F1194
Journal of The Electrochemical Society, 161 (12) F1194-F1201 (2014)
0013-4651/2014/161(12)/F1194/8/$31.00 © The Electrochemical Society
Effective and Stable CoNi Alloy-Loaded Graphene for Ethanol
Oxidation in Alkaline Medium
Nasser A. M. Barakat,a,b,z Moaaed Motlak,c Baek Ho Lim,a Mohamed H. El-Newehy,d,e
and Salem S. Al-Deyabd
a Bionanosystem Engineering Department, Chonbuk National University, Jeonju 561-756,
b Chemical Engineering Department, Minia University, El-Minia, Egypt
c Department of Physics, College of Science, Anbar University, Anbar 31001, Iraq
Republic of Korea
d Petrochemical
Research Chair, Department of Chemistry, College of Science, King Saud University,
Riyadh 11451, Saudi Arabia
e Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt
Low electro-catalytic activity and poor stability are the main dilemmas facing the non-precious electro-catalysts. The super-electric
conductivity and marvelous adsorption capacity strongly recommend graphene to enhance the electro-catalytic activity; however the
alloy structure distinctly improves both of the catalytic activity and chemical stability for the non-precious electro-catalysts. In-situ
decoration of graphene by Cox Niy alloy nanoparticle is introduced to be utilized as an electro-catalyst for ethanol oxidation. Briefly,
cobalt acetate and nickel acetate were added to the reaction media during graphene preparation using a modified chemical route.
Later on, the resultant material was calcined in argon atmosphere at 850◦ C. The utilized physicochemical characteristics affirmed
formation of multi-layer graphene sheets decorated by solid solution Cox Niy alloy nanoparticle. Influence of the nanoparticles
chemical composition and loading on the electro-catalytic activity was investigated. Interesting results were obtained as follow:
utilizing graphene enhances the electro-activity to be 10 times more than the pristine nanoparticles, moreover a negative onset
potential was obtained (−90 mV vs. Ag/AgCl). Furthermore, the obtained results indicated that the metallic nanoparticle composition
and loading have distinct impacts on the electro-catalytic activity; Co0.2 Ni0.2 -decorated graphene revealed the best performance as
well as very good stability.
© 2014 The Electrochemical Society. [DOI: 10.1149/2.0451412jes] All rights reserved.
Manuscript submitted June 5, 2014; revised manuscript received August 14, 2014. Published August 26, 2014.
Undoubtedly, the superior physicochemical characteristics of
graphene open new avenues for the carbonaceous materials to be utilized in various application fields.1–3 The abundant symmetric carboncarbon bonds forming the net strongly enhance the possibility of
conducting electricity very efficiently. Consequently, electrons move
through graphene as if they have no mass. The fantastic properties of
graphene arise from the atomically mesh of carbon atoms arranged
in a honeycomb pattern. Single-layer graphene is predicted to have
a large surface area close to 2600 m2 /g.4,5 Because of this huge surface area, as a carbonaceous nanostructure, graphene has an excellent
adsorption capacity.
Direct alcohol fuel cells (DAFCs) are considered new electrochemical devices for energy production that have recently attracted much
attention. DAFCs have shown that they can produce higher current
densities than proton exchange membrane fuel cells (PEMFCs).6 In
DAFCs, the alcohol is oxidized at the anode to produce carbon dioxide and water. The anodic reactions in the DAFCs are considered to
be a combination of adsorption and electrochemical reactions.7,8 Accordingly, due to the appealing adsorption and electric conductivity
characteristics of graphene, it is the optimum candidate as an electrode
material in the DAFCs.
In principle, the chemistry of graphene is considered similar to
that of graphite. The main difference stems from the fact that unlike
graphite, graphene could be considered as a giant flat molecule able to
undergo chemical reactions on the two faces. Therefore, although the
novel morphology of graphene strongly enhances most of the physicochemical properties, the catalytic activity is still low; resembling the
carbonaceous materials.
Methanol and ethanol are the main alcohols used in the DAFCs.
