Electrochemical carbon dioxide reduction (CO₂) is a promising technology to use renewable electricity to convert CO₂ into valuable carbon-based products. For commercial-scale applications, however, the productivity and selectivity toward multi-carbon products must be enhanced. A facile surface reconstruction approach that enables tuning of CO₂-reduction selectivity toward C₂₊ products on a copper-chloride (CuCl)-derived catalyst is reported here. Using a novel wet-oxidation process, both the oxidation state and morphology of Cu surface are controlled, providing uniformity of the electrode morphology and abundant surface active sites. The Cu surface is partially oxidized to form an initial Cu (I) chloride layer which is subsequently converted to a Cu (I) oxide surface. High C₂₊ selectivity on these catalysts are demonstrated in an H-cell configuration, in which 73% Faradaic efficiency (FE) for C₂₊ products is reached with 56% FE for ethylene (C₂H₄) and overall current density of 17 mA cm^-2. Thereafter, the method into a flow-cell configuration is translated, which allows operation in a highly alkaline medium for complete suppression of CH₄ production. A record C₂₊ FE of ≈84% and a half-cell power conversion efficiency of 50% at a partial current density of 336 mA cm^-2 using the reconstructed Cu catalyst are reported.