Double Layer Energy Storage in Graphene - a Study

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 capacitance retained after 1200 cycle tests. This
demonstrates the commercial potential for high performance,
Electrochemical supercapacitors have important
environmentally friendly and low-cost electrical energy
implications in energy storage [1]. Graphene is an interesting
storage devices based on 2-D graphene material.
2- dimensional material. Studies of graphene are not limited
to mono layer of graphene but also to multilayer of These reports support nanosized thin graphene (NTG) as
graphene. Graphene layers can be grown on a wide variety suitable electrode material for energy storage. Many forms
of transition metal substrates, with a number of layers by of graphene have been used by various groups. The detail of
simple decomposition of hydrocarbons. [2-5] Bi- layer and various forms of graphene, preparation and its various
multilayer graphene can be prepared by thermal exfoliation applications is reviewed by V. Singh et al. [9].
of graphite oxide or reduction of graphene oxide (RGO) by Super capacitors store charge electrostatically by the
using suitable reducing agents [6]. Super capacitors made adsorption of ions on to electrodes that have high accessible
using graphene as electrodes have been studied with specific surface area. Therefore, a high specific capacitance active
capacitance of the order of 120 F/gm. [7]. Graphene electrode plays a vital role in efficient energy storage.
materials as super capacitor electrode material have also Various forms of porous carbon, for instance CNT [10-14],
been investigated by Y. Wang et al. [8]. The Graphene was mesoporous carbon [15, 16], activated carbon [17] and
prepared from graphene oxide sheets that subsequently carbide derived carbon [18] have been studied for electrodes
undergo a gas based hydrazine reduction to restore the in this respect. Graphene and RGO have also been predicted
conducting carbon network. A maximum specific as a potential candidate for super capacitor electrodes due to
capacitance of 205 F/g with a measured power density of 10 very high specific surface area (2630 m2/g), chemical
kW/kg, at energy density of 28.5 Wh/kg, in an aqueous stability, excellent electrical, thermal conductivity and low
electrolyte solution was obtained. These super capacitor cost [7, 8, 19, 20]. Interestingly, graphene has demonstrated
devices exhibit excellent cycle life with ˜ 90% specific 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
*Address correspondence to this author at the Materials Physics Division, have shown CRG’s potential as an electrode for super
School of Advanced Sciences, VIT University, Vellore, TN, India;
Tel: 91-416-2243091; Fax: 91-416-2243092; E-mail: cksubra@gmail.com
capacitor, even though the used surface area was 707 m2/g
and graphene sheets were not fully accessible by the
1876-4037/12 $58.00+.00 © 2012 Bentham Science Publishers

2. Double Layer Energy Storage in Graphene - a Study Micro and Nanosystems, 2012, Vol. 4, No. 3 181
electrolyte. The super capacitor had specific capacitances of D. Pech et al. [40] 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 tetraethyl
electrolytes, respectively. Improved capacitance (191 F/g, in ammonium tetra fluro borate as electrolyte. The discharge
KOH) 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. [41] show that EDLCs can have high storage
m2/g). [14] Wang et al. [8] have achieved specific capacity, but the porous electrodes cause them to perform
capacitance value 205 F/g for hydrazine reduce GO of like resistors in filter circuits that remove ripple from
effective surface area 320 m2/g. It is worth noting that the rectified direct current. They have demonstrated efficient
surface area of graphene sheets plays a significant role which filtering of 120 hertz current with electrodes made from
directly 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 time
agglomeration and restacking due to Van der Waals
constants of less than 200 microseconds, in contrast with ~1
attractive forces between the neighboring layers. The
second for typical EDLCs. Gao et al. [36], have reported scan
aggregation reduces the effective surface area resulting loss
rates of 20V/s and time constants of 10 ms, for all solid EDLC
of capacitance. Therefore, a few researchers have made
with proton conducting polymer and graphite electrodes. They
effort to keep graphene sheets separated by addition of metal
oxide nanoparticles [22]. 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 in
great promise for use in super capacitors with high energy
EDLC by using 1- and 2- dimensional material has become
density and high charge/discharge rates. Recent study of Liu
an increasingly important issue in power applications [42,
et al., [19] has shown graphene-based super capacitor that
exhibits 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 the
temperature and 136Wh/kg at 80 C. They have reported that
electrodes. The parameters that affect solid state EDLC
the mesoporous structure of the curved graphene sheet is
electrode design are pore size, surface area, wettability, and
responsible for this. The curved nature of graphene sheets
conductivity. The gravimetric capacitance Cg is strictly
prevent 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 capacitance
make 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 [44].
Among various conductive polymers, PANI has been Subramaniam and coworkers [15, 31-34] have reported
considered as a most promising conductive electrode electrochemical performance of solid state EDLCs using
material and studied considerably with CNT and other carbon as electrodes with ionic polymers of
carbon system. [24-29] A graphene nanosheets–PANI perfluorosulphonic acid as electrolyte. These carbon based
composite was synthesized by in situ polymerization. [24] EDLCs were assembled using Vulcan XC carbon with typical
The specific capacitance of 1046 F/g was obtained at a scan surface area of 260 m2/gm, mesoporous carbon from silica
rate of 1 mV/s. Conductive graphene nanosheets provide template, (CMK-3), typical surface area of 1260 m2/gm, a
more 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 of
sides, resulting in energy density of 39 kWh/kg at a power Vulcan XC carbon and GNP. In some of these EDLCs high
density of 70 kW/kg. Graphene–PANI composite paper was scan rate capabilities were shown. In this article we present a
also prepared by in situ anodic electro polymerization of short treatise on electrochemical energy storage in Graphene.
polyaniline film on graphene paper. [30] The flexible as
prepared composite paper combined the high conductivity EXPERIMENTAL
showed a gravimetric capacitance of 233 F/g. The assembly of the EDLC has been described in
Subramaniam et al. [31] have shown Graphene detailed elsewhere. [15, 32] Carbon fiber paper TGP-H from
Nanoplatelets (GNP) to have effective energy storage TORAY was used as the base matrix for the electrodes. The
capabilities. These maybe the first reports using solid ionic typical size of the electrode used was 3 cm2. 2.5 mg/cm2 of
polymer 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 the
storage capacities. The EDLCs have good rate capabilities fabrication of the electrodes. It is necessary to optimize the
and reversibility at high scan rates. They have reported quantity of binder. The performance of the electrode is very
performance of EDLCs, which are based on high-surface sensitive to the amount of binder used for a particular particle
area carbon materials, which utilize the capacitance arising size of the active material. Two circular carbon electrodes
from a purely non-Faradaic charge separation at an were assembled on either side of the solid electrolytes. The
electrode/electrolyte interface. [15, 32-34] Solid electrolytes electrodes and the electrolyte were laminated by standard
are elegant to use and very reliable. They can also be shaped lamination process. This assembly was placed between two
and 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 of
composites have been studied by different groups [36-39]. It the end plates to prevent lateral shorting and to delimit the
was possible to achieve high scan rates of a few 100V/s. central capacitor portion and to seal the cell assembly.

