Hydrogen-diffusion-rate-limited hydriding and dehydriding kinetics

Abstract

Hydriding and dehydriding kinetics are derived within the framework of the Johnson-Mehl-Avrami equation. Hydriding is considered to be rate limited by hydrogen diffusion through a growing hydride layer, and dehydriding by hydrogen diffusion through a growing metal layer. Incubation time effects due to surface contamination are taken into account by a delayed nucleation function. The dominant composition-dependent terms in the thermodynamics and in the mobility are taken into account to derive normative diffusivity for hydrogen. The temperature dependence of hydriding is defined by an activation energy (QD−ΔH̄H+ΔH12) where QD is the activation energy for diffusion of hydrogen in the hydride, ΔH̄H is the relative partial molar enthalpy of hydrogen in the hydride, and ΔH12 is the enthalpy of the hydrogen-pressure plateau reaction. For dehydriding the activation energy is (QD+ΔH̄H−ΔH12), but now QD and ΔH̄H refer to hydrogen in the metal. The solution thermodynamics contributions to the activation energy are shown to be very important. It is emphasized that in order to determine a meaningful activation energy, it is not the pressure that must be maintained constant but rather a factor T[1−(Pd/P)1/2] in hydriding and a factor [T[1−(P/Pd)1/2] in dehydriding, where Pd is the dissociation pressure of the hydride. Several hydriding studies from the literature are critically disccused in terms of these results.

Modelling and proper evaluation of volumetric kinetics of hydrogen desorption by metal hydrides

“…Apart from the geometrical suggestion, it was frequently reported that the finite diffusion rate of hydrogen has an effect on desorption kinetics [4]. For the current model this factor was neglected because of the very fast diffusion rate [16,17].…”

“…Conventionally, they are distinguished as in Ref. [4] 2. Diffusion through the a-phase (further in the text adiffusion), 3.…”

Microstructural and Kinetic Evolution of Fe Doped MgH2 during H2 Cycling

“…In order to keep roughly constant the thermodynamic driving force, estimated according to the formulation of Rudman et al [27] the cycling was induced by switching the pressure between 10 bar and 0.1 bar for 22 cycles.…”

Hydrogen diffusion in Mg2NiH4 intermetallic compound

“…in Ref. [21]. Contrary to the conclusion often drawn, namely that the process of desorption is controlled by diffusion [12,13,16,21], quantitative data describing hydrogen diffusion itself are very sparse in the literature.…”