Enhanced hydrogen storage properties of NaAlH ₄ co-catalysed with niobium fluoride and single-walled carbon nanotubes

The effects of single-walled carbon nanotubes (SWCNTs) as a co-catalyst with NbF ₅ on the dehydrogenation and hydrogenation kinetics of NaAlH ₄ were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, differential thermal analysis, temperature-programmed desorption, and isothermal hydrogen ab/desorption techniques. It has been revealed that there is a synergistic effect of SWCNTs and NbF ₅ on the de/rehydrogenation of NaAlH ₄, which improves the hydrogen de/absorption performance when compared to adding either SWCNTs or NbF ₅ alone. For example, the apparent activation energy for the first-step and the second-step dehydrogenation of the co-doped NaAlH ₄ sample is estimated to be 85.9 and 96.2 kJ mol ^-1, respectively, using Kissinger's approach, which is lower than the pristine, SWCNT-, and NbF ₅ doped NaAlH ₄, respectively, indicating a reduced kinetic barrier. These results are attributed to the active Nb-containing species and the function of F anions, as well as the nanosized pores and high specific surface area of the SWCNTs, which facilitates the dissociation and recombination of hydrogen molecules on its surface and the atomic hydrogen diffusion along the grain boundaries and inside the grains, and decreases the segregation of bulk Al after the desorption. Hence, the combined catalytic mechanism is presented. © 2012 The Royal Society of Chemistry.