Anomalous Behaviour in Hydrogen Diffusion through Palladium and Metal-Hydrides

Hydrogen transport through Pd and metal-hydrides has been usually investigated by using potentiostatic current transient technique (chronoamperometry), on the basis of the diffusion-controlled model. This assumes that hydrogen diffusion in the electrode is very slow, while other reactions including interfacial charge transfer and hydrogen transfer are too fast to influence hydrogen transport, and thus hydrogen transport is purely governed by hydrogen diffusion. However, various kinds of anomalous behaviour in hydrogen transport, which could be hardly explained by the diffusion-controlled model, have been quite frequently reported by many researchers. In particular, we propose that the diffusion-controlled model should be no longer valid in case hydrogen transport through such hydride-forming electrodes as Pd and metal-hydrides proceeds under the constraint that hydrogen diffusion is coupled with either interfacial charge transfer or hydrogen transfer reaction or with both reactions. The anomalous hydrogen transport models suggested are theoretically described with the mathematical expressions for the current transient, and then they are exemplified by hydrogen extraction from the Pd and metal-hydrides in aqueous solutions. Finally, some parameters affecting hydrogen transport, e.g. the potential step and the nature of the electrode surface, are discussed in detail.