Improved dehydrogenation of MgH2–Li3AlH6 mixture with TiF3 addition

MgH2–Li3AlH6 mixture shows a mutual activation effect between the components. But the dehydrogenation kinetics is still slow, especially at temperature as low as 250 °C. Hereby, an additive (TiF3) was introduced into the mixture in the present study. The reaction mechanisms were studied by the combined analyses of X-ray diffraction (XRD), thermogravimetric analysis (TGA), as well as thermodynamic calculations. A two-step ball milling method could reduce the mechanical decomposition of Li3AlH6 effectively and was adopted. During milling, Li3AlH6 reacts with TiF3 and produces Al3Ti while MgH2 remains stable. All the species are well mixed after milling and the grain size is as small as 100 nm. During TGA test, all the reactions occur at lower temperatures compared with undoped mixture, especially the dehydrogenation of MgH2, which shows a decrease of 60 °C. Its activation energy is reduced by 32.0 kJ mol−1. The first three isothermal (250 °C) cycles indicate that the kinetics of dehydrogenation has been greatly enhanced, showing a reversible capacity of 4.5 wt.% H2. The time needed for the 1st dehydrogenation has been shortened to 3600 s from 8000 s for the undoped mixture. These improvements are mainly attributed to the catalytic effect of the in-situ formed Al3Ti. But there is no influence on the rehydrogenation kinetics and the enthalpy of the dehydrogenation of MgH2 is unchanged.

► MgH2–Li3AlH6 mixture shows a mutual activation effect.
► The reaction mechanisms were studied by XRD, TGA and thermodynamic calculations.
► Dehydrogenation of MgH2 shows a decrease of temperature by 60 °C with an activation energy reduce by 32.0 kJ mol−1.
► The doped mixture exhibits a relatively stable capacity of 4.5 wt.% H2 after the 1st cycle at 250 °C.