Towards sustainable metal-mediated ammonia electrosynthesis

Ammonia is a key component of fertilizers, a crucial industrial chemical and a carbon-free fuel. Electrosynthesis of ammonia from nitrogen under ambient conditions presents an attractive alternative to the centralized Haber–Bosch process. Although lithium- and calcium-mediated nitrogen reduction (Li-NRR and Ca-NRR) show promise, long-term continuous ammonia electrosynthesis at high rates will be needed for industrial application. In this Perspective we argue that for Li-NRR and Ca-NRR to operate sustainably, the use of continuous-flow reactors—in which NRR is coupled with the hydrogen oxidation reaction, avoiding non-sustainable proton sources and electrolyte oxidation—is essential. Providing the necessary protons via hydrogen oxidation is vital for the sustainable production of ammonia and long-term system stability. We propose strategies such as optimizing the solid–electrolyte interphase design, refining the electrode and reactor engineering to enhance the system stability and ammonia production rate. We also strongly advocate the exploration of electrocatalytic routes for surpassing the theoretical energy efficiency limit of Li/Ca-NRR. Metal-mediated (for example, lithium) electrochemical synthesis of ammonia from N2 is a promising method to electrify the production of this crucial molecule. Here the authors discuss what is needed to make this a viable and sustainable approach.