Electrooxidation of glycerol on nickel and nickel alloy (Ni-Cu and Ni-Co) nanoparticles in alkaline media

Article abstract of DOI:10.1039/c5ra26006j

In the present study, nickel (Ni) and Ni alloy (Ni-Cu and Ni-Co) nanoparticles modified carbon-ceramic electrodes (Ni/CCE, Ni-Cu/CCE and Ni-Co/CCE) were prepared by an electrochemical process for the oxidation of glycerol. In order to obtain the surface and physicochemical information, the Ni/CCE, Ni-Cu/CCE and Ni-Co/CCE were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and electrochemical techniques. Then, cyclic voltammetry and chronoamperometry were employed to characterize the electrocatalytic activity of the modified electrodes, Ni/CCE, Ni-Cu/CCE and Ni-Co/CCE, toward the oxidation of glycerol in 1.0 M NaOH solution. It was found that the Ni alloy nanoparticle modified electrodes are catalytically more active than the Ni/CCE, therefore, the alloying of the Ni with Cu and Co in the form of nanoparticles on the carbon-ceramic electrode, as a homemade substrate, greatly enhances the catalytic activity of the Ni-based electrocatalysts (as the non-platinum electrocatalysts) in glycerol oxidation.

Full text of DOI:10.1039/c5ra26006j

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Electrooxidation of glycerol on nickel and nickel
alloy (Ni–Cu and Ni–Co) nanoparticles in alkaline
media
Cite this: RSC Adv., 2016, 6, 31797
Biuck Habibi* and Nasrin Delnavaz
In the present study, nickel (Ni) and Ni alloy (Ni–Cu and Ni–Co) nanoparticles modified carbon-ceramic
electrodes (Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE) were prepared by an electrochemical process for the
oxidation of glycerol. In order to obtain the surface and physicochemical information, the Ni/CCE, Ni–
Cu/CCE and Ni–Co/CCE were investigated by scanning electron microscopy, energy dispersive X-ray
spectroscopy, X-ray diffraction and electrochemical techniques. Then, cyclic voltammetry and
chronoamperometry were employed to characterize the electrocatalytic activity of the modified
electrodes, Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE, toward the oxidation of glycerol in 1.0 M NaOH
solution. It was found that the Ni alloy nanoparticle modified electrodes are catalytically more active
than the Ni/CCE, therefore, the alloying of the Ni with Cu and Co in the form of nanoparticles on the
carbon-ceramic electrode, as a homemade substrate, greatly enhances the catalytic activity of the Ni-
based electrocatalysts (as the non-platinum electrocatalysts) in glycerol oxidation.
Received 9th December 2015
Accepted 19th March 2016
DOI: 10.1039/c5ra26006j
www.rsc.org/advances
there are few reports about the electrooxidation of alcohols such
as methanol and ethanol on the Ni and Ni-based nano-
1
. Introduction
21–25
Metal and metal alloy nanoparticles due to their high specic particles.
On the other hand, in order to improve the elec-
surface area, reactivity, catalytic and electrocatalytic activities trocatalytic performance of the Ni-based catalysts, bimetallic Ni
can be used extensively in many elds such as medicine, such as Ni–Fe, Ni–Cu, Ni–Co, Ni–Cr and Ni–Mn have been
2
6–30
manufacturing and materials, environmental, energy and elec- investigated and reported.
1–4
tronics. Some of the uses of these nanoparticles in the energy
Glycerol is a highly functionalized molecule containing three
eld include: application in fuel cells, in solar energy systems, hydroxyl groups that are responsible for its solubility in water
in batteries, in coal liquefaction and in transformers. Among and its hygroscopic nature and is widely available from bio-
of the metal and metal alloys nanoparticles, Pt and Pt alloys due sustainable sources and can be produced in a renewable,
to unique chemical and physical characteristics have important environmental-friendly, and cost-effective manner. Glycerol

5
–9
31
applications in these areas especially in direct liquid fuel cells would be promising in fuel cells because it is less toxic and
10,11
(DLFCs).
