The results of a comprehensive computational screening for new electrocatalysts that are stable, active and selective towards electrochemical reduction of nitrogen to ammonia at room temperature and ambient pressure is presented on a range of transition metal nitride surfaces. Density functional theory (DFT) calculations are used to study the thermochemistry of cathode reaction so as to construct the free energy profile and to predict the required onset potential for activation of nitrogen to ammonia via a Mars-­van Krevelen mechanism. Five crucial criteria are investigated in this work to identify promising electrocatalysts for electrochemical ammonia formation in aqueous media; (1) identification of the reaction mechanism and calculation of the nitrogen splitting barrier, (2) investigation of the catalytic activity and quantification of the potential-determining/rate-determining steps, (3) determination of kinetic or thermodynamic barriers for vacancy diffusion into the bulk, (4) investigation of the stability of the surfaces against poisoning in an electrochemical environment, and (5) estimation of the potential required for removal of any oxygen atoms that may adsorb onto the surface upon exposure to electrochemical environment. With this approach, the (100) and (111) facets of the rocksalt as well as the (100) and (110) facets of the zincblende structures of all the naturally occuring d-block elements in the priodic table are scrutinized. The most promising candidates turned out to be the (100) facets of the rocksalt structure of VN, CrN, NbN and ZrN that should be able to form ammonia at ­-0.51 V, ­-0.76 V, -­0.65 V and -­0.76 V vs. SHE, respectively. All are found to be much more active towards nitrogen reduction than towards the competing hydrogen evolution reaction, in contrast to pure metals, which largely evolve hydrogen. Therefore, higher current efficiencies towards ammonia is expected at low onset potentials. An additional benefit to the present analysis is that the method presented in this work may be applicable to other aqueous phase heterogeneous catalytic reactions, where a Mars-van Krevelen mechanism is operative with both selectivity and activity being key catalytic criteria.

Figure caption: Free energy diagram for NH3 formation via a Mars–van Krevelen mechanism on the (100) facet of RS VN where all the possible adsorption sites for H are investigated and the most energetically favourable site and therefore the most stable intermediates are depicted.