Rotating electrode kinetics

This value represents the upper limit of a first order reaction rate constant, k, which may be determined by the RHSE. This limit is approximately one order of magnitude smaller that of a rotating electrode. One way to extend the upper limit is to combine the RHSE with an AC electrochemical technique, such as the AC impedance and faradaic rectification metods. Since the AC current distribution is uniform on a RHSE, accurate kinetic data may be obtained for the fast electrochemical reactions with a RHSE. 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

Fig. 11. Electrode kinetics at the rotating disc electrode [eqn. (122)]. (a) Curve A, iL vs. co1/2 curve B, i vs. CO1/2 at a lower overpotential showing the effect of electrode kinetics where curve B is the line obtained if the reaction were reversible, (b) Analysis of curve B in (a) by plotting T1 vs. G0-1/2. k[ is obtained from the intercept and D from the slope.

All the electrode kinetic methodology described until now has assumed a steady state (or quasi-steady state in the case of the DME). Many techniques at stationary electrodes involve perturbation of the potential or current in combination with forced convection, this offers new possibilities in the evaluation of a wider range of kinetic parameters. Additionally, we have the possibility of modulating the material flux, the technique of hydrodynamic modulation which has been applied at rotating electrodes. Unfortunately, the mathematical solution of the convective-diffusion equation is considerably more complex and usually has to be performed numerically. 

Reversible, quasi-reversible and irreversible electrode processes have been studied at the RDE [266] as have coupled homogeneous reactions without [267] and with the effect of electrode kinetics [268], The theoretical results are very similar to those of a.c. polarography, being very phase-angle sensitive to coupled chemical reactions in the rotation speed range where convection can be neglected, the polarographic results may be directly applied [269]. 

Several cell configurations are common in electrochemical research and in industrial practice. The rotating disk electrode is frequently used in electrode kinetics and in mass-transport studies. A cell with plane parallel electrodes imbedded in insulating walls is a configuration used in research as well as in chemical synthesis. These are two examples of cells for which the current and potential distributions have been calculated over a wide range of operating parameters. Many of the principles governing current distribution are illustrated by these model systems. 

Thus, the half wave potential for electron transfer from the particles to the electrode should shift anodically with increasing electrode rotation speed if the reaction occurs with irreversible electrode kinetics. However, it is found that Via is invariant with rotation speed [168], implying that the particles have reversible electrode kinetics, a result in conflict with the observation of a reaction overpotential of 0.2 V and a transfer coefficient of 0.13 for this set of data. 

A more direct evaluation of the role of mass transfer is obtained by plotting the scaled impedance values as a function of dimensionless frequency p = co/Cl, which is scaled by the rotation speed. The real and imaginary parts of the scaled impedance, shown in Figures 18.2(a) and (b), respectively, are superposed at low frequencies. Thus, the impedance values are, at low frequencies, controlled by convective mass transfer to the rotating disk. Differences are seen at higher frequencies that can be attributed to electrode kinetics. 

This part demonstrates how deterministic models of impedance response can be developed from physical and kinetic descriptions. When possible, correspondence is drawn between hypothesized models and electrical circuit analogues. The treatment includes electrode kinetics, mass transfer, solid-state systems, time-constant dispersion, models accounting for two- and three-dimensional interfaces, generalized transfer functions, and a more specific example of a transfer-function tech-nique.in which the rotation speed of a disk electrode is modulated. 

K. -L. Hsueh, D.-T. Chin, S. Srinivasan, Electrode kinetics of oxygen reduction. A theoretical and experimental analysis of the rotating ring-disk electrode method. J. Electroanal. Chem. 1983, 153, 79-95. 

The boimdary conditions are described in more detail elsewhere. A zero-flux condition is imposed on the electrode for all species except the reactant Cu. A Tafel relationship with a concentration-dependent exchange current density was used to describe the electrode kinetics. The exchange current density was found from rotating disk experiments, and all other model parameters were taken from the literature. No parameters were adjusted for the simulations in the cell. 

The current density measured from the rotating electrode is contributed by both the current densities of electrode electron-transfer reaction and the reactant diffusion. In order to obtain the kinetic parameters of these two processes and their associated reaction mechanisms based on the experiment data, both the theories of electrode electron-transfer reaction and reactant diffusion should be studied and understood. In this chapter, the general theories for electrode kinetics of electron-transfer reaction and reactant diffusion will be given in a detailed level, and we hope these theories will form a solid knowledge for a continuing study in the following chapters of this book. 

The analogy in the mass transport effects in electrode reaction and homogeneous second-order fast reactions in solution becomes clear. In electrode kinetics, however, the charge-transfer rate coefficient can be externally varied over many orders of magnitude through the electrode potential and kd can be controlled by means of hydrodynamic electrodes. For instance the mass transport rate coefficient, kd, for a rotating disc electrode at the maximum practical rotation speed of 10 000 per min is approximately 2 x 10... 

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