However, major barriers in the using, spreading inammable, and also possess relatively high theoretical energy
and commercializing of the Pt and Pt alloy nanoparticles as density. On the other hand, it is a good potential hydrogen
electrocatalyst materials in DLFCs are the sluggish kinetics, source for fuel cells given a rapid growth of biodiesel produc-
12,13
poisoning by carbonaceous species and their high cost.
practice, to solve these problems, the researchers have been during the last recent years
In tion. The oxidation of glycerol has been systematically studied
32,33 29,34–36
and few groups
have been
focused on the alternative nanoparticles (non-platinum) with investigated and shown that the oxidation of glycerol proceeds
signicant electrochemical activity as anodes or cathodes in on the Ni and Ni-based electrocatalysts.
14,15
DLFCs.
The non-Pt-based nanoparticles are commonly used
In this work, the Ni and Ni-based alloys nanoparticles were
as electrocatalysts in fuel cells due to low cost, relatively abun- electrodeposited on the carbon-ceramic electrode as a home-
16–18
dant material and suitable catalytic activity.
Nickel (Ni) is made substrate for the electrooxidation of the glycerol. The
one of the most commonly used non-Pt transition metals and is morphology, structure, composition and electrochemical
used to manufacture the electrocatalysts that are found in behavior of the Ni and Ni-based alloys modied electrodes were
numerous electrochemical systems, such as rechargeable characterized by scanning electron microscopy (SEM), energy
19,20
batteries, supercapacitors and fuel cells.
In fuel cells eld, dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD)
and cyclic voltammetric methods, respectively. Then, the
cyclic voltammetry and chronoamperometry approaches were
employed to characterize the electrocatalytic activity of the
present nanoparticles modied electrodes toward the oxidation
Electroanalytical Chemistry Laboratory, Department of Chemistry, Faculty of Sciences,
Azarbaijan Shahid Madani University, Tabriz, Iran. E-mail: B.Habibi@azaruniv.ac.ir;
biuckhabibi_a@yahoo.com; Fax: +98-41-34327541; Tel: +98-41-34327541
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ꢁ2
of glycerol in 1.0 M NaOH solution. It was found that Ni-based value corresponds to 0.725 mg cm of the Ni on the CCE. The
alloys modied electrodes were electrocatalytically more active Ni/CCE was washed thoroughly with double distilled water and
than Ni-alone nanoparticles modied electrode and had satis- dried before further investigations.
factory stability and reproducibility when stored in ambient
conditions or continues cycling.
2.3.2. Preparation of the Ni–Cu/CCE and Ni–Co/CCE. The
CCE was prepared according to the above mentioned method.
The Ni–Cu and Ni–Co alloys nanoparticles on the CCE were
electrodeposited (potentiostatically) from the aqueous solu-
2
. Experimental
tions of 0.1 M Na
NiCl $6H O and CuCl
Methyltrimethoxysilane (MTMOS), glycerol, methanol, HCl, 1.5 : 0.5 ratio of NiCl $6H
2
SO
4
pH ¼ 3.4 comprising 1 : 1 ratio of
$2H O for Ni–Cu alloy deposition and
O and CoCl for Ni–Co alloy nano-
2.1. Chemicals
6
2
2
2
6
2
2
NaOH, H SO , Na SO , NiCl $6H O, CoCl , CuCl $2H O and particles (total concentration of two salts equal to 2 mM) at ꢁ0.6
2
4
2
4
2
2
2
2
2
high purity graphite powder were obtained from Merck or V versus the SCE for a certain time. The mass values of alloys
Fluka. All solutions were prepared with double distilled water nanoparticles electrodeposited on the surface of the CCE are
and all experiments were carried out at room temperature.
corresponding to an equivalent amount of Ni similar to Ni/CCE
if Ni is considered as the only metal electrodeposited.
