Synthesis and Thermal Characteristics of Complex Metal Hydride: NaAlH4

Abstract

ABSTRACTComplex metal hydrides of general formula, ABH4 (A = alkali metals, B = third group elements such as B, Al, Ga) are potential candidates as hydrogen storage media for transportation. Thermal decomposition of complex hydrides generates hydrogen at elevated temperatures. The by -products of the dehydrogenation process can be regenerated using gaseous hydrogen at suitable temperature and pressure. The initial steps of thermal decomposition of NaAlH4 may be more complicated from the decomposition pathway reported in the literature. Close examination using thermal analysis by TGA, DSC and XRD measurements over the temperature range 30–500°C showed that the initial evolution of hydrogen occurred at a slow rate at ∼80°C, prior to fast decomposition at 190°C and at 260°C. Four regions of weight loss and five major endothermic peaks were measured during the thermal analysis. The effect of heating rate on the thermal analysis response showed that a high resolution of the thermal processes could be achieved at higher heating rates. Thermodynamic data was obtained for the various steps in the decomposition process including the formation of intermediate phases Na 3AlH6, and NaH. We also found that the decomposition of NaH is highly pressure dependent probably due to the high compressibility of the diffuse H− anion. The crystal-chemistry of NaAlH4 during decomposition has been established using X-ray diffraction analysis.