Electrochemical reduction of carbon dioxide (CO₂RR) has attracted intense research attention since it provides an avenue to the renewable-electricity-powered synthesis of value-added carbon-based fuels and feedstocks. Unfortunately, high selectivity in formate electrosynthesis has thus far only been achieved at the expense of low current density and high over-potential. Herein, we report a strategy to create high active sites in tin catalysts by incorporating copper and non-metal sulfur in an intimately integrated fashion. We hypothesized that the presence of copper and sulfur atoms in the catalyst surface could promote under-coordinated metal sites, and thereby improve the electrochemical reduction of CO₂ to formate. We explored, using density functional theory, how the incorporation of copper and sulfur atoms into tin favours formate generation. The resultant under-coordinated tin sites accelerate CO₂RR at record geometric current densities with a Faradaic efficiency of 93% and exhibit prolonged stability. Furthermore, the partial current density for formate on under-coordinated tin sites – normalized by electrochemically active surface area – is also three times higher than that of unmodified tin catalysts. X-ray absorption studies and density functional theory reveal the synergistic role of copper, sulfur and tin in producing local coordination and a consequent electronic structure that enhances electron capturing ability.