CoFe2O4 synthesized via a solvothermal method for improved dehydrogenation of NaAlH4

The effect of cobalt ferrite (CoFe₂O₄) nanopowder synthesised through a solvothermal method on the dehydrogenation properties of sodium alanate (NaAlH₄) was studied for the first time. The onset decomposition temperature of NaAlH₄ is significantly reduced after milling with CoFe₂O₄, in which the 10 wt% CoFe₂O₄-doped NaAlH₄ sample starts to decompose at ～100 °C. In contrast, the as-milled NaAlH₄ begins to decompose at ～200 °C, ～100 °C higher than the doped sample. With respect to desorption kinetic at constant temperature of 150 °C, the 10 wt% CoFe₂O₄-doped NaAlH₄ sample desorbed ～2.2 wt% hydrogen within 30 min, whereas the as-milled NaAlH₄ only desorbed below 0.2 wt% hydrogen. The Kissinger plot exhibited that the apparent activation energy (E_A) for hydrogen release from NaAlH₄ is significantly reduced after adding with 10 wt% CoFe₂O₄-doped NaAlH₄. The E_A values for the first and second stage dehydrogenation of the 10 wt% CoFe₂O₄-doped NaAlH₄ composite are calculated as 80.3 and 88.2 kJ/mol, respectively, and these values are reduced at approximately 34.3 and 30.5 kJ/mol compared with the as-milled NaAlH₄ (114.6 and 118.7 kJ/mol, respectively). Based on the X-ray diffraction result, the enhancement of desorption properties of NaAlH₄ with the presence of CoFe₂O₄ is presumably due to the synergistic catalytic effect played by new active species (Co₃O₄ and Fe) that in situ formed during the desorption process.