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

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 N 2 . However, continuing controversy exists on whether MNs are catalytically active for NRR or produce NH 3 via the reductive decomposition of N lat without N 2 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 N 2 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 NH 3 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.