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 the 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 the 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 the OER, further electrochemical oxidation of the Pt–Ir thin films occurs that clearly differs from that of 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 the ORR and OER.