Effects Of Nanocrystallization On Thethermodynamics And Hydrogen Storage Properties Of Mg-based Hydrogen Storage Alloys

Mg and Mg-based hydrogen storage alloys are one of the most attractive hydrogen storage materials due to their high hydrogen storage capacity and the abundance in resource. However, the high enthalpy of formation leading to the high desorption temperature and the sluggish kinetics limit their practical application. A lot of works have been done in the last few years on improving their hydrogen storage properties. It has been verified that to add catalysts or nanocrystallize the alloys could substantially improve the kinetics of Mg and Mg-based hydrogen storage alloys. The research on the impacts of non-equilibrium structure resulting from nanocrystallization on the thermodynamic properties of Mg and Mg-based hydrogen storage alloys, however, is less addressed. Hence, this thesis investigates the influence of interfacial free energy of nanostructure on the thermodynamic properties of Mg and Mg-based hydrogen storage alloys based on the thermodynamic models and the experiments. The structural change and its impact on hydrogen storage properties of Mg-based hydrogen storage alloys upon hydrogenation were also studied.We have found that the interfacial free energy change from metal state to hydride state is negative for the absorption process of nanocrystalline Mg and Mg2Ni powder systems, which would lower the absorption plateaus and increase hydrogen absorption temperature. For the desorption process,however, the interfacial change free energy is positive which reduces the enthalpy of desorption and decreases the desorption temperature, especially when the grain size is less than 10 nm, the nanosize-effect on the drop of the desorption temperature is significant.Moreover, the Mg1.5NiH4 was found partly dehydrogenates into Mg2NiH0.3 during ball milling. Meanwhile, the particle size of Mg1.5NiH4 were getting smaller and then getting bigger when Mg1.5NiH4 completely decomposed into Mg2Ni. Nevertheless, part of the desorbed hydrogen was absorbed by Mg2Ni to form Mg2NiH0.3 and the particle size becomes smaller again with longer ball milling time. On the contrary, some single-crystal particles, also formed and kept growing up to 100 nm in diameter. As the grain size decreases, the enthalpy of desorption decreases correspondingly. When the grain size is 10 nm, the enthalpy of desorption is 62.6 kJ / mol H₂, which is about 2 kJ / mol H₂ lower than the standard enthalpy of desorption. It is also found that the desorption activation energy of nanocrystalline Mg1.5NiH4 powder decreased to only 53 kJ / mol when the grain size is 10 nm, which is smaller than that of 85 kJ / mol of the initial powder. There is nucleation and growth mechanism for the desorption process of Mg1.5NiH4 and the reaction controlling step is one-dimensional diffusion.In multilayer Mg-based film systems, the interfacial free energy has a linear relationship with the interface thickness and it also relates to the interface fraction of Ni (Pd) and Mg in the multilayer films. The bigger the interface fraction of Ni (Pd) and Mg, the larger interfacial free energy in multilayer film is. In the Mg / Ni multilayer film system, the interfacial free energy is positive, and therefore benefits the hydrogen absorption process. While in the Mg / Pd multilayer film system, the interfacial free energy is negative, which is advantageous to the hydrogen desorption process.Mg / Ni and Mg / Pd multilayer films were prepared using magnetron sputtering. It has been revealed that the Mg layer in the multilayer film was the [001] preferred orientation, which is conducive to hydrogen absorption. The (002) diffraction peak of Mg disappeared first during hydrogen absorption process, at the same time, alloying occurred within the multilayer film. The enthalpy of desorption corresponding to MgH₂ and Mg2NiH4 in the Mg2.9Ni film were 68.6 and 61.3 kJ / mol H₂, respectively, which reduce by about 7 and 3 kJ / mol H₂ comparing to their standard enthalpy. The Mg / Pd multilayer film can absorb and desorb hydrogen at room temperature and it can desorb 2.6 wt.% hydrogen at 373 K. However, the multilayer structure disappeared and phase variation occurred during hydrogen absorption-desorption cycles. The new structure after phase variation was fairly stable in the hydrogen absorption and desorption process.The process of the phase variation within Mg/Pd multilayer film was investigated by x-ray diffraction and transmission electron microscopy. It shows that the interdiffusion between Mg and Pd occurred even at room temperature. The phase variation began by forming Mg6Pd, followed by the forming of Mg₅Pd₂ and MgH₂ during hydrogenation, furthermore, the excess Mg in the Mg / Pd multilayer film hydrogenated into MgH₂. During dehydrogenation, Mg₅Pd₂ reacted with MgH₂ to transform to Mg₆Pd again with releasing of H₂. That is, the reversible phase varied between Mg₆Pd and Mg₅Pd₂ accompanies the reversible phase various between Mg and MgH₂. The process can be expressed as: Mg + Pd→Mg₆Pd + Mg (rich) + H₂ Mg₅Pd₂+MgH₂. The disproportionation reaction plays a role in tailoring the thermodynamics of hydrogen absorption/desorption of Mg, which facilitates the hydrogen absorption/desorption process of Mg, this can be addressed as a mechanism that improves hydrogen absorption/desorption properties.