Activation energy electrochemical

Eor an electrochemical reaction the rate of reaction v and the rate constant k depend on potential E specifically, the potential difference across electrode-solution interphase Acf) through the electrochemical activation energy AGf. Thus, the central problem here is to find the function... 

For many practically relevant material/environment combinations, thennodynamic stability is not provided, since E > E. Hence, a key consideration is how fast the corrosion reaction proceeds. As for other electrochemical reactions, a variety of factors can influence the rate detennining step. In the most straightforward case the reaction is activation energy controlled i.e. the ion transfer tlrrough the surface Helmholtz double layer involving migration and the adjustment of the hydration sphere to electron uptake or donation is rate detennining. The transition state is... 

The detailed mechanism of battery electrode reactions often involves a series of chemical and electrochemical or charge-transfer steps. Electrode reaction sequences can also include diffusion steps on the electrode surface. Because of the high activation energy required to transfer two electrons at one time, the charge-transfer reactions are beheved to occur by a series of one electron-transfer steps illustrated by the reactions of the 2inc electrode in strongly alkaline medium (41). 

It has been known since the early days of electrochemical promotion that upon varying Uwr and thus , not only the catalytic rates, r, are changing in a frequently dramatic manner, but also the activation energy of the catalytic reaction is also significantly affected. An example was already presented in Fig. 4.28 which shows that both C2H4 and CH4 oxidation on Pt/YSZ conform to equation (4.50) with an values of -1 and -3, respectively. 

Such linear or near-linear variations in activation energy E with work function as the one shown in Fig. 4.28 but also in Figures 4.35 to 4.37 are quite common in electrochemical promotion studies and are usually accompanied by a concomitant linear variation in the logarithm of the preexponential factor, r°, defined from ... 

According to Eq. (14.2), the activation energy can be determined from the temperature dependence of the reaction rate constant. Since the overall rate constant of an electrochemical reaction also depends on potential, it must bemeasured at constant values of the electrode s Galvani potential. However, as shown in Section 3.6, the temperature coefficients of Galvani potentials cannot be determined. Hence, the conditions under which such a potential can be kept constant while the temperature is varied are not known, and the true activation energies of electrochemical reactions, and also the true values of factor cannot be measured. 

Alexander N. Frumkin pointed out in 1932 that an electrochemical reaction occnrring at different potentials can be regarded as an ideal set of chemical reactions of the same type, and suggested that the Brpnsted relation be nsed to explain the potential dependence of electrochemical reaction rates. On the basis of Eqs. (14.6) and (14.11), the relation for the activation energy becomes... 

A typical featnre of semicondnctor electrodes is the space charge present in a relatively thick surface layer (see Section 10.6), which canses a potential drop across this layer (i.e., the appearance of a snrface potential %). This potential drop affects the rate of an electrochemical charge-transfer reaction in exactly the same way as the potential drop across the diffnse EDL part (the / -potential) hrst, through a change in carrier concentration in the snrface layer, and second, throngh a change in the effect of potential on the reaction s activation energy. 

Cohen JL, Volpe DJ, Abmna HD. 2007. Electrochemical determination of activation energies for methanol oxidation on polycrystalline platinum in acidic and alkaline electrolytes. Phys Chem Chem Phys 9 49-77. 

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