Electron transfer coefficients current density-overpotential

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. 

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...

The relationship between the net current density, the exehange current density, the overpotential, and the electron transfer coefficient is given by the Butler-Volmer equation for either cathode or anode reaction ... 

Consider the reaction with two consecutive electron-transfer steps described by Eq. (11.12). (a) Show that, if j0,2 j0,1, there is an intermediate range of negative overpotentials in which the apparent transfer coefficient is (2 — ai) and the apparent exchange current density 2j0,i (see Fig. 11.1). (b) Derive the form of the Tafel plot for jo,i > jo,2-... 

They obtained a logarithm of current and potential relationship composed of two linear parts, and rationalized the polarization data. They determined the transfer coefficient in tire higher overpotential region 0.25 confinned later by Hori and Suzuki in their measurements of the partial current densities of HCOO formation at a Hg pool electrode.Tliis value indicates that tire rate determining step is tire first electron transfer to form CO2-" anion radical. The transfer coefficient in the lower overvoltage region is 0.67. 

Equation 6 is referred to as the Butler-Volmer equation. Normally, for significant overpotentials, either one or the other of the two terms is dominant, so that the current-density exponentially increases with r, i.e. In i is proportional to 3tiF/RT in the case, for example, of appreciable positive t] values. Here the significance of Tafel s b coefficient (Equation 1) is seen b = dn/d In i = RT/3F for a simple, single-electron charge-transfer process. 

This is a very useful equation in experimental and applied dectrochemistry and shows that the measured current density is a function of (1) overpotential (2) exchange current density, 1 and (3) the transfer coefficients, and The transfer coefficients are, at least for simple electron transfer processes, not independent variables. In general ... 

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