Rotating disk electrode transients

C. Deslouis, B. Tribollet, M. Duprat, and F. Moran, "Transient Mass Transfer at a Coated Rotating Disk Electrode Diffusion and Electrohydrodynamical Impedances," Journal of The Electrochemical Society, 134 (1987) 2496-2501. 

Although a major advantage of rotating disk electrode techniques, compared to stationary electrode methods, is the ability to make measurements at steady state without the need to consider the time of electrolysis, the observation of current transients at the disk or ring following a potential step can sometimes be of use in understanding an electrochemical system. For example the adsorption of a component. 

A number of experimental electrochemical methods are available. These include voltammetry under transient conditions (e.g. cyclic voltammetry) or under steady state conditions (e.g. rotating disk electrode), and spectroelectrochemistry (e.g. using UV-Vis spectroscopy to monitor an electrochemical process). We focus here on cyclic voltammetry. It is a readily available technique and information that can be gained includes ... 

Electrochemical data (from cychc voltammetry and rotating disk electrode techniques, see Box 8.2) for the 2-electron reduction of tF to Tl in aqueous solution, are consistent with the formation of a transient intermediate T1(1I) species, p-j T1]4+ fonned near the electrode. 

Figure 1-lS. Time domains of double layer charging, charge transfer control, and diffusion control for a potentiostatic transient of a rotating-disk electrode with frequency m. 

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

Newman2°° 2° modeled the transient response of a disk electrode to step changes in current. The solution to Laplace s equation was performed using a transformation to rotational elliptic coordinates and a series expansion in terms of Lengendre polynomials. Antohi and Scherson expanded the solution to the transient problem by expanding the number of terms used in the series expansion. ... 

Prater KB, Bard AJ (1970) Rotating ring-disk electrodes. 1. Fundamentals of the digital simulation approach. Disk and ring transients and collection efficiencies. J Electrochem Soc 117 207. 

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