Nanoconfined Metal Hydrides As Alternatives to Lithium Ion Battery Energy Storage
Authors: Stavila Vitalie , Shivanna Mohana , Taylor William , Torquato Nicole , Allendorf Mark D. 
Published: December 2024

Energy densities of metal hydrides are competitive with lithium ion batteries (Fig. 1), highlighting their potential as contenders for emerging energy storage technologies. Metal hydrides have long been of interest for hydrogen storage for both transportation and stationary applications. However, most of these materials are too stable, i.e. their enthalpy of H2 desorption is too high to use the residual heat of a proton exchange membrane (PEM) fuel cell to drive the H2 release. Alternatively, metastable metal hydrides (MMH), for which H2 release is exothermic or only slightly endothermic, have received much less attention. Compounds such as LiAlH4 and AlH3 are so thermodynamically unstable that direct rehydrogenation following H2 release is thought to be impossible. This is unfortunate because these hydrides have high gravimetric and volumetric energy densities with potential for fast, low-temperature hydrogen release to power a fuel cell.
The goal of this work was to synthesize nanoscale metal hydrides from earth-abundant elements and demonstrate that they can achieve energy densities higher than state-of-the-art lithium ion batteries, achieving full recharge using the residual heat from a PEM fuel cell. We employed a nanoconfinement strategy to improve the thermodynamics and kinetics of hydrogen release and uptake, which has been shown to be effective for many thermally stable metal hydrides.2 We will describe the synthesis and properties of nanoconfined alane (AlH3) using a bipyridine-functionalized covalent triazine framework (CTF) and the Metal-Organic Framework (MOF) UiO-67bpy (Zr6O4(OH)4(bpydc)6; bpydc2− =2,2′-bipyridine-5,5′dicarboxylate), which can coordinate AlH3 through the aromatic linker nitrogens.
We find that the MOF and CTF both maintain the porous structure after infiltration. Brunauer-Emmett-Teller (BET) surface areas derived from nitrogen isotherms indicate significant decreases in the surface area and pore volume. Using a focused ion beam (FIB) to cut a channel in a MOF crystal surface, we obtained cross-sectional transmission electron microscopy (TEM)/energy-dispersive X-ray spectroscopy (EDS) images indicating that AlH3 is uniformly infiltrated throughout the MOF. Electron energy loss (EELS) spectra reveal that signals from MOF linker carbon atoms and aluminum atoms alternate along the FIBed channel, providing convincing evidence that AlH3 is located within the pores. Solid state 27Al-NMR, DFT calculations, and single crystal X-ray diffraction provide additional evidence that AlH3 is located in the vicinity of the bipyridine groups.
We also demonstrate that AlH3 is thermodynamically stabilized by nanoconfinement within the CTF. AlH3@CTF-bipyridine rapidly desorbs hydrogen between 95 – 150 °C. Even more surprising, AlH3@CTF-bipyridine dehydrogenation is reversible at only 60 °C and 700 bar hydrogen, a pressure 36 times lower than the experimentally measured H2 plateau pressure of bulk aluminum. Together, these results suggest that metastable hydrides could be practical materials for H2 storage, and in selected cases with theoretical energy densities exceeding those of Li-ion batteries.

L. Snider, M. Witman, M. E. Bowden, K. Brooks, B. L. Tran, T. Autrey Nat. Chem., DOI: 10.1038/s41557-022-01056-2

Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke J. Phys. Chem. Lett.
2010, 1, 2193–2203.

Figure 1. Comparison of volumetric energy density vs. gravimetric energy density for various energy carriers and batteries. Adapted from Ref. 1.

Figure 1