Improved hydrogen storage performance of MgH2-NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH2-NaAlH4 composite system had improved dehydrogenation performance compared with as-milled pure NaAlH4 and pure MgH2 alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH2-NaAlH4 composite in the presence of TiF3. 10 wt.% TiF3 was added to the MgH2-NaAlH4 mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH2-NaAlH4 composite milled with 10 wt.% TiF3 had lower dehydrogenation temperatures, by 100, 73, 30, and 25 degrees C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH2-NaAlH4 composite. Kinetic desorption results show that the MgH2-NaAlH4-TiF3 composite released about 2.4 wt.% hydrogen within 10 min at 300 degrees C, while the neat MgH2-NaAlH4 sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E-A, for the decomposition of MgH2, NaMgH3, and NaH in the MgH2-NaAlH4-TiF3 composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH2-NaAlH4 composite. The high catalytic activity of TiF3 is associated with in situ formation of a microcrystalline intermetallic Ti-Al phase from TiF3 and NaAlH4 during ball milling or the dehydrogenation process. Once formed, the Ti-Al phase acts as a real catalyst in the MgH2-NaAlH4-TiF3 composite system. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.