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

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 beta-rutile MgH2, gamma-orthorhombic high pressure MgH2 and epsilon-tetragonal TiH2 phases with nanosized crystallites ranging from 4 to 12 nm. In situ hydrogen 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 0.7MgH(2)-0.3TiH(2) 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.