Mercaptoimidazole-Engineered Microenvironment Enables Durable CO₂ Electroreduction in a Zero-Gap PEM Electrolyzer

CO₂ 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 CO₂ electrolyzers.