The electrochemical reduction of carbon dioxide (CO2RR) to formate provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks powered using renewable electricity. Unfortunately, high selectivity in formate electrosynthesis has thus far only been achieved at the expense of highly cathodic potentials, yielding impractically low power conversion efficiencies. In this work, we use density functional theory to investigate the effect of alloying Cu and Sn on the activity and selectivity towards formate. A theoretical thermodynamic analysis of the reaction energetics suggests that the incorporation of copper into tin could suppress hydrogen evolution and CO production, thus favoring formate generation. We investigate a series of CuxSny catalysts prepared through co-electrodeposition. The metallic state of the alloys was confirmed with in situ extended x-ray absorption fine structure (EXAFS). In situ Sn L3-edges and Cu K-edge X-ray absorption spectroscopy measurements indicate the electron donation from Sn to Cu which reveals positive oxidation states of Sn in CuSn3 under operating conditions. Consistent with theoretical trends, we found the designed CuSn3 catalysts exhibit a Faradaic efficiency of 95% towards formate generation at -0.5 V vs. RHE. Furthermore, the catalysts show no degradation over 50 hours of operation.