Investigation Of The Lightweight Hydrogen Storage Materials

The development of hydrogen storage systems with high volumetric and gravimetric hydrogen densities is indeed essential for the on-board fuel cell vehicular applications.This has prompted an extensive effort to develop lightweight hydrogen storage materials.Based on the review of the research and development of lightweight solid-state hydrogen storage materials,the complex hydride NaAIH₄ and Mg-based hydrides were selected as the subject of this work.First,NaAlH₄ was doped with the catalysts,TiF₃,Ti(OBuⁿ)₄,SiO₂ and TiF₃-SiO₂,by the mechanical grinding method. Their microstructures,dehydrogenation/re-hydrogenation performances and the activation energies for the dehydrogenation reaction were systematically investigated by means of X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR), Scanning electron microscopy(SEM),High-resolution transmission electron microscopy(TEM),thermal analyses and the gaseous hydrogen storage properties test methods.The dehydrogenation and re-hydrogenation processes in the Ti-doped NaAIH4 system were then analyzed by in-situ X-ray diffraction.Finally,a novel strategy was proposed,that is by confining NaAlH₄ into an as-synthesized ordered mesoporous silica(OMS) by impregnation and drying techniques.The microstructure and hydrogen storage properties for the space-confined NaAlH₄ particles were systematically studied.On the other hand,two kinds of Ti-containing agents,TiF₃ and Ti(OBuⁿ)₄,were doped to MgH₂ and Mg₂NiH₄.The structure and the bonding property of the doped hydrides were then analyzed by XRD and FTIR.Moreover, X-ray absorption spectroscopy was conducted to gain the information on the interaction of Ti ions with their local environments.Their microstructures,hydrogen storage properties and catalytic mechanism were also investigated.Two kinds of Ti-containing agents,TiF₃ and Ti(OBuⁿ)₄,were doped to NaAlH₄.The microstructure and hydrogen storage properties of Ti-doped NaA1H₄ composite have been systematically studied.Dehydrogenation performances for the hydrides before and after doping were further compared.It is found that the onset temperature for hydrogen release is around 185℃for the pristine NaAlH₄,but decreases to 150℃and 160℃for the TiF₃-and the Ti(OBuⁿ)₄-doped NaAlH₄,respectively.The total amount of hydrogen evolved is 3.8 wt.%for the pristine NaAlH₄,but it is increased to 4.2 wt.%for TiF3-doped NaAlH₄,and 4.5 wt.%for Ti(OBuⁿ)₄-doped NaAlH₄.The kinetics of dehydrogenation for NaAlH₄ can also be enhanced and the activation energies for the dehydrogenation reaction were considerably decreased for the Ti-doped NaAlH₄.It is shown that the activation energies for the first dehydrogenation reaction shift a lower energy by about 30 kJ/mol,and the activation energies for the second dehydrogenation reaction reduce by 138 kJ/mol for TiF₃-doped NaAlH₄,and 133 kJ/mol for Ti(OBuⁿ)₄-doped NaAlH₄.By comparing TiF₃ with Ti(OBuⁿ)₄,it is evident that the former is more efficient than the latter in improving the dehydrogenation performances for NaAlH₄.The dehydrogenation and re-hydrogenation processes in the Ti-doped NaAlH₄ system were analyzed by in-situ X-ray diffraction.It is found that Ti catalyst can reduce not only the dehydrogenation temperature of NaAlH₄,but also that of Na₃AlH₆ from 250℃to 160℃.The rehydrogenation of the decomposed NaAlH₄ is reversible theoretically,but the intermediate hydride,Na₃AlH₆,is not fully converted into NaA1H4 because of the physical separation of NaH and A1 formed in the decomposition,especially the formation of the large A1 crystallites.These,therefore, result in a decrease in the hydrogen storage capacity as observed in the following cycling.Additional measurements have showed that those A1 crystallites larger than 2.3μm are no longer effective for rehydrogenatrion.In order to improve the hydrogen storage properties further,both TiF₃ and mesoporous SiO₂ were doped to NaAlH₄.The catalytic and synergistic effects of the co-dopants on the microstructure and reversible dehydriding properties of NaAlH₄ were systematically investigated.It is found that the onset temperature for hydrogen release is around 185℃for the pristine NaAlH₄,but decreases to 150℃for the co-doped NaAlH₄.The total amount of hydrogen evolved is 3.8 wt.%for the pristine NaA1H4,and around 4.2 wt.%for TiF₃-doped NaAlH₄,but it is increased to 4.9 wt.% after co-doping with TiF₃ and mesoporous SiO₂.The kinetics of dehydrogenation for NaAlH₄ can also be enhanced and the activation energies for dehydrogenation reaction were remarkably decreased for the co-doped NaAlH₄.It is believed that ordered nanometer size pores and high specific surface area may be mainly responsible for the improvement of the dehydrogenation and the catalytic role of SiO₂ is more"physical"than"chemical".A favorable synergistic effect on the dehydrogenation for NaAlH₄ has been achieved when the SiO₂ is added as a co-dopant with the"chemical"catalytic precursor of TiF₃.A novel strategy is proposed,that is by confining NaAlH₄ into the OMS,in which the material is controlled to be nanoscale by the diameter and shape of the pores. NaAlH₄ in the pores of OMS was obtained by impregnation and drying techniques.It has been found that the space-confined NaAlH₄ shows the lower temperature and faster kinetics for dehydrogenation than that of the pristine NaAlH₄.Moreover,in the absence of a catalyst,the re-hydrogenation in a dehydrogenated NaAlH₄ system with nanoscale can be achieved even under the temperatures of 125-150℃and the hydrogen pressures of 3.5-5.5 MPa.The physical limitation of the NaAlH₄ particles, as well as the resulting Al and NaH phases,to be nanoscale by the pores of OMS is believed to be responsible for these.This method of space-confining nanoparticles may provide an approach to improve the hydrogen storage properties,in particular, the cycling stability of NaALH₄ and other complex hydrides.By comparing MgH₂,Mg₂NiH₄ with NaAlH₄,it is evident that these hydrides are qualitatively common in bonding properties.This common feature brings us to consider whether it is possible to reduce the stabilities of MgH₂ and Mg₂NiH₄ via Ti species.Two kinds of Ti-containing agents,TiF₃ and Ti(OBuⁿ)₄,were doped to MgH₂ and Mg₂NiH₄ by ball-milling.Dehydrogenation performances from the hydrides before and after doping were further compared.The structure and the bonding property of the doped hydrides were then analyzed.Moreover,X-ray absorption spectroscopy was conducted to gain the information on the interaction of Ti ions with their local environments.It is shown that the onset temperature for hydrogen release is around 420℃for the blank MgH₂,but decreases to 360℃and 410℃for the TiF₃-and the Ti(OBuⁿ)₄-doped MgH₂,respectively.For Mg₂NiH₄,it is reduced from 230℃to 220℃after doping with TiF₃,but remains unchanged once doping with Ti(OBuⁿ)₄. These indicate that TiF₃ is more efficient than Ti(OBuⁿ)₄ in reducing the stability of MgH2 and Mg₂NiH₄.The enhanced dehydrogenation kinetics is attained in the TiF₃-doped MgH₂,but not in the TiF₃-or Ti(OBuⁿ)₄-doped Mg₂NiH₄.It is believed that the interactions between the hydrides and the Ti-containing agents weaken the Mg(Ni)-H bonds,and thus favors the recombination of hydrogen atoms towards the hydrogen molecules.These interactions are substantially influenced by both the hydrides and the Ti-containing agents.