Coverage enhancement accelerates acidic CO₂ electrolysis at ampere-level current with high energy and carbon...

Acidic CO₂ electroreduction (CO₂R) using renewable electricity holds promise for high-efficiency generation of storable liquid chemicals with up to 100% CO₂ utilization. However, the strong parasitic hydrogen evolution reaction (HER) limits its selectivity and energy efficiency (EE), especially at ampere-level current densities. Here we present that enhancing CO₂R intermediate coverage on catalysts promotes CO₂R and concurrently suppresses HER. We identified and engineered robust Cu₆Sn₅ catalysts with strong ^*OCHO affinity and weak ^*H binding, achieving 91% Faradaic efficiency (FE) for formic acid (FA) production at 1.2 A cm^-2 and pH 1. Notably, the single-pass carbon efficiency reaches a new benchmark of 77.4% at 0.5 A cm^-2 over 300 hours. In situ electrochemical Fourier-transform infrared spectroscopy revealed Cu₆Sn₅ enhances ^*OCHO coverage ~2.8× compared to Sn at pH 1. Using a cation-free, solid-state-electrolyte-based membrane-electrode-assembly, we produce 0.36 M pure FA at 88% FE over 130 hours with a marked full-cell EE of 37%.