2.2. Instrumentation
The electrochemical experiments were carried out using an
AUTOLAB PGSTAT-30 (potentiostat/galvanostat) equipped with
3
. Results and discussion
a USB electrochemical interface and driven GEPS soware was 3.1. Physicochemical characterization of the Ni/CCE, Ni–Cu/
used for electrochemical experiments. A conventional three CCE and Ni–Co/CCE
electrode cell was used at room temperature. The modied
In order to characterize the surface morphology of the present
electrodes (Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE) (3 mm diam-
modied electrodes, the surface of the Ni/CCE, Ni–Cu/CCE and
eter) were used as the working electrode. A saturated calomel
Ni–Co/CCE was investigated by SEM and corresponding images
were shown in Fig. 1. Fig. 1(A) displays the image of the CCE
aer Ni nanoparticles electrodeposition. This image reveals
a high density of globular particles of the Ni nanoparticles with
different sizes. The globular like structure of Ni nanoparticles
probably are not the individual Ni crystallites. Undoubtedly,
electrode (SCE) and a Pt wire were used as the reference and
auxiliary electrodes, respectively. JULABO thermostat was used
ꢀ
to control cell temperature at 25 C. A scanning electron
microscope (SEM), model LEO1430vp (Carl Zeiss, Germany)
equipped with energy dispersive X-ray spectroscopy (EDX) was
used to surfaces and surfaces chemical composition charac-
they are collections, like as a globular, consisting of crystallite
terizations of the Ni, Ni–Cu and Ni–Co nanoparticles. X-ray
aggregates. Fig. 1(B) shows the SEM image of Ni–Cu/CCE. As can
diffraction of the nanoparticles was studied using a Bruker
be seen, the surface of the Ni–Cu/CCE is completely different
AXF (D8 Advance) X-ray power diffractometer with a Cu Ka
with respect to Ni/CCE surface. This image shows the hierar-
radiation source (l ¼ 0.154056 nm) generated at 40 kV and
chical structures with dendritic morphology (with high magni-
35 mA.
40,41

cation in Fig. 1(C)).
Fig. 1(D) displays the SEM image of the
2.3. Preparation of the electrocatalysts
2.3.1. Preparation of the Ni/CCE. We have prepared the Ni
nanoparticles modied CCE by a two-step procedure:
Step 1: The CCE, as a homemade substrate, was produced
33,37,38
according to our previously works.
In briey, the amount
of 0.9 ml MTMOS was mixed with 0.6 ml methanol. Aer
addition of 0.6 ml HCl 0.1 M as the catalyst, the mixture was
magnetically stirred (for about 15 min) until producing a clear
and homogeneous solution. Then, 0.3 g graphite powder was
added and the mixture was stirred for other 5 minutes. Subse-
quently, the homogenized mixture was rmly packed into
a Teon tube (with 3 mm inner diameter and 10 mm length)
and dried for at least 24 h at room temperature. A copper wire
was inserted through the other end to set up electric contact.
The electrode surface was polished with emery paper grade 1500
and rinsed with double distilled water.
Step 2: The Ni nanoparticles were electrodeposited on the
obtained CCE in a 0.1 M Na SO , pH ¼ 3.4, continuous stirring
2
4
solution containing 1 mM NiCl $6H O at ꢁ0.6 V versus SCE for
6
2
39
an optional time. The charge resulting from the complete
Fig. 1 SEM images of the Ni/CCE (A), Ni–Cu/CCE [(B and C) (high
ꢁ2
reduction of the Ni ions at the given time is 2385 mC cm . This magnification)] and Ni–Co/CCE (D).
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CCE aer Ni–Co alloy electrodeposition. The Ni–Co alloy with
spherical structure and high density formed on the CCE. Its
surface investigation shows that the Ni–Co nanoparticles have
the size of 30–80 nm. It seems that, each spherical like struc-
42
tures of Ni–Co is not the individual Ni–Co crystallites. The
surface chemical composition of the Ni/CCE, Ni–Cu/CCE and
Ni–Co/CCE was investigated by EDX analysis (Fig. 2). The EDX
spectrum of Ni/CCE (Fig. 2(A)) contains strong peaks at
approximately 0.8, 7.5, 8.2 keV for Ni element, suggesting that
42
the single Ni nanoparticles are electrodeposited on the CCE.
The EDX analysis of the Ni–Cu/CCE (Fig. 2(B)) and Ni–Co/CCE
Fig. 2(C)) reveals the presence of the Ni, Co and Cu atoms.
