Direct electrode kinetics

The first limitation is related to interference of the anode and the cathode. The finite permeability of the Nation membrane to fuel and oxygen results in crossover of fuel from the anode to the cathode, and oxygen crossover in the opposite direction. This may have a significant influence on electrode kinetics. 

Such an interfacial degeneracy of electron energy levels (quasi-metallization) at semiconductor electrodes also takes place when the Fermi level at the interface is polarized into either the conduction band or the valence band as shown in Fig. 5-42 (Refer to Sec. 2.7.3.) namely, quasi-metallization of the electrode interface results when semiconductor electrodes are polarized to a great extent in either the anodic or the cathodic direction. This quasi-metallization of electrode interfaces is important in dealing with semiconductor electrode kinetics, as is discussed in Chap. 8. It is worth noting that the interfacial quasi-metallization requires the electron transfer to be in the state of equilibrimn between the interface and the interior of semiconductors this may not be realized with wide band gap semiconductors. 

Activation Polarization Activation polarization is present when the rate of an electrochemical reaction at an electrode surface is controlled by sluggish electrode kinetics. In other words, activation polarization is directly related to the rates of electrochemical reactions. There is a close similarity between electrochemical and chemical reactions in that both involve an activation barrier that must be overcome by the reacting species. In the case of an electrochemical reaction with riact> 50-100 mV, rjact is described by the general form of the Tafel equation (see Section 2.2.4) ... 

In deriving eqn. (80), limitations due to mass transport at the interface were not considered. Strictly speaking, this is not realistic and as the reaction rate increases with overpotential in each direction a variation of the concentrations of reactant and product at the surface operates and concentration polarization becomes more important. Each exponential expression in eqn. (80) must be multiplied by the ratio of surface to bulk concentrations, ci s/ci b. The effect of mass transfer in electrode kinetics has been discussed in Sect. 2.4. 

On the other hand, electrode kinetic studies are at a disadvantage compared with investigations of homogeneous kinetics because concentrations are not uniform and surface concentrations can rarely be measured directly (optical methods can sometimes provide direct measurement of the product concentration [1]). This means that the converse situation to that in classical homogeneous kinetics exists in electrode kinetics concentration information needs to be inferred from reaction rates. 

Fig. 7. Direct method for determining electrode kinetics. Contrary to the indirect method portrayed in Fig. 5, no particular kinetic law has been assumed.

Reversible, quasi-reversible and irreversible electrode processes have been studied at the RDE [266] as have coupled homogeneous reactions without [267] and with the effect of electrode kinetics [268], The theoretical results are very similar to those of a.c. polarography, being very phase-angle sensitive to coupled chemical reactions in the rotation speed range where convection can be neglected, the polarographic results may be directly applied [269]. 

Fig. 13.27. Potential vs. current density plots for state-of-the-art fuel cells, o, proton exchange membrane fuel cell , solid oxide fuel cell , pressurized phosphonic acid fuel cell (PAFC) a, direct methanol fuel cell, direct methanol PAFC , alkaline fuel cell. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Un-supported Platinum Microcrystal-lite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 103, Fig. 1, with permission from Elsevier Science.)...

In the preceding sections of this chapter, much attention was devoted to the essential act of electron transfer. Yet although being, for fundamental or analytic purposes, the subject of a large body of the electrochemical literature, pure electron transfer reactions generally are of less interest in terms of preparative electrochemistry. In the opinion of this author, electron transfer at an electrode must be considered as a particular class of activation of molecules to enhance their chemical reactivity. As such, electrode kinetics must be understood (as for other methods of chemical activation) in order to control and eventually direct the overall process to the selected target (compare Chapter 3). 

A more direct evaluation of the role of mass transfer is obtained by plotting the scaled impedance values as a function of dimensionless frequency p = co/Cl, which is scaled by the rotation speed. The real and imaginary parts of the scaled impedance, shown in Figures 18.2(a) and (b), respectively, are superposed at low frequencies. Thus, the impedance values are, at low frequencies, controlled by convective mass transfer to the rotating disk. Differences are seen at higher frequencies that can be attributed to electrode kinetics. 

In suitable cases, pulse techniques such as chronocoulometry or rapid linear-sweep voltammetry also can be employed to monitor the electrode kinetics within the precursor state "i.e., to evaluate directly the first-order rate constant, k, [Eq. (a) in 12.3.7.2] rather than k. Such measurements are analogous to the determination of rate parameters for intramolecular electron transfer within homeogeneous binuclear complexes ( 12.2.2.3.2). Evaluation of k is of particular fundamental interest because it yields direct information on the energetics of the elementary electron-transfer step (also see 12.3.7.5). 

The above does not imply that it is impossible to study the mechanism of alloy deposition it only shows that conclusions cannot be drawn from the usual interpretation of the directly observed current-potential relationship employed in the analysis of electrode kinetics. The partial currents for deposition of each of the alloying elements should be determined as a function of potential and other experimental parameters via determination of the atomic composition of the alloy and the FE. The FE during alloy deposition can be different from that of single-metal deposition of one or both metals involved in the process. Hence, the FE can be expected to depend on the composition of the alloy, and the thickness distribution may differ from that expected according to the current distribution." ... 

An additional merit of the channel electrode set-up is that the exposed calcite surface can be viewed in situ, by light microscopy, allowing the crystallographic nature of the dissolution process to be linked directly to kinetic measurements. Furthermore, it should be noted that ex-situ studies on the morphology of crystal surfaces subjected to dissolution in the channel set-up are likely to provide greater information, by nature of the non-uniform accessibility of the crystal surface to protons, than might be anticipated from the same measurements on a crystal surface exposed to a uniform flux of electrolyte (vide infra). 

---