Ionic Liquid Interface Engineering Enables Ampere-Level CO2-to-Formate Conversion with Faradaic Efficiency Exceeding 99% over Indium Oxyhydroxide

High-rate electrochemical CO2 reduction to formate is strongly limited by the competing hydrogen evolution reaction (HER) and the difficulty of selectively stabilizing formate pathway intermediates under ampere current densities. Herein, we report an ionic liquid interface engineering strategy by immobilizing tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]) on indium oxyhydroxide (InOOH). The resulting [P4441][NTf2]@InOOH catalyst delivers formate Faradaic efficiencies above 90% over a wide current-density range of 0.8-1.2 A cm-2, reaching 99.37% at 1.0 A cm-2, and a formate production rate of up to 22.33 mmol h-1 cm-2 at 1.4 A cm-2. In situ optical characterization and theoretical calculations reveal that [P4441][NTf2] electronically couples with InOOH and induces charge redistribution toward surface indium sites, thereby strengthening CO2 adsorption and activation while significantly lowering the energetic barrier for the key *CO2-to-*OCHO conversion. Meanwhile, [P4441][NTf2] reorganizes interfacial water and weakens the water network near the catalyst surface, markedly suppressing the competitive HER. By coupling electronic regulation with interfacial water management, [P4441][NTf2]@InOOH selectively directs CO2 reduction toward the oxygen-bound *OCHO pathway, enabling high-throughput formate production under ampere-level operation.