Atomic-Layer Thinning of Bismuth Oxide Confers Antireduction Stability and Tunable Protonation Pathway in CO₂-to-Formate Electrocatalysis

Bismuth oxide (Bi₂O₃) holds great potential for the selective electroreduction of CO₂ to formate, yet its practical application is hindered by rapid cathodic reduction to metallic Bi⁰ and competing hydrogen evolution under industrially relevant conditions. Herein, we report a scalable solvothermal method to synthesize free-standing, three-atom-thick (∼1.25 nm) Bi₂O₃ nanosheets (3L-Bi₂O₃) that simultaneously achieve antireduction stability and tunable protonation kinetics for efficient CO₂-to-formate conversion. Potentiodynamic XAS and Raman spectroscopies reveal that compressive strain induced by atomic-layer thinning strengthens Bi–O bonds, as evidenced by ∼11.5% Bi⁰ formation at −1.0 V vs RHE, compared to ∼65.7% for bulk-Bi₂O₃ at −0.6 V vs RHE. Consequently, 3L-Bi₂O₃ maintains a formate Faradaic efficiency of >90% and durability for ∼50 h at 200 mA cm^–2 in 1.0 M KHCO₃ solution. In situ infrared spectroscopy and differential mass spectrometry combined with kinetic analyses identify HCO₃^– as the essential proton donor in the two-step sequential proton-coupled electron transfer (PCET) process. In contrast to bulk-Bi₂O₃, 3L-Bi₂O₃ exhibits a distinct volcano-shaped dependence of formate selectivity on HCO₃^– concentration, reflecting a trade-off between sufficient proton availability for *OCHO formation and suppression of competitive hydrogen evolution. This behavior originates from the weakened *H adsorption and stabilized *OCHO intermediates on an atomically thin Bi₂O₃ surface, which shift the rate-determining step from the initial PCET step (as in bulk-Bi₂O₃) to the subsequent *OCHO protonation, as confirmed by free energy profiles and electronic structure analyses, including charge density differences, Bader charge analysis, and projected density of states.