Reversible Hydrogen Storage in Destabilized LiAlH4-MgH2-LiBH4 Ternary-Hydride System Doped with TiF3

This paper reports the hydrogen storage properties of a ternary hydride system, LiAlH4-MgH2-LiBH4 (molar ratio 1:1:1), both undoped and doped with TiF3 addition. It was found that there is a mutual destabilization among the three hydrides. This new ternary system possesses superior hydrogen desorption properties compared with the unary components (LiAlH4, MgH2, and LiBH4) or binary mixtures of those components (LiAlH4-MgH2, LiAlH4-LiBH4, and MgH2-LiBH4). On doping with TiF3, the system starts to release hydrogen at 60 degrees C and completes dehydrogenation below 400 degrees C. Three major dehydrogenation steps were observed in the undoped and TiF3-doped systems, which corresponds to the decomposition of LiAlH4, MgH2, and LiBH4, respectively. X-ray diffraction (XRD) measurements on the as-dehydrogenated samples were executed to identify the dehydrogenation pathway. The third step decomposition enthalpy of the doped system was determined by pressure-composition-temperature (PCT) measurements and the van't Hoff equation to be 54 kJ/mol H-2, which is smaller than that of LiBH4 alone (74 kJ/mol H-2). In addition, the TiF3-doped system is partially reversible at moderate temperature and pressure (4 MPa, 400 degrees C) with good cycling. The enhancement of the hydrogen sorption properties was attributed to the formation of intermediate compounds, including Li-Mg, Mg-Al, and Mg-Al-B alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway. The TiF3 component in the doped system plays a catalytic role through the formation of Ti-containing and F-containing catalytic species, which strengthens this interaction and thus further improves the dehydrogenation and hydrogenation of this system.