<p>Furfural is one of the most important biomass-based platform molecules as sustainable precursors for chemicals and fuels. The electrochemical hydrodimerization of furfural can generate hydrofuroin, which is a promising precursor for gasoline or jet fuel. However, steering the selectivity toward hydrodimerization under near-neutral conditions that suppress polymerization and disproportionation of furfural remains challenging. Herein, we discovered that tuning the electrolyte cations could be an effective and alternative strategy for modulating furfural reduction to the traditional electrocatalyst design. Under the optimal Ca²⁺ cations, hydrofuroin yield could be much improved from other cations with hydrofuroin selectivity of 95.5%. The systematic characterization of the ion-centered double layer reflected the suitable sizes and charges of hydrated Ca²⁺ for suppressing the hydrogen evolution reaction from activation of surrounding water molecules as well as encouraging furfural accumulation via non-covalent interaction for directing the electron transfer to furfurals. Ions with smaller or larger sizes can either activate local water molecules or form a crowded ionic interface that casts an adverse effect on furfural enrichment. This work not only presents an effective strategy for facilitating biomass valorization via hydrodimerization but also emphasizes the essential role of the ion-centered microenvironment for steering the electrocatalytic pathways.</p>