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 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 electrode
KHz 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 texture
cell were made in the potential range 0 to 1 V at various scan
of the electrode. Graphene nanoplates (GNP), with narrow
rates. All electrochemical measurements were performed at mesopore distribution have been effectively used to enhance
ambient conditions using a Biologic VMP electrochemical
charge storage performance. Wang et al. [46], suggests that
system.
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 forming
thickness of 50 micron. The high electronegativity (i.e. double layers, in different types of electrolytes, because of
electron affinity) of the fluorine atom, bonded to the same difference in size and wettability of the ions to the carbon
carbon 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 be
Nafion® has the maximum electronegative environment used in the electrode microstructure. The surface area of
possible. The proton conductivity is around 0.2 S/cm for a graphene sheets plays a significant role which directly
well humidified sample. affects the performance of the super capacitors.
The properties of the GNP used in the electrode Barbieri et al. [47], have stated using Differential
fabrication have been discussed elsewhere. [31] Functional Theory (DFT) model, that there exists
capacitance limits to high surface area activated carbons for
RESULTS 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 a
various 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 charge
depending on the process of preparation. This has been accommodation inside the pore walls. Capacitance is also
discussed 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, thereby
agglomeration and restacking due to Van der Waals
decreasing the capacitance. Also for SSA around 1200
attractive forces between the neighboring layers. The m2/gm, the average pore wall thickness becomes close to the
aggregation reduces the effective surface area resulting loss
screening length of the electric field. Therefore, for larger
of capacitance. This has been overcome by keeping graphene
SSA values, the pore wall can no longer accommodate the
sheets separated with the addition of metal oxide
same amount of charge at a given electrode potential, thus
nanoparticles [22].
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 electrode
carbon is dominated by space charge layer capacitance [45].
structure can be circumvented by using ordered mesoporous
The total capacitance can be given by equation 1 [15]:
carbons (OMC) or highly ordered mesoporous carbon
C = 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 capacitance
meso and external surface areas, respectively. When we use with scan rates for GNP and normal carbon with specially
different types of carbon, contributions to capacitance from textured electrode structure. It is evident that the percentage
micro pore surface area, Cmicro, which is dependent on drop in specific capacitance with increasing scan rates is
current density and capacitance from meso pore surface area, much smaller for GNP, which makes it an ideal material for
Cmeso, and contributions to capacitance from other large pore, solid state EDLCs.

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 tangential velocity decreases. This approach, both in the
necessary to understand the dynamics of the electric double frequency and time domains, has proved useful for
layer in the frequency and time domain. The problem can be extracting information on the perturbation of the electric
very complicated because the nature of the Nernst–Planck, double layer induced by an external field. This approach is
Poisson, and Navier–Stokes equations makes it impossible to very useful when we want to design solid state EDLC with
obtain analytical time dependent solutions except for very high rate capabilities. The exact nature of the response for
limited situations. Lopez et al. [50] have done some GNP will be available elsewhere.
extensive analysis in this regard. They have used a network
model and have obtained the transient response in the time The EIS measurement provides knowledge of the
domain. At short distances (comparable to the double layer frequency dependence of capacitance of an EDLC. Higher
thickness), the profiles of radial and tangential velocities are the frequency dependence of the EDLC, higher will be the
different: the latter increase with distance much faster than power response. The Nyquist plots for the EDLC assembled
the former, thus demonstrating the existence of electro with GNP and blends of GNP and Vulcan XC are shown in
osmotic slip due to negative counter ions being moved by Fig. (2). The blending of the GNP with Vulcan XC was done
the field toward the left of the particle. Such electro osmotic to see whether the texture of the electrode played a role in
flow is negligible outside the double layer and hence the the frequency dependence of the capacitance of the EDLC. It

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 ACKNOWLEDGEMENTS
double layer interface but not very prominently, as seen from
One of the authors would like to thank Dr. G
the Cyclic Voltammogram. (Fig. 3) From Fig. (3), which
Velayutham and A. M. Prasad, Anabond Sainergy, India, for
presents the Cyclic voltammograms at scan rate of 10 mVs-1
the some of the experimental work carried out and Harini
for GNP and blends of GNP with Vulcan XC, we see no
Ramakrishnan, Cognisant Technologies, India, for the
deviations for the pure GNP and the blends. This has to be
graphic presentations.
studied further to establish its cause. Also a detailed analysis
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Received: December 14, 2011 Revised: March 15, 2012 Accepted: May 14, 2012