(Invited) Electrosynthesis of Ammonia from Dinitrogen in Non-Aqueous Media

Since early 2018, the nitrogen reduction reaction to ammonia (NRR) has become a focus of active research as an approach to sustainable production of ammonia to support and eventually replace the century-old yet highly robust Haber-Bosch catalytic technology. More than one hundred reports on the successful NRR in aqueous electrolyte solutions catalysed by a comparatively wide range of materials have been published by the end of 2019, though the reported ammonia yield rates (<100 pmol s -1 cm -2 , per geometric surface area of the electrode) and faradaic efficiencies (< 20%) are typically low. In fact, the observed amounts of NH 3 produced in aqueous media are most often comparable to the level of adventitious nitrogen-based contaminants, thereby questioning the genuine nature of the reported NRR. The problems of the aqueous NRR, in the first place low faradaic efficiency, can be effectively addressed by employing aprotic electrolyte media for the electrochemical reduction of dinitrogen. 1-2 Under such conditions, the prevalence of the NRR over the competing and undesirable in this context hydrogen evolution reaction is suppressed due to the significantly higher solubility of N 2 than in water and controlled supply of the proton source. Ammonia electrosynthesis in organic media can be realised in at least two ways — either via direct electrocatalytic reaction, 3-4 or through a lithium-mediated process. 5-6 Both approaches have their pros and cons, and both are currently investigated in our groups. The talk will focus on some of the experimental challenges and pitfalls relevant to the non-aqueous NRR and on our recent progress in this area. References 1. Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonov, A. N.; MacFarlane, D. R., Challenges and prospects in the catalysis of electroreduction of nitrogen to ammonia. Nature Catal. 2019, 2 (4), 290-296. 2. Andersen, S. Z.; Čolić, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Nørskov, J. K.; Chorkendorff, I., A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504-508. 3. Zhou, F.; Azofra, L. M.; Ali, M.; Kar, M.; Simonov, A. N.; McDonnell-Worth, C.; Sun, C.; Zhang, X.; MacFarlane, D. R., Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 2017, 10 (12), 2516-2520. 4. Suryanto, B. H. R.; Kang, C. S. M.; Wang, D.; Xiao, C.; Zhou, F.; Azofra, L. M.; Cavallo, L.; Zhang, X.; MacFarlane, D. R., Rational Electrode–Electrolyte Design for Efficient Ammonia Electrosynthesis under Ambient Conditions. ACS Energy Lett. 2018, 3 (6), 1219-1224. 5. Tsuneto, A.; Kudo, A.; Sakata, T., Lithium-mediated electrochemical reduction of high pressure N 2 to NH 3 . J. Electroanal. Chem. 1994, 367 (1–2), 183-188. 6. McEnaney, J. M.; Singh, A. R.; Schwalbe, J. A.; Kibsgaard, J.; Lin, J. C.; Cargnello, M.; Jaramillo, T. F.; Nørskov, J. K., Ammonia synthesis from N 2 and H 2 O using a lithium cycling electrification strategy at atmospheric pressure. Energy Environ. Sci. 2017, 10 (7), 1621-1630.