(
On the other hand, the presence and distribution of the two
components (Ni and Cu in the case of Ni–Cu/CCE and Ni and Co
41,42
in the case of Ni–Co/CCE) registered by the EDX analysis.
In
all spectra in Fig. 2, the peaks at about 0.4, 0.7 and 1.9 keV
correspond to Si, O and C atoms, respectively, which attributed
to the CCE structure.
To characterize the crystal structure of the present modied
electrodes, the XRD experiment was used. Fig. 3 shows the XRD
patterns of the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE. In all XRD
ꢀ
Fig. 3 XRD patterns of the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/
CCE (C).
patterns, the peak at 2q of 55 corresponds to carbon (006) of
the support material (carbon-ceramic). The main peaks in the
ꢀ
ꢀ
ꢀ
XRD pattern of the Ni/CCE (A) at 2q values of 42.6 , 43.6 , 44.76 ,
4
ꢀ
ꢀ
ꢀ
ꢀ
6.41 , 52 , 60.15 and 77.71 corresponding to (010), (002),
(
111), (011), (200), (012) and (220) crystal planes for Ni, respec- of the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE electrocatalysts
ꢁ1
37
tively. Using the Scherrer equation; D ¼ 0.9l/b cos q (where recorded in the supporting electrolyte at a scan rate 50 mV s
;
c
ꢁ
1
l ¼ 0.154056 nm, b is the full width at half-maximum in radians 1.0 mol L NaOH solution. As can be seen in Fig. 4(A), during
and q is the peak position in degrees), the average crystallite size the forward scan, aer a double layer region in the range of
ꢀ
(
D
c
) of Ni nanoparticles form the 2q at the 44.76 (111) is esti- ꢁ1.0 to +0.3 V vs. SCE, an anodic peak appears at about +0.4 V
1
mated to be about 24 nm. In XRD patterns of the Ni–Cu/CCE (B) (wave a ), where the surface of the Ni/CCE is mainly covered by
and Ni–Co/CCE (C), the strong diffraction peaks at 2q values of Ni hydroxides and then the characteristic behavior assigned to
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
2.6 , 43.6 , 44.76 , 46.41 , 51 , 60.15 and 77.71 correspond- redox couple of Ni(II)/Ni(III): [Ni(OH)
2
+ OH 4 NiOOH + H
2
O +
ꢁ
ing to (010), (002), (111), (011), (200), (012) and (220) for pure Ni, e ] happens in higher potential region (from 0.6 to 1.0 V) prior
pure Cu or Ni–Cu alloy in the case of the Ni–Cu/CCE and pure to the oxygen evolution reaction. During the reverse scan,
4
1,43
0
Ni, pure Co or Ni–Co alloy in Ni–Co/CCE, respectively.
As can a cathodic peak at about +0.25 V (wave a
1
), corresponds the a
1
ꢁ
ꢁ
be seen, the XRD pattern of Ni/CCE and also the Ni–Cu/CCE and anodic peak [Ni + 2OH 4 Ni(OH)
2
+ 2e ] and also a small
Ni–Co/CCE are almost similar because these elements have cathodic peak at about ꢁ0.6 V (wave D
1
) occur, represents the
successive atomic number, close atomic weight and same existence of different phases of Ni oxides on the Ni/CCE surface.
44,45
crystal structure in the normal conditions.
For the electrochemical characterization of the Ni/CCE, Ni– be due to the discharge of the g-NiOOH phase to a or b-Ni(OH)
Cu/CCE and Ni–Co/CCE electrocatalysts the cyclic voltammetric obtained during the charging of b-Ni(OH) or partially over-
The electrochemical behavior of
1
On the other hand, the later cathodic peak, D , could possibly
2
2
46,47
studies were used. Fig. 4 shows the cyclic voltammograms (CVs) charging of the b-NiOOH.
Fig. 2 EDX spectrums of the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C).
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Fig. 4 Cyclic voltammograms of the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C) recorded at 50 mV s in 1.0 M NaOH solution.
