Membrane configurations in zero-gap CO2 electrolyzers across ion exchange and non-exchange systems.

Zero-gap membrane electrode assembly (MEA) electrolyzers represent a leading technological platform for industrial electrochemical CO2 reduction. Within these systems, the membrane governs ion and water transport, thereby affecting the local reaction microenvironment, species crossover, and long-term operational stability. This review surveys membrane configurations for zero-gap CO2 electrolyzers, covering both ion-exchange and non-ion-exchange systems. We discuss working principles, performance characteristics, key unresolved challenges, and optimization strategies for each membrane configuration, including anion, cation, and bipolar exchange membranes as well as ion-solvating and porous non-ion-exchange systems. We then provide a holistic comparison of all membrane configurations across multiple performance metrics, followed by an analysis of the techno-economic requirements for industrial deployment. We outline research directions for rational membrane design and device integration, emphasizing novel membrane chemistries, interfacial engineering, and system-level optimization, aiming for high performance and practical viability for CO2 electrolyzers.