Electrode kinetics parameters

Agar had suggested in 1947 that there might be a temperature dependence of p, the electrode kinetic parameter, and Conway took this up in 1982 and showed experimentally that in certain reactions this was the case. 

The most well known work that Conway and his colleagues completed in Ottawa was on the analysis of potential sweep curves. I had been critical of the application of potential sweep theory to reactions which involved intermediates on the electrode surface and, working particularly early with Gilaedi and then with Halina Kozlowska, and to some extent with Paul Stonehart, Conway developed an analysis of the effect of intermediate radicals on the shape and properties of potential sweep showing how interesting electrode kinetic parameters could be thereby obtained. 

In this case, besides the thermodynamic and kinetic parameters of the preceding chemical reaction, the response depends on the kinetics of the electrode reaction represented by the electrode kinetic parameter k = (see Sect. 2.1.2) [60], Figure 2.29 shows the variation of A Fp with e for various k. It is obvious that there is... 

The variation of the peak current with the electrode kinetic parameter k and chemical kinetic parameter e is shown in Fig. 2.31. When the quasireversible electrode reaction is fast (curves 1 and 2 in Fig. 2.31) the dependence is similar as for the reversible case and characterized by a pronounced minimum If the electrode reaction is rather slow (curves 3-5), the dependence A fJ, vs. log( ) transforms into a sigmoidal curve. Although the backward chemical reaction is sufficiently fast to re-supply the electroactive material on the time scale of the reverse (reduction) potential pulses, the reuse of the electroactive form is prevented due to the very low kinetics of the electrode reaction. This situation corresponds to the lower plateau of curves 3-5 in Fig. 2.31. 

Fig. 2.41 Theoretical net voltammograms simulated for different values of to. The electrode kinetic parameter increases from left to right from log (to) = —1.7 to 2.2 with an increment of 0.1. Other conditions of the simulations are a = 0.5, wE-w = 50 mV, A = 5 mV...

Fignre 2.41 indicates that the net peak current is a parabolic function of the electrode kinetic parameter. This is illnstrated in Fig. 2.43. With respect to the electrochemical reversibility of the electrode reaction, approximately three distinct regions can be identified. The reaction is totally irreversible for log(ca) < — 2 and reversible for log(ft)) > 2. Within this interval, the reaction is qnasireversible. The parabolic dependence of the net peak cnrrent on the logarithm of the kinetic parameter asso-... 

Table 2.5 Critical values of the SW amplitude and corresponding potential separations of the split peaks for various values of the electrode kinetic parameter. Conditions of the simulations are the same as for Fig. 2.41...

Here, m = is the electrode kinetic parameter typical for surface electrode processes (see Sect. 2.5.1) and 7= rg is dimensionless diffusion parameter. The latter parameter represents the inflnence of the mass transfer of electroactive species. 

The second-order reaction with adsorption of the ligand (2.210) signifies the most complex cathodic stripping mechanism, which combines the voltammetric features of the reactions (2.205) and (2.208) [137]. For the electrochemically reversible case, the effect of the ligand concentration and its adsorption strength is identical as for reaction (2.205) and (2.208), respectively. A representative theoretical voltammo-gram of a quasireversible electrode reaction is shown in Fig. 2.86d. The dimensionless response is controlled by the electrode kinetic parameter m, the adsorption... 

A quasireversible electrode reaction is controlled by the film thickness parameter A, and additionally by the electrode kinetic parameter k. The definition and physical meaning of the latter parameter is the same as for quasireversible reaction under semi-infinite diffusion conditions (Sect. 2.1.2). Like for a reversible reaction, the dimensionless net peak current depends sigmoidally on the logarithm of the thickness parameter. The typical region of restricted diffusion depends slightly on K. For instance, for log( If) = -0.6, the reaction is under restricted diffusion condition within the interval log(A) < 0.2, whereas for log(if) = 0.6, the corresponding interval is log(A) <0.4. 

Figure 2.96 shows the splitting of the net peak under increasing of the dimensionless electrode kinetic parameter for a given film thickness. The potential separation between split peaks increases in proportion to the electrode kinetic parameter and the amplitude of the potential modulation. The dependence of the peak potential separation on the amplitude is separately illustrated in Fig. 2.97. The analysis of the splitting by varying the amplitude is particularly appeahng, since this instrumental parameter affects solely the split peak without altering the film thickness parameter. Table 2.7 lists the critical intervals of the film thickness and the electrode kinetic parameters attributed with the splitting.

The presence of the reference electrode will also become apparent in electrode kinetics in the analysis of apparent heats of activation (Sect. 3.4) and the impossibility of measuring absolute electrode kinetic parameters for a half reaction such as reaction (2). 

G. K. Rowe, M. T. Carter, J. Richardson, and R. W. Murray, Langmuir 11 1797 (1995). Obtaining electrode kinetic parameters from cyclic voltamograms involving proteins. 

Understanding the activity and selectivity properties of electrocatalysts requires the characterization of catalyst surfaces, determination of adsorption characteristics, identification of surface intermediates and of all reaction products and paths, and mechanistic deliberation for complex as well as model reactions. Electrochemical and classical methods for adsorption studies are well documented in the literature (5, 7-9, 25, 24, 373. Here, we shall outline briefly some prominent electrochemical methods and some nonelectrochemical techniques that can provide new insight into electrocatalysis. Electrode kinetic parameters can be determined by potentionstatic methods using the methodology of Section II1,D,3. 

Thus apparent electrode kinetic parameters are observed (412. ... 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

Equation (5.36) is used to calculate the corrosion rate of a system without knowledge of electrode-kinetic parameters. This approximation may not always result in accurate corrosion estimates. However, this equation provides a basis for rapid corrosion evaluation studies. 

Most of the investigations of the anodic oxidation of oxalic acid were carried out in a potential region where oxide is present on the electrode it was generally held that oxidation does not occur below 0.8 V (N.H.E.). In a recent study, Johnson, Wroblowa, and Bockris have obtained electrode kinetic parameters for the low potential region [0.5-O.8V (N.H.E.)], and this discussion shall essentially follow that investigation. 

The parameters necessary to make a correct model of the double layer are often not easily obtained. However, the uncorrected values of electrode kinetic parameters may retain significance under many circumstances. For instance, for a series of compounds studied under similar conditions, k° and a values can be compared. Double-layer effects are also minimized by large electrolyte concentrations (0.1-5 M). The SE term is smallest near the potential of zero charge (pzc), and largest at extreme potential values, far from the pzc of the electrodes. A good overview of double-layer theory is given in Bard and Faulkner s text and by Rieger. ... 

Simulated Values of Electrode Kinetic Parameters from the Baseline Polarization Curve, in the Absence of Toluene, Using Equation (3.20)... 

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