Ultradiluted (Alkyl)ammonium Enables Acidic CO₂ Electroreduction

Conventional acidic electrolytes for the electrochemical CO2 reduction reaction normally require highly concentrated alkali-metal cations (i.e., K+). However, the usage of high [K+] generally induces a fast pH increase in the catholyte during CO2 electrolysis, thereby leading to an unstable system and frequent electrolyte refreshing. In this work, ultralow-concentrated (alkyl)ammonium-based cations are rationally developed to enable efficient metal-cation-free CO2 electroreduction in strong acids. Compared with traditional K+, the NH4+ cation possesses a higher positively charged center, lower hydration number, and larger surface charge density, leading to significantly lower local pKa (acid dissociation constant). This low local pKa results in a higher percentage of free water that provides sufficient absorbed *H for the hydrogenation of *CO2 to form *COOH (the key intermediate of the CO pathway). As a result, ultradiluted (0.05 M) NH4+ cations added to strong sulfuric acid (bulk pH 1.1) with silver nanoparticles used as catalysts display a substantially higher Faradaic efficiency and partial current density of the CO product than the traditional K+-based electrolyte under the same electrolysis conditions. Moreover, it also shows a substantially lower pH change rate in the catholyte compared with a highly concentrated (3 M) K+-based one during an entire refreshing cycle, leading to a more stable electrolyte system.