Chaotropic Anions Promote Electrochemical C-C Bond Formation by Reshaping Interfacial Solvation

Although anions are known to affect CO and CO₂ electroreduction, the underlying mechanisms have not been clearly defined. We propose that they control the reactivity by altering the interfacial solvation. This hypothesis is informed by the Hofmeister series from chemical biology, which ranks ions by their ability to salt proteins in or out of solution through alteration of the water structure. Kosmotropic anions strengthen hydrogen bonding and promote water ordering, while chaotropic anions disrupt it. We examined four representative anions from this series: CH₃COO^-, Cl^-, CF₃SO₃^-, and ClO₄^-. The most chaotropic anion, ClO₄^-, enhanced C₂⁺ product formation by 3.7 times relative to CH₃COO^-, achieving a total current density of -782 ± 34 mA cm^-2 at -1.50 V versus the Normal Hydrogen Electrode. Temperature-dependent measurements showed that chaotropic anions increase the activation entropy, consistent with greater interfacial water disorder observed by in situ Raman spectroscopy. These results highlight anion-driven water structuring as a powerful lever to enhance multicarbon product formation.