An Investigation On The Preparation And Hydrogen Storage Properties Of High Capacity Al-based Composites

The efficient and safe storage, transportation as well as application of hydrogen energy are thefocus in the field of hydrogen energy research.The research of hydrogenstorage systems withhighgravimetric hydrogendensitieshas important theoretical significance and application value fortheon-boardfuel cellvehicularapplications. However,their practicalapplicationsarehinderedbysluggishkineties. Thus, considerable attentions have been paid on developing light-weight and high-capacity hydrogenstoragematerials. In this paper, hydrogen storage properties of NaAlH4and LiAlH4composite systemswere systematically investigated and discussed.(1) TiN catalyst prepared via a simple "urea glass" route illustrated the high purity and good crystallization. TiN-doped NaAlH4system was synthesized by dry ball milling NaH/Al (the mole ratio of NaH and Al is1:1) with the highly active TiN catalyst under hydrogen pressure under ambient temperature. And a systematic investigation was performed on the dehydrogenation and hydrogenation properties of TiN-doped NaAlH4system.It was found that the percentage of as-synthesized NaAlH4was calculated to be high94wt%.Interestingly, the onset dehydrogenation temperature of TiN-doped NaAlH4was lowered toabout100℃with a peak of138.05℃.The apparent activation energy (Ea) for the first step was estimated to be45.15kJ mol-1by theArrhenius equation. In addition, the results of HP-DSCanalysis clearly showed that excellent properties for dehydrogenation and hydrogenation can be obtained for TiN-doped NaAIH4system.(2) Hydrogen storage properties of commercial NaAlH4doped efficient TiN catalyst have been systematicallyinvestigated.It was found that about2.40wt%H2desorbed in the initial1.5h and5.01wt%H2desorbed within6h at190℃in the TiN-commercial NaAlH4system. And the total dehydrogenation capacity was5.44wt%. However, dehydrogenation capacity of TiN-free NaAlH4was only2.03wt%at190℃.High-pressure differential scanning calorimetry (HP-DSC) results showed thatTiN-commercial NaAlH4system exhibited excellent reversibility under moderate conditions.It might be thathighly active TiN on the composite surface, providing moreedge sites and hydrogen diffusion channels, prevented the nanograins sintering andagglomeration, thus, leading to the improvement of dehydrogenation kinetic, thermodynamics and cyclingstability of commercial NaAlH4.(3) Hydrogen storage properties of LiAlH4doped efficient TiN catalyst were systematicallyinvestigated. It was found that TiN catalyst enhanced the dehydrogenation kinetics anddecreased the dehydrogenation temperature of LiAlH4.Interestingly, the onset hydrogen desorption temperature of2%TiN-LiAlH4sample got lowered from151.0℃to90.0℃with a faster kinetics, and the dehydrogenation ratereached a maximum value at137.2℃. And the total dehydrogenation capacity was7.10wt%. However, the undopedLiAlH4only released5.84wt%of hydrogen at130℃. The hydrogen release of TiN-LiAlH4system could occur at a much lower temperaturenot only by the addition of TiN catalyst but also by thereduction of particle size, modification of crystal structure and increase of defect.(4) NbN catalyst prepared via a simple "urea glass" route illustrated the high purity and good crystallization. NbN-LiAlH4system was prepared by dry ball milling LiAlH4with effectiveNbN catalyst.2%NbN-LiAlH4sample started to decompose at about95℃which was55℃lower than that of as-milled LiAlH4.And about7.10wt%hydrogen was released. In addition, NbN nanoparticles distributed uniformlyon the LiAlH4matrix, suggesting that surface modificationsof LiAlH4occurred after adding the NbN nanoparticles. The isothermal dehydrogenation kinetics measurements showed that2%NbN-LiAlH4could release approximately6.10wt%hydrogen in150min at130℃, whereas commercialLiAlH4only released about0.63wt%hydrogen under the same conditions. It revealed that theenhancements arising upon adding NbN nanoparticles were almost8-9times that of as-milled LiAlH4.It suggested that NbN catalyst could improve the diffusion rate of H in NbN-LiAlH4system.(5)High-active NiCo2O4nanorods were synthesized by hydrothermal and thermal treatment methods. It was found that NiCo2O4nanorod could enhance obviously the dehydrogenation kinetic of LiAlH4. LiAlH4exhibited the superior dehydrogenation performances after adding NiCo2O4nanorobs, which released4.95wt%H2at130℃and6.47wt%H2at150℃within150min. About3.7wt%ofhydrogen can be released from2mol%NiCo2O4-LiAlH4system at90℃, where total7.10wt%of hydrogen was released at150℃.In contrast,the undoped LiAlH4sample just released<1wt%H2after150min. It suggested that NiCo2O4nanorobswith porous structure and high surface areaprevented the nanograins sintering and agglomeration during dehydrogenation, whichenhanced the dehydrogenation and hydrogenation performances of LiAlH4.