Reversible hydrogen storage in destabilized LiAlH₄-MgH ₂-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, LiAlH ₄-LiBH₄, and MgH₂-LiBH₄). On doping with TiF₃, the system starts to release hydrogen at 60 °C and completes dehydrogenation below 400 °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 H₂, which is smaller than that of LiBH₄ alone (74 kJ/mol H₂). In addition, the TiF₃-doped system is partially reversible at moderate temperature and pressure (4 MPa, 400 °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. © 2010 American Chemical Society.