1. Short Communication
Fabrication of a novel graphene nano-sheet electrode
embedded with nano-particles of zirconium dioxide
for electrochemical capacitors: Ions-redeposition on the
surface of nanoporous electrode
Mahdi Nasibi a,b
, Mohammad Reza Shishesaz a
, Mahdi Robat Sarpoushi a,n
,
Mohammad Reza Borhani c
, Zaki Ahmad d,e
a
Technical Inspection Engineering Department, Petroleum University of Technology, Abadan, Iran
b
Health, Safety and Environment (HSE) Engineering Office, NIOPDC, Yazd Region, Yazd 89167-84395, Iran
c
Department of Materials Engineering, Malek Ashtar University of Technology, ShahinShahr, Isfahan, Iran
d
Adjunct Professor, COMSATS University, Lahore, Pakistan
e
Prof Emeritus, KFUPM, Dhahran, Saudi Arabia
a r t i c l e i n f o
Keywords:
Electrode material
Electrochemical capacitors
Microstructure
Energy storage and conversion
a b s t r a c t
In this paper, the effect of charge/discharge cycles on the electrode containing nano-
zirconium oxide, nanoporous carbon black and graphene nanosheets in electrochemical
capacitors has been described. Surface morphology and electrochemical performance of
the prepared electrode have also been conducted. The electrode prepared from graphene
nanosheets (GNS), nanoporous carbon black (NCB), zirconium oxide (ZrO2), and poly-
tetrafluoroethylene (PTFE) in molar ratio of 54:09:27:10 respectively showed a maximum
specific capacitance as high as 11.84 F gÀ1
in the potential range between À0.45 and
0.35 V (V vs. SCE) at a scan rate of 10 mV sÀ1
in a 3 M NaCl electrolyte. The electro-
chemical results show the low ratio of the outer to total charge (qn
O/qn
T) which confirms the
low current response and higher voltage reversal at the end potentials. SEM images
confirms the ions re-deposit as agglomerates and accompanied by a drastic decrease in
the surface area on the surface of the electrode after one charge/discharge cycle.
& 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction
Electrochemical capacitors also known as supercapaci-
tors in recent terminology although known since 1957
[1–3] have undergone a dramatic transformation in recent
years because of their promising potential to deliver more
power than batteries and store more energy than conven-
tional capacitors. An understanding of the greater charge
mechanism role of nano-materials, dissolution kinetics of
solvated ions in the pores has led to the higher capacitance
of electrochemical capacitors by using carbon electrodes
and opened the door for high energy devices. Nano-
materials provide high electrical conductivity, short ion
diffusion pathways, and can provide an excellent inter-
facial integrity to the system. Currently, many laboratories
are actively engaged in the development of new types of
electrode materials, and most of the research has been
focused on the development of nanoporous materials for
electrochemical capacitors [4,5]. Although the demand for
developing new porous electrode materials with higher
specific surface area is increasing, the problem of electro-
lyte ions re-deposition on the pore walls and blocking
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/mssp
Materials Science in Semiconductor Processing
http://dx.doi.org/10.1016/j.mssp.2014.11.007
1369-8001/& 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
n
Corresponding author.
E-mail address: Mehdi.sarpoushi@gmail.com (M.R. Sarpoushi).
Materials Science in Semiconductor Processing 30 (2015) 625–630

2. them is very challenging. Re-deposition of electrolyte ions
and active materials on the electrode decreases the active
specific surface area and the charge stored on the elec-
trode. Transition metal oxides are considered to be the
most suitable candidate materials for electrochemical
capacitors. This stems from the high specific capacitance
coupled with very low resistance resulting in a high
specific power which makes them suitable for commercial
applications. As investigated by Nasibi et al. zirconium
oxide demonstrate capacitive behavior in 2 M KCl electro-
lyte [6].
The aim of this paper is to investigate the charge distribu-
tion ability of nanoporous NCB/GNS/ZrO2 electrode and the
effect of electrolyte ions and active material re-deposition on
this electrode. Mechanical pressing was used as a fast and
easy method to fabricate the electrode. The products were
then evaluated as possible candidate electrode materials for
electrochemical capacitors using techniques including cyclic
voltammetry, electrochemical impedance spectroscopy and
scanning electron microscopy.
