Ag-induced Phase Transition of Bi₂O₃ Nanofibers for Enhanced Energy Conversion Efficiency towards Formate in CO₂ Electroreduction

Bi-based electrocatalysts have been widely investigated in the CO₂ reduction reaction (CO₂RR) for the formation of formate. However, it remains a challenge to achieve high Faradaic efficiency (FE) and industrial current densities at low overpotentials for obtaining both high formate productivity and energy efficiency (EE). Herein, we report an Ag−Bi₂O₃ hybrid nanofiber (Ag−Bi₂O₃) for highly efficient electrochemical reduction of CO₂ to formate. Ag−Bi₂O₃ exhibits a formate FE of >90% for current densities from −10 to −250 mA ⋅ cm⁻² and attains a yield rate of 11.7 mmol ⋅ s⁻¹ ⋅ m⁻² at −250 mA ⋅ cm⁻². Moreover, Ag−Bi₂O₃ increased the EE (52.7%) by nearly 10% compared to a Bi₂O₃ only counterpart. Structural characterization and in-situ Raman results suggest that the presence of Ag induced the conversion of Bi₂O₃ from a monoclinic phase (α-Bi₂O₃) to a metastable tetragonal phase (β-Bi₂O₃) and accelerated the formation of active metallic Bi at low overpotentials (at > −0.3 V), which together contributes to the highly efficient formate formation.