Electrode reaction, activation energy

Brensted Polanyl Relation and Electrode Reaction Activation Energy... 

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). 

Sepa, D. B. Energies of Activation of Electrode Reactions A Revisited Problem 29... 

This is an example of a reversible reaction the standard electrode potential of the 2PS/PSSP + 2c couple is zero at pH 7. The oxidation kinetics are simple second-order and the presence of a radical intermediate (presumably PS-) was detected. Reaction occurs in the pH range 5 to 13 with a maximum rate at pH 6.2, and the activation energy above 22 °C is zero. The ionic strength dependence of 2 afforded a value for z Zg of 9 from the Bronsted relation... 

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. 

The second effect is that of a change in the potentiaf difference effectively influencing the reaction rate. By its physical meaning, the activation energy should not be influenced by the full Galvani potential across the interface but only by the potential difference (cpo ) between the electrode and the reaction zone. Since the Galvani potential is one of the constituent parts of electrode potential E, the difference - j/ should be contained instead of E in Eq. (14.13) ... 

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. 

The activation energy for the reaction, a, was determined for the above Pt-porous nanoparticles from the first cycle of CV measurement in the temperature range between 30 and 60 °C, Figure 13c. The activation energy was obtained from the slope, —EJR, of the Arrhenius relationship and equal to SOklmoP. This value was similar to some of those obtained for the electro-oxidation of methanol on electrodes of Pt particles dispersed in Nation [50, 51]. 

Shubina and co-workers calculated the activation energy for the reaction between CO and OH on a Pt(l 11) surface in the absence of water, and obtained a value of about 0.6 eV [Shubina et al., 2004]. Janik and Neurock [2007] calculated the barrier for this reaction on Pt(l 11) in the presence of water and as a function of the surface charge of the Pt(l 11) electrode. They found a value of 0.50 eV in the absence of a surface... 

This last equation contains the two essential activation terms met in electrocatalysis an exponential function of the electrode potential E and an exponential function of the chemical activation energy AGj (defined as the activation energy at the standard equilibrium potential). By modifying the nature and structure of the electrode material (the catalyst), one may decrease AGq, thus increasing jo, as a result of the catalytic properties of the electrode. This leads to an increase in the reaction rate j. 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

The functional dependence of the activation energy of the anodic electrode reaction can be derived as follows. According to the definition of the rate of the electrode reaction, the partial current density... 

The determination of the activation energies of electrode reactions is especially important for the theory of electrode reactions and for study of the relationship between the structure of the reacting substances and the electrode reaction rates. 

Several descriptions of electrode reaction rates discussed on the preceding pages and the difficulty to standardize electrode potential scales with respect to different temperatures imply several definitions of activation energies of electrode reactions. The easiest way to determine this quantity, for example, for an irreversible cathodic process, employs Eqs (5.2.9), (5.2.10) and (5.2.12) at a constant electrode potential,... 

The standard activation energy of the electrode reaction Hi is defined as... 

Table 5.2 Activation energies of electrode reactions (From R. Tamamushi and... 

Table 5.2 lists examples of the activation energies of electrode reactions. 

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