Enhanced electrocatalytic performance of cobalt oxide nanocubes incorporating reduced graphene oxide as a modified platinum electrode for methanol oxidation

Article abstract of DOI:10.1039/c4ra08952a

Herein, we report a facile hydrothermal method for the preparation of cobalt oxide nanocubes incorporating reduced graphene oxide (rGO-Co₃O₄ nanocubes) for electrocatalytic oxidation of methanol. The synthesized rGO-Co₃O₄ nanocubes were characterized using transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and Raman techniques. The electrochemical behavior of an rGO-Co₃O₄ nanocube modified electrode was studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The electrocatalytic performances of rGO-Co₃O₄ nanocube-modified electrodes with different wt% of GO were investigated in relation to methanol oxidation in an alkaline medium. The rGO-Co₃O₄ nanocube modified electrode showed enhanced current density due to oxidation of methanol when compared to the bare Pt, rGO, and Co₃O₄ nanocube modified electrodes. The optimal GO content for an rGO-Co₃O₄ nanocube-modified electrode to achieve a high electrocatalytic oxidation of methanol was 2 wt%, and it showed an anodic peak current density of 362 μA cm^-2.

Full text of DOI:10.1039/c4ra08952a

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Page 1 of 26
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Enhanced electrocatalytic performance of cobalt oxide nanocubes incorporated reduced
graphene oxide modified platinum electrode for methanol oxidation
Muhammad Shahid Mehmood, Alagarsamy Pandikumar, Amir Moradi Golsheikh,
Huang Nay Ming, Lim Hong Ngee
Graphical Abstract
Cobalt oxide nanocubes incorporated reduced graphene oxide prepared by hydrothermal method
and used for the electrocatalytic oxidation of methanol.

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Enhanced electrocatalytic performance of cobalt oxide nanocubes
incorporated reduced graphene oxide modified platinum electrode for
methanol oxidation
†
Muhammad Shahid Mehmood1, Alagarsamy Pandikumar1*, Amir Moradi Golsheikh1,
Huang Nay Ming1*, Lim Hong Ngee2, 3*
1 Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science,
University of Malaya, 50603 Kuala Lumpur, Malaysia
2 Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM
Serdang, Selangor, Malaysia
3Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia,
43400 UPM Serdang, Selangor, Malaysia
Corresponding author’s: huangnayming@um.edu.my (Huang N.M.),
pandikumarinbox@gmail.com (Pandikumar. A), and
janet_limhn@science.upm.edu.my (Lim H.N.)
Abstract
Herein, we report a facile hydrothermal method for the preparation of cobalt oxide
nanocubes incorporated reduced graphene oxide (rGOꢀCo3O4 nanocubes) for electrocatalytic
oxidation of methanol. The synthesized rGOꢀCo3O4 nanocubes were characterized using
transmission electron microscopy (TEM), field emission scanning electron microscopy
(FESEM), Xꢀray diffraction (XRD), and Raman techniques. The electrochemical behavior of
rGOꢀCo3O4 nanocubes modified electrode was studied using cyclic voltammetry (CV) and
electrochemical impedance spectroscopy (EIS) techniques. The electrocatalytic performances
of rGOꢀCo3O4 nanocubeꢀmodified electrodes with different wt. % of GO were investigated in
relation to methanol oxidation in an alkaline medium. The rGOꢀCo3O4 nanocubes modified
electrode showed enhanced current density due to oxidation of methanol when compared to
the bare Pt, rGO, and Co3O4 nanocubes modified electrodes. The optimal GO content for an
rGOꢀCo3O4 nanocubeꢀmodified electrode to achieve a high electrocatalytic oxidation of
methanol was 2 wt. %, and it showed an anodic peak current density of 362 ꢀA/cm2.