2. Experimental
2.1. Materials
Nanoporous (o10 nm in diameter) carbon black (NCB)
micro-sized particles (o2 μm) were purchased from
Degussa, Germany. Graphene nanosheets (GNS) (60 nm
Flakes, multi-layered) with the specific surface area of
15 m2
/g and a purity of 98.5% were purchased from
Graphene Supermarket, USA. Polytetrafluoroethylene
(PTFE) micro-sized powder (o2 μm) and high-purity
(499%) nano-sized zirconium oxide (ZrO2) particles
(o100 nm) were purchased from Aldrich, Germany. All
of the other chemicals used were purchased from
Germany (to purchase these materials, visit the websites
www.graphenesupermarket.com, http://www.sigmaal
drich.com and www.merckgroup.com(. In order to prepare
the electrode, a mixture containing 54 wt% of GNS, 9 wt%
NCB, 27 wt% ZrO2 nanoparticles and 10 wt% PTFE was
mixed well in ethanol to form a paste and then was
pressed (at 50 MPa pressure) onto the 316 L stainless steel
substrate, which served as a current collector (having a
surface area of 1.4 cm2
). The typical mass of electrode
material was 45 mg. The electrolyte used was 3 M NaCl.
2.2. Characterization
The electrochemical behavior of the prepared electrode
was characterized using cyclic voltammetry (CV) and
electrochemical impedance spectroscopy (EIS) tests. The
electrochemical measurements were conducted using an
Autolab (Netherlands) potentiostat Model PGSTAT 302N.
CV tests were carried out within the range of À0.45 to
Fig. 1. SEM images obtained from 54:09:27:10 electrode before (up) and after (down) charging/discharging cycles.
M. Nasibi et al. / Materials Science in Semiconductor Processing 30 (2015) 625–630626

3. þ0.35 V (V vs. SCE), using scan rates of 10, 20, 50, 100 and
200 mV sÀ1
. EIS measurements were carried out in a
frequency range of 0.010 Hz–100 kHz at OCP with an AC
amplitude of 10 mV.
The specific capacitance can be estimated from the
voltammetric charge surrounded by the CV curve accord-
ing to the following formula [7,8]:
C ¼
qa þjqcj
2mΔV
ð1Þ
where, qa, qc are the sum of anodic and cathodic voltam-
metric charges on positive and negative sweeps, respec-
tively, m is the mass of active material (regardless of the
mass of PTFE) and ΔV is the potential window of CV. The
morphology and nature of the porous electrode were
studied using scanning electron microscopy (TESCAN,
USA). Also the real (C0
) and imaginary (C″) capacitances
of the electrode are calculated using the following equa-
tion [9,10]:
C0
¼
Z″ðωÞ
ωjZðωÞj2
ð2Þ
C″ ¼
Z0
ðωÞ
ωjZðωÞj2
ð3Þ
where Z0
(ω) and Z″ (ω) are the respective real and
imaginary parts of the complex impedance Z (ω), ω is
the angular frequency and it is given by ω¼2πf.
3. Results and discussion
Electrodes which contain carbon base materials and
metal oxides store the electrical charges through two
mechanisms: (a) double layer formation on carbon base
materials and (b) Faradic redox reactions of transition
metal oxides. Transition metal oxides are the best elec-
trode materials for redox pseudocapacitors, whereas elec-
trical double layer capacitors mainly focusing on carbon
materials [11,12]. The principal redox reaction involved in
charging and discharging processes of zirconium dioxide
particles in an aqueous electrolyte can be described by the
following equation:
ZrIV
O2 þλþ
þeÀ
2ZrIII
OOλ ð4Þ
where λ denotes Naþ
or Hþ
. Two mechanisms can be
proposed for charge storage in the presence of ZrO2
particles. The first mechanism is based on the intercala-
tion/extraction of protons or alkali cations into/from the
oxide particles (reaction (5)), whereas the second mechan-
ism probably involves the surface adsorption/desorption of
protons or alkali cations (reaction (6))
ZrO2 þMþ
þeÀ
2ZrOOM (5)
(ZrO2)surfaceþMþ
þeÀ
2(ZrOOM)surface (6)
Fig. 2. (a) CV curve and (b) Nyquist diagram obtained from prepared electrode in 3 M NaCl electrolyte.
