Electrochemical and surface science investigations of PtCr alloy electrodes

Abstract

Electrodes of supported Pt, modified with Cr, have shown an increase in electrochemical activity for oxygen reduction in phosphoric acid fuel cells over supported Pt only electrodes. To clarify the role of chromium and its chemical nature at the electrode surface, we have characterized a series of Pt x Cr( 1- x ) bulk alloys ( x = 0.9, 0.65, 0.5, 0.2) by electrochemical and ex-situ surface science methods. In this paper we report the surface characterization of native and post-electrochemical electrodes by XPS, cyclic voltammetry in 0.05 M H 2SO 4 and 85% H 3PO 4, and analysis of 0.05 M H 2SO 4 electrolyte following electrochemical treatment. The surface Cr(1 to 2 nm) was oxidized to Cr 3+ oxide for surfaces at open circuit and those exposed to potentials < + 1.3 V vs DHE in 0.05 M H 2SO 4 and < + 1.55 V vs. DHE in 85% H 2PO 4. In 0.05 M H 2SO 4 the Cr component was electrooxidized to solube Cr 6+ species at potentials > +1.3 V with the extent of Cr dissolution dependent on initial alloy stoichiometry. Alloys with Cr content ⪖ 0.5 are capable of producing (dependent on time spent at potentials above +1.3 V in 0.05 M H 2SO 4) very porous Pt-rich surfaces. Loss of Cr was also observed in 85% H 3PO 4 for the alloys with Cr content ≧ 0.5, although at the more positive potential limit of +1.55 V. For the Pt 0.2Cr 0.8, treatment in 85% H 3PO 4 at +1.4 V and above led to the appearance of Pt 4+ and Cr 6+ species, apparently stabilized in a porous phosphate overlayer up to 5 nm thick (dependent on time spent at potentials above this limit). The enhancement reported for supported Pt+Cr oxygen cathodes is discussed in the light of these results.