A comparative study of the electrooxidation of C1 to C3 aliphatic alcohols on Ni modified graphite electrode

Article abstract of DOI:10.1007/s11426-012-4731-6

Nickel modified graphite electrodes (G/Ni) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol, ethanol, 1-propanol and 2-propanol in alkaline solutions. The methods of cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, the electrochemical response, peak current varied in the order of MeOH > EtOH > 1-PrOH > 2-PrOH. Under the CA regime, a higher catalytic rate constant obtained for methanol oxidation was in agreement with CV measurements. Lower charge transfer resistance was obtained for low carbon alcohols oxidation and significantly higher exchange current density was obtained for methanol oxidation. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-012-4731-6

SCIENCE CHINA
Chemistry
•
ARTICLES •
September 2012 Vol.55 No.9: 1819–1824
doi: 10.1007/s11426-012-4731-6
A comparative study of the electrooxidation of C1 to C3 aliphatic
alcohols on Ni modified graphite electrode
1
1
2*
2
3
JAFARIAN M. , MIRZAPOOR A. , DANAEE I. , SHAHNAZI Sangachin A. A. & GOBAL F.
1
Departement of Chemistry, K. N. Toosi University of Technology, Tehran, Iran
2
Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
3
Departement of Chemistry, Sharif University of Technology, Tehran, Iran
Received April 4, 2011; accepted December 19, 2011; published online August 9, 2012
Nickel modified graphite electrodes (G/Ni) prepared by galvanostatic deposition were examined for their redox process and
electrocatalytic activities towards the oxidation of methanol, ethanol, 1-propanol and 2-propanol in alkaline solutions. The
methods of cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV
studies, the electrochemical response, peak current varied in the order of MeOH > EtOH > 1-PrOH > 2-PrOH. Under the CA
regime, a higher catalytic rate constant obtained for methanol oxidation was in agreement with CV measurements. Lower
charge transfer resistance was obtained for low carbon alcohols oxidation and significantly higher exchange current density
was obtained for methanol oxidation.
alcohols, electrocatalytic, nickel, modified electrode, equivalent circuit
1
Introduction
propriate fuel for DAFCs [5] in spite of its toxicity and en-
vironmental problems because of high miscibility with wa-
ter, has several offsetting advantages such as high efficiency,
very low polluting emissions and potential renewability in
addition to fast and convenient refueling [6, 7]. Methanol
can undergo electrooxidation to formaldehyde, formic acid
Because of energy crisis and environmental affinity, fuel
cells (FCs) are widely investigated and considered as one of
the most important power recourses. Among several alco-
hols such as methanol, ethanol and propanol [1], which can
be used in direct alcohol fuel cells (DAFCs), ethanol is the
most promising one due to considerable energy density (≈8
kWh/kg), nontoxicity, biological renewability, easy han-
dling and storage [2, 3]. It has been reported that ethanol is
and CO
2
[5].
The electrooxidation of 2-propanol is known to start at
much more negative potentials than that of methanol, while
there has been relatively little research regarding the
2
-propanol electrooxidation [8, 9]. 1-propanol and 2-pro-
panol have different oxidation mechanisms; 1-propanol
produces propanal, propionic acid, and CO
, whereas
-propanol produces acetone and CO . 2-propanol does not
oxidized to acetaldehyde and CO
nism in which the oxidation to acetaldehyde and CO
2
by a dual-path mecha-
pro-
2
2
ceeds through the dehydrogenation of adsorbed ethanol and
adsorbed intermediates, respectively [4]. However, its oxi-
dation is much more complex than methanol in view of the
need to break the C-C bond to achieve total oxidation.
Methanol, which is generally considered as the most ap-
2
2
produce CO as a reaction intermediate because of the diffi-
culties in carbon chain breaking due to the central position-
ing of the OH bond in 2-propanol. The absence of CO ad-
sorption during the 2-propanol oxidation leads to a lower
oxidation onset potential than that in methanol, ethanol, and
1
-propanol [8]. As such, 2-propanol is a prospective fuel for
*
Corresponding author (email: danaee@put.ac.ir)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

