Unraveling the electrolyte-free interface in membrane CO2 electrolysers

Abstract

Zero-gap membrane electrode assembly electrolysers represent the benchmark architecture for scalable CO₂ electrolysis and beyond. However, their device-level performance, particularly regarding energy efficiency and long-term stability, remains inadequate for practical deployment. Here, we argue that a key constraint of membrane electrolysers lies in the absence of catholytes, which creates a local reaction environment fundamentally distinct from that of aqueous H-type or flow cell systems, thereby reshaping electrocatalytic behaviour at the device level. We highlight the profound impacts of this catholyte-free interface, including altered proton availability, carbonate issues, mass transport limitations, product crossover, and re-oxidation—each representing a forefront challenge for CO₂ electrolysis. By examining these emerging interfacial phenomena, we propose key strategies for advancing membrane CO₂ electrolysers, including membrane-anolyte integration, ionomer engineering, and in situ device diagnostics. Collectively, these insights aim to bridge the interfacial gap between traditional half-cell studies (catalyst–electrolyte interfaces) and modern full-cell devices (catalyst–membrane interfaces).