Mechanochemical synthesis of magnesium-based hydrogen storage materials

Abstract

A systematic investigation of the structural stability, evolution and hydrogenstorage properties of Mg-based hydrides was carried out, involving mechanical milling and chemical alloying. The effects of milling on particle size, lattice parameter, microstructure, and phase composition of the powder mixtures were characterised using SEM, X-Ray diffraction and quantitative Rietveld analyses. Mechanical milling was shown to be an effective method of refining the particle size, particularly when MgH2 is involved. The influences of the selected chemical elements, including transition metals, graphite carbon and rare-earth metals, on hydrogen desorption/absorption of various milled mixtures were clearly identified using coupled Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC). The as-received MgH2 shows an onset desorption temperature of 420°C. Mechanical milling reduces the onset temperature to 330°C. Chemical alloying, via surface catalysis and/or solid-solutioning, further increases the desorption kinetics and reduces the desorption temperature down to 250°C. The degree of such effect decreases from Ni, Al, Fe, Nb, Ti, to Cu. Further comparison of desorption characteristics of MgH2 mixed and mechanically alloyed with Ni clearly shows that the kinetic improvement and the effective reduction of the desorption temperature is mainly due to a catalytic effect, rather than solid-solutioning of Ni. Although posing little influence on desorption characteristics, graphite improves the absorption behaviour of MgH2. The rare earth metals, Y and Ce, do not seem to influence hydrogen desorption of MgH2 due to the formation of stable hydride phases, but CeO2 in the (MgH2+Ce) mixture provides a beneficial effect on desorption kinetics. Multi-component mixtures of (MgH2+15Fe+5Ce) and (MgH2+Al+Ni+Y+Ce) exhibit relatively fast desorption kinetics and the lowest desorption temperature at about 240°C and 220°C, respectively. Finally, mechanical alloying of the non-stoichiometric compositions of (3MgH2+Fe) and (4MgH2+Fe) effectively generated a new ternary hydride, Mg2FeH6, with a very high yield of about 80wt% from the (3MgH2+Fe) mixture, which is a promising candidate for heat-storage. The research findings laid a clear and valuable foundation for future development of new and cost-effective Mgbased hydrogen storage materials with a high capacity, a low desorption temperature and rapid kinetics.