Reversible De/hydriding Reaction Mechanism And Hydrogen Storage Properties Of Mg Based Solid Solution

The development and key problem of hydrogen storage materials, especially for Mg-based hydrogen storage alloys, were reviewed firstly. On the whole, the de/hydriding kinetics of Mg-based hydrogen storage alloys was significantly improved through modifying alloy composition, nanostructuring, doping catalysts and so on. While upon improvement of the thermodynamic properties, it was still not so satisfactory. Although the dehydriding enthalpy of MgH₂ was lowered in some degree by alloying and nanostructing, the thermodynamics of MgH₂ is still too high. The large-scale practical application of MgH₂ as a hydrogen storage material was strictly restricted by its high stability and sluggish dehdyriding kinetics. In this work, for the first time we presented that the dehydriding enthalpy of Mg-based solid solution could be lowered by its reversible de/hydriding phase transformation.In this dissertation, a series of Mg-based solid solutions, such as Mg-Sn, Mg-Al, Mg-In and Mg-In-Al, were prepared by the method of sinter-milling. Their hydrogen storage properties were measured by Sievert method and DSC measurement. The microstructure and phase transformation in the de/hydriding process were analyzed and characterized by XRD, SEM, TEM, and so on. In-situ analysis on dehydriding of the solid solutions was performed by high temperature X-ray diffraction. The de/hydriding mechanism of Mg-Sn, Mg-Al and Mg-In binary solid solutions and their effects on the hydrogen storage properties were revealed. Based on those binary systems, the de/hydriding mechanism and hydrogen storage properties of Mg（In, Al） ternary solid solution were further explored.The study on the preparation of mestable Mg（Sn） supersaturated solid solution indicated that the dissolving of Sn in Mg lattice was limited by the readily formation of Mg₂Sn in the milling process. Although the solubility of Mg（Sn） could not be remarkably extended by ball milling, the in-situ formed Mg₂Sn facilitated the refinement of particle and grain of Mg, and then nanostructured Mg/Mg₂Sn composite were synthesized finally. The activation properties and de/hydriding kinetics of Mg were improved, and dehydriding enthalpy was slightly lowered. However, the reduction of dehydriding entropy offsetted some destabilization effect of the lowered enthalpy.The hydrogen storage properties of Mg were improved by alloying with Al. In this work, the metastable Mg（Al） supersaturated solid solutions were prepared by ball milling,and the solubility of Mg（Al） at ambient temperature was extended to about 8 at.%. The Mg（Al） supersaturated solid solution transformed into low solubility Mg（Al）_L solid solution and Mg₁₇Al₁₂ phase through one hydriding and dehydriding. The followed de/hydriding of the Mg-Al alloy divided into three reversible steps. The dehydriding enthalpy of MgH₂ was lowered by the formation of Al₂Mg₃ and Mg₁₇Al₁₂ intermetallic compounds, together with the phase transformation of Al redissolving in Mg lattice forming Mg（Al）_L solid solution in the dehydriding process. For Mg₉₀Al₁₀ alloy, the dehydriding enthalpy was 70.8 kJ/mol, and its apparent activation energy of hydriding was drastically reduced to 56 kJ/mol.In order to improve the reversibility of hydrogenation and further lowing the reaction enthalpy of Mg-based solid solution, we prepared Mg（In） binary solid solution. The de/hydriding mechanism of Mg（In） solid solution above 573 K could be depicted by: Mg（In） + H₂（?）MgH₂+ββis an intermediate disordered solid solution phase with FCC-structure, which transforms to low temperatureβ′′phase in cooling process. Mg（In） solid solution decomposes into MgH₂ and indium when hydriding below 573 K, which is incompletely reversible. During dehydriding process, indium mainly reacts with MgH₂ forming Mg₃In, and trace of indium re-dissolves in Mg lattice forming Mg（In） solid solution. The intermediateβphase accelerates the decomposition of MgH₂ and reversibly transforming to Mg（In） solid solution, and improves the reversibility of the hydriding reaction of Mg（In） solid solution. The dehydriding enthalpy of Mg（In） solid solution was lowered by its reversible de/hydriding phase transformation with the participation of theβphase. The dehydriding enthalpy of Mg（In） solid solution was further lowered with the increasing of indium content. The dehydriding enthalpy of Mg_0.9In_0.1 solid solution was 65.2 kJ/mol.We further investigated the de/hydriding mechanism and hydrogen storage properties of Mg（In, Al） ternary solid solution based on the Mg-based binary solid solutions. Mg（In, Al） ternary solid solution disproportionates into MgH₂ andβphase by hydriding, and then recovered in dehydrogenation. Al was dissolved in the intermediateβphase, which inhibited the formation of Mg₁₇Al₁₂, Al₂Mg₃ and Al. The problem that Mg（Al） solid solution could not completely recover after hydrogenation was successfully resolved, and the dehydriding enthalpy was further reduced. As for Mg_0.85In_0.05Al_0.1, the dehydriding enthalpy was lowered to 62.9 kJ/mol.The de/hydriding kinetics of Mg-based solid solution was improved by doping with catalysts furtherly. The research indicated that the de/hydriding kinetics of Mg-based solid solution could be significantly improved by the in-situ formed dispersive TiH_1.7 which originated from the precursor of TiCl₃ in de/hydriding, and that the apparent activation energy of de/hydrogenation of Mg-based solid solution was significantly lowered. For Mg_0.9In_0.05Al_0.05 ternary solid solution doped with 10 wt.%TiCl₃, the apparent activation energy of hydrogen desorption was drastically reduced to 48 kJ/mol, and it could quikly absorb and desorb hydrogen at 493 K.