Electrochemical CO₂ Reduction on a Bi-Sn Eutectic Alloy in Acidic Media for Formic Acid Production

Electrochemical CO₂ reduction (eCO₂R) offers a sustainable route for carbon utilization, but most electrolyzers operate in neutral or alkaline media, where (bi)carbonate formation limits long-term operation and complicates product recovery. Here, we show that operating eCO₂R under acidic conditions enables direct formic acid production while minimizing (bi)carbonate accumulation. A eutectic Bi-Sn gas-diffusion electrode (GDE) achieved a faradaic efficiency (FE) of 81.3% toward formic acid at -100 mA cm^-2 and in a pH 3 electrolyte, outperforming Bi and Sn GDEs, with formic acid remaining the dominant product up to -400 mA cm^-2. Density functional theory calculations revealed a synergistic Bi-Sn interfacial effect, where weakened hydrogen adsorption and intermediate binding of CO₂-to-formate intermediates collectively suppress hydrogen evolution and promote formic acid formation. The GDE maintained stable performance with <10% FE loss in a 100 h continuous operation, using a periodic electrolyte replacement strategy. These results establish acidic eCO₂R as a viable strategy for high-purity formic acid production and demonstrate how interfacial alloy engineering can advance CO₂ electrolysis toward scalable, renewable energy-powered chemical manufacturing.