Active Site Ensembles on Transition Metal Phosphide Nanocrystals for NO₃^- Electroreduction

Transition metal phosphides (TMPs) are an exciting class of electrocatalytic materials due to their active site ensembles, i.e. surface charge distribution and variety of surface binding sites. While HER and CO₂ electroreduction have been investigated thoroughly, the mechanism and performance of Ni₂P for the electroreduction of NO₃^- to NH₃ remains understudied. In our work, we prepare 5 nm Ni₂P nanocrystals using a heat-up colloidal synthesis and demonstrate a 100% Faradaic efficiency for NO₃^- reduction over HER at -0.4 V vs. RHE in neutral, buffered conditions. NH₃ selectivity is maximized (> 80% FE) at -0.2 V vs. RHE, where phosphate is the H-source for the hydrogenation of NO_x. Our rate order analysis and DFT calculations support a sequential deoxygenation-hydrogenation pathway of NO₃^- to NH₃ that involves competitive co-adsorption of H* and NO_x* intermediates. Interestingly, we also find that ΔG_H* can be modulated by as much as 0.4 eV depending on the neighboring NO_x* species. Encouraged by the positive impact of active site ensembles on Ni₂P nanocrystals for NO₃^- electroreduction activity, we also explore methods to synthesize multimetal TMP electrocatalysts via post-synthetic cation exchange. Ni_2-xM_xP nanocrystals (M = Cu, Co) were prepared and evaluated as NO₃^- reduction electrocatalysts, where dopant identity and concentration modulate selectivity and activity significantly. This work demonstrates the importance of active site ensembles in the development of earth abundant, selective catalysts for NO₃^- electroreduction to NH₃ and beyond with other electrosynthetic reactions.