Microstructures and Hydrogen Desorption Properties of the MgH2鈥揂lH3 Composite with NbF5 Addition

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• 作者：Haizhen Liu ; Xinhua Wang ; Yongan Liu ; Zhaohui Dong ; Shouquan Li ; Hongwei Ge ; Mi Yan
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：August 21, 2014
• 年：2014
• 卷：118
• 期：33
• 页码：18908-18916
• 全文大小：530K
• ISSN：1932-7455

文摘

Niobium fluoride (NbF₅) is introduced into the MgH₂ + 1/4AlH₃ hydride composite by ball milling to improve the hydrogen desorption properties of the Mg鈥揂l鈥揌 system. It is found that after being ball milled with 1 mol % of NbF₅, AlH₃ in the composite has almost fully decomposed and forms metallic Al, which indicates that NbF₅ can significantly destabilize AlH₃. DSC results show that NbF₅ addition also helps reduce the peak desorption temperature of MgH₂ in the composite from 324 to 280 掳C. Isothermal desorption measurements demonstrate that MgH₂ in the doped composite can rapidly release 98% of its hydrogen after desorption at 300 掳C for 1 h, while the valued is only 46% for the undoped composite. The activation energy for hydrogen desorption of MgH₂ in the doped composite is calculated to be 104.5 kJ/mol, much lower than that in the undoped composite (127.4 kJ/mol). These results suggest that NbF₅ addition dramatically improves the hydrogen desorption kinetics of the MgH₂ + 1/4AlH₃ composite. Dehydrogenation鈥揾ydrogenation measurements reveal that the hydrogen desorption kinetics of the undoped composite declines with cycle number, whereas the NbF₅-doped composite maintains good cycling stability. Microstructure studies indicate that the decline of the kinetics is attributed to the grain growth and particle agglomeration of MgH₂ during hydrogen sorption cycling. However, NbF₅ addition can suppress this grain growth through the formation of Nb/NbH layers surrounding the particles of MgH₂ and acting both as the impediment to grain growth of Mg/MgH₂ and as the gateway for hydrogen diffusion. Finally, the role that AlH₃ plays in the hydrogen desorption process of the Mg鈥揂l鈥揌 composites is discussed.