Significantly Improved Dehydrogenation of LiAlH₄ Destabilized by MnFe₂O₄ Nanoparticles

The effects of nanosized MnFe₂O₄ additive on the dehydrogenation properties of LiAlH₄ prepared by ball milling were investigated for the first time. It was found that the LiAlH₄ + 7 mol % MnFe₂O₄ sample started to decompose at 62 and 119 °C for the first two dehydrogenation stages and released 7.45 wt % hydrogen, which is 88 and 71 °C lower than those of as-received LiAlH₄, respectively. The isothermal dehydriding kinetics show that the doped LiAlH₄ sample could release about 4.7 wt % hydrogen in 70 min at 90 °C. Furthermore, the first two dehydrogenation steps could be finished within 80 min with 7.44 wt % hydrogen released at 120 °C, whereas as-received LiAlH₄ only released about 0.5 wt % hydrogen for the same temperature and time. From differential scanning calorimetry (DSC) and Kissinger desorption kinetics analyses, the apparent activation energies, E_a, of the doped sample were 66.7 kJ/mol for the first dehydrogenation stage and 75.8 kJ/mol for the second dehydrogenation stage, resulting in decreases of 40.2% and 58.1% compared with those of as-received LiAlH₄, which are much higher than those of LiAlH₄ doped with other reported catalysts calculated by Kissinger method. Through X-ray diffraction (XRD) and Fourier transform infrared (FTIR) observations, in situ formed Fe_0.9536O and amorphous Mn or Mn-containing phases together provide a synergetic catalytic effect for the remarkably improved dehydrogenation properties of LiAlH₄.