(Digital Presentation) Towards Li-Mediated Nitrogen Reduction Reaction at High Current-to-Ammonia Efficiency

Abstract

Apart from its role as a main precursor of the synthetic fertiliser production, ammonia is likely to become an even more important chemical in the future due to its prospects as an energy carrier.(1) To meet the associated growing demand while meeting the CO2 emission reductions targets, recent efforts have aimed to translate the conventional polluting NH3 synthesis to low CO2 emissions technology by supplying the Haber-Bosch reactors with H2 derived from renewable-powered water electrolysis rather than steam methane reforming.(2) This solution will likely to be implemented in many NH3 synthesis plants soon, but the most preferred future technology for green ammonia synthesis is through a 100% renewable-powered, one step electrochemical process involving nitrogen reduction at the cathode and water oxidation at the anode. However, the possibility of direct dinitrogen reduction with traditional heterogeneous catalysts in both aqueous and aprotic electrolyte solutions at practical rates and faradaic efficiencies is yet to be proven,(3, 4) which calls for investigations of alternative pathways. One prominent possibility, with clear practical prospects, is the indirect lithium redox mediated process in organic electrolytes, which has been evidenced as genuine and by far the most efficient process to electrochemically convert N2 to NH3.(5) However, several key challenges, including insufficiently high yield rate and faradaic efficiency as well as instability of the system, are yet to be resolved before the lithium-mediated nitrogen reduction reaction (Li-NRR) can be considered a process of applied significance. Our first step towards this aim was to introduce a stable phosphonium cation/ylide proton shuttle that delivers protons to the cathode to support rapid and controllable conversion of lithium nitride into ammonia.(6) It was demonstrated that phosphonium cation provides favourable proton activity in the system, which enables high Li-NRR faradaic efficiency of 69 ± 1 %. Moreover, the phosphonium shuttle exhibits high stability under the reaction conditions, i.e. was genuinely able to cycle between anode and cathode without being consumed.Our further efforts have focussed on understanding the effects of the electrolyte-electrode environment on the Li-NRR kinetics. As a result of this investigation, a system configuration that enables the electroreduction of N2 to ammonia at a faradaic efficiency closely approaching 100% was discovered. Moreover, the synthesis can run in both uninterrupted and interrupted regimes on a timescale of days. The rate of ammonia electrosynthesis in the Li-NRR with the optimised electrode-electrolyte interface can achieve as high as ca 500 nmol s-1 cm-2 (per geometric electrode surface area) with electrodes of cm2 scale.Finally, we examined the degradation of the electrolyte solution components during the high-performance Li-NRR. It was found that the major source of these undesired processes are the anode processes rather than the cathode. This highlights the urgent need for the development of effective H2- or H2O-feed anodes that can be integrated with the lithium mediated process. This and other future challenges on our way towards achieving sustainable and stabile NH3 electrosynthesis system to support the development of the Ammonia Economy will be highlighted. D. R. MacFarlane, J. Choi, B. H. R. Suryanto, R. Jalili, M. Chatti, L. M. Azofra and A. N. Simonov, Adv. Mater., 32, 1904804 (2020).D. R. MacFarlane, P. V. Cherepanov, J. Choi, B. H. R. Suryanto, R. Y. Hodgetts, J. M. Bakker, F. M. Ferrero Vallana and A. N. Simonov, Joule, 4, 1186 (2020).H.-L. Du, T. R. Gengenbach, R. Hodgetts, D. R. MacFarlane and A. N. Simonov, ACS Sustainable Chem. Eng., 7, 6839 (2019).H.-L. Du, R. Y. Hodgetts, M. Chatti, C. K. Nguyen, D. R. Macfarlane and A. N. Simonov, J. Electrochem. Soc., 167, 146507 (2020).B. H. R. Suryanto, H.-L. Du, D. Wang, J. Chen, A. N. Simonov and D. R. MacFarlane, Nat. Catal., 2, 290 (2019).B. H. Suryanto, K. Matuszek, J. Choi, R. Y. Hodgetts, H.-L. Du, J. M. Bakker, C. S. Kang, P. V. Cherepanov, A. N. Simonov and D. R. MacFarlane, Science, 372, 1187 (2021).