Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 341

rate of the electrochemical reactions occurring at both electrodes. This is typical in
electrocatalysis, where the actions of the electrode potential and the catalytic electrode
material will synergistically increase the reaction rate. The current intensity j is pro-
portional to the rate of reaction v, i.e., j ¼ nFv. For a first-order electrochemical
reaction (the rate of which is proportional to the reactant concentration c) the current
density can be expressed as

j ¼ nFv ¼ nFk(T, E)c ¼ nFk 0 ce DG 0 = RT e anFE = RT ¼ j 0 e anFE = RT

(11 : 11)

This last equation contains the two essential activation terms met in electrocatalysis: an
exponential function of the electrode potential E and an exponential function of the
chemical activation energy DG 0 (defined as the activation energy at the standard equi-
librium potential). By modifying the nature and structure of the electrode material (the
catalyst), one may decrease DG 0 , thus increasing j 0 , as a result of the catalytic
properties of the electrode. This leads to an increase in the reaction rate j.

11.3 EFFECT OF CATALYST STRUCTURE AND COMPOSITION:
METHANOL ELECTRO-OXIDATION

Platinum - ruthenium systems are known to be the most effective bimetallic catalysts
in terms of activity with regard to methanol oxidation and selectivity with regard to
complete oxidation to CO 2 [Kabbabi et al., 1998; Schmidt et al., 1999; Hamnett,
1999; Dinh et al., 2000]. Radioactive labeling experiments provided some evidence
for the effects of Ru on methanol adsorption and oxidation. The presence of Ru
increases the rate of methanol adsorption, the maximum CO coverage of the surface
being reached more rapidly at a Pt-Ru than at a Pt catalyst (Fig. 11.1) at a given adsorp-
tion potential. Moreover, the presence of Ru leads to a higher rate of CO oxidative des-
orption (Fig. 11.2), i.e., CO 2 evolution, than at a pure Pt catalyst. The reason for such
an enhancement of activity with regard to methanol electro-oxidation is generally
related to the higher CO tolerance of Pt-Ru. This is the bifunctional mechanism accord-
ing to which adsorbed CO species are oxidized by OH species generated on Ru surface
atoms at lower anode potentials [Watanabe and Motoo, 1975a, b; Iwasita et al., 2000].
Another explanation for the enhanced CO tolerance is the electronic effect [Waszczuk
et al., 2001a, 2002] by which the presence of Ru brings about a change in the electronic
state of Pt, leading to a change in CO binding. A mechanism combining both effects is
also possible. However, Lu et al. showed on the basis of UHV, electrochemical NMR
and electrochemical studies that the bifunctional mechanism for the enhancement of
CO oxidation at Pt/Ru fuel cell catalysts was four times larger than the ligand effect
[Lu et al., 2002].
A bifunctional catalyst should be able to activate two different reaction steps
(methanol and water adsorption and surface reaction between adsorbed species),
and so active sites with different properties are necessary. As an example, investi-
gations of possibility of enhancing activity with regard to methanol electro-oxidation
with Pt-Ru-based electrodes are of great interest with regard to improving the electrical
efficiency of DMFCs. Several approaches have been considered: the effect of Pt-Ru