Acidic CO ₂ Electrolysis With Near‐Ideal Selectivity and Carbon Efficiency Enabled by Overcoming Its Inherent Trade‐Off

Carbon dioxide electroreduction (CO₂R) in acid tends to be a promising route to avoid CO₂ loss in alkaline and neutral electrolytes; however, high alkali cation concentrations (typically ≥3 M) are required to activate CO₂ and suppress water electroreduction, causing carbonate formation and thus unsatisfied single-pass carbon efficiency (SPCE). Based on theoretical and experimental analyses, we show that an inherent trade-off exists: increasing cation concentrations improves Faradaic efficiency (FE) toward CO₂R products but comes at the expense of reduced SPCE. We demonstrate a polyimide-modification strategy to overcome this trade-off by taking advantage of the amino groups that can effectively capture protons, creating a local alkaline microenvironment surrounding the electrode surface. In a proof-of-concept experiment, SnO₂ nanoparticles were modified with polyimide and acted as a CO₂R catalyst, which achieved, simultaneously, near-ideal SPCE of 95.7% and FE of 96% (toward HCOOH) at pH 1.36 with dilute potassium ions down to even 0.1 M. We expect that these findings will accelerate the development of carbon- and electron-efficient acidic CO₂ electrolysis.