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The electrochemical reduction of CO₂ to multi-carbon products has attracted much attention because it provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the efficiency of CO₂ conversion to C₂ products remains below that necessary for its implementation at scale. Modifying the local electronic structure of copper with positive valence sites has been predicted to boost conversion to C₂ products. Here, we use boron to tune the ratio of Cu^δ+ to Cu⁰ active sites and improve both stability and C₂-product generation. Simulations show that the ability to tune the average oxidation state of copper enables control over CO adsorption and dimerization, and makes it possible to implement a preference for the electrosynthesis of C₂ products. We report experimentally a C₂ Faradaic efficiency of 79 ± 2% on boron-doped copper catalysts and …