Methanol is nonrenewable, volatile and a flammable substance; moreover it has a high toxicity. Compared to methanol, ethanol is less toxic
and has a higher energy density. Methanol and ethanol have energy
densities of 6.09 and 8.00 kW.h.kg−1 , respectively.9 Moreover, ethanol
is considered a source of renewable energy as it can be produced from
agricultural bioprocesses.10 In practical use, direct methanol fuel cells
(DMFCs) have several disadvantages including their sluggish reaction
kinetics for methanol oxidation, methanol crossover through Nafion
z
E-mail: [email protected]
membranes to the oxygen electrode and anode poisoning by strongly
adsorbed intermediates (mainly CO).11 On the other hand, ethanol is
well known for having a lower crossover rate and affecting cathode
performance less severely than methanol because of its smaller permeability through the Nafion membrane and its slower electrochemical
oxidation kinetics on the Pt/C cathode.11,12 Furthermore, in alkaline
media, the direct ethanol fuel cells (DEFCs) show better polarization
characteristics, using alkali electrolytes allows for a greater possibility for application of non-noble and less expensive metal catalysts.
However, it is also known that ethanol requires more active electrocatalysts because, compared to methanol, there is more difficulty to
break ethanol C-C bond and thus achieving full oxidation to produce CO2 (i.e. produce 12 e-); usually main ethanol electrooxidation
products are acetaldehyde (2 e-) and acetic acid (4 e-) which reactions generate fewer electrons than methanol total electrooxidation
(6 e-).
At present, almost all pre-commercial low-temperature fuel cells
use Pt-based electro-catalysts.13–16 In this regard, graphene-Pt composite showed very good performance.17–21 Actually, utilizing precious metals distinctly increases the manufacturing cost and constrains
wide applications. Moreover, the catalyst poisoning by CO or CHO
species is another real problem facing most of the Pt-based electrocatalysts.22–24 Because of its surface oxidation properties, nickel reveals good performance as an electro-catalyst. Nickel is commonly
used as an electro-catalyst for both anodic and cathodic reactions in organic synthesis, water electrolysis and alcohol electro-oxidation.25–27
To exploit the synergetic effect of the alloy structure, Ni-based alloys
were investigated; NiCu, NiCr and NiMn.28–30 It is known that cobalt
has low electro-catalytic activity, so it was not used as a main catalyst
in the DAFCs.31 However, Co was used as a co-catalyst to annihilate
the Pt poisoning.31,32
In general, in DAFCs, alcohol oxidation is considered to be a
combination of adsorption and electrochemical reaction on the anode
surface.7,8 Accordingly, because of the adsorption capacity of carbon,
it has been incorporated in many recently reported electrocatalytic
materials.33–38 Accordingly, as carbonaceous material, graphene is
expected the best substrate due to the huge surface area which creates
excellent adsorption capacity.
Nickel and cobalt are neighbors in the periodic table, so they
likely form solid solution alloys.39–41 Recently, Wang et al.42
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Journal of The Electrochemical Society, 161 (12) F1194-F1201 (2014)
introduced CoNi-doped graphene as electro-catalyst for ethanol, however because the utilized procedure was not proper to produce pure
metal the corresponding performance was low.
Based on a modified Hummer’s method,43,44 we introduce full
study about utilizing of Cox Niy -decorated graphene as electro-catalyst
for ethanol oxidation including the influence of the metallic nanoparticle composition and loading. The results indicated that graphene
strongly enhances the catalytic performance. Moreover, there are distinct effects for the composition and loading of the metallic counterpart
on the electro-catalytic activity.
Experimental
Materials.— The utilized precursors for the metallic nanoparticle
in the introduced Cox Niy -decorated graphene were cobalt (II) acetate
tetra-hydrate (CoAc, 98% assay, Junsei Chemical Co., Ltd, Japan), and
nickel acetate tetrahydrate (NiAc, 99.0% assay, Sigma Aldrich) which
were utilized without any further modification. However, graphene
was synthesized using graphite powder (<20 μm), hydrogen peroxide,
hydrazine monohydrate and sulphuric acid (95–97%) which were
purchased from Sigma–Aldrich. Distilled water was used as a solvent.
Procedures.—Preparation of graphene oxide (GO).— Graphene
was prepared chemically from reduction of exfoliated GO. The GO
was synthesized from natural graphite powder by a modified Hummer’s method.43,44 Briefly: 5 g of graphite treated twice by 5% HCl
was placed in cold (0◦ C) concentrated H2 SO4 (130 mL), then 15 g of
KMnO4 was added gradually to the mixture in the ice bath with stirring
for 2 h. After dilution with DI water, the temperature was increased to
98◦ C. Later on, the mixture was cool down to room temperature and
H2 O2 (50 mL, 30 wt%) was added, the mixture was left overnight.
