Analysis of stresses generated during hydrogen charging into and discharging from Pd foil electrode

Abstract

The present work involves the analysis of stresses generated during hydrogen charging into and discharging from Pd foil electrode. In chapter Ⅲ, the build-up/decay of compressive and tensile stresses has been analysed as a function of hydrogen charging potential during hydrogen ingress into and egress from palladium (Pd) foil electrode in the presence of a single phase (α-Pd phase) and a mixture of two phases (α-Pd and β-Pd phases) in 0.1 M NaOH solution by using a laser beam deflection technique, combined with current transient technique. The transient of 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 pre-charged 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 characterised 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 $4\times10^{-8}$ to $5\times10^{-7}cm^2 s^{-1}$. It is indicated that the elastic tensile stress field adjacent to the dislocation developed around the β-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 α-Pd phase are always exceeded by those stresses exerted across the electrode during hydrogen i...