Novel Ultrathin Mg Nanoblades for Hydrogen Storage
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1216-W05-02
Novel Ultrathin Mg Nanoblades for Hydrogen Storage Fu Tang , Gwo-Ching Wang, and Toh-Ming Lu Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute 110 8th Street, Troy, New York 12180-3590
ABSTRACT We describe the growth of novel ultrathin Mg crystalline nanoblades by oblique angle vapor deposition. These nanoblades were then coated with catalyst Pd and hydrogenated into magnesium hydride MgH2. In situ thermal desorption spectroscopy study showed a low H desorption temperature at ~365 K. In situ reflection high energy electron diffraction patterns were used to study the temperature dependent structure and composition changes during the dehydrogenation of Pd coated MgH2 nanoblades. The diffraction rings reveal the formation of alloys of Pd and Mg when the temperature is over ~480 K. Transmission electron microscopy diffraction also supports the formation of Pd and Mg alloys. This alloying reduces the cycling capability of Mg hydride. The de-hydrogenation of MgH2 introduces a strain at the bilayer interface between MgH2 and Mg resultant from 30% volume reduction from MgH2 to Mg and formed curved nanoblades as evident by scanning electron microscopy images. Designing factors of recyclable simple hydrides will be discussed. INTRODUCTION Various simple metal hydrides, complex metal hydrides [1], metal organic framework compounds [2], and activated carbon aerogels, have been studied for hydrogen storage [3]. The keys for successful hydrogen storage materials are high hydrogen storage capacity, fast kinetics of hydrogenation and dehydrogenation under low reaction temperature and pressure, minimum deterioration and reversal during cycling, safety, low weight, space-efficiency, and affordable cost. Magnesium (Mg) is an attractive candidate for a hydrogen storage material. Mg is abundant and has a low density (1.738 g/cm3). It is weight- and space-efficient and therefore good for mobile applications. Theoretically, magnesium hydride (MgH2) contains 7.6 wt. % of hydrogen, which is above the 6 wt. % system requirement of the DOE hydrogen plan. In a recent study, Leon et al. successfully stored 7.5 % hydrogen in air-exposed, highly textured Mg thin films [4, 5]. However, the hydrogenation-dehydrogenation process of Mg is slow and requires a high temperature (~623 K or 350 oC), possibly because the hydrogen molecules do not readily dissociate on the Mg surface [6, 7] and magnesium hydride has high thermodynamic stability [812]. The poor diffusion of hydrogen in the hydride also kinetically limits the sorption rate [13]. Until now, the slow kinetics and high sorption and desorption temperatures have prevented the practical use of Mg as a hydrogen storage material. Various ways to improve hydrogen kinetics, sorption and desorption temperatures, and pressure have been explored. Hydrogen adsorbing materials with nanoscale grain structures that have high surface areas are emerging. Among them, two methods have been demonstrated to accelerate the reaction rate significantly: ball-milling o
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