The ellipsometric characterization of Pt + Cr alloy surfaces in acid solutions

Abstract

Results of a combined electrochemical and ellipsometric investigation of Pt + Cr alloys in 0.05 M H 2SO 4 and in 85% H 3PO 4 electrolytes are presented. Potential multicycling beyond the onset of a sharp increase in the anodic current is shown to result in the electrooxidation of Cr in Cr-rich alloys (>50 at.%) that forms a metal deficient surface layer which can reach a thickness of several tens of nm. The surface layer thus formed in 0.05 M H 2SO 4 has been analyzed optically and shown to fit a model of a composite, metalelectrolyte film, with a residual well connected metal network occupying ca. 30% of the volume. Increase in effective surface area due to the formation of such porous surface layers is one possible reason for the increase in the rate of electrochemical processes brought about by Cr additions to Pt catalysts. Cr-rich alloys were found to be better passivated in the 85% H 3PO 4 electrolyte, in which the anodic end potential required to cause significant Cr electro-oxidation at room temperature is 200 mV higher than in 0.05 M H 2SO 4 Multicycling with a limited anodic end potential can be performed for the same Cr-rich Pt + Cr alloys with no drift in surface optical properties. The ellipsometric measurements demonstrated that even without the exposure of such alloys to excessive anodic potentials, the surface is still covered by a Cr oxide species, which undergoes a redox conversion process, probably involving Cr(III)/(IV). The possible role of such a Cr oxide species in the electrocatalysis of oxygen reduction is discussed.