Synthesis of bimetallic Ni-Cr nano-oxides as catalysts for methanol oxidation in NaOH solution

Article abstract of DOI:10.1166/jnn.2015.9528

Bimetallic Ni-Cr nano-oxide catalysts were synthesized by thermal decomposition method and were investigated as the anode electrocatalysts for the oxidation of methanol. The catalysts were characterized by X-ray diffraction and transmission electron microscopy. The electroactivity of the catalysts towards methanol oxidation in a solution containing 0.25 M NaOH and 1.0 M MeOH was examined using cyclic voltammetry and chronoamperometry. The results indicate that a mixture of rhombohedral-structured NiO and Cr ₂ O₃ nanocrystals generated at the calcination temperature of 500-700 ° C while octahedral-structured spinel NiCr ₂ O₄ formed at higher temperature. The influence of metallic molar ratio on the electrocatalytic performance of the catalysts was studied. The Ni-Cr nano-oxides prepared at comparatively low temperature displayed significantly higher catalytic activity and durability in alkaline solution toward electrooxidation of methanol compared with the pure nano NiO. The results indicate a synergy effect between NiO and Cr₂ O₃ enhancing the electrocatalytic properties of the bimetallic Ni-Cr nano-oxide catalysts. Meanwhile, NiCr₂ O₄ hardly increased the activity and durability of the catalyst. In addition, the Ni-Cr catalyst also exhibited excellent stability and good reproducibility. Therefore, Ni-Cr nano-oxide catalyst may be a suitable and cheap electrocatalyst for methanol oxidation in alkaline medium.

Full text of DOI:10.1166/jnn.2015.9528

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 3743–3749, 2015
Copyright © 2015 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Synthesis of Bimetallic Ni–Cr Nano-Oxides as Catalysts
for Methanol Oxidation in NaOH Solution
Yingying Gu∗, Jing Luo, Yicheng Liu, Haihong Yang, Ruizhou Ouyang, and Yuqing Miao
College of Science and School of Environment and Architecture, University of Shanghai for
Science and Technology, Shanghai 200093, China
Bimetallic Ni–Cr nano-oxide catalysts were synthesized by thermal decomposition method and
were investigated as the anode electrocatalysts for the oxidation of methanol. The catalysts were
characterized by X-ray diffraction and transmission electron microscopy. The electroactivity of the
catalysts towards methanol oxidation in a solution containing 0.25 M NaOH and 1.0 M MeOH was
examined using cyclic voltammetry and chronoamperometry. The results indicate that a mixture of
rhombohedral-structured NiO and Cr2O3 nanocrystals generated at the calcination temperature of
ꢀ
500–700 C while octahedral-structured spinel NiCr2O4 formed at higher temperature. The influ-
ence of metallic molar ratio on the electrocatalytic performance of the catalysts was studied. The
Ni–Cr nano-oxides prepared at comparatively low temperature displayed significantly higher cat-
alytic activity and durability in alkaline solution toward electrooxidation of methanol compared with
the pure nano NiO. The results indicate a synergy effect between NiO and Cr2O3 enhancing the
Delivered by Publishing Technology to: West Virginia University/ Health Sci Ctr LIb
electrocatalytic properties of the bimetallic Ni–Cr nano-oxide catalysts. Meanwhile, NiCr2O4 hardly
IP: 178.77.183.196 On: Tue, 20 Oct 2015 01:28:40
increased the activity and durability of the catalyst. In addition, the Ni–Cr catalyst also exhibited
Copyright: American Scientific Publishers
excellent stability and good reproducibility. Therefore, Ni–Cr nano-oxide catalyst may be a suitable
and cheap electrocatalyst for methanol oxidation in alkaline medium.
Keywords: Direct Methanol Fuel Cell, Methanol Oxidation, Nano Oxide, Bimetallic Oxide.
1. INTRODUCTION
activity and durability, many research works have studied
the modification of Ni-based catalysts through incorporat-
ing non-precious metallic elements, complexes and oxides.
