On the nature of platinum oxides on carbon-supported catalysts

Abstract

In fuel cells, the formation of platinum oxides on the Pt surface interferes with the Oxygen Reduction Reaction (ORR). The chemical composition of Pt oxides formed on carbon-supported Pt nanoparticles has been postulated for the first time to be hydroxylated platinum PtOH and hydroxylated platinous PtOH+ species. The thermodynamics of electrocapillary was applied to determine the charge number for Pt oxidation in various pH under a constant ionic strength perchlorate solutions. The charge number of the Pt oxide increased with the holding potential from −0.9 up to −1.5 electrons/H+ at 0.765 and 0.89V vs. SHE, respectively. This indicates the coexistence of multi oxides, where PtOH is initially formed and is further converted at higher potentials to PtOH+. The charge number for H adsorption was measured as expected to be +1, increasing the confidence in the technique. The charge number allows the calculation of individual oxide coverages, explaining the previously intriguing reports that the fraction coverage was larger than 1 under the simplified assumption of one-electron transfer per Pt atom and without the need to assume a place-exchange mechanism. It is concluded that under increasing holding potential the coverage of PtOH+ increases while that of PtOH decreases. The formation of both oxides can be periodically reversed by a short switch to lower potentials, E<0.6V vs. SHE. The reduction of the oxides and consequently the recovery of Pt activity are found to be relatively fast.