Electrooxidation of methanol on NiMn alloy modified graphite electrode

Article abstract of DOI:10.1016/j.electacta.2009.11.039

Nickel and nickel-manganese alloy modified graphite electrodes (G/Ni and G/NiMn) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol in alkaline solutions. The methods of cyclic voltammetery (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, in the presence of methanol NiMn alloy modified electrode shows a significantly higher response for methanol oxidation. The peak current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of methanol. The anodic peak currents show linear dependency upon the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol was found to be 4 × 10^-6 cm² s^-1. A mechanism based on the electro-chemical generation of Ni³⁺ active sites and their subsequent consumptions by methanol have been discussed and the corresponding rate law under the control of charge transfer has been developed and kinetic parameters have been derived. The charge transfer resistance accessible both theoretically and through the EIS have been used as criteria for derivation of the rate constant.

Full text of DOI:10.1016/j.electacta.2009.11.039

Electrochimica Acta 55 (2010) 2093–2100
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrooxidation of methanol on NiMn alloy modified graphite electrode
I. Danaeea, M. Jafariana,∗, A. Mirzapoora, F. Gobalb, M.G. Mahjania
a Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran
b Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel and nickel–manganese alloy modified graphite electrodes (G/Ni and G/NiMn) prepared by gal-
vanostatic deposition were examined for their redox process and electrocatalytic activities towards the
oxidation of methanol in alkaline solutions. The methods of cyclic voltammetery (CV), chronoamperom-
etry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, in the presence of methanol
NiMn alloy modified electrode shows a significantly higher response for methanol oxidation. The peak
current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding
cathodic current in presence of methanol. The anodic peak currents show linear dependency upon the
square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the
CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol was
found to be 4 × 10−6 cm2 s−1. A mechanism based on the electro-chemical generation of Ni3+ active sites
and their subsequent consumptions by methanol have been discussed and the corresponding rate law
under the control of charge transfer has been developed and kinetic parameters have been derived. The
charge transfer resistance accessible both theoretically and through the EIS have been used as criteria for
derivation of the rate constant.
Received 1 September 2009
Received in revised form
10 November 2009
Accepted 12 November 2009
Available online 11 December 2009
Keywords:
Methanol
Electrocatalytic
Nickel
Modified electrode
Manganese
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
electrocatalyst is needed. As described previously [10–15], a con-
siderable increase in power density and fuel utilization was
Investigation of electrooxidation processes of alcohols is critical
in attaining a better understanding of the direct alcohol fuel cell
(DAFC). Methanol is one of the interesting future fuels for fuel cell
application. Compared with other cells, the direct methanol fuel
cell (DMFC) has several advantages such as high efficiency, very
low polluting emissions, a potentially renewable fuel source, fast
and convenient refueling, simple operation and ease of fuel storage
and distribution. The low operating temperature of a DMFC (typi-
cally <95 ◦C) allows for easy start up and rapid response to changes
in the load or operating conditions [1–3]. However, compared to
the hydrogen based fuel cells, DMFC still remains to be further
developed. One of the problems still unsolved is the slow kinetics
of methanol electrooxidation [4]. Considerable efforts have been
directed towards the study of methanol electrooxidation at high
pH. The use of alkaline solutions in a fuel cell has many advantages
such as increasing its efficiency [5,6], a wider selection of possible
electrode materials, almost no sensitivity to surface structures [7]
and negligible poisoning effects in alkaline solutions [8,9].
obtained by optimizing different components of fuel cells. Dif-
ferent electrode materials based on Pt and Pt-binary electrodes
were commonly used as a catalyst for the electrochemical oxida-
tion of methanol. As catalysis is a surface effect, the catalyst needs
to have the highest possible surface area. Carbon is a common
choice for supporting electrocatalyst particles in DAFCs because of
its large surface area, high electrical conductivity, and porous struc-
ture [16–19]. Meanwhile carbon is electrocatalytically inactive and
serves only as a mechanical support. Therefore, carbon-supported
electrocatalysts [16], such as Pt–Ru and Pt–Ru–P/carbon nano-
composites [17], Pt/Ni and Pt/Ru/Ni alloy nanoparticles [20] are
generally used as catalyst. Ni has commonly been used as an
electrocatalyst for both anodic and cathodic reactions in organic
synthesis and water electrolysis [21–24]. Ni based electrodes have
lower poisoning effect than Pt based electrode and also are cheap.
One of the very important uses of nickel as a catalyst is for the oxida-
tion of alcohols. Several studies of the electro-oxidation of alcohols
on Ni have been reported [25,26]. Taraszewska and Roslonek [27]
found that Ni(OH)2 modified glassy carbon electrode acts as an
effective catalyst for the oxidation of methanol. Most oxidizable
organic compounds were found to oxidize at the same poten-
tial and this potential coincided exactly with that at which the
surface of the nickel anode becomes oxidized [26–30]. Van Effen
and Evans [31] found that the oxidation of ethanol on Ni in KOH
In the electrochemical oxidation of methanol, the electrode
material is clearly an important factor where a highly efficient
∗
Corresponding author. Tel.: +98 21 22853551; fax: +98 21 22853650.