ꢁ
ꢁ
Ni electrode in alkaline medium has been reported previ- couple of the Ni + 2OH 4 Ni(OH) + 2e , appear on it. The
ously.
2
48–52
Comparison the reported results in the literature with presence of Co atoms in the electrocatalyst structure changes
our results shows a good agreement between them. Based on the peak currents and peak potentials; the anodic peak current
0
the stated results, Ni electrode is oxidized to a highly hydrated (wave a
3
) and corresponding cathodic peak current (wave a
3
) in
ꢁ
ꢁ
a-Ni(OH)
lower potentials in the rst scan of cyclic voltammetry studies the Ni/CCE [Iwave a1 ¼ 0.5 mA in Fig. 4(A)] with sharp appear-
and the resulted hydroxides [a-Ni(OH) ] cover the electrode ances and in less positive potentials. Also, in the reverse scan
surface. The electrooxidation of Ni to a-Ni(OH) is a reversible the cathodic peak (wave D ) was increased more than two orders
2
according to: Ni + 2OH 4 Ni(OH)
2
+ 2e , at the CV of the Ni–Co/CCE (Iwave a3 ¼ 1.8 mA) are higher than that of
2
2
3
0
process; wave a1 and a in Fig. 4(A). With increasing the with respect to Ni/CCE, indicating the existence of different
1
potential in the positive direction the a-Ni(OH) convert to the phases of Ni and Co oxides on the Ni–Co/CCE surface. These
2
b-Ni(OH)
2
, which is more stable and less hydrated than a- changes in the CV prole of the Ni–Co/CCE are probably due to
, via an irreversible process. So that the conversion of b- the formation of Co(OH) from the corresponding Co atoms
to a-Ni(OH) and also to atomic Ni is not possible. At that occurs earlier than that of Ni(OH) . It is reported that the
Ni(OH)
Ni(OH)
2
2
2
2
2
the more positive potentials, the b-Ni(OH)
NiOOH.
2
converted to addition of Cu or Co to Ni particles such as Ni–Cu or Ni–Co
alloys modied electrodes or even other metals such as Cd, Zn,
23,30,34,51–53
Fig. 4(B) shows the CV of the Ni–Cu/CCE in the same sup- Ca and Ti elements as solid solutes to the Ni, represents a very
porting electrolyte. Besides the anodic (wave a ) and cathodic efficient strategy of suppressing the formation of the b-NiOOH
2
0
47,56–58
(
wave a ) peaks in the CV of the Ni–Cu/CCE, there are two other phase.
Therefore, the electrochemical behavior of the pure
2
anodic peaks, A
peak, C
reactions:
1 2
and A
, from ꢁ0.6 to 0.0 V and one cathodic electrodeposited Ni and Ni–Cu and Ni–Co alloy modied elec-
1
, at the about ꢁ0.9 V correspond to the following trodes reveals good stabilization of the b oxyhydroxide form on
54–57
the alloy nanoparticles and the effects of this phenomenon
obviously appearance on the CV proles of the Ni–Cu and Ni–Co
ꢁ
ꢁ
Cu + OH / Cu(OH) + e : wave A
47
1
alloy modied electrodes.
ꢁ
ꢁ
Cu (OH) + OH / CuO + H
2
O + e : wave A
2
3
.2. Electrocatalytic activity of the Ni/CCE, Ni–Cu/CCE and
ꢁ
ꢁ
ꢁ
O + 2e
CuO + H
2
O + e / Cu(OH) + OH or 2CuO + H
2
Ni–Co/CCE
ꢁ
/
Cu
2
O + 2OH : wave C
1
Fig. 5 shows the CVs of the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE
electrocatalysts in the absence and presence of the glycerol in
alkaline solution. Red lines in Fig. 5 display the CVs of Ni/CCE,
Ni–Cu/CCE and Ni–Co/CCE in 1.0 M NaOH solution containing
and also some cumulative effects of the presence of the Cu
atoms on the anodic and cathodic peak currents of Ni system.
