Hydrogen Transport through Pd Electrode : Current Transient Analysis

Abstract

The hydrogen transport through a palladium electrode in the coexistence of two palladium hydride phases (alpha- and beta-PdH) has been investigated by analysing current decay transients based on the modified McNabb and Foster's physical model of hydrogen trapping, considering interstitial sites in beta-phase as reversible trap sites, supplemented by cyclic polarization curve and open-circuit potential transients. From the appearance of the three-staged current decay transients and hydrogen content in the electrode, it is inferred that the beta-phase is formed just beneath the electrode surface during the hydrogen injection into the electrode at overpotentials between 0.08 and -0.02 V(rhe) as the beta-PdH patches sporadically embedded in alpha-PdH matrix and below -0.02 V(rhe) as the beta-phase layer completely embedded in alpha-PdH matrix. During the hydrogen extraction from the electrode, it follows the hydrogen transport initially proceeds to decomposition of the complete beta-phase layer into alpha-phase by ''up-hill diffusion'' of hydrogen from inner hydrogen-poor alpha-phase towards outer hydrogen-rich beta-phase, accompanied by interface-controlled phase boundary movement, intermediately it proceeds to complete decomposition of the alpha-phase into Pd by simple diffusion through alpha-phase, finally followed by complete decomposition of the remaining sporadic beta-PdH patches into alpha-PdH or Pd. The ''up-hill diffusion'' is accomplished by transferring interstitial hydrogen in the alpha-phase to hydrogen trapped in the beta-phase. The stress gradient across the alpha/beta phase boundary developed during the hydrogen injection helps the hydrogen transport during the hydrogen extraction. (C) 1997 Elsevier Science Ltd.