Ni–Pd nanoparticles,21 Ni/TiO2 nanotubes,22 AuNi sup-
ported on carbon,23 Au nanoparticle-coated Ni/Al lay-
ered double hydroxides,24 Ni–Cd coated graphite25 and
Ni–Cu alloy26 were investigated and showed enhanced
electrocatalytic activity and stability for methanol oxida-
tion reaction (MOR). According to the reports that the
electroactivity of Ni-based catalysts depends mainly on
the catalytic role of Ni(OH)2/NiOOH redox as an effec-
tive electron transfer mediator for the oxidation process of
alcohols, carbohydrates and many other organics in alka-
line solutions.27–31 The electrooxidation mechanisms on
these catalysts involve the formation of a higher valence
Ni oxide, which acts as a chemical oxidizing agent.
Direct methanol fuel cells (DMFCs) have been attracted
much attention for its high theoretical energy density of
methanol, using liquid fuel, working at low temperature,
and have presented opportunities for use as a power source
for portable electronic devices such as notebook comput-
ers, cellular phones.1–3 Pt and Pt-based alloys have been
proved to be highly efficient anode catalysts for fuel cells.
However, the high cost and the apt to be poisoned by
even trace CO originated from the oxidation of methanol
prevent their practical applications in DMFCs.4–6 Conse-
quently, many attentions have been focused on the devel-
opment of non-precious metal catalysts to overcome the
technical barriers,7–18 among which, nickel based materi-
als are of the most promising candidates because of their
good chemical stability, electrical properties and the abil-
ity of removing intermediate COad in alkaline media.19ꢀ20
However, these catalysts still show much lower activity
than the Pt catalysts. In order to improve the catalytic
Herein, bimetallic Ni–Cr nano-oxide catalysts were syn-
thesized by a thermal decomposition method in order to
enhance the electrocatalytic performance of nickel-based
catalyst for the oxidation of methanol in alkaline medium.
The catalysts were characterized by X-ray diffraction
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 5
1533-4880/2015/15/3743/007
doi:10.1166/jnn.2015.9528
3743

Synthesis of Bimetallic Ni–Cr Nano-Oxides as Catalysts for Methanol Oxidation in NaOH Solution
Gu et al.
(XRD) and transmission electron microscopy (TEM). The
electrocatalytic properties of the Ni–Cr nano-oxides for
MOR were studied by cyclic voltammetry and chronoam-
perometry. An attempt was made to determine the interre-
lation between the phase and chemical composition of the
catalysts and their activities in methanol oxidation.
10 ꢅL 5% Nafion solution was dispersed ultrasonically for
15 min to obtain a well-dispersed ink suspension. 10 ꢅL of
the resulting suspension was then dropped onto the surface
of GC electrode, which was dried at 30 ꢀC for 15 min. For
cyclic voltammetry experiments, the electrode was condi-
tioned in an aqueous solution of 0.25 M NaOH and 1.0 M
methanol at 30 ꢀC by the potential cycling from 0 to 0.9 V
(vs. SCE) at a scan rate of 100 mV·s−1 for 100 cycles.
2. EXPERIMENTAL DETAILS
2.1. Preparation of Catalyst
3. RESULTS AND DISSCUSSION
3.1. Characterization of Ni–Cr Catalysts
Nafion solution (5.0%) was obtained from Shanghai
Yibang technology Co., Ltd. All other chemicals used were
of analytical grade and were used as received without
any further purification from Sinopharm Chemical Reagent
Co., Ltd.
TEM images of the Ni1–Cr1 samples calcined at differ-
ent temperatures are shown in Figure 1. Well dispersed
nanoparticles with size around 30 nm are obtained at the
ꢀ
calcination temperature of 500 C (Fig. 1(a)). The parti-
The catalysts were synthesized via thermal decomposi-
tion method. A typical process is described as follows: a
mixture containing 3.3–7.5 mM NiCl2 ·6H2O, 2.5–6.7 mM
CrCl3 · 6H2O and 5 mL deionized water was stirred for
5 minutes to obtain a homogeneous solution, followed by
the addition of 10 mM citric acid monohydrate, 10.5 mM
ethylenediaminetetraacetic acid disodium salt and 10 mL
ammonium hydroxide into the above-prepared mixtuꢀre.
