Effects of iron oxide (Fe2O3, Fe3O4) on hydrogen storage properties of Mg-based composites

When magnesium powder containing small additions of certain multiple valence transition metal oxides (TMOs) is ball milled in hydrogen, the hydrogenated Mg-based product can show remarkable improvements in hydrogen absorption/desorption properties. Using a magnetically controlled Uni-Ball-Mill, small amounts of the iron oxides, Fe2O3 and Fe3O4, were ball milled with Mg powder in a hydrogen atmosphere (Mg to Fe atomic ratio; 20: 1). Milling products as well as samples used in hydrogen absorption/desorption experiments were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. TG analysis combined with DSC revealed a higher hydrogen storage capacity for the Mg + Fe2O3 + H(2)(.)milled product (6 wt% H) compared with 5 wt% H for Mg + Fe3O4 + H-2. XRD revealed that during heating, both iron oxides were reduced to pure Fe, a result not previously reported for similar materials milled using different milling devices. For both samples, there was little difference found in the decomposition temperature of the as-prepared MgH2 and rehydrogenated composites. However, storage capacity degradations were observed for the rehydrogenated composites (4 wt% H storage capacity for MgH2 + Fe2O3 and 4.4 wt% H for MgH2 + Fe3O4). The higher capacity degradation of rehydrogenated MgH2 + Fe2O3 composite is also believed to be a result of the reduction reaction, during which more magnesium was consumed than was consumed by the same amount of Fe3O4. The results also were related to the particular ball-milling equipment and low-energy shearing milling mode employed, which promoted the development of a nanostructural hydride product which subsequently changed structure significantly during the first desorption cycle. (c) 2005 Elsevier B.V. All rights reserved.