Synthesis, hydrogen storage properties and thermodynamic destabilization of Mg-TixCr0.8-xV0.2 (x=0.25, 0.35, 0.45, 0.55) nanocomposites

Mg-20 wt% Ti_xCr_0.8-xV_0.2 (x = 0.25, 0.35, 0.45, 0.55) nanocomposites were synthesized by Reactive Ball Milling (RBM) under 5.0 MPa H₂ for 8 h at room temperature. The morphology, crystal structure, and isothermal hydrogen ab-/desorption properties of the as-milled nanocomposites were investigated. X-ray diffraction confirms that the composites were composed of nanocrystalline β-MgH₂, γ-MgH₂ and Ti-Cr-V_0.2H_y (1.91 < y < 2.01) phases. High-Resolution Transmission Electron Microscopy (HRTEM) images reveal that nanocrystalline β-MgH₂, γ-MgH₂ and Ti_0.35Cr_0.45V_0.2H_y phases coexist on nano-level in the as-milled composites. Differential scanning calorimeter (DSC) results show that MgH₂-20 wt% Ti_0.35Cr_0.45V_0.2 composite presents the minimum decomposition temperature (256 °C) among the studied composites and a relatively low activation energy of 86.43 kJ/mol. The hydrogen desorption capacity of MgH₂-20 wt% Ti_0.35Cr_0.45V_0.2 composite reach to 5.30 wt% within 20 min at 270 °C under 0.01 MPa H₂, compared with that only 0.1 wt% H₂ for pure MgH₂ powders under the same conditions. Besides, the hydrogenation and dehydrogenation enthalpies are determined to be −69.35 and 70.60 kJ/(mol⋅H₂) respectively, which show a significant decrease compared with that of Mg/MgH₂ systems. More importanly, the β-MgH₂ phase is found to decompose along with γ-MgH₂ and Ti-Cr-V_0.2H_y phases at the temperature ranging from 210 to 270 °C. It reveals the favorable synergetic desorption effect among γ-MgH₂, β-MgH₂ and Ti_0.35Cr_0.45V_0.2H_y phases, which possibly leads to better dehydrogenation kinetics and thermodynamics properties of Mg/MgH₂ systems.