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

Abstract

Mg-20 wt% TixCr0.8-xV0.2 (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-V0.2Hy (1.91 < y < 2.01) phases. High-Resolution Transmission Electron Microscopy (HRTEM) images reveal that nanocrystalline β-MgH2, γ-MgH2 and Ti0.35Cr0.45V0.2Hy phases coexist on nano-level in the as-milled composites. Differential scanning calorimeter (DSC) results show that MgH2-20 wt% Ti0.35Cr0.45V0.2 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% Ti0.35Cr0.45V0.2 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-V0.2Hy phases at the temperature ranging from 210 to 270 °C. It reveals the favorable synergetic desorption effect among γ-MgH2, β-MgH2 and Ti0.35Cr0.45V0.2Hy phases, which possibly leads to better dehydrogenation kinetics and thermodynamics properties of Mg/MgH2 systems.