A TEM based study of the microstructure during room temperature and low temperature hydrogen storage cycling in MgH2 promoted by Nb-V

Magnesium hydride combined with a new bimetallic Nb-V catalyst displays remarkably rapid and stable low temperature (200 degrees C) hydrogen storage kinetics, even after 500 full volumetric absorption/desorption cycles. The system is also able to fairly rapidly absorb hydrogen at room temperature at a pressure of 1 bar. This unprecedented absorption behavior was demonstrated for 20 cycles. We employed extensive cryo-stage transmission electron microscopy (TEM) analysis on fully and partially sorbed materials to provide insight into the rapid Mg to MgH2 phase transformation. After extended cycling of what was initially a 1.5 mu m thick fully dense alloy film, the sample structure becomes analogous to that of a weakly agglomerated nanocomposite powder. The cycled Mg-V-Nb structure consists of a dense distribution of catalytic Nb-V nanocrystallites covering the surfaces of larger Mg and MgH2 particles. The partially absorbed 20 degrees C and 200 degrees C microstructures both show this morphology. TEM results combined with Johnson-Mehl-Avrami-type kinetic analysis point to the surface catalyst distribution and stability against coarsening as being a key influence on the two-stage hydriding kinetics. Remarkably, the mean size of the Nb0.5V0.5H nanocrystallites stays essentially invariant throughout cycling; it is 3 nm after 45 cycles and 4 nm after 500 cycles. A mechanistic description is provided for the cycling-induced microstructural evolution in the ternary alloy as well as in the binary baselines. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.