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Electrochemical CO2 reduction is of tremendous interest for storing chemical energy from renewable sources while reducing CO2 emissions. While copper is one of the most effective catalysts, it suffers from low selectivity and limited long‐term durability. Here, these limitations are overcome by engineering Nafion coatings on CuO nanoparticle‐based catalysts supported on glassy carbon. By tuning the Nafion thickness and internal structure, it is shown that both the selectivity to multicarbon (C2+) products and long‐term stability can be dramatically enhanced. Optimized catalyst layers reach Faradaic efficiencies for C2+ products of 86% during long‐term testing for 200 h, with no evidence for performance degradation. Indeed, the C2+ Faradaic efficiency increases during testing, which is attributed to favorable in situ electrochemical fragmentation of catalytic nanoparticles. Finally, the optimized Nafion/Cu catalytic coatings are utilized to create scalable membrane electrode assemblies for CO2 electrolysis, yielding significantly enhanced C2H4 selectivity (≈58%) and activity at technologically‐relevant currents of 1–2 A. These results highlight the potential for creating multi‐functional Nafion coatings on CO2 reduction catalysts to favorably tune the reaction environment, while also promoting in situ transformations to active and selective nanoscale structures and morphologies, not just on model surfaces but also in state‐of‐the‐art gas diffusion electrodes. [ABSTRACT FROM AUTHOR]