E-mail address: mjafarian@kntu.ac.ir (M. Jafarian).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.11.039

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I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
Fig. 1. Metallograph of electrode surface after electrodeposition of (a) Ni and (b) NiMn alloy from a solution composed of 0.22 M NiSO4·6H2O + 0.4 M C6H5Na3O7·2H2O + 0.22 M
(NH4)2SO4 for Ni deposition at the current density of 7 mA cm−2 and for 300 s and addition of 0.2 M MnSO4·5H2O for NiMn alloy deposition. (c) WDX results of the chemical
composition of NiMn alloy at the surface. (d) X-ray diffractogram (XRD) of electrodeposited NiMn alloy, in the range 10◦ < 2ꢀ < 100◦.
solution involved the formation of a higher valent nickel oxide,
which acts as a chemical oxidizing agent. This fact was confirmed
by both cyclic voltammetry and impedance spectroscopy [32].
The study of alloy electrodes is motivated primarily from the
anticipation of a synergistic electrocatalytic benefit from the com-
bined properties of the components of alloys. Furthermore, the
use of preanodized alloy electrodes offered the advantages of ease
of preparation and long-term stability in comparison with ther-
mally prepared and electrolytically deposited mixed-oxide film
electrodes. Kuwana and co-workers investigated the alloy elec-
trodes in their studies of carbohydrate reaction at Ni-based alloys
containing high percentages of Cu and Cr [33,34]. One promising
characteristic of such alloy electrodes is the resistance to corrosion
in the alkaline media. However, semiconductive oxide films, e.g.
Cu2O and NiO, if formed might cause a slowly increasing ohmic
drop at the electrode surface as the thickness of these oxide films
increase with time [35,36]. The addition of small amounts of alloy-
ing elements to Cu has been demonstrated to provide increased
electrocatalytic activity in comparison to the pure Cu electrode.
and Solartron model 1255 frequency response analyser. The system
is run by a PC through M270 and M398 commercial software via
a GPIB interface. Fitting of experimental impedance spectroscopy
data to the proposed equivalent circuit was done by means of home
written least square software based on the Marquardt method for
the optimization of functions and Macdonald weighting for the real
and imaginary parts of the impedance [38,39].
A dual Ag–AgCl saturated KCl, a Pt wire and a graphite (G) disk
electrode were used as the reference, counter and working elec-
trodes, respectively. All studies were carried out at 298 2 K. The
G disk electrode was polished with 0.05 mm alumina powder on
a polishing microcloth and rinsed thoroughly with distilled water
prior to modification.
Films of nickel were formed on the graphite surface by
galvanostatic deposition from a solution composed of 0.22 M
NiSO4·6H2O + 0.4 M C6H5Na3O7·2H2O + 0.22 M (NH4)2SO4 where a
current density of 7 mA cm−2 for 300 s was employed. Addition of
MnSO4·5H2O to achieve the concentration of 0.2 M in the previous
solution and applying the same condition was used for NiMn depo-
sition. Working electrode was placed in the middle of the cell and
electrolyte was stirred with magnetic stirrer during electrodepo-
sition. The surface morphology and chemical composition of the
deposit was evaluated by Leica Cambridge S360 SEM equipped
with wavelength dispersive X-ray (WDX). Structural investigation
was conducted by X-ray diffraction (XRD) method using a Philips
(Xpert) diffractometer and Cu K␣ radiation.
For example, the presence of 5% Mn to Cu, denoted as Mn5Cu95
,
shows a much improved electrochemical activity for the oxidation
of glucose in alkaline media in comparison to that of the pure Cu
electrode [37].
The purpose of the present work is to study the electrochem-
ical oxidation of methanol on a nickel–manganese alloy modified
graphite electrode in a solution of 1 M NaOH aiming at the elucida-
tion of the mechanism, derivation of the kinetic parameters of the
process and the usefulness of the electrocatalytic process.
3. Results and discussion
2. Experimental
The micrographs presented in Fig. 1a and b display the morphol-
ogy of the electrodeposited Ni and NiMn alloy obtained by SEM.
Mirror finished surfaces are observed for the electrodeposited Ni
and NiMn alloy although the surfaces are rough under microme-
ter scale. The films are fairly compact with virtually no pores and
cavities. The result of chemical composition analysis obtained by
WDX revealed that the composite contains 91% Ni and 9% Mn. The
compositions are given in weight percents (Fig. 1c).
Sodium hydroxide, nickel sulfate, manganese sulfate, sodium
citrate, ammonium sulfate and methanol used in this work were
Merck product of analytical grade and were used without fur-
ther purification. Electrochemical studies were carried out in a
conventional three electrode cell powered by an electrochemical
system comprising of EG&G model 273 potentiostat/galvanostat

I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
2095
Fig. 2. Consecutive cyclic voltammogram of G/NiMn oxidation in 1 M NaOH com-
prised of first and fiftieth cycle at a scan rate of 100 mV s−1
.