The solution was kept in a resistance furnace at 300 C
for 3 h to get a hardened gel, which was then grinded to
powder. The powder was further calcined in the resistance
furnace at 500 ꢀC, 600 ꢀC, 700 ꢀC, 800 ꢀC, 900 ꢀC, respec-
cle size increases as the increasing of calcination temper-
ature and the prepared nanoparticles are almost spherical
when the temperature is not higher than 700 ꢀC (Figs. 1(b)
and (c)). Much larger particles generate above 700 ꢀC
due to the surface fusion and congregation and a parti-
cle size range 100–900 nm is obtained. Clear crystalline
facets with octahedral structure can be seen in Figures 1(d)
and (e), indicating that the Ni1–Cr1 samples may be trans-
formed into the spinel structure.32ꢆ33
Figure 2 shows the XRD patterns of Ni1–Cr1 nano-
oxides obtained at different calcination temperatures. The
peaks in the patterns indicate that the Ni1–Cr1 nano-oxides
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tively, for 2 h with a heating rate of 2 C·min−1. Finally,
ꢀ
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obtained from the temperature between 500 ꢀC and 700 ꢀC
the powder was washed with deionized water and dried in
air. The obtained nano-oxide materials were then marked
as Nia–Crb (a/b refers to the molar ratio of Ni/Cr).
Copyright: American Scientific Publishers
are the mixtures of the crystalline NiO and Cr2O3.34 The
diffraction peaks at 2ꢃ values of 37.3ꢀ, 43.3ꢀ, 62.9ꢀ, 75.4ꢀ
and 79.4ꢀ are corresponding to the NiO with a rhombo-
hedral structure. And the diffraction peaks at 2ꢃ values of
24.5ꢀ, 33.6ꢀ, 36.3ꢀ, 41.5ꢀ, 43.3ꢀ, 50.3ꢀ, 54.9ꢀ and 65.2ꢀ are
ascribed to the rhombohedral-structured Cr2O3. The aver-
age particle sizes calculated from the Scherer formula are
about 28.9 nm, 31.4ꢀ nm and 38.5 nm at the calcination
2.2. Apparatus
The size and morphology of catalysts were observed using
a TECNAI G220 (JEOL) transmission electron microscope
operated at 200 kV. The X-ray powder diffraction patterns
of the products were investigated with a Rigaku D/max
22009C/pc (Japan) diffractometer with CuKꢀ radiation
(ꢁ = 1ꢂ5406 Å). The diffraction patterns were collected at
room temperature by step scanning in the range of 10ꢀ ≤
2ꢃ ≤ 90ꢀ, at the scan rate of 2ꢀ ·min−1. All electrochemical
experiments were carried out using a CHI660D electro-
chemical working station.
ꢀ
ꢀ
temperatures of 500 C, 600 C and 700 C, respectively.
The Ni1–Cr1 materials obtained at higher temperature have
the comparatively larger crystal sizes because of more
energy for the crystal growing stage.32 Characteristic peaks
of spinel-structured NiCr2O4 are observed in the XRD pat-
tern with decreasing intensities of Cr2O3 and NiO at the
ꢀ
temperature of 800 C indicating the inset of NiO into the
lattice of Cr2O3. The characteristic peaks of Cr2O3 almost
disappear at 900 C, which means that the mixture has
ꢀ
2.3. Electrochemical Measurements
Electrochemical measurements were performed using a
three-electrode configuration consisting of a catalyst mod-
ified glass carbon (GC, ꢄ = 3 mm), a Pt wire and a sat-
urated calomel electrode (SCE) as working, counter and
reference electrodes, respectively. Cyclic voltammograms
(CVs) were recorded using a modulated potentiostat.
The GC electrodes were first polished to a mirror
like surface using a standard electrode polishing kit and
cleaned with acetone and distilled water successively.
