Doctor of Philosophy
Institute for Superconducting & Electronic Materials, Faculty of Engineering
Mao, Jianfeng, Light metal hydrides for reversible hydrogen storage applications, Doctor of Philosophy thesis, Institute for Superconducting & Electronic Materials, Faculty of Engineering, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3445
Developing new practical hydrogen storage system with high volumetric and gravimetric hydrogen densities is necessary to implement fuel cell technology for transportation applications. One very promising option regarding safety and volumetric capacity is storage in metal hydrides. However, no single metal hydride fulfils all the requirements relating to hydrogen capacity, reaction enthalpy, and reaction kinetics for efficient hydrogen storage in the required operating temperature ranges. The objective of this thesis is to promote the kinetics and tailor the thermodynamics of the light metal hydrides such as magnesium hydride, borohydrides, and alanates by employing material design strategies such as catalytic doping and additive destabilization. In this study, a series of combined systems such as NaBH4-MgH2, LiAlH4-NaBH4, NaBH4-CaH2, NaBH4-Ca(BH4)2, LiBH4-MgH2, LiAlH4-LiBH4, LiAlH4-MgH2, and LiAlH4-MgH2-LiBH4; and a series of single metal hydrides such as Ca(BH4)2, Ti-catalyzed NaBH4, Ni and Co-catalyzed MgH2, and NbF5 and/or single-walled carbon nanotube (SWCNT) catalyzed NaAlH4 have been systemically investigated for hydrogen storage. For NaBH4, we found that the hydrogen de/absorption properties of NaBH4 can be improved by doping with different Ti-based additives including Ti, TiH2 and TiF3, in which TiF3 possessed the highest catalytic activity for the hydrogen sorption reaction of NaBH4. The observed promotion effect of TiF3 on the reversible dehydrogenation of NaBH4 could be explained by the combined effects of active Ti and F- containing species. To further improve the kinetic and thermodynamic properties of NaBH4, a series of destabilizing agents such as MgH2, LiAlH4, CaH2, and Ca(BH4)2, were combined with NaBH4 to form reactive hydride composites (RHCs) such as NaBH4-MgH2, LiAlH4-NaBH4, NaBH4-CaH2 and NaBH4-Ca(BH4)2. iii It was found that the dehydrogenation thermodynamics of NaBH4 could be modified by forming MgB2, AlB2, and CaB6 compounds upon dehydrogenation. The formation of boride compounds also led to improved reversibility. In particular, the de/rehydrogenation properties of these combined systems can be further improved by use of dopant catalysts such TiF3 and NbF5. For the LiBH4-MgH2 composite system, it was demonstrated that the dehydrogenation and rehydrogenation kinetics, as well as the capacity can be significantly improved by combined use of hydrogen back pressure and NbF5 addition. It was also found that the hydrogen de/absorption kinetics of the LiBH4- MgH2 system was enhanced by doping with a novel catalyst, ruthenium nanoparticles supported on multiwalled carbon nanotubes (Ru/C). Meanwhile, we also demonstrated that the hydrogen de/resorption properties of the LiBH4-MgH2 system were modified by introducing Ni nanoparticles, through the catalytic effect of Ni and the formation of Mg-Ni-B ternary alloy upon dehydrogenation. We have also found that the binary hydride systems such as LiAlH4-MgH2 and LiAlH4-LiBH4, and ternary hydride system LiAlH4-MgH2-LiBH4, display superior de/rehydrogenation performance compared to their individual hydrides (e.g. LiAlH4, MgH2 and LiBH4) or binary mixtures of those components (LiAlH4-MgH2, LiAlH4-LiBH4, MgH2-LiBH4). The enhancement of the hydrogen sorption properties was likewise attributed to the formation of intermediate compounds, including Li-Mg, Mg-Al, Al-B, and Mg-Al-B alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway. It was also demonstrated that doping with TiF3 can further promote this interaction and thus enhanced the hydrogen sorption properties of these systems. iv It was also found that the dehydrogenation temperature and the absorption/desorption kinetics of MgH2 were improved by adding either NiCl2 or CoCl2, and a significant enhancement was obtained in the case of the NiCl2 doped sample. Kinetic investigation indicated that the formation of Mg from the dehydrogenation of the milled MgH2 is controlled by a slow, random nucleation and growth process. However, after NiCl2 or CoCl2 doping, the nucleation process of Mg is transformed into two-dimensional growth. The results suggested that the additives reduced the kinetic barrier and lowered the driving forces for nucleation. We have also systematically investigated the dehydrogenation kinetics and thermodynamics of Ca(BH4)2. It was found that the dehydriding reaction of Ca(BH4)2 starts at approximately 320 °C, and about 9.6 wt % hydrogen is desorbed through the two-step reaction. Furthermore, the apparent activation energy (Ea), enthalpy of reaction (ÄH), and entropy of reaction (ÄS) has been estimated based on the results of the TPD and PCT measurements. It was also found that the dehydrogenation/hydrogenation properties of NaAlH4 were significantly enhanced by mechanically milling with NbF5. Furthermore, it was revealed that there is a synergistic effect of SWCNTs and NbF5 on the de/rehydrogenation of NaAlH4, which improves the hydrogen de/absorption performance when compared to adding either SWCNTs or NbF5 alone. These results are attributed to the active Nb-containing species and the function of F anions, as well as the nanosized pores and high specific surface area of the SWCNTs, which facilitates the dissociation and recombination of hydrogen molecules on their surfaces and the atomic hydrogen diffusion along the grain boundaries and inside the grains, and decreases the segregation of bulk Al after the desorption.