Effect of annealing temperature on the microstructure of Pr2Co7 alloy and its hydrogen absorption-desorption kinetics

Abstract

In this work, we studied the effect of annealing temperature on the kinetics of hydrogen absorption/desorption in nanocrystalline Pr2Co7 alloy. The pressure-composition (P–C) isotherm measurements revealed that the plateau pressure of Pr2Co7 alloy after annealing becomes comparatively flat. The maximum hydrogen absorption content of as-milled alloy and the annealed one reached 1.01 and 1.2 H/M (H/M: hydrogen per metal), respectively. The derived formation enthalpy ΔH and Gibbs free energy ΔG of the hydride in the annealed alloy are higher than those of the as-milled compound, indicating that the hydrides stability increases after annealing. The mechanism of hydrogen absorption-desorption process in the as-milled and annealed alloys was further investigated according to the Johnson-Mehl-Avrami formula. The hydrogenation of the as-milled and annealed Pr2Co7 alloys was found to be controlled by the three-dimensional-interface process. The activation energies Ea were determined and indicate that annealing is a very beneficial way to improve hydrogen absorption-desorption kinetics of the nanocrystalline Pr2Co7 alloy. The latter has a very fast absorption kinetic at 298 K. Throughout the study, it has been shown that Pr2Co7 alloy exhibits excellent hydrogenation properties which open a new perspective to be a new candidate for important uses in the field of hydrogen storage such as negative electrodes for nickel metal hydride (Ni-MH) batteries, mobile and short range vehicular applications.