Discovery of A New Al Species in Hydrogen Reactions of NaAlH₄

Abstract

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.

On the effect of thermal treatment and hydrogen vibrational dynamics in sodium alanates: An inelastic neutron scattering study

“…3 we report their fits showing how the s (Q,E) (see also Appendix A) spectra from samples 2 and 4 can be quite satisfactorily reconstructed by a simple linear combination of their thermally untreated components: Na 3 AlH 6 plus NaAlH 4 or NaH [24] ( 2 = 0.61 and 0.62, respectively), leaving no unaccounted vibrational features. Thus, under the conditions of our experiments, after our thermal treatment we have found no evidence for the recently discovered by 27 Al NMR [25] highly defective AlH 5 2-species. However, these may exist only locally, as an out-ofequilibrium short-lived species near defects created by Ti atoms, for instance.…”

Nanoconfined LiBH4 and Enhanced Mobility of Li+ and BH4– Studied by Solid-State NMR

“…28 Similar effects might occur in LiBH 4 . The mobile BH 4 − species could possibly be formed in the bulk material too without the presence of a nanoporous material.…”

Native Defect Concentrations in NaAlH4 and Na3AlH6

“…At this point, Reaction 2 is thermodynamically favorable, and there is a coexistence of molten NaAlH 4 with solid Na 3 AlH 6 , Al, and NaH. In addition, Gross 27,33 and the existence of a new γ-NaAlH 4 phase has been proposed based on theoretical calculations. 34 In contrast, Gross et al 32 clearly showed that Ti-doped systems decompose according to Reaction 1 when the temperature is only 100°C.…”