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

Abstract

Two kinds of novel materials, Mg–1·6 mol%Ni–0·4 mol%NiO–2 mol%MCl (MCl = NbCl ₅ , CrCl ₃ ), 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 NbCl ₅ and CrCl ₃ on hydrogen storage performance were investigated. A microstructure analysis showed that besides the main Mg and Ni phases, NiO and Mg ₂ Ni phases were present in all samples. MgCl ₂ was only found in halide-doped samples and NbO was only found in NbCl ₅ -doped samples after ball milling. The particle size decreased significantly after 7 h of cryomilling. MgH ₂ , Mg ₂ NiH ₄ and Mg ₂ NiH_0·3 were present in all the samples, while NbH ₂ was only observed in the NbCl ₅ -doped sample afterabsorption. The NbCl ₅ -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 NbCl ₅ -containing composite desorbed hydrogen faster than the no-halide doped sample. The hydriding–dehydriding kinetics performance of the CrCl ₃ -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 NbH ₂ played important roles in improving absorption and desorption performances.