Characterization of aluminum hydride polymorphs: a potential hydrogen storage material for use with hydrogen fuel cells

Abstract

The α-, β-, and γ-polymorphs of AIH3 have been successfully synthesized and the intrinsic and mechanically modified thermal stabilities of each polymorph have been experimentally determined. The thermogravimetric profiles of the α- and γ-phases exhibit dehydriding reactions in the temperature range of 370-450 K. The profile of the β-phase shows the continuous dehydriding reactions, which differs from the other two phases. The values of the enthalpy of dehydriding reactions ΔHdehyd are determined to be 6.0 ± 1.5, -3 to approximately -5, and 1.0 ± 0.5 kJ/mol H2 for the α-, β-, and γ-phases, respectively. The milling-time dependences of the powder X-ray diffraction measurement and thermal analyses indicate the occurrence of the dehydriding reactions both in the a- and y-phases during milling, but there is no drastic change in the β-phase. The temperature at which dehydrogenation occurs for the a- and y-phase is drastically reduced by approximately 50 K upon milling, results that are similar to doping. Infrared spectroscopy was used in order to develop a convenient, low-cost alternative to X-ray diffraction for differentiating between the various polymorphs of AIH3. The spectroscopy data also indicates that γ- AIH) may have a combination of bridging and terminal hydrogens, and β- AIH3 may lack any long-range ordering of the crystal1ine structure. The dehydrogenation reaction for each phase was found to be nearly 9 mass %. The β- and γ-phase have improved kinetics relative to the α-phase, results that are similar to doping. Titanium dopants did not appear to improve the dehydrogenation reaction in , this study, nor did they allow for the rehydrogenation of AIH) or metallic aluminum. Preliminary results from Raman and neutron vibrational spectroscopy, in addition to powder neutron diffraction data are also presented.