So that, the anodic and cathodic peak currents of Ni oxidation
and reduction on the Ni–Cu/CCE were increased two orders
with respect to Ni/CCE [Fig. 4(A)]. Therefore the surface oxides
layer on the Ni–Cu alloy nanoparticles modied electrode is
subsequently transformed to a mixture of NiOOH and CuO or
ꢁ1
0.1 M glycerol at a scan rate 50 mV s . As can be seen in the CVs
of the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C), the
oxidation reaction of glycerol has the same appearance and
starts concomitantly with the formation of the Ni/Ni(OH) on
2
the Ni/CCE (Fig. 5(A)) and corresponding peaks at the alloy
57,58
Cu(OH). Also, according to the literature,
some of the
3
+
nanoparticles on the Ni–Cu/CCE (Fig. 5(B)) and Ni–Co/CCE
Cu(OH) and CuO can be oxidized further to the Cu oxide in
high potentials prior to the oxygen evolution reaction.
(Fig. 5(C)). The electrooxidation of glycerol commences (onset
oxidation potential) at about 0.4 V, 0.3 V and 0.219 V vs. SCE on
the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE electrocatalysts,
respectively. At potentials more than ca. 0.4 V on the Ni/CCE, 0.3
V on the Ni–Cu/CCE and 0.219 V on the Ni–Co/CCE, the reaction
becomes accelerated and maximum currents occur at ca. 0.79 V
While the Ni–Co/CCE electrocatalyst in supporting electro-
lyte exhibits a CV [Fig. 4(C)] that is very similar to the CV of the
Ni/CCE in the same electrolyte. However, some differences,
especially in the peak currents and potential region of the redox
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ꢁ1
Fig. 5 Cyclic voltammograms of the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C) recorded at 50 mV s in absence (black lines) and presence
red lines) of 0.1 M glycerol in 1.0 M NaOH solution.
(
(Ni/CCE), 0.72 V (Ni–Cu/CCE) and 0.55 V (Ni–Co/CCE), respec- possible intermediates and nal products including glyceral-
tively. The performance of the present electrocatalysts toward dehyde, dihydroxyacetone and carboxylic compounds such as
the oxidation of the glycerol was evaluated by three parameters: glyceric, glycolic, formic, tartronic, hydroxypyruvic, oxalic, and
(
1) onset oxidation potential, (2) forward anodic peak potential ketomalonic acids are formed from the glycerol electro-
61,62
and (3) forward anodic peak current. These electrochemical oxidation (Scheme 1).
As can be seen in Scheme 1, the
parameters in the glycerol oxidation at the Ni/CCE, Ni–Cu/CCE glycerol oxidation is made up of complex pathway reactions that
and Ni–Co/CCE electrocatalysts are listed in Table 1. As can be can produce a large number of useful intermediates or valuable
63
seen in Table 1, the lower onset oxidation potential, forward ne chemicals along with the exchange of electrons.
anodic peak potential and higher anodic peak current for Ni- In order to determine of kinetic parameters of the electro-
based alloys disclose that the Ni-based alloys have the high oxidation of glycerol at the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE,
electrocatalytic activity toward the glycerol oxidation with the Tafel analysis was planned. Fig. 6 shows the Tafel plots
respect to alone Ni modied electrode (Ni/CCE). As mentioned resulted from the CVs of 0.1 M glycerol in 1.0 M NaOH solution
above, due to the higher surface concentration of the b-NiOOH on the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C) at a scan
ꢁ
1
form in the Ni–Cu/CCE and Ni–Co/CCE because of the presence rate of 20 mV s . The slope of Tafel plots are 253, 245 and 175
ꢁ
1
of the Cu and Co, the alloy modied electrodes generate mV dec for glycerol electrooxidation at the Ni/CCE, Ni–Cu/
a higher electrocatalytic activity toward glycerol oxidation in CCE and Ni–Co/CCE, respectively. The ana values can be
6
4,65
NaOH solution with respect to Ni/CCE. On the other hand, calculated from the Butler–Volmer equation:
log i ¼ log i +
o
comparison of the black lines (absence of glycerol) and red lines an Fh/2.303RT; the constants R and F denote the universal gas
a
(presence of glycerol) in Fig. 5 shows that the oxygen evolution constant and the Faraday constant, respectively, T is the
