Stabilizing sub-2 nm δ-Bi₂O₃ via strong lanthanide-oxide-support interaction for durable CO₂ electroreduction to formate.

Abstract

Stabilizing metal oxides is a prerequisite for elucidating their intrinsic mechanistic roles and sustaining high electrocatalytic activity. Here, we synthesize a high-temperature-phase La₂O₃-socketed sub-2 nm δ-Bi₂O₃ heterojunction (δ-Bi₂O₃/La₂O₃) that suppresses Bi³⁺ reduction to metallic Bi, achieving ≥95% formate Faradaic efficiency for ~200 hours in industrial-level electrolyzers. Electronic structure analyses reveal that strong electrostatic interactions between δ-Bi₂O₃ and La₂O₃ drive oxygen migration to the interface, contracting δ-Bi₂O₃ domains and enhancing La-Bi d-p orbital hybridization. This structural relaxation stabilizes interfacial Bi-O-La linkages and electron-deficient Bi₂O_3+x species under cathodic potentials, as confirmed by in situ X-ray absorption spectroscopy. Pourbaix diagrams and in situ infrared spectroscopy demonstrate that La₂O₃ promotes water dissociation to form a hydroxylated δ-Bi₂O₃ surface under working potentials, enhancing protonation propensity. Consequently, the energy barrier for the rate-determining step (*CO₂ → *HCOO) is lowered to +0.15 eV on δ-Bi₂O₃/La₂O₃, significantly lower than the +0.83 eV barrier on pristine δ-Bi₂O₃. This work establishes a sub-nanoscale oxide/oxide heterojunction strategy to stabilize high-valent metal sites, enabling sustainable electrochemical conversion.