The mixture was filtered under vacuum, the obtained precipitate was
washed with 10% aqueous HCl several time and dried at 50◦ C.
Preparation of CoNi-loaded graphene.—In 250 mL round flask,
300 mg of the prepared GO were dispersed in 400 mL distilled water
and ultrasonicated for 40 min, then 0.5 mL of hydrazine hydrate was
added to the suspended GO. Specific amounts from CoAc and NiAc
were dissolved individually in the minimum amounts of water and
mixed with the GO suspension. Typically, to study the loading, 0.1,
0.2 and 0.4 g from each salt was utilized. However, to study the influence of the metallic nanoparticle composition, the loading was fixed
at 0.4 g (total salts), the utilized NiAc/CoAc ratios were 0/100, 25/75,
50/50, 75/25 and 100/0. In all experiments, the utilized GO powder
was kept as 300 mg. The slurry was refluxed at 150◦ C for 10 h. Then,
the solution was filtered. The obtained filter cake was washed several
times by plenty amounts of water, dried under vacuum at 80◦ C for
one night, and then calcined under argon atmosphere at 850◦ C for two
hours.
Characterization.— Information about the phase and crystallinity
was obtained by using Rigaku X-ray diffractometer (XRD, Rigaku,
Japan) with Cu Kα (λ = 1.5406 Å) radiation over Bragg angle
ranging from 10 to 100◦ . Normal and high resolution images were
obtained with transmission electron microscope (TEM, JEOL JEM2010, Japan) operated at 200 kV equipped with EDX analysis. The
Raman spectra were measured using Nanofinder 30 spectrometer
(Tokyo Inst. Co., Japan) equipped with a He:Ne (lambda = 633 nm
laser) and the scattering peaks were calibrated with a reference peak
from a Si wafer (520 cm−1 ). Raman spectra were recorded under
a microscope with a 40x objective in range of 0–1600 cm−1 and 3
mW of power at the sample. The electrochemical measurements were
performed on a VersaSTAT 4 (USA) electrochemical analyzer and a
conventional three-electrode electrochemical cell. A Pt wire and an
Ag/AgCl electrode were used as the auxiliary and reference electrodes,
respectively. All potentials were quoted regarding to the Ag/AgCl
electrode. Glassy carbon electrode was used as working electrode.
Preparation of the working electrode was carried out by mixing 2 mg
of the functional material, 20 μL Nafion solution (5 wt%) and 400 μL
F1195
isopropanol. The slurry was sonicated for 30 min at room temperature.
15 μL from the prepared slurry was poured on the active area of the
glassy carbon electrode which was then subjected to drying process
at 80◦ C for 20 min. The active surface area of the utilized glassy carbon working electrode was 0.07 cm2 . Actually, the normalization was
estimated based on the active surface area of the working electrode.
The functional material weight used on the prepared electrode was
0.07143 mg.
Results and Discussion
Characterization of the prepared graphene.— Among the various
reported methods for graphene preparation, chemical reduction of GO,
or chemically converted graphene, provides not only an established,
low-cost and scalable approach, but also a highly flexible method
for the chemical functionalization of graphene materials.45,46 Accordingly, the chemical route was invoked in this study. XRD analysis can
be easily differentiate between graphite, GO and graphene. Typically,
the graphitic structure is assigned by a sharp peak at 2θ of 26.5◦ , which
is indexed to the (0 0 2) crystal plan.47 However, a new diffraction
peak appears at 2θ of 10.5◦ , along with the disappearance of the (0 0 2)
diffraction peak when graphite is oxidized to GO.47 Graphene can be
identified by a typical diffraction peak at 2θ of 27.5◦ ,44,48 usually this
peak is broad which indicates the smaller crystalline size of graphene
in a single or few layers structure. Accordingly, from Fig. 1A, one can
claim that the obtained powder composes of graphene which indicates
that the utilized chemical procedure has been successfully achieved.
