High electrocatalytic performance of platinum and manganese dioxide nanoparticle decorated reduced graphene oxide sheets for methanol electro-oxidation

Article abstract of DOI:10.1039/c4ra05507a

In this study we report the synthesis of a novel Pt-MnO₂-ERGO electrocatalyst by the deposition of MnO₂and Pt nanoparticles decorated on reduced graphene oxide sheets using a simple electrochemical method. The as prepared MnO₂and Pt nanoparticles decorated on the reduced graphene oxide sheets (Pt-MnO₂-ERGO electrocatalysts) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The cyclic voltammetric (CV), chronoamperometric and electrochemical impedance spectroscopic (EIS) measurements show high electrocatalytic activity and stability of the electrodes towards the methanol oxidation reaction in nitrogen saturated sulfuric acid aqueous solutions and in mixed sulfuric acid and methanol aqueous solutions. The voltammetric results show the electrocatalytic characteristics of the Pt-MnO₂-ERGO electrocatalysts, which exhibit superior electrocatalytic activity (including good poison tolerance, and low onset potential) and stability toward electro-oxidation of methanol in a model reaction. The electrochemical impedance spectroscopic result shows good electrocatalytic activity in relation to methanol oxidation and improved tolerance of CO. In addition, the as designed Pt-MnO₂-ERGO nanocomposite modified electrode with a novel structure can be directly employed for fuel cells. This journal is

Full text of DOI:10.1039/c4ra05507a

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PAPER
High electrocatalytic performance of platinum and
manganese dioxide nanoparticle decorated
reduced graphene oxide sheets for methanol
electro-oxidation†
Cite this: RSC Adv., 2014, 4, 41387
a
b
a
A. T. Ezhil Vilian, Muniyandi Rajkumar, Shen-Ming Chen, Chi-Chang Hu,b
*
Karunakara Moorthy Boopathicd and Chih-Wei Chucd
In this study we report the synthesis of a novel Pt–MnO2–ERGO electrocatalyst by the deposition of MnO2
and Pt nanoparticles decorated on reduced graphene oxide sheets using a simple electrochemical
method. The as prepared MnO2 and Pt nanoparticles decorated on the reduced graphene oxide sheets
(Pt–MnO2–ERGO electrocatalysts) were characterized by scanning electron microscopy (SEM),
transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy
(EDX) and X-ray photoelectron spectroscopy (XPS). The cyclic voltammetric (CV), chronoamperometric
and electrochemical impedance spectroscopic (EIS) measurements show high electrocatalytic activity
and stability of the electrodes towards the methanol oxidation reaction in nitrogen saturated sulfuric acid
aqueous solutions and in mixed sulfuric acid and methanol aqueous solutions. The voltammetric results
show the electrocatalytic characteristics of the Pt–MnO2–ERGO electrocatalysts, which exhibit superior
electrocatalytic activity (including good poison tolerance, and low onset potential) and stability toward
electro-oxidation of methanol in a model reaction. The electrochemical impedance spectroscopic result
shows good electrocatalytic activity in relation to methanol oxidation and improved tolerance of CO. In
addition, the as designed Pt–MnO2–ERGO nanocomposite modified electrode with a novel structure
can be directly employed for fuel cells.
Received 9th June 2014
Accepted 20th August 2014
DOI: 10.1039/c4ra05507a
www.rsc.org/advances
with the Pt catalysts, and poor kinetics due to catalyst poisoning
by the carbon intermediate species produced during the
Introduction
oxidation of methanol. The unstable catalytic activity, poor
durability, and high cost have limited the commercial prospects
of DMFCs.6,7 A great deal of effort has been devoted to reducing
the use of Pt and enhancing the catalytic efficiency of Pt for
methanol oxidation in fuel cells and practicality in industrial
applications.8
Over the past few decades, direct methanol fuel cells (DMFCs)
have generated tremendous interest as green power sources for
portable electronics owing to their advantages of high-energy
conversion efficiency, system simplicity, environmental friend-
liness, low operating temperature and the storage convenience
of liquid fuel cells.1–3 However, the successful commercial
application of DMFCs is still hindered by several technological
challenges, including the high cost and insufficient durability
of the widely used metal-based catalysts,4,5 critical problems
In recent years, many bi- or trimetallic Pt-based catalysts
such as Pt–Ru, Pt–Co–Ru, and Pt–Co–Sn have been devel-
oped.9–12 On the other hand, graphene, a two-dimensional (2D)
carbon material with a single-atom thick sheet of hexagonally
arrayed sp2-bonded carbon atoms has attracted a great deal of
attention from both the scientic and industrial communi-
ties.13–15 It is emerging as one of the most appealing catalytic
