The consequences of electrolyte engineering in electrochemical nitrogen reduction to ammonia

Anthropogenic nitrogen fixation and ammonia-derived fertiliser have fed the growth of society. However, the nature and economies of scale of the Haber-Bosch process made it a major source of energy consumption and greenhouse gas emissions, while benefiting only a portion of the global population. Electrochemical ammonia production holds potential for decarbonisation and decentralisation. Aqueous electrolysers are likely impractical due to the intense competition between nitrogen reduction, and proton reduction to hydrogen. Non-aqueous systems circumvent this problem, with a lithium-mediated approach having been rigorously validated to reduce nitrogen at viable rates. In this system, in situ reduced metallic lithium reacts with nitrogen and protons to produce ammonia. Alike battery anodes, lithium reacts with its electrolyte, forming a passivation layer (the solid electrolyte interphase or SEI), which is thought to prompt the selectivity of this reaction. This thesis examines and addresses the significant energy burden required to maintain the active lithium metal surface through the lens of electrolyte engineering. A true reference electrode was developed to measure this energy burden and elucidate the role of the electrolyte in mitigating it. Electrolytes were formulated based on this understanding, and the interplay between electrolyte properties, SEI chemistry and selectivity was illustrated with overlapping non-linear relations. Such relations can be translated into a linear correlation between the selectivity of nitrogen reduction to ammonia and the activity of Li-ions and protons. While providing descriptors for identifying higher-performing electrolytes, it highlighted that significant energy efficiency gains can only be achieved by breaking free from lithium. Guided by simulations, new electrolytes based on promising alkali metals such as Ca and Mg were investigated to achieve nitrogen fixation beyond lithium. The fundamental insights gained from these studies are expected to enhance the alkali metal chemistries and help overcome their energy bottleneck, toward sustainable and affordable ammonia production.