The mechanism of the evolution of hydrogen on palladium and associated interval damage phenomena

The mechanism of the hydrogen evolution reaction has been examined and the knowledge thus obtained applied to the interpretation of various aspects of internal damage found above a critical overpotential in alkaline solution. Tafel slopes showed a change at eta = -0.3 V; the stoichiometric number is 1 < v < 1.2, separation factor (S-TH = 8 +/- 1), FTIR measurements give partial derivative ln theta/partial derivative eta = (4.7R T/F)(-1), and permeation measurements show partial derivative lnP(infinity)/partial derivative eta = (6.1 RT/F)(-1). These facts are consistent (up to an overpotential (eta) of -0.3 V) with a rate determining proton discharge, coupled to Tafel recombination; at eta more negative than -0.3 V, the mechanism changes to a fast proton discharge with rate determining electrochemical desorption to H-2. Damage within the Pd electrode appears markedly at pinholes along the hexagonal sides of grain boundaries. The number of hexagonal units and pinholes increases with overpotential and time. An increased solubility of hydrogen at points of high stress is seen as the cause of damage discernible at triaxial points and that generated along the sides of grain boundaries. The high fugacities of hydrogen in voids around these regions serves to interpret the further deterioration of the structure observed with increasing overpotential. Calculations of pressures inside the voids suggest that initiation of damage to the Pd corresponds to a change in reaction mechanism at eta similar or equal to -0.3 V. (C) 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.