1

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Introduction
The constantly increasing energy demand due to the depletion of conventional fossil
fuel reserves and the rapid growth of environmental issues have led to search for alternative
power sources.1,2 These include direct methanol fuel cells (DMFCs) as alternative green,
environmentally friendly, and sustainable power sources that can directly convert the
chemical energy of methanol into electrical energy.3,4 A polycrystalline Pt electrode is the
most commonly used electrode material in DMFCs. However, its high cost, rarity, low power
density, and poor COꢀpoisoning tolerance significantly hinder their commercialization and
practical application.5,6 In order to reach a high power density and low production cost for
DMFCs, wellꢀdesigned and fabricated highꢀperformance electrocatalysts with controlled
compositions and morphologies have been emerged as a key point for realizing high device
performance.5,6
In this area, carbonꢀbased nanomaterials, including activated carbon, carbon
nanotubes (SWCNT, DWCNT, MWCNT), GO, and rGO have enormous potential to boost
the performance of DMFCs.3,7,8 Among these, graphene plays a crucial role in the fabrication
of advanced composite/hybrid electrocatalysts for high performance DMFCs because of its
unique physicochemical properties, including a high specific surface area, superior electronic
conductivity, and excellent stability.8ꢀ13 Several attempts have been made to apply metal and
metal oxide nanomaterials incorporated graphene modified electrodes in DMFC applications
to attain a high power density.14,15
Metal oxide incorporated graphene composite gained tremendous attention because of
their lowꢀcost, facile synthesis, high electrocatalytic activity, and durability in DMFC
applications.4,8,11,14,15 Currently, cobalt oxide (Co3O4) nanomaterials are more attractive due
2

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to their simple preparation method, diverse morphology, high catalytic activity, and
applicability in electrocatalysis.16 Recently, Co3O4 nanomaterials combined with graphene
have shown excellent electrocatalytic activities. Hence, these have been applied in the field of
electrochemical energy conversion and storage.17 Xiao et al. reported the influence of the
surface structure on the electrocatalytic activity of Co3O4 anchored graphene sheets toward an
oxygen reduction reaction for a fuel cell application.18
In the present investigation, cobalt oxide nanocubes incorporated reduced graphene
oxide (rGOꢀCo3O4 nanocubes) were prepared using GO (simplified Hummer’s method19)
with Co(II) ions by a simple hydrothermal method and used for electrocatalytic methanol
oxidation. The asꢀprepared rGOꢀCo3O4 nanocubes were characterized using TEM, FESEM,
XRD, and Raman techniques. The electrochemical behavior of rGOꢀCo3O4 nanocubes
modified electrode was investigated using the CV and EIS techniques with a [Fe3(CN)6]
redox couple as an electrochemical probe. The electrocatalytic performance toward methanol
oxidation was investigated in an alkaline medium for rGOꢀCo3O4 nanocubes modified
electrodes with different wt. % of GO content and the optimized GO content for high
performance was found to be 2 wt. %, with a high peak current density of 362 ꢀA/cm2
during a forward scan. The high electrochemical performance, lowꢀcost, and ease of
fabrication make these rGOꢀCo3O4 nanocubes modified electrode a potential candidate for
direct methanol fuel cell applications.
Experimental Methods
Materials
Graphite flakes were purchased from Asbury Inc. (USA). Sulfuric acid (H2SO4, 98%),
potassium permanganate (KMnO4, >99%), hydrochloric acid (HCl, 35%), and ammonia
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solution (NH3, 25%) were purchased from R & M Chemicals. Cobalt acetate tetrahydrate
(Co(CH3COO)2.4H2O) was purchased from Sigma Aldrich. Hydrogen peroxide (H2O2, 35%)
and methanol (CH3OH) were obtained from Systerm, Malaysia. Distilled water was used
throughout the experimental work.
Synthesis of rGO-Co3O4 nanocubes
In the typical process for the preparation of rGOꢀCo3O4 nanocubes, the graphene
oxide (GO) was prepared by using a simplified Hummer’s method.19 A 12 mL of
Co(CH3COO)2.4H2O (83 mM) was mixed with different wt. % of GO (1, 2, 4, 8, and 12
wt. %) and stirred for 2 h to obtain a homogeneous solution. After that, 15 mL of 6 %
ammonia was slowly added dropꢀwise into the above reaction mixture under vigorous stirring.
Then, 75 mL of the reaction mixture was transferred to a 100 mL Teflonꢀlined stainless steel
autoclave and subjected to hydrothermal treatment at 180 °C for 12 h. Finally, the obtained
precipitate of rGOꢀCo3O4 nanocubes was washed five times with DI water and ethanol and
dried in a hot air oven at 60 °C. For comparison, Co3O4 and rGO were prepared using a
similar method without using GO and (Co(CH3COO)2. 4H2O), respectively.
Preparation of modified electrode
An rGOꢀCo3O4 nanocubes modified electrode was fabricated by dispersing 1 mg of
the synthesized rGOꢀCo3O4 nanocubes in 1 mL of deionized water and then sonicating it for
30 min to ensure a homogeneous dispersion. A 5 µL of the colloidal rGOꢀCo3O4 nanocubes
solution was cast on a polycrystalline Pt electrode and then dried in a hot air oven at 65 °C
for 1 h. This Pt/rGOꢀCo3O4 nanocubes modified electrode was used for the electrocatalytic
oxidation of methanol.
