Hydrogen Sorption Mechanisms in Magnesium-based Nanolayers

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0932249ZhangOn-board hydrogen storage is considered to be one of the most challenging barriers to the realization of a hydrogen economy. Mg is one promising hydrogen storage candidate materials due to its relatively high hydrogen capacity (7.6 wt %). Unfortunately, hydrogen absorption and desorption in bulk Mg-based materials is only thermodynamically possible at temperatures above 573K, outside of desired operating range of PEM fuel cells. At the same time, the kinetics of hydrogen loading and release are limited by several factors, including the formation of impermeable barriers, the low diffusion rate of H inside Mg, the mobility of the metal/hydride interface during hydrogenation and the limited dissociation rate of H2 molecules on the Mg surface. While there have been numerous efforts to improve the kinetics of hydrogen sorption by reducing the characteristic hydrogen transport length as well as through the incorporation of catalysts, few of these approaches have been able to lower the temperature necessary for hydrogen desorption as this necessarily requires the destabilization of the hydrogen-carrying phase. Intellectual Merits: The PIs propose to investigate the hydrogen storage and release/loading behavior of nanostructured multi-layered thin films in which stress/strain as well as chemistry can be tuned to optimize the kinetics of hydrogen sorption and lower the stability of hydrogen-carrying phases. Tools such as HRTEM, STEM and EELS will be used to understand the fundamental mechanism of hydrogen sorption at nanometer length scale. Concurrently, advanced computational methods based on density functional theory will be used to provide a fundamental understanding of the effects of interface structure, strain and chemistry on the thermodynamics and kinetics of hydrogen storage. The specific objectives of this project are to: 1) investigate the influence of interface area density and structure on hydrogen desorption kinetics; 2) explore microstructural evolution of the synthesized thin films during hydrogenation; 3) predict the influence of chemistry, stress and film structure on the hydrogen diffusion kinetics through computational methods; 4) investigate the influence of metastable intermediate hydride phases on the kinetics of hydrogen sorption through characterization and first principles calculations; 5) explore H sorption at the molecular level through in situ XPS; and 6) investigate the influence of stress and stress evolution on the thermodynamic stability of Mg-carrying phases via microstructural characterization, computational investigation and in situ XPS studies.Broader impacts: The project will have broader impact in several areas, such as (a) training graduate and undergraduate students (NSF-REU) in materials and energy-related research through experimental and computational approaches; (b) providing students experience of working at national laboratories, (c) the development of courses at both the undergraduate and graduate levels; (d) recruiting minority students; and (f) disseminating the knowledge to broader audience through NSF-RET program.