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