1820
Jafarian M, et al. Sci China Chem September (2012) Vol.55 No.9
DAFCs, as has been suggested in other studies [10, 11].
However, its reaction mechanism has not been fully de-
scribed [11, 12, 13].
From a kinetic point of view, the anodic reaction is slug-
gish at moderate operating temperatures of the PEM type
fuel cells because complex reaction steps are involved dur-
ing the complete oxidation reaction; for instance, multi-
electron transfers proceed together with consecutive adsorp-
tion/surface reactions/desorption of several reaction prod-
ucts and byproducts. In addition, some poisonous species
three-electrode cell powered by an electrochemical system
comprising of EG&G model 273 potentiostat/galvanostat
and Solartron model 1255 frequency response analyzer. The
system is run by a PC through M270 and M398 commercial
software via a GPIB interface. A dual Ag/AgCl-saturated
KCl, a Pt wire and a graphite (G) disk electrode were used
as the reference, counter and working electrodes, respec-
tively. All studies were carried out at 298 K. The G disk
electrode was polished with 0.05 mm alumina powder on a
polishing microcloth and rinsed thoroughly with doubly
distilled water prior to modification.
(
e.g., CO), which are produced during the electrooxidation
of alcohols, are irreversibly adsorbed on the active surface
sites of the Pt-based catalysts, thereby impeding the elec-
trocatalysis. Even the complete oxidation of the simplest of
alcohols (MeOH) involves six electrons pointing to the
multi-steps nature of the process with several products or
reaction intermediates.
Electrocatalytic oxidation of alcohols over various cata-
lysts, which are mainly based on platinum, is the subject of
a great number of studies [14]. Pt is one of the best anodic
materials that exhibit catalytic properties for the oxidation
process of alcohols to proceed at a sufficient rate in fuel
cells [15], but the high cost of Pt makes it not economic for
industrial applications. Consequently some research activi-
ties have been conducted to find new and less expensive
materials as anodes for DAFCs [16].
Films of nickel were formed on the graphite surface by
galvanostatic deposition from a solution composed of 0.22
M NiSO
4
.6H
2 6 5 3 7 2
O, 0.4 M C H Na O .2H O and 0.22 M
(NH SO for Ni deposition at the current density of 7
4
)
2
4
2
mA cm for 300 s. The working electrode was placed in
the middle of the electrolyte, which was stirred with mag-
netic stirrer during electrodeposition. Fitting of experi-
mental impedance spectroscopy data to the proposed equiv-
alent circuit was done by means of home written least
squares software based on the Marquardt method for the
optimization of functions and Macdonald weighting for the
real and imaginary parts of the impedance [30].
3
Results and discussion
Carbon is a common choice for supporting nanosized
electrocatalyst particles in DAFCs because of its large sur-
face area, high electrical conductivity, and pore structures
Figure 1 presents consecutive cyclic voltammograms (CV)
of a G/Ni electrode after 50 cycles in 1 M NaOH solution
1
[
1720]. However, this inert material (carbon) does not help
recorded at a potential sweep rate of 100 mV s . In the first
sweep, a pair of redox peaks appeared at 445 and 350 mV
electrocatalytic activities, but serves only as a mechanical
support. A few studies have been reported in this regard,
mostly using oxide materials as active or promoting sup-
ports [21, 22].
It is well established that nickel can be used as a catalyst
due to its surface oxidation properties [2326]. One of the
very important uses of nickel as a catalyst is for the oxida-
tion of alcohols. Ni produces very high anodic current but
does not have suitable oxidation potential. Several studies
of the electro-oxidation of alcohols on Ni and glassy car-
bon/Ni-oxides have been reported [2729].
2+
3+
vs. Ag/AgCl, which are assigned to Ni /Ni redox couple
in alkaline media. In the subsequent cycles, the peaks shift
cathodically and stabilize at 420 and 305 mV vs. Ag/AgCl,
respectively. The entire behavior is in accord with the data
reported previously in the literature concerning the for-
2
mation and interconversion of α and ß-phases of Ni(OH) ,
3
+
its conversion to NiOOH and the enrichment of Ni species
on or just beneath the surface [3135]. In the subsequent
cycles, both the anodic and cathodic peaks shift negatively
and stabilize pointing to higher energies (potential) required
for nucleation of NiOOH in the first cycle. The enhanced
base line current of the first cycle is associated with the ox-
The purpose of the present work is to compare the elec-
trochemical oxidation of methanol, ethanol, 1-propanol and
2+
2
-propanol on a nickel modified graphite electrode in a so-
idation of Ni to Ni . The current grows with the number of
potential scans indicating the progressive enrichment of the
lution of 1 M NaOH in order to investigate the potential
usefulness of higher alcohols as fuels in DAFCs.
2+
3+
accessible electroactive species Ni and Ni on or near the
surface.
Figure 2 shows cyclic voltammograms of G/Ni electrode
in 1 M NaOH solution in the presence of 0.1 M methanol,
ethanol, 1-propanol and 2-propanol at a potential sweep rate
of 10 mV s . The activity order of alcohol oxidation on
G/Ni electrode is methanol > ethanol > 1-propanol >
2-propanol. It seems that longer carbon branch suppresses
the adsorption of alcohols to the surface [36]. At G/Ni elec-
trode, oxidation of alcohols appeared as a typical electro-
2
Materials and methods
1
Sodium hydroxide, nickel sulfate, sodium citrate, ammo-
nium sulfate and alcohols used in this work were Merck
product of analytical grade and were used without further
purification. Doubly distilled water was used throughout.
Electrochemical studies were carried out in a conventional

