A proposed kinetic model for hydrogen sorption in MgH2

Abstract

The role of ternary Mg–Nb oxides on H2 absorption and desorption properties of ball milled MgH2 is discussed on the basis of a kinetic model involving the formation of reaction pathways of ternary oxide species with easier splitting of H2 that facilitate the transport of H2 into the solid structure. The kinetic model comprised three key steps: dissociation of molecular hydrogen into H atoms, diffusion of H into Mg to form MgH2 during adsorption, and successive recombination of H–H on the surface of the additives during desorption to make a pathway for H2 sorption in the structure. The presence of transition metal oxides remarkably increases the H2 sorption rates of nano-structured MgH2, as determined by a volumetric Sievert apparatus. The H2 desorption rate increases with increasing temperature from 593 to 673 K and under the same conditions, the absorption rate decreases. The most promising results were obtained for Mg3Nb6O11-doped MgH2 nanoparticles. The MgH2/Mg3Nb6O11 system is completely dehydrogenated at 673 K under 0.1 MPa H2 and the samples fully rehydrogenated at 613 K under 2.5 MPa H2 pressure. Activation energies were obtained for MgH2/Mg3Nb6O11 mixture, and without additive (ball milled MgH2), to be 88 and 127 kJ mol−1, respectively, underlying the effect of the ternary additive.