reaction occurs at more positive potentials when glycerol is temperature (in K), a is the charge transfer coefficient of the
59
present in NaOH solution. As reported in the literature, this reaction, i
o
is the exchange current density, n is the number of
a
may be due to a higher adsorption affinity of elevated valence Ni electrons transferred in the rate-determining step and h is the
or mixed Ni–Cu and N–Co intermediate species for glycerol than overpotential. The an
for hydroxyl species (OH ). Alternatively, in presence of glycerol are 0.23, 0.24 and 0.34 for glycerol electrooxidation at the Ni/
a
evaluated from the slope of Tafel plots,
ꢁ
(
red lines) the cathodic peaks originally observed in the absence CCE, Ni–Cu/CCE and Ni–Co/CCE, respectively. In the electro-
0
0
0
of glycerol (black lines) (a , a and a and also D ) during the chemical oxidation of glycerol, the rate-determining step is
reverse scan disappear completely. These effects could be a one-electron process,
associated to the dependency of the beginning of the glycerol 0.24 and 0.34 for glycerol electrooxidation at the Ni/CCE, Ni–Cu/
oxidation with the preliminary steps of the transformation of CCE and Ni–Co/CCE, respectively.
Ni(OH)
1
2
3
1
29,30
n
a
¼ 1; thus, a values should be 0.23,
60
2
to NiOOH at the surface of electrocatalysts. Therefore,
The effect of scan rate on the electrooxidation of glycerol at
these results indicate that the Ni-based electrocatalysts can the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE was done and results
catalyze the glycerol oxidation in alkaline media; where the were shown in Fig. 7(A)–(C), respectively. As can be seen, the
anodic peak currents in all cases increase with the scan rate.
The relation between the anodic peak currents of glycerol
Table 1 Electrochemical parameters of glycerol electrooxidation on
the present electrocatalysts
oxidation obtained in forward scan and square root of scan
1/2
rates (v ) are shown in the insets of Fig. 7(A)–(C) (I for the Ni/
0
00
CCE, I for Ni–Cu/CCE and I for Ni–Co/CCE). The anodic peak
currents are linearly proportional to the square root of scan
rates, which indicate the electrocatalytic oxidation of glycerol at
the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C) may be
Fuel
Electrocatalyst
Eonset (V)
Epf (V)
Ipf (mA)
Glycerol
Ni/CCE
Ni–Cu/CCE
Ni–Co/CCE
0.40
0.30
0.22
0.79
0.70
0.55
3.44
4.12
5.74
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Scheme 1 Schematic diagram for the oxidation of the glycerol and resulted products at the present modified electrodes in alkaline media based
on the literature.
controlled by a diffusion process. In addition, the peak poten- Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE was rstly examined in
tials of the oxidation of glycerol at the Ni/CCE, Ni–Cu/CCE and 1.0 M NaOH containing 0.1 M glycerol by cyclic voltammetry.
Ni–Co/CCE shi to high potentials with increasing of the scan The obtained results indicate that the anodic peak currents in
0
00
rate (insets of II (A), II (B) and II (C) in Fig. 7 for the Ni/CCE, the forward scan remain constant with an increase in scan
Ni–Cu/CCE and Ni–Co/CCE, respectively). These results indi- numbers at the initial stage and then start to decrease aer
cate that the electrooxidation of glycerol at the Ni/CCE, Ni–Cu/ 30 scans. The peak currents of the 500th scan are about
CCE and Ni–Co/CCE is an irreversible electrode process.
between 93 and 95% than that of the rst scan for the elec-
trocatalysts (not shown here). In general, the loss of the
catalytic activity aer successive number of scans may result
from the consumption of glycerol during the CV scan. It may
also be due to poisoning and the structure change of the Ni
and Ni-based nanoparticles as a result of the perturbation of
the potentials during the scanning in aqueous solutions,
3.3. Long-term stability of the Ni/CCE, Ni–Cu/CCE and Ni–
Co/CCE
In view of practical applications, long-term stability of the
electrocatalysts is important. The long-term stability of the
ꢁ1
Fig. 6 Tafel plots from the electrooxidation of 0.1 M glycerol in 1.0 M NaOH solution at scan rate of 20 mV s on the Ni/CCE (A), Ni–Cu/CCE (B)
and Ni–Co/CCE (C).