Jafarian M, et al. Sci China Chem September (2012) Vol.55 No.9
1821
tends to decrease the overall rate of alcohols 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 corresponding current goes
through the maximum due to the regeneration of active sites
for adsorption of alcohols as a result of the removal of ad-
sorbed intermediates and products. The rate of alcohols ox-
idation as signified by the anodic current in the cathodic
half cycle drops as the unfavorable cathodic potentials are
approached.
According to the high current density in the presence of
alcohols and also observation of a new oxidation peak for
alcohols oxidation at a potential much more positive than
that of the oxidation of Ni(OH)
2
potential, we assume that
part of the anodic current is due to alcohols oxidation by
NiOOH due to the disappearance of the NiOOH reduction
peak in the negative sweep, and part of the current is due to
alcohols oxidation on the surface of the oxide layer by di-
rect electro-oxidation [37, 38]. The redox transition of nick-
el species present in the film is
Figure 1 Consecutive cyclic voltammogram of G/Ni oxidation in 1 M
NaOH composed of (1) first and (2) fiftieth cycle at a scan rate of 100 mV

1
s . The potential is against the Ag/AgCl reference electrode.
2+
3+



Ni
Ni +e
(1)


and alcohol is oxidized on the modified surface via the fol-
lowing reaction
3
+
2+
Ni + alcohol   Ni + intermediate
(2)
(3)
3+
2+
Ni + intermediate   Ni + products
3
+
3+
Ni -alcohol   Ni -intermediate + e
(4)
(5)
3+
3+
Ni -intermediate   Ni -products + e
Observation of a high current density in the presence of
Figure 2 (a) Cyclic voltammograms in the absence (1) and presence of
alcohol in comparison to the Ni(OH) is according to Eqs.
2
0
.1 M of (2) 2-propanol, (3) 1-propanol, (4) ethanol and (5) methanol on
(
4) and (5).
the G/Ni electrode in 1M NaOH solution. The potential sweep rate was 10

1
Setting the working electrode potentials to desired values,
mV s ; (b) initial potential of alcohols oxidation. The potential is against
the Ag/AgCl reference electrode.
we measured the catalytic rate constant of alcohol under
chronoamperometric regime. Figure 3 shows double steps
chronoamperograms for the G/Ni in the absence (1) and
presence (26) of alcohols at the concentration of 0.1 M
with an applied potential step of 650 and 300 mV, respec-
tively. The current is negligible when the potential is
stepped down to 300 mV, indicating the irreversibility of
alcohols oxidation process. Chronoamperometry can also be
used for evaluation of the catalytic rate constant according
to [39]
catalytic response. The anodic current increased with re-
spect to that observed for the modified surface in the ab-
sence of alcohols and it was followed by decreasing the
cathodic current.
The decreased cathodic current that ensured the oxidation
process in the reverse cycle indicates that the rate determin-
ing step certainly involves alcohols and that it is incapable
of reducing the entire high valent nickel species formed in
the oxidation cycle. The electrocatalytic oxidation of alco-
hols not only occurs in the anodic but also continues in the
initial stage of the cathodic half cycle. Alcohol molecules
adsorbed on the surface are oxidized at higher potentials
Icat

exp( )
1/2
1/2
1/2



erf ( ) 
(6)


