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 3.6 Co 0.7 Mn 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 AB 5 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.

Electrochemical Determination of Physicochemical and Mechanical Properties in ZrCr1-xNiMoxHydrides

“…The equilibrium potentials for AB 2 M0, AB 2 M1 and AB 2 M2 alloys, E j=0 , were determined from mid-plateau values as the arithmetic means between the free charge E opc,charge and discharge potentials, E opc,discharge (Table III). Hysteresis for AB 2 alloys were also calculated as indicated elsewhere [37][38][39] between 20 and 80% SOC using data from Figure 3. Dimensionless hysteresis was evaluated as:…”

“…It has been established that manganese substitution by molybdenum (0.10 < x < 0.25) in LaNi 3.6 Co 0.7 Mn 0.4-x Mo x Al 0.3 (AB 5 ) alloys produces an increase in the hysteresis, but holding the same crystal structure. 38 P-c-T hysteresis in Ti 12.5 Zr 21 V 10 Cr 8.5 Mn x Co 1.5 Ni 46.5-x (AB 2 ) hydrides is closely related to the unit cell aspect ratio (a/c) of the major C14 phase. 41 It was argued that the increase in the a/c ratio decreases the distance for protons to hop along the c-axis.…”