1. Short Communication
Graphite nanosheets as an electrode material
for electrochemical double layer capacitors
Mahdi Nasibi a
, Melika Irankhah b
, Mehdi Robat Sarpoushi a,n
,
Mohammad Ali Golozar c
, Masoud Moshrefifar d
, Mohammad Reza Shishesaz a
a
Technical Inspection Engineering Department, Petroleum University of Technology, Abadan, Iran
b
Chemistry Engineering Department, Amirkabir University of Technology, Tehran, Iran
c
Materials Science and Engineering Department, Isfahan University of Technology, Isfahan, Iran
d
Materials and Mining Engineering Department, Yazd University, Yazd, Iran
a r t i c l e i n f o
Keywords:
Electronic materials
Nanostructures
Electrochemical measurement
Electrical properties
Energy storage
a b s t r a c t
In this paper, the effect of ion sizes of cations and anions on the charge storage capability
of graphite nanosheets is investigated. Electrochemical properties of prepared electrodes
are studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy
(EIS) techniques, in 3 M NaCl, NaOH and KOH electrolytes. A scanning electron microscope
(SEM) is used to characterize the microstructure and nature of prepared electrodes.
SEM images and XRD patterns confirm the layered structure (12 nm thickness) of the used
graphite with an interlayer distance of 3.36 Å. The electrochemical results and the ratio of
qn
O=qn
T confirm a better charge storage and charge delivering capability of prepared
electrodes in 3 M NaCl electrolyte. The charge/discharge cycling test shows a good
reversibility and confirms that the solution resistance will increase after 500 cycles.
& 2014 Published by Elsevier Ltd.
1. Introduction
Recently, electrochemical capacitors have attracted
worldwide research interest. Depending on charge storage
mechanisms, capacitors can be classified into three types:
electrochemical double layer capacitors (EDLCs), faradaic
pseudocapacitors and hybrid capacitors [1,2]. Especially in
EDLCs and at high charge/discharge rates, since no chemical
reaction is involved, the effects are easily reversible with
minimal degradation in deep discharge or overcharge and
the typical life cycle is hundreds of thousands of cycles [3,4].
With respect to the electrode materials there are three main
categories: carbon based materials, transition metal oxides
and conductive polymers [5,6]. Among carbon based materi-
als—due to good conductivity, superior chemical stability,
large surface area-to-volume ratio and unique layered struc-
ture—graphite has competed with carbon nanotubes, acti-
vated carbon, etc. which are used as electrode material for
ECs. High surface area of graphite does not depend on the
distribution of pores in solid state, but comes from the
interconnected open channels between graphite layers dis-
tributed in a two-dimensional architecture [7–9]. However,
the major problem of such a material is that not all the BET
surface area is electrochemically accessible [10]. Ion sizes in
electrolyte and the charge/discharge rate are significant
parameters affecting the ratio accessible to the total surface,
energy storage and power capability of graphite electrodes.
Therefore, choosing a proper electrolyte in accordance with
the morphology may be effective.
In this paper, the effect of ion size on charge storage,
charge delivering capability and reversibility of graphite
electrodes was investigated using cyclic voltammetry and
electrochemical impedance spectroscopy techniques. The
morphology and nature of the prepared electrodes were
investigated employing a scanning electron microscope.
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/mssp
Materials Science in Semiconductor Processing
1369-8001/$ - see front matter & 2014 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.mssp.2014.01.002
n
Corresponding author. Tel.: þ98 9155725460.
E-mail address: mehdi.sarpoushi@gmail.com (M.R. Sarpoushi).
Materials Science in Semiconductor Processing 20 (2014) 49–54

2. 2. Experimental
2.1. Materials
Graphite nanopowder (12 nm Flakes, multi-layered)
with the specific surface area of 15 m2
/g and purity of
98.5% was purchased from graphite supermarket and poly-
tetrafluoroethylene (o2 μm) from Aldrich company. All
other chemicals used in this study were purchased from
Merck. 90 wt% graphite nanopowder and 10 wt% polytetra-
fluoroethylene (PTFE) were well mixed by ultrasonic wave
in ethanol in paste form for about 60 min. Paste form was
chosen for better dispersion of PTFE in graphite nanoflakes.
After drying the paste and powdering, the composite was
pressed onto a 316L stainless steel plate (5 Â 107
Pa) which
served as a current collector (surface area was 1.22 cm2
).
