New research from an international team of scientists co-led by Drs. Di Zhou and Dmitrii Semenok from HPSTAR proposed a new synthesis route for metal hydrides with high hydrogen content via laser heating of amidoboranes under pressure and this method can be extended to binary and ternary polyhydrides of other alkali and alkaline earth metals. The team’s findings, published in Advanced Energy Materials, demonstrate cesium and rubidium polyhydrides, can absorb four times more hydrogen than current top contenders. Their discoveries open up possibilities for modifying existing compounds for hydrogen storage to increase their efficiency.

Hydrogen 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. However, one of the major complications to the widespread adoption of hydrogen power is the lack of safe, sustainable, and cost-effective technology for storing this extremely diffusive gas, which is reactive, hard-to-contain, and explosive.

“The alternative is chemical storage,” said Dr. Dmitrii Semenok, a lead author of the study and an advanced postdoc of HPSTAR. “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 during heating. However, there is a relatively hard limit on how much hydrogen you can place into those materials: about two hydrogen atoms per one metal atom.”

“The compounds we synthesized — cesium heptahydride CsH₇ and rubidium nonahydride RbH₉ — contain as many as seven and nine hydrogen atoms, respectively, per metal atom. And we expect them to be among the first such hydrogen-rich materials stable at near ambient conditions, although the latter requires further confirmation. The proportion of hydrogen atoms in these compounds is twice as high as in methane CH₄,” Dmitrii added.

“We react the powder of ammonia borane with either Cs or Rb, which produces salts known as cesium or rubidium amidoboranes,” explained Dr. Di Zhou, the first author of the study and a postdoc of HPSTAR. “Then heating under pressure decomposes those salts into cesium or rubidium monohydrides and lots of hydrogen. Since the experiment was performed in a diamond anvil cell at about 100,000 times normal atmospheric pressure, the extra hydrogen is forced into the crystal lattice voids, forming cesium heptahydride and rubidium nonahydride — the latter, in two distinct crystal modifications.”

“Cs and Rb elements are “predestined for this” because of how large their atoms are, resulting in bigger voids in the crystal structure for hydrogen to occupy” added Dr. Zhou. The formation of the compounds agrees well with the predictions of both the simulations and the DFT calculations. The presence of the compounds was also confirmed with multiple analytical techniques: X-ray analysis, Raman spectroscopy, and reflection/transmission spectroscopy.

The team now intends to repeat the experiment using large volume presses at a lower pressure — about 10,000 atmospheres — to obtain larger samples of Cs and Rb polyhydrides and verify that once synthesized, these compounds remain stable even at atmospheric pressure, unlike the other polyhydrides known to date.

Caption: The study looks at hydrogen-rich compounds formed at high pressures by hydrogen — the dumbbell-shaped molecules — and the alkali metals Cs and Rb. Similar compounds of Sr were considered by the team in a prior study.

氢化物因其卓越的超导特性而受到广泛深入的研究。然而通过氨硼烷或者氢气直接与金属反应而生成金属氢化物的传统合成方法需要的压力非常高，也仅适用于部分金属氢化物的合成。近日，北京高压科学研究中心的周迪，Dmitrii Semenok博士与俄罗斯斯科尔科沃科学技术学院的Oganov教授的合作研究提出了一种新的金属氢化物的合成方法，即通过直接加热金属的氨基硼烷，使其直接分解获得金属氢化物。通过该方法合成的 RbH₉等氢化物可以在低于10 GPa的压力下稳定存在，CsH₇甚至能稳定存在于近常压环境。合成的氢化物的氢含量最高可以达到93 at%，有望为改进现有储氢材料并提高储氢效率提供了新思路。相关结果以“Raisins in a Hydrogen Pie: Ultrastable Cesium and Rubidium Polyhydrides”为题发表于近期的《Advanced Energy Materials》上。