support materials due to its unique structure and excellent
properties such as superior electrical conductivity, excellent
mechanical exibility, high thermal and chemical stability, and
extremely large surface area.16,17 Graphene is being integrated
with other materials in order to harness its favorable properties
for practical applications including in metals, semiconductors,
ceramics, polymers and other carbon materials.18 Graphene
oxide (GO) is a derivative of graphene, which has many oxygen-
aDepartment of Chemical Engineering and Biotechnology, National Taipei University of
Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, Republic of
China. E-mail: smchen78@ms15.hinet.net; Fax: +886 2270 25238; Tel: +886 2270
17147
bLaboratory of Electrochemistry & Advanced Materials, Department of Chemical
Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of
China
cNanoscience and Technology Program, Taiwan International Graduate Program,
Academia Sinica, Taipei 115, Taiwan, Republic of China
dResearch Center for Applied Science, Academia Sinica, Taipei115, Taiwan, Republic of
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra05507a
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containing functional groups on its surface (hydroxyl, epoxide, analysis was carried out using a PHI 5000 Versa Probe equipped
carbonyl and carboxyl groups) which provide sites for the with an Al Ka X-ray source (1486.6 eV). Raman spectra were
anchoring and dispersion of metal nanoparticles.19 It has been measured with a Raman spectrometer (Dong Woo 500i, Korea)
found that graphene-supported Pt-based catalysts exhibit equipped with a charge-coupled detector. XRD analysis was
improved performance and create less poisoning by CO-like carried out using an XPERT-PRO diffractometer (PANalytical
˚
intermediates during methanol oxidation than catalysts sup- B.V., the Netherlands) using Cu Ka radiation (k ¼ 1.54 A). The
ported on carbon.20 Several forms of carbon materials have been current and power were measured using precision multimeter
considered in robust strategies for the creation of DMFCs, such (Keithley instruments; model 2400) in a room atmosphere.
as hollow carbon hemispheres,21 carbon nanotubes (CNTs),22
carbon nanobers,23 carbon nanorods,24 carbon nanospheres,25
Synthesis of graphene oxide
graphene,26 and so on. Among the carbon materials, graphene
The GO used in the experiments was synthesized from natural
is regarded as a suitable supporting material for the loading of
ake graphite powder by a modied Hummers method.28
Pt nanoparticles in fuel cells, due to the large specic surface
area, excellent electronic conductivity, thermal stability and
H2SO4 (150 mL) and then 25 g of KMnO4 was slowly added
durability.27
ꢀ
Briey, graphite powder (5.0 g) was put into 0 C concentrated
under ice cooling. The mixture was stirred continuously for 30
To the best of our knowledge, there has been no study on the
min. Aer the addition of 600 mL of deionized water while the
effects of MnO2 and Pt nanoparticles decorated on reduced
temperature was kept under 50 ꢀC, 250 mL of water and 6 mL of
graphene oxide sheets (Pt–MnO2–ERGO) and its electrocatalytic
H2O2 (30 weight%) were subsequently added to reduce the
applications towards methanol oxidation. The experimental
residual KMnO4. The reaction was allowed to continue for 30
conditions related to the preparation of the Pt–MnO2–ERGO
min, yielding a brilliant yellow solution. Finally, the solid
electro catalyst nanoparticles are discussed and the samples
suspension was rst washed using 2 M of HCl solution and then
were characterized using X-ray diffraction (XRD), energy-
washed 3–4 times with ethanol and dried overnight in a vacuum
dispersive X-ray spectroscopy (EDX), scanning electron micros-
at 60 ꢀC. The graphite oxide slurry was then dried in a vacuum
copy (SEM), Transmission electron microscopy (TEM) and X-ray
ꢀ
oven at 60 C for 48 h before use. Aerwards, the sample was
photoelectron spectroscopy (XPS) analysis. The catalytic activity
prepared by dispersing 0.5 mg mLꢁ1 of GO in deionized water
of the as prepared MnO2 and Pt nanoparticles decorated on the
with the aid of ultra-sonication for 30 minutes.
reduced graphene oxide sheets for methanol oxidation were
studied using cyclic voltammetry (CV), chronoamperometric
measurements and electrochemical impedance spectroscopy
Preparation of the ERGO–MnO2–Pt modied electrodes
(EIS) techniques. The Pt–MnO2–ERGO nanocomposite elec- The glassy carbon electrode (GCE) with a diameter of 3 mm was
trode demonstrate good electrocatalytic activity towards meth- polished with an alumina (particle size of about 0.05 mm)/water
anol oxidation, improved tolerance of CO and also could be slurry using a Buehler polishing kit. It was then washed with
directly employed for fuel cell applications.
