Double Layer Energy Storage in Graphene - a Study Micro and Nanosystems, 2012, Vol. 4, No. 3 181electrolyte. The super capacitor had specific capacitances of D. Pech et al.  has reported high scan rates (250V/s)135 F/g and 99 F/g in aqueous KOH and organic using onion nano carbon electrodes and Organic tetraethylelectrolytes, respectively. Improved capacitance (191 F/g, in ammonium tetra fluro borate as electrolyte. The dischargeKOH) was obtained after using microwave power to expand current shows a linear behavior up to scan rates of 100V/s.GO layers and reduce the GO to RGO (surface area 463 Miller et al.  show that EDLCs can have high storagem2/g).  Wang et al.  have achieved specific capacity, but the porous electrodes cause them to performcapacitance value 205 F/g for hydrazine reduce GO of like resistors in filter circuits that remove ripple fromeffective surface area 320 m2/g. It is worth noting that the rectified direct current. They have demonstrated efficientsurface area of graphene sheets plays a significant role which filtering of 120 hertz current with electrodes made fromdirectly affects the performance of the super capacitors. vertically oriented graphene nanosheets grown directly on The main drawback of using graphene and RGO is the metal current collectors. This design minimized electronic and ionic resistances and produced capacitors with RC timeagglomeration and restacking due to Van der Waals constants of less than 200 microseconds, in contrast with ~1attractive forces between the neighboring layers. The second for typical EDLCs. Gao et al. , have reported scanaggregation reduces the effective surface area resulting loss rates of 20V/s and time constants of 10 ms, for all solid EDLCof capacitance. Therefore, a few researchers have made with proton conducting polymer and graphite electrodes. Theyeffort to keep graphene sheets separated by addition of metaloxide nanoparticles . have used a silicotungstic acid based HPA–H3PO4–PVA polymer electrolyte, and focused on high scan rates. The metal oxide–graphene nano composite have shown a The reduction of ionic and electronic resistance ingreat promise for use in super capacitors with high energy EDLC by using 1- and 2- dimensional material has becomedensity and high charge/discharge rates. Recent study of Liu an increasingly important issue in power applications [42,et al.,  has shown graphene-based super capacitor thatexhibits specific energy density of 85.6 Wh/kg at room 43]. This means that the storage capacity depends on the texture of the carbon used and the construction of thetemperature and 136Wh/kg at 80 C. They have reported that electrodes. The parameters that affect solid state EDLCthe mesoporous structure of the curved graphene sheet is electrode design are pore size, surface area, wettability, andresponsible for this. The curved nature of graphene sheets conductivity. The gravimetric capacitance Cg is strictlyprevent face to face restacking and maintain large pore size determined by the electrode material and electrode structure.(2–25 nm). High scan rates with minimal loss of capacitancemake them good electrode materials for energy storage. No simple relation exists between Cg and specific surface area (SSA). This may also be due to ion size effects . Among various conductive polymers, PANI has been Subramaniam and coworkers [15, 31-34] have reportedconsidered as a most promising conductive electrode electrochemical performance of solid state EDLCs usingmaterial and studied considerably with CNT and other carbon as electrodes with ionic polymers ofcarbon system. [24-29] A graphene nanosheets–PANI perfluorosulphonic acid as electrolyte. These carbon basedcomposite was synthesized by in situ polymerization.  EDLCs were assembled using Vulcan XC carbon with typicalThe specific capacitance of 1046 F/g was obtained at a scan surface area of 260 m2/gm, mesoporous carbon from silicarate of 1 mV/s. Conductive graphene nanosheets provide template, (CMK-3), typical surface area of 1260 m2/gm, amore active sites for nucleation of PANI and is blend of Vulcan XC carbon and mesoporous carbon (90:10 %homogeneously coated by PANI nano particles on both by weight), Graphene nano platelets (GNP) and blends ofsides, resulting in energy density of 39 kWh/kg at a power Vulcan XC carbon and GNP. In some of these EDLCs highdensity of 70 kW/kg. Graphene–PANI composite paper was scan rate capabilities were shown. In this article we present aalso prepared by in situ anodic electro polymerization of short treatise on electrochemical energy storage in Graphene.polyaniline film on graphene paper.  