Using lightning to make ammonia out of thin air

University of Sydney researchers have harnessed human-made lightning to develop a more efficient method of generating ammonia – one of the world’s most important chemicals. Ammonia is also the main ingredient of fertilisers that account for almost half of all global food production.

The team have successfully developed a more straightforward method to produce ammonia (NH₃) in gas form. Previous efforts by other laboratories produced ammonia in a solution (ammonium, NH₄⁺), which requires more energy and processes to transform it into the final gas product.

The current method to generate ammonia, the Haber-Bosch process, comes at great climate cost, leaving a huge carbon footprint. It also needs to happen on a large scale and close to sources of cheap natural gas to make it cost-effective.

The chemical process that fed the world, and the Sydney team looking to revolutionise it

Naturally occurring ammonia (mostly in the form of bird droppings), was once so high in demand it fuelled wars.

The invention of the Haber-Bosch process in the 19th century made human-made ammonia possible and revolutionised modern agriculture and industry. Currently 90 percent of global ammonia production relies on the Haber-Bosch process.

“Industry’s appetite for ammonia is only growing. For the past decade, the global scientific community, including our lab, wants to uncover a more sustainable way to produce ammonia that doesn’t rely on fossil fuels.

“Currently, generating ammonia requires centralised production and long-distance transportation of the product. We need a low-cost, decentralised and scalable ‘green ammonia’,” said lead researcher Professor PJ Cullen from the University of Sydney’s School of Chemical and Biomolecular Engineering and the Net Zero Institute. His team has been working on ‘green ammonia’ production for six years.

“In this research we’ve successfully developed a method that allows air to be converted to ammonia in its gaseous form using electricity. A huge step towards our goals.”

The research was published in AngewandteChemie International edition.

Ammonia contains three hydrogen molecules, meaning it can be used as an effective carrier and source of hydrogen as an energy source, even potentially as an effective means of storing and transporting hydrogen. Industry bodies have found they can access the hydrogen by ‘cracking’ ammonia to separate the molecules to use the hydrogen.

Ammonia is also a strong candidate for use as a carbon-free fuel due to its chemical make-up. This has caught the interest of the shipping industry which is responsible for about 3 percent of all global greenhouse gas emissions.

Cracking a chemical conundrum

Professor Cullen’s team’s new method to generate ammonia works by harnessing the power of plasma, by electrifying or exciting the air.

But the star is a membrane-based electrolyser, a seemingly non-descript silver box, where the conversion to gaseous ammonia happens.

During the Haber-Bosch process, ammonia (NH₃) is made by combining nitrogen (N₂) and hydrogen (H₂) gases under high temperatures and pressure in the presence of catalyst (a substance that speeds up a chemical reaction).

The plasma-based method Professor Cullen’s team developed uses electricity to excite nitrogen and oxygen molecules in the air. The team then passes these excited molecules to the membrane-based electrolyser to convert the excited molecules to ammonia.

The researchers said this is a more straightforward pathway for ammonia production.

Professor Cullen said the findings signal a new phase in making green ammonia possible. The team is now working on making the method more energy efficient and competitive compared to the Haber-Bosch process.

“This new approach is a two-step process, namely combining plasma and electrolysis. We have already made the plasma component viable in terms of energy efficiency and scalability.

“To create a more complete solution to a sustainable ammonia productive, we need to push the energy efficiency of the electrolyser component,” Professor Cullen said.

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