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Characterization techniques
The size and shape of the rGOꢀCo3O4 nanocubes were studied using a JEOL JEMꢀ
2100F highꢀresolution transmission electron microscope. The surface morphology and
elemental composition were examined using a JEOL JSMꢀ7600F field emission scanning
electron microscope. Raman spectra were acquired from a Renishaw inVia 2000 system with
a green laser emitting at 532 nm. The crystalline nature and phase identification of the rGOꢀ
Co3O4 nanocubes were recorded using a Philips X’pert Xꢀray diffractometer with copper Kα
radiation (λ = 1.5418 Å) at a scan rate of 0.02 degree/sec. All the electrochemical
measurements were carried out using a VersaSTATꢀ3 electrochemical analyzer (Princeton
Applied Research, USA) with a conventional threeꢀelectrode system under a nitrogen
atmosphere at room temperature (27 °C). The rGOꢀCo3O4 nanocubes modified electrode was
used as a working electrode, a platinum wire served as a counter electrode, and a Ag/AgCl
electrode was the reference electrode. The supporting electrolyte and target substrate were
composed of 0.1 M KOH and 0.1 M CH3OH, respectively. All the potentials are quoted
against the Ag/AgCl electrode unless otherwise mentioned.
Results and Discussion
Formation of rGO-Co3O4 nanocubes
Fig. 1 shows a schematic illustration of the formation of Co3O4 nanocubes
incorporated rGO sheets fabricated using a hydrothermal method. In this typical synthetic
process, a Co(II) precursor was added to the preformed GO solution, and the medium was
changed to basic with the help of an NH3·H2O solution. Upon the hydrothermal process, the
GOꢀCo(OH)2 transformed into rGOꢀCo3O4 nanocubes. The Co3O4 nanocubes were
incorporated like spacers between the disorderedly stacked graphene sheets.
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Morphological studies of rGO-Co3O4 nanocubes
The FESEM images of the rGOꢀCo3O4 nanocubes show that the Co3O4 nanocubes are
incorporated onto the surfaces of graphene sheets. As can clearly be seen in Fig. 2(c), the
rGOꢀCo3O4 nanocubes with 2 wt. % of GO lacked sufficient graphene sheet surfaces to
anchor the Co3O4 nanocubes, which resulted in a large number of Co3O4 nanocubes being
exposed on the surface. When the GO content was increased to 12 wt. % in the rGOꢀCo3O4
nanocubes (Fig. 2(c)) many of the Co3O4 nanocubes were sandwiched between the disorderly
stacked graphene sheets, and few of the Co3O4 nanocubes were exposed on the surface. For
comparison, the disorderly stacked graphene sheets and Co3O4 nanocubes prepared using the
hydrothermal method are shown in Fig. 2(a and b).
TEM images of the Co3O4 nanocubes and rGOꢀCo3O4 nanocubes are shown in Fig. 3.
There is a clear distinction between the bare and grapheneꢀsupported Co3O4 nanocubes. Fig.
3(a) shows the Co3O4 nanocubes without any graphene sheets. As seen, the obtained
materials had irregular shapes. It should be emphasized that the absence of graphene affected
the Co3O4 nanocubes formation. The Co3O4 nanocubes incorporated graphene is shown in
Fig. 3(b). It can be seen that the Co3O4 nanocubes are anchored on the transparent graphene
sheet surfaces. This result clearly reveals that the graphene plays a role in the formation of
Co3O4 nanocubes.
X-Ray diffraction analysis of rGO-Co3O4 nanocubes
The crystalline natures of the prepared GO, rGO, Co3O4 nanocubes, and rGOꢀCo3O4
nanocubes were studied by recording the XRD patterns, and their results are shown in Fig.
S1†. The GO (Fig. S1(a)†) shows a strong and sharp diffraction peak at the 2θ value of 10°
due to the (001) plane.20 After the hydrothermal process for GO, an intense sharp peak
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disappears at the 2θ value of 10°, and two new broad peaks appear at the 2θ values of 26.8°
and 42.7°, which can be attributed to the (002) and (100) planes for rGO, respectively (Fig.
S1(b)†). The two observed diffraction peaks originate from the disorderedly stacked graphene
sheets, which confirms the successful reduction of GO into rGO.20,21 The Co3O4 nanocubes
and rGOꢀCo3O4 nanocubes (Fig. S1(cꢀd)†) show a characteristic diffraction pattern at the 2θ
values around 31.2°, 36.8°, 44.7°, 55.5°, 59.2°, and 65.1°, which were readily indexed to the
respective (220), (311), (222), (400), (422), (511), and (440) planes of face centered cubic
Co3O4 (JCPDS card No. 42ꢀ1467).21,22 For the rGOꢀCo3O4 nanocubes (Fig. S1(d)†), the
diffraction peak of rGO is undistinguishable, which clearly indicates that the faceꢀtoꢀface
stacking of the rGO sheets is absent because of the formation of Co3O4 nanocubes on both
sides of the rGO sheets.22
Raman studies of rGO-Co3O4 nanocubes
Raman analysis is more suitable tool and most commonly employed to differentiate
the graphene (graphene oxide and reduced graphene oxide) from the very thin graphite sheets.
