Origin of Degradation in the Reversible Hydrogen Storage Capacity of V_1–<i>x</i>Ti_<i>x</i> Alloys from the Atomic Pair Distribution Function Analysis

Reduction in reversible hydrogen storage capacity with increasing hydrogenation and dehydrogenation cycle number is observed in numerous hydrogen storage materials, but the mechanism behind this unfavorable change has not been elucidated yet. In this study, we have investigated the development of structural defects or disorders in V_1–<i>x</i>Ti_<i>x</i>H₂, <i>x</i> = 0, 0.2, and 0.5, during the first 15 hydrogen absorption and desorption cycles using the atomic pair distribution function (PDF) analysis of synchrotron X-ray total scattering data to find out the possible structural origin of the poor cyclic stability of V_1–<i>x</i>Ti_<i>x</i> alloys. While pure vanadium shows no significant change in the PDF, alloy samples subject to several hydrogenation and dehydrogenation cycles display fast decaying of the PDF profile due to a progressive increase in the PDF peak width with increasing <i>r</i>. This <i>r</i>-dependent PDF peak broadening effect becomes stronger with cycle number. Molecular dynamics (MD) simulations demonstrated that dislocation defects explain characteristic features in our experimental PDFs very well and suggested that a large number of dislocations are formed during hydrogen cycling. We found there is a close relation between the reduced amount of the reversible hydrogen content of V_0.8Ti_0.2 and the amount of generated dislocations. On the basis of the PDF analysis results, a possible mechanism behind degradation in the reversible hydrogen storage capacity of V_1–<i>x</i>Ti_<i>x</i> is discussed