Fig. 3. (a) Typical cyclic voltammograms of a G/NiMn electrode in 1 M NaOH in the
potential sweep rates of 2, 5, 7, 10, 20, 30, 40, 50, 75, 100, 200, 350, 500, 750, 1000,
1250, 1500, 1750, 2000 mV s−1, (b) the dependency of anodic and (c) cathodic peak
currents to the sweep rate at lower values (2–40 mV s−1). (d) The proportionality of
anodic and (e) cathodic peak currents to the square roots of sweep rate at higher
values (50–2000 mV s−1).
Structural analysis of the deposit was performed using XRD. X-
ray diffractogram of the deposit in the range of 15◦ < 2ꢀ < 100◦ is
shown in Fig. 1d. Peaks at 2ꢀ = 26.5◦ and 77.8◦ are due to the car-
bon substrate. Peaks centered at 44.5◦, 51.8◦, 76.4◦, 93◦ and 98.4◦
are related to nickel and peaks at 2ꢀ = 25.5◦ and 42◦ are due to
the electrodeposited manganese. Also, it seems that deposit have
face-centered cubic (fcc) structure oriented in (1 1 1) and (2 0 0)
directions. The grain size of the deposit was found to be about
8.5 nm which was estimated from the width at half height of the
most intense diffraction line by Scherrer’s equation [40].
6.2 × 10−8 mol cm−2 have been derived that correspond to the pres-
ence of around 65 monolayers of surface species for the NiMn
electrode. In the higher potential sweep rates this dependency is
of square root form (Fig. 3d and e) signifying the dominance of the
diffusion controlled processes.
Fig.
2 presents consecutive cyclic voltammograms of a
Fig. 4 shows cyclic voltammograms of G/Ni and G/NiMn elec-
trode in 1 M NaOH solution in the presence of 0.5 M methanol at a
potential sweep rate of 10 mV s−1. As can be seen in 0.5 M methanol
G/NiMn electrode generates higher current density for electro-
oxidation in NaOH solution. The larger response at the G/NiMn
electrode in comparison with G/Ni electrode is proposed to be the
result of pre-adsorption of methanol molecules at Mn sites by inter-
action of non-bonded electron pairs of the O-atom of methanol with
the partially vacant d-orbital of the Mn species within the oxide
film.
Fig. 5a shows cyclic voltammograms of G/NiMn electrode in
1 M NaOH solution in the presence of various concentrations of
methanol (0.1–0.9 M) at a potential sweep rate of 10 mV s−1. At
G/NiMn electrode, oxidation of methanol appeared as a typical elec-
trocatalytic response. The anodic charge increased with respect to
that observed for the modified surface in the absence of methanol
andit was followedbydecreasingthe cathodic charge uponincreas-
ing the concentration of methanol in solution. In the presence of
nickel–manganese electrode in 1 M NaOH solution recorded at a
potential sweep rate of 100 mV s−1. In the first cycle a pair of
redox peaks appearing at 490 and 382 mV/Ag–AgCl is assigned to
Ni2+/Ni3+ redox couple in alkaline media. In the subsequent cycles
both the anodic and cathodic peaks shift cathodically and stabilize
at 450 and 360 mV/Ag–AgCl, pointing to higher energies (potential)
required for nucleation of NiOOH in the first cycle. The entire behav-
ior is in accord with the data reported previously in the literature
concerning the formation and interconversion of ␣ and ␤-phases
of Ni(OH)2, its conversion to NiOOH and the enrichment of Ni3+
species on or just beneath the surface [41–45]. The enhanced base
line current of the first cycle is associated with the oxidation of Ni
to Ni(II). The current growth with the number of potential cycles
indicates the progressive enrichment of the accessible electroactive
species Ni2+ and Ni3+ on or near the surface. The current rise follow-
ing the anodic wave is the result of the oxygen evolution reaction.
The changes of the peaks position are likely due to the changes
[41] in the crystal structures of the nickel hydroxide and the nickel
oxyhydroxide constituents of the electro-chemically formed sur-
face film. It has been reported [42,43] that at the initial stages of
electrooxidation ␣-Ni(OH)2 forms and is further slowly converted
to ␤-Ni(OH)2 form. Both the electrodeposited nickel hydroxide and
nickel oxyhydroxide phases are believed to be non-stoichiometric
[44,45].
Fig. 3a presents typical cyclic voltammograms of a G/NiMn elec-
trode in 1 M NaOH solution at various potential sweep rates of
2–2000 mV s−1. The peak’s currents are proportional to sweep rates
in the range of 2–40 mV s−1 (Fig. 3b and c) pointing to the electro-
chemical activity of the surface redox couple [46]. From the slope
of these lines and using [46]
ꢀ
ꢁ
n2F2
Ip
=
vAꢁ ∗
(1)
4RT
where ꢁ * is the surface coverage of the redox species
(mol cm−2) and v being the potential sweep rate. Taking aver-
age of both cathodic and anodic results, ꢁ * values of around
Fig. 4. Cyclic voltammograms in the absence (1) and the presence of 0.5 M of
methanol on (2) G/Ni and (3) G/NiMn electrode in 1 M NaOH solution. Potential
sweep rate was 10 mV s−1. (b) Initial potential of methanol oxidation.

