NaBX4-MgX2 Composites (X= D, H) Investigated by IN Situ Neutron Diffraction
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NaBX4-MgX2 Composites (X= D, H) Investigated by In Situ Neutron Diffraction D. Pottmaier1, S. Garroni2, M. Brunelli3, G. B. M. Vaughan4, A. Castellero1, E. Menéndez2, M.D. Baró2, and M. Baricco1 1 Dipartimento di Chimica IFM - NIS - Università di Torino - Turin, Italy; 2 Departament de Física - Universitat Autònoma de Barcelona - Barcelona, Spain; 3 D20 - Institut Laue-Langevin - Grenoble, France; 4 ID11 - European Synchrotron Radiation Facility - Grenoble, France. ABSTRACT Light element complex hydrides (e.g. NaBH4) together with metal hydrides (e.g. MgH2) are considered two primary classes of solid state hydrogen storage materials. In spite of drawbacks such as unfavorable thermodynamics and poor kinetics, enhancements may occur in reactive hydride composites by nanostructuring of reactant phases and formation of more stable product phases (e.g. MgB2) which lower overall reaction enthalpy and allow reversibility. One potential system is based on mixing NaBH4 and MgH2 and subsequent ball milling, which in a 2:1 molar ratio can store considerable amounts of hydrogen by weight (up to 7.8 wt%). A study of the 2NaBX4 + MgX2 → MgB2 + 2NaX + 4X2 (X=D,H) reaction is assessed by means of in-situ neutron diffraction with different combinations of hydrogen and deuterium on the X position. The desorption is established to begin at temperatures as low as 250 °C, starting with decomposition of nanostructured MgX2 due to joint effects of nanostructured MgX2 and its reducing effect at NaBX4. Analyses of background profile, due to the high incoherent neutron scattering of hydrogen, as a function of temperature demonstrate direct correlation of H/D desorption reactions with relative phases amount.
INTRODUCTION Hydrogen gas has the highest heating value among all chemical fuels, but it has also low energy content per unit volume [1]. Therefore, finding an alternative way to store hydrogen is of critical importance for the implementation of an efficient hydrogen energy system. Hydrogen can be stored in the solid state in two modes: in ad/physorption - hydrogen remains in its molecular form interacting with compounds (e.g. Carbon polymorphs) by weak bonds; whereas in ab/chemisorption - hydrogen in its atomic form reacts with other elements (e.g. Alkali metals) by forming primary bonds. With its high hydrogen storage capacity of 10.8 wt%, NaBH4 is an excellent candidate for solid hydrogen storage applications, but its high thermal stability and lack of reversibility hinder its use. Enhancements may occur in reactive hydride composites (RHC), a novel approach consisting on mixing a complex hydride (e.g. NaBH4) with a single hydride (e.g. MgH2) which lowers overall reaction enthalpy and allows reversibility of this hydrogen storage composite system [2]. Significantly, improved ab- and desorption kinetics are achieved through RHC systems compared to single complex hydrides, while maintaining high hydrogen storage capacities. For NaBH4-MgH2 system, possible decomposition reactions are the following: 2NaBH4 + MgH2← → 2NaH + Mg
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