It is known from the previous reports23ꢀ26, the graphite sheets has inꢀphase vibration of the
highly ordered graphite lattice (G band) at 1575 cmꢀ1 and a very weak D band at 1355 cmꢀ1
due to the disorder graphite edges. In contrast, the graphene oxide and reduced graphene
oxide has well defined D and G band at 1355 and 1595 cm–1 in comparison with graphite23ꢀ26
.
The Raman spectra of the GO, rGO, Co3O4 nanocubes, and rGOꢀCo3O4 nanocubes were
recorded and are shown in Fig. 4. The Raman spectrum for GO shows D and G band peaks at
1355 and 1595 cm–1 due to the sp3 defects and inꢀplane vibrations of sp2 carbon atoms,
respectively.27,28 A small bump in Fig. 4(a,b) was observed related to the band 2D for GO
(2680 cmꢀ1) and rGO (2694 cmꢀ1) respectively. This shows the multilayer GO and rGO, also a
shift to higher wavelengths in the 2D band of rGO was observed after reduction from GO
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which indicates the restacking of graphene sheets due to the removal of functional group
from GO which may prevent restacking of GO sheets after it was reduced.28 It was observed
that 2D band of rGO in Fig. 4 (d) retain the same position as for GO showing no stacking of
graphene layers. It may because of Co3O4 nanocubes behave like spacer in between the
graphene sheets as confirmed by FESEM image. It was also demonstrated that D band
remained unchanged, while the G band of rGO was shifted to 1610 cm–1 because of the “selfꢀ
healing” characteristics of the rGO, which recovers the hexagonal network of carbon atoms
with defects. In addition D/G band intensity ratio for rGO greater than that of GO suggesting
decrease in sp2 domain of GO after reduction.27,29 This observation clearly indicates the
successful transformation of GO to rGO (Fig. 4(aꢀb)). The Raman spectrum of the rGOꢀ
Co3O4 nanocubes (Fig. 4(d)) shows four peaks at approximately 482, 525, 615, and 686 cm−1,
which correspond to the Eg, F2g, F2g, and A1g modes of Co3O4, along with the D and G bands
for rGO.30ꢀ32
Electrochemical behavior of rGO-Co3O4 nanocubes modified electrode
The rGO, Co3O4 nanocubes and rGOꢀCo3O4 nanocubes modified electrodes were
electrochemically characterized by recording the cyclic voltammograms in 0.1 M KOH at a
scan rate of 50 mVs–1 and their results are shown in Fig. 5(A). All the cyclic voltammograms
show a clear peak at ꢀ0.2 V and a small peak at ꢀ0.1 V due to the hydrogen
adsorption/desorption processes in this potential region. The anodic peaks observed for
Co3O4 nanocubes and rGOꢀCo3O4 nanocubes modified electrodes around 50 mV were due to
the adsorption of oxygen containing species such as, H2O and OH⁻33,34, and another peak
observed between 200–300 mV was attributed to the formation of higher valence oxides or
hydroxides of cobalt in the alkaline medium.34,351 The observed cathodic peaks at 70 and 480
mV for the Co3O4 nanocubes and rGOꢀCo3O4 nanocubes modified electrodes are related to
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the regeneration of Co(II) species.35 The overall electrochemical processes36ꢀ39 during the
anodic and cathodic scans are given in Eqn. (1) and (2) below:
Co3O4 + OH− + H2O
CoOOH + OH−
3CoOOH + eꢀ
… (1)
… (2)
CoO2 + H2O + eꢀ
The cyclic voltammograms for an rGOꢀCo3O4 nanocubes modified electrode in the
presence of 1 mM K3[Fe(CN)6] in 0.1 M KCl at a scan rate of 50 mVsꢀ1 shows a smaller peak
potential separation (ꢁEp) and larger peak current than those of a bare Pt electrode (Fig.
