The effects of the nanometric interstitial compounds TiC, ZrC and TiN on the mechanical and thermal dehydrogenation and rehydrogenation of the nanocomposite lithium alanate (LiAlH₄) hydride

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• 作者：R.A. Varin ; R. Parviz
• 关键词：Solid state hydrogen storage ; Lithium alanate (LiAlH4) ; Nanometric interstitial compounds (TiC ; TiN and ZrC) ; Ball milling ; Mechanical and thermal dehydrogenation and rehydrogenation
• 刊名：International Journal of Hydrogen Energy
• 出版年：14 February, 2014
• 年：2014
• 卷：39
• 期：6
• 页码：2575-2586
• 全文大小：2048 K

文摘

Profuse mechanical dehydrogenation occurs during controlled high energy ball milling of LiAlH₄ containing 5聽wt.% of the nanometric interstitial compounds such as n-TiC, n-TiN and n-ZrC which involves a gradual decomposition of LiAlH₄ to the mixture of Li₃AlH₆ and Al (Stage I) followed by a further decomposition of Li₃AlH₆ to the mixture of Al and LiH (Stage II). XRD reveals that the interstitial compounds remain stable in the hydride matrix during entire ball milling duration. The effectiveness of the nanometric interstitial compound additives for mechanical dehydrogenation increases on the order of n-TiN聽>聽n-TiC聽>聽n-ZrC. X-ray diffraction (XRD) reveals that there is no measurable change in a unit cell volume of LiAlH₄ after ball milling which indicates that an accelerated mechanical dehydrogenation of LiAlH₄ containing the nanometric interstitial compounds is unrelated to the lattice expansion as we have already reported for the nanometric metal Fe (n-Fe). In addition, the observed strong catalytic activity of the nanometric interstitial compounds for mechanical dehydrogenation is not related to their valence electron concentration (VEC) number. However, the n-TiN additive, which is the most effective one for mechanical dehydrogenation, has the smallest average particle size of 20聽nm and the largest Specific Surface Area (SSA聽>聽80聽m²/g). For thermal dehydrogenation in Stage I the average apparent activation energy, E_A, for the interstitial compound additives is within the range of 87-96聽kJ/mol whereas, for comparison, the nanometric metallic additives, n-Fe and n-Ni, exhibit drastically smaller apparent activation energy on the order of 55-70聽kJ/mol. The average apparent activation energy for thermal dehydrogenation in Stage II is in the range of 63-80聽kJ/mol in the order of E_A(n-ZrC) < E_A(n-Ti聽=聽n-TiC) and is lower than that for the nanometric metal additives n-Ni and n-Fe. In summary, the nanometric interstitial compounds do not substantially affect the apparent activation energy of Stage I but are able to reduce the apparent activation energy of thermal dehydrogenation in Stage II. XRD reveals that the interstitial compounds remain stable in the hydride matrix up to the dehydrogenation temperature of at least 165聽掳C. Ball milled LiAlH₄ containing 5聽wt.% n-TiC, n-TiN and n-ZrC is able to slowly discharge large quantities of H₂ up to 5-6聽wt.% during storage at 40聽掳C. Unfortunately, the results of rehydrogenation at 165聽掳C under 95聽bar for 5聽h indicate that LiAlH₄ containing the nanometric interstitial compounds exhibits no rehydrogenation.