the commercial potential for high performance, environmentally friendly and low-cost electrical energy storage devices based on 2-D graphene material. These reports support nanosized thin graphene (NTG) as suitable electrode material for energy storage. Many forms of graphene have been used by various groups. The detail of various forms of graphene, preparation and its various applications is reviewed by V. Singh et al. [9]. Super capacitors store charge electrostatically by the adsorption of ions on to electrodes that have high accessible surface area. Therefore, a high specific capacitance active electrode plays a vital role in efficient energy storage. Various forms of porous carbon, for instance CNT [10-14], mesoporous carbon [15, 16], activated carbon [17] and carbide derived carbon [18] have been studied for electrodes in this respect. Graphene and RGO have also been predicted as a potential candidate for super capacitor electrodes due to very high specific surface area (2630 m2/g), chemical stability, excellent electrical, thermal conductivity and low cost [7, 8, 19, 20]. Interestingly, graphene has demonstrated intrinsic capacitance near 21F/cm2, that set new upper limit for capacitance. [21] Stoller et al. [7] pioneered the use of chemically reduced graphene, CRG, in super capacitor. They have shown CRG’s potential as an electrode for super capacitor, even though the used surface area was 707 m2/g and graphene sheets were not fully accessible by the © 2012 Bentham Science Publishers
  2. 2. Double Layer Energy Storage in Graphene - a Study electrolyte. The super capacitor had specific capacitances of 135 F/g and 99 F/g in aqueous KOH and organic electrolytes, respectively. Improved capacitance (191 F/g, in KOH) was obtained after using microwave power to expand GO layers and reduce the GO to RGO (surface area 463 m2/g). [14] Wang et al. [8] have achieved specific capacitance value 205 F/g for hydrazine reduce GO of effective surface area 320 m2/g. It is worth noting that the surface area of graphene sheets plays a significant role which directly affects the performance of the super capacitors. The main drawback of using graphene and RGO is the agglomeration and restacking due to Van der Waals attractive forces between the neighboring layers. The aggregation reduces the effective surface area resulting loss of capacitance. Therefore, a few researchers have made effort to keep graphene sheets separated by addition of metal oxide nanoparticles [22]. The metal oxide–graphene nano composite have shown a great promise for use in super capacitors with high energy density and high charge/discharge rates. Recent study of Liu et al., [19] has shown graphene-based super capacitor that exhibits specific energy density of 85.6 Wh/kg at room temperature and 136Wh/kg at 80 C. They have reported that the mesoporous structure of the curved graphene sheet is responsible for this. The curved nature of graphene sheets prevent face to face restacking and maintain large pore size (2–25 nm). High scan rates with minimal loss of capacitance make them good electrode materials for energy storage. Among various conductive polymers, PANI has been considered as a most promising conductive electrode material and studied considerably with CNT and other carbon system. [24-29] A graphene nanosheets–PANI composite was synthesized by in situ polymerization. [24] The specific capacitance of 1046 F/g was obtained at a scan rate of 1 mV/s. Conductive graphene nanosheets provide more active sites for nucleation of PANI and is homogeneously coated by PANI nano particles on both sides, resulting in energy density of 39 kWh/kg at a power density of 70 kW/kg. Graphene–PANI composite paper was also prepared by in situ anodic electro polymerization of polyaniline film on graphene paper. [30] The flexible as prepared composite paper combined the high conductivity showed a gravimetric capacitance of 233 F/g. Subramaniam et al. [31] have shown Graphene Nanoplatelets (GNP) to have effective energy storage capabilities. These maybe the first reports using solid ionic polymer as electrolyte and GNP as electrode material. The main features of these EDLCs are fast scan rates and high storage capacities. The EDLCs have good rate capabilities and reversibility at high scan rates. They have reported performance of EDLCs, which are based on high-surface area carbon materials, which utilize the capacitance arising from a purely non-Faradaic charge separation at an electrode/electrolyte interface. [15, 32-34] Solid electrolytes are elegant to use and very reliable. They can also be shaped and sized to suit the application. All solid EDLCs using proton conducting polymer electrolyte and various composites have been studied by different groups [36-39]. It was possible to achieve high scan rates of a few 100V/s. Micro and Nanosystems, 2012, Vol. 4, No. 3 