The effects of the micrometric and nanometric iron (Fe) additives on the mechanical and thermal dehydrogenation of lithium alanate (LiAlH₄), its self-discharge at low temperatures and rehydrogenation

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• 作者：Robert A. Varin ; ^{ravarin@mecheng1.uwaterloo.ca} ; Roozbeh Parviz
• 关键词：Solid state hydrogen storage ; Lithium alanate (LiAlH4) ; Micrometric and nanometric iron ; Ball milling energy ; Dehydrogenation and rehydrogenation
• 刊名：International Journal of Hydrogen Energy
• 出版年：2012
• 期刊代码：42_03603199
• 类别：ch
• 出版时间：June, 2012
• 卷：37
• 期：11
• 页码：9088-9102
• 文件大小：2241 K

摘要

LiAlH₄ containing 5聽wt.%of nanometric Fe (n-Fe) shows a profound mechanical dehydrogenation by continuously desorbing hydrogen (H₂) during high energy ball milling reaching 鈭?.5聽wt.%H₂ after 5聽h of milling. In contrast, no H₂ desorption is observed during low energy milling of LiAlH₄ containing n-Fe. Similarly, no H₂ desorption occurs during high energy ball milling for LiAlH₄ containing micrometric Fe (渭-Fe) and, for comparison, both the micrometric and nanometric Ni (渭-Ni and n-Ni) additive. X-ray diffraction studies show that ball milling results in a varying degree of the lattice expansion of LiAlH₄ for both the Fe and Ni additives. A volumetric lattice expansion larger than 1%results in the profound destabilization of LiAlH₄ accompanied by continuous H₂ desorption during milling according to reaction: LiAlH₄ (solid)聽鈫捖?/3Li₃AlH₆聽+聽2/3Al聽+聽H₂. It is hypothesized that the Fe ions are able to dissolve in the lattice of LiAlH₄ by the action of mechanical energy, replacing the Al ions and forming a substitutional solid solution. The quantity of dissolved metal ions depends primarily on the total energy of milling per unit mass of powder generated within a prescribed milling time, the type of additive ion e.g. Fe vs. Ni and on the particle size (micrometric vs. nanometric) of metal additive. For thermal dehydrogenation the average apparent activation energy of Stage I (LiAlH₄ (solid)聽鈫捖?/3Li₃AlH₆聽+聽2/3Al聽+聽H₂) is reduced from the range 76 to 96聽kJ/mol for the 渭-Fe additive to about 60聽kJ/mol for the n-Fe additive. For Stage II dehydrogenation (1/3Li₃AlH₆聽鈫捖燣iH+1/3Al聽+聽0.5H₂) the average apparent activation energy is within the range 77-93聽kJ/mol, regardless of the particle size of the Fe additive (渭-Fe vs. n-Fe). The n-Fe and n-Ni additives, the latter used for comparison, provide nearly identical enhancement of dehydrogenation rate during isothermal dehydrogenation at 100聽掳C. Ball milled (LiAlH₄聽+聽5聽wt.%n-Fe) slowly self-discharges up to 鈭?聽wt.%H₂ during storage at room temperature (RT), 40 and 80聽掳C. Fully dehydrogenated (LiAlH₄聽+聽5聽wt.%n-Fe) has been partially rehydrogenated up to 0.5聽wt.%H₂ under 100聽bar/160掳C/24聽h. However, the rehydrogenation parameters are not optimized yet.