A Catalyst-Coated Mesoporous Carbon-Membrane Electrode Assembly for In Situ Soft X‑ray XPS and NEXAFS Studies of...

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 IrO _x catalyst layer, providing continuous electronic and protonic pathways and stable hydration through the membrane. By tuning the chamber water vapor 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 vapor 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 electrolyzers. This approach establishes a reliable methodology for coupling electrochemistry with the element specific soft X-ray spectroscopy under realistic reaction conditions.