Synthesis mechanism and properties of Mg-Mg₂Ni composite hydrogen storage alloy produced by hydriding combustion synthesis

Hydriding combustion synthesis (HCS) has been regarded as an innovative process to produce magnesium based hydrogen storage alloys. In the present paper, a Mg-Mg₂Ni composite hydrogen storage alloy was prepared by the HCS process under moderate conditions. Phase composition, synthesis mechanism and hydrogen storage properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure composition temperature (PCT). The results indicated that the HCS product was composed mainly of MgH₂, Mg₂NiH₄ and Mg ₂NiH_0.3. A gas-solid reaction mechanism played an important role in the HCS process. The hydriding activity of the as synthesised product was very high so that it absorbed 4.61 mass% hydrogen in the first hydriding process without any activation treatment and the maximum hydrogen storage capacity achieved was 5.24 mass%. Moreover, the hydriding rate was excellent. For example, it could absorb >3.20 and 4.05 mass% hydrogen within 4 min in the first and forth hydriding/dehydriding cycle respectively. The relationships between the plateau pressure and temperature were: IgP(0.1 MPa)=-4250.5/T+7.9635 (hydriding), IgP(0.1 MPa)= -4125.4/T+ 7.6502 (dehydriding) for the lower plateau, and IgP(0.1 MPa)= -2894.6/T+ 5.9656 (hydriding), IgP(0.1 MPa)= -3855.9/T+ 7.3435 (dehydriding) for the higher plateau.