A steel rod and a hollow cylinder of epoxy were used for
pressing. The composite is pressed onto the epoxy properly
by the steel rod. A Teflon paper was used at the bottom of
the rod because of its very low adhesion, in order to stick
the composite material to the stainless steel substrate. The
typical mass load of the electrode material was 45 mg. The
electrolytes investigated were 3 M NaCl, NaOH and KOH.
3. Characterization
Electrochemical behavior of prepared electrodes was
characterized using CV and EIS tests. Electrochemical
measurements were performed using an Autolab (Nether-
lands) Model PGSTAT302N. CV tests were performed
within the range of À0.55 and þ0.3 V (vs. SCE). EIS
measurements were carried out in the frequency range
of 100 kHz–0.02 Hz at OCP with an AC amplitude of 10 mV.
The specific capacitance C (F gÀ1
) of the active material was
determined by integrating either the oxidative part or the
reductive part of the cyclic voltammogram curve (Q(C)). This
charge was subsequently divided by the mass of the active
material m (g) and the width of the potential window of
cyclic voltammogram ΔE (V), i.e.,C ¼ Q= mΔVð Þ [11–13].
4. Results and discussion
Ion size and diffusion of anions and cations are effective
parameters of specific capacitance. When the KCl and NaCl
electrolytes are compared, the pronounced difference is of
cation nature. Ionic radii of Kþ
and Naþ
are 146 and
135 pm, respectively. In water solutions hydroxide ions are
bigger than protons (ionic radius of Hþ
and OHÀ
is 35 and
153 pm, respectively) which may lead to hindrance in the
diffusion of potassium or sodium ions and lead to non-
capacitive behavior. On the other hand, ionic mobility of
Kþ
is larger than Naþ
but ionic radius is smaller. However,
electrochemical nature of KCl electrolyte is close to that of
NaCl electrolyte. In addition to the ionic characteristics,
nature of the active material is an effective parameter of
capacitance. The SSA (specific surface area) of graphite
used in this paper is about 15 m2
/g but graphene (in all
forms) almost shows a higher SSA. The higher SSA of
graphene than that of graphite is the main reason for
increasing the capacitance (Table 1). Additionally, CB is
characterized by high porosity and small particle size.
These porosities are also varied from mild surface pitting
to the actual hollowing out of particles. The surface area of
CB is considered to be more accessible generally than the
other forms of high surface area carbons; this fine highly
branched structure of CB particles makes them ideally
suitable for filling inter-particle voids created between
coarse particles which improves the electrical contact
between them and increases the capacitance (Table 1).
Specific surface area and conductivity are two important
parameters to prepare highly efficient electrodes for capa-
citors. But only a part of the surface is always accessible by
electrolyte ions to be adsorbed. This would increase the
solution resistance. Fig. 1(a) shows the SEM image of
prepared electrodes, confirming the 2D graphite nanosheets
consisting of several carbon atom layers with a total
thickness of about 12 nm. Graphite layers interact with
each other to form open pore systems through which ions
easily access the surfaces between the graphite nanosheets
to form an electric double layer. Distance between these
nanosheets was measured using XRD and was about 3.36 Å
(Fig. 1(b)). The graphite used was perpendicular to these
nanosheets, showed no porosity and was completely flat
(Fig. 1 (a)). Thus, the used material is in its 2D porous and
1D flat surfaces completely. With this morphology, it seems
that the charge storage depends directly on the charge
separation on flat part (which is the most accessible surface
of electrode) and on open pore systems (which are less
accessible and ion size dependant). Ion size and ion diffu-
sion through these pores would affect the activation of
these less accessible surfaces, especially at high scan rates
[14]. Increasing the ionic radius would decrease the number
of adsorbed ions on the unit surface area of the electrode.
This would decrease the charge stored on the outer
Helmholtz layer. Therefore, for further investigations 3 M
electrolytes of KOH, NaOH and NaCl were employed. The
main difference in these electrolytes is the effective radius
of their anions and cations. Naþ
, Kþ
, ClÀ
and OHÀ
ions have
effective radii of 102, 138, 181 and 153 pm, respectively. The
ratio of interlayer distance of graphite 3.36 Å to ionic radius
(α) for these ions would be 3.29, 2.43, 1.86 and 2.20,
respectively.
