New material for hydrogen storage confines this clean yet troublesome fuel

Scientists from China, Russia, Japan, and Italy have discovered a material for chemical storage of hydrogen that can “soak up” four times as much of this gas as the current best contenders (Figure 1).

Hydrogen (H) is expected to play a major role in the future low-carbon economy. It can be produced renewably and consumed to generate electricity or heat via fuel cells or combustion. Finding efficient ways to confine hydrogen is crucial for integrating this promising fuel into the sustainable economy of the future. With proper storage technology, hydrogen could one day replace natural gas in high-temperature industrial processes, and serve for balancing supply and demand on the power grid. The study came out in Advanced Energy Materials.

A major problem preventing the widespread use of hydrogen power is the lack of safe, sustainable, and economical technology for storing this light, reactive, hard-to-contain, and explosive gas. To accumulate and transport hydrogen in gas cylinders, tubes, cryogenic tanks, and pipelines, it can be compressed or liquefied. But such processing is very expensive. The compression and refrigeration involved expend the equivalent of about 20-40 % of the total energy ultimately provided by the hydrogen — a very high penalty. And even then, it still packs only about half as much energy per unit volume as natural gas — compressed or liquefied.

“The alternative is chemical storage,” says one of the lead authors of the study, Dr. Dmitrii Semenok. “Certain materials, for example Mg-Ni and Zr-V alloys, can store hydrogen in the voids between the metal atoms that make up the crystal structure. Such accumulators provide relatively dense and safe storage and release hydrogen fairly quickly upon demand if heated. However, there is a relatively hard limit on how much hydrogen you can put into those materials: about two H atoms per one metal atom. And that is the chief figure of merit.”

“The compounds we synthesized — cesium heptahydride CsH₇ and rubidium nonahydride RbH₉ — pack as many as seven and nine hydrogen atoms, respectively, per metal atom. The proportion of hydrogen atoms in these compounds is one of the highest among all known hydrides, twice as high as in methane CH₄,” Dmitrii added.

The first author of the study, Dr. Di Zhou, explained the details of the experiment: “We react the hydrogen-rich powder of ammonia borane (NH₃BH₃) with either cesium or rubidium. This produces salts known as amidoboranes. Heat decomposes those salts into cesium or rubidium monohydrides and lots of hydrogen. Since the experiment is run in a diamond anvil cell under pressure of 100,000 atmospheres, the extra hydrogen is forced into the crystal lattice voids, forming cesium heptahydride and rubidium nonahydride — the latter, in two distinct crystal lattice modifications.” X-ray diffraction studies of the polyhydrides in diamond anvil cells were carried out at the Xpress beamline of Elettra, and were complemented by Raman, and reflection/transmission spectroscopy.

Figure 1: Sketch depicting the general concept of this work: “raisins” of heavy cesium and rubidium atoms located in the hydrogen “pie” of the polyhydrides CsH₁₇, CsH₇ and RbH₉, whose structures are shown on the right

According to the researchers, cesium and rubidium are “predestined for this,” because of how large their atoms are, resulting in bigger voids in the crystal structure for hydrogen to occupy. The formation of the compounds agrees with the predictions of both the team’s simulations and the calculations based on fundamental physical laws. The team now intends to repeat the experiment using large-scale hydraulic presses at a lower pressure — about 10,000 atmospheres — to obtain larger amounts of cesium and rubidium polyhydrides and verify that once synthesized, these compounds remain stable even at atmospheric pressure, unlike the other polyhydrides known to date.

This research was conducted by the following research team:

Di Zhou¹, Dmitrii Semenok¹, Michele Galasso², Frederico Gil Alabarse³, Denis Sannikov⁴, Ivan A. Troyan⁵, Yuki Nakamoto⁶, Katsuya Shimizu^6, and Artem R. Oganov^4
1 Center for High Pressure Science & Technology Advanced Research, Beijing, China.
² Institute of Solid-State Physics, University of Latvia, Latvia.
³ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy.
⁴ Skolkovo Institute of Science and Technology, Moscow, Russia.
⁵ A.V. Shubnikov Institute of Crystallography of the Kurchatov Complex of Crystallography and Photonics (KKKiF), Moscow, Russia.
⁶ KYOKUGEN, Graduate School of Engineering Science, Osaka University, Osaka, Japan.

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The authors thank Nicolas Posunko () for the help with writing of this text.