181 D. Pech et al. [40] has reported high scan rates (250V/s) using onion nano carbon electrodes and Organic tetraethyl ammonium tetra fluro borate as electrolyte. The discharge current shows a linear behavior up to scan rates of 100V/s. Miller et al. [41] show that EDLCs can have high storage capacity, but the porous electrodes cause them to perform like resistors in filter circuits that remove ripple from rectified direct current. They have demonstrated efficient filtering of 120 hertz current with electrodes made from vertically oriented graphene nanosheets grown directly on metal current collectors. This design minimized electronic and ionic resistances and produced capacitors with RC time constants of less than 200 microseconds, in contrast with ~1 second for typical EDLCs. Gao et al. [36], have reported scan rates of 20V/s and time constants of 10 ms, for all solid EDLC with proton conducting polymer and graphite electrodes. They have used a silicotungstic acid based HPA–H3PO4–PVA polymer electrolyte, and focused on high scan rates. The reduction of ionic and electronic resistance in EDLC by using 1- and 2- dimensional material has become an increasingly important issue in power applications [42, 43]. This means that the storage capacity depends on the texture of the carbon used and the construction of the electrodes. The parameters that affect solid state EDLC electrode design are pore size, surface area, wettability, and conductivity. The gravimetric capacitance Cg is strictly determined by the electrode material and electrode structure. No simple relation exists between Cg and specific surface area (SSA). This may also be due to ion size effects [44]. Subramaniam and coworkers [15, 31-34] have reported electrochemical performance of solid state EDLCs using carbon as electrodes with ionic polymers of perfluorosulphonic acid as electrolyte. These carbon based EDLCs were assembled using Vulcan XC carbon with typical surface area of 260 m2/gm, mesoporous carbon from silica template, (CMK-3), typical surface area of 1260 m2/gm, a blend of Vulcan XC carbon and mesoporous carbon (90:10 % by weight), Graphene nano platelets (GNP) and blends of Vulcan XC carbon and GNP. In some of these EDLCs high scan rate capabilities were shown. In this article we present a short treatise on electrochemical energy storage in Graphene. EXPERIMENTAL The assembly of the EDLC has been described in detailed elsewhere. [15, 32] Carbon fiber paper TGP-H from TORAY was used as the base matrix for the electrodes. The typical size of the electrode used was 3 cm2. 2.5 mg/cm2 of the active material was coated on the surface of this matrix. Solution of the ionic polymer was used as binder in the fabrication of the electrodes. It is necessary to optimize the quantity of binder. The performance of the electrode is very sensitive to the amount of binder used for a particular particle size of the active material. Two circular carbon electrodes were assembled on either side of the solid electrolytes. The electrodes and the electrolyte were laminated by standard lamination process. This assembly was placed between two grafoil® end plates which were used as current terminals. Insulating gaskets were placed on both the internal faces of the end plates to prevent lateral shorting and to delimit the central capacitor portion and to seal the cell assembly.
  3. 3. 182 Micro and Nanosystems, 2012, Vol. 4, No. 3 Subramaniam and Maiyalagan Table 1. Specific Capacitance of Various Graphene Material No Material Specific Capacitance F/g Electrolyte Reference 1 Graphene Surface area: 2675 m2/g 550 Aq KOH 7 2 Graphene - PANI 1046 6M KOH 30 3 Graphene: Sheet Curved 100 - 250 4 Graphene: Hydrazine Reduced GO 200 KOH 8 5 Graphene: Thermally reduced 31 Aq KOH 35 6 GNP 70 PEM 31 Electrochemical Impedance spectroscopy (EIS) measurements were performed in the frequency range 100 KHz to 10 mHz. EIS was used to understand the interface characteristics. Cyclic voltammetric measurements of the cell were made in the potential range 0 to 1 V at various scan rates. All electrochemical measurements were performed at ambient conditions using a Biologic VMP electrochemical system. The electrolyte used was Nafion 212 CS with typical thickness of 50 micron. The high electronegativity (i.e. electron affinity) of the fluorine atom, bonded to the same carbon atom as the SO3H group makes the sulfonic acid a superacid (similar to trifluoromethane sulfonic acid). Nafion® has the maximum electronegative environment possible. The proton conductivity is around 0.2 S/cm for a well humidified sample. The properties of the GNP used in the electrode fabrication have been discussed elsewhere. [31] RESULTS AND DISCUSSIONS Table 1, presents the specific capacitance value of various graphene material. The specific capacitance can vary from a few tens of F/g to as high as 1046 F/g, depending