Kinetics Tafel slope

Tafel equation Tafel kinetics Tafel slope Taffy process Taft s SV function Tagamet [51481-61-9] d-Tagatose... 

Vetter and Schultze also proposed that the reduction of the oxide occurs pathwise (in a localized way) and therefore with kinetics (Tafel slopes) which are almost independent of thickness or average coverage. 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

If we want to use the Tafel slopes to obtain the empirical kinetics of polymerization, we have to use a metallic electrode coated with a previously electrogenerated thin and uniform film of the polymer in a fresh solution of the monomer. In some cases experimental Tafel plots present the two components (Fig. 4) before and after coating. 

Both initiation and polymerization kinetics obtained from Tafel slopes (Fig. 5) are related to the formation of very thin films, which are not useful for most applications of conducting polymers. A similar restriction can be attributed to the combination of electrochemical and gravimet-... 

Gnanamuthu DS, Petrcelli JV. 1967. A generalized expression for the Tafel slopes and the kinetics of oxygen reduction on nohle metal and alloys. J Electrochem Soc 114 1036-1041. 

Equation (1.37) is of the form n = a + b log / an empirical observation first reported by Tafel. Thus, a Tafel plot of rj vs loge/ giving a straight line at high overpotentials is indicative of quasi-reversible kinetics. The slope gives / and the intercept (obtained via the extrapolation back to n = 0) gives /0 see Figure 1.8. 

The kinetics of MeOH oxidation of a 1 1 PfRu in an MEA has been well established by Vidakovic, Christov, and Sundmacher. At low overpotentials, the MeOH oxidation reaction was found to be zero order in MeOH concentration, indicating that CO oxidation is the rate-determining step. A Tafel slope of 50-60 mV dec was found at 60°C. At higher overpotentials, positive reaction orders were found, suggesting that MeOH adsorption becomes rate determining. An activation energy of 55 kj moP was found this agrees well with the values found for similar bulk PtRu electrodes. 

For the kinetic region, the values of the theoretical and experimental Tafel slopes have been shown to agree with Uc equal to i.9.io.i5o.i52-i57 If eq 13 were to be written with respect to the surface overpotential, as defined by eq 11, instead of the electrode overpotential, then it would read... 

The equations used in these models are primarily those described above. Mainly, the diffusion equation with reaction is used (e.g., eq 56). For the flooded-agglomerate models, diffusion across the electrolyte film is included, along with the use of equilibrium for the dissolved gas concentration in the electrolyte. These models were able to match the experimental findings such as the doubling of the Tafel slope due to mass-transport limitations. The equations are amenable to analytic solution mainly because of the assumption of first-order reaction with Tafel kinetics, which means that eq 13 and not eq 15 must be used for the kinetic expression. The different equations and limiting cases are described in the literature models as well as elsewhere. 

Figure 5. Measurement and analysis of steady-state i— V characteristics, (a) Following subtraction of ohmic losses (determined from impedance or current-interrupt measurements), the electrode overpotential rj is plotted vs ln(i). For systems governed by classic electrochemical kinetics, the slope at high overpotential yields anodic and cathodic transfer coefficients (Ua and aj while the intercept yields the exchange current density (i o). These parameters can be used in an empirical rate expression for the kinetics (Butler—Volmer equation) or related to more specific parameters associated with individual reaction steps.(b) Example of Mn(IV) reduction to Mn(III) at a Pt electrode in 7.5 M H2SO4 solution at 25 Below limiting current the system obeys Tafel kinetics with Ua 1/4. Data are from ref 363. (Reprinted with permission from ref 362. Copyright 2001 John Wiley Sons.)...

The observed slope of 60 mV can only be explained with step (7.29) as rate determining. In this case, the mechanism is EC and the predicted Tafel slope 60 mV. However, if the rate-determining step shifts to step (7.30), the mechanism becomes ECE, which is kinetically equivalent to EE, and the Tafel slope turns again to 40 mV. 

The performance of oxide electrodes depends on both factors, electronic and geometric. The latter is especially important since the preparation of oxide layers as a rule produces very high surface areas. A way to disentangle the two factors is to scrutinize the behavior of an intensive property. In electrochemical kinetics, the Tafel slope is the most appropriate, since it depends closely on the reaction mechanism and not on the extension of the surface area. 

The interpretation of the anodic branch of LSV for p-Si is apparently more simple because the current increases following an exponential variation with a Tafel slope of 60-80 mV/decade. In this case, an accumulation layer is generated, and then the current is only controlled by the kinetics of the electrochemical reaction, which involves several successive steps. It is not necessary to account for the various reaction paths proposed by many authors. 

Hendrikx et al. [36] investigated the reaction kinetics and mechanism of zinc and amalgamated zinc electrode in KOH solutions in the concentration range 1.5-10 M using galvanostatic methods. On the basis of Tafel slopes and reaction orders for OH , the following rate determining step (rds) in anodic and cathodic processes was postulated ... 

The steady-state Tafel lines for methanol oxidation in acid solution are 55-60 mV/decade over the potential range 0.4-0.5 (SHE). When the potential of the working electrode on this scale is made more positive, the Tafel slope changes and becomes 110 mV/decade. These two numerically stated Tafel slopes can readily be reexpressed in electrode kinetic terms to correspond, respectively, to ... 

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