In situ copper faceting enables efficient CO 2 /CO electrolysis.

The copper (Cu)-catalyzed electrochemical CO 2 reduction provides a route for the synthesis of multicarbon (C 2+ ) products. However, the thermodynamically favorable Cu surface (i.e. Cu(111)) energetically favors single-carbon production, leading to low energy efficiency and low production rates for C 2+ products. Here we introduce in situ copper faceting from electrochemical reduction to enable preferential exposure of Cu(100) facets. During the precatalyst evolution, a phosphate ligand slows the reduction of Cu and assists the generation and co-adsorption of CO and hydroxide ions, steering the surface reconstruction to Cu (100). The resulting Cu catalyst enables current densities of > 500 mA cm -2 and Faradaic efficiencies of >83% towards C 2+ products from both CO 2 reduction and CO reduction. When run at 500 mA cm -2 for 150 hours, the catalyst maintains a 37% full-cell energy efficiency and a 95% single-pass carbon efficiency throughout.