A mixture containing 20 mg catalyst, 2 mL ethanol and
been almost completely transformed into nickel chromite
spinel after the thermal process. The formation mechanism
can be expressed by the following chemical reaction:35ꢆ36
Cr2O3 +NiO → NiCr2O4
3.2. Electrocatalytic Performance of Ni–Cr
Catalysts for Oxidation of Methanol
A consecutive sweep from 0 to 0.9 V was conducted
for 100 cycles on Ni1–Cr1 electrode in alkaline solution.
3744
J. Nanosci. Nanotechnol. 15, 3743–3749, 2015

Gu et al.
Synthesis of Bimetallic Ni–Cr Nano-Oxides as Catalysts for Methanol Oxidation in NaOH Solution
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Copyright: American Scientific Publishers
Figure 1. TEM images of Ni1–Cr1 nano-oxides calcined at (a) 500 ꢀC, (b) 600 ꢀC, (c) 700 ꢀC, (d) 800 ꢀC and (e) 900 ꢀC.
Figure 3 shows that the peak current density increases with
increasing scan cycle within 50 cycles and thereafter, no
obvious changes are found in the oxidation potential and
maximal current density with more scan cycle done. The
CV curves between 50th and 100th scan cycle are almost
overlapped. Therefore, the CV data of the 100th cycle are
present in the following CV figures.
In the consecutive CVs of Ni1–Cr1 catalyst, except
the first cycle, the peak potentials are almost invariable,
suggesting that the phase transformation of Ni oxyhy-
droxide from ꢀ to ꢁ is inhibited.26 It is reported that
there are four phase produced over the lifetime of a
nickel hydroxide electrode substrate, namely, ꢀ-Ni(OH)2,
ꢂ-Ni(OH)2, ꢀ-NiOOH and ꢁ-NiOOH.37ꢃ38 It has been
identified that phase transformations are likely to occur
during a normal cycle. ꢀ-Ni(OH)2 is first oxidized to
ꢀ-NiOOH and reduced back during the cell discharge.
Meanwhile, the reduction of ꢁ-NiOOH forms the hydrated
ꢂ-Ni(OH)2 phase, which is unstable in strong alkali and
ages to the ꢀ-form.39 It is well-known that the forma-
tion of ꢁ-NiOOH phase is associated with swelling or
volume expansion of nickel film electrodes leading to the
subsequent microcracks and disintegration of the nickel
film. Lower inter-electrode spacing results in lower inter-
nal resistance and thus better efficiency of the electrode.
Therefore, ꢀ-NiOOH phase is considered to be a bet-
ter electroactive material for high electrochemical perfor-
mance in alkaline solution. Thus, the peak current density
of the Ni1–Cr1 electrode increases continuously before the
50th round can be ascribed to the consecutively formation
of ꢀ-NiOOH from ꢁ-NiOOH.
The CV behaviours of Ni1–Cr1 nano-oxide catalysts cal-
cined at different temperatures are shown in Figure 4.
Peaks for MOR appear at around 0.66 V (vs. SCE) of
Ni1–Cr1 catalysts, while no significant peak is observed
under the same condition for bare GC electrode. There-
fore, the peaks can be attributed to the catalytic oxidation
of methanol by Ni1–Cr1 nano-oxides in alkaline solution.
And the current density for MOR on Ni1–Cr1 calcined at
700 ꢀC is higher than that at 500 and 600 ꢀC owing to the
higher crystallinity of the catalysts, which is evidenced by
sharper peaks at higher decomposition temperature in the
J. Nanosci. Nanotechnol. 15, 3743–3749, 2015
3745

Synthesis of Bimetallic Ni–Cr Nano-Oxides as Catalysts for Methanol Oxidation in NaOH Solution
Gu et al.
∆
Cr2O3 NiO
NiCr2O4
Without catalysts
500 ºC
1
2
3
4
5
6
4
∆
∆
12
8
∆
∆
3
∆
∆
∆
∆
900 ºC
*
*
600 ºC
∆
2
700 ºC
∆
∆
800 ºC
800 ºC
700 ºC
900 ºC
5
4
6
+
+
600 ºC
500 ºC
1
0
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
80
100
E(vs. SCE) (V)
2 theta (deg.)