5(B)), indicating a rapid electron transfer and larger electroactive surface area for the rGOꢀ
Co3O4 nanocubes. After modification with rGO, Co3O4 nanocubes, and rGOꢀCo3O4
nanocubes, the ꢁEp values decreased by 76, 67, and 60 mV, respectively, when compared to
the bare Pt (80 mV), and the peak current increased on the order of rGOꢀCo3O4 nanocubes >
Co3O4 nanocubes > rGO when compared to the bare Pt electrode (Fig. 5(B)). The noticeable
increase in the anodic and cathodic peak current of the rGOꢀCo3O4 nanocubes compared to
the rGO confirmed the contribution of the Co3O4 nanocubes to an increase in the
electroactive surface area and the promotion of electron transfer. Notably, the rGOꢀCo3O4
nanocubes modified electrode showed a ꢁEp value that was very similar to the ideal kinetics
(59 mV) of a oneꢀelectron reversible process, suggesting the excellent conductivity and ideal
reversibility of the redox reaction.
The scan rate was varied in the range of 10–500 mVsꢀ1 for the rGOꢀCo3O4 nanocubes
modified electrode with 1 × 10ꢀ3 M of K3[Fe(CN)6] in 0.1 M KCl, and the results are shown
in Fig. S2(A)†. It can be seen that the redox peak current for the anodic and cathodic scan
increased linearly with the increasing scan rate, and no obvious shift is observed for the
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anodic and cathodic peak potentials. This clearly suggests that the electrochemical redox
reaction that occurs for the rGOꢀCo3O4 nanocubes modified electrode is reversible. A plot of
the anodic and cathodic peak current vs. square root of the scan rate is shown in Fig. S2(B)†,
which shows the linear relation for the rGOꢀCo3O4 nanocubes modified electrode (R2 =
0.9938 and 0.9942), indicating an adsorption controlled redox process.40
In order to investigate the electrochemical behavior of bare Pt, rGO, Co3O4 nanocubes
and rGOꢀCo3O4 nanocubes modified electrode, electrochemical impedance spectroscopic
(EIS) analyses were performed by dipping the electrodes into a solution containing 1 × 10ꢀ3
M K3[Fe(CN)6] in 0.1 M KCl at a scanning frequency range of 0.01–120,000 Hz. The
Nyquist plots for the bare Pt, rGO, Co3O4 nanocubes and rGOꢀCo3O4 nanocubes modified
electrodes show a line in the lowꢀfrequency region and an arc in the high frequency region
(Fig. 6(A) and Fig. S3). The bare Pt electrode shows a large semicircle compared to the other
modified electrodes, which is a result of the large chargeꢀtransfer resistance (Rct) at the
electrode/electrolyte interface due to the sluggish electron transfer kinetics. It can obviously
be seen that Rct decreased for the rGO, Co3O4 nanocubes and rGOꢀCo3O4 nanocubes
modified electrodes, which can be attributed to the presence of high conductive rGO sheets
and catalytically active Co3O4 nanocubes on the electrode surface. Fig. 6(B) shows the Bodeꢀ
phase plots of various modified electrodes in the frequency range of 0.1–10000 Hz. A higher
phase peak intensity can be seen at a frequency range of 100–1000 Hz, which is attributed to
the chargeꢀtransfer resistance of the modified electrode.41,42 Interestingly, the phase peaks in
the Bode plots for the rGO, Co3O4 nanocubes and rGOꢀCo3O4 nanocubes modified electrode
are shifted to a low frequency region of 0.1–100 Hz as a result of the high electron transfer
behavior of the highly conductive rGO sheets and the catalytically active Co3O4 nanocubes
based modified electrodes.
