Cobalt concentration effect in Pt1−xCox on the reversible potential for forming OHads from H2Oads in acid solution

We present results of a periodic spin-density-functional theory study of the effect of the cobalt concentration in Ptx−1Cox surfaces on the reversible potential for forming OH(ads) from H2O(ads). Adsorbed OH is recognized as a poison to oxygen cathodes that contributes to the high O2 reduction overpotential. Five surface compositions with a cobalt concentration of 0%, 25%, 50%, 75% and 100% were studied, and these surfaces were covered with a monolayer Pt skin. The OH(ads) formation potential was found to depend significantly on the sub-surface composition. Alloying platinum with cobalt atoms led to an increasingly positive shift of the reversible potential for forming OH(ads), as the amount of cobalt is increased. Beyond 75% cobalt concentration the effect levels off. Experimentally, the Pt3Co alloy has been well studied and is believed to be covered by a Pt skin and the potential for OH(ads) formation on it, from water oxidation, is shifted positive. The present theoretical results suggest that if Pt skins form on alloys more concentrated in Co than Pt3Co, the positive shift in the potential for OH(ads) formation will be greater and, on an oxygen cathode in a fuel cell, the overpotential for O2 reduction will be even less.