NbCl5 and CrCl3 catalysts effect on synthesis and hydrogen storage performance of Mg-Ni-NiO composites

Two kinds of novel materials, Mg-1.6 mol%Ni-0.4 mol%NiO-2 mol%MCl (MCl = NbCl5, CrCl3), along with Mg-1.6 mol%Ni-0.4 mol%NiO for comparison, were examined for their potential use in hydrogen storage applications, having been fabricated via cryomilling. The effects of NbCl5 and CrCl3 on hydrogen storage performance were investigated. A microstructure analysis showed that besides the main Mg and Ni phases, NiO and Mg2Ni phases were present in all samples. MgCl2 was only found in halide-doped samples and NbO was only found in NbCl5-doped samples after ball milling. The particle size decreased significantly after 7 h of cryomilling. MgH2, Mg2NiH4 and Mg2NiH0.3 were present in all the samples, while NbH2 was only observed in the NbCl5-doped sample after absorption. The NbCl5-containing composite exhibited a low onset absorption temperature of 323 K, which was 10 K lower than that of the no-halide doped catalyst. It absorbed 5.32 wt% of hydrogen in 370 s at 623 K under 4 MPa hydrogen pressure and can absorb 90% of its full hydrogen capacity in 50 s. Having an onset desorption temperature of 483 K in vacuum, the NbCl5-containing composite desorbed hydrogen faster than the no halide doped sample. The hydriding-dehydriding kinetics performance of the CrCl3-doped sample did not improve, but it did exhibit a lower onset desorption temperature of 543 K under 0.1 MPa, which was 20 K lower than that of the no-halide doped sample. NbO, NiO and NbH2 played important roles in improving absorption and desorption performances. (author)