Optimizing the Lattice Nitrogen Coordination to Break the Performance Limitation of Metal Nitrides for Electrocatalytic Nitrogen Reduction.

Abstract

Metal nitrides (MNs) are attracting enormous attention in the electrocatalytic nitrogen reduction reaction (NRR) because of their rich lattice nitrogen (N_lat) and the unique ability of N_lat vacancies to activate N₂. However, continuing controversy exists on whether MNs are catalytically active for NRR or produce NH₃ via the reductive decomposition of N_lat without N₂ activation in the in situ electrochemical conditions, let alone the rational design of high-performance MN catalysts. Herein, we focus on the common rocksalt-type MN(100) catalysts and establish a quantitative theoretical framework based on the first-principles microkinetic simulations to resolve these puzzles. The results show that the Mars-van Krevelen mechanism is kinetically more favorable to drive the NRR on a majority of MNs, in which N_lat plays a pivotal role in achieving the Volmer process and N₂ activation. In terms of stability, activity, and selectivity, we find that MN(100) with moderate formation energy of N_lat vacancy (E _vac) can achieve maximum activity and maintain electrochemical stability, while low- or high-E _vac ones are either unstable or catalytically less active. Unfortunately, owing to the five-coordinate structural feature of N_lat on rocksalt-type MN(100), this maximum activity is limited to a yield of NH₃ of only ∼10^-15 mol s^-1 cm^-2. Intriguingly, we identify a volcano-type activity-regulating role of the local structural features of N_lat and show that the four-coordinate N_lat can exhibit optimal activity and overcome the performance limitation, while less coordinated N_lat fails. This work provides, arguably for the first time, an in-depth theoretical insight into the activity and stability paradox of MNs for NRR and underlines the importance of reaction kinetic assessment in comparison with the prevailing simple thermodynamic analysis.