The flexible asprepared composite paper combined the high conductivity EXPERIMENTALshowed a gravimetric capacitance of 233 F/g. The assembly of the EDLC has been described in Subramaniam et al.  have shown Graphene detailed elsewhere. [15, 32] Carbon fiber paper TGP-H fromNanoplatelets (GNP) to have effective energy storage TORAY was used as the base matrix for the electrodes. Thecapabilities. These maybe the first reports using solid ionic typical size of the electrode used was 3 cm2. 2.5 mg/cm2 ofpolymer as electrolyte and GNP as electrode material. The the active material was coated on the surface of this matrix.main features of these EDLCs are fast scan rates and high Solution of the ionic polymer was used as binder in thestorage capacities. The EDLCs have good rate capabilities fabrication of the electrodes. It is necessary to optimize theand reversibility at high scan rates. They have reported quantity of binder. The performance of the electrode is veryperformance of EDLCs, which are based on high-surface sensitive to the amount of binder used for a particular particlearea carbon materials, which utilize the capacitance arising size of the active material. Two circular carbon electrodesfrom a purely non-Faradaic charge separation at an were assembled on either side of the solid electrolytes. Theelectrode/electrolyte interface. [15, 32-34] Solid electrolytes electrodes and the electrolyte were laminated by standardare elegant to use and very reliable. They can also be shaped lamination process. This assembly was placed between twoand sized to suit the application. All solid EDLCs using grafoil® end plates which were used as current terminals.proton conducting polymer electrolyte and various Insulating gaskets were placed on both the internal faces ofcomposites have been studied by different groups [36-39]. It the end plates to prevent lateral shorting and to delimit thewas possible to achieve high scan rates of a few 100V/s. central capacitor portion and to seal the cell assembly.
182 Micro and Nanosystems, 2012, Vol. 4, No. 3 Subramaniam and MaiyalaganTable 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 19 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) Cext, which is almost a constant term has to be evaluated.measurements were performed in the frequency range 100 This will give us an idea of the structure of electrodeKHz to 10 mHz. EIS was used to understand the interface required for optimal charge discharge performance.characteristics. Cyclic voltammetric measurements of the The total capacitance is a function of the micro texturecell were made in the potential range 0 to 1 V at various scan of the electrode. Graphene nanoplates (GNP), with narrowrates. All electrochemical measurements were performed at mesopore distribution have been effectively used to enhanceambient conditions using a Biologic VMP electrochemical charge storage performance. Wang et al. , suggests thatsystem. the role of micro pores (< 2nm) and that of meso pores (2 – The electrolyte used was Nafion 212 CS with typical 50 nm) and other pore sizes may be different when formingthickness of 50 micron. The high electronegativity (i.e. double layers, in different types of electrolytes, because ofelectron affinity) of the fluorine atom, bonded to the same difference in size and wettability of the ions to the carboncarbon atom as the SO3H group makes the sulfonic acid a surface. This becomes very important for solid state EDLCs.superacid (similar to trifluoromethane sulfonic acid). One has to optimize the quantity of the ionic binder to beNafion® has the maximum electronegative environment used in the electrode microstructure. The surface area ofpossible. The proton conductivity is around 0.2 S/cm for a graphene sheets plays a significant role which directlywell humidified sample. affects the performance of the super capacitors. The properties of the GNP used in the electrode Barbieri et al. , have stated using Differentialfabrication have been discussed elsewhere.  