Tuning CO ₂ RR Pathways through Water Control in Ionic Liquids

The electrochemical reduction of CO2 (CO2RR) into value-added chemicals offers a compelling route toward fuel and chemical production. In this study, the influence of water concentration on CO2RR performance is explored using a Cu/Cu2O catalyst interfaced with a CO2-philic ionic liquid (IL), methyltributylphosphonium bis(trifluoromethylsulfonyl)imide (P1444TFSI). Catalyst characterization prior to electrolysis confirmed the presence of the Cu2O (111) facet and Cu+ surface species, which remained on the surface even after electrolysis. This does not normally occur with aqueous electrolytes, which typically reduce almost all the oxide during the initial stages of electrolysis. Electrochemical tests across different water concentrations (<1–80 mmol L–1) demonstrated that water modulates both proton availability and surface composition. At 20 mmol L–1, CO2RR selectivity shifted toward formate/formic acid (86% of product distribution) with suppressed methanol production, suggesting that the inhibition of the *CHO pathway and the promotion of *CO intermediates is favorable for C–C coupling. In contrast, 80 mmol L–1 water enables the *CHO pathway, yielding methanol and methane. At highly negative potentials (−1.3 to −1.6 V vs Fc/Fc+), extended hydrocarbon products, including propylene and butane, emerged, enabled by stabilized *C2 intermediates and the wide electrochemical stability window of the IL. Butane formation was attributed to ethylene dimerization, independent of *H coverage, and the formation of propylene indicates that C–C coupling can occur among three intermediates that originate from *CO. Faradaic efficiencies for hydrogen remained low throughout, especially at 20 mmol L–1, affirming HER suppression by the IL. Importantly, the IL showed no signs of degradation postelectrolysis. This work highlights the pivotal role of water in tuning CO2RR selectivity and reaction pathways, demonstrating that careful control of proton donors in nonaqueous environments enables tailored hydrocarbon formation via stabilized intermediates and the mechanistic steering of complex multicarbon reactions.