Storing hydrogen with aluminum hydrides

A team of scientists from the U.S. Lawrence Livermore National Laboratory and Sandia National Laboratories has proposed to use aluminum hydride (AlH₃) to store hydrogen. This solid-state metal hydride, which is also known as “alane,” is commonly used for rocket fuel, explosives, as a reducing agent in alkali batteries, and as a hydrogen source for low-temperature fuel cells. 

According to the researchers, this material is able to overcome the challenge represented by the thermodynamic limitation of hydrides in storing hydrogen. “Many high-capacity metal hydrides suffer from poor thermodynamics of hydrogen uptake after initial release, which necessitates extreme hydrogen pressures to regenerate,” the scientists explained. “Such a limitation is often tied to their metastable nature and hinders their real-world applications.”

At room temperature, aluminum hydride is a metastable crystalline solid and when it goes through an endothermic reaction it decomposes
to generate aluminum and hydrogen gas. The research team developed a nanoconfined material with improved thermodynamics and placed it within the nanopores of a highly porous bipyridine-functionalized covalent triazine framework, which they called AlH₃@CTF-bipyridine. 

This configuration made it possible to convert bulk metallic aluminum into alane at 60 degrees Celsius and at a hydrogen pressure of only 700 bar, which compares to 6,900 bar in conventional processes. “This pressure is readily achievable in commercial hydrogen fueling stations, although further improvements are necessary to achieve rapid fueling,” the scientists further explained, noting that current processes used to produce alane are carried out under “extreme conditions.”

The academics found that the alane they developed offered more stability. They observed that intrinsically stable radicals and tiny alane clusters interact chemically with the nanopores of the confining framework, which creates thermodynamic conditions that are completely different from those of bulk metallic aluminum. “Nanoconfinement is a really interesting approach for stabilizing metastable hydrogen-storage materials, particularly given the wide palette of potential host materials,” said LLNL materials scientist Brandon Wood.

A full description of the technology can be found in the study Defying Thermodynamics: Stabilization of Alane Within Covalent Triazine Frameworks for Reversible Hydrogen Storage, which was recently published in Angewandte Chemie.