Japanese researchers report the development of a simple-structured, aluminum-based interstitial hydride for hydrogen storage in a paper in the AIP Publishing journal APL Materials. Their compound, Al2CuHx, was synthesized by hydrogenating Al2Cu at an extreme pressure of 10 gigapascals (1.5 million pounds per square inch) and a high temperature of 800 °C (1,500 °F).
Lightweight interstitial hydrides with high hydrogen content—such as Mg-based hydrides, alanates, borohydrides, and amino boranes—have been proposed as a safe and efficient means for storing hydrogen for fuel cell vehicles, but so far, none have proven practical as a hydrogen repository.
An aluminum-based alloy hydride offers a more viable candidate because it has the desired traits of light weight, no toxicity to plants and animals, and absence of volatile gas products except for hydrogen. Although lightweight complex aluminum hydrides have been explored extensively, the researchers note, because of their thermodynamic properties and slow kinetics, those suited for practical applications have not yet been developed.
Interstitial aluminum-based alloy hydrides are expected to show properties different from those of the hydrides mentioned above. The hydrogenation and dehydrogenation reactions would simply proceed by a one-step process similar to conventional interstitial alloys such as LaNi5 showing LaNi5 + x/2H2 ⇔ LaNi5Hx suitable for practical application. In addition, the thermodynamic properties of interstitial aluminum-based alloy hydrides can be modified by partially or completely replacing their counterpart metal atoms; hence, the pressure–temperature conditions for hydrogen desorption and absorption reactions would become tunable. However, no interstitial aluminum-based alloy hydride has been synthesized so far, probably because aluminum tends to form Al–H covalent bonds.
In this letter, we demonstrate the formation of Al2CuHx (x ∼ 1) interstitial hydride. The crystal structure of the hydride proposed on the basis of X-ray diffraction experiments and first-principles calculations shows the characteristic alignment of hydrogen atoms. … The present results will help expanding the variety of aluminum-based alloy hydrides, helping in further developing practical hydrogen-storage materials.—Saitoh et al.
The synthesis using high-temperature and high-pressure hydrogen atmosphere to form the alloy hydride twists the Al 8 Cu square antiprisms in Al 2 Cu around the c axis of a tetragonal unit cell.The twist enlarges the interstitial spaces for accommodating hydrogen atoms which align linearly parallel to the c axis in Al2CuHx.
The researchers characterized the conditions of the hydrogenation reaction using in-situ synchrotron radiation X-ray diffraction measurement, while the crystal and electron structures of the compound formed were studied with powder X-ray diffraction measurement and first-principle calculations, respectively. Together, these examinations confirmed the first-ever formation of an interstitial hydride of an aluminum-based alloy.
Although its synthesis requires very extreme conditions and its hydrogen content is low, our new compound showed that an aluminum-based alloy hydride is achievable. Based on what we’ve learned from this first step, we plan to synthesize similar materials at more moderate conditions—products that hopefully will prove to be very effective at storing hydrogen.—Hiroyuki Saitoh, lead author
Hiroyuki Saitoh, Shigeyuki Takagi, Naruki Endo, Akihiko Machida, Katsutoshi Aoki, Shin-ichi Orimo and Yoshinori Katayama (2013) “Synthesis and formation process of Al2CuHx: A new class of interstitial aluminum-based alloy hydride,” APL Mat. 1, 032113 doi: 10.1063/1.4821632