What Drives the Selectivity of 5-(Hydroxymethyl)furfural Electroreduction over Copper in Mildly Basic Medium?

Abstract 2,5-Bishydroxymethylfuran (BHMF) is an important bio-based monomer that can be produced via the electrochemical reduction of 5-(hydroxymethyl)furfural (HMF) on Cu electrocatalysts. Under mildly alkaline conditions, however, BHMF formation competes with hydrodimerization to 5,5′-bis(hydroxymethyl)hydrofuroin (BHH) and oligomerization, particularly at HMF concentrations relevant for scale-up. Here, we combine electrochemical experiments and grand-canonical density functional theory (GC-DFT) to elucidate the origin of these competing pathways during HMF electroreduction on Cu-based foam catalysts at pH 9.2. Our results suggest that BHH formation and oligomerization arise from two distinct mechanisms: hydrodimerization might proceed via surface-catalyzed C–C coupling between adsorbed HMF species, whereas oligomerization is associated with the desorption of a ketyl intermediate into the electrochemical double layer. In contrast, BHMF formation preferentially occurs through the hydrogenation of an alkoxyl intermediate. The relative contribution of these pathways is modulated by catalyst morphology and the electroactive Cu surface area. Increasing the surface area through Cu nanoparticle electrodeposition enhances BHMF selectivity from 40% to 58% at 50 mM HMF and significantly suppresses oligomerization at –0.51 V vs RHE. Overall, our results highlight the importance of balancing HMF surface availability and H* availability and provide a mechanistic framework linking catalyst morphology and electrochemical conditions to control the selective HMF electroreduction on Cu and offer design principles for maximizing BHMF yields while minimizing hydrodimerization and carbon loss.