deionized water and ultrasonicated for 3 min each in water and
ethanol to remove any adsorbed alumina particles or dirt from
the electrode surface and nally dried. The typical procedure
for constructing the ERGO–MnO2–Pt modied electrode was
schematically shown in Scheme 1. A 5 mL of GO dispersion was
drop casted on the pre-cleaned GCE and dried in air oven at
30 ꢀC. The modied GCE was then rinsed with water to remove
loosely adsorbed GO. Higher amounts of GO could agglomerate
on the electrode surface, affecting the catalytic activity and
stability. Therefore, an optimal concentration of 0.5 mg mLꢁ1
was used. The GO lm modied GCE was gently washed with
water and transferred to an electrochemical cell containing 0.05
M PBS (pH 5) aer which 30 successive cycles of electrochemical
reduction were performed in the potential range between 0 and
Experimental section
Chemicals
Graphite (powder, <20 mm), H2PtCl6$6H2O, KMnO4, H2SO4,
methanol and commercial Pt–C were purchased from Sigma-
Aldrich and used without further purication. Double
distilled water (with a resistivity of 18.25 MU cm) was employed
throughout the experiments. All other analytical grade reagents
were used without further purication.
Apparatus
The CV measurement was carried out at a CH Instrument 405A ꢁ1.5 V at a scan rate of 0.05 V sꢁ1 (see Fig. S1†). The rst large
electrochemical workstation (Shanghai Chenhua Co., China). A cathodic peak appeared at ꢁ1.1 V, corresponding to the elec-
three-electrode system was employed, including a working trochemical reduction of oxygen functionalities of GO. The
ERGO–MnO2–Pt modied GCE electrode, a saturated Ag/AgCl/ epoxy and hydroxyl groups on the basal plane were mostly
KCls reference electrode and a platinum wire counter elec- decorated with GO sheets, while carbonyl and carboxyl groups
trode. SEM images were measured with a Hitachi S-3000 H and were located at the edges.29 Then, the electrochemically reduced
EDX images were recorded using a HORIBA EMAX X-ACT Model graphene oxide (ERGO) modied GCE was dried under an
51-ADD0009. Transmission electron microscopy (TEM) images infrared lamp for few minutes. Electro deposition of MnO2 was
were collected by using a Philips TECNAI 20 microscope (200 performed on the as-ERGO modied GCE electrode using
kV). EIS was carried out at a frequency range of 0.1 Hz to 100 scanning between the potentials 0.5 V and ꢁ0.3 V at a rate of
kHz with a ZAHNER instrument (Kroanch, Germany). XPS 0.02 V sꢁ1 in a N2-saturated a solution containing 10 mm
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Scheme 1 Schematic representation of fabrication of Pt–MnO2–ERGO nanocomposite modified electrode.
KMnO4 + 0.04 M H2SO4 for 6 cycles.30 Later, electrochemical are in agreement with those obtained in previous studies.31
deposition of Pt nanoparticles on the MnO2–ERGO modied Further evidence for the formation of MnO2 can be seen in the
electrode surface was carried out by immersing in an aqueous XPS spectrum of Mn for the MnO2–ERGO sample, where Mn
solution containing 1 mM K2PtCl6 in 0.5 M H2SO4 for 10 cycles 2p3/2 and Mn 2p1/2 peaks are observed at 642.2 eV and 654.0 eV,
in the potential range of ꢁ0.25 to 1.0 V (see Fig. S2†). The as respectively, as presented Fig. 1C. In addition, as can be
prepared catalyst can posses a better electrocatalytic perfor- observed in Fig. 1D, there are two peaks in the Pt 4f binding
mance under the above-mentioned conditions. Each single fuel energy region of Pt–MnO2–ERGO at 70.6 eV and 73.9 eV, which
cell set up consists of an anode and cathode compartments. The are attributed to 4f7/2, and 4f5/2 of metallic Pt, respectively. To
as prepared nanocomposite electrode posses higher open evaluate the surface oxidation states of Pt, the Pt 4f spectra were
circuit voltage and power density at 1 M methanol solution. On deconvoluted into three doublets, which are assigned to the
the other hand, The stability of the modied Pt–MnO2–ERGO different oxidation states of Pt. The most intense doublet
electrode was assessed by immersing it in a nitrogen-saturated (around 71 eV and 74 eV) is assigned to metallic Pt.32 Thus, it
0.1 M H2SO4 solution using a scan rate of 50 mV sꢁ1. The can be concluded that Pt–MnO2–ERGO are present in the
electrocatalytic activity of the methanol oxidation reaction was prepared nano electro catalyst.
measured by immersion in a nitrogen-saturated 0.1 M H2SO4 + 1
Fig. 2A shows the results of Raman spectroscopy, another
M CH3OH solution at a scan rate of 50 mV sꢁ1 until repeatable powerful method widely used in studies of GO and ERGO. The
cyclic voltammograms were attained. Furthermore, all experi- results reveal two prominent peaks at the typical D band (1330
ments were carried out at ambient temperature.
cmꢁ1) and G band (1588 cmꢁ1) for GO, which correspond to the
presence of sp3 defects and tangential vibrations of sp2 carbon
atoms in the hexagonal plane, respectively. While the intensity
ratio of the D and G bands (ID/IG) of GO is about 0.98, the ID/IG of
ERGO has increased to 1.31 due to a decrease in the average size
of the sp2 carbon network upon the electrochemical reduction
of the exfoliated GO. This is in agreement with that of XRD
results above.