Figure 4. CVs of Ni1–Cr1 nano-oxides prepared at different calcination
temperatures in 0.25 M NaOH and 1.0 M methanol solution at the scan
rate of 100 mV·s−1 at 30 ꢀC.
Figure 2. XRD patterns of Ni1–Cr1 nano-oxides calcined at different
temperatures.
XRD patterns.25ꢀ40 On the other hand, in the cases of the
samples calcined at 800 and 900 ꢀC, the peak current den-
sities are quite lower than the others, due to the formation
of NiCr2O4 with large sizes. It is obvious that the mixture
of Cr2O3 and NiO nanoparticles exhibites a higher activity
toward methanol oxidation in comparison with NiCr2O4,
which indicates Cr2O3 is a good promoter of the Ni–Cr
catalysts for MOR, while NiCr2O4 can hardly increase the
of 1.29, which is higher than that of Pt/C (0.92),33 and
even higher than that of Pt–Co nanowire catalyst (1.20)43
and porous Pt–Ni nanotubes (1.26).44 This comparatively
higher value of If /Ib for Ni1–Cr1 catalyst indicates the
highly active and resistant to the poisoning of carbona-
ceous species. Obviously, the catalysts calcined at rel-
atively lower temperature show higher current densities
as well as higher I /I values than the samples calcined
above 800 ꢀC, suggesting that the catalysts containing both
NiO and Cr2O3 have a higher resistant to the poison car-
bonaceous species than NiCr2O4.
The CVs of Ni–Cr nano-oxides with different metallic
molar ratios toward MOR are shown in Figure 5. Here, the
electroactivity of monometallic nano-oxides, pure NiO and
Cr2O3, was also tested for methanol oxidation, which were
synthesised using the same method as Ni–Cr nano-oxides.
All catalysts show electroactivity for MOR, while the pure
nano-Cr2O3 is the poorest catalyst among all the samples
and catalytic peak current density of pure nano-NiO is also
much lower than that of the Ni–Cr nano-oxides. Notably,
the catalytic oxidation peak current density exhibited by
Ni–Cr nano-oxides are even much higher than the sum
of peak current densities given by the pure nano-Cr2O3
and the pure nano-NiO, and the highest one is found at
the molar ratio of 2:1 as listed in Table II. The results
reveal the higher electrocatalytic activity of the bimetal-
lic Ni–Cr nano-oxides toward methanol oxidation than the
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catalytic activities of the catalysts.
The peak current ratio of the forward scan (If ) to back-
IP: 178.77.183.196 On: Tue, 20 Oct 2015 01:28:40
Copyright: American Scientific Publishers
ward scan (Ib) has generally been used to evaluate the
poisoning tolerance for methanol oxidation.41 The higher
If /Ib ratio indicates more effective removal of the poison-
ing species on the catalyst surface, thus the better toler-
ance toward poisoning of the catalyst.42 The If /Ib ratios of
Ni1–Cr1 nano-oxides are shown in Table I. Among these
samples, Ni1–Cr1 calcined at 700 ꢀC has a high If /Ib value
16
1st
10th
20th
30th
40th
12
50th
60th
8
70th
80th
4
90th
100th
Table I. Electrocatalytic properties of Ni1–Cr1 nano-oxides prepared at
different calcination temperatures.
Sample (ꢀC)
Peak current density (mV·cm−2
)
If /Ib
0
0.0
0.2
0.4
0.6
0.8
1.0
500
600
700
800
900
8ꢁ3
11ꢁ5
12ꢁ6
4ꢁ2
1.11
1.14
1.29
1.06
1.05
E (vs. SCE) (V)
Figure 3. CVs of Ni1–Cr1 electrode for 100 consecutive cycles in
0.25 M NaOH and 1.0 M methanol solution at the scan rate of 100 mV·
s−1 at 30 ꢀC.
3ꢁ5
3746
J. Nanosci. Nanotechnol. 15, 3743–3749, 2015

Gu et al.
