Superscalar Phase Boundaries Derived Multiple Active Sites in SnO2/Cu6Sn5/CuO for Tandem Electroreduction of CO2 to Formic Acid

The electrocatalytic CO2 reduction reaction (CO2RR) to fuels driven by electrocatalysts is a viable strategy for efficient utilization of emitted CO2. CO2RR involves multiple‐steps, including adsorption, activation, hydrogenation, etc. At present, copper‐tin alloy catalysts have shown the capability to reduce CO2 to formic acid or formate. However, their poor adsorption and activation capacities for CO2 molecules, as well as the sluggish kinetics in *H supply restrict the proton‐coupled electron transfer processes in the electrocatalytic CO2RR to produce formic acid. In order to solve the above problems, the ultra‐small SnO2/Cu6Sn5/CuO nanocatalysts with superscalar phase boundaries are fabricated by laser sputtering. The introduction of SnO2 enhances the adsorption and activation of CO2, while CuO promotes H2O decomposition and provides abundant *H intermediates, resulting in tandem catalytic sites on the SnO2/Cu6Sn5/CuO composite catalysts and thus leading to excellent CO2RR activity and high selectivity to formic acid. The Faradic efficiency of formic acid (FEHCOOH) at the SnO2/Cu6Sn5/CuO electrode reaches 90.13% along with a high current density of 25.2 mA cm−2 at −0.95 V versus reversible hydrogen electrode. The role of the multiphase boundaries constructed by introduction of oxides is confirmed by in situ infrared spectroscopy and kinetic isotope effects experiments, which is consistent with the design concept. SnO2/Cu6Sn5/CuO nanocatalysts with superscalar phase boundaries and multiple active catalytic sites are carefully designed for the CO2 reduction reaction (CO2RR). SnO2 enhances the adsorption and activation of CO2, while CuO promotes *H supply. The ultra‐small nanoscale size and continuous catalytic active sites greatly reduce the transfer obstacles and activity loss of intermediates, effectively improving the CO2RR activity and HCOOH selectivity.