AN INVESTIGATION OF THE ELECTROCHEMICAL KINETICS OF DEUTERIUM INSERTION INTO A PD MEMBRANE-ELECTRODE IN 0.1 M LIOD SOLUTION BY THE AC-IMPEDANCE TECHNIQUE

Abstract

The absorption and diffusion of deuterium into and through a palladium membrane electrode respectively in 0.1 M LiOD solution have been studied by means of an a.c. impedance technique in combination with an electrochemical permeation cell. A.c. impedances of the electrode were measured in the overpotential range from -0.07 to 0.23 V (vs. a reversible hydrogen electrode) applied to the cathodic side of the palladium membrane after the deuterium permeation current had achieved a steady state. The measured impedance spectra were analysed using a complex non-linear least-squares (CNLS) fitting method based upon a faradaic admittance equation derived previously for a model of diffusion-controlled indirect hydrogen absorption into the palladium membrane via an adsorbed phase. Assuming tridimensional adsorption of deuterium in the interfacial region, the steady state current and deuterium coverage were theoretically calculated as a function of overpotential with the kinetic rate constants of Volmer adsorption, deuterium absorption reaction and deuterium diffusivity in palladium best fitted to the measured impedance spectra. The value of forward rate constant of Volmer adsorption and the ratio of bulk concentration of absorbed deuterium to surface concentration of adsorbed deuterium were determined to be 2.16 x 10(-9) mol cm-2 s-1 and 4.88 x 10(2) cm-1 respectively, smaller by one order of magnitude than those of hydrogen (1.26 x 10(-8) mol cm-2 s-1 and 2.35 x 10(3) cm-1 respectively), whereas the value of backward rate constant was 1.74 X 10(-9) mol cm-2 s-1, similar to that of hydrogen (3.85 x 10(-9) mol cm-2 s-1). The diffusivity of deuterium in palladium was found to be (5.10 +/- 1.04) X 10(-7) cm2 s-1, slightly higher than that of hydrogen ((3.49 +/- 0.69) x 10(-7) cm2 s-1), representing the reverse isotope effect.