MgH2 is one of the most promising hydrogen storage materials due to its high capacity andlow cost. In an effort to develop MgH2 with a low dehydriding temperature and fastsorption kinetics, doping MgH2 with NiCl2 and CoCl2 has been investigated in this paper.Both the dehydrogenation temperature and the absorption/desorption kinetics have beenimproved by adding either NiCl2 or CoCl2, and a significant enhancement was obtained inthe case of the NiCl2 doped sample. For example, a hydrogen absorption capacity of5.17 wt% was reached at 300℃ in 60 s for the MgH2/NiCl2 sample. In contrast, theball-milled MgH2 just absorbed 3.51 wt% hydrogen at 300℃ in 400 s. An activation energyof 102.6 kJ/mol for the MgH2/NiCl2 sample has been obtained from the desorption data,18.7 kJ/mol and 55.9 kJ/mol smaller than those of the MgH2/CoCl2, which also exhibits anenhanced kinetics, and of the pure MgH2 sample, respectively. In addition, the enhancedkinetics was observed to persist even after 9 cycles in the case of the NiCl2 doped MgH2sample. Further kinetic investigation indicated that the hydrogen desorption from themilled MgH2 is controlled by a slow, random nucleation and growth process, which istransformed into two-dimensional growth after NiCl2 or CoCl2 doping, suggesting that theadditives reduced the barrier and lowered the driving forces for nucleation

The effect of Ti0.4Cr0.15Mn0.15V0.3 (termed BCC due to the body centered cubic structure) alloy on the hydrogen storage properties of MgH2 was investigated. It was found that the hydrogenated BCC alloy showed superior catalysis properties compared to the quenched and ingot samples. As an example, the 1 h milled MgH2 t 20 wt.% hydrogenated BCC shows a peak temperature of dehydrogenation of about 294℃. This is 16, 27 and 74℃ lower than those of MgH2 ball milled with quenched BCC, ingot BCC and an uncatalysed MgH2 sample, respectively. The hydrogenated BCC alloy is much easier to crush into small particles, and embed in MgH2 aggregates as revealed by X-ray diffraction and scanning electron microscope results. The BCC not only increases the hydrogen atomic diffusivity in the bulk Mg but also promotes the dissociation and recombination of hydrogen. The activation energy, Ea, for the dehydrogenation of the MgH2/hydrogenated BCC mixture was found to be 71.2 5 kJ mol H21 using the Kissinger method. This represents a significant decrease compared to the pure MgH2 (179.7 5 kJ mol H21), suggesting that the catalytic effect of the BCC alloy significantly decreases the activation energy of MgH2 for dehydrogenation by surface activation.

We have investigated the hydrogen storage properties of the LiAlH4-LiBH4 system, both un-doped and doped with titanium based catalysts. It was found that TiF3 exhibited the superior catalytic effects in terms of enhancing the hydriding/dehydriding kinetics and reducing the dehydrogenation temperature of the LiAlH4-LiBH4 system. Compared to the un-doped LiAlH4-LiBH4 system, the onset temperatures of the 5mol% TiF3-doped sample for the first and second dehydrogenation steps were decreased by 64 and 150℃, respectively. X-ray diffraction patterns of the dehydrogenated samples revealed that the produced Al from LiAlH4 could react with B from the decomposition of LiBH4 to form AlB2 and LiAl compounds. Pressure-composition-temperature (PCT) and van’t Hoff plots made it clear that the decomposition enthalpy of LiBH4 in the TiF3-doped LiAlH4-LiBH4 system is decreased from 74 kJ/(mol of H2) for the pure LiBH4 to 60.4 kJ/(mol of H2). The dehydrogenation products of the TiF3-doped LiAlH4-LiBH4 sample can absorb 3.76 and 4.78 wt.% of hydrogen in 1h and 14h, respectively, at 600℃ and under 4MPa of hydrogen. The formation of LiBH4 was detected by X-ray diffraction in the rehydrogenated sample.

The hydrogen storage properties of 2NaBH4 +MgH2 system were studied. It was found that the presence of MgH2 could destabilize the decomposition of NaBH4, decreasing the dehydrogenation temperature about 40℃ compared with the pure NaBH4. It is believed that the formation of MgB2 upon dehydrogenation stabilizes the dehydrogenated state and, thereby, destabilizes the NaBH4. For the desorption the following two-step reaction was observed: 2NaBH4 +MgH2→2NaBH4 +Mg+H2→2NaH +MgB2 +4H2. Furthermore, various catalysts such as TiF3, TiO2, Zr, Si and BCC alloy were doped to the NaBH4-MgH2 system. Among these catalysts, TiF3 exhibited the optimum behavior in terms of fast kinetics and lowering the dehydrogenation temperature of the NaBH4-MgH2 system. The rehydrogenation experiments of TiF3-doped NaBH4-MgH2 system were investigated at 600℃ with an initial hydrogen pressure of about 4MPa. It showed that 5.89 wt.% hydrogenwas rehydrogenated within 12 h. XRD results demonstrated the formation of NaBH4 and MgH2 in the rehydrogenated sample.

The hydrogen storage and electrochemical properties of the Ti0.32Mn0.22Cr0.1V0.28Nix (x=0, 0.05, 0.15, 0.2) alloyswere investigated. XRD results revealed that the addition of Ni resulted in the formation of C14 Laves phase. With the increasing Ni content, the lattice parameter a of BCC phase and the lattice parameter a and c of C14 Laves phase decreased. The Ni addition reduced the hydrogen absorption/desorption capacities at gas solid reaction, but improved the electrochemical discharge capacity significantly. The maximum discharge capacity of 392mAhg-1 was obtained for the Ti0.32Mn0.22Cr0.1V0.28Ni0.15 alloy. Cycling measurement revealed that the discharge capacity of the Ti0.32Mn0.22Cr0.1V0.28Ni0.15 alloy degraded with the increasing cycle number, decreasing to 63% of the maximum discharge capacity after 100 cycles. Further improvement on the activation and cycle life of this alloy can be achieved by surface modification with 20 wt.% LaNi3.55Co0.75Mn0.4Al0.3 addition.