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Abstract: The hydrogen desorption/absorption of the 2:1 mole ratio of LiBH4 and MgH2 with and without a catalyst was investigated. In the case of the uncatalyzed LiBH4/MgH2 mixture, the first hydrogen desorption started at 50°C. The amount of released hydrogen was 8.6wt% at the first hydrogen desorption and dramatically reduced to 1.8wt% at the tenth hydrogen desorption. The decrease in the hydrogen capacity in the subsequent hydrogen desorption may be due to the irreversibility of LiBH4. To investigate effects of a catalyst on the hydrogen desorption, 3mol% of TiCl3, HfCl4, ZrCl4, or VCl3 was added to the LiBH4/MgH2 mixture. The lowest hydrogen desorption temperature, 260°C, was from the sample with TiCl3. An amount of the catalyst also influenced the kinetics of the hydride mixture and 5mol% seems to be an optimum amount of TiCl3 that resulted in the lowest hydrogen desorption temperature, 240°C. In addition, the higher the amount of a catalyst, the lower the amount of the released hydrogen. [Copyright &y& Elsevier]

Abstract: The segregation of bulk Al after the hydrogen desorption may be one of many reasons why there is the incomplete hydrogen re-absorption and the need for higher desorption temperature in subsequent desorption of HfCl4- and TiO2-doped NaAlH4. In this work, we attempted to improve hydrogen storage capacity of 4mol% HfCl4- and 4mol% TiO2-doped NaAlH4 by adding a co-dopant (graphite, activated carbon, and MCM-41) using ball milling technique. It was found that the co-dopants significantly affect the hydrogen desorption/re-absorption of the hydride. Results show that the hydrogen re-absorption capacity of HfCl4- and TiO2-doped NaAlH4 added with the co-dopants increases 10–50% as compared with that without a co-dopant, and graphite seems to be the best co-dopant. Moreover, the hydrogen desorption temperature in the subsequent cycle of co-dopant doped samples decreases about 10–15°C compared to the sample without a co-dopant. It could be explained that high porosity and large surface area of the co-dopants would decrease the segregation of bulk Al after the desorption and improve hydrogen diffusion in/out bulk of desorbed/re-absorbed samples. [Copyright &y& Elsevier]

The main objective of this work was to investigate the effect of two transition metals (0–9mol% ZrCl4 and HfCl4) on hydrogen desorption/absorption of NaAlH4 and LiAlH4. The hydrogen desorption was carried out over a wide temperature range of 25– while the hydrogen absorption took place at and between 50– for NaAlH4 and LiAlH4, respectively, with the same pressure of 11MPa. The result revealed that the transition metals (Zr and Hf) could improve the kinetics of the hydrogen desorption on NaAlH4. NaAlH4 doped with HfCl4 released hydrogen at the lower temperature than that of ZrCl4-doped NaAlH4. In addition, the rate of hydrogen desorption increased with increasing the amount of HfCl4 doping. However, the maximum hydrogen capacity of was obtained from the first desorption, and dropped to 2.2–2.6wt% in the subsequent cycles. This may be because of, after the hydrogen desorption, the hydrides melt due to the high temperature resulted in agglomeration of the hydrides and segregation of Al. XRD results showed the formation of NaCl after milling of NaAlH4 with 4mol% HfCl4 and no evidence of any Hf-containing phase. However, with 10mol% HfCl4, the formation between Al and Hf in the form of Al3Hf was observed. This compound may act as catalyst in the reversible hydrogen desorption/absorption. For LiAlH4, ZrCl4 or HfCl4 also enhanced the kinetics of desorption of LiAlH4. Moreover, it was observed that LiAlH4 released hydrogen during the milling. After the hydrogen desorption from LiAlH4, no hydrogen absorption was observed for the undoped hydride or that doped with HfCl4 or ZrCl4. This may be because of the instability of LiAlH4. Moreover, milling and dopant may also destabilize the structure of LiAlH4 causing the irreversibility of hydrogen desorption/absorption on LiAlH4. [Copyright &y& Elsevier]