Mercaptoimidazole‐Engineered Microenvironment Enables Durable CO ₂ Electroreduction in a Zero‐Gap PEM Electrolyzer

CO 2 conversion in proton exchange membrane (PEM) electrolysis systems offers a sustainable pathway for chemical production by eliminating carbonate formation; however, it faces a trade‐off between suppressing the hydrogen evolution reaction and preventing salt precipitation. Here, we resolve this paradox through a molecular‐level engineering strategy by anchoring a mercaptoimidazole ligand on lead‐based catalyst. Operando spectroscopic analyses and theoretical studies reveal that this ligand shell creates a local alkaline microenvironment and establishes a proton‐shielding effect at the catalyst surface. When integrated into a zero‐gap PEM electrolyzer, the catalyst achieves a peak formate Faradaic efficiency of 95.8% and sustains over 90% selectivity at a current density of 600 mA cm −2 . This performance persists under strongly acidic (pH 1.0) and cation‐starved (0.001 M ) conditions. The PEM system delivers extended stability, with over 300 h of continuous operation at industrially relevant current densities. Our work establishes a design strategy that decouples the catalytic microenvironment from the bulk electrolyte and provides a route for durable and selective acidic CO 2 electrolyzers.