Page 91 - MODERN ASPECTS OF ELECTROCHEMISTRY

Claude LamyAet al.
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reformate, a low level of CO is still present in the hydrogen-rich gas. In
order to avoid the complexity of the fuel processing and the consequences
of weight, particularly for electric vehicle applications, the direct oxida-
tion of methanol is a simple and attractive way to directly convert the
chemical energy of the methanol oxidation reaction to electricity. The
direct electrooxidation of methanol is advantageous for a fuel cell appli-
cation only if the reaction is complete and leads to the formation of carbon
dioxide and water at low anode potentials. Platinum is the best electro-
catalyst in an H 2-O 2 PEMFC, but in a DMFC it is subjected to poisoning
by CO; the challenge is thus to develop electrocatalysts able to oxidize
methanol without poisoning its surface at such low potentials.
The overall reaction for methanol electrooxidation is expressed by
o
Eq. (1) and its standard reversible potential by Eq. (6). The value of E r =
1.21 V for the methanol oxidation reaction is very close to that for a
hydrogen-oxygen fuel cell (e.g., 1.23 V). Under standard equilibrium
o
conditions, an anode potential E a of 0.016 V vs. SHE can be easily
calculated from thermodynamic data. This means that theoretically metha-
nol can be oxidized at very low potentials. Conversely, it is well known
that methanol is only oxidized at potentials greater than 0.5 V, in acid
medium on a platinum electrocatalyst, owing to the slow kinetics of its
electrooxidation reaction, which results in high overpotentials. The chal-
lenge—to increase the methanol oxidation kinetics at an electrode/elec-
trolyte interface—has been most difficult for practically all
electrochemists working in this field, and even at present, this irreversible
loss due to activation overpotential is high for several mechanistic reasons.
Thus, worldwide efforts have focused on the elucidation of the
reaction mechanism. For this purpose, knowledge about the following
items is vital: (1) identification of reaction products and the electrode
kinetics of the reactions involved, (2) identification of adsorbed interme-
diate species and their distribution on the electrode surface, and (3)
dependence of the electrode kinetics of the intermediate steps in the
overall and parasitic reactions on the structure and composition of the
electrocatalyst. It is only after a better knowledge of the reaction mecha-
nisms is obtained that it will be possible to propose modifications of the
composition and/or structure of the electrocatalyst in order to significantly
increase the rate of the reaction.