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Electrocatalytic oxidation of methanol at rGO-Co3O4 nanocubes modified electrode
The schematic representation of methanol oxidation at the Pt/rGOꢀCo3O4 modified
electrode is shown in Fig. 7. During the electrooxidation of methanol, the anodic oxidation
peak current is obtained due to the oxidation of freshly chemisorbed species coming from the
adsorption of methanol and the formation of intermediate carbonaceous species in the
forward scan and further it reacts with adsorbed CO on the electrode to produce CO2, H2O,
electron (e−). In the reverse scan, the oxidation peak is primarily associated with
removal/oxidation of carbonaceous species not completely oxidized in the forward scan,
rather than caused by freshly chemisorbed species. In order to investigate the electrocatalytic
activity of the prepared rGOꢀCo3O4 nanocubes modified Pt electrode, a cyclic voltammogram
was recorded in the presence of 0.1 M CH3OH and 0.1 M KOH at a scan rate of 50 mVs–1,
and its results is shown in Fig. 8(d). For comparison purposes, cyclic voltammograms were
also recorded for polycrystalline Pt, rGO and Co3O4 nanocubes modified electrodes under the
same conditions (Fig. 8(aꢀc)). Interestingly the rGOꢀCo3O4 nanocubes modified electrode
showed a higher electrocatalytic oxidation current density during forward (358 ꢀA/cm2) and
backward (196 ꢀA/cm2) scans compared to the other modified electrodes (poly crystalline Pt,
rGO, and Co3O4 nanocubes). The peak currents observed during the forward and reverse
scans were due to the methanol oxidation and removal of the residual carbonaceous species
formed in the forward scan, respectively.43,44 The higher electrocatalytic activity of the rGOꢀ
Co3O4 nanocubes modified electrode was mainly due to the combination of the cubic Co3O4
structure and highly conductive reduced graphene oxide (Fig. 2 and 4) and this catalytic
activity is good in agreement with the FESEM and Raman results (Fig. 2 and 4). The
enhanced structural features of Co3O4 nanocubes incorporated rGO resulted in the efficient
electron transport to the active sites on the catalyst than the bare Co3O4 nanocubes. Hence,
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the FESEM images (Fig. 2) supported the fact that the high dispersion of Co3O4 nanocubes
into the rGO matrix leads to significantly enhanced the electrocatalytic performance. Xiao et
al. reported that the electrocatalytic activity was significantly influenced by the graphene
when it was incorporated into different Co3O4 nanostructures.18
Optimization of rGO content in rGO-Co3O4 nanocubes modified electrode for efficient
methanol oxidation
Optimizing the weight percentage of the GO in the rGOꢀCo3O4 nanocubes is very
important to attain a high electrocatalytic performance toward methanol oxidation. In this
regard, the wt. % of the GO content in the rGOꢀCo3O4 nanocubes was varied and the obtained
results are shown in Fig. S4(A)†. The observed results clearly suggest that the oxidation
current density was increased during the forward and reverse scans with the increasing of the
GO content until it reached a maximum of 2 wt. % of GO in composite of rGOꢀCo3O4. After
this, a further increase in the GO content of the rGOꢀCo3O4 nanocubes led to a decrease in the
current density during the forward and reverse scans (Fig. S4(A)†). The observed current
densities for methanol oxidation during the forward and reverse scan for rGOꢀCo3O4
nanocubes with different GO contents are summarized in Fig. S4(B)†. The decrease in the
current density with increasing GO content, which was due to the decrease in the anodic peak
current that resulted from the restacking of the graphene nanosheets, substantially decreased
the active catalytic sites on the electrocatalysts.45,46
Influence of scan rate and methanol concentration on electrocatalytic activity of rGO-
Co3O4 nanocubes modified electrode
The effect of the scan rate on the electrocatalytic oxidation of methanol was
investigated for the rGOꢀCo3O4 nanocubes modified electrode in the presence of 0.1 M
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CH3OH and 0.1 M KOH at different scan rates in the range of 10–200 mVsꢀ1, and the results
are shown in Fig. 9(A). It can be seen that the oxidation peak current density increased with
increasing scan rate. The observed anodic peak current density is plotted against the scan rate
for the rGOꢀCo3O4 nanocubes modified electrode and is shown in the inset of Fig. 9(A). An
increase in the scan rate likely enhanced the electron movement. The rGOꢀCo3O4 nanocubes
modified electrode showed a linear relationship between the anodic peak current density
obtained from the forward scans and the square root of the scan rate toward methanol
oxidation. This suggests that the oxidation of methanol at the rGOꢀCo3O4 nanocubes
modified electrode is a diffusionꢀcontrolled process.
The methanol concentration is a crucial factor in the practical application of direct
methanol fuel cells. The influence of the methanol concentration was studied by varying the
methanol content in an rGOꢀCo3O4 nanocubes modified electrode at a scan rate of 50 mVsꢀ1
in the presence of 0.1 M KOH and the results are shown in Fig. 9(B). It can be observed that
the anodic peak current density increases and the anodic peak potential have a slightly
positive shift with an increase in the methanol concentration. The positive shift in the anodic
peak potential with increasing methanol concentration is due to the saturation of active sites
at the surface of the rGOꢀCo3O4 nanocubes modified electrode.
Reversibility and stability of the rGO-Co3O4 nanocubes modified electrode towards
methanol oxidation
The reliability of the Pt electrode modified with the rGOꢀCo3O4 nanocubes towards
the electrocatalytic oxidation of methanol was investigated by repeating the CV
measurements continually up to 100 cycles. Fig. 10(A) shows the CV curves for the proposed
Pt/rGOꢀCo3O4 modified electrode electrode in 0.1 M KOH and 0.1 M CH3OH. It can be
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clearly seen that the CV curves are stable and the current density at ꢀ150 mV (vs. Ag/AgCl)
shows 108 % retention (increased from 350 to 380 ꢀA/cm2) after 100 cycles. Thus, the
proposed Pt/rGOꢀCo3O4 modified electrode shows good sensitivity and better stability
towards the electrocatalytic oxidation of methanol.
