Metal Oxide Clusters on Nitrogen-Doped Carbon are Highly Selective for CO2 Electroreduction to CO

The electrochemical reduction of CO2 (eCO2RR) using renewable energy is an effective approach to pursue carbon neutrality. The eCO2RR to CO is indispensable in promoting C–C coupling through bifunctional catalysis and achieving cascade conversion from CO2 to C2+. This work investigates a series of M/N–C (M = Mn, Fe, Co, Ni, Cu, and Zn) catalysts, for which the metal precursor interacted with the nitrogen-doped carbon support (N–C) at room temperature, resulting in the metal being present as (sub)nanosized metal oxide clusters under ex situ conditions, except for Cu/N–C and Zn/N–C. A volcano trend in their activity toward CO as a function of the group of the transition metal is revealed, with Co/N–C exhibiting the highest activity at −0.5 V versus RHE, while Ni/N–C shows both appreciable activity and selectivity. Operando X-ray absorption spectroscopy shows that the majority of Cu atoms in Cu/N–C form Cu0 clusters during eCO2RR, while Mn/, Fe/, Co/, and Ni/N–C catalysts maintain the metal hydroxide structures, with a minor amount of M0 formed in Fe/, Co/, and Ni/N–C. The superior activity of Fe/, Co/, and Ni/N–C is ascribed to the phase contraction and the HCO3– insertion into the layered structure of metal hydroxides. Our work provides a facile synthetic approach toward highly active and selective electrocatalysts to convert CO2 into CO. Coupled with state-of-the-art NiFe-based anodes in a full-cell device, Ni/N–C exhibits >80% Faradaic efficiency toward CO at 100 mA cm–2.