Proton transfer modulation via electrolyte additives for suppressing hydrogen evolution and enhancing C2+ selectivity during acidic CO2 electroreduction

Abstract

Suppressing the hydrogen evolution reaction (HER) is a critical challenge for electrochemical reduction of carbon dioxide (CO2RR) in acidic media, where abundant protons drive rapid proton delivery via the Grotthuss mechanism. Although electrolyte additives have been reported to modulate the HER in neutral and alkaline systems, their effects and roles remain largely unexplored under acidic conditions, where proton transport pathways are fundamentally distinct. Here, we identify hydrogen-bond acceptor (HBA) ability as a molecular-level descriptor governing HER suppression in acidic CO2RR. Using glycol-based additives with an identical backbone but varied terminal groups, such as diethylene glycol (DEG), diethylene glycol monomethyl ether (DEGME), and diethylene glycol dimethyl ether (DEGDE), we show that the HBA ability monotonically correlates with HER suppression in acidic media. Additives with high HBA ability accept protons from neighboring water molecules but cannot effectively relay them further, interrupting long-range proton hopping. When we translated additive effects to acidic CO2RR on Cu catalysts, similar HER suppression and concomitant enhancement of the CO2RR were observed. Moreover, the presence of additives also alters selectivity toward C2+ products. In situ Raman spectroscopy confirms that additives with high HBA ability elevate local pH due to regulated proton transfer, and enhance *CO coverage, collectively favoring C-C coupling over C1 formation. The correlation between HBA ability and HER suppression is preserved across various electrolyte conditions. This work establishes HBA ability as a rational design criterion for electrolyte additives that simultaneously suppress the HER and promote C2+ electrosynthesis in acidic media, which can be applicable to other proton-coupled electrochemical systems.