A technology has been developed that can produce highly efficient ammonia using only solar energy.

UNIST (President Yong-Hoon Lee) announced on the 16th that the research team of Professors Ji-Wook Jang and Seong-Yeon Jang of the Department of Energy and Chemical Engineering, in collaboration with Professor Thomas F. Jaramillo of Stanford University, has developed an eco-friendly ammonia production photoelectrode system based on perovskite.

It exceeded the commercialization standard for eco-friendly ammonia production speed set by the U.S. Department of Energy by about 1.7 times. It is the world's highest record.

The principle is to reduce nitrate (NO3-), a major component of wastewater, in water to produce ammonia (NH3) using solar energy. The developed system has recorded the world's highest solar-to-ammonia conversion speed and has overcome the weakness of perovskite, which is weak to water.

Ammonia, an industrial raw material, is used to synthesize high value-added compounds such as fertilizers, foods, and pharmaceuticals. Currently, most ammonia is produced by the 'Haber-Bosch process'. On the other hand, there is the problem that it is not environmentally friendly and requires the consumption of a huge amount of fossil fuels.

The research team developed a photoelectrode system with high performance and durability to protect perovskite solar cells and a high-performance ammonia production catalyst by placing ruthenium on titanium nanosheets.

Perovskite can absorb light well and generate a lot of charges, but it is easily decomposed in water. The research team protected the perovskite with a Fields metal that easily turns into liquid, and strongly bound it to a high-performance ammonia production catalyst. Fields metal has a low melting temperature of 63 degrees, so it melts easily and becomes solid at room temperature, making this design possible.

The fabricated photoelectrode prevents direct contact of the perovskite with water. The perovskite and the ammonia production catalyst are electrically connected and fixed. The photoelectrode, which receives light in water, generates charges. The charges are efficiently transferred to the ammonia production catalyst exposed on the electrode surface, and ammonia is stably produced with high efficiency.

The research team used glycerol, which reacts at a lower voltage than water, to produce ammonia without an external voltage. First, they placed a platinum catalyst on a titanium nanosheet to increase the oxidation reaction rate of glycerol.

The voltage produced by the photoelectrode and the minimum voltage required for ammonia conversion were matched through the oxidation reaction of glycerol. In other words, if the photoelectrode is immersed in water with glycerol added and exposed to light, ammonia can be produced spontaneously. At the same time as ammonia production, glyceric acid, which is nine times more valuable than glycerol, was also produced as a byproduct.

The ammonia production rate using the developed photoelectrode was recorded at a maximum of 1,745 μgNH3 cm⁻²h⁻¹. This far exceeds the 1,000μgNH3 cm⁻²h⁻¹ standard for commercialization of eco-friendly ammonia production speed set by the U.S. Department of Energy.

By changing the nitrate reduction catalyst used in the developed system to a different type, it can also be used for the production of various high value-added materials.

Professor Jang Ji-wook explained, “Through this research, we produced nitrate, a main component of wastewater, glycerol, a byproduct of biodiesel, ammonia, and high value-added glyceric acid.” He continued, “The developed technology can produce highly efficient ammonia without external voltage, and can also be used to produce various eco-friendly fuels.”

Professor Jang Seong-yeon explained, “This study is very important in that it is applied to the production of high-efficiency solar fuel,” and “It is very significant in that it exceeds the solar fuel production speed standard for commercialization of solar fuel.”

The results of this study were published on April 1 in Nature Catalysis, the top journal in catalyst research. Postdoctoral researcher Ahmad Tayyebi, Rashmi Mehrotra, Muhibullah Al Mubarok, and Ji-Eun Kim, a graduate student in the UNIST Integrated Masters and Doctoral Program, participated as co-first authors.

The research was conducted with the support of the Brainlink Project for Excellent Researchers and the Global Basic Research Laboratory Support Project (BRL) of the National Research Foundation of Korea, Ministry of Science and ICT.