Synthesis, structural and hydrogenation properties of Mg-rich MgH2–TiH2 nanocomposites prepared by reactive ball milling under hydrogen gas

Abstract

MgH₂–TiH₂ nanocomposites have been obtained by reactive ball milling of elemental powders under 8 MPa of hydrogen pressure. The composites consist of a mixture of β-rutile MgH₂, γ-orthorhombic high pressure MgH₂ and ε-tetragonal TiH₂ phases with nanosized crystallites ranging from 4 to 12 nm. In situhydrogen absorption curves on milling reveal that nanocomposite formation occurs in less than 50 min through the consecutive synthesis of the TiH₂ and MgH₂ phases. The abrasive and catalytic properties of TiH₂ speed up the formation of the MgH₂ phase. Thermodynamic, kinetic and cycling hydrogenation properties have been determined for the 0.7MgH₂–0.3TiH₂ composite and compared to nanometric MgH₂. Only the MgH₂ phase desorbs hydrogen reversibly at moderate temperature (523 to 598 K) and pressure (10⁻³ to 1 MPa). The presence of TiH₂ does not modify the thermodynamic properties of the Mg/MgH₂ system. However, the MgH₂–TiH₂ nanocomposite exhibits outstanding kinetic properties and cycling stability. At 573 K, H-sorption takes place in less than 100 s. This is 20 times faster than for a pure nanometric MgH₂ powder. We demonstrate that the TiH₂ phase inhibits grain coarsening of Mg, which allows extended nucleation of the MgH₂ phase in Mg nanoparticles before a continuous and blocking MgH₂ hydride layer is formed. The low crystallinity of the TiH₂ phase and its hydrogenation properties are also compatible with a gateway mechanism for hydrogen transfer from the gas phase to Mg. Mg-rich MgH₂–TiH₂ nanocomposites are an excellent media for hydrogen storage at moderate temperatures.