On-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.
The goal of this project is to explore---thorough the synergistic combination of experimental and computational methods---the influence of strain, interfacial structure and alloying on the kinetics and thermodynamics of hydrogen storage in nanostructured Mg-based materials. We proposed to use innovative approaches to synthesize multi-component, Mg-based, nanolayer films that contain significant interfacial areas. Hydrogen storage technology is vital for the application of hydrogen as an alternative fuel for sustainable energy related applications. Mg is a promising light-weight material that has superior hydrogen storage capacity (7.6 wt. % of hydrogen) and low cost for large scale applications. However, the stability of Mg hydride is a key challenge for on-board applications of Mg. Bulk Mg hydride has a tetragonal crystal structure (referred to as T-MgH2), and desorbs H2 at ~ 573K. In contrast the application of H for automobile fuel cells requires a H2 desorption temperature at ~ 350K. In spite of active studies in the past decade, such a goal has not been achieved for practical application of T-MgH2. Intellectual merit. The major scientific findings of this NSF-hydrogen project can be summarized as follows. First, we demonstrate that stress-induced orthorhombic Mg hydride (O-MgH2) is thermodynamically destabilized at ~ 373K or lower. Such drastic destabilization arises from large tensile stress in single layer O-MgH2 bonded to rigid substrate, or compressive stress due to large volume change incompatibility in Mg/Nb multilayers. H desorption occurred at room temperature in O-MgH2 10 nm / O-NbH 10 nm multilayers. Ab inito calculations show that constraints imposed by the thin-film environment can significantly reduce hydride formation enthalpy, verifying the experimental observations. These studies provide key insight on the mechanisms that can significantly destabilize Mg hydride and other type of metal hydrides. Second, discovery of size effect on hydrogen sorption in Mg films. We show that sputtered Mg films absorbed H2 at 373K and evolved into a metastable orthorhombic Mg hydride phase. Thermal desorption spectroscopy studies show that thinner dense MgH2 films desorb H2 at lower temperature than thicker porous MgH2 films. Meanwhile MgH2 pillars with greater porosity have degraded hydrogen sorption performance contradictory to general wisdom. The influences of stress on formation of metastable MgH2 phase and consequent reduction of H sorption temperature are discussed. Third, investigation of phase stability via a combination of experimental and theoretic approach. Our experiments suggest that bcc Mg can be stabilized when grown in Mg/Nb multilayers. Bcc Mg has only been observed under very high pressures and is in fact (mechanically) unstable under room conditions. Density functional theory calculations (DFT) were performed to gain insight into the stability of Mg in the bcc structure. Calculations show that bcc Mg is in fact metastable under thin film conditions, when Mg grows epitaxially on bcc Nb, in agreement with experiments. Furthermore the classical thermodynamic approach has been used for describing the pseudomorphic growth in Mg/Nb multilayer films. The bi-phase diagram of these films has been predicted theoretically and the predictions were verified experimentally by studying multiple Mg/Nb nanolayers thin films. Broader impact. (a) Outreach to scientific community. During this NSF project, we collaborated with scientists from Argonne National Laboratory, Los Alamos National Laboratory, University of North Texas, and Liaoning Shihua University (China). (b) Dissemination of results to broader audience. The PIs have presented these studies at numerous national and international conferences. They have also presented recent studies at their undergraduate and graduate classes with more than 100 undergraduate and graduate students. (c) Professional preparation for students. Two Ph.D. students (including 1 female student) were directly involved in this project. One of them has become a processing engineer at Intel (Oregon), and one female student becomes a scientist at a national laboratory in Thailand. One undergraduate student was involved in this project. One visiting professor from China was on sabbatical for one year to work on this project. (d) Research outcome. 6 articles were published in peer-reviewed scientific journals (plus more in revision and in preparation).
