Phase-Controlled Bi₂O₃ Nanosheets for Highly Efficient Electrochemical CO₂ Reduction to Formate

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a sustainable route to convert CO₂ into valuable chemicals, with formic acid being one of the most economically attractive products. Bismuth (Bi)-based materials have shown promise as CO₂RR electrocatalysts, yet their practical application is hindered by structural instability, competing side reactions, and the lack of a systematic understanding of phase-dependent catalytic behavior. Herein, we report the synthesis of β-Bi₂O₃, δ-Bi₂O₃, and α-Bi₂O₃ with uniform 2D nanosheet morphologies, enabling a direct and fair comparison of their CO₂RR performance. Notably, the β-Bi₂O₃ nanosheets (NSs), which have not been previously reported, deliver a formate faradaic efficiency of 99.0% at -0.8 V versus the reversible hydrogen electrode in 1.0 m KOH, along with a formate production rate of 3321 µmol cm^-2 h^-1, surpassing most reported Bi-based catalysts. Through comprehensive structural characterization and combined experimental and theoretical investigations covering CO₂-to-formate reduction, formate decomposition, and the hydrogen evolution reaction, we establish a clear phase-activity relationship. This work not only demonstrates the performance of β-Bi₂O₃ NSs but also provides new insights into the rational design of phase-engineered Bi-based electrocatalysts for CO₂RR.