NMR and X-ray Diffraction Studies of Phases in the Destabilized LiH-Si System
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N3.6.1
NMR and X-ray Diffraction Studies of Phases in the Destabilized LiH-Si System R. C. Bowman, Jr.1, S.-J. Hwang2, C. C. Ahn3, and J. J. Vajo4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. 2 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, U.S.A. 3 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, U.S.A. 4 HRL Laboratories, LLC, Malibu, CA 90265, U.S.A. 1
ABSTRACT Hydrogen absorption and desorption isotherms have been measured on ballmilled mixtures of LiH+Si powders to evaluate the thermodynamic parameters of these reversible reactions. The phase compositions at the various stages of reaction have been examined by Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR) of the 7Li, 1H, and 29Si nuclei and powder x-ray diffraction (XRD). The initial mixtures of LiH and Si were found to convert into known Li-Si silicide intermetallics (i.e., Li12Si7, Li7Si3, and Li13Si4) as well as providing evidence for a previously unknown ternary Li-Si-H phase as hydrogen was first desorbed and then absorbed. While the absorption reactions are reversible over portions of the Li-Si-H composition range, incomplete recovery of the original LiH + Si phases was also observed under some test conditions. INTRODUCTION In order to meet U.S. DOE hydrogen storage goals proposed for years 2005 and 2010, metal hydrides will need to be composed mainly of light elements (i.e., Li, B, Mg, Al, etc.). The novel approach of destabilizing hydrogen-rich but strongly bound hydrides such as LiH via alloying with Si has been shown to improve substantially their potential as hydrogen storage materials in fuel cell powered vehicles [1]. The LiH + Si system produces a reversible hydrogen storage capacity totaling ~5.0 wt.% with a 4-to-5 order-of-magnitude increase [1] in the equilibrium pressure compared to just LiH for temperatures below 800 K. Volumetric measurements of the hydrogen absorption and desorption isotherms in the 650 K – 780 K temperature range reveal two and three plateau regions on lightly ballmilled mixtures of 2.5LiH+Si and 4.4LiH+Si, respectively. In general, for two components, hydride destabilization through alloy formation upon dehydrogenation involves the reaction [1] nAH x + mBH y ↔ An Bm + 1/2(nx +my)H 2
(1)
where AHx and BHy are binary or more complex hydrides, and n and m are specified by the stoichiometry of the AB alloy. For the LiH/Si system, a prototypical reaction would be 4LiH + Si↔ Li 4Si + 2H2
(2)
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Because the formation of SiH4 is endothermic, hydrogenation of Si is not expected and thus reaction 2 could be reversible. Because crystalline silicon does not hydrogenate, the Si addition reduces the gravimetric hydrogen density from that of the pure hydrides. For reaction 2, the hydrogen capacity is 6.6 wt.% compared to 12.7 wt.% for pure LiH. Since there are several Li-Si phases [2-4], which are listed in Table I along with their structure references and known
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