Tuning Proton Activity in Organic Electrolytes for Selective CO₂-to-Long-Chain Hydrocarbon Conversion

The electrochemical reduction of carbon dioxide (CO₂) into long-chain hydrocarbons (HCs), using protons from water and renewable electricity, is a potential pathway to synthesizing carbon-neutral liquid fuels. Recent studies have shown that nickel-based catalysts can drive this transformation; however, their ability to selectively generate hydrocarbons is often limited by the competing water reduction to hydrogen gas (H₂). In this work, proton activity in an aprotic dimethyl sulfoxide (DMSO) electrolyte is systematically tuned by introducing a series of alcohol proton donors. Across this donor series, the selectivity of HCs exhibits a volcano-shaped dependence on the acidity of the proton donors. The Faradaic efficiency of C₁-C₆ HCs increases from 4.0% in aqueous 0.1 M KHCO₃ electrolyte to 22.1% in DMSO electrolyte spiked with 0.2 M ethylene glycol (EG). Nuclear magnetic resonance (NMR) spectroscopy shows that at 0.2 M, EG preferentially forms DMSO-EG hydrogen bonds that inhibit EG-EG clustering, thereby maintaining a well-dispersed proton donor environment. This solvation structure establishes a moderate proton-activity regime that facilitates CO₂ activation and C-C chain growth, while suppressing H₂ formation. Collectively, our results reveal that tuning both the molecular structure of the proton donor and its concentration-dependent solvation behavior in DMSO provides a strategy to delineate and access the proton-activity window that maximizes the CO₂-to-HC conversion.