Journal of New Materials for Electrochemical Systems 5, 83-90 (2002)
c J. New. Mat. Electrochem. Systems
Osx (CO)n Based Methanol Tolerant Electrocatalyst for O2 Electroreduction in Acid
Electrolyte
R. H. Castellanos∗, A. L. Ocampo, and P. J. Sebastian.
CIE-UNAM, Solar Hydrogen Fuel Cell, 62580 Temixco, Morelos, México.
( Received June 12, 2001 ; received in revised form December 5, 2001 )
Abstract: A transition metal cluster electrocatalyst based on Osx (CO)n was synthesized by pyrolysis of Os3 (CO)12 in 1,2-Dichlorobenzene (b.p.
≈ 180◦ C) under inert atmosphere ( N2 ). The oxygen electroreduction reaction (OER) for an Osx (CO)n was studied with a rotating disk electrode
in 0.5 M H2 SO4 electrolyte containing different concentrations of methanol. The diffusion coefficient and solubility of oxygen (DO2 and CO2 ) in
different solutions of 0.5 M H2 SO4 with methanol were evaluated. The electrocatalytic activity of the material did not show appreciable variation
due to the presence of methanol. Cyclic voltamperometry studies did not reveal CH3 OH oxidation, i.e. the electrocatalyst is methanol tolerant.
Linear voltamperometry studies showed high electrocatalytic activity for OER in pure and CH3 OH containing 0.5 M H2 SO4 electrolyte. Analysis
of the linear voltamperometry data showed that the reaction follows first-order kinetics and the value of the Koutecky-Levich slope indicates a
multielectron charge transfer of 4e− during the OER in the presence and absence of methanol. The values of the Tafel slopes obtained from the
mass transfer corrected Tafel plots were in the range of 119-123 mV/decade. This catalyst is attractive for application in the cathode of the direct
methanol fuel cells.
Key words : Methanol tolerant electrocatalyst, Oxygen electroreduction, cathode, DFMC.
1.
INTRODUCTION
Osx (CO)n in the presence and absence of methanol in different
concentrations by measuring with a paste of graphite rotating
disk electrode (RDE) at room temperature.
Direct fuel cell systems using fuels such as methanol have received much attention during these days. Direct methanol fuel
cells (DMFCs) offer an attractive way for conversion of chemical energy into electrical energy, because methanol is an inexpensive, readily available, easily storable and transportable
liquid[1,2]. But, the DMFC performance present different problems, one basic problem is the permeation of methanol
(crossover). This effect causes depolarization of the cathode and
hence serious power losses in the cell [3,4]. For the resolution of
this problem one approach is to use an oxygen electroreduction
selective cathode catalyst, i.e., to suppress methanol electrooxidation on the fuel cell cathode.[5] In the present work the
authors report the synthesis and characterization of the OER
kinetic of the transition metal cluster electrocatalyst based on
2.
EXPERIMENTAL
The Osx (CO)n cluster electrocatalyst was synthesized by chemical precipitation reaction (pyrolysis) of 0.142 mmol tri-osmium
dodecacarbonyl Os3 (CO)12 in oxygen free 1,2-dichlorobenzene
(bp≈180◦ C). The synthesis was carried out in a round bottom
flask where the transition metal carbonyl compound was dissolved in 100 ml of the solvent. The chemical reaction was
performed under refluxing condition for 20 h, afterwards the
product of the reaction was centrifuged and washed with diethyl
ether in order to eliminate solvent residues and dried in vacuum
at room temperature. The rotating disk electrode (RDE) studies
were carried out at 25◦ C in a conventional glass cell with a water jacket, which had three separate compartments for the work-
∗ To whom correspondence should be addressed: Fax: +5273250018
e-mail: [email protected]
83
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R. H. Castellanos et al./ J. New Mat. Electrochem. Systems 5, 83-90 (2002)
ing, counter and reference electrodes. A mercury sulfate electrode (MSE), Hg/Hg2 SO4 , 0.5M H2 SO4 (MSE=0.680 V/ENH)
was used as the reference electrode. The electrode potentials are
referred to the normal hydrogen electrode (NHE). A graphite
rod and platinum mesh were used as the counter- electrodes.
