| 其他摘要 | Ti-NaAlH4 system has attracted considerable interests owing to its favorable thermodynamics, relatively high H-capacity (5.5wt.%), as well as the markedly improved dehydriding/rehydriding kinetics under moderate temperature arising upon Ti-catalyst doping. The overall properties of Ti-NaAlH4 system stands for the state-of-the-art of hydrogen storage materials. A key topic in the study of Ti-NaAlH4 system is to understand better the catalytic mechanism, and on the basis of which to improve H-storage properties via developing advanced doping technologies. Currently, the dominant doping technology involves the usage of high-valence Ti compounds. While possessing high catalytic effectiveness, using Ti compounds brings about serious problems on both property and mechanism aspects: generation of inert by-product that causes capacity loss, and invisibility of Ti-containing species that largely hinders the catalytic mechanism understanding. In this work, we focus on solving these problems via developing novel doping technologies. Systematic property/phase/microstructure studies, in combination with theoretical efforts, on Ti-doped NaAlH4 prepared under various conditions have resulted in considerable progresses in both improving H-storage properties and developing better understanding of the nature of catalytically active Ti-species.
(1) Catalytically enhanced NaAlH4 system was prepared by mechanically doping the hydride with metallic Ti powder. This novel doping process offers a clear potential to achieve high hydrogen capacity due to the elimination of the problematic generation of by-product and the release of gas-impurities when using high-valence Ti compounds. Combined property/phase/microstructure investigations on the metallic Ti-doped NaAlH4 suggest that a Ti-H-Al metastable species generated via interaction between TiH2 and Al matrix may act as active species to catalyze the reversible dehydrogenation of NaAlH4. In contrast to the lack of experimental evidence in the other hypotheses, the present mechanism understanding originates from definite identification of TiH2 phase, and is well supported by related property/microstrutural results. Furthermore, it is, at least partially, validated by First-Principles calculation results. Clearly, experimental efforts aiming at improving H-storage properties of Ti-NaAlH4 system may benefit from such better mechanism understanding.
(2) On the basis of understanding of catalytically active species, a novel selective two-step doping method was developed. By simply controlling the addition time of ductile Al, the distribution state of Ti hydride in the NaAlH4 matrix was markedly improved. As a result, a pronounced improvement on dehydriding/rehydriding performances of the doped hydrides is achieved. For example, doping NaH/Al mixture with 4 mol%Ti by this novel two-step method results in a 3 times higher recharging rate, a 5 times higher dehydriding rate and a nearly 40% increase on H-capacity, from 3 to 4.3wt.%, compared to the sample prepared under Ar atmosphere by traditional one-step method. The success achieved by using two-step method further validates the rationality of the proposed nature of active Ti-species.
(3) Another novel KH+Ti co-doping method was proposed and utilized to prepare high-capacity Ti-NaAlH4 system. Under cycling conditions, the practical H-capacity of KH+Ti co-doped NaAlH4 reaches up to 4.7wt.%, about 1.4wt.% higher than that of the sample solely doped with Ti powder prepared under H2 atmosphere. As demonstrated, the pronounced capacity enhancement mainly comes from the markedly improved dehydriding performance of Na3AlH6, associated with a favorable thermodynamic modification arising upon K+ partial substitution of the hydride lattice.
(4) A novel dopant precursor TiF3 was also developed. As a dopant precursor, TiF3 is superior to the currently popular choice, TiCl3, in accelerating the reversible hydriding/dehydriding process of NaAlH4. The utilization of TiF3 results in marked improvement on cycling H-capacity, hydriding/dehydriding kinetics, and operation temperature/pressure conditions, compared with the TiCl3-doped hydride. According to the combined experimental/theoretical studies, doping hydride with TiF3 results in a partial substitution of H- by F anion in the hydride lattice, and accordingly, a favorable thermodynamic modification. A novel "Functional Anion" concept has been put forward on the basis of this finding. It renews the mechanism understanding that is currently confined within metal cations and/or their derivatives. Furthermore, they may pave a new way for pursuing improved hydrogen storage properties of other related high H-capacity complex hydride systems.
Clearly, these findings have contributed to developing better catalytic mechanism understanding and to enhancing H-storage properties of Ti-NaAlH4 system. Furthermore, they may guide the research efforts on other related high H-capacity complex hydride systems. |
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