Besides XRD, Raman microscopy can also provide a trustable
comparison between graphite, GO and graphene. For instance,
graphite displays a prominent G peak at 1581 cm−1 , corresponding to the first-order scattering of the E2 g mode.49 However, in the
Raman spectrum of GO, the G band is broadened and shifted to 1594
cm−1 . Moreover, because of the reduction in size of the in-plane sp2
domains due to the extensive oxidation, the D band at 1363 cm−1
becomes prominent. On the other hand, the Raman spectrum corresponding to graphene contains both G and D bands (at ∼1580 and
∼1350 cm−1 , respectively) with an increase in the D/G intensities
ratio compared to that in GO. This change suggests a decrease in
the average size of the sp2 domains upon reduction of the exfoliated
GO.49 Fig. 1B displays the Raman spectrum of the prepared reduced
graphene oxide. As shown in the figure, the pattern typically matches
the standard graphene.
HR TEM image for the prepared graphene is shown in Fig. 1C;
the exfoliation clearly appears. Moreover, Fig. 1D (HR TEM image)
indicates that the formed graphene is multilayer; around 3 layers.
From XRD and Raman and TEM analyzes, it can be confirmed that the
utilized procedure was successfully achieved to produce the graphene.
Characterization of the produced Cox Niy -decorated graphene.—
Our previous studies and also for other researchers indicated that the
metal acetate is the best precursor to produce pristine metal rather
than other metallic compounds (e.g. oxides or hydroxides).50–53 Actually, the abnormal decomposition of the acetate anion in the inert
atmosphere leads to produce strong reducing gases (namely, CO and
H2 ) which results in complete reduction for the salt to form pure
metal.50–53 Briefly, formation of pure nickel can be explained by the
following reactions:54
Ni(CH3 COO)2 · 4H2 O → 0.86Ni(CH3 COO)2 · 0.14Ni(OH)2
+ 0.28CH3 COOH + 3.72H2 O
[1]
0.86 Ni(CH3 COO)2 · 0.14Ni(OH)2
→ NiCO3 + NiO + CH3 COCH3 + H2 O
[2]
NiCO3 → NiO + CO2
[3]
NiO + CO → Ni + CO2
[4]
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Figure 1. Physicochemical characterizations for the prepared graphene; (A) XRD, (B) Raman, and (C) and (D) areTEM and HR TEM images, respectively.
However, it was explained that cobalt can be obtained from acetate
as follow:55
Co(CH3 COO)2 · 4H2 O → Co(OH)(CH3 COO) + 3H2 O + CH3 COOH
[5]
Co(OH)(CH3 COO) → 0.5CoO + 0.5CoCO3 + 0.5H2 O
+ 0.5CH3 COCH3
CoCO3 → CoO + CO2
CoO + CO → Co + CO2
[6]
spectra of Ni and Co are almost similar. In fact, XRD can provide important information about whether Ni and Co are present separately
or in the form of an alloy structure. The little differences in the cell
parameters, 3.544 Å of cobalt and 3.523 Å of Ni should have the (2
0 0) peak at 51.53 and 51.86 degree, respectively using CuKa radiation. A single peak within this range would indicate formation of a
solid solution. As shown in the inset (Fig. 2B) which displays the (2
0 0) peak, single peak is observed which indicates formation of CoNi
solid solution. The average grain size was calculated by applying the
Scherrer equation.
[7]
τ=
[8]
The XRD analysis (Fig. 2A) reveals that the utilized reducing
agents could not achieve complete reduction for the utilized metal
precursors added to the GO suspension. The results indicated that the
metallic counterpart in the precipitate obtained after the reflux step
composed of Co(OH)2 (JCDPS # 45-0031) and Ni(OH)2 (JCDPS #
14-0117). However, as shown in Fig. 2B, calcination of the obtained
powder in argon atmosphere led to produce the pristine metals which
matches the previous studies.50–53 The strong diffraction peaks at 2θ
values of 44.30, 51.55, 76.05 and 92.55◦ corresponding to (1 1 1),
(2 0 0), (2 2 0) and (3 1 1) crystal planes, respectively indicate formation of pure nickel or pure cobalt or both (JCDPS 15-0806; Co
and 04–0850; Ni). Cobalt and nickel have successive atomic numbers, close molecular weights, and same crystal structure in the normal conditions; FCC crystal lattice with space group class (S.G) of
Fm3m(225). Moreover, the cell parameters are very close; 3.544 and
3.523 Å for the cobalt and nickel, respectively. Accordingly, the XRD
kλ
βcosθ
[9]
Where K is constant (0.9), λ is the X-ray wavelength, β is the full
width half maximum (FWHM), θ is the Bragg angle and τ is the mean
size of the crystalline domains. The estimated value was 28.56 nm.