The electrocatalytic stability of the Pt/rGOꢀCo3O4 modified electrode was studied
using chronoamperometry. Fig. 10(B) showed the chronoamperometric curve of 1 mM
CH3OH in 0.1 KOH for the Pt/rGOꢀCo3O4 modified electrode at an applied potential of ꢀ150
mV and the electrode shows the rapid decrease in the potentiostatic current at the initial stage
during the oxidation of methanol due to the formation of intermediate species, such as COads
and CHOads etc.,47,48. Then, the current was gradually decayed and achieved the pseudoꢀ
steady state. Similar observation also reported to illustrate the stability of the modified
48
electrode for the electrocatalytic oxidation of methanol with the Pt/graphene , Pt/Vulcan 48,
Pt/C–MoC–Gl 49.
Conclusions
The Co3O4 nanocubes incorporated in reduced graphene oxide successfully
synthesized using a simple hydrothermal method. The asꢀprepared rGOꢀCo3O4 nanocubes
combined the unique properties of graphene with the effects of Co3O4 nanocubes and showed
a superior electrocatalytic activity and high poisoning tolerance for methanol electroꢀ
oxidation compared with bare Pt, rGO and Co3O4 nanocubes modified electrodes. In
particular, the best electrocatalytic activity for the rGOꢀCo3O4 nanocubes modified electrode
in an alkaline environment could be achieved with an optimized GO content of 2 wt. %, and
it showed an anodic peak current density of 362 ꢀA/cm2 during the forward scan. Further, the
modified electrode showed good reversibility with the retention of 108 % and stability
14

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towards methanol oxidation. Considering the lowꢀcost, facile, and controllable method for the
preparation of rGOꢀCo3O4 nanocubes and the improved electrocatalytic activity toward
methanol oxidation, this direct strategy can applied to the development of metal oxideꢀ
graphene composites as new electrode materials for high performance direct methanol fuel
cells.
Acknowledgements
This work was financially supported by a University of Malaya Research Grant
UMRG Programme (RP007Cꢀ13AFR) Science Fund from the Ministry of Science,
Technology and Innovation (06ꢀ01ꢀ04ꢀSF1513); and a High Impact Research Grant from the
Ministry of Higher Education of Malaysia (UM.C/625/1/HIR/MOHE/SC/21).
†Electronic supplementary information (ESI) available: XRD analysis, electrochemical
properties and electrocatalytic methanol oxidation are available in ESI.
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Figures and Caption
Fig. 1 Schematic representation for the preparation of rGO-Co3O4 nanocubes.
Fig. 2 FESEM images of (a) rGO, (b) Co3O4 nanocubes, and (c and d) rGO-Co3O4 nanocubes
with 2 and 12 wt. % of GO.
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Fig. 3 TEM images of (a) Co3O4 nanocubes and (b) rGO-Co3O4 nanocubes.
G
a
D
b
G
D
500
1000
1500
2000
2500
3000
500
1000
1500
2000
2500
3000
A
A
1g
1g
d
c
D
G
E
g
F
2g
F
2g
E
F
g
2g
F
2g
400 450 500 550 600 650 700 750
450
500
550
600
650
700
750
800
500
1000
1500
2000
2500
3000
500
1000
1500
2000
2500
3000
Wavelength (cmꢀ1)
Fig. 4 Raman spectra obtained for the (a) GO, (b) rGO, (c) Co3O4 nanocubes, and (d) rGO-
Co3O4 nanocubes.
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ꢀ5
1.5x
10
A
ꢀ5
x10
1.0
ꢀ6
10
5.0x
a
0.0
b
c
ꢀ6
ꢀ5.0x
10
ꢀ5
x10
ꢀ1.0
ꢀ5
10
ꢀ1.5x
ꢀ0.
4
ꢀ0.
2
0.0
0.2
0.4
E (V) vs Ag/AgCl
B
4.0ꢁ
2.0ꢁ
0.0
ꢀ2.0ꢁ
ꢀ4.0ꢁ
ꢀ6.0ꢁ
(a) Bare Pt
(b) Pt/rGO
(c) Pt/Co3O4
(d) Pt/rGOꢀCo3O4
ꢀ0.2
0.0
0.2
0.4
0.6
E(V) vs Ag/AgCl
Fig. 5 (A) CVs obtained for the (a) rGO, (b) Co3O4 nanocubes and (c) rGO-Co3O4 nanocubes
modified Pt electrodes in the presence of 0.1 M KOH at scan rate of 50 mVs-1. (B) CVs
recorded at (a) bare Pt, (b) rGO, (c) Co3O4 nanocubes and (d) rGO-Co3O4 nanocubes
modified electrodes for 1 × 10-3 M K3[Fe(CN)6] in 0.1 M KCl at a scan rate of 50 mVs-1.