Functional Theory (DFT) model, that there exists capacitance limits to high surface area activated carbons forRESULTS AND DISCUSSIONS EDLCs. Large Specific Surface Area, SSA, leads to large Table 1, presents the specific capacitance value of gravimetric capacitance, Cg. The model predicts that above avarious graphene material. The specific capacitance can value of 1200 m2/gm, the variation of Cg with SDFT plateaus.vary from a few tens of F/g to as high as 1046 F/g, These limitations are due to space constriction for chargedepending on the process of preparation. This has been accommodation inside the pore walls. Capacitance is alsodiscussed in detail in literature. [2-35] limited by the space charge capacitance of the solid. Below a pore wall width of 1nm ( 1000 m2/g) the two adjacent space The main drawback of using graphene and RGO is the charge region inside the pore begin to overlap, therebyagglomeration and restacking due to Van der Waals decreasing the capacitance. Also for SSA around 1200attractive forces between the neighboring layers. The m2/gm, the average pore wall thickness becomes close to theaggregation reduces the effective surface area resulting loss screening length of the electric field. Therefore, for largerof capacitance. This has been overcome by keeping graphene SSA values, the pore wall can no longer accommodate thesheets separated with the addition of metal oxide same amount of charge at a given electrode potential, thusnanoparticles . causing saturation. A detailed analysis using DFT has to The specific capacitances of the carbons depend on done for graphene and its blends.various factors, and that of high surface area activated Some of the limitations in performance and in electrodecarbon is dominated by space charge layer capacitance . structure can be circumvented by using ordered mesoporousThe total capacitance can be given by equation 1 : carbons (OMC) or highly ordered mesoporous carbonC = Cmicro Smicro + Cmeso Smeso + Cext Sext (1) (HOMC) [48, 49]. In these systems, high specific capacitance can be obtained and the capacitance show Where, C micro, Cmeso and Cext are the contribution to negligible dependence on potential sweep rate.capacitance from micro pore, meso pore and external,surface area, respectively. Smicro, Smeso and Sext are the micro, Fig. (1), presents the variation of specific capacitancemeso and external surface areas, respectively. When we use with scan rates for GNP and normal carbon with speciallydifferent types of carbon, contributions to capacitance from textured electrode structure. It is evident that the percentagemicro pore surface area, Cmicro, which is dependent on drop in specific capacitance with increasing scan rates iscurrent density and capacitance from meso pore surface area, much smaller for GNP, which makes it an ideal material forCmeso, and contributions to capacitance from other large pore, solid state EDLCs.
Double Layer Energy Storage in Graphene - a Study Micro and Nanosystems, 2012, Vol. 4, No. 3 183Fig. (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 tangential velocity decreases. This approach, both in thenecessary to understand the dynamics of the electric double frequency and time domains, has proved useful forlayer in the frequency and time domain. The problem can be extracting information on the perturbation of the electricvery complicated because the nature of the Nernst–Planck, double layer induced by an external field. This approach isPoisson, and Navier–Stokes equations makes it impossible to very useful when we want to design solid state EDLC withobtain analytical time dependent solutions except for very high rate capabilities. The exact nature of the response forlimited situations. Lopez et al.  have done some GNP will be available elsewhere.extensive analysis in this regard. They have used a networkmodel and have obtained the transient response in the time The EIS measurement provides knowledge of thedomain. At short distances (comparable to the double layer frequency dependence of capacitance of an EDLC. Higherthickness), the profiles of radial and tangential velocities are the frequency dependence of the EDLC, higher will be thedifferent: the latter increase with distance much faster than power response. The Nyquist plots for the EDLC assembledthe former, thus demonstrating the existence of electro with GNP and blends of GNP and Vulcan XC are shown inosmotic slip due to negative counter ions being moved by Fig. (2). The blending of the GNP with Vulcan XC was donethe field toward the left of the particle. Such electro osmotic to see whether the texture of the electrode played a role inflow is negligible outside the double layer and hence the the frequency dependence of the capacitance of the EDLC. It