Results and discussion
Surface characterization of the Pt–MnO2–ERGO
Fig. 1A shows the XPS spectra for Pt–MnO2–ERGO, in
which elements of Pt, C, Mn, and O are detected in the
Pt–MnO2–ERGO electro catalyst and normalized to produce the
graphite carbon peak at 284.6 eV. The C 1s XPS spectra of GO
show binding energies at 284.6 (C]C), 285.48 (C–OH), 286.68
(C–O–C), 287.39 (C]O), and 288.55 eV (O]C–O). These values
Fig. 2B shows the XRD patterns of Pt–MnO2–ERGO. The
diffraction peaks at 2q angles of 24.28ꢀ and 24.35ꢀ in the XRD
pattern of graphite can be assigned to the (002) facets of the
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Fig. 1 (A) XPS spectra of Pt–MnO2–ERGO electrocatalysts (B) high-resolution XPS spectra of C 1s, (C) the Mn 2p, and (D) Pt 4f core-level spectra
of the as fabricated Pt–MnO2–ERGO electrocatalysts.
hexagonal crystalline graphite, respectively, and indicate that conrming the existence of MnO2 between the Pt nanoparticles.
the GO has been reduced to ERGO. Furthermore, the diffraction The wrinkles on the moderately reduced ERGO sheets on the
peaks at around 26.6ꢀ, 33.8ꢀ, 51.9ꢀ, and 61.8ꢀ are due to ITO help to maintain the high surface area important for pre-
diffraction at the (100), (101), (102), and (110) planes of MnO2, venting aggregation of the ERGO. Furthermore, The MnO2
(JCPDS card no. 44-0141) respectively, and conrm that the as- anchored on the moderately reduced ERGO–ITO surface. This
prepared MnO2 nanoparticles are well-crystallized.33 The strong can be attributed to the oxygen functionalities and negative
diffraction peaks at 2q ¼ 39.81ꢀ, 46.09ꢀ, 67.79ꢀ and 81.33ꢀ charges at the ERGO surface, which favor the adsorption of
observed on the Pt–MnO2–ERGO are assigned to the charac- MnO2. Further removal of these oxygen-containing groups from
teristic (111), (200), and (220) crystalline planes of Pt, the ERGO surface leads to a remarkable increase in MnO2
respectively.
particle size due to aggregation, indicating that the oxygen-
containing groups on ERGO surfaces do play an important
role in enhancing the loading of MnO2 as can be seen in Fig. 3B.
Further conrmation of the existence of ERGO–MnO2 is found
in the EDX spectra, which show peaks corresponding to Mn
(20%), O (65%), and C (15%), as shown in Fig. 3E. Furthermore,
the SEM micrographs show uniformly distributed platinum
nanoparticles that have directly grown on the moderately
reduced external ERGO surface (see Fig. 3C) due to the existence
of an electronic interaction between the negatively charged
[PtCl6]4ꢁ and oxygen functionalities at the ERGO and MnO2
surface. This provides a larger active surface area. In addition,
the morphologies of the Pt–MnO2–ERGO nanocomposite are
illustrated. These improve the electrocatalytic activity as well.
Morphological studies
The morphology and the size of the ERGO, MnO2–Pt, MnO2–ERGO
and Pt–MnO2–ERGO on the ITO electrodes were examined by
SEM, TEM and EDX analysis. As can be seen from Fig. 2C ERGO
akes with broad lamellar structures or folds, which provide the
large surface-area with thickness 3–4 nm are formed. Fig. 3A
shows the morphologies of the MnO2 and Pt nanoparticles,
which are highly agglomerated on the ITO surface, so the
morphologies between them cant able to distinguish separately
in the gure. In order to conrm the presence of these materials
we do EDX spectrum. Fig. 3D shows the EDX spectrum shows
peaks corresponding to Mn (40%), O (50%) and Pt (10%),
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Fig. 2 Raman spectra of GO, ERGO. (B) XRD patterns of (a) ERGO, (b) MnO2–ERGO, (c) ERGO–Pt, (d) Pt–MnO2–ERGO electro catalysts. (C) SEM
images of ERGO and TEM images of Pt–MnO2–ERGO electro catalysts.