Its design is similar to the one reported by Will [6]. A “salt
bridge” was used to protect against methanol contamination [7].
The electrode for the OER was prepared by mixing the graphite
powder (Aldrich), paraffin oil (Merck) and the electrocatalyst
in the form of powder as reported elsewhere. The cross section area of the disk electrode was 0.19 cm2 . But, the experiments for evaluation of CO2 and DO2 were done with a microelectrode of diameter 500 µm. The H2 SO4 solutions were prepared from sulfuric acid (J. T. Baker 98%) with deionized water
(18.2 MΩ-cm ). The electrolytes used in this study were either 0.5 M sulfuric acid or 0.5 M sulfuric acid + (0.5 M, 1.0 M,
2.0 M) methanol (Aldrich) respectively. Prior to the electrochemical measurements the electrolyte was out-gassed with nitrogen for the electrode activation. Scanning the potential from
0 to 0.880 V/NHE for 45 min did the electrode activation. After
that, the electrolyte was saturated with oxygen and maintained
the oxygen flux over the electrolyte during the current-potential
measurements. A minimum of three measurements were realized for each experiment. All experiments were carried out at
room temperature (25◦ C) and atmospheric pressure.
3.
3.1
RESULTS AND DISCUSSION
Cyclic Voltametry
Figure 1 compares the cyclic voltamperograms (20 mV/s, 25◦ C)
for the Pt-Ru 20:10 (%) (A and C) and Osx (CO)n (B and D)
disk electrodes in the absence and presence of 0.5 M CH3 OH
in N2 saturated 0.5 M H2 SO4 at room temperature. Curves A
and C represents the voltamperograms for Pt-Ru in the absence
and presence of methanol respectively, where as B and D for
Osx (CO)n in a similar way. This plot shows that the methanol
oxidation onset potential is around 0.6 V/NHE for the Pt-Ru catalyst, meanwhile the Osx (CO)n catalyst does not show CH3 OH
oxidation during the anodic sweep. The base voltamperograms
of Osx (CO)n is characterized by two main features that determine the shape of the cyclic voltamperograms. The anodic peak
at 0.81 V/NHE and the cathodic peak at 0.4 V/NHE. These
peaks are associated with changes of oxidation state of this compound (i.e. Os(0)→Os(1)) [8]. These peaks do not change with
the time of cycling ( for more than 1h ), but remains stable. At
potentials below 0.2 V/NHE there are no distinct Hupd region,
which indicate no hydrogen adsorption. This result shows that
Osx (CO)n is insensitive to methanol oxidation.
3.2 Oxygen Electroreduction Reaction
Figure 2a displays the polarization curves for the Osx (CO)n catalyst in oxygen saturated electrolyte in the absence and pres-
Figure 1: Cyclic voltamperograms for a Pt-Ru (20:10 atomic %)
and Osx (CO)n catalyst in N2 -saturated 0.5 M H2 SO4 without
(A and B respectively) and with methanol presence (C and D
respectively) in the electrolyte at different concentrations.
ence of different concentrations of methanol and figure 2b displays the polarization curves for the commercial catalyst Pt-Ru
(20/10) obtained at 1600 rpm and under similar experimental
conditions. For the Osx (CO)n the current decrease with the increase of the methanol concentration, but the order of magnitude is the same. In the case of Pt-Ru, the polarization curves
indicate not only a decrement in current, but also a displacement in the open-circuit potential. This is indicative of the influence of methanol presence on the oxygen reduction reaction.
It may be observed in the figure 2a that the open circuit potential (Eoc ) is shifted only a little towards the cathodic direction with increasing methanol concentration in the electrolyte.
The values are shown in table 1. This cathodic shift is only
0.02 V/NHE in the whole range of concentrations under consideration. This is in contrast with the noticeable cathodic shift
observed (0.15 V/NHE) in the case of Pt-Ru even at 0.5M of
methanol in the solution. There is a small decrease in the cathodic current due to oxygen electroreduction for Osx (CO)n when
there is methanol in the electrolyte. But there is no anodic peak
corresponding to methanol oxidation during the whole range
of potential sweep. This small decrease in current may be attributed to the probable co-lateral chemical reaction between
methanol and sulfuric acid leading to the partial blocking of the
active sites of the catalyst [9].