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I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
Fig. 5. (a) Cyclic voltammograms of the G/NiMn electrode in 1 M NaOH solution in
the presence of (1) 0.1 M; (2) 0.3 M; (3) 0.5 M; (4) 0.7 M; (5) 0.9 M of methanol in the
solution. Potential sweep rate was 10 mV s−1. (b) Dependency of the anodic peak
current on the concentration of methanol in solution.
Fig. 6. Repeated cyclic voltammograms of 0.5 M methanol oxidation on G/NiMn
electrode at 100 mV s−1, cycle number: 1, 5, 10, 50, 100, 150, 200, 250, 300, 350.
potential much more positive than that of the oxidation of Ni(OH)2
potential. Meanwhile, the anodic peak currents that are linearly
proportional to the square root of scan rate (Fig. 7b) suggest that the
overall oxidation of methanol at this electrode is controlled by the
diffusion of methanol from solution to the surface redox sites. The
value of electron transfer coefficient for the reaction which is totally
irreversible-diffusion controlled can be obtained from the follow-
ing equation [47] where ˛ is transfer coefficient, v is the potential
sweep rate and other symbols have their usual meaning:
0.9 M methanol with the potential sweep rate of 10 mV s−1, the
ratio of anodic to cathodic charge in the presence of methanol was
99/1 while in its absence it was 55/45 as derived by integrating the
anodic and cathodic peaks after background correction.
The anodic current in the positive sweep was proportional to
the bulk concentration of methanol (Fig. 5b) and any increase in
the concentration of methanol caused an almost proportional linear
enhancement of the anodic current. So, catalytic electrooxidation
of methanol on G/NiMn seems to be certain.
ꢂ
ꢃ
The decreased cathodic current that ensued the oxidation pro-
cess in the reverse cycle indicates that the rate determining step
certainly involves methanol and that it is incapable of reducing
the entire high valent nickel species formed in the oxidation cycle.
The electrocatalytic oxidation of methanol occurs not only in the
anodic but also continues in the initial stage of the cathodic half
cycle. Methanol molecules adsorbed on the surface are oxidized at
higher potentials parallel to the oxidation of Ni(II) to Ni(III) species.
The later process has the consequence of decreasing the number of
sites for methanol adsorption that along with the poisoning effect
of the products or intermediates of the reaction tends to decrease
the overall rate of methanol oxidation. Thus, the anodic current
passes through a maximum as the potential is anodically swept. In
the reverse half cycle, the oxidation continues and its correspond-
ing current goes through the maximum due to the regeneration of
active sites for adsorption of methanol as a result of the removal of
adsorbed intermediates and products. Surely, the rate of methanol
oxidation as signified by the anodic current in the cathodic half
cycle drops as the unfavorable cathodic potentials are approached.
Fig. 6 shows the cyclic voltammograms of G/NiMn electrode in
1 M NaOH in presence of 0.5 M methanol at a scan rate of 100 mV s−1
for 350 cycles. It is observed that the current density of electro-
oxidation of methanol is almost constant in 350 cycles due to the
stability of electrocatalyst in this cycle number and indicating that
methanol reacted with the surface and no poisoning effect on the
surface was observed.
RT
Ep
=
lnv + constant
(2)
n˛F
Using the dependency of anodic peak potential on the natu-
ral logarithm of the potential sweep rate (Fig. 7c), the value of ˛
was obtained as 0.25. Moreover, a plot of the scan rate-normalized
current (I/v1/2) with respect to scan rate (Fig. 7d) exhibited a typi-
cal shape of an electrochemical–chemical (ECꢀ) catalytic processes
[48].
A number of mechanisms have been proposed for the electro-
oxidation of alcohols on Ni in alkaline solutions. While Fleischmann
et al. [26,49] assumed catalytic/intermediate role for NiOOH, others
[27,28] reported that methanol oxidation takes place after com-
Cyclic voltammograms of G/NiMn in the presence of 0.5 M
methanol at various potential sweep rates and the proportional-
ity of anodic peak currents to the square root of sweep rates in a
range of 2–1500 mV s−1 are illustrated in Fig. 7a and b, respectively.
The cathodic peak was not observed in low scan rates, but appeared
upon increasing the sweep rate. The phenomenon indicates that the
electrooxidation of nickel species to higher valence state is much
faster than the catalytic oxidation of methanol. This reveals that the
oxidation of methanol on Ni may be a slow process. In higher scan
rates a new oxidation peak observed for methanol oxidation at a
Fig. 7. (a) Typical cyclic voltammograms of the G/NiMn in1 M NaOH in the presence
of 0.5 M methanol at various potential sweep rates of 2, 5, 7, 10, 20, 30, 40, 50, 75,
100, 200, 350, 500, 750, 1000, 1250 and 1500 mV s−1. (b) Dependence of anodic peak
current during the forward sweep on the square roots of sweep rate. (c) Dependence
of the peak potential on ln ꢂ for the oxidation of methanol at G/NiMn electrode. (d)
The anodic current function (I/ꢂ1/2) vs. potential sweep rate v.

