Metal Nitride as a Mediator for the Electrochemical Synthesis of NH₃

The Haber-Bosch process has a massive carbon footprint, and it is highly desired to decentralize NH3 synthesis in a more sustainable manner. Li-mediated NH3 electrosynthesis is a promising approach to making NH3 under ambient conditions, but it suffers from poor energy efficiency owing to the highly reducing electroplating potential of Li. In this Letter, we report a combined theoretical and experimental investigation into other mediators beyond Li, such as Ca, Mg, Sr, Y, and V. The density functional theory results suggest that, besides Li, Ca and Mg can also activate N2 at room temperature and have a stable surface nitride vacancy necessary for NH3 synthesis. NH3 faradaic efficiencies of 50% ± 0.2% and 27% ± 2% are obtained from Ca and Mg, respectively, at an applied current density of 15 mA/cm2. This Letter serves as a proof-of-concept for Li-free NH3 synthesis and will motivate further research in metal-nitride-mediated processes.