Understanding Electrochemical Ammonia Production: Ramblings of a Perplexed Electrochemist

Ammonia is the most essential chemical commodity globally, with prospects for even greater significance in the future. For over a century, it has been produced via the Haber-Bosch process. Today, several alternative production strategies are being researched, and this thesis focuses on the electrochemical approach, particularly the indirect method, which utilises electroplated lithium or calcium in non-aqueous electrolytes. Research in this area could also have implications for fields such as battery technology and organic electrosynthesis.
In this thesis, a fundamental framework of electrochemistry is presented. The electrochemical system, including the electrode-electrolyte interface, is described, followed by an in-depth discussion of the thermodynamics, kinetics, and mass transport processes that govern electrochemical reactions. In the subsequent chapter, the focus shifts to various reference electrodes. Their application is less well studied and understood in non-aqueous electrolytes, and given the importance of potential in electrochemical systems, gaining this understanding is crucial. It is concluded that the most suitable reference electrodes for lithium-mediated nitrogen reduction reactions are based on ferrocene or lithium iron phosphate, a common battery material.
On the cathode side, the lithium-mediated nitrogen reduction reaction system is explored. Initial emphasis is placed on the challenges posed by the high levels of impurities in the system, complicating result interpretation. The interface is then characterised, with a discussion on reactions and double-layer formation preceding lithium plating. A conceptualisation of the formation and composition of the solid electrolyte interphase is provided, supported by various techniques, including linear sweep voltammetry, potentiostatic electrochemical impedance spectroscopy, electrochemical quartz crystal microbalance, in operando mass spectrometry, and in operando grazing-incidence wide-angle X-ray scattering. Finally, the parameters related to the performance of a lithium-mediated nitrogen reduction reaction system are discussed, elucidating how proton- or anode-induced solvent oxidation are key factors for long-term electrolyte stability. Anode-induced solvent oxidation can be mitigated by employing diglyme and the hydrogen oxidation reaction. However, proton-induced oxidation presents a more challenging issue, leading to an investigation of the hydrogen oxidation reaction in non-aqueous electrolytes. Interpretation is complicated by both inherent solvent impurities and those generated during the experiment. It is demonstrated that hydrogen oxidation is possible without the addition of a proton shuttle, though it becomes energetically more efficient in its presence, and potentially resulting in a more sustainable system.
Finally, alternative strategies beyond lithium- or calcium-mediated nitrogen reduction are proposed. These include the ”true” nitrogen reduction reaction, which is argued to occur in liquid ammonia, analogous to the oxygen reduction reaction in water. The thesis discusses how nitrogen reduction may be achieved in aqueous media through electrochemical control of interfacial reaction dynamics and proton flux.