I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
2097
Fig. 8. (a) Double steps chronoamperograms of G/NiMn electrode in 1 M NaOH solu-
tion with different concentrations of methanol of: (1) 0 M, (2) 0.1 M, (3) 0.3 M, (4)
0.5 M, (5) 0.7 M and (6) 0.9 M. Potential steps were 650 mV and 300 mV, respectively.
(b) Dependency of transient current on t−1/2
.
plete conversion of Ni(OH)2 to NiOOH in the course of an anodic
potential sweep. Vertes and Horanyi [50] criticized Fleischmann
mechanism and raised doubts on the role of NiOOH as electrocat-
alyst. In their experimental observation a new oxidation peak for
the alcohol was obtained at a potential much more positive than
that of the oxidation of Ni(OH)2 potential, whereas the redox peaks
due to Ni(OH)2/NiOOH conversion remained unaffected.
According to the high current density in presence of methanol
and also observation of a new oxidation peak for methanol oxida-
tion at a potential much more positive than that of the oxidation
of Ni(OH)2 we assume that part of the anodic current is due to
methanol oxidation by NiOOH due to the disappearance of the
NiOOH reduction peak in the negative sweep and part of the cur-
rent is due to methanol oxidation on the surface of oxide layer by
direct electro-oxidation.
Fig. 9. (a) Nyquist diagrams and (b) phase shift plot of G/NiMn electrode in different
concentration of methanol in 1 M NaOH: (1) 0.1 M, (2) 0.2 M, (3) 0.3 M. DC potential
is 650 mV/Ag–AgCl.
the background current presents a linear dependency (Fig. 8b). The
dominance of a diffusion controlled process is evident. Using the
slope of this line in Cottrell equation [53]
The redox transition of nickel species present in the film is
I = nFAD1/2Cꢃ−1/2t−1/2
(9)
v
1
Ni(II)←→ Ni(III) + e−
(3)
the diffusion coefficient of methanol has been obtained to be
4 × 10−6 cm2 s−1. The current is also negligible when potential is
stepped down to 300 mV, indicating the irreversibility of methanol
oxidation process.
and methanol is oxidized on the modified surface via the following
reaction
v
2
Ni3+ + methanol−→ Ni2+ + intermediate
(4)
Fig. 9 shows the Nyquist diagrams of G/NiMn electrode recorded
at the oxidation peak potential as dc-offset for some selected
concentrations of methanol (0.1–0.3 M). The Nyquist diagrams con-
sisted of two slightly depressed overlapping capacitive semicircles
in the high and low frequency sides of the spectrum. The depressed
semicircle in the high frequency region can be related to the
combination of charge transfer resistance and the double layer
capacitance. The low frequency semicircle was attributed to the
adsorption of reaction intermediate on the electrode surface. Bode
phase plots for the same sample are shown in Fig. 4b. Two distin-
guishable peaks are observed in the Bode plots corresponding to
two depressed semicircles in the Nyquist plot.
The equivalent circuit compatible with the Nyquist diagram
recorded in the presence of methanol is presented in Fig. 10. To
obtain a satisfactory impedance simulation of methanol electro-
oxidation, it is necessary to replace the capacitor C with a constant
phase element (CPE), Q, in the equivalent circuit. The most
widely accepted explanation for the presence of CPE behavior and
depressed semicircles on solid electrodes is microscopic roughness
causing an inhomogeneous distribution in the solution resistance
as well as in the double-layer capacitance [54]. In this electrical
equivalent circuit, Rs, CPEdl and Rct represent solution resistance, a
constant phase element corresponding to the double layer capac-
itance and the charge transfer resistance. CPEads and Rads are the
electrical elements related to the adsorption of reaction interme-
v
3
Ni3+ + intermediate−→ Ni2+ + products
(5)
where Ni3+ sites are regenerated by the power source and on the
Ni3+ oxide surface by direct electro-oxidation [51]
v
4
Ni3+ − methanol−→ Ni3+ − intermediate + e
(6)
(7)
v
5
Ni3+ − intermediate−→ Ni3+ − products + e
Eqs. (4) and (5) are according to Fleischmann mechanism and in
Eqs. (6) and (7), Ni3+ used as active surface for methanol oxidation.
Observation of a new oxidation peak for methanol oxidation at a
potential much more positive than that of the oxidation of Ni(OH)2
potential is according to Eqs. (6) and (7). According to above equa-
tion the Faradaic current density can be written as [52]
IF = (v1 + v4 + v5)F
(8)
Setting the working electrode potentials to desired values, the
measurement of the catalytic rate constant as well as the diffusion
coefficient of methanol was performed under chronoamperomet-
ric regime. Fig. 8a shows double steps choronoamperograms for the
G/NiMn in the absence (1) and presence (2)–(6) of methanol over
a concentration range of 0.1–0.9 M with applied potential steps
of 650 and 300 mV, respectively. Plotting of the net current with
respect to the inverse of the square roots of time after removing

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I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
methanol oxidation and we can dispense with Eqs. (6) and (7).
