Reimagining acidic CO2 electroreduction via anion-mediated proton transfer

Acidic CO2 electroreduction reaction (CO2RR) offers a carbon-negative pathway for synthesizing value-added chemicals with high carbon efficiency but is significantly hindered by the competing hydrogen evolution reaction (HER). While concentrated K+ cations have been extensively employed to suppress HER and improve CO2RR selectivity, they inevitably trigger catastrophic salt precipitation that degrades electrolyzer durability. Here, we pioneer an anion engineering strategy that breaks the cation-concentration paradigm through manipulating proton transfer dynamics. Combining mass spectrometry, spectroscopic techniques and theoretical calculations, we reveal that hydrolyzable anions improve proton transfer via forming protonated species that simultaneously strengthen hydrogen-bond networks and lower the kinetic barrier for *H intermediate formation, thereby promoting HER. By leveraging this fundamental insight, we achieved highly selective CO2RR with 87.3% Faradaic efficiency in strong acidic conditions (pH 1) at a low K+ concentration (0.2 M) using non-hydrolyzable Cl−. Our work provides a paradigm shift from cation-centric to anion-dominated electrolyte design, and establishes anion hydrolysis tendency as a crucial descriptor of electrocatalytic performance in acidic CO2RR systems.