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 + H₂-milled product (6 wt% H) compared with 5 wt% H for Mg + Fe₃O₄ + H₂. 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. © 2005 Elsevier B.V. All rights reserved.