The capacitance of each electrode was calculated from
the CV curves using
Cs ¼
Z
i dV=msΔV ð1Þ
where Cs is the specific capacitance,
R
i dV is the integrated
area of the CV curve, m is the mass of the active material
(mass of electrode material regardless of mass of PTFE),
ΔV is the potential range, and s is the scan rate.
Table 1
Comparison of different carbon based materials in KCl electrolyte.
Active material Electrolyte Specific capacitance (F gÀ1
)
Carbon black [22] 3 M KCl 33.58
Activated carbon [23] 1 M KCl 29
Multilayer graphene [24] 1 M KCl 15.6
Few layer graphene [24] 1 M KCl 14.9
Single layer graphene [24] 1 M KCl 10.9
M. Nasibi et al. / Materials Science in Semiconductor Processing 20 (2014) 49–5450

3. Fig. 1(c) shows the first cyclic voltammetry curves of
the prepared electrodes at the scan rate of 100 mV sÀ1
in
different electrolytes. All CVs exhibit a rectangular shaped
profile which is a good characteristic of ideal capacitive
behavior. All the electrode/electrolyte systems exhibited a
potential almost independent of the double layer capaci-
tance. Prepared electrodes showed a low current density
in 3 M KOH electrolyte which increased in 3 M NaOH and
NaCl electrolytes (Fig. 1(c)). Increasing the ionic radius
could decrease the ionic diffusion through the pores
and cause a lower current reversal at the final potentials
(Fig. 1(c)). Table 2 represents capacitance at the scan rate
of 100 mV sÀ1
in all three electrolytes. NaCl solution
possessed the maximum capacitance among these elec-
trolytes. The point intersecting the Nyquist curves with the
real axis in the range of high frequency shows the
equivalent series resistance (ESR) (Fig. 1(d)). It indicates
the total resistance of electrode, the bulk electrolyte
resistance and the resistance at the electrolyte/electrode
interface [15–18]. Therefore, two counter-acting para-
meters would act simultaneously as the ionic radius
increases: increase in electrical resistance and decrease
in charge adsorption on the surface of prepared electrodes.
In order to obtain quantitative information on the utiliza-
tion of prepared graphite electrodes, voltammograms
were analyzed as a function of scan rate, using the
procedure reported by Ardizzone et al. Fig. 2(a), (b), and
(c) exhibits cyclic voltammetry curves obtained using
different scan rates in 3 M KOH, NaOH and NaCl electro-
lytes, respectively. Table 3 represents capacitance in dif-
ferent scan rates in NaCl solution (optimum solution). In
charge and discharge cycles, the total charge can be
written as the sum of an inner charge from the less
accessible reaction sites and an outer charge from the
more accessible reaction sites, i.e.,qn
T ¼ qn
I þqn
O, where qn
T, qn
I
and qn
O are the total charge and charges related to inner
and outer surfaces, respectively [19,20]. Extrapolation of q*
to s¼0 from 1/q* vs. s1/2
plot (Fig. 2(d)) gives the total
charge qn
T which is the charge related to the entire active
surface of the electrode. In addition, extrapolation of q* to
s¼1 (sÀ1/2
¼0) from q* vs. sÀ1/2
plot (Fig. 2(e)) gives the
outer charge qn
O, which is the charge on the most acces-
sible active surface. Prepared electrodes show the ratio of
outer to total charge qn
O=qn
T
À Á
of 0.005, 0.008 and 0.239 in
KOH, NaOH and NaCl electrolytes, respectively. These
ratios confirm the low current response of graphite in
KOH and NaOH electrolytes which is due to the large
radius of Kþ
ions and low mobility of hydroxyl ions.
Among these electrolytes, NaCl shows the lowest electrical
resistance and therefore was chosen to investigate the
effect of charge/discharge cycle.
As scan rate increases (Fig. 3(a)) the capacitance versus
potential relation will deviate from the classical square
waveform, expected for a pure capacitor. As discussed by
Table 2
Capacitance (100 mV sÀ1
) in different electrolytes.
Electrolyte Capacitance (F gÀ1
at 100 mV sÀ1
)
NaCl 1.99
KOH 1.09
NaOH 1.84
Fig. 1. (a) Scanning electron microscopy image, (b) XRD pattern obtained
from graphite electrodes, (c) cyclic voltammetry curves and (d) Nyquist
diagrams obtained from graphite electrodes in different electrolytes.
M. Nasibi et al. / Materials Science in Semiconductor Processing 20 (2014) 49–54 51

4. some researchers this is due to the resistance effects down
the pores [4]. In addition, efficiency is another important
parameter affecting the capacitance at high sweep rates.
