Dehydrogenation Kinetics and Modeling Studies of 2LiBH4 + MgH2 Enhanced by NbF5 Catalyst

Although LiBH4-MgH2 composite shows excellent hydrogen storage capacity, it still has the problem of sluggish kinetics. To improve the hydrogen desorption kinetics, mixtures of LiBH4-MgH2 (2:1) were doped with 4 mol % NbF5, Nb2O5, and Mg2Ni. The hydrogen desorption kinetics of these mixtures were compared using constant pressure thermodynamic forces in which the ratio of the equilibrium plateau pressure to the opposing plateau was the same for all of the reactions studied. The results showed NbF5 to be vastly superior to the other catalysts for improving the kinetics of the composite mixture with the hydrogen desorption rates being in the order: NbF5 >> Nb2O5 > Mg2Ni. Desorption enthalpies that were obtained from van't Hoff plots indicated that the thermodynamic stabilities of the catalyzed mixtures were in the order: NbF5 < Nb2O5 < Mg2Ni. Modeling studies indicated that chemical reaction at the phase boundary was the most likely process controlling the reaction rates for the catalyzed mixtures.