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

Stabilizing metal oxides is a prerequisite for elucidating their intrinsic mechanistic roles and sustaining high electrocatalytic activity. Here, we synthesize a high-temperature-phase La 2 O 3 -socketed sub-2 nm δ-Bi 2 O 3 heterojunction (δ-Bi 2 O 3 /La 2 O 3 ) that suppresses Bi 3+ 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-Bi 2 O 3 and La 2 O 3 drive oxygen migration to the interface, contracting δ-Bi 2 O 3 domains and enhancing La–Bi d-p orbital hybridization. This structural relaxation stabilizes interfacial Bi–O–La linkages and electron-deficient Bi 2 O 3+x species under cathodic potentials, as confirmed by in situ X-ray absorption spectroscopy. Pourbaix diagrams and in situ infrared spectroscopy demonstrate that La 2 O 3 promotes water dissociation to form a hydroxylated δ-Bi 2 O 3 surface under working potentials, enhancing protonation propensity. Consequently, the energy barrier for the rate-determining step (*CO 2 → *HCOO) is lowered to +0.15 eV on δ-Bi 2 O 3 /La 2 O 3 , significantly lower than the +0.83 eV barrier on pristine δ-Bi 2 O 3 . This work establishes a sub-nanoscale oxide/oxide heterojunction strategy to stabilize high-valent metal sites, enabling sustainable electrochemical conversion.