More hydrogen, more ammonia, more fertilizer, all using less energy

Researchers led by Genki Kobayashi at the RIKEN Pioneering Research Institute (PRI) in Japan recently discovered a way to max out the amount of hydrogen that can be stored in perovskite crystalline powder. The trick is to introduce the hydrogen into the perovskite lattice structure using mechanochemistry – chemical reactions that occur by physically grinding and mixing compounds together. This process also affects the crystalline structure of the powder, making it an even better catalyst for producing ammonia. Because this process requires less energy than traditional non-mechanical methods, the discovery is eco-friendly and good for future sustainability. The findings were reported in the Journal of the American Chemical Society.

Scientists are currently striving to store hydrogen more efficiently for a variety of reasons, and one of the best mediums is a type of crystal called perovskite. Chemical reactions can be used to replace the oxygen ions in the crystalline powder with hydride (H-), turning it into a perovskite oxyhydride. Once hydrogen is stored in this way, it is easily transportable and can be used as a catalyst to create ammonia. As ammonia is the main ingredient in most fertilizers, is needed for many plastics, and is itself a type of hydrogen fuel, the perovskite oxyhydride power has numerous potential benefits. However, whether they use high temperature or high pressure, currently known chemical reactions only replace about 17% of the oxygen with hydride, meaning that the powder has the potential to store much more hydrogen than is currently possible.

The team led by Chief Scientist Kobayashi is researching ways to increase the hydrogen saturation limit from 17% and get more hydrogen into perovskite powder. Rather than using high temperatures or high-pressure techniques, they have been experimenting with physical mechanochemical reactions, which work well at room temperature and make them a more attractive option for maintaining the environment. Now, they have found a way to greatly increase hydrogen saturation, with twice as many oxygen ions in the crystalline structure being replaced by hydrides. This means that the new method virtually doubles the hydrogen-storage capacity of perovskite powder.

In the experiments, the researchers produced barium titanate oxyhydride in two ways: mechanochemically and topochemically. They found that the mechanochemical way–physically grinding and mixing the ingredients–had two advantages over the standard high-temperature method. First, the lattice structure of the crystalline powder contained more hydride. Second, even when taking pieces of each with the same number of hydrides, the mechanochemically produced version was a better catalyst; more ammonia was produced. Analysis showed that this was because the grinding process induced beneficial deformations in the lattice that high heat could not.

“This advancement is good news for environmental sustainability and will eventually help us achieve a real hydrogen-based economy,” says Kobayashi. In the short term, he says that their new findings provide valuable material design guidelines that will be useful in the development of new functional materials that contain hydride ions.

The new hydrogen saturation limit of 34% is likely the maximum that can be achieved using barium titanate, but even better results might be possible starting with another perovskite. “In the long term,” Kobayashi says, “our mechanochemical approach is expected to yield even better catalysts for ammonia synthesis, as well as materials for electrochemical devices such as fuel cells, a field in which the Kobayashi Laboratory specializes.”

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