M. Nasibi et al. / Materials Science in Semiconductor Processing 30 (2015) 625–630 627

4. where Mþ
denotes as Naþ
or Hþ
. Re-deposition of
dissolved ions on the surface of the electrodes may block
a part of the active surface of carbon base materials and
decrease the charge storage ability of the prepared elec-
trodes. The SEM images of the 54:09:27:10 (GNS/NCB/
ZrO2/PTFE) electrode with different magnifications are
shown in Fig. 1. The uniform distribution of NCB and
ZrO2 particles over the whole area of GNSs suggests
uniform distribution of properties such as conductivity,
electroactivity, etc. Addition of ZrO2 nanoparticles and NCB
particles to graphene can increase the distance between
graphene layers and it also makes the surface of the
electrodes porous. Addition of NCB particles into the
electrode material can act as a mixer due to its large
particle size and increases distances between GNSs more
and arrange them in different directions. Therefore, the
prepared electrode confirms the 3D structure. Fig. 1 shows
the SEM images of the electrode (Fig. 1(a–c)) before and
after (Fig. 1(d–f)) the charging/discharging cycle. Fig. 1(d–f)
shows agglomeration of depositions on the surface of the
electrode. Blocked pores are expected to be less likely,
because large particles of depositions would not block
micropores. Therefore, this phenomenon has no predomi-
nant effect on the reduction of the specific surface area of
nanoporous carbon particles and graphene nanosheets.
However agglomeration of ZrO2 particles reduces the
specific surface area of the particles dramatically and
results in a decrease in active materials for further cycling.
Earlier work demonstrates that zirconium oxide/carbon
black composite showed electrochemical capacitance char-
acteristics [6]. Fig. 2(a) shows the second CV obtained from
the prepared electrode at a scan rate of 10 mV sÀ1
in 3 M
NaCl electrolyte. The total capacitance of the electrode
under these conditions was about 11.84 F gÀ1
. The elec-
trode exhibited satisfactory pseudocapacitive behavior.
These results indicate the high electroactivity and favor-
able kinetic properties of the GNS/NCB/ZrO2 electrode in
NaCl electrolyte.
The point of intersecting Nyquist curves with the real
axis in the high frequency range is the equivalent series
resistance (ESR). It indicates the total resistance of the
Fig. 3. Equivalent circuit of NCB/GNS/ZrO2 electrode in 3 M NaCl electro-
lyte at OCP.
Table 1
Electrochemical parameters obtained from electrochemical impedance
spectroscopy measurement from GNS/NCB/ZrO2 electrode.
Parameters Rs (Ω) Cdl (F/g) Rct (Ω) CF (F/g) Rl (Ω)
Value 9.93 2.39 0.69 6.80 223.4
Errors (%) 0.5037 31.46 13.18 1.06 8.062
Fig. 4. (a) Phase angle vs. frequency, (b) imaginary capacitance and real capacitance vs. frequency.
M. Nasibi et al. / Materials Science in Semiconductor Processing 30 (2015) 625–630628

5. electrode, the bulk electrolyte resistance and the resis-
tance at the electrolyte/electrode interface. As shown in
Fig. 2(b), it could be obviously seen that the impedance
spectra are almost similar in form, composed of one
semicircle at the high frequency end followed by a nearly
vertical line at the low frequency end. The radius of the
semicircle in the high frequency region reflects the impe-
dance on the electrode/electrolyte interface [9]. The
equivalent circuit of the GNS/NCB/ZrO2 electrode is shown
in Fig. 3. The equivalent circuit contains the bulk solution
resistance element (Rs), the double layer capacitance (Cdl)
which is parallel with the charge transfer resistance (Rct),
the Faradaic capacitance (CF) and leakage current (Rl).
