Superior Catalytic Effects of Nb2O5, TiO2, and Cr2O3 Nanoparticles in Improving the Hydrogen Sorption Properties of NaAlH4

Sodium alanate (NaAlH4) is a promising complex metal hydride due to its reasonable hydrogen storage capacity (7.4 wt %). However, the pristine NaAlH4 suffers from the inherent limitations of unfavorable thermodynamics (high desorption temperature), slow kinetics, and poor reversibility. In the present work, the efficacy of Nb2O5, TiO2., and Cr2O3 nanoparticles in ameliorating hydrogen sorption properties of NaAlH4 was evaluated. The use of Nb2O5 and TiO2 displayed superior catalytic effects in terms of enhancing dehydriding/rehydriding kinetics and reducing the dehydrogenation temperature of NaAlH4. Isothermal volumetric measurements at 150 degrees C revealed that kinetics of hydrogen desorption with Nb2O5 and TiO2 were almost 11-12 times that of pristine NaAlH4. The apparent activation energy as well as enthalpy of dehydrogenation were considerably lowered by addition of Nb2O5 and TiO2 nanopowders. Moreover, the pronounced enhancement on hydrogen capacity arising upon adding Nb2O5 and TiO2 was observed to persist well in rehydrogenation/dehydrogenation cycles. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and FESEM-EDS analyses demonstrated that reduction of Nb2O5 and TiO2 during the first desorption was coupled to the migration of reduced titanium oxide species from surface to bulk during cycling. It was suggested that these finely dispersed oxygen-deficient niobium and titanium species might contribute to kinetic improvement by facilitating the hydrogen diffusion during both dehydrogenation/rehydrogenation.