Enhanced activity and stability of Pt–La and Pt–Ce alloys for oxygen electroreduction: the elucidation of the active surface phase

Abstract

Three different Pt–lanthanide metal alloys (Pt₅La, Pt₅Ce and Pt₃La) have been studied as oxygen reduction reaction (ORR) electrocatalysts. Sputter-cleaned polycrystalline Pt₅La and Pt₅Ce exhibit more than a 3-fold activity enhancement compared to polycrystalline Pt at 0.9 V, while Pt₃La heavily corrodes in 0.1 M HClO₄ electrolyte. Angle Resolved X-ray Photoelectron Spectroscopy (AR-XPS) and Low Energy Ion Scattering (LEIS) have been extensively combined with electrochemical techniques to follow the chemical and structural changes at the surface. The highly reactive lanthanide atoms are not stable in the presence of oxygen and readily oxidize. The surface oxides are completely dissolved in the electrolyte. In Pt₅La and Pt₅Ce the so formed Pt overlayer provides kinetic stability against the further oxidation and dissolution. At the same time, it ensures a very high stability during ORR potential cycling, suggesting that these alloys hold promise as cathode catalysts in Proton Exchange Membrane Fuel Cells (PEMFCs).