Searching for the Rules of Electrochemical Nitrogen Fixation

Li-mediated ammonia synthesis is – thus far – the only electrochemical method to decentralised ammonia production, an alternative to one of the largest thermal heterogeneous catalytic processes, for its unique selectivity on a solid electrode. However, it is burdened with intrinsic energy losses, operating at Li plating potential. In this work, we survey the periodic table to understand the fundamental features that make Li stand out. Through density functional theory calculations and experimentation on chemistries analogous to lithium (e.g. Na, Mg, Ca), we find that lithium is unique in several ways. It combines a stable nitride that readily decomposes to ammonia, with an ideal solid electrolyte interphase, balancing reagents at the reactive interface. We propose descriptors based on simulated formation and binding energies of key intermediates, and further on hard and soft acids and bases (HSAB principle) to generalize such features. The survey will help the community towards new electrochemical systems for nitrogen fixation.