Thermodynamics modeling of ZrCo-H systems with biaxial compressive strain during desorption

As known, inferior cycling performance and deteriorated de-/hydrogenation kinetics resulting from by hydrogen-induced disproportionation severely impedes engineering applications of zirconium-cobalt (ZrCo) alloys used in the industry of hydrogen isotope storage. In this work, the first principles calculations are conducted to investigate the thermodynamic behavior of ZrCo hydrides with and without biaxial compressive strain during desorption. Importantly, the whole dehydrogenation process is divided into two steps due to the presence of intermediate ZrCoH _x ( x = 2, 2.25, 2.50, and 2.75). During Step I, the hydrided phase ZrCoH ₃ is decomposed into ZrCoH _x (ZrCoH ₃ → ZrCoH _x), and then the phase transformation from ZrCoH _x to ZrCo (ZrCoH _x → ZrCo) occurs in Step II. Afterwards, the equilibrium pressures, enthalpy changes, and entropy changes involved in the reactions are determined through the thermodynamic relations. The results demonstrate that compared to the second step, there is a superiority of dehydrogenation performance for the first step, which is derived from higher equilibrium pressure. Moreover, the equilibrium pressure increases with the decrease of the H content in ZrCoH _x because of the intensification of vibrational degrees of freedom of H atoms. Further study concerns on the influence of biaxial strain (-3% - 0%) along [010] and [001] on thermodynamics properties of the ZrCo-H systems, indicating that the strain deteriorates the lattice stability and improves the equilibrium pressure. By contrast, it is more favorable to enhancing the dehydrogenation performance when the compressive strain is beyond 1.5%.