Using pH Dependence to Understand Mechanisms in Electrochemical CO Reduction

Electrochemical conversion of CO(2)into hydro-carbons and oxygenates is envisioned as a promising path towardclosing the carbon cycle in modern technology. To date, however,the reaction mechanisms toward the plethora of products aredisputed, complicating the search for alternative catalyst materials.To conclusively identify the rate-limiting steps in CO reduction onCu, we analyzed the mechanisms on the basis of constant-potentialdensity functional theory (DFT) kinetics and experiments at a widerange of pH values (3-13). Wefind that*CO dimerization isenergetically favored as the rate-limiting step toward multicarbonproducts. Thisfinding is consistent with our experiments, wherethe reaction rate is nearly unchanged on a standard hydrogenelectrode (SHE) potential scale, even under acidic conditions. Formethane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from thefirst protonation stepunder acidic/neutral conditions to a later one under alkaline conditions. We also show, through a detailed analysis of themicrokinetics, 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.

Automated summary

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is a promising approach to closing the carbon cycle in modern technology. However, the reaction mechanisms for different products are disputed, making it difficult to find suitable catalyst materials. In this study, the rate-limiting steps in CO reduction on Cu were conclusively identified through experiments and theoretical analysis. The findings provide insights for future mechanistic studies and catalyst design.