Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the reaction mechanisms towards the plethora of products are disputed, complicating the search for novel catalyst materials. In order to conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant potential DFT kinetics and experiments at a wide range of pH values (3 - 13). We find that *CO dimerization is energetically favoured as the rate limiting step towards multi-carbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on an SHE potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step in acidic/neutral conditions to a later one in alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.
Experimental methods, Computational details, Energetics towards C2+ products via *CO dimerization on varying facets, Energetics towards CH4 on varying facets, Simulated *CO coverage at varying potential and pH, Considerations on methane production being limited by a chemical step, Measured current densities of hydrogen evolution reaction (HER), Qualitative scheme for estimating the potential and pH dependence of electrochemical reactions, pH dependence and energetics on the RHE scale, Individual Tafel slopes of measurements in experimental database, pH corrected experimental database for CH4, Tabulated DFT based energetics