Chemical engineers at UNSW Sydney and University of Sydney have developed a hybrid plasma electrocatalytic process for the production of sustainable (“green”) ammonia. The method makes green ammonia from air, water and renewable electricity and does not require the high temperatures, high pressure and huge infrastructure currently needed to produce this essential compound.

Traditional production of ammonia via the Haber-Bosch process consumes about 2% of the world’s energy and accounts for 1% of the industrial world’s carbon dioxide emissions.

The new production method—demonstrated in a laboratory-based proof of concept—also has the potential to play a role in the global transition towards a hydrogen economy, in which ammonia is seen as a possible solution to the problem of storing and transporting hydrogen energy.

Hybrid plasma-electrochemical technology for green ammonia production. Schematics illustrating (A) the Haber-Bosch process and (B) a novel ammonia synthesis approach via NO_x intermediaries. Non-thermal plasma activates water and air, producing NO_x dissolved in solution as an intermediary for ammonium’s electrochemical synthesis. Sun et al.

In an open-access paper published in Energy and Environmental Science, the authors from UNSW and University of Sydney note that ammonia synthesis was one of the critical achievements of the 20^th century. When used in fertilizers that quadrupled the output of food crops, it enabled agriculture to sustain an ever-expanding global population.

But since the beginning of the 1900s when it was first manufactured on a large scale, production of ammonia has been energy intensive—requiring temperatures higher than 400 ˚C and pressures greater than 200 atm—and all powered by fossil fuels.

Dr Emma Lovell, a co-author on the paper from UNSW’s School of Chemical Engineering, says the Haber-Bosch process is only cost-effective when produced on a massive scale due to the huge amounts of energy and expensive materials required. “The current way we make ammonia via the Haber-Bosch method produces more CO₂ than any other chemical-making reaction,” she says.

Dr Lovell says that in addition to the big carbon footprint left by the Haber-Bosch process, having to produce millions of tonnes of ammonia in centralized locations means even more energy is required to transport it around the world, not to mention the hazards that go with storing large amounts of it in the one place. She and her colleagues therefore looked at how to produce it cheaply, on a smaller scale and using renewable energy.

The way that we did it does not rely on fossil fuel resources, nor emit CO₂. And once it becomes available commercially, the technology could be used to produce ammonia directly on site and on demand—farmers could even do this on location using our technology to make fertilizer—which means we negate the need for storage and transport. And we saw tragically in Beirut recently how potentially dangerous storing ammonium nitrate can be.

So if we can make it locally to use locally, and make it as we need it, then there’s a huge benefit to society as well as the health of the planet.

—Dr Lovell

ARC DECRA Fellow and co-author Dr Ali (Rouhollah) Jalili says trying to convert atmospheric nitrogen (N₂) directly to ammonia using electricity “has posed a significant challenge to researchers for the last decade, due to the inherent stability of N₂ that makes it difficult to dissolve and dissociate”.

Dr Jalili and his colleagues devised proof-of-concept lab experiments that used plasma to convert air into NO_x intermediaries: either NO₂^- (nitrite) or NO₃^- (nitrate). The nitrogen in these compounds is much more reactive than N₂ in the air.

Once we generated that intermediary in water, designing a selective catalyst and scaling the system became significantly easier. The breakthrough of our technology was in the design of the high-performance plasma reactors coupled with electrochemistry.

—Dr Jalili

The NO_x intermediaries were converted to ammonia at a rate of 23.2 mg/h (42.1 nmol/cm²s), using a scalable electrolyzer operating at a low cell voltage of 1.4 V, current densities of over 50 mA/cm², and specific energy consumption of 0.51 kWh/mol NH₃.

The team will next turn its attention to commercializing this breakthrough, and is seeking to form a spin-out company to take its technology from laboratory-scale into the field.

Resources

• Jing Sun, David Alam, Rahman Daiyan, Hassan Masood, Tianqi Zhang, Renwu Zhou, Patrick Cullen, Emma Catherine Lovell, Ali Rouhollah Jalili and Rose Amal (2021) “A hybrid plasma electrocatalytic process for sustainable ammonia production” Energy Environ. Sci. doi: 10.1039/D0EE03769A