Shuttling protons in ammonia synthesis: An electrochemical route to ammonia could substantially lower the greenhouse gas emissions associated with the current thermal Haber-Bosch process. One relatively promising option under study involves reductive formation of lithium nitride, which can be protonated to ammonia. However, the ethanol used to date as a local proton source in these studies may degrade under the reaction conditions. Suryanto et al. report the use of a tetraalkyl phosphonium salt in place of ethanol (see the Perspective by Westhead et al. ). This cation can stably undergo deprotonation–reprotonation cycles and, as an added benefit, it enhances the ionic conductivity of the medium. Science , abg2371, this issue p. 1187 ; see also abi8329, p. 1149

Ionic liquids and their various analogues are without doubt the scientific sensation of the last few decades, paving the way to a more sustainable society. Their versatile suite of properties, originating from an almost inconceivably large number of possible cation and anion combinations, allows tuning of the structure to serve a desired purpose. Ionic liquids hence offer a myriad of useful applications from solvents to catalysts, through to lubricants, gas absorbers, and azeotrope breakers. The purpose of this review is to explore the more unexpected of these applications, particularly in the energy space. It guides the reader through the application of ionic liquids and their analogues as i) phase change materials for thermal energy storage, ii) organic ionic plastic crystals, which have been studied as battery electrolytes and in gas separation, iii) key components in the nitrogen reduction reaction for sustainable ammonia generation, iv) as electrolytes in aluminum‐ion batteries, and v) in other emerging technologies. It is concluded that there is tremendous scope for further optimizing and tuning of the ionic liquid in its task, subject to sustainability imperatives in line with current global priorities, assisted by artificial intelligence.

Electrochemical lithium-mediated nitrogen reduction can enable synthesis of ammonia from renewables in a distributed fashion on various scales, but its integration into electrolyser devices presents an ongoing challenge, in particular due to the lack of understanding of the interrelation between the performance and the proton transport parameters. Herein, we use a top-performance N₂ electroreduction system with lithium bis(trifluoromethylsulfonyl)imide electrolyte to correlate the reaction metrics to the properties of the proton carrier, focusing on alcohols, a phosphonium cation, tetrahydrofuran, a Brønsted acid, ammonium and water. We demonstrate that productive carriers require optimised electrolyte compositions, which define the interplay of key reaction steps. Through this understanding, we achieved ammonia electrosynthesis with the phosphonium cation and iso-propanol at performance metrics close to those provided by the ethanol benchmark. Critically, we demonstrate that the latter undergoes irreversible degradation through reaction with oxidised solvent, which is not the case for the more robust iso-propanol and phosphonium cation proton carriers.

Aiming at inexpensive Brønsted-acidic ionic liquids, suitable for industrial-scale catalysis, a family of protonic ionic liquids based on nitrogen bases and sulfuric acid has been developed. Variation of the molar ratio of sulfuric acid, χ H₂SO₄ , was used to tune acidity. The liquid structure was studied using ¹H NMR and IR spectroscopies, revealing the existence of hydrogen-bonded clusters, [(HSO₄)(H₂SO₄)]⁻, for χ H₂SO₄ > 0.50. Acidity, quantified by Gutmann Acceptor Number (AN), was found to be closely related to the liquid structure. The ionic liquids were employed as acid catalysts in a model reaction; Fischer esterification of acetic acid with 1-butanol. The reaction rate depended on two factors; for χ H₂SO₄ > 0.50, the key parameter was acidity (expressed as AN value), while for χ H₂SO₄ > 0.50 it was the mass transport (solubility of starting materials in the ionic liquid phase). Building on this insight, the ionic liquid catalyst and reaction conditions have been chosen. Conversion values of over 95% were achieved under exceptionally mild conditions, and using an inexpensive ionic liquid, which could be recycled up to eight times without diminution in conversion or selectivity. It has been demonstrated how structural studies can underpin rational design and development of an ionic liquid catalyst, and in turn lead to a both greener and economically viable process.