Discovery of A New Al Species in Hydrogen Reactions of NaAlH4

The NaAlH4 system remains the archetype of hydrogen storage solids. However, the detailed mechanisms of the hydrogen reactions of NaAlH4 remain unknown. We report 27Al in situ nuclear magnetic resonance (NMR) spectroscopic data revealing an Al-bearing mobile species that could provide the large scale metal-atom transport needed for rehydriding. This new species forms under reaction conditions but can be captured at ambient temperature using excess H2 pressure. The NMR measurements demonstrate that the species is highly mobile (at 20 °C) and carries both Al and H atoms. On the basis of the 27Al shift (close to NaAlH4) and the disorder evident in X-ray diffraction, the species is identified as highly defective NaAlH4, likely having a large AlH3 vacancy concentration.