Roles of metal catalysts on the hydrogen storage behaviors of LiAlH₄/LiBH₄

Abstract

The hydrogen storage capacities of both undoped and doped Li-Al-H, Li-B-H, and Li-Al-B-H systems were studied through thermo-volumetric analysis. All samples were mixed by mechanical ball milling. The hydrogen desorption was performed from room temperature to 300 ℃ (to 350℃ for Li-B-H systems) with a heating rate of 2℃ min-1 and the absorption was done at 300℃ (at 120℃ for Li-Al-H systems) and 8.5 MPa hydrogen for 6 h. In the case of the undoped systems, LiAIH₄ decomposes into two steps starting at 145℃ and continues to 220℃ with the total hydrogen amount of 7.6 wt%. LiBH₄ desorbs a small amount of hydrogen of 0.1-1.0 wt% between 95 and 300℃. For the LiAlH₄-LiBH₄ mixtures, a 2: 1 LiAlH₄: LiBH₄ molar ratio releases the highest amount of hydrogen at 6.6 wt% in the temperature range of 100-220℃. In the case of the doped systems, 1 mol% of metal catalysts (TiCl₃, TiO₂, VCI₃, or ZrCl₄) was doped to the systems. For LialH₄, all of the additives lower the temperature in the first and second steps of the hydrogen desorption and improve the amount of hydrogen released. For LiBH₄, a small amount of a catalyst can improve the reversibility for at least three cycles. The LiAlH₄-LiBH₄ mixture in the presence of TiCfi desorbs hydrogen at the lowest temperature (40 ° C). Furthermore, 3 and 5 mol% TiCfi were added to the LiAlH₄-LiBH₄ mixture. The hydrogen desorption capacity decreases with the increase in the doping amount. No hydrogen absorption was observed for any of the doped LiAlH₄-LiBH₄ samples. In addition, XRD patterns indicate A1 and LiH in the Li-Al-H and Li-Al-B-H systems after the desorption corresponding to the decomposition reaction of LiAlH₄.