Hydroxyl Adsorption as a Selectivity Descriptor for Electrocatalytic Nitrate Reduction over Cu Catalysts

The electrocatalytic nitrate reduction reaction (NO 3 – RR) driven by renewable electricity has recently emerged as a promising route for ammonia (NH 3 ) synthesis under mild conditions. Yet, the energy-efficient NH 3 electrosynthesis from NO 3 – is hindered by high overpotential and competing nitrite (NO 2 – ) formation. Here, we demonstrate potential- and concentration-dependent NO 3 – RR product selectivity over a size- and shape-selected Cu nanocube model catalyst. More negative potentials and lower NO 3 – concentrations are favorable for improving selectivity toward NH 3 over NO 2 – . Such apparent dependence is closely associated with weakened hydroxyl (*OH) adsorption. In combination with operando spectroscopic characterization and density functional theory calculation results, *OH adsorption has been proposed as a descriptor that determines product selectivity by affecting the availability of surface adsorbed hydrogen (*H) produced through water dissociation. Our work underscores the importance of the *OH adsorption as a selectivity descriptor for designing more efficient systems toward NO 3 – RR to NH 3 .