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Abstract

CO₂ reduction is of significant interest for the production of nonfossil fuels. The reactivity of eight Cu foams with substantially different morphologies was comprehensively investigated by analysis of the product spectrum and in situ electrochemical spectroscopies (X‐ray absorption near edge structure, extended X‐ray absorption fine structure, X‐ray photoelectron spectroscopy, and Raman spectroscopy). The approach provided new insight into the reactivity determinants: The morphology, stable Cu oxide phases, and *CO poisoning of the H₂ formation reaction are not decisive; the electrochemically active surface area influences the reactivity trends; macroscopic diffusion limits the proton supply, resulting in pronounced alkalization at the CuCat surfaces (operando Raman spectroscopy). H₂ and CH₄ formation was suppressed by macroscopic buffer alkalization, whereas CO and C₂H₄ formation still proceeded through a largely pH‐independent mechanism. C₂H₄ was formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.