Porously Reduced 2‐Dimensional <scp>Bi₂O₂CO₃</scp> Petals for Strain‐Mediated Electrochemical <scp>CO₂</scp> Reduction to <scp>HCOOH</scp>

Here we introduce bismuth‐based catalysts for the efficient electrochemical reduction of CO 2 to formic acid (HCOOH), which are composed of petal‐shaped Bi 2 O 2 CO 3 (BOC) that spontaneously formed from Bi thin film in aqueous carbonate solution at room temperature. During the electrochemical reduction process, the BOC petals transform to reduced BOC (R‐BOC) consisting of individual BOC and Bi domains. Lattice mismatch between both domains induces biaxial strain at the interfaces. Density functional theory calculations suggest that the tensile strain on the Bi domain stabilizes the *OCHO intermediate, reducing the thermodynamic barrier toward CO 2 conversion to HCOOH. Together with the thermodynamic benefit and the unique nanoporous petal‐shaped morphology, R‐BOC petals have a superior Faradaic efficiency of 95.9% at −0.8 V RHE for the electrochemical conversion of CO 2 to HCOOH. This work demonstrates that the spontaneously formed binary phases with desirable lattice strain can increase the activity of bismuth catalysts to the CO 2 reduction reaction; such a strategy can be applicable in design of various electrocatalysts.