The copper (Cu)-catalyzed electrochemical CO₂ reduction provides a route for the synthesis of multicarbon (C₂₊) 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 C₂₊ 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 C₂₊ products from both CO₂ 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.