Synthesis of Bimetallic Ni–Cr Nano-Oxides as Catalysts for Methanol Oxidation in NaOH Solution
strongly reduced poisoning and hence improved stability,49
3
Bare-GC
Ni3-Cr1
1
2
16
12
8
which may be attributed to the enhancement of the oxida-
tion of CO by Cr atom.36
Ni2-Cr1
Ni1-Cr1
Ni1-Cr2
Cr2O3
NiO
3
4
5
6
7
4
CVs of Ni1–Cr1 electrode at different scan rates and
the plot of If against the square root of the scan rates
are shown in Figures 6(a) and (b). The catalytic oxida-
tion peak potentials of methanol shift gradually towards
2
more positive values with increasing scan rate, suggest-
5
ing a kinetic limitation on surface diffusion of methanol.
A linear dependence of If on the square root of the scan
7
4
rate is observed, conforming that the electrocatalytic oxi-
dation of methanol at the Ni1–Cr1 electrode is controlled
6
1
0
by a semi-infinite linear diffusion process.
0.0
0.2
0.4
0.6
0.8
1.0
Reproducibility was tested by measuring CVs of 5 inde-
pendently prepared Ni1–Cr1 electrodes in 0.25 M NaOH
containing 1.0 M methanol, as shown in Figure 7. A rel-
ative standard deviation (RSD) of 0.72% is estimated for
the oxidation peak currents, which indicates good fabri-
cation reproducibility. The stability of the electrode was
also studied. The electrode employed in this measurement
was kept in room temperature and repeated assays every
day in 9 days, as shown in Figure 8. The response signal
retained 93.9% of the initial one after 9 days and the RSD
E(vs. SCE) (V)
Figure 5. CVs of Ni–Cr nano-oxides with different molar ratio of Ni/Cr
in 0.25 M NaOH and 1.0 M methanol solution at the scan rate of 100 mV·
s−1 at 30 ꢀC.
monometal oxide catalyst of NiO or Cr2O3 due to the sig-
nificant synergy effect of NiO and Cr2O3ꢀ
In the electrochemical process, the ꢁ-Ni(OH)2 of the
electrodes is oxidized to ꢁ-NiOOH in low potential and
transformed to ꢂ-NiOOH in high potential during the pos-
itive sweep. Then methanol is oxidized on the electrode
surface by the intermediate product NiOOH. The compar-
ison of the electrochemical behaviour between the pure
3.0
(a)
7
2.5
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1
1 mV/s
nickel oxide and Ni–Cr oxide reveals the lower ꢃ/ꢂ redox
IP: 178.77.183.196 On: Tue, 20 Oct 20151001m:V2/s8:40
6
5
2
contribution with a good stabilization of ꢁ/ꢁ nickel oxy-
Copyright: American Scien2ti.0fic Publis8h0emrVs/s
3
hydroxide form on the Ni–Cr electrode. This is due to the
inhibition of variation of peak potentials after prolonged
electrochemical treatment in alkaline solution. Thus the
addition of chromium to the nickel oxyhydroxide species
represents a very efficient strategy to suppress the for-
mation of ꢂ-NiOOH phase. Additionally, the presence of
Cr2O3 nanoprticles enhances the formation of NiIII/NiII
redox couple, therefore, the catalytic activity and mechan-
ical stability are improved.45ꢄ46 Similar electrochemical
behaviour was observed for alloys obtained by adding sev-
eral percent of Cd, Zn, Co, Ca, Ti and other elements to
the nickel hydroxide.26ꢄ47ꢄ48
4
5
6
7
100 mV/s
150 mV/s
200 mV/s
250 mV/s
4
3
1.5
1.0
0.5
0.0
2
1
0.0
0.2
0.4
0.6
0.8
1.0
E(vs. SCE) (V)
3.0
2.5
2.0
1.5
1.0
0.5
(b)
The peak current densities and the If /Ib ratios show
interesting differences between the catalysts in Table II.
All the Ni–Cr electrodes exhibit much higher If /Ib ratios
than the pure Cr2O3 and NiO electrodes, highlighting their
Table II. Electrocatalytic properties of Ni–Cr nano-oxides with differ-
ent molar ratio of Ni/Cr.
