Beyond the surface: Investigating CO2 electroreduction pathways on copper foil

Cation effects in the electrochemical CO2 reduction reaction (CO2RR) are often attributed to modifications of the electric double layer, yet their role in driving catalyst surface restructuring remains insufficiently understood. Here, we investigate CO2RR on polycrystalline Cu foil in non-buffering sulfate electrolytes containing Li+, Na+, K+, or Cs+, enabling a direct comparison of alkali cation identity under otherwise similar conditions. We observe a systematic shift in selectivity between CH4 and C2H4, with larger cations favoring C-C coupling and higher C2H4/CH4 ratios. Ex situ grazing-incidence x-ray diffraction, scanning electron microscopy, and x-ray photoelectron spectroscopy reveal pronounced cation-dependent reconstruction of the Cu surface, including changes in near-surface crystallographic texture, morphology/roughness, and surface chemical signatures. An electrolyte-exchange experiment further shows that key features of the reconstructed state can be partially retained and continue to influence selectivity after transfer between electrolytes, indicating that restructuring contributes to the observed cation trends. Together with density functional theory calculations of key intermediate stabilization on Cu facets, these results highlight that alkali cations act through multiple coupled mechanisms-including surface restructuring and interfacial effects-to govern CO2RR pathways and product selectivity.