Stabilization of Undercoordinated Cu Sites in Strontium Copper Oxides for Enhanced Formation of C2+ Products in Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (ECO2RR) is a promising approach to generate renewable fuels and commodities with the integration of renewable energy. Cu-based catalysts produce an array of products resulting from the transfer of 2e– to 18e– during ECO2RR. Value-added C2+ products are of great interest yet difficult to selectively produce. Oxide-derived Cu (OD-Cu)-type catalysts have shown improved selectivity and activity over metallic Cu catalysts that have not been preoxidized. Undercoordinated Cu sites on OD-Cu-type catalysts are suggested to be the active sites for enhanced C2+ production. However, the stability of undercoordinated Cu sites remains largely unexplored in alkaline ECO2RR conditions. In this work, we prepare strontium copper oxide catalysts of varying Sr–Cu ratios and crystalline phases. We identify a SrCuO2 tetragonal phase catalyst to be highly selective toward C2+ products, exhibiting a 53% C2+ Faradaic efficiency at −0.83 V vs reversible hydrogen electrode (RHE). Ex situ X-ray absorption spectroscopy (XAS) indicates that SrCuO2 catalysts are able to retain or recover oxidized Cu species after exposure to reductive ECO2RR conditions for almost 1 hour, whereas OD-Cu remains in a metallic state. Furthermore, operando XAS of SrCuO2 catalysts under alkaline ECO2RR conditions in a gas diffusion electrode-type flow cell reveals the formation and enhanced stabilization of Cu metallic moieties with a low coordination number of 6.3. This work suggests that tuning copper oxides via incorporation of secondary cations in the crystal lattice can further improve the stability of undercoordinated and higher-valence-state Cu sites for improved ECO2RR performance.