Year

2011

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Because it is a promising energy carrier, intensive efforts have been made to realize the potential of hydrogen to become a major energy carrier, for both mobile and stationary applications. Solid-state hydrogen storage has become an attractive option due to its high volumetric hydrogen capacity and favorable safety considerations. The purposes of this work are to enhancement the kinetics and tailor the thermodynamics of the light metal hydrides, LiAlH4 and MgH2, using different types of catalyst and the destabilization concept. In this study, a series of single metal hydrides such as NbF5- catalyzed LiAlH4, SWCNTs-metal-catalyzed LiAlH4, TiO2 nanopowder-catalyzed LiAlH4, and HfCl4 and FeCl3-catalyzed MgH2; and a series of combined systems such as MgH2-NaAlH4 and MgH2-LiAlH4 have been systemically investigated for hydrogen storage.

For LiAlH4, we found that the hydrogen desorption properties of LiAlH4 can be improved by doping with NbF5. The observed promotion effect of NbF5 on the dehydrogenation of LiAlH4 could be explained by combined effect of active Nbcontaining species and the function of F anions, which facilitates the dissociation of hydrogen molecules on their surfaces. It was also found that the dehydrogenation temperature and the desorption kinetics of LiAlH4 were improved by adding with SWCNTs-metal catalyst. The enhancement of the hydrogen desorption properties was likewise due to the combined influence of the SWCNT structure itself, hydrogen spillover effect, and high contact area between carbon and the hydride. All these are responsible for the weakened the Al–H bond, consequently improving the dehydrogenation properties of LiAlH4. We have also found that the dehydrogenation properties of LiAlH4 were improved by doping with TiO2 nanopowder. The result shows that TiO2 nanopowders remain stable during the milling process. The significant improvement is most likely attributable to the TiO2 nanoparticles act as a surface catalyst, increases the surfaces defects by decreasing crystal grain size in the LiAlH4 powder, creating a larger surface area for hydrogen to interact, thereby decreasing the temperature for decomposition.

For MgH2, it was found that the de/rehydrogenation properties of MgH2 were significantly improved by mechanically either HfCl4 or FeCl3, and a significant improvement was obtained in the case of the HfCl4 doped sample. From the x-ray diffraction and x-ray photoelectron spectroscopy results, it appears likely that the significant improvement of MgH2 sorption properties was due to the catalytic effects of Hf species and Fe that formed during the dehydrogenation process. These species may interact with hydrogen molecules, which may lead to the dissociation of hydrogen molecules and the improvement of the desorption/absorption rate. Besides that, the formation of MgCl2 may also play a critical role, and there are more likely to be synergetic effects when it is combined with Hf species and Fe.

Another method to improve the hydrogen storage properties of MgH2 is based on the combined system (destabilization concept). A MgH2–NaAlH4 (4:1) composite system was prepared by mechanical milling to investigate the destabilization effect between MgH2 and NaAlH4. It was found that this composite system showed improved dehydrogenation performance compared with those of as-milled NaAlH4 and MgH2 alone. The dehydrogenation process in the MgH2–NaAlH4 composite can be divided into four stages. X-ray diffraction patterns indicate that the second, third, and fourth stages are fully reversible. The formation of NaMgH3 and Mg17Al12 phase during the dehydrogenation process, which alter the dehydrogenation pathway furthermore change the thermodynamic of the reaction play a critical role in the enhancement of dehydrogenation in MgH2–NaAlH4 composite.

We have also systematically investigated the dehydrogenation kinetics and thermodynamics of MgH2-LiAlH4 combined system with and without additives. The improvement of the dehydrogenation properties was likewise attributed to the formation of intermediate compounds, including Al-Mg and Li-Mg, upon dehydrogenation, which change the thermodynamics of the reaction through altering the dehydrogenation pathway. Ten different additives, including TiF3, NbF5, NiF2, CrF2, YF3, TiCl3·1/3(AlCl3), HfCl4, LaCl3, CeCl3, and NdCl3, were added to the MgH2-LiAlH4 (4:1) mixture. Among the additives examined, the titanium-based metal halides, TiF3 and TiCl3·1/3AlCl3, exhibited the best improvement in term of reducing the dehydrogenation temperature and enhancing the dehydrogenation rate. It is believed that the formation of Ti-containing and F-containing species during the ball milling or the dehydrogenation process may be actually responsible for the catalytic effects and thus further improve the dehydrogenation of the TiF3 and TiCl3·1/3AlCl3-added MgH2- LiAlH4 composite system.

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