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

The electroreduction of CO 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. 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, 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.