Fast and slow dehydrogenation of ball milled lithium alanate (LiAlH4) catalyzed with manganese chloride (MnCl2) as compared to nanometric nickel catalyst

The results of the studies on the dehydrogenation behavior of the ball milled LiAlH4 catalyzed with 5 wt.% of manganese chloride (MnCl2) are reported. During ball milling for 15 min the LiAlH4 + 5 wt.% MnCl2 nanocomposite releases a miniscule amount of ∼0.25 wt.% H2. However, no products of the possible reaction between LiAlH4 and MnCl2 (e.g. LiCl) are observed by X-ray diffraction (XRD). In a DSC test most of LiAlH4 decomposes exothermically to Li3AlH6 in a solid state while a small fraction of retained LiAlH4 melts and decomposes in a liquid state. During dehydrogenation at 100 °C under 0.1 MPa H2 the ball milled LiAlH4 + 5 wt.% MnCl2 nanocomposite is able to desorb ∼4.6 wt.% H2 within ∼30,000 s in a solid state but only in Stage I (reaction: LiAlH4 (solid) → 1/3Li3AlH6 + 2/3Al + H2). The apparent activation energy of dehydrogenation for this solid state reaction is equal to ∼80 kJ/mol as compared to ∼70 kJ/mol obtained for LiAlH4+5 wt.% n-Ni [7]. However, during decomposition at 100 °C a chemical reaction occurs between LiAlH4 and MnCl2 producing LiCl and most likely an amorphous Mn metal catalyzing the reaction in Stage I. The ball milled LiAlH4+5 wt.% MnCl2 nanocomposite is capable of desorbing substantial quantities of H2 during long term storage at room temperature (RT; ∼21 °C), 40 and 80 °C.

Research highlights▶ Dehydrogenation behavior of ball milled LiAlH4 with 5 wt.% of MnCl2 catalytic precursor are reported. ▶ A small amount of hydrogen desorbs during milling although no products of the possible reaction between LiAlH4 and MnCl2 (e.g. LiCl) are observed by X-ray diffraction. ▶ In a DSC test most of LiAlH4 decomposes exothermically to Li3AlH6 in a solid state while a small fraction of retained LiAlH4 melts and decomposes in a liquid state. ▶ At 100°C under 0.1 MPa H2 the ball milled LiAlH4 + 5 wt.% MnCl2 nanocomposite is able to desorb ∼4.6 wt.% H2 within ∼30,000 s in a solid state (LiAlH4 (solid)→1/3Li3AlH6 + 2/3Al + H2). ▶ The apparent activation energy of dehydrogenation for this solid state reaction is equal to ∼80 kJ/mol. ▶ During decomposition at 100°C a chemical reaction occurs between LiAlH4 and MnCl2 producing LiCl and most likely an amorphous Mn metal catalyzing the reaction. ▶ The ball milled LiAlH4 + 5 wt.% MnCl2 nanocomposite is capable of desorbing substantial quantities of H2 during long term storage at room temperature (RT; ∼21 °C), 40 and 80 °C.