Low‐Coordination Indium Single‐Atom Sites Anchored on a Metal‐Organic Framework Single‐Layer Boosts Electroreduction of CO ₂ Into Formic Acid

Abstract The electroreduction of CO 2 under acidic conditions presents both scientific significance and technical challenges. Herein, we developed a post‐synthetic modification strategy to anchor unexpected tetrahedral In(III) ions onto Zr‐oxo clusters of a single‐layer zirconium metal‐organic framework (denoted as Zr‐MOF‐In ). Operating under harsh acidic conditions (pH = 1.67) at −1.8 V versus RHE, the Zr‐MOF‐In catalyst demonstrates exceptional performance with 95.7% Faradaic efficiency for formic acid production and a current density of 213.3 mA cm −2 . The system maintains operational stability over 20 h without notable activity decay. Remarkably, when integrated into a membrane electrode assembly electrolyzer with solid‐state electrolyte at 4 V full‐cell voltage, Zr‐MOF‐In continuously generates pure formic acid solution (505.5 mmol L −1 , 100% purity) free of electrolyte contamination, achieving a concentration 1.5‐fold higher than the current state‐of‐the‐art. Mechanistic investigations identify dual enhancement mechanisms: i) the low‐coordinated In(III) single‐atom sites facilitate dual *HCOO intermediate binding, enhancing reaction kinetics beyond conventional single‐intermediate adsorption on high‐coordination metal centers; and ii) the single‐layer MOF configuration optimizes active site exposure, synergistically maximizing catalytic efficiency.