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.