Recent Advances to Metal-Mediated Nitrogen Conversion to Ammonia

Introduction

Fig. 1: Schematic representation of Li-N2R in a continuous-flow electrolyzer. (Source: Y. Cornejo Carrillo, after Fu et al. [3])

Metal-mediated nitrogen reduction to ammonia (M-N₂R) has been gaining popularity as an alternative to the conventional Haber-Bosch process, which consumes around 1% of the world's energy supply and emits over 450 million tons of carbon dioxide (CO₂) every year. [1] The mechanism of M-N₂R operation relies on the electrodeposition of specific metal ions in solution, the formation of a stable metal-nitride (MxN) species, and the protonation of that intermediate to evolve ammonia (see Fig. 1). This process offers the opportunity to use electrical energy (from a sustainable source) to drive the reaction and produce ammonia (NH₃) in a decentralized manner. In this brief report, novel advances using lithium and calcium as mediators for ammonia synthesis will be discussed.

Lithium as Mediator

One of the most promising results when using lithium (Li) as the mediator comes from the work of Hoang-Long Du and co-workers. When using lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in tetrahydrofuran (THF) as the electrolyte and pressurizing nitrogen (N₂) to 15 bar, these researchers achieved a current-to-ammonia efficiency (also known as Faradaic efficiency, FE) close to 100% after 96 hours of continuous operation. [2] They attributed this high performance to the imide-rich electrode-electrolyte interface region which provides a compact ionic layering that controls the selectivity of species that can react with Li to yield NH₃.

Another more recent with Li-based electrolytes is the work by Xianbiao Fu and collaborators. They achieved a maximum NH₃ FE of 64% using a diglyme-based solvent with lithium tetrafluoroborate (LiBF₄) in a continuous-flow reactor. [3] And after 300 hours of operation, this system produced more than 4.6 g of ammonia (95% of that being in the gas phase) at room temperature and ambient pressure. Compared to previous publications using a similar electrolyte, the addition of a continuous source of protons via hydrogen oxidation reaction at the anode was a key factor to improve the stability of Li-N₂R. Moreover, by multiplying the cell voltage (5 V) by the total charge passed (120,000 C) and dividing it by the ammonia yield, this results in an energy requirement of approximately 13 MJ/kg_NH3.

Calcium as Mediator

The search for other metal species that can activate N₂ has led to the investigations of calcium (Ca) as the mediator. Not only is the fifth most prevalent element in the Earth's crust with an abundance of 4.7%, but Ca can also be electrodeposited at a lower energy input compared to Li (-2.87 V_RHE vs -3.04 V_RHE). [4] In one recent study that tested a continuous NH₃ synthesis using an electrolyte containing calcium tetrakis(hexafluoroisopropyloxy)borate, or Ca[B(hfip)₄]₂, dissolved in THF, the resulting NH₃ FE was 40% under ambient conditions. [5] Although the performance for Ca is lower than for Li-based electrolytes, these findings further motivate the improvement of this system via studies and engineering of the Ca solid electrolyte interphase (SEI) layer.

Outlook

The M-NRR field is still in the early stages of development and commercialization for decentralized ammonia production at ambient conditions. In both systems, one critical parameter to consider is the stability of the solvent during operation to establish continuous NH₃ production. Nevertheless, all of these findings motivate exploration of other metal alternatives to lithium (such as magnesium and titanium) to reach cost-effective and scalable technologies (see Table 1).

© Yamile Cornejo Carrillo. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] X. Cai et al., "Lithium-Mediated Electrochemical Nitrogen Reduction: Mechanistic Insights to Enhance Performance," iScience. 24, 103105 (2021).

[2] H. L. Du et al., "Electroreduction of Nitrogen With Almost 100% Current-to-Ammonia Efficiency," Nature 609, 722 (2022).

[3] X. Fu et al, "Continuous-Flow Electrosynthesis of Ammonia By Nitrogen Reduction and Hydrogen Oxidation," Science. 379, 707 (2023).

[4] R. Aversa et al.,' "The Basic Elements of Life," Am. J. Eng. Appl. Sci. 9, 1189 (2017).

[5] X. Fu et al., "Calcium-Mediated Nitrogen Reduction For Electrochemical Ammonia Synthesis," Nat. Mater. 23, 101 (2024).

[6] J. M. McEnaney et al, "Ammonia Synthesis From N₂ and H₂O Using a Lithium Cycling Electrification Strategy at Atmospheric Pressure," Energy Environ. Sci. 10, 1621 (2017).