Understanding the oxygen reduction/evolution reactions (ORR/OER) on bimetallic Pt-Ir electrocatalysts using in-situ Raman spectroscopy and DFT

Abstract

Bimetallic platinum-iridium electrocatalysts for oxygen reduction/evolution reactions (ORR/OER) are of great interest for unitized regenerative fuel cells. The potential-dependent formation of catalytically active Pt-Ir species and their resulting electronic structure to accelerate both ORR and OER are still unknown. Raman spectroscopy is used to monitor in-situ the potential-resolved electronic and structural interactions of Pt and Ir in sputtered Pt-Ir thin films as model catalyst systems for ORR and OER. The low coverage of oxygen-based intermediates on the Pt surface sites for Pt-Ir films correlates with enhanced ORR activity. At potentials before and during OER further electrochemical oxidation of the Pt-Ir thin films occurs that clearly differs from the monometallic films. DFT calculations indicate the formation of hydrous PtO₆-IrO₆ edge-sharing chains with µ-oxo bond linkages to couple Pt and Ir centers electronically. The theoretical stretching and bending motions of these Pt-O-Ir bonds are the most intense vibrations at 494, 542 and 682 cm^-1 and are in very good agreement with the experimental data (~510, ~540 and 657 cm^-1). Very remarkably, theoretical and experimental data uncover a clear shift to lower energy by ~38 cm^-1 for symmetric stretching of Ir-O-Pt (657 cm^-1 ) compared to Ir-O-Ir (695 cm^-1 ) under OER conditions. In other words, this change in bond strength can be correlated with lower OER activity and indicates a weakening of the Ir-O-Pt bond by ≈ 0.48 kJ mol^-1 compared to IrO_x . Overall, our comprehensive in-situ Raman and DFT investigations provide new mechanistic insights into the potential-resolved formation of catalytically active Pt-Ir sites for both ORR and OER.