Mobile Species in NaAlH₄

Abstract

Sodium alanate (NaAlH4) is the archetypical complex (ionic-covalent) hydride compound for hydrogen storage applications. However, the details of the reactions of this compound upon hydrogen cycling remain unclear. Recently, a new mobile aluminum-containing species termed S105 was discovered in the hydrogen chemistry of sodium alanate using in situ NMR with S105 a likely facilitator of Al-atom transport in NaAlH4. Here, we find that hydroxide impurities play a crucial role in the formation of the mobile species. Specifically, in bare NaAlH4, S105 is present after melting (under sufficient hydrogen pressure to block the dehydriding reaction) only in samples either exposed to H2O vapor or mixed with metal hydroxides. We find that the 27Al line position of S105 is close to that of NaAlH4 (after correcting for second-order quadrupole effects), indicating that S105 involves very mobile AlH4– tetrahedra (rotationally and translationally). We propose that hydroxide impurities promote fast diffusion of nearby AlH4– units, similar to enhanced motions seen in NaH; the hydroxides also react with NaAlH4 to form NaH and subsequently produce Na3AlH6, which is always found to accompany S105. Our measurements reveal that the only chemical components of S105-containing alanate apart from hydroxides are NaAlH4 and Na3AlH6. Presence of the S105 species in NaAlH4 samples also leads to faster dehydriding in hot, undoped NaAlH4 solid, pointing to an enhancement of the hydrogen reaction kinetics by S105.