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The hydrogen desorption/absorption behaviors of LiAlH4/LiBH4 with a focus on the effects of catalysts, namely TiCl3, TiO2, VCl3, and ZrCl4, were investigated using a thermal-volumetric apparatus. The hydrogen desorption was performed from room temperature to 300 °C with a heating rate of 2 °C min-1. The LiAlH4-LiBH4 mixture with a molar ratio of 2:1 decomposed between 100 and 220 °C, and the hydrogen desorption capacity reached up to 6.6 wt %. Doping 1 mol % of a catalyst to the mixture resulted in the two-step decomposition and a decrease in the hydrogen desorption temperature. All the doped samples provided lower amountz of desorbed hydrogen than that obtained from the undoped one. No hydrogen absorption was observed under 8.5 MPa of hydrogen pressure and 300 °C for 6 h. Despite the fact each of the catalysts may affect the hydrogen storage behaviors of the mixture differently, none resulted in a change in the sample reversibility [ABSTRACT FROM AUTHOR]

Sodium aluminum hydride or sodium alanate (NaAlH4) has been considered as a potential material for hydrogen storage. Although its theoretical hydrogen storage capacity is 5.5 wt.% at 250 °C, the material still has its drawback in the regeneration issue. With the use of certain catalysts, the regeneration problem can somewhat be alleviated with added benefits in the decrease in the hydrogen decomposition temperature and the increase in the decomposition rate. This work summarizes what we have learned from the decomposition of NaAlH4 with/without catalysts and co-dopants. The decomposition was carried out using a thermovolumetric apparatus. For the tested catalysts—HfCl4, VCl3, TiO2, TiCl3, and Ti—the decomposition temperature of the hydride decreases; however, they affect the temperature in the subsequent cycles differently and TiO2 appears to have the most positive effect on the temperature. Sample segregation and the morphological change are postulated to hinder the reversibility of the hydride. To prevent the problems, co-dopants—activated carbon, graphite, and MCM-41—were loaded. Results show that the hydrogen reabsorption capacity of HfCl4- and TiO2-doped NaAlH4 added with the co-dopants increases 10–50% compared with that without a co-dopant, and graphite is the best co-dopant in terms of reabsorption capacity. In addition, the decomposition temperature in the subsequent cycles of the co-dopant doped samples decreases about 10–15 °C as compared to the sample without a co-dopant. Porosity and large surface area of the co-dopant may decrease the segregation of bulk aluminum after the desorption and improve hydrogen diffusion in/out bulk of desorbed/reabsorbed samples. [ABSTRACT FROM AUTHOR]

Abstract: Although hydrogen has a great potential as clean energy, safe practical storage of hydrogen for applications such as fuel cells has been a major challenge. NaAlH4 is one of the metal hydrides, which are candidates for hydrogen storage in vehicles. However, the rather slow absorption/desorption kinetics is still a significant drawback. To alleviate this problem, purified NaAlH4 was ground with TiCl3, ZrCl4, or HfCl4. Desorption kinetics and capacities were observed under TPD-like operation. Absorption efficiency was determined by raising the temperature up to 125°C. Of the three doped metals investigated for the positive effect on facilitating NaAlH4 decomposition, TiCl3 assists the best on the first reaction while ZrCl4 and HfCl4 do for the second one. Despite the kinetics enhancement directly involves with the ZrCl4 amount, there is a threshold of ZrCl4-content which affects. 6% ZrCl4 is considered as an appropriate amount to improve the hydrogen release because it simultaneously decreases the desorption temperature and gives the outstanding rate. In hydrogen desorption, ZrCl4 provides the most amount of released hydrogen, but for hydrogen absorption TiCl3-doped NaAlH4 possesses the highest capacity. It is believed that the metal size is one of the key factors resulting in such the behavior. [Copyright &y& Elsevier]

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]

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]