As cobalt and nickel are neighbors in the periodic table, their
atomic and molecular weights are close, also both metals have higher
melting point than the calcination temperature (i.e. no vaporization
was expected during the performed calcination process), moreover
there is no big difference between molecular weights of the utilized
precursors for the two metals, so it was assumed that the final Co/Ni
ratio matches the ratio between the two precursors. On the other hand,
according to TGA results for GO (data are not shown), the percentage
of weight decrease due to calcination is almost 60%, so the formed
rGO in the final powder is ∼120 mg. It is noteworthy mentioning that
the total metals precursor in each sample was fixed at 0.4 g to 300
mg GO. Therefore, the final weight percentages of graphene and the
metallic alloy in the introduced electrocatalyst can be estimated as
55.3 and 44.7 wt%, respectively.
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Journal of The Electrochemical Society, 161 (12) F1194-F1201 (2014)
F1197
incorporation on the current density can be observed. Ni-decorated
graphene reveals maximum current density 10 times more than Ni
NPs. Moreover, it can be concluded that pristine graphene has almost
negligible electro-catalytic activity toward ethanol oxidation as it is
expected from the carbonaceous materials in general.
The onset potential indicates the electrode overpotential, it is an
important indicator among the invoked parameters to demonstrate the
electro-catalytic activity. In other words, the efficacy of the electrocatalyst can be evaluated from the onset potential. In alcohol electrooxidation, high activity and less overpotential corresponds to more
negative onset potential.57 Usually, OH and CO adsorbed layer on the
surface of the electrodes is the main reason behind increasing the onset
potential, this gas layer leads to overpotential.31,58 Due to its polarity
which enhances the adsorbability, carbon monoxide is the most intermediate compound leading to high overpotential. Carbon monoxide
accumulates on the surface of the electrode till further oxidation step
to carbon dioxide.31,59 Revealing small value of the corresponding onset potentials is one important advantage for the Pt-based electrodes
over the non-precious electro-catalysts. Interestingly, using graphene
as a supporter for the nickel nanoparticles led to decrease the onset
potential from ∼390 mV (for the pristine Ni NPs) to – 90 mV [vs.
Ag/AgCl RE] (Fig. 4A) which is a promising results as according
to our best knowledge, for the non-precious electro-catalysts, it is
the lowest onset potential value reported in the literature so far. It is
also clear form Fig. 4B that the onset potential is independent on the
ethanol concentration.
Figure 2. XRD results for the synthesized CoNi-decorated graphene before;
(A), and after calcination; (B) process. The inset represents the (2 0 0) peak.
To further investigate the real composition of the formed metallic compound and the obtained morphology, TEM analyzes were
carried out (Fig. 3). Fig. 3A and 3B display normal TEM images,
as shown dark spots from crystalline nanoparticles are distributed
along the graphene sheet. Elemental analysis along a randomly chosen line was carried out to detect Co and Ni distribution (Fig. 3C). As
shown, Co and Ni have same distribution which verifies the aforementioned hypothesis about formation of NiCo alloy nanoparticle attached
with graphene sheet and simultaneously supports the XRD results.
Fig. 3D displays the metallic particle size distribution, from the obtained results indicated that the average size is ∼68 nm.
Attraction of the positively-charged metal ions by the polarized
bonds of the functional groups on the GO is an accepted mechanism
for the synthesis of inorganic nanostructures decorated graphene.56
Accordingly, it can be claimed that during the reflux step, Co+2 and
Ni+2 ions attached the GO surface which resulted in formation of
CoNi NPs during the calcination process.