The corresponding EDX spectra for Pt–MnO2–ERGO nano-
composite appearing in Fig. 3F show peaks corresponding to
the elements of C (10%), O (45), and Mn (35%), Pt (10%),
conrming the existence of metallic Pt nanoparticles on the
surface of the MnO2–ERGO nanosheets, which is very condu-
cive to the electro-oxidation of methanol. In addition Fig. 2D
shows HRTEM images of the Pt–MnO2–ERGO, respectively.
The image reveals MnO2 nanostructure coated on the ERGO
sheet surface. Which will direct the carbon–MnO4 reactions
occur preferentially at the defective sites in the ERGO, and
therefore a signicant amount of framework defects are
consumed in the rst step. The residual carbon–oxygen func-
tional groups on the MnO2–ERGO sheets play an important
role in distributing the Pt nanoparticles. Moreover, the pres-
ence of hydrous MnO2 can create large hydrophilic regions on
the surface of ERGO, which can facilitate the diffusion of Pt2+
ions and effectively prevent agglomeration of the metal
nanoparticles.
Electrochemical investigation of Pt–MnO2–ERGO modied
GCE
EIS is an exact method to elucidate the electrochemical prop-
erties of the proposed lm. The EIS analysis has been studied by
analyzing the Nyquist plots of the corresponding lms. Here the
respective semicircle parameters correspond to the electron
transfer resistance (Ret), solution resistance (Rs) and double
layer capacity (Cdl) of the lms. The plot of the real component
(Z0) and the imaginary component ꢁZ00(imaginary) resulted in
the formation of a semi-circular Nyquist plot. From the shape of
an impedance spectrum, the electron-transfer kinetics and
diffusion characteristics can be extracted. The respective semi-
circle parameters correspond to the electron transfer resistance
(Ret) and the double layer capacity (Cdl) nature of the modied
electrode. Fig. 4A displays the Nyquist plot for the comparison
of different modied electrodes (a) Pt–C, (b) Pt–MnO2, (c)
Pt–ERGO and (d) Pt–MnO2–ERGO modied electrodes in a
nitrogen saturated solution of 0.1 M H2SO4 containing 1 M
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Fig. 3 SEM images of (A) Pt–MnO2, (B) MnO2–ERGO, (C) Pt–MnO2–ERGO, and EDX spectra of (D) Pt–MnO2, (E) MnO2–ERGO (F) Pt–MnO2–ERGO
electro catalysts.
CH3OH. Fig. 4A inset shows the randles equivalent circuit on its surface. Therefore, the composite lm could be efficiently
model for the proposed lm. On comparison with different used for the electrocatalytic reactions. A simplied randles
modied electrodes, the Pt–MnO2–ERGO exhibits a smaller circuit model (Fig. 4A, inset) has been used to t the impedance
semicircle when compare with Pt–ERGO, Pt–MnO2, and Pt–C spectra. The randles circuit model well suites with the imped-
electrocatalysts in the 0.1 M H2SO4 + 1 M CH3OH electrolyte ance spectroscopic results and the t model error for the lm
solution. From these results, we can clearly indicate that the was found as 6.3%. Finally the electrochemical impedance
loading of Pt nanoparticles on the MnO2–ERGO modied GCEs spectroscopic analysis clearly illustrates that the electro-
can facilitate electron transfer reaction at the electrode inter- chemical behavior of the proposed Pt–MnO2–ERGO composite
face, which implies the Pt–MnO2–ERGO modied GCE may act lm is excellent.
as an excellent electro catalyst for methanol oxidation reaction
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active surface area (ESA) of the electro catalysts can be calcu-
lated from the hydrogen adsorption and desorption area by34
ECSA ¼ QH/(0.21 ꢂ WPt),
(1)
where QH (C) represents the average charge for hydrogen
adsorption and desorption; WPt is the loading of Pt electro-
deposited on the electrode; and 0.21 (mC cmꢁ2) represents
the transferred coefficient for a monolayer of H adsorbed on the
Pt surface. The ECSA is essential for comparing electrocatalytic
activity and it provides information regarding the number of
available electrochemically active sites. The hydrogen desorp-
tion peak area is commonly used to determine ECSA in the
potential region of ꢁ0.2
V to 1.2 V (vs. Ag/AgCl). A
strong desorption peak in the corresponding potential range on
the Pt–MnO2–ERGO electro catalyst is observed during the
positive-going potential scan. The calculated results show that
the Pt–MnO2–ERGO electro catalyst exhibits a higher ECSA
value (62.8 m2 gꢁ1) > Pt–ERGO with an ECSA of 42.4 m2 gꢁ1
>
Pt–MnO2 with an ECSA of 38.9 m2 gꢁ1 > Pt–carbon blacks with
an ECSA of 32.5 m2 gꢁ1; see Table 1, suggesting better perfor-
mance for methanol oxidation. This outstanding performance
of Pt–MnO2–ERGO can explain the higher electrocatalytic
activity. This result can be attributed to more effective utilization
of the smaller size of the Pt nanoparticles and more uniform
deposition of Pt NPs loaded on the MnO2–ERGO sheets.
