Researchers at Monash University in Australia have developed an electrochemical process for the conversion of nitrogen to ammonia with nearly 100% current-to-ammonia efficiency. A paper on their work is published in the journal Nature.

Beyond its use in the fertiliser and chemical industries, ammonia is currently seen as a potential replacement for carbon-based fuels and as a carrier for worldwide transportation of renewable energy. Implementation of this vision requires transformation of the existing fossil fuel based technology for NH₃ production to a simpler, scale-flexible technology, such as the electrochemical lithium-mediated nitrogen reduction reaction (Li-NRR).

This provides a genuine pathway from N₂ to ammonia, yet is hampered by limited yield rates and efficiencies. Here we investigate the role of the electrolyte in this reaction and present a high-efficiency, robust process enabled by compact ionic layering in the electrode-electrolyte interfacial region.

—Du et al.

There are several means of producing ammonia, but the Haber-Bosch process remains the most prevalent, accounting for about 90% of total production. Haber-Bosch and the other processes involved in industrial-scale production require high temperatures (more than 400 °C) and high pressure (more than 150 bar). Those conditions are needed to break the strong bonds in nitrogen and react with hydrogen to form ammonia (NH₃). (Earlier post.)

These processes, taking up around 1% of global energy consumption, are largely fossil fuel-based. Hence, ammonia is the most greenhouse gas-intensive chemical-making reaction globally, totalling roughly 1.5% of total global CO₂ emissions.

The Monash team improved an electrochemical, lithium-mediated N₂ reduction reaction. In lithium-mediated nitrogen reduction, lithium ions are reduced to lithium metal, which spontaneously reacts with nitrogen to form lithium nitride (Li₃N). The Li₃N then reacts with a proton source to form ammonia to make NH₃.

However, a significant portion of the current can cause other reactions—auch as Li metal deposition onto the electrode and the reductive degradation of the electrolyte.

The Monash team switched to a high-concentration imide-based lithium salt electrolyte (bis(trifluoromethylsulfonyl)imide) that shut off the unwanted side reactions, enabling stabilized ammonia yield rates of 150±20 nmol s^-1 cm^-2 and current-to-ammonia efficiency closely approaching 100%.

The ionic assembly formed at the electrode surface suppresses electrolyte decomposition and supports stable N₂ reduction. Our study highlights the interrelation between the performance of the Li-NRR and the physicochemical properties of the electrode-electrolyte interface. We anticipate that these findings will guide the development of a robust, high-performance process for sustainable ammonia production.

—Du et al.

Resources

• Du, HL., Chatti, M., Hodgetts, R.Y. et al. (2022) “Electroreduction of nitrogen at almost 100% current-to-ammonia efficiency.” Nature doi: 10.1038/s41586-022-05108-y

• Nikifar Lazouski, Zachary J. Schiffer, Kindle Williams, Karthish Manthiram (2019) “Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction,” Joule, doi: 10.1016/j.joule.2019.02.003