Crystalline solid could function as a hydrogen carrier by adsorbing and releasing ammonia

All around the world, scientists are striving towards next-generation energy technologies that can help us move away from fossil fuels. Using hydrogen as an energy carrier and clean energy source is perhaps one of the most promising solutions on the horizon.

However, there is a major challenge to overcome before hydrogen economies become a reality: Hydrogen gas is remarkably difficult to store and transport safely, which severely limits its applicability across many fields.

Against this backdrop, a research team from Tokyo Institute of Technology, Japan, and Tokyo University of Science, Japan has been working hard to reach an alternative solution to the hydrogen storage problem. Led by Associate Professor Kosuke Ono, they recently developed a novel compound—simply called 1a—that can adsorb at high density and desorb ammonia (NH₃) repeatedly, making it easy to recover ammonia.

This gas is much more convenient to move around and can provide chemical energy just like hydrogen. Their findings have been published in the Journal of the American Chemical Society.

Compared to hydrogen, NH₃ does not require cold storage or extremely high pressure, which already saves a lot of energy. Moreover, existing industrial NH₃ infrastructure could be readily repurposed for emerging NH₃ applications as an energy carrier.

These are but a few of the advantages of NH₃. Ono explains, "NH₃ is not only a source of hydrogen but also considered a carbon-free energy carrier that produces N₂ and H₂O upon combustion without producing CO₂. Thus, the capture and recovery of NH₃ are highly desirable, both from an environmental perspective and with respect to efficient resource use."

However, materials for NH₃ storage should be chemically stable while also supporting energy-efficient ways of adsorbing and releasing captured gas. To realize such a material, the researchers created a crystalline solid out of 1a molecules, which are cyclic oligophenylenes with CO₂H functional groups in the inner portion of their ring-like structure.

When forming this porous crystalline solid, referred to as 1a (N), the 1a molecules organize themselves into bundles of parallel nanochannels. Thanks to the CO₂H groups, the channels are acidic, which in turn helps adsorb NH₃. Worth noting, the packing density for NH₃ in 1a (N) is 0.533 g/cm³ at room temperature—almost as high as the density of pure liquid NH₃.

Interestingly, simply lowering the pressure around 1a (N) is enough to make it release almost all the stored NH₃, which addresses a main limitation of previously reported materials.

"Crystalline 1a (N) is a stable NH₃-adsorption material with the ability for repeated usage. The issue of residual NH₃ during desorption, which has often plagued conventional NH₃-adsorption materials, can be resolved when using 1a (N) via a simple decompression operation," says Ono. In addition to these qualities, 1a (N) is also easy to prepare, which extends its applicability and cost-effectiveness.

Overall, this innovation could serve as a much-needed stepping stone toward efficient and scalable NH₃ storage, thereby paving the way to sustainable hydrogen economies. Moreover, by substituting the CO₂H functional groups with different compounds, it may be possible to adsorb other types of highly reactive gases that typically pose practical challenges, such as HCl or Cl₂.