Subnanometric Nickel Phosphide Heteroclusters with Highly Active Ni^δ+-P^δ- Pairs for Nitrate Reduction toward Ammonia

The development of efficient electrocatalysts for the neutral nitrate reduction reaction (NO₃^-RR) toward ammonia (NH₃) is essential to address the environmental issues caused by NO₃^- but remains considerably challenging owing to the sluggish reaction kinetics of NO₃^-RR in neutral media. Herein, we report subnanometric heteroclusters with strongly coupled nickel-phosphorus (Ni-P) dual-active sites as electrocatalysts to boost the neutral NO₃^-RR. Experimental and theoretical results reveal that the subnanometric feature of Ni-P heteroclusters promotes the electron transfer from Ni to P, generating Ni^δ+-P^δ- active pairs, in which Ni^δ+ species are highly active for the NO₃^-RR and P^δ- tunes the interfacial water hydrogen bonding network to promote the water dissociation step and accelerate proton transfer during the NO₃^-RR. Consequently, in the neutral NO₃^-RR, Ni-P heteroclusters exhibit a large NH₃ yield rate of 0.61 mmol h^-1 cm^-2 at -0.8 V versus reversible hydrogen electrode, which is 2.8- and 3.3-fold larger than those on Ni-P nanoparticles and Ni clusters, respectively, and the generated NH₃ exists as NH₄⁺ in electrolytes. This study offers an efficient approach to boosting electrocatalytic reactions with multiple intermediates by designing subnanometric heteroclusters with strongly coupled active sites.