Impedance Analysis of Hydrogen Absorption Reaction on Pd Membrane Electrode in 0.1 M LiOH Solution under Permeable Boundary Condition

Abstract

The hydrogen absorption and diffusion into and through a palladium membrane electrode has been investigated in 0.1M LiOH solution by using an ac impedance method combined with an electrochemical hydrogen permeation technique. The ac impedance measurements were carried out in the overpotential range of - 0.07 to 0.28 V(rhe) applied to the cathodic side of the palladium membrane after the hydrogen permeation achieved a steady-state. The experimental impedance spectra were analysed by using complex non-linear least squares (CNLS) fitting method on the basis of two Faradaic hydrogen absorption admittance equations derived in the preceding work. As a result, the indirect hydrogen absorption model accounts for the impedance spectra measured in the overpotential range of 0.1-0.28 V(rhe), whereas the direct absorption model dominates the absorption at the overpotentials below 0.08 V(rhe), accompanied by the anomalous behaviour of hydrogen permeation transients. Under the assumption of tridimensional hydrogen adsorption on the palladium electrode, the hydrogen coverage was theoretically calculated as a function of overpotential with the kinetic rate constants of Volmer adsorption, hydrogen absorption reaction and the hydrogen diffusivity in palladium, best-fitted to the measured impedance spectra. The calculated hydrogen coverage suggested the formation of B-phase palladium hydride at the overpotentials below 0.08 V(rhe). The model change to the direct hydrogen absorption, accompanied by the anomalous hydrogen permeation, accounts for the formation of P-palladium hydride at the overpotential of 0.08 V(rhe).