Table 1 shows numerical values for the equivalent circuit
of the GNS/CB/ZrO2 electrode by the Nyquist plot and
fitting it using the equivalent circuit. Since a low value of
Rct (0.69 Ω) is obtained, the pseudocapacitive nature of the
ZrO2 is confirmed. The capacitance value obtained from CV
plots is confirmed by fitting Nyquist plots using the
equivalent circuit (Table 1). This confirmation indicates
the high accuracy of the selected equivalent circuit.
It is known that a phase angle of 901 signifies an ideal
capacitive behavior; otherwise the material would show a
pseudocapacitive behavior [13,14]. From Fig. 4(a), it is
evident that the phase angle at the tail is nearly 681 which
further confirms the pseudocapacitive nature of the ZrO2-
containig electrode. τ0 is the minimum amount of time
required to discharge all the energy from the device with
Fig. 5. Cyclic voltammogram curves obtained from 54:09:27 electrode at different scan rates in 3 M NaCl electrolyte.
Fig. 6. Extrapolation of (a) qn
to υ¼0 from the qÀ1
vs. υ0.5
plot given the total charge and (b) qn
to υ¼1 from the q vs. υÀ0.5
plot given the outer charge
for different electrodes.
M. Nasibi et al. / Materials Science in Semiconductor Processing 30 (2015) 625–630 629

6. an efficiency of more than 50% of its maximum value, and
it can be derived from the frequency at maximum C″ [9]. τ0
for the electrode in NaCl electrolyte is 26.88 s (Fig. 4(b)).
The low ion diffusion in NaCl electrolyte was confirmed by
the corresponding large relaxation time constant (τ0).
The CV curves obtained at scan rates of 100 and
200 mV s-1
deviated from the classical square waveform,
expected for pure capacitance, due to a marked decrease in
the accessible surface area at such a high scan rates [15]
(Fig. 5). It was found that the redox current progressively
increased by increasing scan rate [16]. As shown in Fig. 5,
at high scan rates, the shape of the CVs is elliptic
which indicates that the capacitance was mainly due to
pseudocapacitance.
Total charge (qt) stored on the electrodes can be divided
into two parts: outer charges (qO) and inner charges (qI).
Outer charges are stored on the external surface of the
electrode but inner charges need to diffuse through the
pores which open on the surface. Therefore, blocking pores
will decrease some of the charge storage ability of the
electrodes. In order to obtain quantitative information
about the charge distribution on the proposed electrodes
during charging, voltammograms were analyzed as a
function of scan rate (Fig. 6), according to the procedure
reported by Ardizzone et al. [13]. Extrapolation of q to
υ¼1 (υÀ1/2
¼0) from the q vs. υÀ1/2
plot (Fig. 6(a)) gives
the outer charge, qO, of 1.29 for 54:09:27:10 electrode,
which corresponds to charges on the most accessible
active surface of the electrode. Besides, extrapolation of q
to υ¼0 from 1/q vs. υ1/2
plot (Fig. 6(b)) gives the total
charge, qT, of 90.90 C gÀ1
cmÀ2
, which corresponds to
charges related to the entire active surface of the electrode.
This confirms that about 98% of charges are stored on
inner pore walls of the electrode.
4. Conclusion
SEM images confirmed the redox reactions involved in
charging and discharging processes of zirconium dioxide
particles in an aqueous electrolyte caused agglomeration
of ZrO2 particles, which caused a dramatic decrease in the
specific surface area of the particles and a consequent
decrease in the amount active materials available for
further cycling. The proposed electrode from a combina-
tion of GNS, NCB, ZrO2, and PTFE showed a maximum
capacitance of as high as 11.84 F gÀ1
at 10 mV sÀ1
in 3 M
NaCl electrolyte. The electrode also showed a low ratio of
the outer charge to total charge (qO/qT) of 0.014 which
confirms the low current response on voltage reversal and
less accessibility to the electrolyte for internal surface
adsorption. As expected, data obtained from electrode
material confirmed both double layer capacitance and
pseudocapacitance. The investigations presented above
show an easy method to fabricate electrodes for electro-
chemical capacitors with embedded nano-materials.
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