Effect of scan rate and electrochemical oxidation of methanol
on the Pt–MnO2–ERGO electrodes
Further investigation is carried out to explore the electro-
chemical characterization of the Pt–MnO2–ERGO modied GCE
electrode; by measuring the CV response in the N2 saturated 0.1
M H2SO4 + 1 M methanol solution, as shown in Fig. 5A. It can be
seen that the peak currents of methanol oxidation increased
with the increase in the scan rate. Fig. 5A displays the methanol
oxidation at the Pt–MnO2–ERGO modied GCE electrode. There
is a linear relationship between the peak current density (Ip)
obtained from the forward CV scans and square root of the scan
rate (n1/2) (see Fig. 5B). In addition, the Ep moves slightly to
higher potentials with an increasing scan rate. This indicates
that the process of methanol diffusion controls the oxidation of
methanol at the Pt–MnO2–ERGO modied GCE electrode.
Fig. 4 (A) Nyquist plots of the EIS for the (a) Pt–C, (b) Pt–MnO2, (c)
Pt–ERGO and (D) Pt–MnO2–ERGO modified electrodes in a nitrogen
saturated solution of 0.1 M H2SO4 containing 1 M CH3OH in a
frequency range from 0.1 Hz to 100 kHz. The inset shows the randles
equivalent circuit for the modified electrodes. (B) Cyclic voltammo-
grams obtained for the (a) Pt–C, (b) Pt–MnO2, (c) Pt–ERGO and (D)
Pt–MnO2–ERGO modified electrodes in a nitrogen saturated solution
of 0.1 M H2SO4 at a scan rate of 50 mV sꢁ1
.
Fig. 4B shows the CVs for the Pt–MnO2–ERGO modied GCE
electrodes and Pt–ERGO, Pt–MnO2 and Pt–C electro catalysts in
0.1 M H2SO4 at a sweep rate of 50 mV sꢁ1. The electrochemically
Table 1 Electrochemical parameters of as-prepared different electro catalystsa
Electrocatalysts
Onset potential/V
ECSA/m2 gꢁ1
If/Ir ratio
Ref.
Pt–tungsten carbide–ERGO
Core–shell-like PbPt/graphene
Pt–Ru–ERGO–CCE
Pt–graphene and b-cyclodextrin (NaBH4)
Pt–Ni–graphene
Pt–ERGO–amine functionalized Fe3O4 magnetic nanospheres
Pt–7% CeO2–graphene
Pt–MnO2–ERGO
0.25
0.29
—
0.35
0.12
—
253.12
49.7
10.28
36.20
98
59.29
66.4
62.8
1.26
1.16
1.30
1.32
1.33
0.95
1.48
1.98
36
37
38
39
40
41
42
0.659
0.36
This work
a
If and Ir represent the forward and backward anodic peak current density.
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Fig. 5 (A) CVs for the Pt–MnO2–ERGO modified electrodes in a nitrogen saturated solution of 0.1 M H2SO4 containing 1 M CH3OH at different
scan rates, from a to g, the scan rates were 10 to 50 mV sꢁ1. (B) The relationship between the peak currents and the square root of the scan rates.
(C) Cyclic voltammograms of (a) Pt–C, (b) Pt–MnO2, (c) Pt–ERGO and (D) Pt–MnO2–ERGO modified electrodes in a nitrogen saturated solution
of 0.1 M H2SO4 containing 1 M CH3OH at a scan rate of 50 mV sꢁ1
.
The catalytic activity of the Pt–MnO2–ERGO modied GCE which is higher than for the Pt–ERGO, Pt–MnO2, and Pt–C
electrode and Pt–ERGO, Pt–MnO2, and Pt–C electro catalysts was electro catalysts. The signicantly higher If/Ib value for the
characterized by CV in a 0.1 M H2SO4 + 1 M CH3OH aqueous Pt–MnO2–ERGO electro catalysts than that of Pt–ERGO indicates
solution at a potential scan rate of 50 mV sꢁ1; the corresponding that the presence of MnO2 improves the electrocatalytic activity
results are shown in Fig. 5C. All results for the Pt–MnO2–ERGO of the Pt–ERGO for methanol oxidation. A comparison of the
modied GCE electrodes and the Pt–ERGO, Pt–MnO2, and Pt–C proposed method to the reported ones, presented in Table 1,
electro catalysts show the characteristic peaks for pure Pt in the indicates that the proposed method is superior to the previous
forward peak and backward peak scans. The onset potential for ones with regard to superior catalytic activity toward methanol
methanol oxidation occurs at 0.36 V, which is less than that of oxidation. This suggests that here is less accumulation of
the Pt–ERGO (0.40 V), Pt–MnO2 (0.42 V) or Pt–C (0.45 V) electro carbonaceous species on the Pt–MnO2–ERGO electro catalysts.
