The effects of the micrometric and nanometric iron (Fe) additives on the mechanical and thermal dehydrogenation of lithium alanate (LiAlH_4), its self-discharge at low temperatures and rehydrogenation

摘要

LiAlH_4 containing 5 wt.% of nanometric Fe (n-Fe) shows a profound mechanical dehydrogenation by continuously desorbing hydrogen (H_2) during high energy ball milling reaching ~3.5 wt.% H_2 after 5 h of milling. In contrast, no H_2 desorption is observed during low energy milling of LiAlH_4 containing n-Fe. Similarly, no H_2 desorption occurs during high energy ball milling for LiAlH_4 containing micrometric Fe (μ-Fe) and, for comparison, both the micrometric and nanometric Ni (μ-Ni and n-Ni) additive. X-ray diffraction studies show that ball milling results in a varying degree of the lattice expansion of LiAlH_4 for both the Fe and Ni additives. A volumetric lattice expansion larger than 1% results in the profound destabilization of LiAlH_4 accompanied by continuous H_2 desorption during milling according to reaction: LiAlH_4 (solid) →V3Li_3AlH_6 + 2/3A1 + H_2. It is hypothesized that the Fe ions are able to dissolve in the lattice of LiAlH_4 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 (LiAlH_4 (solid)→1/3Li_3AlH_6 + 2/3Al + H_2) is reduced from the range 76 to 96 kJ/mol for the μ-Fe additive to about 60 kJ/mol for the n-Fe additive. For Stage Ⅱ dehydrogenation (l/3Li_3AlH_6 → LiH+l/3Al + 0.5H_2) the average apparent activation energy is within the range 77-93 kj/mol, regardless of the particle size of the Fe additive (μ-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 ℃. Ball milled (LiAlH_4 + 5 wt.% n-Fe) slowly self-discharges up to ~5 wt.% H_2 during storage at room temperature (RT), 40 and 80 ℃. Fully dehydrogenated (LiAlH_4 + 5 wt.% n-Fe) has been partially rehydrogenated up to 0.5 wt.% H_2 under 100 bar/160℃/24 h. However, the rehydrogenation parameters are not optimized yet.