Zebra-Striped Fibers in Relation to the H2 Sorption Properties for MgH2 Nanofibers Produced by a Vapor-Solid Process

MgH2 is a promising candidate for solid-state H-2 storage applications. To improve the H-2 sorption kinetics, highly pure MgH2 nanofibers were synthesized by a vapor solid process. The fibers showed improved sorption kinetics compared with bulk MgH2, which were investigated by a volumetric pressure-composition-temperature method. To choose a meaningful kinetic model that represented the physical reality and intrinsic kinetic parameters for the unique fiber structure, theoretical modeling of the sorption data and metallographic examinations of the in situ dehydrogenation process and partially hydrogenated samples were carried out. The Johnson-Mehl-Avrami (JMA) model, which is based on the theory of nucleation and growth transformation, was selected for the kinetic mechanism analysis of the hydrogenation/dehydrogenation of the fibers. The theoretical modeling by the JMA model indicated that the phase transformation of Mg to MgH2 (or of MgH2 to Mg) in the individual fibers proceeded one-dimensionally along the zebra-striped MgH2-Mg interfaces in the longitudinal direction of the fibers, as confirmed by the observation of the in situ dehydrogenation process and the partially hydrogenated samples. Interestingly, during the H-2 absorption/desorption steps, the fibers were separated into periodic white-black (MgH2-Mg) zebra stripes, and the phase growth proceeded along the white-black interfaces in the longitudinal direction of the fibers. The activation energies for hydrogenation and dehydrogenation were estimated to be 116.55 and 150.78 kJ/mol, respectively, which were within the range of previously reported values for bulk MgH2.