Oxygen Reduction at Pt0.65Cr0.35, Pt0.2Cr0.8 and Roughened Platinum

Abstract

Oxygen reduction in has been investigated at , and at surfaces produced from them by selective dissolution of the Cr component. Rotating disk electrodes (RDE) were used to examine the oxygen reduction reaction (ORR). The published (1–2) surface analysis using x‐ray photoelectron spectroscopy (XPS) combined with sputter profiling and Rutherford backscattering spectrometry (RBS) convincingly demonstrated that the surface region can be selectively depleted of the Cr component by electrochemical excursions to potentials positive of ∼+1.25V vs. RHE. For the more Cr‐rich alloy very severe depletion (>500Å) occurs upon prolonged potential excursions above +1.25V. For the sample, the surface depletion extends about 2–3 monolayers into the surface. Electrochemical characterization by cyclic voltammetry also confirms that the surface becomes progressively roughened as the potential exceeds the region where chromium is passive. Extensive roughening of the surface results in a significant increase of the measured ORR current per geometric area of the RDE. Tafel slopes of the ORR for moderately rough surfaces (roughness factor, ), produced from either starting alloy, are nearly identical to pure smooth Pt. For rougher surfaces (produced only at the more Cr‐rich starting alloy) the Tafel slopes determined from a mass‐transfer corrected plot of are also nearly identical to those obtained at smooth Pt. It is concluded that the measured increase of ORR current following the roughening of the RDE alloy electrode is due solely to the increase of Pt surface area. The mechanistic and practical implications of this effect in fuel cell electrochemistry are discussed.