These curves show three distinct regions; 1) the kinetic region
where the current ik is independent of the rotation velocity ω, 2)
the mixed control region where the electrochemical process is
determined by the kinetic as well as diffusional processes and
3) the mass transfer region where the diffusion current id is a
function of the rotational velocity. But in this last region, the polarization curves do not show flat plateau current and at higher
rotation rates the curve appears more inclined. This behavior
Osx (CO)n Based Methanol Tolerant Electrocatalyst for O2 Electroreduction in Acid Electrolyte . / J. New Mat. Electrochem. Systems 5, 83-90 (2002)
85
ity, DO2 the oxygen diffusion coefficient and CO2 the oxygen
concentration in the electrolyte and ω the rotation rate [12].
The kinetic current ( ik ) and Koutecky-Levich slope ( 1/B =
1/0.62nFν −1/6 DO2 2/3 CO2 ) can be calculated from a plot of
1/i vs 1/ω 1/2 , but for calculating the rates constans of OER,
CO2 and DO2 are required. The diffusion coefficient and oxygen
concentration are a function of methanol concentration in the
electrolyte.
Figure 2: a) Polarization curves for a rotating disk Osx (CO)n
electrode in: A) O2 saturated O.5 M H2 SO4 and B) O2 saturated 0.5 M H2 SO4 with 0.5, 1.0, 2.0 M. CH3 OH. Rotation rates: 100-1600 rpm; sweep rate: 5 mV/s. b) O2 reduction
on a rotating Pt-Ru disk electrode at 1600 rpm and different
methanol concentrations as indicated in the plot. Scan rates is
5 mV/s.
Table 1: Values of open circuit potential (Eoc ) in the absence
and presence of CH3 OH in O2 saturated 0.5 M H2 SO4 at room
temperature.
Methanol Concentration
(Molarity)
0.0
0.5
1.0
2.0
3.3 Measurement of Oxygen Solubility and Diffusivity
The current transients (plots of i vs t) for oxygen electroreduction at different methanol concentration (0.0, 0.5, 1.0 and 2.0 M
CH3 OH) were obtained by chronoamperometric measurements
(figure 3) [13]. Each measurement was recorded after the electrode potential had been changed by steps of 0.2 V/NHE from
0.8 V/NHE, where the oxygen reduction rate is negligible, and
the O2 reduction is assumed to be diffusion controlled. In the
case of Semi-Infinite Spherical Diffusion the transient current is
[14]
Open Circuit Potential
in O2 (V/NHE)
0.820
0.800
0.800
0.800
is exhibited by irreversible electrode processes catalyzed by an
attached catalyst [10].
It has been established that to obtain the total electrocatalytic
activity of a material for OER, the kinetic and diffusion current
should be known. The Koutecky-Levich equation at a given
potential is
1
1
1
=
+
i
ik
id
(1)
where i, ik and id are the experimental, kinetic and diffusion
controlled currents respectively. This equation may be used to
separate ik and id and get the true electrocatalyst activity [11].
The equation 1 can be rewritten as
1
1
1
=
+
2/3
i
nF KCO2
0.62nF ν −1/6 DO2 CO2 ω 1/2
Figure 3: Current transient for the reduction of oxygen from
saturated solution of O2 in O.5 M H2 SO4 with 0.0, 0.5, 1.0,
2.0 M. CH3 OH at 25◦ C on Osx (CO)n microelectrode.
(2)
where k is the reaction rate constant, n is the number of electrons
per mol of O2 , F the Faraday constant, ν the kinematic viscos-
i(t) = nAs F DO2 CO2
1
(πDO2 t)1/2
+
1
r
(3)
where As is the surface area of the electrode and r is the radius
of the electrode. Winlove et al [15] showed that equation (3) is
valid for (4Dt/r)−1/2 >0.5 and that a plot of i vs 1/t1/2 should be
linear for the time domain 1/t1/2 >D1/2 /r. With these assumptions the equations for calculating CO2 and DO2 , on the basis
of the theory for diffusion in a microelectrode, were deduced.
