Enhanced hydrogen storage performance for MgH2–NaAlH4 system—the effects of stoichiometry and Nb2O5 nanoparticles on cycling behaviour

Abstract

Nowadays, the technological utilization of reactive hydride composites (RHC) as promising hydrogen storage materials is hampered by their reaction kinetics. In the present work, effects of reactant stoichiometry on ensuing hydrogen sorption properties and pathway of the MgH2–NaAlH4 (mole ratios 1:2, 1:1 and 2:1) system, both undoped and doped with Nb2O5 nanoparticles, were investigated. It was found that the as-prepared reactant stoichiometry of MgH2/NaAlH4 system had a profound impact on its dehydrogenation kinetics and reaction mechanism. Variable temperature dehydrogenation data revealed that undoped binary composites possessed enhanced hydrogen desorption properties compared to that of pristine NaAlH4 and MgH2. The use of Nb2O5 displayed superior catalytic effects in terms of enhancing dehydriding/rehydriding kinetics and reducing the dehydrogenation temperature of MgH2–NaAlH4 system. Isothermal volumetric measurements at 300 °C revealed that enhancements arising upon adding Nb2O5 were almost double that of undoped MgH2–NaAlH4 composites. The apparent activation energies for NaAlH4, Na3AlH6, MgH2, and NaH relevant decompositions in doped composite were found to be much lower than that for the undoped one. Moreover, Nb2O5 doping also markedly enhanced the reversible capacity of MgH2–NaAlH4 composites under moderate conditions, persisting well during three de/rehydrogenation cycles. XRD, XPS, and FESEM-EDS analyses demonstrated that reduction of Nb2O5 during first desorption was coupled to the migration of reduced niobium oxide species from the bulk to the surface of the material. It was suggested that these finely dispersed oxygen-deficient niobium species might contribute to kinetic improvement by serving as the active sites to facilitate hydrogen diffusion through the diffusion barriers both during dehydrogenation and rehydrogenation.