Catalytic Duality of Platinum Surface Oxides in the Oxygen Reduction and Hydrogen Oxidation Reactions

In polymer electrolyte membrane fuel cells (PEMFCs), the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) take place on the surface of platinum nanoparticles (Pt-NPs) residing on carbon support. Polycrystalline platinum (Pt(poly)) serves as a model polyoriented system due to its randomly oriented grains separated by grain boundaries, and research using Pt(poly) creates important background knowledge that is used to identify and understand electrochemical phenomena occurring in fuel cells. In this study, we report new results on the electrochemical behavior of Pt(poly) in 0.50 M H2SO4 aqueous solution saturated with reactive gases, namely O-2(g) and H-2(g). We analyze the influence of the potential scan rate over a broad range of values (1.00-50.0 mV s(-1)) on the cyclic voltammetry (CV) behavior of Pt(poly). A comparative analysis of the impact of dissolved O-2 and H-2 on the electrochemical behavior of Pt(poly) is performed using CV profiles and capacitance transients. Their analysis reveals the existence of new features that are observed in the potential range corresponding to the Pt surface oxide formation and reduction. The results indicate that the Pt surface oxide reveals catalytic duality because it acts both as an inhibitor and a catalyst in both the ORR and HOR. In the case of the ORR, the anodic-going transients reveal that the process becomes inhibited as the Pt surface oxide develops, while in the cathodic-going transients, the reduction of Pt surface oxide significantly (ca. 65%) increases the reaction rate. In the case of the HOR, the anodic-going transients also reveal that the process becomes inhibited as the Pt surface oxide develops, while in the cathodic-going transients, the reduction of Pt surface oxide increases (ca. 15%) the reaction rate. The catalytic effect can be attributed either to changes in the surface electronic structure that accompanies the surface oxide reduction or to short-lived increase in the electrochemically active surface area.