In the above sequence of reactions (3)–(5), k1 and k are obvi-
−1
ously potential dependent rate constants and are of the forms:
ꢄ
ꢅ
˛nF(E − Eo)
k1(E) = ko exp
(10)
1
RT
ꢄ
ꢅ
(˛ − 1)nF(E − Eo)
k
−1(E) = ko exp
(11)
−1
RT
where ko’s are the chemical rate constants measured at equilibrium
potential, ˛ being the anodic symmetry factor and other parameters
have their usual meanings. The rate laws for the reactions (3)–(5)
have the forms of:
Fig. 10. Equivalent circuits compatible with the Nyquist diagrams in Fig. 9 for
methanol electrooxidation on G/NiMn electrode.
v1 = k1ꢁꢀII − K−1ꢁꢀIII
v2 = k2ꢁꢀIIIC∗
(12)
(13)
where ꢁ is the total number of adsorption sites per unit area of the
electrode surface, ꢀ’s represent the fractional coverage’s of different
nickel valence states and C* is the bulk concentration of methanol.
With only the (II) and (III) valence states of nickel prevailing:
ꢀII + ꢀIII = 1
(14)
and the rates of changes of their coverage’s as well as that of the
intermediate compounds being:
dꢀII
dt
dꢀIII
dt
= −
= −k1ꢀII + k−1ꢀIII + k2ꢀIII
∗ − k3ꢀIIICi
C
∗ + k3ꢀIIICi
(15)
(16)
dCi
dt
= k2ꢀIII
C
where Ci is the concentration of intermediate.
Assuming that the steady state approximations dominating:
dꢀII
dt
dꢀIII
dt
= −
= 0
(17)
(18)
dCi
dt
= 0
one arrives at the values of the coverage’s:
k
−1 + 2k2C∗
ꢀII
=
(19)
(20)
k1 + k−1 + 2k2C∗
k1
ꢀIII
=
(k1 + k−1 + 2k2C∗)
Fig. 11. (a) Nyquist diagrams and (b) phase shift plot of G/NiMn electrode in differ-
ent concentration of methanol in 1 M NaOH: (1) 0.1 M, (2) 0.3 M, (3) 0.5 M, (4) 0.7 M,
(5) 0.9 M. DC potential is 450 mV/Ag–AgCl.
On the basis of this rate equation the faradic current will be:
2FAk1ꢁk2C∗
k1 + k−1 + 2k2C∗
if =
(21)
diates. In this circuit the charge transfer resistance of the electrode
reaction has a simple physical meaning indicating how fast charge
transfer occurs during methanol electrooxidation at the surface of
the electrode.
where A being the surface area of the electrode and the correspond-
ing charge transfer resistance is:
ꢂ
ꢃ−1
dif
dE
(k1 + k−1 + 2k2C∗)2
To corroborate the equivalent circuit, the experimental data
are fitted to equivalent circuit and the circuit elements are
obtained. Table 1 illustrates the equivalent circuit parameters for
the impedance spectra of methanol oxidation different concentra-
tion of methanol. As can be seen from Table 1, increasing methanol
concentrations decrease the diameters of both semicircles.
Fig. 11 shows the Nyquist diagrams of G/NiMn electrode
recorded at the low overpotential as dc-offset over a concentra-
tion range of 0.1–0.9 M methanol. The Nyquist diagrams consisted
of two slightly depressed overlapping capacitive semicircles due to
charge transfer resistance and adsorption intermediates. Table 2
illustrates the equivalent circuit parameters for the impedance
spectra of methanol oxidation different concentration of methanol.
In low applied overpotential the current density of oxidation
processes is related to Ni2+/Ni3+ redox couple in the presence of
ꢄ
ꢂ
ꢃꢅ
Rct
=
=
(22)
∗
k
F
2˛Fk
C
−1
2
2FAꢁk2C∗k1
+
RT
RT
The methanol concentrations dependency of Rct is presented in
Fig. 12 where an initial sharp drop is terminated to a very slow
change as the concentration of methanol is approaching 0.9 M. The
results are further fitted to Eq. (22) to estimate the values of the
rate constants. From this equation the values of the rate constants
as k1 = 4.1 × 10−2 s−1, k−1 = 0.3 s−1 and k2 = 3.1 × 102 cm3 mol−1 s−1
have been calculated.
The steady state polarization curves of the electro-oxidation of
methanol on G/NiMn electrode at a number of methanol concentra-
tions are presented in Fig. 13a. The rotation rate of the electrode is
maintained at 500 rpm. The oxidation process was found to begin at
nearly 413 mV/Ag–AgCl and to reach a plateau at 600 mV/Ag–AgCl

I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
2099
Table 1
Equivalent circuit parameters of electrooxidation of methanol on G/NiMn electrode in NaOH solution obtained from Fig. 9.
