Development of NaAlH4 and carbon-based materials for hydrogen storage

Abstract

Hydrogen storage in a solid state material has been considered as a more suitable way for a fuel cell driven car as compared with compressed hydrogen and liquid hydrogen storage. Metal hydrides seem to be the promising materials to store hydrogen. However, their properties, such as the reversible hydrogen capacity, need to be developed to reach the hydrogen storage target. In this work, sodium aluminum hydride (NaAlH4, 5.6 wt% H2) was selected as a medium for hydrogen storage. The main purpose of this thesis was to develop the reversible hydrogen capacity of NaAlH4. Firstly, transition metals (TiCl3, ZrCl4, HfCl4 and VCl3) were used as a dopant to improve the hydrogen desorption/absorption in NaAlH4. Ball milling was used as a means to mix NaAlH4 with the dopant. It was found that doping metal decrease the desorption temperature of NaAlH4 and the reversible hydrogen capacity of doped NaAlH4 is about 30-75% of their original hydrogen capacity. TiCl3 exhibits the best effective dopant among the tested transition metals with the reversible hydrogen capacity of 3.85 wt%. The role of transition metals on the hydrogen absorption involves catalyzing the hydrogen dissociation in the desorbed hydride. Moreover, the formation of a by-product, NaCl, from TiCl3 lowers the reversible hydrogen capacity of NaALH4. Therefore, other forms of the metal, Ti and TiO2, were used as an additive. The result reveals that TiO2 doped NaAlH4 has the reversible hydrogen capacity as same as the one doped with TiCl3 while the rate of hydrogen absorption of TiO2 doped NaAlH4 is higher than that of TiCl3 doped NaAlH4. This is due to the porosity of TiO2 that facilitates the hydrogen diffusion in the desorbed sample. This result also indicates the segregation of the desorbed hydride after the hydrogen desorption. Consequently, carbon materials (graphite, activated carbon, and carbon nanotubes) were co-doped in metal doped NaAlH4 to prevent the segregation and to increase the hydrogen diffusion in the desorbed hydride. The hydrogen capacity of metal doped NaAlH4 can be increased by co-doping with carbon materials. Graphite is the best co-dopant for TiCl3 doped NaAlH4 with hydrogen re-absorption capacity up to 4 wt%. Moreover, hydrogen storage capacity of carbon nanotube deposited by Pd or V was measured to study their possibility in using as a medium for hydrogen storage.