Discussion

The polarization behavior of the present DMFC appears to be mainly affected by the methanol oxidation proccss. It is known that the chemisorption of methanol on Pt occurs at potential values below 0.2 V vs. NHE. On the other hand, the chemisorption process of methanol on Ru sites is significantly less favored than on Pt sites but it is strongly activated by the temperature [2], Besides, the water displacement reaction producing oxygen species chemisorption on Ru sites occurs at potentials as low as 0.2 V vs. NHE [2]. From these evidences, it is thought that at 95° C a significant Traction of Ru sites can suitably chemisorb OH groups, as the overpotcntial slightly increases. The polarization curves of the present DMFC show a potential drop of about 0.2 V at very low currcnt density. Afterwards, the current increases steeply with potential. This behavior could be related to a mechanism involving a first dehydrogenation step followed by oxidation of cheinisorbcd residues as the anode overpotcntial bccomes sufficiently high for a fast water displacement on Ru sites.

As described above, a lower Ru content (20%) was experimentally determined for the Pt-Ru alloy in the present catalyst with rcspcct to the nominal composition (50%). This suggests that the remaining Ru atoms arc present as RuO* spccics as accounted by XPS analysis. Accordingly, it could be suggested that the formation of RuO* species occurs at the surface of the catalyst. It is observed that formation of "labile" oxygen groups on Ru sites being not directly alloyed with Pt but close enough to Pt sites, can allow the chemical interaction between these groups and the adsorbed mcthanolic residues to produce CO2. It is thought that both mcchanisms involving Pt-Ru alloy and RuOx species could actively participate to the methanol oxidation reaction. RuO^ spccics arc probably less poisoned by adsorbed mcthanolic residues with rcspcct to Ru sites in the Pt-Ru alloy at high temperatures. Yet. it is likely that the abscncc of an atomic mixing of Pt with RuOx spccics imposes an activation barrier for the migration of adsorbed intermediates reducing the activity of the Pt-Ru/C catalyst.

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