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200000
150000
100000
50000
0
A
d
c
b
a
0
25000
50000
75000
100000
125000
Zre (ohms)
80
70
60
50
40
30
20
10
0
B
d
c
b
a
0.01
0.1
1
10
100
1000
10000
Frequency (Hz)
Fig. 6 (A) Nyquist plots and (B) Bode phase plots obtained for (a) bare Pt, (b) rGO, (c) Co3O4
nanocubes and (d) rGO-Co3O4 nanocubes modified Pt electrodes for 1 × 10-3 M K3[Fe(CN)6]
in 0.1 M KCl.
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Fig. 7. Schematic mechanism for the electrocatalytic oxidation of methanol at Pt/rGO-Co3O4
modified electrode.
3.75x10ꢀ4
d
3.00x10ꢀ4
c
2.25x10ꢀ4
b
1.50x10ꢀ4
7.50x10ꢀ5
a
0.00
ꢀ0.5 ꢀ0.4 ꢀ0.3 ꢀ0.2 ꢀ0.1
0.0
0.1
0.2
E (V) vs Ag/AgCl
Fig. 8 CVs obtained for the (a) bare Pt, (b) rGO, (c) Co3O4 nanocubes and (d) rGO-Co3O4
nanocubes modified Pt electrodes in the presence of 0.1 M CH3OH and 0.1 M KOH at a scan
rate of 50 mVs-1.
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2.5x10ꢀ4
2.0x10ꢀ4
1.5x10ꢀ4
1.0x10ꢀ4
5.0x10ꢀ5
2.5x10ꢀ4
2.0x10ꢀ4
1.5x10ꢀ4
1.0x10ꢀ4
A
0
50
100
150
200
Scan rate (mV/sec)
10 mV/s
25 mV/s
50 mV/s
75 mV/s
100 mV/s
125 mV/s
150 mVs
175 mV/s
200 mV/s
5.0x10ꢀ5
0.0
ꢀ5.0x10ꢀ5
ꢀ0.6
ꢀ0.4
ꢀ0.2
0.0
0.2
E (V) vs Ag/AgCl
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2x10ꢀ3
1.0x10ꢀ3
8.0x10ꢀ4
6.0x10ꢀ4
4.0x10ꢀ4
B
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Methanol Concentration (M)
2.0x10ꢀ4
0.0
ꢀ0.6 ꢀ0.5 ꢀ0.4 ꢀ0.3 ꢀ0.2 ꢀ0.1 0.0 0.1 0.2
E (V) vs Ag/AgCl
Fig. 9 (A) CV recorded for rGO-Co3O4 nanocubes modified Pt electrode in presence of 0.1 M
CH3OH and 0.1 M KOH at different scan rates in range of 10–200 mVs-1. (Inset) Plot of scan
rate versus anodic peak current density obtained from the CV measurements. (B) CV
recorded for rGO-Co3O4 nanocubes modified electrode at scan rate 50 mVs-1 in presence of
0.1 M KOH and various concentrations of CH3OH. (Inset) Plot of methanol concentration
versus anodic peak current density for rGO-Co3O4 nanocubes modified Pt electrode.
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4.0x10ꢀ4
3.0x10ꢀ4
2.0x10ꢀ4
Cycleꢀ1
A
Cycleꢀ10
Cycleꢀ20
Cycleꢀ30
Cycleꢀ40
Cycleꢀ50
Cycleꢀ75
Cycleꢀ100
Cycle#100
Cycle#1
1.0x10ꢀ4
0.0
ꢀ0.5 ꢀ0.4 ꢀ0.3 ꢀ0.2 ꢀ0.1
0.0
0.1
0.2
E (V) vs Ag/AgCl
1.5ꢁ
1.0ꢁ
Pt/rGOꢀCo O
3 4
B
500.0n
0.0
400.0
800.0
1.2k
1.6k
2.0k
Elapsed Time (Sec)
Fig. 10. (A) Cyclic voltammograms obtained for the rGO-Co3O4 nanocubes modified Pt
electrode in the presence of 0.1 M CH3OH and 0.1 M KOH at a scan rate of 50 mVs-1 with 1
to 100 cycles. (B) Chronoamperometric curve obtained for the Pt/rGO-CO3O4 modified
electrode in the presence of 1 µM CH3OH and 0.1 M KOH at a scan rate of 50 mVs-1 at a
fixed potential of -150 mV vs. Ag/AgCl.
7