catalysts, indicating that methanol oxidation is easier on the This result indicates that the moderately reduced ERGO sup-
Pt–MnO2–ERGO. In addition, the ratio of the forward oxidation ported electro catalysts possess better poison tolerance. This can
peak current (If) to the reverse anodic peak current (Ib) is an be ascribed to the presence of residual oxygen-containing
important index that can be used to gauge the tolerance of groups on the moderately reduced ERGO, which act as
catalysts to the accumulation of carbonaceous species. There- binding sites for Pt–MnO2. The oxygen-containing groups (such
fore, the higher If/Ib ratio for the methanol oxidation suggests as OH) might also promote the oxidation of CO adsorbed by the
more effective removal of the poisoning species on the catalyst active Pt sites and benet the regeneration of Pt. The ERGO
surface, which implies that the methanol can be oxidized much sheets are negatively charged and contain –COOH and –OH
more efficiently. Furthermore, the ERGO affects the catalytic functional groups on the surface and at the edge of the carbon
activity of the Pt–MnO2 electro catalysts (see Fig. 5C). the more sheets. The negatively charged ERGO sheets can bond strongly
negative onset potential for methanol oxidation on the with the Mn2+ through electrostatic attraction. This indicates the
Pt–MnO2–ERGO electro catalysts is indicative of superior cata- suitability of the modied electrode for electronic analysis. The
lytic activity toward methanol oxidation than is the case with the adsorption of OHad species onto the MnO2 is more favorable.
Pt–ERGO (If/Ib value is approximately 1.12), Pt–MnO2 (If/Ib value The catalytic effect of MnO2 towards the oxidation of methanol
is approximately 1.32), or Pt–C (If/Ib value is approximately 99) oxidation is probably due to a parallel catalytic reaction. In the
electro catalysts. As can be seen in Fig. 5C, the If/Ib value is presence of methanol, MnO2 acts as both the catalyst support
approximately 1.98 for the Pt–MnO2–ERGO electro catalysts, and a catalyst for methanol oxidation. As soon as the MnO2 is
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for 3000 s. As shown in Fig. 6A, the currents decreased quickly at
the beginning. Tenacious reaction intermediates such as COads
will begin to accumulate if the kinetics of the removal reaction
cannot keep pace with that of methanol oxidation, possibly
owing to the continuous oxidation of methanol on the surface.
The electro catalyst has good anti-poisoning ability due to a
more gradual decay of current density with time. In Fig. 6A, it
can be seen that the current density for methanol oxidation on
the Pt–MnO2–ERGO modied GCE electrode and the Pt–ERGO,
Pt–MnO2, and Pt–C electro catalyst electrode decreases slowly
throughout the process, whereas the corresponding decay on
the Pt–MnO2 and Pt–C electro catalyst electrodes is very fast.
The Pt–MnO2–ERGO modied GCE electrode shows improved
anti-poison ability and the Pt–ERGO, Pt–MnO2 and Pt–C electro
catalysts show excellent current stability over 3000 s duration.
However, as can be observed in Fig. 6A, the current densities at
1500 s are 2.19 mA cmꢁ2 for Pt–MnO2–ERGO, 1.76 mA cmꢁ2 for
Pt–ERGO, 1.24 mA cmꢁ2 for Pt–MnO2 and 0.65 mA cmꢁ2 for
Pt–C. respectively. The highest oxidation current density can be
found for the Pt–MnO2–ERGO modied electro catalyst and the
steady-state current density among the Pt–ERGO, Pt–MnO2 and
Pt–C electro catalysts throughout all ranges up to 3000 s, this
result further proves the Pt–MnO2–ERGO electro catalyst has
better electrocatalytic activity than the latter for methanol
electro-oxidation.
