LiAlH4–ZrCl4 mixtures for hydrogen release at near room temperature

The dehydrogenation temperature of LiAlH₄ was significantly reduced by the production of mixtures with ZrCl₄. Stoichiometric 4:1, and 5 mol % mixtures of LiAlH₄ and ZrCl₄ were produced by ball milling at room temperature and ?196 °C, and tested for dehydrogenation at low temperature. Cryogenic ball-milling resulted in an effective way to produce reactive mixtures for hydrogen release; because of achieving small aggregates size (5–20 μm) in 10 min of cryomilling while preventing substantial decomposition during preparation. Dehydrogenation reaction in the mixtures LiAlH₄/ZrCl₄ started around 31–47 °C under different heating rates. Partial dehydrogenation was proved at 70 °C: 4.4 wt % for the 5 mol% ZrCl₄–LiAlH₄ mixture, and 3.4 wt % for the best 4:1 stoichiometric mixture. Complete dehydrogenation up to 250 °C released 6.4 wt% and 4.1 wt%, respectively. Dehydrogenation reactions are exothermic, and the LiAlH₄/ZrCl₄ mixtures are unstable and difficult to handle. The activation energy of the exothermic reactions was estimated as 113.5 ± 9.8 kJ/mol and 40.6 ± 6.6 kJ/mol for 4LiAlH₄+ZrCl₄ and 5%mol ZrCl₄+LiAlH₄ samples milled in cryogenic conditions, respectively. The dehydrogenation pathway was changed in the LiAlH₄/ZrCl₄ mixtures as compared to pure LiAlH₄. Dehydrogenation reaction is proposed to form Al, LiCl, Zr, and H₂ as main products. Modification of the dehydrogenation reaction of LiAlH₄ was achieved at the cost of reducing the total hydrogen release capacity.