Magnesium hydride (MgH2) is one of the most attractive hydrogen storage materials because of its high hydrogen storage capacity (7.6 wt %), light weight and low cost. However, the high desorption temperature (above 300ºC) and the slow hydrogen absorption and desorption kinetics limit the practical application of this hydride.
In our work, we have shown that hydrogen sorption kinetics of nanocrystalline MgH2 powders can be significantly improved by adding nanosized metal oxides as catalysts. Recently, we have found , that introduction of Cr2O3 nanoparticles in the course of the MgH2 milling process results in the synthesis of nanocrystalline/nanoparticle composite, which shows very fast hydrogen sorption properties. Therefore, following this approach, we have used high energy (ball) milling to fabrication of MgH2-MexOy nanocomposites (MexOy=Cr2O3, TiO2, Fe2O3, Fe3O4, In2O3, ZnO). The hydrogen sorption kinetics of investigated nanocomposites was evaluated using a volumetric Sievert apparatus. The phase structure, morphology and chemical composition were investigated by XRD, SEM, EDS and DSC-TG.
Microstructural investigations of mechanical (ball) milled MgH2 powders without and with nano-catalysts showed a MgH2 particle size of 0.5-10μm. The milled MgH2 exhibited a homogeneous crystallite size distribution with average crystallite size smaller than 100 nm. The brittle nano-catalyst particles were embedded by the MgH2 matrix, forming a nanocomposite structure. The particle size of catalysts was found to be 10-300 nm. Our results indicate that in both absorption and desorption reactions, the superior catalytic effect of Cr2O3 and TiO2 was observed. The catalytic effect of investigated metal oxides is also expressed in lowering activation energy of dehydriding reaction. The mechanism of catalysis is not clear so far and some proposals will be presented and discussed.
1. M. Polanski, J. Bystrzycki, T. Plocinski, IJHE, 33 (2008) 1859-1867