Tuning nitrate electroreduction selectivity via doping of nickel phosphide nanocrystals

Metal phosphide nanomaterials have been demonstrated as electrocatalysts for various electrosynthetic transformations, where their activity for hydrogenation reactions is attributed to their surface site heterogeneity. In this study, we modify Ni2P’s surface ensembles using a cation exchange method to synthesize doped Ni2-xMxP (M = Cu, Co) nanocrystals as catalysts for nitrate electroreduction. We modulate Ni2P’s hydrogen affinity based on d-band theory, where we expect cobalt to introduce more strongly bound hydrogen onto the surface and copper to do the opposite. Cobalt doping suppresses hydrogen evolution and achieves faradaic efficiencies of ≥ 80% for NH3, nearly a two-fold increase relative to undoped Ni2P, while copper doping promotes hydrogen and nitrite formation over ammonia. This work demonstrates the synthetic tunability of metal phosphide nanocrystals and the impact of hydrogen affinity on nitrate electroreduction selectivity.