The Role of Ensembles in Metal Phosphide Electrocatalysis for Modulating Nitrate Electroreduction Behavior

Abstract

Transition metal phosphides (TMPs) are an exciting class of electrocatalytic materials due to their active site ensembles, i.e., the surface charge distribution and variety of adsorption sites. While the importance of surface ensembles has been thoroughly investigated for HER and CO2 electroreduction, the mechanism and performance of Ni2P for the electroreduction of NO3– to NH3 remains understudied. In this work, we prepared 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 hydrogen source for the hydrogenation of NOx* intermediates. 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 NOx* intermediates. Encouraged by the positive impact of active site ensembles on Ni2P nanocrystals for NO3– electroreduction behavior, we explored methods to modify the ensembles via nanocrystal doping. To that end, we prepared a series of Ni2-xMxP nanocrystals (M = Cu, Co) using a two step, post-synthetic cation exchange method and evaluated them as NO3– electroreduction catalysts. We observed that cobalt doped Ni2P nanocrystals are able to suppress HER to less than 10% Faradaic Efficiency in favor of NH3 production (> 80% FE). In contrast, copper-doped Ni2P nanocrystals favor HER and NO2–; H2 Faradaic efficiency increases from 0 to 60% and correspondingly, NH3 selectivity decreases by 60% at −0.4 V vs. RHE. We rationalize the selectivity trends based on the relative H-binding strengths of these dopants, where copper introduces more weakly-bound hydrogens on the surface and inhibits NH3 formation. Conversely, cobalt introduces strongly-bound hydrogens on the surface that enables hydrogenation of reaction intermediates to form NH3. These works demonstrate 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.