Efficient Electrocatalytic Conversion of CO₂ to Pure Formic Acid Solutions via Strain-Engineered Bismuth Nanosheets

The electrochemical reduction of CO₂ to formic acid (HCOOH), a valuable chemical feedstock, offers a promising pathway toward carbon neutrality. However, achieving high selectivity, industrial current densities, long-time stability, and the direct production of pure products simultaneously remains difficult. Here we present tensile-strained bismuth nanosheets (TS-BiNs) created through a scalable mechanochemical ball-milling and carbonation process, effectively addressing all of these challenges. The TS-BiNs catalyst demonstrates outstanding performance, maintaining over 92% Faradaic efficiency for HCOO^- across a wide current density range from -50 to -1000 mA cm^-2 and an excellent stability at -100 mA cm^-2 for over 100 h. When used in a solid-state electrolyte reactor (4 cm²), it ensures the continuous and stable production of pure HCOOH solutions with widely adjustable concentrations (40-1500 mM) at 400 mA over 100 h. Combined experimental and theoretical analyses reveal that the tensile strain enhances the adsorption of the ^*OCHO intermediate and inhibits the competing hydrogen evolution reaction, thereby directing the process toward highly efficient HCOOH formation. This work underscores the significant potential of strain engineering in catalyst design and introduces an integrated catalyst-reactor strategy for practical electrochemical CO₂ upgrading.