Hydrogen transport through metals determined by electrochemical methods

Abstract

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 under the appropriate boundary conditions experimentally accessible. It was shown that direct hydrogen absorption into the Pd foil without adsorbed intermediate state of hydrogen does occur at certain hydrogen overpotentials. The nature of the subsurface hydride just beneath the metal surface is discussed 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 generation at the interface of two hydride phases as well as the movement of the boundary between two phases. In addition, the effects of hysteresis and microstructure on the hydrogen absorption are dealt with. Finally, hydrogen transport through the metal covered by the oxide film is reviewed, considering the electric field across the oxide film and the electrochemical equilibrium at the interface between oxide film and underlying metal. Mathematical expressions 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 together with the location of charge transfer in the metal/oxide composite.