In-situ soft X-ray spectroscopy of OER metal oxide electrocatalysts for water splitting

Water electrolysis requires a detailed understanding of the structural and electronic transformations of oxygen evolution reaction (OER) catalysts under operating conditions. X ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) are powerful tools to probe composition, oxidation state, and electronic structure, and near-ambient pressure instrumentation enables in-situ measurements under realistic electrochemical conditions. Here, we report the development of two specialized working electrode assemblies (WEAs) designed to maintain ionic conductivity and ensure photoelectron detection: (1) a three-layered carbon-based membrane comprising an ion exchange membrane, a mixed ionomer/mesoporous carbon scaffold, and a deposited electrocatalyst, and (2) a gold-coated polycarbonate track-etched (PCTE) membrane with Nafion-filled pores and electrocatalyst deposition. These assemblies enabled systematic operando XPS and NEXAFS studies of iridium oxide catalysts of varying crystallinity. Amorphous and semi-crystalline oxides exhibited mixed Ir4+/Ir5+ states and significant formation of coordinatively unsaturated oxyl species under OER conditions. Valence band measurements revealed previously unreported changes in the Ir 5d electronic structure. Aging studies demonstrated increased Ir3+ content and modifications to the Ir 4f satellite structure, highlighting structural degradation in aged catalysts. The Au/PCTE WEA further allowed the first in-situ XPS and NEXAFS investigation of mixed Ru–Ir oxide catalysts, revealing potential-dependent ruthenium surface segregation and Ir4+/Ir5+ redox transitions. A novel potential-programmed NEXAFS (PP-NEXAFS) approach resolved μ1-, μ2-, and μ3-O species, identifying distinct onset potentials, hysteresis, and an oxygen redox process that forms electrophilic oxygen without further oxidation of Ir5+. Collectively, these findings refine the mechanistic understanding of OER on state-of-the-art catalysts, highlighting the coupled roles of metal-centered and oxygen-ligand redox processes. The work establishes advanced operando methodologies and provides design principles for efficient, stable, and cost-effective OER electrocatalysts for sustainable hydrogen production. In-situ soft X-ray spectroscopy of OER metal oxide electrocatalysts for water splitting