Growth of Oxide Films at Pt Anodes at Constant Current, Density in  H 2 SO 4

Abstract

Constant current charging curves from 10 -6 to 10 -1 A-cm -2 are used to study the mechanism of growth of anodic oxide films at platinum in H2SO4 solutions. At any constant current density the potential initially changes linearly with time. In this linear region no 02 is evolved. The linear region is followed by a region Jn which potential changes logarithmically with time while oxygen evolution becomes the predominant reaction. The thickness of the oxide films at which the transition occurs depends on the applied current density but for all current densities it is less than 10A. Both in the linear and the logarithmic region the ellipsometric thickness-time relationships essentially parallel the V-t relationships. The rate of growth is described bywhere d is the thickness and Vo the potential at which oxide film starts to grow. From the transfer coefficient a, that is equal to 158 A-V -1, it was deduced that the oxide phase is composed of divalent platinum ions, i.e., the oxide phase is Pro. It is suggested that the rate-determining step is a process at the metal/oxide interface rather than a process at the oxide/solution interface or a process within the bulk of oxide. The distribution of the potential between the oxide film and the double layer is discussed. It is shown that the potential difference across the solution double layer is constant for the growth at current densities examined here. This is possible if the electrochemical reaction in the double layer is fast. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.218.248.200 Downloaded on 2015-04-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.218.248.200 Downloaded on 2015-04-13 to IP

A model for anodic hydrous oxide growth at iridium

“…Thus, one must conclude that several layers of some other form of oxide (the inner oxide) were produced and reduced back to the metal during each of these cycles. This inner oxide growth is presumably similar to oxide growth at Pt [32][33][34][35][36] and probably occurs by a place-exchange mechanism [32]. A difference from the behaviour of Pt is that no well-defined reduction peak can be obtained for the inner oxide at Ir.…”

“…+1 V, slow place exchange occurs [8,32], but virtually all the place-exchanged oxide is reduced back to metal during the negative part of the cycle. We assume here that place exchange occurs at Ir as it does at Pt [32,35], producing a thin, compact oxide layer.…”

“…Thus, the inner oxide accumulates so that on each positive scan, the oxidation currents become diminished and higher potentials are required to cause further growth of the compact oxide. This interpretation is based on oxide growth at Pt, where the growth rate is proportional to the applied potential and inversely proportional to the oxide film thickness [35].…”

On the Kinetics and Mechanism of O 2 Reduction at Oxide Film Covered Pt Electrodes: I . Effect of Oxide Film Thickness on Kinetics

“…Effect of oxide film thickness on kinetics.--When a constant anodic current is applied to a Pt electrode cathodically stripped of its oxide film, following an initial stage of adsorption with oxygen species, an oxide film begins to grow at potentials higher than about 1.1V vs. RHE without any noticeable oxygen evolution, even at potentials as high as 1.6V (6,20,(21)(22)(23)(24). In this oxide growth region, the potential increases linearly with time with the dE/dt slope increasing with the applied anodic current (6,20,21,23). This type of growth has been interpreted in terms of the Cabrera-Mott model of high field assisted migration of ions through an insulating oxide film.…”

“…This type of growth has been interpreted in terms of the Cabrera-Mott model of high field assisted migration of ions through an insulating oxide film. Eventually, at a higher potential, e.g., 1.6V depending on the applied current, oxygen starts to evolve and soon becomes the predominant electrode reaction (6,20). In this OE region, potential still increases, but now as the logarithm of time (19).…”