This equations are applicable when a i vs 1/t1/2 plot is linear
(figure 4). The slope and the intercept of this plot are related to
CO2 and DO2 by
86
R. H. Castellanos et al./ J. New Mat. Electrochem. Systems 5, 83-90 (2002)
The little discrepancy in the values of Do2 and Co2 reported by
different authors may be attributed to two reasons; 1) the effect
due to the adsorption of species like HSO4 − and SO4 = on the
electrode surface. These species are formed by the dissociation
of H2 SO4 by the following schemes
H2 SO4 → H2 SO4 → H + + HSO4−
HSO4− → H + + SO4= :
Figure 4: Current vs time−1/2 for O2 electroreduction on
Osx (CO)n microelectrode in a O.5 M H2 SO4 solution mixture
with 0.0, 0.5, 1.0 and 2.0 M. concentration at 25◦ C.
r2 A2
πB 2
(4)
B2
nF Ar3
(5)
DO2 =
CO2 =
pKa = 1.92
[23].
The adsorbed species diminish the active area of the electrode
surface for the oxygen reduction reaction and hence the current, 2) the electrodes used at different occasions may not be
completely plane and polished, which implies that the geometric area is not well defined and hence variation in the Co2 and
Do2 values reported [24].
Figure 5 gives the variation in Co2 (oxygen concentration) and
Do2 (oxygen diffusion coefficient) values as a function of the
methanol concentration in the electrolyte. The figure indicates
that the diffusion coefficient increases with increase in methanol
concentration, while the oxygen concentration decreases with
increase in methanol concentration in the electrolyte.
Where A and B are the intercept and the slope of i vs 1/t1/2
plot, r is the radius of a planar electrode used. The oxygen diffusion coefficient and oxygen solubility in 0.5 M H2 SO4 for n =
4 electrons, were found to be 1.36X10−5 cm2 /s and
1.49X10−6 mol/cm3 for Osx (CO)n . Table 2 shows the CO2
and DO2 valued reported by different authors along with those
found in the present study.
Evidently the values obtained in our study conform well with
those reported in the literature.
Table 2: Report of oxygen transport parameters in 0.5 M
H2 SO4 .
Diffusion
coefficient
(cm2 /s)
1.8 X 10−5
1.9 X 10−5
—
1.7 X 10−5
1.04 X 10−5
1.34 X 10−5
—1.4 X 10−5
1.36 X 10−5
Solubility
(mol/cm3 )
1.13 X 10−6
—
1.2 X 10−6
—
1.22 X 10−6
1.16 X 10−6
1.02 X 10−6
1.1 X 10−6
1.49 X 10−6
Figure 5: a) O2 diffusion coefficient and b) O2 solubility vs
methanol concentration determined using Osx (CO)n microelectrode.
Source
S. Gottesfeld et al [16]
S.K. Zecevick et al [17]
J.C.K. Ho, D.L. Piron [18]
R. Jiang, F.C. Anson [19]
R.N. Itoe et al [20]
R.N. Itoe et al [20]
K. Gubbins, R. Walker [21]
W.F. Linke [22]
This Work
This result is in agreement with the equation 1, where it is evident that DO2 and CO2 are inversely proportional to each other.
The experimentally calculated values of these oxygen transport
parameters show a a marked deviation from their average values
shown in figure 5. This may be attributed to the modification
of the electrode surface due to the adsorption of the chemical
species formed in the electrolyte as a result of the oxidative dehydrogenation of methanol in presence of H2 SO4 and the catalysts [25]. The chemical species formed during the oxidation
of methanol are formaldehyde and formic acid, according to the
following reactive scheme [26],
Osx (CO)n Based Methanol Tolerant Electrocatalyst for O2 Electroreduction in Acid Electrolyte . / J. New Mat. Electrochem. Systems 5, 83-90 (2002)
87
Table 3: Theoretical and Experimental values for electron transfer according to equation 2 for oxygen reduction reaction in the
absence and presence of CH3 OH in O2 saturated 0.5 M H2 SO4 at room temperature.
