Ren, W.Zheng, Y.Qiao, S.-Z.2025-07-242025-07-242025Energy and Environmental Science, 2025; 18(15):7402-74121754-56921754-5706https://hdl.handle.net/2440/146288First published 07 Jul 2025Zero-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).enThis journal is © The Royal Society of Chemistry 2025Journal article10.1039/d5ee02408k744902Ren, W. [0000-0002-8769-3131]Zheng, Y. [0000-0002-2411-8041]Qiao, S.-Z. [0000-0002-1220-1761] [0000-0002-4568-8422]