Charge transfer coefficient chemical

The study of Li28 + DMF solutions [60] also allowed to characterize the electrochemical properties of polysulfides only redox couples of the type 8 /8 are involved. The chemical reactions coupled to charge transfers are classical dissociation and disproportionation equilibria no complex rearrangement reaction or transient species has been necessary. Redox potentials and charge-transfer coefficients of the redox couples involved in sulfur and polysulfide solutions are summarized in Table 2. 

Experimentally, it is often found that the anodic and cathodic charge transfer coefficients are about 1/2. This is typically the case for outer-sphere electron transfer. Values between zero and one are found for several more complex reactions. We now consider whether this behavior is reasonable in the framework of the phenomenological model presented here. In an outer-sphere process, the oxidized and reduced species are outside the electrochemical double layer. The chemical potential of these species is then not influenced by the electrode potential, and the following is valid ... 

In the previous chapter finite difference methods were introduced for one of the simplest situations from a theoretical point of view cyclic voltammetry of a reversible E mechanism (i.e., charge transfer without chemical complications) at planar electrodes and with equal diffusion coefficients for the electroactive species. However, electrochemical systems are typically more complex and some refinements must be introduced in the numerical methods for adequate modelling. 

Here, AG ch and AG chrepresent the chemical part of the Gibbs free energy of activation, which does not depend on the applied potential. The proportionality constant a is called the charge transfer coefficient. Its value is situated between 0 < < 1. 

The electrode reaction of Cr / CyDTA has been studied in detail by Tanaka et al. [lb]. They report a value of kob = 0.029 cm s K We have tried to estimate this kob in the pH-range 4 to 8 where E is independent of pH. Using cyclic voltammetry (CV) and faradaic impedance measurements we confirm that kob is smaller than 0.1 cm s and a (charge transfer coefficient) < 0.4. Up to now we are unable to get exact values for these constants because a chemical pre-equilibrium of unidentified origin interferes with the charge transfer process. Further work is underway to clarify this point. 

The parameters in Equations 12.10 through 12.14 are defined as follows. AG(, and AG are the standard-state free energy of activation for chemisorptions (J/mol) at the cathode and anode, respectively, and A is the active cell area (cm ). k° and are the intrinsic rate constants (cm/s) for the cathode and anode reaction, respectively, and Ch o respectively the concentration of hydrogen ion and water at the membrane gas interface on the cathode side of the cell, is called the cathodic charge transfer coefficient or chemical activity parameter. R is the gas constant. The ohmic overvoltage can be represented using Ohm s law as discussed in Chapter 7 as... 

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

The reason for the disparate range of standard rate coefficients is to be found in the intrinsic chemical changes on the reacting species upon charge transfer as well as the electronic states involved at the electrode. 

The ratios given in Eq. (4.66) are only dependent on the electrode shape and size but not on parameters related to the electrode reaction, like the number of transferred electrons, the initial concentration of oxidized species, or the diffusion coefficient D. For fixed time and size, the values of f or Qf2 are characteristic for a simple charge transfer (see Fig. 4.4 for the plot of Qf2 calculated at time (ti + T2) for planar, spherical, and disc electrodes) and, as a consequence, deviations from this value are indicative of the presence of lateral processes (chemical instabilities, adsorption, non-idealities, etc.) [4, 32]. Additionally, for nonplanar electrodes, these values allow to the estimation of the electrode radius when simple electrode processes are considered. 

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