Electron transfer coefficient

The rate constant of electron transfer (ks) and anodic and cathodic electron transfer coefficients (aa and ac) of the SODs at various pH values were estimated with Laviron s equation and summarized in Table 6.5. Interestingly, the fastest electron transfer of the SODs was essentially achieved in a neutral solution, probably in agreement with the biological conditions for the inherent catalytic mechanisms of the SODs for 02" dismutation, although the electrode processes of the SODs follow a different mechanism. 

D0 and DR are the respective diffusion coefficients k° and a are known as the standard (electron transfer) rate constant and electron transfer coefficient respectively, and both are kinetic parameters characterizing the feasibility of the electron transfer x is the distance away from the electrode surface. 

The exact values of the critical kinetic parameter depend on the electron-transfer coefficient and the amplitnde. These values are listed in Table 2.3. If the electron-transfer coefficient is not known, an average value of the critical kinetic parameter (fflbax)avr e nsed. The values of (fflbax)avr fo different amplitudes are given in Table 2.4. The error in the estimation of sur by using bax is close to 10%. 

Table 2.3 Dependence of the critical kinetic parameter CO sx on the normalized amplitude and electron-transfer coefficient Oa. Conditions of the simulations are the same as for Fig. 2.41...

Table 2.4 Dependence ofthe average critical kinetic parameter (minax)avr normalized amplitude nEs valid for the electron-transfer coefficient 0.1 < cq < 0.9. Conditions ofthe simulations as in Fig. 2.41...

In addition, the split peaks can be used for estimation of electron-transfer coefficient as well as for precise determination of the formal potential of the surface electrode reaction. The potential separation between split peaks is insensitive to the electron-transfer coefficient. However, the relative ratio of the heights of the split peaks depends on the electron-transfer coefficient according to the following function ... 

In the presence of interactions, besides the kinetic parameter co and the electron-transfer coefficient, the response is controlled by the interaction product a . The kinetic parameter and the interaction product can be unified into a single complex... 

The physical meaning of the kinetic parameter m is identical as for surface electrode reaction (Chap. 2.5.1). The electrochemical reversibility is primarily controlled by 03 (Fig. 2.71). The reaction is totally irreversible for log(m) < —3 and electrochemically reversible for log(fo) > 1. Between these intervals, the reaction appears quasireversible, attributed with a quasireversible maximum. Though the absolute net peak current value depends on the adsorption parameter. Fig. 2.71 reveals that the quasireversible interval, together with the position of the maximum, is independent of the adsorption strength. Similar to the surface reactions, the position of the maximum varies with the electron transfer coefficient and the amphtude of the potential modrrlation [92]. 

Similar to the surface electrode processes (Chap. 2.5.1) the peak current ratio of the split peaks ( fp,c/ lp,a) is a function of the electron transfer coefficient o. Note that the anodic and the cathodic peak is located at the more negative and more positive potentials, respectively. This type of dependence is given in Fig. 2.98. Over the interval 0.3 < < 0.7 the dependence vs. is hnear, associated with the... 

In this equation, SLm represents the Tafel slope for the mixture of X plus Y, and Slx,Siy represents the Tafel slopes for the individual components. This equation enables a determination of / from Tafel representations, providing that the quotients between the individual electrochemical rate constants, kx and ky, and the electron transfer coefficients, ocxnax,ocYnaY, are known. 

Table 4.1 Heterogeneous rate constant (k°), electron transfer coefficient (a), and formal potential ) corresponding to the best fit of theoretical working curves (Eq. 4.120) to the RPV experimental results [48]... 

In Fig. 4.18b, the effects of the electron transfer coefficient (a) on the ADDPV curves is shown. A decrease of a leads to the decrease of the peak currents together with the shift of the peak and crossing potentials toward more negative values. The ratio of the peak currents (l/ he// hel) also varies with a, and the greater the a value, the greater the l/ he//j, ll llL ratio. It is worth highlighting the anomalous... 

Here, a represents the electron transfer coefficient, is the apparent charge-transfer... 

A rotating platinum disk electrode was used for gathering data for the calculation of the kinetic constants. Tafel plots (5) were developed (Figure 2), from which the heterogeneous rate constants, k , and the electron-transfer coefficients, a, were determined. In all determinations of kinetic parameters, a blank voltammogram, run under identical conditions, was subtracted from the data to remove current due to background reactions or charging of the double layer. 

Based on Eqn (2.22) or Eqn (2.23), we can discuss several important concepts of electrochemical kinetics, including the overpotential, the Nernst reversible electrode potential, the exchange current density, the standard reaction rate, the electron-transfer coefficient, and the reversible and irreversible reactions. 

Electron-transfer coefficient (a). As discussed previously, this a is called the electron-transfer coefficient, which is one of the important parameters for the electrode electron-transfer kinetics. For majority of electrochemical reaction systems, the value of this a is in the range of 0.2—0.8, depending on the nature of the studied system. However, in the electrochemical research, if this value is not measured, people normally assume its value to be 0.5. 

Figure 2.4 Current density—overpotential curves of 0 -i- nae <- / reaction at three different electron-transfer coefficients (a = 0.25, 0.5, and 0.75, respectively), calculated according Eqn (Z28a) using the parameter values of = / =8.314 J mol, T=298 K, F=96,487 CmoP, and /=1.0x 10 Acm . (For color version of this...

Using these equations, the experiment data of current density at various electrode potentials can be analyzed, from which the kinetic parameters of electron-transfer reaction such as the electron-transfer coefficient and the exchange current density can be obtained. 

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