Theoretical model for AB₅ alloy hydride formation: the electrochemical activation of the hydrogen diffusion process

Abstract

This paper examines the hydrogen diffusion process during activation cycles in LaNi_3.6Co_0.7Mn_{0.4 − x} Mo _x Al_0.3 alloys with x = 0; 0.1 and 0.25 in strong alkaline solutions. The effect of molybdenum substitution was analyzed by potential, composition, and temperature (E-c-T) and charge/discharge curves. Hysteresis values and the potential between charge/discharge curves have been used as parameters for efficiency loss. Further, electrochemical impedance spectroscopy was carried out in order to visualize the charge transfer and diffusion component during activation process. A mathematical approach is presented here based on the mechanism involved in the course of the hydriding reaction solving a spherical second Fick’s law. Thus, adsorbed hydrogen is considered to be transferred from the surface to inner sites, and the diffusion into the bulk of the alloy is demonstrated to be the slow route. Diffusion overpotentials and time constants for AB₅ alloys were calculated as a function of the number of hydrogenation cycles. These results show that the molybdenum content has positive effects in kinetic behavior.