Acidic CO2 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 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.