MgH2 with Nb2O5 as additive, for hydrogen storage:: Chemical, structural and kinetic behavior with heating

The influence of heating/cycling on MgH2 samples milled together with 2 mol% of Nb2O5 was investigated. X-ray diffraction (XRD) shows a reduction of the Nb2O5 to metallic niobium and a possible MgNb2O3.67 phase, along with an increased formation of MgO compared to a pure MgH2 sample. It was shown that the additive during the heating process is preventing the MgH2 phase from grain growth. Using X-ray photoelectron spectroscopy metallic niobium and niobium oxides were observed emerging to the surface only after cycling. All reported reactions had taken place already during the first cycle. Using annealing experiments at different temperatures, samples of the same composition were generated but with different XRD crystal sizes. These samples showed similar sorption kinetics, which indicates that the prevention of grain growth by the Nb2O5 additive can only have a minor effect on the kinetics for this range of crystal sizes. Since the Nb2O5 is reduced during the heating, a possible catalytic effect has to be due to metallic Nb or those of its oxides with oxygen deficiency, like MgNb2O3.67, formed during the first cycle. However, other processes like particle size reduction or a decrease of agglomeration by the additive during milling also have to be taken into account. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.