Ampere‐Level CO 2 ‐to‐Formate Electrosynthesis Over Grain‐Boundary‐Rich Sulfur‐Doped SnO 2 in Alkaline–Neutral–Acidic Electrolytes

ABSTRACT Electrocatalytic reduction of CO 2 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 SnO 2 catalyst that enables CO 2 ‐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 SnO 2 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.