Ni2P active site ensembles tune electrocatalytic nitrate reduction selectivity.

We demonstrate that active site ensembles on transition metal phosphides tune the selectivity of the nitrate reduction reaction. Using Ni2P nanocrystals as a case study, we report a mechanism involving competitive co-adsorption of H* and NOx* intermediates. A near 100% faradaic efficiency for nitrate reduction over hydrogen evolution is observed at -0.4 V, while NH3 selectivity is maximized at -0.2 V vs. RHE.