Ampere-Level CO2-to-Formate Electrosynthesis Over Grain-Boundary-Rich Sulfur-Doped SnO2 in Alkaline-Neutral-Acidic Electrolytes.

Electrocatalytic reduction of CO2 to formate offers a sustainable route for carbon utilization, converting a greenhouse gas into a valuable chemical feedstock. Although Sn‑based catalysts are among the most promising candidates, their formate selectivity often suffers a marked decline at ampere‑level current densities, and operating efficiently in various electrolytes, especially acidic media, remains a major challenge. In this work, we report a grain boundary‑rich sulfur (S)-doped SnO2 catalyst that enables CO2-to-formate electrosynthesis at ampere-level current densities across alkaline, neutral, and acidic electrolytes. The catalyst achieves formate Faradaic efficiencies exceeding 90% at 1 A cm-2 in acidic, neutral, and alkaline electrolytes. In situ spectroscopy and theoretical calculations reveal that S doping reduces the reaction free energy change for the key *OCHO intermediate and promotes its protonation to formate. Meanwhile, S‑doped SnO2 with abundant grain boundaries facilitates the formation of an enhanced interfacial hydrogen-bonding network, which promotes efficient proton transfer while suppressing the competing hydrogen evolution reaction (HER). This work provides new insights into the design of high-performance electrocatalysts through elemental doping and elucidates the governing role of the interfacial microenvironment in reaction activity and selectivity, offering design principles applicable to broader electrochemical syntheses.