The nitrogen reduction reaction to ammonia is of vital importance to humanity as a source of most manufactured fertilisers. However, it is also a source, currently, of around 1.5% of global greenhouse gas emissions. While green hydrogen as a replacement for the steam-methane reforming reaction is a viable approach to producing green ammonia, issues of scale and intermittency mean that the path towards this solution is fraught. On the other hand, the direct electrochemical approach to the reduction of nitrogen offers, in principle, a route to ammonia at various levels of scale and without any sensitivity to intermittency. For this reason the Nitrogen Reduction Reaction (NRR) has attracted intense activity in the last five years or so.¹

However, the obvious and appealing aqueous electrolyte approach to this reaction suffers from serious competition from the parallel proton reaction to hydrogen reaction, meaning that the Faradaic Efficiency (or selectivity) of the NRR process is typically quite low. In many reports the yield is also so low that there is a major issue with contamination as a source of the ammonia,² leading to concerns that many reports are actually false positives. The situation has been reviewed by us and others recently and protocols for reliable results proposed. ^3-5 The first part of this talk will summarise and update on this unfortunate situation.

On the other hand, the relatively well known Li-mediated approach to the NRR appears to be making progress towards practical levels of selectivity and rate. This electrolysis reaction uses aprotic solvent media and thus intrinsically avoids major competition from H₂ evolution. Recent work from our group⁶ has shown that phosphonium salts offer an appropriately low degree of proton acidity such that they can act as excellent recycling proton carriers for this process, allowing us to achieve ~70% Faradaic Efficiency and rates above 50 nmol s^-1cm^-2. This proton acidity occurs via the well known phosphonium <-> ylide transformation, as shown by extensive NMR studies of the process. This section of the talk will discuss this proton carrier mechanism, as well as present an overview of very recent work in our laboratories that has allowed us to closely approach 100% Faradaic Efficiency in the Li mediated NRR process. All of the work reported from our laboratories is based on rigorous controls to address contamination from NOx sources and is verified by calibrated ¹⁵NH₃ experiments.

1. MacFarlane, D. R.; Cherepanov, P. V.; Choi, J.; Suryanto, B. H. R.; Hodgetts, R. Y.; Bakker, J. M.; Ferrero Vallana, F. M.; Simonov, A. N., A Roadmap to the Ammonia Economy. Joule 2020, 4, 1186-1205.
2. Choi, J.; Du, H.-L.; Nguyen, C. K.; Suryanto, B. H. R.; Simonov, A. N.; MacFarlane, D. R., Electroreduction of Nitrates, Nitrites, and Gaseous Nitrogen Oxides: A Potential Source of Ammonia in Dinitrogen Reduction Studies. Acs Energy Letters 2020, 5, 2095-2097.
3. Choi, J.; Suryanto, B. H. R.; Wang, D.; Du, H.-L.; Hodgetts, R. Y.; Ferrero Vallana, F. M.; MacFarlane, D. R.; Simonov, A. N., Identification and Elimination of False Positives in Electrochemical Nitrogen Reduction Studies. Nature Communications 2020, 11, 5546.
4. 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 Catalysis 2019, 2, 290–296.
5. Andersen, S. Z., et al., A Rigorous Electrochemical Ammonia Synthesis Protocol with Quantitative Isotope Measurements. Nature 2019, 570, 504-508.
6. Suryanto, B. H. R.; Matuszek, K.; Choi, J.; Hodgetts, R. Y.; Du, H.-L.; Bakker, J. M.; Kang, C. S. M.; Cherepanov, P. V.; Simonov, A. N.; MacFarlane, D. R., Nitrogen Reduction to Ammonia at High Efficiency and Rates Based on a Phosphonium Proton Shuttle. Science 2021, 372, 1187-1191.