Electro-catalytic activity investigation.—Influence of graphene
incorporation.— Fig. 4A displays the cyclic voltogramms for three
electrodes; unsupported nickel nanoparticles (<25 nm, Aldrich), pristine graphene and Ni-decorated graphene prepared by the same aforementioned procedure (the initial precursors were 400 mg NiAc and
300 mg GO). The utilized electrolyte was 3 M ethanol (in 1 M KOH),
and the cyclic voltammetery analysis was carried out at 25◦ C and scan
rate of 50 mV/s. As shown in the figure, strong impact for graphene
Influence of alloy structure.—As aforementioned, the alcohol electrooxidation process is based on adsorption of the original reactants and
the intermediates compounds, and then successive dissociation steps
carry out. Typically, in case of ethanol dehydrogenation, the first step is
the cleavage of O–H bond, forming ethoxy species CH3 CH2 O. Further
transformation of ethoxy species gives acetaldehyde CH3 CHO, which
then can be oxidized by numerous reactions, forming acetate ion
CH3COO− , acetone CH3 COCH3 , crotonaldehyde CH3 CHCHCHO,
acetyl CH3 CO, methane, other hydrocarbons, carbonate ion CO3 2− ,
CO and CO2 .60,61 The optimum dehydrogenation means releasing the
maximum number of electrons (e.g. 12), in such a case, the C–C bond
is dissociated resulting C1 species are oxidized to CO2 .31
Fig. 5 displays the cyclic voltammograms for all the prepared
Cox Niy -decorated graphene with different concentrations of the metallic nanoparticle in presence of 3 M ethanol (in 1 M KOH solution). It
is noteworthy mentioning that the loading of the metallic nanoparticle
was kept constant for all formulations used in Fig. 5. The obtained results indicated that the graphene decorated by Ni0.2 Co0.2 nanoparticles
has the best electro-catalytic activity toward ethanol oxidation. It was
reported that oxidation of ethanol process takes place in the anodic
and cathodic directions, the cathodic peak is probably due to further
oxidation of ethanol or the intermediate products of its oxidation.62,63
As shown in Fig. 5, for the graphene decorated by Ni0.2 Co0.2 alloy
NPs, the oxidation peaks (as marked by the arrows) are clearly appear
at relatively low potential. However, these peaks are not shown for the
other formulations.
Influence of metallic NPs loading.—As it was aforementioned in the
introduction section, the advantage of incorporation of carbonaceous
material with the active electro-catalysts is carried out to exploit the
adsorption capacity. Fig. 6 represents the influence of the metallic
nanoparticle loading for alloys having equal amounts from the two
metals on the electro-catalytic activity. As shown in the figure, the
graphene decorated by Ni0.2 Co0.2 alloy NPs reveal the best performance among the utilized samples. For the sample having higher
loading (Ni0.4 Co0.4 -decorated graphene), it can be claimed that the
comparatively low performance attributes to hiding more surface from
the graphene sheet as shown in the TEM image of this sample (the
inset). In other words, one can say that graphene adsorption capacity
has a distinct role in the electrooxidation process as graphene can
adsorb the alcohols molecules and/or the reactions intermediates to
complete the oxidation process. It is noteworthy mentioning that the
approximate loading percentages for the investigated formulations are
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Journal of The Electrochemical Society, 161 (12) F1194-F1201 (2014)
Figure 3. TEM images for the prepared CoNi-graphene; (A and B), and line EDX analysis; (C). The linear distribution of nickel and cobalt along with the selected
line are shown. The initial amounts for NiAc and CoAc were similar; 0.2 g. Panel D displays the size distribution of the metallic nanoparticles.
29.5, 44.7 and 62.5 wt% for Ni0.1 Co0.1 -, Ni0.2 Co0.2 -, and Ni0.4 Co0.4 loaded graphene electrocatalysts, respectively.
Maximum ethanol concentration.—Generally, utilizing highly concentrated alcohol solution in the DAFCs is strongly preferable because
it distinctly diminishes the cells size and simultaneously improves the
power density. Theoretically, using absolute ethanol is not possible as
water is a reactant in the anode reactions, so ethanol/water ratio is a
process parameter for the electro-catalysts. The ethanol electrooxidation can be displayed as follow:
C2 H5 OH + 3H2 O → 12H+ + 12e− + 2CO2
[10]
Each electro-catalyst corresponds to a maximum concentration
from the alcohol, beyond this concentration threshold the performance
decreases. Fig. 7 displays the influence of ethanol concentration on the
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Journal of The Electrochemical Society, 161 (12) F1194-F1201 (2014)
F1199
Figure 4. Cyclic voltammograms at a scan rate of 50 mV/s and 25◦ C for Ni NPs, pristine graphene and Ni-decorated graphene in 1 M KOH and in presence of
3.0 M ethanol; (A) and for Ni-decorated graphene at different ethanol concentration; (B).
Figure 5. Cyclic voltammograms at a scan rate of 50 mV/s and 25◦ C for CoNidecorated graphene with different concentrations for the metallic nanoparticle
in 3 M ethanol (in 1 M KOH). The arrows point to the ethanol oxidation peaks.
