Page 100 - MODERN ASPECTS OF ELECTROCHEMISTRY

83
Direct Methanol Fuel Cells
methanol residues and in hydroxyl species can be expected to depend on
the electrode potential. However, in the case of CO adsorbed on platinum,
the CO ads coverage is high and rather constant over a large potential range
from 0.1 to 0.7 V vs. RHE. Thus, one may infer that the current density
versus potential relation is dependent mainly on the coverage in adsorbed
OH. As a consequence, the first approach to improving the overall kinetics
is to increase the adsorbed OH coverage at low potentials. This can be
done by increasing the dispersion of platinum (i.e., by decreasing the
particle sizes) or by adding to platinum a second metal more easily
oxidized at lower potentials. These two possibilities are discussed in the
next section.
In Eq. (28), the methanol residue is mainly adsorbed CO. As discussed
earlier, other kinds of adsorbed species can also be present on the electrode
surface. The different species and their degree of coverage on the Pt
surface depend on the electrode potential and also on the nature and the
structure of the electrode surface. For the sake of simplicity, if only two
types of adsorbed species, COads and CHOads, are considered, Eq. (28)
becomes:

j = nF θ OH [k θ exp(α n FE/RT)+ k θ CHO exp(α n FE/RT)]
7
10
7
7
CO
10
10
(29)
, and n i are the corresponding parameters for steps (23) and
where k i , α i
(26), respectively. For this simple model, an optimized electrocatalyst is
one with a θ value as low as possible, and with θ CHO and θ each being
OH
CO
equal to 0.5 at low potentials. Since it is impossible to avoid the formation
of adsorbed CO on pure platinum, it is obvious that at least a second metal
is necessary to fulfill the conditions of optimal coverage.
4. Structure and Composition of the Electrode in Relation to its
Electroactivity
(i) Effect of the Platinum Crystallographic Structure
Since oxidation of methanol is an electrocatalytic reaction with
different adsorption steps, interactions of the adsorbed species with the
metallic surface are important. Using platinum single-crystal electrodes,
it has been proven that the electrooxidation of methanol is a surface-
sensitive reaction. 23,24 The initial activity of the Pt(110) plane is much
higher than that of the other low-index planes, but the poisoning phenome-
non is so rapid that it causes a fast decrease in the current densities. The