Oxygen Electroreduction Activity and X-Ray Photoelectron Spectroscopy of Platinum and Early Transition Metal Alloys

The oxygen reduction reaction (ORR) was studied experimentally on sputter-cleaned, polycrystalline electrodes of Pt ₃Y, Pt ₅Y, Pt ₂Y, Pt ₃Sc, Pt ₃Hf, Pt ₃Zr and Pt under conditions relevant for low-temperature fuel cells. The surfaces were characterised insitu by means of electrochemical methods and exsitu under ultrahigh vacuum conditions. The ORR activity was established in an electrochemical cell containing 0.1M HClO ₄ by use of a rotating ring disk electrode assembly. The surface composition was characterised before and after the electrochemical measurements by using angle-resolved X-ray photoelectron spectroscopy. The ORR activity of the electrodes increased in the following order: Pt ₂Y≈Pt ₃Zr≈Pt≈<Pt ₃Hf<Pt ₃Sc≪Pt ₅Y<Pt ₃Y. At 23°C, the most active catalyst, Pt ₃Y, exhibited a six- to ninefold improvement in activity over Pt in the potential range 0.9-0.87V with respect to a reversible hydrogen electrode. Over the same potential range at 60°C, Pt ₃Y exhibited a four- to fivefold improvement in activity over Pt. The angle-resolved X-ray photoelectron spectroscopy analyses show that Pt ₃Y and Pt ₅Y formed a Pt overlayer under ORR conditions. In contrast, the surfaces of Pt ₃Hf and Pt ₃Zr comprised a mixture of Pt and HfO _x or ZrO _x, respectively, which explained their poor performance.