on the process of preparation. This has been discussed in detail in literature. [2-35] The main drawback of using graphene and RGO is the agglomeration and restacking due to Van der Waals attractive forces between the neighboring layers. The aggregation reduces the effective surface area resulting loss of capacitance. This has been overcome by keeping graphene sheets separated with the addition of metal oxide nanoparticles [22]. The specific capacitances of the carbons depend on various factors, and that of high surface area activated carbon is dominated by space charge layer capacitance [45]. The total capacitance can be given by equation 1 [15]: C = Cmicro Smicro + Cmeso Smeso + Cext Sext (1) Where, C micro, Cmeso and Cext are the contribution to capacitance from micro pore, meso pore and external, surface area, respectively. Smicro, Smeso and Sext are the micro, meso and external surface areas, respectively. When we use different types of carbon, contributions to capacitance from micro pore surface area, Cmicro, which is dependent on current density and capacitance from meso pore surface area, Cmeso, and contributions to capacitance from other large pore, 19 Cext, which is almost a constant term has to be evaluated. This will give us an idea of the structure of electrode required for optimal charge discharge performance. The total capacitance is a function of the micro texture of the electrode. Graphene nanoplates (GNP), with narrow mesopore distribution have been effectively used to enhance charge storage performance. Wang et al. [46], suggests that the role of micro pores (< 2nm) and that of meso pores (2 – 50 nm) and other pore sizes may be different when forming double layers, in different types of electrolytes, because of difference in size and wettability of the ions to the carbon surface. This becomes very important for solid state EDLCs. One has to optimize the quantity of the ionic binder to be used in the electrode microstructure. The surface area of graphene sheets plays a significant role which directly affects the performance of the super capacitors. Barbieri et al. [47], have stated using Differential Functional Theory (DFT) model, that there exists capacitance limits to high surface area activated carbons for EDLCs. Large Specific Surface Area, SSA, leads to large gravimetric capacitance, Cg. The model predicts that above a value of 1200 m2/gm, the variation of Cg with SDFT plateaus. These limitations are due to space constriction for charge accommodation inside the pore walls. Capacitance is also limited by the space charge capacitance of the solid. Below a pore wall width of 1nm ( 1000 m2/g) the two adjacent space charge region inside the pore begin to overlap, thereby decreasing the capacitance. Also for SSA around 1200 m2/gm, the average pore wall thickness becomes close to the screening length of the electric field. Therefore, for larger SSA values, the pore wall can no longer accommodate the same amount of charge at a given electrode potential, thus causing saturation. A detailed analysis using DFT has to done for graphene and its blends. Some of the limitations in performance and in electrode structure can be circumvented by using ordered mesoporous carbons (OMC) or highly ordered mesoporous carbon (HOMC) [48, 49]. In these systems, high specific capacitance can be obtained and the capacitance show negligible dependence on potential sweep rate. Fig. (1), presents the variation of specific capacitance with scan rates for GNP and normal carbon with specially textured electrode structure. It is evident that the percentage drop in specific capacitance with increasing scan rates is much smaller for GNP, which makes it an ideal material for solid state EDLCs.
  4. 4. Double Layer Energy Storage in Graphene - a Study Micro and Nanosystems, 2012, Vol. 4, No. 3 183 Fig. (1). Variation of specific capacitance with scan rate for GNP and normal carbon. Fig. (2). Nyquist plots for the EDLCs with GNP, and blends of GNP with Vulcan XC. For applications which require high scan rates, it is necessary to understand the dynamics of the electric double layer in the frequency and time domain. The problem can be very complicated because the nature of the Nernst–Planck, Poisson, and Navier–Stokes equations makes it impossible to obtain analytical time dependent solutions except for very limited situations. Lopez et al. [50] have done some extensive analysis in this regard. They have used a network model and have obtained the transient response in the time domain. At short distances (comparable to the double layer thickness), the profiles of radial and tangential velocities are different: the latter increase with distance much faster than the former, thus demonstrating the existence of electro osmotic slip due to negative counter ions being moved by the field toward the left of the particle. Such