AbstractA composite of Ca(BH4)2−MgF2 is proposed as a reversible hydrogen storage system. The dehydrogenation and rehydrogenation reaction mechanisms are investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) and Raman spectroscopy. The formation of an intermediate phase (CaF2−xHx) is observed during rehydrogenation. The hydrogen content of 4.3 wt % is obtained within 4 h during the first dehydrogenation at isothermal and isobaric conditions of 330 °C and 0.5 bar H2, respectively. The cycling efficiency is evaluated by three release and uptake cycles together with absorbed hydrogen content in the range 5.1−5.8 wt % after 2.5 h (T = 330 °C and p(H2) = 130 bar). The kinetic properties on the basis of hydrogen absorption are comparable for all cycles. As compared to pure Ca(BH4)2 and Ca(BH4)2−MgH2 composite, Ca(BH4)2−MgF2 composite reveals the kinetic destabilization and the reproducibility of hydrogen storage capacities during cycling, respectively.