First-principles investigation on the role of interstitial site preference on the hydrogen-induced disproportionation of ZrCo and its doped alloys

There are two phase structures involved in ZrCo hydrides (ZrCoHx). When x <= 1, the alpha-phase hydride is generated when hydrogen atoms occupy the 3c and 12i sites. When 1 < x <= 3, three interstitial sites of 4c(2), 8f(1), and 8e are occupied by H, and in turn the beta-phase hydride is formed. There is a disproportionation reaction in beta-phase hydrides during hydrogen discharging process to produce the ZrH2 phase with higher thermal stability, leading to inferior hydrogen storage performance. In this study, the influence of hydrogen storage capacity on thermodynamic and lattice stabilities of alpha- and beta-phase hydrides for each occupancy position is investigated under the framework of the first-principles study. The results indicate that the binding energy in the 3c site is higher compared with the 12i site under the condition of identical hydrogen storage capacity. Similarly, the binding energy is the largest for the 8e site compared with the other two sites, indicating that there is the least energy released in the reaction process. Thus, the 8e site is proved as the most unfavorable site in beta-phase ZrCo hydrides, which is due to its degraded thermodynamic stability. Also, comparisons of mechanical properties and total density of states for each site in two hydride phases are presented to demonstrate that compound lattice stability in the 8e site is the poorest, suggesting that it is more likely to produce disproportionation. Furthermore, the dependence of hydrogen storage performance of beta-phase hydrides on Ti/Rh doping is examined as well. It is discovered that there is improved thermodynamic stability and lattice stability in the 8e site for Zr0.875Ti0.125Co after Zr is partially substituted by Ti, which significantly enhances the disproportionation resistance. In contrast, when Co is partially replaced by Rh, there is a deterioration in the thermodynamic stability of ZrCo0.875Rh0.125 in the 8e site, but its lattice stability is somewhat improved. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.