Methanol Concentration
(M)
0.0
0.5
1.0
2.0
Theoretical values
Koutecky-Levich Slope
for 2 e−
(mA−1 rpm1/2 )
74.32
57.92
53.50
51.00
Theoretical values
Koutecky-Levich Slope
for 4 e−
(mA−1 rpm1/2 )2
37.16
28.96
26.75
25.50
Experimental values of
Koutecky-Levich Slope
(mA−1 rpm1/2 )
32.74
24.30
21.11
20.23
The adsorption of these kinds of species provoke the blocking of
the active sites of the electrocatalyst, which leads to the decrease
in the catalytic current and also affects the rate of electróntransfer reactions [27].
Figure 6 displays the 1/i vs 1/ω 1/2 plot (Koutecky – Levich
plot) for oxygen electroreduction reaction on Osx (CO)n at different methanol concentrations. The experimental curves show
a linear relation between 1/i and 1/ω 1/2 with intercepts different from zero in the same range of potential, which indicates
that the current is not diffusion controlled [28] The KouteckyLevich slope decreases with increase in methanol concentration,
which in turn is associated with the decrease in the CO2 values
for different methanol concentration, as indicated in equation 2.
Figure 7 shows the theoretical Koutecky-Levich curves for two
as well as four electron transfer reactions. The values of the
slopes calculated from the Koutecky-Levich plot agree well with
the theoretical values calculated for four electron transfer using
equation 2. The oxygen transport parameters calculated from
the Koutecky-Levich plot is shown in table 3. From KouteckyLevich plot some authors [29] assumed that it is possible to estimate the order of kinetics reaction for OER. However, this analysis is not sensitive to the reaction order, as shown by Vesovic
et al [30]. The parallel nature of the Koutecky-Levich lines and
the fact that the number of electrons exchanged is not altered not
necessarily means that the reaction mechanism is not altered by
the presence of methanol, which is discussed in the following
sections.
The reaction order with respect to O2 reduction was evaluated
using the cathodic current observed in the mixed control region
of the polarization curves [31]. In this method the O2 concentra-
Figure 6: Koutecky-Levich plots for oxygen electroreduction
(at different potentials) at Osx (CO)n RDE in O2 saturated 0.5 M
H2 SO4 solution containing 0.0, 0.5, 1.0 and 2.0 M methanol at
25 ◦ C.
tion is changed in the vicinity of the electrode surface, whereas
in the bulk of the solutions it remains unaltered. By varying the
rotation rate, the reaction order with respect to dissolved oxygen can be determined from the plot log im vs log (1-im /iL ) of
equation (6)
log im
im
= log ik + m log 1 −
iL
(6)
where ik is the kinetic current, im is the current of the mixed
control region, iL is the limiting current and m is the reaction
order with respect to dissolved oxygen concentration. The values of im for different methanol concentration were taken from
the polarization curves at different potential ranges and the values of iL were calculated using the levich equation
88
R. H. Castellanos et al./ J. New Mat. Electrochem. Systems 5, 83-90 (2002)
Figure 7: Koutecky-Levich plot for O2 electroreduction on an
Osx (CO)n RDE in the absence and presence of 0.5, 1.0, 2.0 M
CH3 OH in the O2 saturated 0.5 M H2 SO4 . The solid lines are
the theoretically calculated values for four and two electrons
electroreduction of oxygen by a diffusion-convection controlled
process.
2/3
iL = 0.62nF ν −1/6 DO2 CO2 ω −1/2
[32].
Figure 8 shows the plot of log im vs log (1-im /iL ), which gives
a strainght line with the slopes ( m ) ranging from 1.1 to 0.94,
implying a first order reaction (FOR) for the potentials indicated
in the plots. In the case of 0.5 M H2 SO4 without methanol the
slopes were mostly between 1.11and 0.94 indicating a FOR for
potentials below 0.580 V/NHE. When there is methanol present
in the electrolyte the potential range is displaced to more cathodic. For 0.5 M methanol presence, the slopes were between
1.06 and 0.99 for potentials below 320 V/NHE and for 1.0 M
and 2.0 M methanol the slopes were 1.10 -1.00 and 1.05 - 1.00
respectively for potentials below 300 V/NHE. In this last case
there is a general problem in using the equation 6, because
the co-lateral product from the oxidative dehydrogenations of
methanol poisoning of the electrode caused continous changes
in the surfaces. In the cases examined with methanol in the
range betwen 0.580 and 0.400 V/NHE a fractional reaction orde
with respect to O2 reduction from 0.5 to 0.8 was obtained. These
values do not correspond to reaction order values because the
adsortion effect modificated the electrode surface.
