Hydrogen transport through metals determined by electrochemical methods

The topics related to electrochemical hydrogen absorption reaction into the metal electrode are reviewed, with particular attention to palladium as a model system. Faradaic admittance expressions involving hydrogen transport through the metal in the low hydrogen concentration range are reviewed unde r the appropriate boundary conditions experimentally accessible. It was sho wn that direct hydrogen absorption into the Pd foil without adsorbed interm ediate state of hydrogen does occur at certain hydrogen overpotentials. The nature of the subsurface hydride just beneath the metal surface is discuss ed together with its role in hydrogen transport through Pd metal. Transport of hydrogen through the metal in the presence of two coexisting phases of hydrogen-rich and -deficient phases is discussed in terms of stress generat ion at the interface of two hydride phases as well as the movement of the b oundary between two phases. In addition, the effects of hysteresis and micr ostructure on the hydrogen absorption are dealt with. Finally, hydrogen tra nsport through the metal covered by the oxide film is reviewed, considering the electric field across the oxide film and the electrochemical equilibri um at the interface between oxide film and underlying metal. Mathematical e xpressions concerning hydrogen transport through an oxide/metal bilayer are introduced for the steady state and transient states. Interaction between absorbed hydrogen and the anion of the oxide is discussed in detail togethe r with the metal/oxide composite.