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 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 pumping system and closed-loop alumina-bed water vapor dosing maintain steady-state hydration at 6–10 mbar with