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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 0.89 V 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 |
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