Effect of metal type and loading on hydrogen storage on NaAlH₄

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 8C. 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.