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Humanity has an insatiable appetite for ammonia: This substance is used to make fertilizer, which in turn is used in most modern agriculture. Until now, the Haber-Bosch process has been the method of choice for extracting nitrogen from the seemingly inexhaustible atmosphere and binding it in the form of ammonia. However, this method requires an extremely large amount of methane gas and energy.

Prof. Nikolay Kornienko from the University of Bonn has discovered a more climate-friendly alternative for producing ammonia from renewable energy sources. The findings are in Nature Communications.

Like in the Garden of Eden, grain, beets, and potatoes should sprout as luxuriantly as possible so that plates are well filled. This is ensured by regular fertilization—especially with nitrogen. A source of nutrients that seems to never run dry. At the beginning of the 20th century, Fritz Haber and Carl Bosch developed a process that extracts nitrogen from the seemingly inexhaustible air. This achievement earned them the Nobel Prize in Chemistry in 1918.

Using an iron-based catalyst, very high pressure, and temperatures of up to 500 degrees Celsius, the Haber-Bosch process binds nitrogen from the air to hydrogen, producing ammonia. As an aside, some plants also master the art of binding atmospheric nitrogen with tiny bacteria in their roots and making it available for their growth. However, green plants do this in a climate-neutral way, whereas humans have not yet managed to do so.

"The Haber-Bosch process is extremely energy-intensive," says Prof. Dr. Nikolay Kornienko from the Institute of Inorganic Chemistry at the University of Bonn. Ammonia production is based predominantly on fossil fuels, which means that greenhouse gas emissions are correspondingly high.

"In order to achieve the goal of a sustainable and climate-neutral society, the search for alternative ammonia synthesis processes is a priority," says Kornienko, who is also a member of the transdisciplinary research area "Matter" at the University of Bonn.

Nitrogen fertilizer from sun and wind

Alternative methods? These have been experimented with for some time. The aim is to replace the Haber-Bosch ammonia synthesis with a process that uses renewable energy from sources such as the sun and wind. The hydrogen required would then no longer come from methane gas, but would be obtained directly from the electrical splitting of water (H₂O) into hydrogen (H₂) and oxygen (O₂).

Sounds simple? It's not. Anyone who wants to produce ammonia on a large scale using wind and solar power has to navigate a number of pitfalls in the chemical reaction pathways.

"The lithium-mediated nitrogen reduction reaction (LiNRR) is considered the most robust way to electrify ammonia synthesis," says Hossein Bemana, the lead author of the study. In this system, lithium ions (Li⁺) are electrochemically reduced to a lithium metal layer. This lithium metal can then react with nitrogen gas (N₂) to form a lithium-nitrogen compound.

If a hydrogen source is available, the lithium-nitrogen compound is converted into ammonia (NH₃) and dissolved lithium ions. Then the process starts all over again. That's the theory, at least.

"We generally view this system as a model for the time being, as there are several practical difficulties," says Kornienko. Because high voltage is required to reduce lithium ions to metallic lithium, energy efficiency is limited to around 25%.

In addition, the system must operate in an air- and water-free environment, as lithium metal is highly reactive. Another challenge is that, similar to batteries, a porous solid electrolyte interphase (SEI) grows on the lithium layer. This layer must allow nitrogen gas and hydrogen to pass through as reactants to the lithium.

The wrong thing is sacrificed

Ideally, the hydrogen would come directly from the splitting of water. However, in this system, alcohols are usually used as the hydrogen source. In some cases, the solvent also decomposes and then serves as a hydrogen source itself. "This makes the system impractical, as several alcohol or solvent molecules have to be sacrificed to produce ammonium," says the chemist.

However, the researchers have found a way to extract hydrogen directly from the splitting of water and transfer it to nitrogen. They used a palladium (Pd) foil as both an electrode and a membrane. "Palladium can serve as a membrane because it allows hydrogen atoms to pass through," reports Kornienko.

In the experiment, the Pd foil separated an anhydrous reaction environment, in which the LiNRR reactions take place, from a water-based reaction environment. "In the end, we were able to extract hydrogen atoms electrochemically directly from the water and transfer them to the reactive lithium/lithium-nitrogen material to produce ammonia," says the chemist.

The researchers used infrared spectroscopy and mass spectrometry to verify that this really works as intended. They used a heavy isotope of hydrogen (deuterium = D) as a water source and produced ND₃ instead of NH₃. Conversely, the researchers labeled all molecules in the LiNRR compartment with D instead of H—as desired, NH₃ was produced in this case and not ND₃ as before.

Bemana and Kornienko have already filed a patent application for this process. The research team used only electricity for its experiments to produce ammonia (NH₃). However, there is still a long way to go before the desired nitrogen fertilizer can be produced economically from renewable energy sources. To achieve this, scientists would have to achieve a yield 1,000 times greater than in their current experiments. "We are still in the early stages," says the chemist. "In general, research needs to be done on the reaction rates and selectivity of the system—the control of electrons to the desired target."