A Catalyst-Coated Mesoporous Carbon–Membrane Electrode Assembly for In-situ Soft X-ray XPS and NEXAFS Studies of Electrocatalytic Interfaces

In-situ soft X-ray spectroscopy provides direct insight into the electronic structure of electrocatalysts under realistic reaction conditions but remains technically challenging due to the need to combine aqueous electrochemistry with ultra-high-vacuum detection. Here we present a mesoporous carbon–membrane working electrode assembly (WEA) that enables window-free in-situ XPS and NEXAFS measurements during electrochemical reactions. The design integrates a Nafion™ proton-exchange membrane with a mesoporous carbon–ionomer contact layer and a thin IrOₓ catalyst layer, providing continuous electronic and protonic pathways and stable hydration through the membrane. By tuning the chamber water vapour pressure to 8 mbar, the WEA maintains a nanometer-thin water layer sufficient for the oxygen evolution reaction (OER) while preserving photoelectron detection efficiency. A robust peristaltic pump integrated with an alumina-bed water vapour dosing system maintains steady-state hydration at 6–10 mbar with <±0.1 mbar variation, enabling reproducible in-situ spectra over extended periods. In-situ Ir 4f and O 1s XPS reveal oxidation of Ir³⁺/Ir⁴⁺ to Ir⁴⁺/Ir⁵⁺ and dynamic changes in hydroxyl and lattice oxygen species, while O K-edge NEXAFS identify the formation of potential-stabilized μ₂–O and μ₁–O oxygen ligand species at OER. The WEA thus provides a quantitative, window-free platform for probing electrochemical interfaces under near-ambient conditions and establishes a general methodology for in-situ soft X-ray studies of functional electrocatalysts, closely resembling the architecture and operation of industrial membrane-based water electrolysers. This approach establishes a reliable methodology for coupling electrochemistry with the element-specific soft X-ray spectroscopy under realistic reaction conditions.