1/2

IL


L
where Icat and I are the currents of the G/Ni in the presence
2
+
3+
parallel to the oxidation of Ni to Ni species. The latter
process has the consequence of decreasing the number of
sites for alcohols adsorption, which, along with the poison-
ing effect of the products or intermediates of the reaction,
and absence of alcohols and γ = kC*t is the argument of the
error function. k is the catalytic rate constant, C* is bulk
concentration of alcohols and t is elapsed time (s). In the
1/2
cases where γ > 1.5, erf(γ ) is almost equal to unity and the

1822
Jafarian M, et al. Sci China Chem September (2012) Vol.55 No.9
above equation simplifies to
Figure 5(b). Two distinguishable peaks are observed in the
Bode plots corresponding to two depressed semicircles in
the Nyquist plot.
Icat
1/2
1/2
1/2
1/2



  (kC *t)
(7)
IL
The equivalent circuit compatible with the Nyquist dia-
gram recorded in the presence of alcohols is depicted in
Figure 6. To obtain a satisfactory impedance simulation of
alcohols electro-oxidation, it is necessary to replace the ca-
pacitor 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
1
/2
From the slope of the Icat/I
, the mean value of k for methanol, ethanol, 1-propanol
and 2-propanol at the concentration of 0.1 M was obtained
L
vs. t plot, presented in Figure
4
3
3
1 1
as 3.1, 2.6, 1.4, and 0.93×10 cm mol s , respectively.
Figure 5(a) shows the Nyquist diagrams of G/Ni elec-
trode recorded at the oxidation peak potential as dc-offset
for 0.1 M selected low carbon alcohols. The Nyquist dia-
grams consisted of two slightly depressed overlapping ca-
pacitive semicircles in the high and low frequency sides of
the spectrum for all alcohols. The depressed semicircle in
the high frequency region might be related to the combina-
tion of charge transfer resistance and the double layer ca-
pacitance. The low frequency semicircle was attributed to
the adsorption of the reaction intermediate on the electrode
surface. Bode phase plots for the same sample are shown in
Figure 3 Double steps chronoamperograms of G/Ni electrode in 1 M
NaOH solution in the absence (1) and presence of 0.1 M of (2) methanol,
(3) ethanol, (4) 1-propanol and (5) 2-propanol. Potential steps were 650
and 300 mV, respectively.
Figure 5 (a) Nyquist diagrams; (b) phase shift plot of the G/Ni electrode
for (1) 2-propanol, (2) 1-propanol, (3) ethanol and (4) methanol in 1 M
NaOH. DC potential is 650 mV vs. Ag/AgCl.
1
/2
L
Figure 4 Dependence of Icat/I on t derived from the data of chrono-
Figure 6 Equivalent circuits compatible with the Nyquist diagrams in
Figure 5 for low carbon alcohols electrooxidation on the G/Ni electrode.
amperograms of (1) methanol, (2) ethanol, (3) 1-propanol and (4)
-propanol from Figure 3.
2

Jafarian M, et al. Sci China Chem September (2012) Vol.55 No.9
1823
Table 1 Equivalent circuit parameters of electrooxidation of alcohols on the G/Ni electrode in NaOH solution obtained from Figure 5
3
3
3
R
s
R
ct
Q
dl×10
Rads×10
Q
ads×10
Alcohol
n
1
n
2
(
Ω)
(Ω)
251
420
615
730
(F)
(Ω)
(F)
Methanol
Ethanol
15.5
15.8
15.7
15.5
1.05
1.02
1.1
2.45
4.82
7.2
1.08
1.6
0.81
0.85
0.8
0.8
0.75
0.78
0.82
1
-Propanol
-propanol
1.8
2
1.1
8.65
1.95
0.82
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