Stability studies
The long-term cycle stabilities of the Pt–MnO2–ERGO modied
Fig. 6 (A) Chronoamperometric curves showing methanol oxidation
on the (a) Pt–C, (b) Pt–MnO2, (c) Pt–ERGO and (D) Pt–MnO2–ERGO
electro catalysts CH3OH + 0.1 M H2SO4 were also tested by
modified electrodes at 0.65 V in a 0.1 M H2SO4 + 1 M CH3OH solution. cycling the electrode potential between 0 and 0.8 V at 50 mV sꢁ1
(B) Cyclic voltammograms of Pt–MnO2–ERGO modified electrodes
for 200 cycles. The representative CVs and estimated peak
current densities as obtained in forward scans (ia) with
with 200 cycle in a nitrogen saturated solution of 0.1 M H2SO4 con-
taining 1 M CH3OH at a scan rate of 50 mV sꢁ1
.
increasing cycle numbers are shown in Fig. 6B. We can observe
that the Pt–MnO2–ERGO modied electro catalysts have higher
activity for methanol oxidation. The peak current for methanol
oxidation in the rst scan for the Pt–MnO2–ERGO modied
electro catalysts reaches a peak value of 9.11 mA cmꢁ2 (ia). Aer
200 CV test cycles, the current density of the Pt–MnO2–ERGO
modied electro catalyst remains at 72.5% of the rst scan. The
decrease in the electrocatalytic peak currents is mainly due to
the agglomeration of Pt particles in the reaction process, which
leads to a decrease of the reaction activity. Some of the Pt
particles might fall on reduced graphene oxide carriers, which
would lead to an accumulation of carbonaceous residues on the
electro catalyst surface. The observations imply that the
Pt–MnO2–ERGO modied electro catalysts possesses signi-
cantly enhanced long-term cycle stability for methanol oxidation.
A simple methanol fuel cell has been composed by assem-
bling Pt–MnO2–ERGO modied GCE as anode and commer-
cially available Pt–C as a cathode with Naon 112 as the
membrane in our homemade fuel cell setup in our laboratory.
reduced by methanol to the lower valence state of MnOOH, it is
electro oxidized back to MnO2 on the electrode surface. The
reactions are described in the following equations:35
CH3OH + Pt / Pt–COads + 4H+ + 4eꢁ
H2O + MnO2 / MnO2–OHads + H+ + eꢁ
(2)
(3)
Pt–COads + MnO2–OHads / Pt + MnO2 + CO2 + H+ + eꢁ (4)
On the other hand, The Pt–MnO2–ERGO electrocatalysts can
facilitate electron transport and accelerate the mass transfer
kinetics at the electrode surface and thus promote higher
catalytic activity.
Current transients
The catalytic activity and stability of the Pt–MnO2–ERGO The performance of the direct methanol fuel cell was carried out
modied GCE electrodes and Pt–ERGO, Pt–MnO2, and Pt–C in the presence of 0.1 M H2SO4 + 1 M CH3OH aqueous solutions
electro catalysts for methanol oxidation were further investi- at 30 ꢀC. The results shows that the as prepared Pt–MnO2–ERGO
gated through chronoamperometry in a 0.1 M H2SO4 + 1 M composite modied electro catalysts provides better power
CH3OH aqueous solution under a constant potential of 0.65 V performance, by as much as 148 mW cmꢁ2 compared with the
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commercial Pt–C value of 92 mW cmꢁ2 (see Fig. S3†). The
improved power performance of the as prepared nano-
composite is attributed to the high oxygen reduction activity
and the enhanced tolerance towards the oxidation of methanol,
which transferred from the anode to the cathode through the
Naon membrane. Moreover, the open-circuit voltage for
Pt–MnO2–ERGO modied electro catalysts is higher than that of
Pt–C. The open circuit voltage (VOC) of the methanol fuel cell is
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Conclusion
In summary, we have reported on a way to synthesize reduced
graphene oxide (ERGO) supported Pt–MnO2 electro catalysts
using a simple electro deposition technique, a layer-by-
layer method. CV and chronoamperometric were used to
measure the electrocatalytic activity. Results showed that the
Pt–MnO2–ERGO modied electro catalyst presented greatly
enhanced catalytic activity and long-term stability toward
methanol electro-oxidation. Its specic activity could reach
1.98 mA and cmꢁ2, respectively, which is signicantly higher
than that of the Pt–ERGO or Pt–MnO2 electro catalysts or the
commercially available Pt black. The electrochemical catalytic
activity of these Pt–MnO2–ERGO modied electro catalysts
towards methanol oxidation was also evaluated in comparison
with the activity of Pt–ERGO, Pt–MnO2 electro catalysts and
commercial Pt black. The Pt–MnO2–ERGO modied electro
catalysts demonstrated superior electrocatalytic activity. More-
over, SEM and EDX results conrmed the transparent structure
of the Pt–MnO2–ERGO modied electro catalysts. The electro-
chemical synthesis of Pt–MnO2–ERGO modied electro cata-
lysts could be a promising system for methanol oxidation.
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Acknowledgements
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We wish to express our appreciation to the Ministry of Science
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