Fluoride-modified Stern layer on NiOOH for steering ethylene glycol electrooxidation toward formate

The Stern layer, the innermost region of the electrochemical double layer, directly governs interfacial potential gradients, solvent organization, and intermediate stabilization at electrocatalytic surfaces. Yet it is seldom exploited as a chemically designable entity for controlling reaction selectivity. Here, we show that trace fluoride ions construct a fluoride-modified Stern layer on Ni(OH) 2 -derived NiOOH, which steers the ethylene glycol electrooxidation reaction toward formate. On the electrolyte side, this fluoride-modified Stern layer disrupts the rigid hydrogen-bonding network of interfacial water, reduces the fraction of tetrahedrally coordinated water, accelerates charge transfer, and promotes ethylene glycol enrichment. On the electrode side, fluoride within the Stern layer perturbs the interfacial electric field, which tunes the electronic properties of surface Ni and accelerates the catalyst’s intrinsic local surface reconstruction. Density functional theory and constant-potential molecular dynamics show that the fluoride-modified interface lowers this barrier by stabilizing cleaved intermediates within the Stern layer. A physics-informed anion descriptor analysis further reveals that hydration, hardness, and speciation collectively influence Stern layer regulation, guiding sulfate as another compatible hard-hydrated additive that improves formate selectivity. Collectively, these results reveal that electrolyte-side anion engineering induces interfacial electric-field perturbations to tune surface electronic behaviors and regulate C−C scission as well as product distribution.