Cmethanol (M)
Rs (ꢄ)
16.1
16
Rct (ꢄ)
153
97
56
Qdl (F × 103)
Rads (ꢄ × 10−2
)
Qads (F × 103)
n1
n2
0.1
0.2
0.3
1.4
1.6
1.6
16.2
9.5
1.5
2.1
2.5
0.8
0.82
0.8
0.79
0.79
0.82
16
7.6
Table 2
Equivalent circuit parameters of electrooxidation of methanol on G/NiMn electrode in NaOH solution obtained from Fig. 11.
Cmethanol (M)
Rs (ꢄ)
16
16.5
16.2
16
Rct (ꢄ)
760
411
295
240
Qdl (F × 103)
Rads (ꢄ × ×10−3
)
Qads (F × 103)
n1
n2
0.8
0.75
0.78
0.82
0.8
0.1
0.3
0.5
0.7
0.9
0.9
0.9
1
3.98
2.75
1.5
1
0.85
0.8
0.8
0.82
0.8
1.4
2
2.7
3.1
1.2
1.3
1.32
1.12
16.1
210
while the oxygen evolution starts at still higher potentials. Accord-
ing to Eq. (21) the plots of the inverse of current against the inverse
of methanol concentration should be linear:
of the inverse of current against inverse of the square roots of
rotation rate is linear indicating that the oxidation process was
controlled by diffusion of methanol.
ꢄ
ꢅ
k1 + k
−1
if−1 = (FAk1ꢁ )−1
+
C∗−1
(23)
4. Conclusions
2FAk1k2ꢁ
Fig. 13b presents the inverse of current versus inverse of con-
centration dependencies where straight lines at various potentials
have been obtained in good agreement with Eq. (23). Fig. 13c shows
the Koutecky–Levich plots [46] obtained in different rotation rate
for methanol oxidation on G/NiMn electrode. As expected, the plot
The nickel oxide film was formed electrochemically on elec-
trodeposited nickel and nickel–manganese alloy in a regime of
cyclic voltammetry on a graphite electrode and tested for electro-
oxidation of methanol in alkaline media. The modified electrodes
showed electrocatalytic activity for the oxidation of methanol at
around 650 mV/Ag–AgCl, while the graphite electrode presents
no activity. More specifically, the response for methanol electro-
oxidation at the NiMn alloy modified electrode is significantly
larger than the response obtained for pure electrodeposited Ni.
The anodic peak currents for methanol oxidation at G/NiMn are
linearly proportional to the square root of scan rate. Double
steps choronoamperograms for the G/NiMn in the presence of
methanol show irreversible process and the dominance of a dif-
fusion controlled process is evident. The diffusion coefficient of
methanol has been obtained to be 4 × 10−6 cm2 s−1. A kinetic
model was developed and using the methods of cyclic voltamme-
try, choronoamperometry and impedance spectroscopy, the kinetic
parameters such as transfer coefficient (˛), the catalytic reaction
rate constants (k), and the diffusion coefficient of methanol in the
bulk of solution were determined.
Fig. 12. Dependence of experimental Rct (᭹) on methanol concentration derived
from the data of Nyquist diagrams in Fig. 11 and theoretical curve (—) on the basis
Eq. (22).
References
[1] K. Scott, W.M. Taama, P. Argyropoulos, J. Power Sources 79 (1999) 43.
[2] K. Kordesch, G. Simander, Fuel Cells and their Application, Weinheim, VCH
Verlagsgesellschaft, 1996, p. 151.
[3] J. Kim, T. Momma, T. Osaka, J. Power Sources 189 (2009) 999.
[4] T.D. Jarvis, E.M. Stuve, in: J. Lipkowski, P. Ross (Eds.), Electrocatalysis, Wiley-
VCH, 1998, p. 109.
[5] M. Jafarian, F. Forouzandeh, I. Danaee, F. Gobal, M.G. Mahjani, J. Solid State
Electrochem. 13 (2009) 1171.
[6] K. Nishimura, K. Machida, M. Enyo, J. Electroanal. Chem. 251 (1988) 117.
[7] E. Morallon, F.J. Cases, J.L. Vazquez, A. Aldaz, Electrochim. Acta 37 (1992) 1883.
[8] I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal, M.G. Mahjani, Int. J. Hydrogen
Energy 34 (2009) 859.
[9] I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal, M.G. Mahjani, Int. J. Hydrogen
Energy 33 (2008) 4367.
[10] X. Ren, P. Zelenay, S. Thomas, J. Davey, S. Gottesfeld, J. Power Sources 86 (2000)
111.
[11] T. Schultz, S. Zhou, K. Sundmacher, Chem. Eng. Technol. 24 (2001) 1223.
[12] L. Carrette, K.A. Friedrich, U. Stimming, Fuel Cells 1 (2001) 5.
[13] A.S. Arico, P. Crety, V. Baglio, E. Modica, V. Antonucci, J. Power Sources 91 (2000)
202.
[14] H. Dohle, J. Divisek, R. Jung, J. Power Sources 86 (2000) 469.
[15] M. Baldauf, W. Preidel, J. Appl. Electrochem. 31 (2001) 781.
[16] E. Antolini, Chem. Phys. 78 (2003) 563.