Sample
Peak current density (mV·cm−2
)
If /Ib
Ni3–Cr1
Ni2–Cr1
Ni1–Cr1
Ni1–Cr2
NiO
9ꢀ2
16ꢀ0
12ꢀ6
6ꢀ3
3ꢀ1
0ꢀ9
1.27
1.14
1.29
1.19
1.05
1.03
0.0
0.1
0.2
0.3
0.4
0.5
ν
1/2 (V1/2 s–1/2
)
Figure 6. (a) CVs of Ni1–Cr1 nano-oxides in 0.25 M NaOH and 1.0 M
methanol solution at the scan rate of 1, 10, 80, 100, 150, 200, 250 mV·
Cr2O3
s−1, (b) Plot of If versus v1/2
.
J. Nanosci. Nanotechnol. 15, 3743–3749, 2015
3747

Synthesis of Bimetallic Ni–Cr Nano-Oxides as Catalysts for Methanol Oxidation in NaOH Solution
Gu et al.
60
16
)
– 2
14
12
10
40
20
0
8
6
4
2
0
C u r r e n t d e n s i t y ( m A c m
5
4
3
Catalyst
2
1
0
5
10
15
20
25
0
Time (h)
Figure 7. Results of five repetitive measurements of Ni1–Cr1 electrode
in 0.25 M NaOH and 1.0 M methanol solution at the scan rate of 100 mV·
s−1 at 30 ꢀC.
Figure 9. Chronoamperometric curve of Ni1–Cr1 electrode in 0.25 M
NaOH and 1.0 M methanol solution, applied potential 0.67 V.
4. CONCLUSIONS
is calculated to be 2.0%, reflecting a good stability of the
electrode.
Ni–Cr electrocatalysts for MOR with different molar ratios
have been synthesized via the thermal decomposition
synthesis process. The XRD and TEM characterizations
indicate that the bimetallic Ni–Cr nanoparticles are well
dispersed with average particle size of about 30–40 nm
Figure 9 shows the chronoamperometric curve for MOR
on the Ni1–Cr1 electrode in alkaline solution. The initial
high current density mainly corresponds to the double-
layer charging. The electrode presents continuous decay
in activity during the first few minutes, which can be
attributed to the remaining of the adsorbed intermediate
products of methanol oxidation such as CO poisoning
on the surface of electrode.50ꢀ51 And the current density
behaves a more stable trend through a long time test-
ing, owing to the adsorption of OH−, which may help to
diminish CO poisoning and also facilitate the oxidation of
MeOH to HCOO− and CO2.52 Accordingly, these results
illustrate that the Ni–Cr catalyst display favorable stabil-
ity, high catalytic activity and high anti-poisoning ability
for methanol oxidation, in good agreement with the CV
results.
ꢀ
below 700 C. The electrochemical measuremꢀents show
that the Ni–Cr nano-oxides calcinated at 700 C exhibit
remarkably high activity towards methanol oxidation in
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alkaline media. Meanwhile, good stability is observed over
Copyright: American Scientific Publishers
a moderately extended period. The values of current den-
sity and If /Ib for Ni1–Cr1 electrode are 12.6 mV · cm−2
and 1.29, respectively. The kinetic results indicate that the
reaction of the electrooxidation of methanol is a diffusion-
controlled process. The relatively high current density,
high CO tolerance as well as stability suggest that the
Ni–Cr nano-oxides may act as a promising catalyst for
DMFCs in alkaline media.
Acknowledgment: The authors greatly appreciate the
support from the Shanghai Committee of Science and
Technology (No. 11430502100), the National Natural Sci-
ence Foundation of China (No. 21305090), the Natural
Science Foundation of Shanghai (No. 13ZR1428300) and
the Innovation Program of Shanghai Municipal Education
Commission (Nos. 14ZZ139, 14YZ086).
15
)
– 2
12
9
6
3
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Received: 19 November 2013. Accepted: 11 January 2014.
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