HYDROGEN STORAGE IN MAGNESIUM HYDRIDE-BASED MATERIALS PREPARED BY HIGH ENERGY BALL MILLING AND BY MELT SPINNING

The implementation of fuel cell driven light vehicles may represent an improvement of the quality of environment and life, but one of the major technical problems for their commercial introduction is represented by the onboard hydrogen storage. The alternative solution to high pressure vessels or liquid hydrogen reservoirs may consist in storing hydrogen into solid absorbers which allow volumetric densities comparable or higher than liquid hydrogen and are intrinsically safe. Nanosized Mg hydride samples, mixed with suitable catalysts, are promising candidates for this purpose. Magnesium hydride powders mixed with 0.5 mol% of Nb₂O₅, with and without graphite, were ball milled in argon atmosphere. Hydrogen absorption/desorption tests, as well as pressurecomposition isotherms, were obtained on the treated material by a Sievert volumetric device. It is found that the coexistence of niobium pentaoxide and graphite improves the kinetics of MgH₂ with respect to the literature data. Magnesium-rich Mg-Ni-Fe compounds were prepared by the melt spinning technique and subsequently ball milled for short times. Samples with similar composition were obtained by simply ball milling elemental powders. The hydrogen desorption kinetics were found better for the former samples than the latter ones. By Mossbauer spectroscopy measurements it is found that iron dissolves into the Mg₂Ni lattice in the samples prepared by the melt spinning procedure, and that after multiple hydrogen absorption/desorption cycles the main part of iron separates from the Mg₂NiH₄ lattice.