Enhanced hydrogen storage properties of NaAlH4 co-catalysed with niobium fluoride and single-walled carbon nanotubes

Abstract

The effects of single-walled carbon nanotubes (SWCNTs) as a co-catalyst with NbF₅ on the dehydrogenation and hydrogenation kinetics of NaAlH₄ 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 NbF₅ on the de/rehydrogenation of NaAlH₄, which improves the hydrogen de/absorption performance when compared to adding either SWCNTs or NbF₅ alone. For example, the apparent activation energy for the first-step and the second-step dehydrogenation of the co-doped NaAlH₄ 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 NbF₅ doped NaAlH₄, 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.