Mixed-Electrode Reactions

In the following chapter, two general electrode mechanisms are cortsidered. In the first one, only the reactant adsorbs at the electrode strrface [92,110, 111, 114,115]  

The second mechanism is more general, since both reactant and product adsorbs on 

As follows from the initial condition (2.148) no adsorbed form is present on the electrode surface prior to the voltammetric experiment. The boundary condition (2.151) shows that the current is produced by the flux of the dissolved reactant as well as by its adsorbed form. The solution for the R form at the electrode surface is  

In the experimental analysis the SW potential scan is preceded by a certain delay period (/delay) to allow the reactant to adsorb on the electrode surface. Besides, the reactant adsorbs additionally in the course of the voltammetric scan starting from the initial (Es) to the peak potential (fp). Thus, the total accumulation period (face) 

Note that the product /A yields the scan rate of the square-wave potential modulation. If the delay period is sufficiently long, the additional adsorption during the potential scan is negligible. Otherwise, the additional adsorption complicates the theoretically expected dependencies, in particular the relationships between the net peak currents and potentials on the frequency [114]. 

Here Pr = y 7 is a dimensionless adsorption parameter, (RR)m is an integration parameter defined as 

---

Fig. 11-2. Electron energy leveb for a mixed electrode reaction of iron corrosion in acidic solution = Fermi level of iron electrode Sfw/hj) = Fermi level of hydrogen redox...

In order for this mixed electrode reaction shown in Fig. 11-2 to proceed, the Fermi level efod of the iron electrode must be higgler than the Fermi level erh-zhj) of the hydrogen redox reaction and also must be lower than the Fermi level for the transfer reaction of iron ions. In other words, the potential E of the iron electrode must be lower than the equilibrium potential of the... 

The presence of a metal surface can catalyze redox reactions which also constitute mixed electrode reactions. For example, the oxidation of hydrogen molecules (2H2, , + 02, , - > 2H2O ) does not occur in aqueous solutions but this oxidation catalyticaUy proceeds on platinum electrodes as a coupled process of the anodic and the cathodic reactions shown in Eqn. 11—2 and in Fig. 11—3 ... 

Fig. 11-3. Catalytic oxidation of hydrogen molecules on a metal electrode (a) oxidation of hydrogen does not occur in aqueous solution (h) oxidation of hydrogen occurs catalyticaUy on metal as a mixed electrode reaction.

Such bio-redoz reactions are catalyzed by the transfer of electrons or holes via electron levels in membranes and enzymes reactions proceed by the same mechanisms as those of mixed electrode reactions. 

Fig. 11-14. (a) Corrosion rate of metallic iron in nitric acid solution as a function of concentration of nitric add and (b) schematic polarization curves for mixed electrode reaction of a corroding iron in nitric add W p, = iron corrosion rate CHNO3 = concentration of nitric add t" (t ) = current of anodic iron dissolution (cathodic nitric add reduction) dashed curve 1= cathodic current of reduction of nitric add in dilute solution dashed ciuve 2 s cathodic current of reduction of nitric add in concentrated solution. [From Tomashov, 1966 for (a).]... 

---