Electrochemical CO Reduction Using Membrane Electrode Assemblies: Progress, Challenges, and Opportunities.

Electrochemical CO reduction (COR) offers a promising route for converting waste CO into valuable short-chain (C-C) hydrocarbon chemicals using renewable electricity. Substantial progress has been made in elucidating COR reaction mechanisms and in designing high-performance electrocatalysts and electrode structures. Building on these developments, recent efforts have increasingly focused on system-level optimization to fully harness the potential of electrocatalysts for achieving new benchmark efficiencies under practical conditions. Among different COR device configurations, zero-gap membrane electrode assembly (MEA) electrolyzers-typically consisting of catalyst-coated gas diffusion electrodes (GDEs) pressed tightly against an ion-exchange membrane-have shown promise for achieving high COR current densities at low cell voltages. However, critical challenges remain in the MEA-based COR systems that must be addressed before large-scale deployment. This review discusses recent advances in MEA-based COR, providing cross-scale analyses that connect microscale reaction kinetics, mesoscale mass transport, and device-level integration. It identifies key performance indicators that capture the complex interplay between catalysts, electrode structures, and the overall reaction system, serving as a foundation for the rational design of components and MEA systems toward efficient and scalable operation. With these insights, this review discusses opportunities and challenges for advancing MEA devices toward sustainable and practical CO-to-chemical conversion.