184 Micro and Nanosystems, 2012, Vol. 4, No. 3 Subramaniam and MaiyalaganFig. (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 ACKNOWLEDGEMENTSdouble layer interface but not very prominently, as seen from One of the authors would like to thank Dr. Gthe Cyclic Voltammogram. (Fig. 3) From Fig. (3), which Velayutham and A. M. Prasad, Anabond Sainergy, India, forpresents the Cyclic voltammograms at scan rate of 10 mVs-1 the some of the experimental work carried out and Harinifor GNP and blends of GNP with Vulcan XC, we see no Ramakrishnan, Cognisant Technologies, India, for thedeviations for the pure GNP and the blends. This has to be graphic presentations.studied further to establish its cause. Also a detailed analysisat increasing scan rates will give us a better understanding of REFERENCESthe texture of the EDLC electrode. Equivalent series  Conway, B.E., Electrochemical Super capacitors: Scientificresistance (ESR) is important in evaluating EDLC interface Fundamentals and Technological Applications, Kluwercharacteristics. The effective ESR is revealed as the intercept Academic/Plenum Publishers, New York, 1999.on the Z axis as . It is evident from the Fig. (2) that  Geim, A.K.; Novoselov, K.S. The Rise of Graphene. 2007 Nat. Mater. 6, 183-191.the ESR value is lower for the blend with 40% Vulcan XC.  Geim, A.K. Graphene: Status and Prospects. 2009 Science, 324,Since creating a good double layer interface is important the 1530-1534.reason for this has to be examined.  Rao, C.N.R.; Sood, A.K.; Subrahmanyam, K.S.; Govindaraj, A. Graphene: The New Two Dimensional Nanomaterial. 2009CONCLUSIONS Angew. Chem. Int. Ed. 48, 7752-7777.  Allen, M.J.; Tung, V.C.; Kaner, R.B. Honeycomb Carbon: A From the present study we can state that EDLCs Review of Graphene, Chem. Rev. 2009 DOI: 10.1021/cr900070d.assembled with GNP and Blends of GNP with Vulcan XC  Park, S.; Ruoff, R.S. Chemical Methods for Production of Graphene. Nat. Nanotechnol, 2009 4, 217-224.and Solid polymer electrolyte like Nafion show exceptional  Stoller, M.D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R.S. Grapheneenergy storage capabilities. They can also support high scan Based Ultra Capacitor. Nano Lett. 2008 8(10), 3498-3502.rates with substantial smaller capacitance drop with  Wang, Y.; Shi, Z.; Huang, Y.; Ma, Y.; Wang, C.; Chen, M.; Chen,increasing scan rates. There exist different distinct regions of Y. Super capacitor Based on Graphene Material. J. Phys. Chem. C, 2009 113, 13103-13107.scan rates like low medium or high scan rates where we can  Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S.I.; Seal, S.operate depending on the application. Blending GNP with Graphene Based Materials: Past Present and Future. Progress incarbon like Vulcan XC does not change the storage Mat. Sci, 2011 56, 1178-1271.capabilities. However, optimization of the electrode structure  Frackowiak, E.; Metenier, K.; Bertagna, V.; Beguin, F. Super capacitor electrodes from multiwalled carbon nanotubes. Appl.in terms of blend percentage, binder content and interface Phys. Lett., 2000 77(15), 2421-2423.character in the frequency and time domain is essential.  Niu, C.; Sichel, E.K.; Hoch, R.; Moy, D.; Tennent, H. High powerDepending on these characteristic features the EDLCs can be electrochemical capacitors based on carbon nanotube electrodes. 1997 Appl. Phys. Lett., 70, 1480-1482.very commercially viable.  An, K.H.; Kim, W.S.; Park, Y.S.; Choi, Y.C.; Lee, S.M.; Chung, D.C. Super capacitors using single-walled carbon nanotubeCONFLICT OF INTEREST electrodes. Adv. Mater, 2001 13, 497-500. The author(s) confirm that this article content has no  Du, C.; Yeh, Y.; Pan, N. High power density super capacitors using locally aligned carbon nanotube electrodes, Nanotechnol., 2005 16,conflicts of interest. 350-353.
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