Kinetic and structural studies of Mg-based materials for hydrogen storage and switchable mirror applications [electronic resource]
Keywords: Materials Science and Engineering.

Hydrogen has long been identified as a promising energy carrier in our efforts to usher in a new era of energy technologies to reduce our dependence on carbon emitting fossil fuels. There is already a lot of activity towards bringing the hydrogen economy to fruition, but significant innovations are still necessary in the materials used for hydrogen production, storage, delivery and use to make a cost-competitive case for widespread adoption. Metal hydrides are an attractive alternative to store hydrogen for on-board use in vehicles, with good volumetric energy density as compared to the energy intensive compression or liquefaction used today. Magnesium hydride based materials in particular has been proposed as candidates to explore for hydrogen storage applications. The structure of these hydrides and the thermodynamics and kinetics of hydrogen sorption and desorption from them need to be well understood to improve them to meet the necessary targets for practical use. Although MgH2 has a good theoretical storage capacity, it suffers from poor hydrogenation and dehydrogenation kinetics at room temperature. Our unique Pd-catalyst 'nanoportal' model structure allows controlled nucleation and growth of the hydride in the underlying Mg films, thus enabling determination of the hydrogenation reaction mechanism. Using kinetic modeling based on the well-known Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis, I discovered that the hydrogenation reaction in our structure was limited by the flux of hydrogen atoms at the Pd catalyst-Mg hydride interface. Interfacial engineering to improve this contact would help improve hydrogenation kinetics. Mg can be improved upon as a hydrogen storage material by alloying additives into it. A number of transition metal additives to Mg are seen to decrease the dehydrogenation temperature of MgH2 as well as improve the H2 sorption and desorption kinetics. The transparent nature of the hydrides make them attractive candidates for switchable gasochromic mirror applications as well. In this work, I synthesized and characterized the heretofore unexplored Mg-Ta alloy system, to obtain information about its phase diagram, which correlated well with theoretical calculations. I found that the Mg-Ta system forms metastable solid solutions across a wide composition range, even though they have unfavorable thermodynamics of mixing. I describe the difficulties we faced in synthesis of Mg-Ta alloys due to the phenomenon of resputtering, and our way to quantify this phenomenon and overcome it. I investigated the use of these Mg-Ta alloys for hydrogen storage and switchable mirror applications. These alloy thin films were seen to reversibly switch from reflective metal to transparent hydride states within a few seconds due to dramatically enhanced hydrogen sorption and desorption kinetics, compared to Mg. I found that the Ta additive plays a catalytic role in these structures and its location plays an important part in influencing these reactions. Kinetic modeling revealed that the mechanism of hydrogen desorption in these thin film structures is a one-dimensional growth, which is enhanced by the presence of Ta and limited by diffusion through the MgH2 in its absence. Structural characterization revealed that the Ta segregates out of the Mg-Ta alloy as early as the second dehydrogenation cycle, but still enhances the kinetics of hydrogenation and dehydrogenation by acting as a pathway to enhance diffusion of the hydrogen out of the metal structure.