electro osmotic flow is negligible outside the double layer and hence the tangential velocity decreases. This approach, both in the frequency and time domains, has proved useful for extracting information on the perturbation of the electric double layer induced by an external field. This approach is very useful when we want to design solid state EDLC with high rate capabilities. The exact nature of the response for GNP will be available elsewhere. The EIS measurement provides knowledge of the frequency dependence of capacitance of an EDLC. Higher the frequency dependence of the EDLC, higher will be the power response. The Nyquist plots for the EDLC assembled with GNP and blends of GNP and Vulcan XC are shown in Fig. (2). The blending of the GNP with Vulcan XC was done to see whether the texture of the electrode played a role in the frequency dependence of the capacitance of the EDLC. It
  5. 5. 184 Micro and Nanosystems, 2012, Vol. 4, No. 3 Subramaniam and Maiyalagan Fig. (3). Cyclic voltammograms at scan rate of 10 mVs-1 for GNP and blends of GNP with Vulcan XC. is evident from Fig. (2) that blending only changed the double layer interface but not very prominently, as seen from the Cyclic Voltammogram. (Fig. 3) From Fig. (3), which presents the Cyclic voltammograms at scan rate of 10 mVs-1 for GNP and blends of GNP with Vulcan XC, we see no deviations for the pure GNP and the blends. This has to be studied further to establish its cause. Also a detailed analysis at increasing scan rates will give us a better understanding of the texture of the EDLC electrode. Equivalent series resistance (ESR) is important in evaluating EDLC interface characteristics. The effective ESR is revealed as the intercept on the Z axis as . It is evident from the Fig. (2) that the ESR value is lower for the blend with 40% Vulcan XC. Since creating a good double layer interface is important the reason for this has to be examined. ACKNOWLEDGEMENTS One of the authors would like to thank Dr. G Velayutham and A. M. Prasad, Anabond Sainergy, India, for the some of the experimental work carried out and Harini Ramakrishnan, Cognisant Technologies, India, for the graphic presentations. REFERENCES [1] [2] [3] [4] CONCLUSIONS From the present study we can state that EDLCs assembled with GNP and Blends of GNP with Vulcan XC and Solid polymer electrolyte like Nafion show exceptional energy storage capabilities. They can also support high scan rates with substantial smaller capacitance drop with increasing scan rates. There exist different distinct regions of scan rates like low medium or high scan rates where we can operate depending on the application. Blending GNP with carbon like Vulcan XC does not change the storage capabilities. However, optimization of the electrode structure in terms of blend percentage, binder content and interface character in the frequency and time domain is essential. Depending on these characteristic features the EDLCs can be very commercially viable. [5] [6] [7] [8] [9] [10] [11] [12] CONFLICT OF INTEREST The author(s) confirm that this article content has no conflicts of interest. [13] Conway, B.E., Electrochemical Super capacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1999. Geim, A.K.; Novoselov, K.S. The Rise of Graphene. 2007 Nat. Mater. 6, 183-191. Geim, A.K. Graphene: Status and Prospects. 2009 Science, 324, 1530-1534. Rao, C.N.R.; Sood, A.K.; Subrahmanyam, K.S.; Govindaraj, A. Graphene: The New Two Dimensional Nanomaterial. 2009 Angew. Chem. Int. Ed. 48, 7752-7777. Allen, M.J.; Tung, V.C.; Kaner, R.B. Honeycomb Carbon: A Review of Graphene, Chem. Rev. 2009 DOI: 10.1021/cr900070d. Park, S.; Ruoff, R.S. Chemical Methods for Production of Graphene. Nat. Nanotechnol, 2009 4, 217-224. Stoller, M.D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R.S. Graphene Based Ultra Capacitor. Nano Lett. 2008 8(10), 3498-3502. Wang, Y.; Shi, Z.; Huang, Y.; Ma, Y.; Wang, C.; Chen, M.; Chen, Y. Super capacitor Based on Graphene Material. J. Phys. Chem. C, 2009 113, 13103-13107. Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S.I.; Seal, S. Graphene Based Materials: Past Present and Future. Progress in Mat. Sci, 2011 56, 1178-1271. Frackowiak, E.; Metenier, K.; Bertagna, V.; Beguin, F. Super capacitor electrodes from multiwalled carbon nanotubes. Appl. Phys. Lett., 2000 77(15), 2421-2423. Niu, C.; Sichel, E.K.; Hoch, R.; Moy, D.; Tennent, H. High power electrochemical capacitors based on carbon nanotube electrodes. 1997 Appl. Phys. Lett., 70, 1480-1482. An, K.H.; Kim, W.S.; Park, Y.S.; Choi, Y.C.; Lee, S.M.; Chung, D.C. Super capacitors using single-walled carbon nanotube electrodes. Adv. Mater, 2001 13, 497-500. Du, C.; Yeh, Y.; Pan, N. High power density super capacitors using locally aligned carbon nanotube electrodes, Nanotechnol., 2005 16, 350-353.
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