Even though the results obtained from the calculations based
on the Koutecky-Levich criteria establishes that the number of
electrons transferred during the oxygen reduction reaction and
hence the reaction mechanism is not affected by the presence
of methanol in the electrolyte, there is a variation in the values
obtained for the order of reaction due to the methanol presence.
The methanol presence provokes a displacement of the poten-
Figure 8: Dependence log(i) vs log(1-i/iL ) for oxygen electroreduction on an Osx (CO)n RDE in the absence and presence of
0.5, 1.0, 2.0 M CH3 OH in the O2 saturated 0.5 M H2 SO4 at
specified potentials.
tial range associated with the reaction order. This is because,
the kinetics of the oxygen reduction reaction displaces to more
cathodic potentials due to the methanol presence.
One of the criteria to check these possibities is to calculate the
slope of the tafel plot. The tafel plot can be obtained after the
kinetic current is calculated, i.e. after the measured current is
corrected for difussión effects. The kinetic current can be calculated from the equation [33]
ik =
iL · i
iL − i
(7)
Figure 9 shows the mass transfer corrected Tafel plots for different methanol concentrations and represents the average curve
obtained from six measurements at six different RDE rotation
rates ranging from 100 to 1600 rpm. The Tafel slopes were
calculated from the linear part of the graph, in the range 0.7900.650 V/NHE for 0.5 M sulfuric acid and in the range, 0.760 –
0.580 V/NHE for sulfuric acid with different methanol concentrations. This graph shows that O2 reduction rate is lightly faster
in 0.5M sulfuric acid than sulfuric acid with different methanol
concentrations. This phenomenon in the case of methanol is
caused by the different degrees of adsorption of the collateral
products from the oxidation dehydrogenation of methanol. The
values of the Tafel slopes obtained in the present study is shown
in table 4.
There is not much variation in the tafel slope values even if
methanol is present in different concentrations or not. This indicates that the reaction mechanism for O2 reduction is the same
Osx (CO)n Based Methanol Tolerant Electrocatalyst for O2 Electroreduction in Acid Electrolyte . / J. New Mat. Electrochem. Systems 5, 83-90 (2002)
89
methanol concentration in O.5 M H2 SO4 electrolyte. These values are lower than those corresponding to without CH3 OH in
the electrolyte. A decrease in O2 electroreduction current due
to methanol presence in the electrolyte was observed, but the
kinetics of oxygen electroreduction reaction (OER) were not
modified. The electrode kinetic study showed that OER follows a first order reaction with respect to O2 reduction reaction and that the rate determining step in the velocity of oxygen
electroreduction reaction corresponds to single electron transfer. The catalysts were insensitive to methanol oxidation and
selective for oxygen reduction reaction with a favorable multielectron charge transfer (n=4e− ), indicating that the material is
a potential candidate as cathode in a direct methanol fuel cell.
Figure 9: Mass-transfer corrected Tafel plots for the data obtained from the polarization curves (figure 2) for O2 electroreduction in 0.5 M H2 SO4 : A) without and B) with 0.5, 1.0, 2.0
M methanol present in the electrolyte at 25◦ C.
Table 4: Values of the kinetics parameters for oxygen reduction
reaction in the absence and presence of 0.5 M CH3 OH in O2
saturated 0.5 M H2 SO4 at room temperature.
Methanol
Concentration
(M)
0.0
0.5
1.0
2.0
Tafel
Slope (b)
V/decade
0.119
0.123
0.121
0.122
Exchange current
density
io (mA/cm2 )
5.33X10−6
2.81X10−6
2.37X10−6
2.57X10−6
Charge Transfer
coefficient
(nα)
0.495
0.482
0.483
0.486
5.
ACKNOWLEDGMENTS
The authors acknowledge the financial support received from
CONACYT through the projects 25519 A and I 35945-U
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