[17] W.D. King, J.D. Corn, O.J. Murphy, D.L. Boxall, E.A. Kenik, K.C. Kwiatkowski, S.R.
Stock, C.M. Lukehart, J. Phys. Chem. 107 (2003) 5467.
[18] A. OliveiraNeto, R.R. Dias, M.M. Tusi, M. Linardi, E.V. Spinace, J. Power Sources
166 (2007) 87.
Fig. 13. (a) Typical pseudo-steady state polarization curves of G/NiMn electrode
obtained in (1) 0.1 M, (2) 0.3 M, (3) 0.5 M, (4) 0.7 M, (5) 0.9 M methanol in 1 M NaOH.
(b) Plot of the inverse of current versus the inverse of concentration of methanol.
(c) Koutecky–Levich plot for methanol oxidation.

2100
I. Danaee et al. / Electrochimica Acta 55 (2010) 2093–2100
[19] E.V. Spinace, L.A. Farias, M. Linardi, A. Oliveira Neto, Mater. Lett. 60 (2008)
2099.
[20] K.W. Park, J.H. Choi, B.K. Kwon, S.A. Lee, Y.E. Sung, H.Y. Ha, S.A. Hong, H. Kim,
A. Wieckowski, J. Phys. Chem. 106 (2002) 1869.
[37] S. Mho, D.C. Johnson, J. Electroanal. Chem. 500 (2001) 524.
[38] J.R. Macdonald, Solid State Ionics 13 (1984) 147.
[39] M. Jafarian, F. Gobal, I. Danaee, M.G. Mahjani, Electrochim. Acta 52 (2007) 5437.
[40] M. Alper, H. Kockar, M. Safak, M. Celalettin Baykul, J. Alloys Compd. 453 (2008)
15.
[41] A.A. El-Shafei, J. Electroanal. Chem. 471 (1999) 89.
[42] G.W.D. Briggs, P.R. Snodin, Electrochim. Acta 27 (1982) 565.
[43] F. Hahn, B. Beden, M.G. Croissant, C. Lamy, Electrochim. Acta 31 (1986) 335.
[44] J. Desilvestro, D.A. Corrigan, M.J. Weaver, J. Electrochem. Soc. 135 (1988) 885.
[45] R. Barnard, C.F. Randell, J. Appl. Electrochem. 13 (1983) 89.
[46] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2001, p.
591.
[21] T.C. Wen, S.M. Lin, J.M. Tsai, J. Appl. Electrochem. 24 (1994) 233.
[22] C. Fan, D.L. Piron, A. Sleb, P. Paradis, J. Electrochem. Soc. 141 (1994) 382.
[23] I.A. Raj, K.I. Vasu, J. Appl. Electrochem. 20 (1990) 32.
[24] M.A. Casadei, D. Pletcher, Electrochim. Acta 33 (1988) 117.
[25] S. Berchmans, H. Gomathi, G. Prabhakara Rao, J. Electroanal. Chem. 394 (1995)
267.
[26] M. Fleischmann, K. Korinek, D. Pletcher, J. Electroanal. Chem. 31 (1971) 39.
[27] J. Taraszewska, G. Roslonek, J. Electroanal. Chem. 364 (1994) 209.
[28] J.R. Allen, A. Florido, S.D. Young, S. Daunert, L.G. Bachas, Electroanalysis 7 (1995)
710.
[47] J.A. Harrison, Z.A. Khan, J. Electroanal. Chem. 28 (1970) 131.
[48] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706.
[49] M. Fleischmann, K. Korinek, D. Pletcher, J. Chem. Soc., Perkin Trans. 2 (1972)
1396.
[29] A. Bettelheim, B.A. White, S.A. Raybuck, R.W. Murray, Inorg. Chem. 26 (1987)
1009.
[30] K.A. Macor, T.G. Spiro, J. Am. Chem. Soc. 105 (1983) 5601.
[31] R.M. Van Effen, D.H. Evans, J. Electroanal. Chem. 103 (1979) 383.
[32] A.J. Motheo, S.A.S. Machado, F.J.B. Rabelo, J.r. Santos, J. Braz. Chem. Soc. 5 (1994)
161.
[33] J.M. Marioli, P.F. Luo, T. Kuwana, Anal. Chim. Acta 282 (1993) 571.
[34] J.M. Marioli, T. Kuwana, Electroanalysis 5 (1993) 11.
[35] S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part B, Elsevier, New
York, 1980.
[50] G. Vertes, G. Horanyi, J. Electroanal. Chem. 52 (1974) 47.
[51] I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal, M.G. Mahjani, J. Phys. Chem. B
112 (2008) 15933.
[52] I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal, M.G. Mahjani, Electrochim. Acta
53 (2008) 6602.
[53] A.J. Bard, L.R. Faulkner, in: A.J. Bard (Ed.), Electrochemical Methods, Fundamen-
tals and Applications, Wiley, New York, 2001, p. 209 (Chapter 5).
[54] A. Maritan, F. Toigo, Electrochim. Acta 35 (1990) 141.
[36] K.C. Khulbe, R.S. Mann, A. Manoogian, Chem. Rev. 80 (1980) 417.