Tailoring the Ionomer Type to Optimize Catalyst Microenvironment for Enhanced CO 2 Reduction in Membrane Electrode Assemblies

ABSTRACT Electrocatalytic CO 2 reduction reaction (CO 2 RR) represents an advanced technology for converting CO 2 into highly valuable chemicals. Although significant progress has been achieved in producing multi‐carbon chemicals such as ethylene (C 2 H 4 ), addressing (bi)carbonate salt formation and precipitation in alkaline electrolytes remains a critical challenge for achieving long‐term stability during industrialization. We developed a Cu 2 (OH) 2 CO 3 /Mg 2+ /C pre‐catalyst, which transforms into a catalytically active Cu 0 /Cu 2+ /Mg 2+ composite by electroreduction. Crucially, the application of different ionomers (specifically Sustainion XA‐9) on this composite catalyst effectively alleviates salt precipitation issues, thereby enabling high‐selectivity, durable CO 2 ‐to‐C 2+ conversion. In a membrane electrode assembly, the maximum Faradaic efficiency for C 2+ products reaches 80%, with stable operation at 200 mA cm −2 for 50 h. In situ Raman spectroscopy reveals that only top‐type *CO intermediate exists on the Cu 0 /Cu 2+ /Nafion cathode, whereas both bridge‐type and top‐type of *CO sites coexist on the Cu 0 /Cu 2+ /Mg 2+ /Sustainion XA‐9 cathode. This dual adsorption configuration facilitates the C─C coupling kinetics on the catalyst, inducing a favorable microenvironment for selective C 2+ formation. Therefore, strategic optimization of catalyst architectures and ionomer engineering enables CO 2 RR with improved efficiency and durability, advancing green chemistry and carbon‐neutral technologies.