Complex quaternary hydrides Mg2(Fe,Co)Hy for hydrogen storage

Complex ternary hydrides based in Mg and transition metals are very attractive materials for hydrogen and energy storage due to their large volumetric capacity, up to 150 kgH₂/m³ in Mg₂FeH₆ and their high dissociation enthalpies. These compounds may be produced at room temperature by mechanical milling of the constituents in H₂ atmosphere. This technique has also served to explore the synthesis of quaternary hydrides Mg₂T_1−zT’_zH_y, combining two transition metals to optimize the properties of the resulting hydride. In the present work we analyze the mechanical synthesis of the compounds Mg₂Fe_1−zCo_zH_y (z = 0, ¼, ½, ¾, 1) by mechanical alloying at room temperature Mg-Fe-Co powder mixtures in adequate proportion, at 0.3 MPa H₂. We follow the mechanosynthesis process trough the analysis of the hydrogen absorption kinetic curves. Samples obtained after a steady state was reached were characterized by X ray diffraction and Mössbauer spectroscopy. The different stages in the mechanosynthesis of these complex hydrides are discussed in terms of the composition and initial state of the powder mixture.