核壳结构纳米复合镁基储氢合金的制备及热力学调控机理研究

The essential issue for improving the hydrogen storage properties of Mg-based alloys is how to reduce the dehydriding enthalpy by nanostructuring while keeping the structural stability of nanosized magnesium particles in the de-/hydrogenation cycles. The present project proposes a novel core-shell nanocomposite structure which could provide unique interface characteristic and stress regulation to alter the thermodynamics of magnesium hydride. The main research contents of this project include: the synthesis of core-shell nanostructured Mg-based hydrogen storage alloys with different shell materials, size and interface structure; The characterization and determination of microstructure and de-/hydriding kinetic and thermodynamic properties; The effect of shell materials on the core-shell interface structure and hydrogen storage performances; The de-/hydriding reaction mechanism of core-shell nanostructured Mg-based alloys; The hydriding and dehydriding characteristics of core-shell interface and its influence on the hydrogen sorption of magnesium based alloy; The formation mechanism of stress, the microstructure factors such as core size, shell thickness and core-shell interface, and the evolution of stress in the core-shell nanocomposite structure. The abovementioned study would elucidate the effects of interfacial stress on the thermodynamic properties, i.e. de-/hydriding equilibrium pressure,hydriding and dehydriding plateau hysteresis, reaction enthalpy, reaction entropy of Mg-based hydrogen storage alloys. The successful implementation of this project would open a new way to tune the thermodynamic properties of Mg-based hydrogen storage alloys, and promote the rapid development of other high-capacity hydrogen storage materials.

如何利用纳米结构改善镁基合金的储氢性能并保持纳米结构和储氢性能的循环稳定性是解决镁基储氢合金实用化的关键问题。本项目提出利用核壳纳米结构中独特的界面和应力作用来调控镁基储氢合金的热力学性能。项目拟制备不同壳层材料、不同核/壳层尺寸的纳米镁基合金，准确表征核壳纳米复合镁基合金的微观结构、储氢热力学和动力学特性，揭示其吸/放氢反应机理；研究核壳界面结构的吸放氢特性及其对镁基合金储氢性能的影响；阐明核壳结构中应力的产生机制、微观结构影响因素（核层尺寸、壳层厚度、核壳界面等）及演变规律，全面揭示应力作用和界面特性对镁基合金吸/放氢平衡压、吸/放氢平台滞后、反应焓、反应熵等热力学参数的调控机理。本项目研究将为镁基储氢合金的热力学调控提供新的思路和理论指导，并为其它高容量储氢材料体系的热力学调控提供借鉴作用。

镁氢化物是一类极具应用前景、具有较高容量的储氢材料，但由于过高的热力学稳定性导致脱氢温度，不能得到实际应用。本项目围绕镁氢化物的热力学储氢性能改善，基于纳米化、纳米核壳结构、合金化等策略，开展了纳米镁及镁氢化物的制备、镁氢化物纳米核壳结构的制备、Mg-X(Ag, Al, Zn)多元合金制备等研究工作，研究了纳米化、核壳结构、多相复合结构对镁氢化物放氢热力学性能的影响规律，取得以下主要研究结果：1）采用创新的热场辅助球磨工艺和二丁基镁还原转化方法，获得镁氢化物平均晶粒尺寸仅为6nm，是目前文献报道的最小镁氢化物晶粒尺寸。PCI测试结果表明，超细纳米镁氢化物的可在255℃放氢，放氢平衡压为0.56 bar，远高于普通粗晶及纳米镁氢化物（0.25bar）。van’t Hoff拟合结果则表明，超细纳米镁氢化物的放氢反应焓变从74.7 kJ/mol H下降到66.8 kJ/mol，表明其热力学性能得到了大幅改善。这一工作首次证实了镁氢化物的纳米尺寸效应，表明当镁氢化物的晶粒尺寸小于6nm，镁氢化物的热力学稳定性大幅降低，这与理论计算结果是一致的。2）采用溶胶-凝胶法在球形纳米镁颗粒（颗粒200-500nm）的表面包覆钛氧化物，钛氧化物壳层厚度为60-100nm。与未包覆的纳米Mg颗粒相比，Mg@TiOx平衡压均有所升高。结果表明，球形纳米镁材料的放氢反应焓变由ΔH=-74.5 kJ/mol H降低至ΔH=-73.4 kJ/mol。首次证实了核壳结构对镁氢化物的热力学性能改善效果。项目在学术刊物上发表论文 11 篇，获得授权专利2项，培养研究生 5 名，完成了预定研究目标。