Mg鈥揊e Thin Films: A Phase-Separated Structure with Fast Kinetics of Hydrogenation

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• 作者：Shiyou Zheng ; Ke Wang ; Vladimir P. Oleshko ; Leonid A. Bendersky
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：October 11, 2012
• 年：2012
• 卷：116
• 期：40
• 页码：21277-21284
• 全文大小：522K
• 年卷期：v.116,no.40(October 11, 2012)
• ISSN：1932-7455

文摘

In this paper we suggest a new approach to improve the kinetics of hydrogenation based on a material with a high density of interfaces between hydride-forming solid (HFS) and hydrogen-diffusing solid media (HDM). Such materials can be realized for phase-separated systems when synthesized in the conditions limiting kinetics of phase separation, e.g., in thin film deposition. Mg鈥揊e was selected as a model system: (1) It has phase separation; (2) MgH₂ is a high-capacity hydride; (3) Fe has high diffusivity of hydrogen. Mg_1-xFe_x thin films (x = 0鈥?.30) capped with Pd were prepared by electron beam codeposition and their hydrogenation/dehydrogenation kinetics and cycling properties were studied at 413 K. The structures of the thin films before and after hydrogenation during different cycles were investigated by X-ray diffraction and transmission electron microscopy. It has been found that there is a substantial improvement in hydrogen absorption and desorption kinetics for the Mg_1-xFe_x films in comparison to pure Mg film. The improvement is attributed to the presence of Fe layers percolating throughout the Mg matrix. For the Mg_1-xFe_x films with x = 0.05鈥?.15 more than 3.5% mass fraction hydrogen can be absorbed under hydrogen pressures of 0.1 MPa in less than 2 min, and above 3.0% mass fraction hydrogen can be desorbed in 15 min. For x > 0.15 films, the reversible hydrogen storage properties degrade significantly; structural study of the higher concentration films shows the presence of a stable ternary hydride Mg₂FeH₆, formation of which is responsible for interruption of fast hydrogen delivery.