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. 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. 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. 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. 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.
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From the present study we can state that EDLCs
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energy storage capabilities. They can also support high scan
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The author(s) confirm that this article content has no
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