| Mg-based materials have been widely considered as promising candidates for hydrogen,heat and energy storage because of the high storage capacity of hydrogen and heat, excellentreversibility, abundant resources, low cost and environmentally friendly etc.. However, thehigh working temperature and difficult manufacturing hurdle their practical applications.Moreover, to understand the mechanism of hydrogen storage behavior through catalysis is acritical issue for improving the hydrogen storage property of MgH2.This doctoral dissertation has intensely reviewed the history of hydrogen economy andhydrides, then the current research on Mg-based hydrogen storage materials has beenreviewed, and the key issues and the corresponding strategy have been pointed out. Theauthor proposed a strategy to enhance the hydrogen storage of Mg-based materials throughrapid solidification. Two types of materials, including nanocomposites and metallic glasseshave been produced with improved hydrogen storage properties. On one side, hydrides and/oroxides nanocomposites with excellent hydrogen storage properties have been fabricated viahydrogenation and oxidation treatments upon the melt-spun alloys, and the mechanism of theimprovements on hydrogen storage performances has been revealed. On the other side, anovel and promising pathway using Mg-based metallic glasses for hydrogen storage has beenproposed. The study of the interaction between hydrogen and metallic glasses could also betheoritical and practical base for the H-related materials, microstructure and physicalproperties of the amorphous alloys etc.In chapter2, hydrogen storage property of the melt-spun Mg3LaNi0.1alloy has beenstudied. The phase structure and hydrogen storage properties of the obtained alloy have beenstudied by XRD, TEM, DSC analysis and PCI measurements. The melt-spun Mg3LaNi0.1alloy is composed of Mg3La phase, while fine LaMg2Ni grains are precipitated after agingtreatment at room temperature. Both Mg3La and LaMg2Ni undertake disproportion reactionsupon hydrogenation and decomposed into MgH2, LaH3and Mg2NiH4. The melt-spun alloyshows excellent hydrogen absorption kinetics and lowered desorption temperature comparingwith the induction melted alloy, and a maximum reversible hydrogen storage capacity of3.1wt.%and a minimum hydrogen desorption temperature of224C have been achieved.This improvement on the hydrogen storage properties is attributed to the catalytic role of insitu formed nanocrystalline Mg2Ni and LaH3. The effect of Ce and Ni contents on theglass-forming ability (GFA) of Mg-Ce-Ni system in the Mg-rich corner of Mg-Ce-Ni systemhas been revealed. Amorphous alloy with the highest Mg content, Mg90Ce5Ni5, is prepared by melt-spinning. With the amorphous alloy as precursor, nanostructure multi-phases compositehas been prepared by crystallizing it in hydrogenation process. The study could be used as areference for materials design of Mg-based hydrogen storage materials.In chapter3, Mg-based nanocomposite with controllable crystallite and particle sizeshave been fabricated via controlling activation treatments upon the amorphous alloy. HigherH2pressures and lower temperatures are beneficial for obtaining finer hydrides with lowerhydrogen desorption temperatures compared to those induced under lower hydrogen pressuresand/or higher temperatures. The nanocomposite with MgH2, Mg2NiH4and CeH2.73incrystallite size of~10nm could be prepared, and its hydrogen storage kinetics have been wellimproved. Moreover, the evolution of the grain and particle sizes of the composite has beenstudied. The growth of grain and particle sizes during de/hydrogenation cycles deteriorates thehydrogen storage kinetics, and the thermodynamics of MgH2could not be altered in crystallitesize of tens of nanometers.In chapter4, a simple method has been proposed to induce a novel symbioticCeH2.73/CeO2catalyst in Mg-based hydrides, which is capable of massive fabrication.Moreover, a spontaneous hydrogen release effect at the CeH2.73/CeO2interface, which leads todramatic increase of catalysis than either sole CeH2.73or CeO2catalyst, has been revealed.Maximum hydrogen desorption temperature reduction of MgH2could reach down to~210oCas molar ratio of CeH2.73to CeO2was1:1. The dynamic boundary evolution during hydrogendesorption was observed in the symbiotic CeH2.73/CeO2at atomic resolution using in situHRTEM. Combining the ab-initio calculations, which show significant reduction of theformation energy of VH(hydrogen vacancy) in the CeH2.73/CeO2boundary region incomparison to those in the bulk MgH2and CeH2.73, we demonstrate that the outstandingcatalytic activity can be attributed to the spontaneous hydrogen release effect at theCeH2.73/CeO2interface.In chapter5, a series of Mg-based metallic glasses have been prepared and theirhydrogen storage behaviors at different temperatures have been studied. At room temperature,hydrogen absorption content shows close relation with the amorphous structure,hydrogenation capacity of the glassy Mg90Ce5Ni5alloy is twice larger than that of thecorresponding crystalline alloy. At higher temperatures (lower than Tg), Mg-based metallicglasses could aborb~5wt.%-H and transform into glassy hydrides, which shows aintermediate state between crystalline Mg and MgH2. The glassy alloys present higherhydrogen storage capacity than that of the corresponding crystalline alloys. With increas ofhydrogen content, the Mg atoms became more and more ordered. AFM and Cs-HRTEM experiments indicate there are remarkable phase segregation after hydrogen absorption.Hydrogen occupation in the metallic glasses have been ellucidated via both experimental andtheoritical study. Moreover, the hydrogen storage properties of the Mg-based metallic glasseshave been enhanced via addition of catalysts, size reduction and materials design etc. In thiswork, Mg65Ce10Ni20Cu5metallic glass shows the lowest hydrogen desorption temperature aslow as150oC. It has been shown that metallic glasses could be a novel and promising familyfor hydrogen storage with huge potentials for hydrogen-related applications. |
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