Characterization and mechanistic studies of titanium-doped sodium alanate

Abstract

A crucial obstacle to the realization of a "hydrogen economy" is the development of practical means of on-board hydrogen storage. Storing hydrogen as a gas at high pressures, or as a liquid at cryogenic temperatures, is impractical due to safety concerns and volumetric constraints. This has prompted extensive efforts to develop solid hydrogen-storage systems, such as metal hydrides, advanced carbons, and metal organic frameworks, for vehicular use. In 1997, Bogdanović and Schwickardi reported that hydrogen could be reversibly evolved from solid sodium alanate (NaAlH4) under moderate conditions upon mixing the hydride with a few mole percent of selected transition metal complexes. Yet, over a decade later, the fundamental basis of the kinetic enhancement of hydrogen cycling that arises upon doping remains unresolved. An understanding of this novel solid-state chemistry would be significant to the efforts of developing other complex metal hydrides for hydrogen storage. Thus, our studies have focused on elucidating the nature of the dopants and the structural effects they exert on NaAlR4 through various spectroscopic techniques. The titanium dopant was characterized through electron paramagnetic resonance studies. Anelastic spectroscopy studies revealed the formation of point defects during dehydrogenation of Ti-doped NaAlR4. The formation of defects was subsequently supported by positron annihilation spectroscopy. Muon spin rotation studies further elucidated the hydrogen dynamics of dehydrogenation. The results of these studies, which connect previous findings and lead to a better understanding of the enigma of Ti-doped NaAlR4, will be presented.