Electrolyte Engineering Enables Selective Electrocatalytic Hydrogenation of Furfural to 2-Methylfuran

The electrocatalytic hydrogenation (ECH) of furfural (FF) to 2-methylfuran (2MF) is a promising route for producing sustainable biofuels and aviation fuels from nonedible biomass. However, achieving high selectivity and Faradaic efficiency (FE) for 2MF remains a significant challenge due to the competing hydrogen evolution reaction and the formation of furfuryl alcohol (FOH). In this study, we demonstrate that electrolyte engineering can greatly enhance the selectivity of FF ECH toward 2MF. Using a copper electrocatalyst at pH 2, we show that a phosphate-based electrolyte boosts 2MF selectivity (72.1%) compared to a sulfate-based electrolyte (54.8%) at -0.5 V vs RHE. Using in situ surface-enhanced Raman spectroscopy (SERS), we demonstrate that H₃PO₄ can form hydrogen bonds with FF adsorbates, thus acting as a proton relay. This promotes a proton-coupled electron transfer (PCET) mechanism steering selectivity toward 2MF. We further establish a direct correlation between 2MF selectivity and the intensity ratio of Raman bands associated with PCET and FOH intermediates under a wide range of conditions (pH, potential, and electrolyte). Finally, by introducing bromide ions into the phosphate electrolyte, the competing hydrogen atom transfer (HAT) pathway is further suppressed, achieving a record-high Faradaic efficiency of 91.4 ± 5.7% and a relative selectivity of 89.0 ± 4.3% for 2MF. Taken together, these results highlight the critical role of electrolyte composition in optimizing the ECH process for sustainable fuel production.