High-pressure Ball Milling Research Articles

Improved Hydrogen Storage Properties of LiBH4 Destabilized by in Situ Formation of MgH2 and LaH3

A reactive composite of LiBH4–xLa2Mg17 was successfully prepared by means of mechanochemical reaction under 40 bar of H2. It was found that MgH2 and LaH3 were readily formed in situ during high-pressure ball milling, and a strong dependency of hydrogen storage performance of the LiBH4–xLa2Mg17 composites on the content of La2Mg17 was observed. The as-prepared LiBH4–0.083La2Mg17 composite under 40 bar of H2 exhibits superior hydrogen storage properties as ∼6.8 wt % of hydrogen can be reversibly desorbed and absorbed below 400 °C. It was also purposed that the self-decomposition of MgH2 first occurred to convert into Mg with hydrogen release upon dehydrogenation and subsequently catalyzed the reaction of LiBH4 and LaH3 to liberate additional hydrogen along with the formation of LaB6 and LiH. The in situ formed MgH2 and LaH3 provide a synergetic thermodynamic and kinetic destabilization on the de/hydrogenation of LiBH4, which is responsible for the distinct reduction in the operating temperatures of the as-prepared LiBH4–xLa2Mg17 composites.

Hydrogen storage properties of nanostructured MgH 2/TiH 2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH 2/0.1TiH 2 composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH 2 from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH 2/0.1TiH 2 nano-composite system are 77.4 kJ mol −1 H 2 and 137.5 J K −1 mol −1 H 2, respectively. These values are in agreement with those obtained for a commercial MgH 2 system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.

Synthesis of Metal Complex Hydrides for Hydrogen Storage

A new procedure for the direct synthesis of metal complex hydrides, NaAlH4 and LiAlH4, has been developed through hydrogenation of their respective metal hydrides, NaH and LiH, with Al, both in the presence of a catalyst and in the presence of a liquid complexing agent. This procedure involves three steps: blending the TiCl3 catalyst precursor with reactants using dry high-pressure ball milling (HPBM) in hydrogen, hydrogenation using HPBM in hydrogen and tetrahydrofuran (THF), and vacuum filtration and drying. Essentially, pure NaAlH4 and LiAlH4 products were produced in high yield at ambient temperature and near ambient pressure. For the Na and Li systems, the Ti nanoparticles, stabilized in THF, together with the formation of a MAlH4·xTHF adduct (M = Na or Li), both facilitated the hydrogenation of MH at such mild conditions. For the Na system, x was equal to 4 or 8, but the interaction was very weak, and for the Li system, x was equal to 4, but the interaction was very strong. In general, THF, the Ti catalyst, and wet HPBM all played essential roles in promoting the hydrogenation reactions, which proceeded through either a direct pathway (2MH + 2Al + 3H2 → 2MAlH4), or a sequential pathway (6MH + 2Al + 3H2 → 2M3AlH6 followed by M3AlH6 + 2Al + 3H2 → 3MAlH4), with the formation of sodium hexahydridoaluminate limiting in the latter case. The same procedure was tried unsuccessfully for the direct synthesis of Mg(AlH4)2 from MgH2 and for AlH3 from Al.