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Fig. 7 Effect of scan rate on the electrooxidation of 0.1 M glycerol in 1.0 M NaOH at the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C). The
0
00
insets [I, I and I respectively for the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C)] show the dependence of the forward anodic peak currents
0
00
on the square root of scan rates and insets [II, II and II respectively for the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C)] show the dependence
potentials of the forward anodic peak on the square root of scan rates.
Fig. 8 Chronoamperometric curves for the electrooxidation of 0.1 M glycerol at the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C) in 1.0
M NaOH solution at two applied potentials (0.6 V and 0.8 V) for Ni/CCE, (0.6 V and 0.75 V) for Ni–Cu/CCE and (0.4 V and 0.55 V) for Ni–Co/CCE.
especially in presence of the organic compound. Another
factor might be due to the diffusion process occurring
4. Conclusions
between the surface of the electrocatalysts and the bulk A simple electrochemical method was used to fabricate the Ni
solution. With an increase in scan numbers, glycerol mole- and Ni alloys (Ni–Cu and Ni–Co) nanoparticles on the carbon-
cules diffuse gradually from the bulk solution to the surface of ceramic electrode as the electrocatalysts for the oxidation of
the electrocatalysts. Aer the long-term CV experiments, the glycerol. The prepared modied electrodes, Ni/CCE, Ni–Cu/CCE
present electrocatalysts were stored in water for a week; then and Ni–Co/CCE, were characterized by XRD, SEM, EDX and
glycerol oxidation was carried out again by CV, and excellent electrochemical techniques. The SEM studies show the well-
catalytic activities toward glycerol oxidation were still dispersed and spherical nanoparticles of the Ni and Ni–Co
observed on the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE electro- alloy on the CCE; the size of these nanoparticles were about 24
catalysts. In order to further evaluation the stability of nm for alone Ni and between 30 and 80 nm for the Ni–Co alloy,
the electrocatalytic activity of the Ni/CCE, Ni–Cu/CCE and respectively. While, the Ni–Cu alloy nanoparticles were elec-
Ni–Co/CCE toward glycerol oxidation, chronoamperometric trodeposited on the CCE as the hierarchical structures with
measurements were performed by applying two different dendritic morphology. The EDX analyses conrm the presence
applied potentials for 700 s. Fig. 8 displays the chro- of the alone Ni and its alloy in the Ni/CCE, Ni–Cu/CCE and
noamperometric curves of 0.1 M glycerol oxidation in 1.0 M Ni–Co/CCE, respectively. On the other hand, the XRD patterns
NaOH at the Ni/CCE (A), Ni–Cu/CCE (B) and Ni–Co/CCE (C) show that the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE display the
under constant potentials of (0.6 V and 0.8 V) for Ni/CCE, characteristic diffraction peaks of a face-centered cubic Ni
(
0.6 V and 0.75 V) for Ni–Cu/CCE and (0.4 V and 0.55 V) for structure. The cyclic voltammetric and chronoamperometric
Ni–Co/CCE electrocatalysts. It was found that the currents studies reveal that the Ni/CCE, Ni–Cu/CCE and Ni–Co/CCE have
observed from chronoamperograms were in good agreement high electrocatalytic activities toward the oxidation of glycerol.
with the currents observed from cyclic voltammetry. These The obtained results show that, relative to alone Ni, Ni-based
results indicate that the present electrocatalysts have good alloys nanoparticles modied electrodes exhibit a greatly
long-term stability and storage properties.
enhanced performance in terms of the low onset potential,
higher anodic peak current and low positive peak potential in
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the electrooxidation of glycerol. Therefore the Ni-based alloy 15 A. Serov and C. Kwak, Review of non-platinum anode
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Acknowledgements
The authors gratefully acknowledge the research council of
Azarbaijan Shahid Madani University for nancial support.
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