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

Carbon dioxide electroreduction (CO 2 R) in acid tends to be a promising route to avoid CO 2 loss in alkaline and neutral electrolytes; however, high alkali cation concentrations (typically ≥3 M) are required to activate CO 2 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 CO 2 R 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, SnO 2 nanoparticles were modified with polyimide and acted as a CO 2 R 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 CO 2 electrolysis.