The electrocatalytic reduction conversion of CO ₂ to produce methane (CH ₄ ) as a fuel has attracted intensive attention for renewable energy.

Achieving large-scale electrochemical CO₂ reduction to multicarbon products with high selectivity using membrane electrode assembly (MEA) electrolyzers in neutral electrolyte is promising for carbon neutrality. However, the unsatisfactory multicarbon products selectivity and unclear reaction mechanisms in an MEA have hindered its further development. Here, we report a strategy that manipulates the interfacial microenvironment of Cu nanoparticles in an MEA to suppress hydrogen evolution reaction and enhance C₂H₄ conversion. In situ multimodal characterizations consistently reveal well-stabilized Cu^δ+-OH species as active sites during MEA testing. The OH radicals generated in situ from water create a locally oxidative microenvironment on the copper surface, stabilizing the Cu^δ+ species and leading to an irreversible and asynchronous change in morphology and valence, yielding high-curvature nanowhiskers. Consequently, we deliver a selective C₂H₄ production with a Faradaic efficiency of 55.6% ± 2.8 at 316 mA cm⁻² in neutral media.