As sweep rate increases, loss of energy increases and the
charge stored on the electrode surface decreases causing
the capacitance to decrease (Fig. 3(a)). Graphite electrode
shows a low capacitance of 3.62 F gÀ1
in 3 M NaCl electro-
lyte. It seems that the low ionic diffusion due to small
interlayer distances which is one of the unique properties
of graphite nanosheets may be the main reason for this
low capacitance.
Regarding the practical applications, the cyclic stability
of supercapacitors is a crucial parameter. The life cycles of
both conducting polymers and metal oxides, as candidates
for pseudocapacitive materials, are much shorter than
those of the carbon based materials because of the loss
of active materials. In the case of graphite nanosheets, the
cyclic stability was evaluated by repeating the CV at a scan
rate of 100 mV sÀ1
for 500 cycles (Fig. 3(b)). Simulta-
neously, EIS tests were used to evaluate the electrode
changes (Fig. 3(c)). Graphite was found to exhibit excellent
stability over the entire cycle numbers (Fig. 3(b)). The
anodic and cathodic currents decreased, but the cyclic
voltammetry curves remained in their rectangular shaped
profiles (Fig. 3(b)). Conversely the charge stored on the
electrode decreased only by $23% of the initial charge
stored on the electrode (Table 4). These behaviors demon-
strated a good cycling stability. Most capacitors, except
those of vacuum or air type, do not exhibit ideal pure
capacitive behavior according to the criterion that the real
and imaginary components of their impedance should be
out of phase by 901 independent of the frequency between
periodic changes of current and voltage when addressed
by a sinusoidally alternating voltage. Most practical capa-
citors deviate from this requirement because their impe-
dance is not that of a pure capacitance but of a capacitance
linked in series with an element exhibiting ohmic behavior
that in combination with the capacitance gives rise to a
phase angle that is less than 901 and is usually frequency
dependent. This ohmic component is referred to as the
equivalent series resistance [21]. In all three electrolytes
the phase angle is less than 901 because of the ESR
involved in all three electrolytes. Fig. 4 shows that the
phase angle (φ) between E (voltage) and I (current)
depends on the frequency of NaCl electrolyte.
In this case, a good coating attributed to the proper
applied pressure was one of the reasons for good cyclic
stability. Although applying a higher pressure may increase
Fig. 2. Cyclic voltammograms curves obtained using different scan rates
in 3 M (a) KOH, (b) NaOH and (c) NaCl electrolytes. (d) Extrapolation of q*
to s¼0 from the 1/q* vs. s0.5
plot, which gives the total charge and (e)
extrapolation of q* to s¼1 from the q* vs. sÀ0.5
plot, which gives the
outer charge for graphite electrodes in different electrolytes.
Table 3
Capacitance at different scan rates in NaCl solution.
Scan rate (mV sÀ1
) Capacitance (F gÀ1
)
5 3.62
30 2.67
50 2.38
l00 1.99
200 1.40
300 1.05
500 0.71
M. Nasibi et al. / Materials Science in Semiconductor Processing 20 (2014) 49–5452

5. the cyclic stability due to the better adhesion of coating on
the substrate, capacitance would decrease dramatically.
During the 500 charge/discharge cycles the equivalent series
resistance increased (Fig. 3(c)) and Nyquist plots shifted to
higher values which can be attributed to the electrolyte
decomposition on the electrode surface.
5. Conclusions
In summary, studies confirmed the presence of flat and
porous (having about 3.36 Å interlayer distance) surfaces,
and showed ion size and charge/discharge rate dependent
on energy storage and power capability of the used
material. It showed a good cycling performance stemmed
from controlled thickness and subsequent changing of the
substrate reactive state. The relatively high ratio of qn
O=qn
T
(0.236) confirms the high current response on voltage
reversal in 3 M NaCl electrolyte. The rectangular shaped
profile which is a good characteristic of ideal capacitive
behavior is obvious over the entire cycle numbers. Charge
stored on the electrode decreased only by $23% of the
initial charge stored on the electrode which represents
good reversibility. The proposed electrodes provided a
double layer capacitance and showed a capacitance as
high as 3.62 F gÀ1
at 5 mV sÀ1
in 3 M NaCl electrolyte. The
low specific surface area of graphite nanosheets was
responsible for this low capacitance.
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