UNSW 'artificial leaf' creates ammonia from wastewater and sunlight

Engineers from UNSW Sydney have converted a traditional silicon solar panel into a device that produces ammonia in a significantly more environmentally friendly way.

Ammonia is essential for producing fertilisers that support global agriculture and food production. However, the traditional way of making it creates large amounts of greenhouse gas emissions, since fossil fuels are required for the hydrogen production and the energy to power the high-temperature process.

UNSW Scientia Professor Rose Amal, collaborating with Professor Xiaojing Hao, and their teams have now developed a way to generate ammonium ions from nitrate-containing wastewater using only a specially designed solar panel that works like an artificial leaf. Their research has recently been published in a paper in the Journal of Energy and Environmental Science.

The process is known as photoelectrocatalytics (PEC) and utilises a nano-structured thin layer of copper and cobalt hydroxide on the panel that acts as a catalyst to assist the chemical reaction needed to produce ammonium nitrate from the wastewater.

The research team, which includes lead author of the paper Chen Han and Dr Jian Pan (a DECRA Fellow), have built a 40 cm² artificial leaf system on the roof of Tyree Energy Technologies building at UNSW which has been able to produce ammonium ions that can satisfy 1.49 m² of cropland.

The results encourage development of the size of the system in pursuit of creating ammonia without the associated greenhouse gas emissions.

“Traditional ammonia production requires high temperatures — around 400 to 500 degrees Celsius — and high pressure, historically necessitating the use of fossil fuels,” said Professor Amal from the School of Chemical Engineering. “This system works at ambient conditions and just uses sunlight to produce ammonium from nitrate-containing wastewater, which is an important chemical used in fertiliser.

“We think this new technology could be implemented on a relatively small scale in agricultural locations to produce ammonium onsite, which would decentralise the production process and further reduce CO₂ emissions that are associated with the transportation process.”

In a real leaf, photosynthesis is the way that plants use sunlight, water and carbon dioxide to create oxygen and energy in the form of sugar. In this new photoelectrocatalytic process, the solar panel is acting like an artificial leaf to use sunlight and nitrate-containing wastewater to create ammonium nitrate.

“We are combining photovoltaics’ expertise from UNSW’s School of Photovoltaics & Renewable Energy Engineering and our expertise in chemical engineering to take nitrate waste and turn it into an important commodity in the form of ammonia,” Han said. “We’ve developed a very efficient catalyst with some special nanostructures and incorporated that with a traditional silicon solar panel which results in a highly effective process.

“You do not need a high concentration of ammonia in fertiliser so we believe the amounts of ammonia we are producing using our system make it a viable application in the real world, although we definitely still have some ways to further improve it.”

Image: Artificial leaf system developed at UNSW to create ammonia from wastewater using only the sun. Credit: Chen Han/UNSW.