Analysis of the compressive and tensile stresses generation/relaxation during hydrogen ingress into and egress from Pd foil electrode

The build up/decay of compressive and tensile stresses has been analyzed as a function of the hydrogen charging potential during hydrogen ingress into and egress from a palladium (Pd) foil electrode in the presence of a single phase (alpha-Pd phase) and a mixture of two phases (alpha-Pd and beta-Pd phases) in 0.1 M NaOH solution, by using a laser beam deflection technique, combined with a current transient technique. The transient of the hydrogen concentration profile across the electrode is derived from the compressive and tensile deflection transients, measured simultaneously with cathodic and anodic charge transients during hydrogen ingress into the fresh electrode and hydrogen egress from the precharged electrode, respectively. The time to the maximum compressive and tensile deflections measured during hydrogen ingress into and egress from the electrode, respectively, is assigned to specific hydrogen concentration profiles which are characterized by the 'break-through time' indicating in a first approximation the arrival of hydrogen at the opposite side of the interface. From the value of the time to the maximum deflection, hydrogen diffusivity in the Pd foil electrode was determined to be 4x10(-8) to 5x10(-7) cm(2) s(-1). It is indicated that the elastic tensile stress field, adjacent to the dislocation developed around the beta-Pd phase formed in the Pd foil electrode, introduces the additional trap sites for hydrogen. Larger hydrogen diffusivity determined during hydrogen discharging than hydrogen charging is discussed in terms of how much the trap sites are filled with hydrogen during hydrogen charging. Stresses exerted due to local molar volume change across the electrode during hydrogen ingress into and egress from a single alpha-Pd phase are always exceeded by those stresses exerted across the electrode during hydrogen ingress into and egress from a mixture of alpha-Pd and beta-Pd phases. (C) 1999 Elsevier Science Ltd. All rights reserved.