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

Abstract

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