The loading percentages is 44.7 wt%.
Figure 6. Cyclic voltammograms at a scan rate of 50 mV/s and 25◦ C for CoNidecorated graphene with different loading for the metallic nanoparticle in 3 M
ethanol (in 1 M KOH). The inset displays TEM image for Co0.4 Ni0.4 -decorated
graphene sample. The loading percentages are 29.5, 44.7 and 62.5 wt% for
Ni0.1 Co0.1 -, Ni0.2 Co0.2 -, and Ni0.4 Co0.4 - loaded graphene, respectively.
Figure 7. Cyclic voltammograms at a scan rate of 50 mV/s and 25◦ C for (A) Co0.1 Ni0.1 - and (B) Co0.2 Ni0.2 - decorated graphene samples with different ethanol
concentrations (in 1 M KOH). The loading percentages are 29.5 and 44.7 wt% for Ni0.1 Co0.1 -, and Ni0.2 Co0.2 – loaded graphene, respectively.
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Journal of The Electrochemical Society, 161 (12) F1194-F1201 (2014)
Figure 8. Selected 40 cycles from the during the cyclic voltammograms of the proposed washing process; A, and the cyclic voltammograms in presence of 0.5
M ethanol at a scan rate of 50 mV/s and 25◦ C for fresh and washed in 1 M KOH for 1200 cycles Co0.2 Ni0.2 -decorated graphene electrode; B.
electro-catalytic activity of two samples; Ni0.1 Co0.1 - and Ni0.2 Co0.2 decorated graphene; Fig. 7A and 7B, respectively. As shown in the
figures, the maximum ethanol concentration in case of the sample
having low metallic loading (Ni0.1 Co0.1 -decorated graphene) was 3M
(Fig. 7A), while increasing the loading led to improve the maximum
concentration threshold to be 4 M as can be observed in Fig. 7B. This
is an interesting finding as the sample giving the best performance in
all investigations (Ni0.2 Co0.2 -decorated graphene) is still prominent.
Catalyst stability.—In contrast to the precious metals, low stability of
the transition metals electro-catalyst is a main problem facing wide
commercial application. Low stability can be translated as dissolution
of the catalyst due to reaction with utilized electrolyte. Therefore, to
properly investigate the stability of the introduced catalyst in the utilized alkaline media, a comparison between the electro-catalytic performance of a fresh electrode and a washed one was carried out. Typically, the electro-catalytic activity of Ni0.2 Co0.2 -decorated graphene
electrode was first investigated, then the same electrode was washed
by subjecting to cyclic voltammetry process for 1200 successive cycles (scan rate 100 mV/s) in presence of 1 M KOH solution, after that
the electro-catalytic performance of this electrode was re-investigated.
Fig. 8A demonstrates a selected 40 cycles from the utilized 1200 cycles for the proposed washing process, good stability can be observed.
Fig. 8B displays the obtained cyclic voltammograms of the washed
and fresh electrodes. As shown in the figure, there is almost no difference in the electro-catalytic activity of the fresh and washed electrodes
toward ethanol oxidation. Actually, the observed good stability can
be attributed to the alloy structure which reveals good resistance to
corrosion in the alkaline media.64
Conclusions
CoNi-decorated graphene can be produced by adding cobalt acetate and nickel acetate in the reaction media during graphene preparation using the chemical route. However, calcination of the resultant
decorated graphene in argon atmosphere is an important step to produce solid solution CoNi alloy nanoparticle. Utilizing graphene as
a supporter for the introduced Ni-based alloy nanoparticle strongly
enhances the electro-catalytic activity toward ethanol oxidation. The
composition and the loading of the alloy nanoparticle have a distinct
influence on electro-catalytic activity; Ni0.2 Co0.2 -decorated graphene
reveals the best results. Due to the alloy structure, the proposed electrocatalyst has good stability in the alkaline media.
Acknowledgment
This project was supported by NSTIP strategic technologies programs, number (11-ENE1917–02) in the Kingdom of Saudi Arabia.
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