From Electricity to Fuels: Descriptors for C1 Selectivity in Electrochemical CO2 Reduction

Electrochemical reduction of carbon dioxide (CO₂) over transition metals follows a complex reaction network. Even for products with a single carbon atom (C₁ products), two bifurcated pathways exist: initially between carboxyl (COOH*) and formate (HCOO*) intermediates and the COOH* intermediate is further bifurcated by pathways involving either formyl (CHO*) or COH*. In this study, we combine evidence from the experimental literature with a theoretical analysis of energetics to rationalize that not all steps in the reduction of CO₂ are electrochemical. This insight enables us to create a selectivity map for two-electron products (carbon monoxide (CO) and formate) on elemental metal surfaces using only the CO and OH binding energies as descriptors. In the further reduction of CO^*, we find that CHO* is formed through a chemical step only whereas COH* follows from an electrochemical step. Notably on Cu(100), the COH pathway becomes dominant at an applied potential lower than −0.5V vs. RHE. For the elemental metals selective towards CO formation, the variation of the CO binding energy is sufficient to further subdivide the map into domains that predominantly form H₂, CO, and ultimately more reduced products. We find Cu to be the only elemental metal capable of reducing CO₂ to products beyond 2e⁻ via the proposed COH pathway and we identify atomic carbon as the key component leading to the production of methane. Our analysis also rationalizes experimentally observed differences in products between thermal and electrochemical reduction of CO₂ on Cu.