Highly efficient and selective electrochemical reduction of carbon dioxide represents one of the biggest scientific challenges in artificial photosynthesis, where carbon dioxide and water are converted into chemical fuels from solar energy. However, our fundamental understanding of the reaction is still limited and we do not have the capability to design an outstanding catalyst with great activity and selectivity a priori. Here we assemble uniform gold–copper bimetallic nanoparticles with different compositions into ordered monolayers, which serve as a well-defined platform to understand their fundamental catalytic activity in carbon dioxide reduction. We find that two important factors related to intermediate binding, the electronic effect and the geometric effect, dictate the activity of gold–copper bimetallic nanoparticles. These nanoparticle monolayers also show great mass activities, outperforming conventional carbon dioxide reduction catalysts. The insights gained through this study may serve as a foundation for designing better carbon dioxide electrochemical reduction catalysts.

Solid–liquid interface engineering has recently emerged as a promising technique to optimize the activity and product selectivity of the electrochemical reduction of CO₂. In particular, the cation identity and the interfacial electric field have been shown to have a particularly significant impact on the activity of desired products. Using a combination of theoretical and experimental investigations, we show the cation size and its resultant impact on the interfacial electric field to be the critical factor behind the ion specificity of electrochemical CO₂ reduction. We present a multi-scale modeling approach that combines size-modified Poisson–Boltzmann theory with ab initio simulations of field effects on critical reaction intermediates. The model shows an unprecedented quantitative agreement with experimental trends in cation effects on CO production on Ag, C₂ production on Cu, CO vibrational signatures on Pt and Cu as well as Au(111) single crystal experimental double layer capacitances. The insights obtained represent quantitative evidence for the impact of cations on the interfacial electric field. Finally, we present design principles to increase the activity and selectivity of any field-sensitive electrochemical process based on the surface charging properties: the potential of zero charge, the ion size, and the double layer capacitance.