Anomalous behaviour of hydrogen extraction from hydride-forming metals and alloys under impermeable boundary conditions

Hydrogen injection into and extraction from hydride-forming electrodes has been routinely investigated under the assumption that the electrode is homogeneous in structure and hydrogen transport through such electrodes is purely controlled by hydrogen diffusion. However, various kinds of abnormal behaviours in hydrogen transport, which could hardly be explained in terms of the diffusion-control model, have been quite frequently reported by many researchers. This review provides a comprehensive survey of the anomalous behaviours of hydrogen transport observed in such hydride-forming electrodes as Pd and metal-hydrides, with particular emphasis on hydrogen extraction under the impermeable boundary conditions during the potentiostatic current transient measurement involving the potential stepping. After a brief discourse on the conventional diffusion-control model, the topics related to the boundary conditions at the electrode surface during hydrogen extraction are extensively reviewed. In particular, it is shown that the diffusion-controlled constraint should be no longer valid at the electrode surface for hydrogen extraction in case hydrogen diffusion is influenced by either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. Subsequently, the atypical behaviours of current transient due to hydrogen diffusion in the presence of traps and in the coexistence of two hydride phases are treated in detail. Each of the hydrogen extraction models suggested is discussed with the aid of the anodic current transients numerically calculated based upon the theoretical electrode potential curve, and then it is exemplified by hydrogen extraction from Pd and metal-hydrides in aqueous solutions.