Tafel kinetic parameters determination

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 current 7 is an extensive quantity, in that it depends on the size of the electrode. For this reason, the reaction rate is conveniently referred to the unit surface area (7/S=j, current density). Even so, the current density continues to be an extensive quantity if referred to the geometric (projected) surface area since electrodes are as a rule rough and the real surface does not coincide with the geometric surface [23]. Conversely, b is an intensive quantity, in that it depends only on the reaction mechanism and not on the size of the electtode. The term b is the most important kinetic parameter in electrochemistry also because of the easy and straightforward procedure for its experimental determination. Most electrode mechanisms can be resolved on the basis of Tafel lines only. 

The -> polarization curves for irreversible and quasireversible systems are shown in Figure (a). The respective -> Tafel plots are presented in Figure (b). Tafel plots can be constructed only for electrochemically irreversible systems, and kinetic parameters can be determined only when irreversible kinetics prevails. A switching from reversible to irreversible behavior and vice versa may occur. It depends on the relative values of ks and the -> mass transport coefficient, km. If km ks irreversible behavior can be observed. An illustration of the reversibility-irreversibility problem can be found in the entry -> reversibility. 

Raney Ni particles become entrapped in the electrodeposited Ni under the influence of a cathodic current and stirring. The electrocatalytic behavior of this material was characterized by the Tafel parameters for H2 evolution for various quantities (mg cm" ) of the Raney particles deposited. Particle size and aging effects were also determined. Kinetic parameters for the HER on various coatings were determined and compared (181). A related process for binding and cementing electrocatalytic Ni powders used a three-dimensional aluminium phosphate polymer (182). The Ni active material developed in the form of spiky filaments. 

A rotating platinum disk electrode was used for gathering data for the calculation of the kinetic constants. Tafel plots (5) were developed (Figure 2), from which the heterogeneous rate constants, k , and the electron-transfer coefficients, a, were determined. In all determinations of kinetic parameters, a blank voltammogram, run under identical conditions, was subtracted from the data to remove current due to background reactions or charging of the double layer. 

Determination of kinetic parameters. The important kinetic parameters are the Tafel slope, exchange current density, transfer coefficient, stoichiometric number, and reaction orders. 

The anode and cathode corrosion currents, fcorr.A and fcorr,B. respectively, are estimated at the intersection of the cathode and anode polarization of uncoupled metals A and B. Conventional electrochemical cells as well as the polarization systems described in Chapter 5 are used to measure electrochemical kinetic parameters in galvanic couples. Galvanic corrosion rates are determined from galvanic currents at the anode. The rates are controlled by electrochemical kinetic parameters like hydrogen evolution exchange current density on the noble and active metal, exchange current density of the corroding metal, Tafel slopes, relative electroactive area, electrolyte composition, and temperature. 

The charge transfer control conditions are limited to moderate polarisations, however if the system is slow enough, the corresponding polarisation zone can extend to over several hundred millivolts. By using the experimental data one can then easily determine the kinetic parameters, as outlined below. To do this, a Tafel plot is often used log / = f (77) as illustrated in figure 4.30. 

These asymptotes enable one to determine the kinetic parameters by using the experimental data easily extracted from the Tafel plot (see figure 4.30) ... 

In order to obtain general expressions for various kinetic parameters such as the Tafel slope, it is necessary to take into account the stoichiometric correlation among the steps involved in the overall reaction. The stoichiometric number of the constituent step was thus introduced by Horiuti and employed in steady state reaction rate theory. " For the case where a unique rate-determining step (rds) exists in the reaction route, it directly follows from Eq. (12)... 

In the case of the Volmer-Tafel reaction route, the equivalent pressure is readily obtained by substituting Eq. (71) into Eq. (109) and employing the relation between m and rj, as calculated (see Figure 7). The resultant log Ph2 vs. 7] relation becomes practically linear, and its slope is determined by mo and other kinetic parameters. ... 

Kinetic parameters other than the Tafel slopes and exchange current density already given have not been determined. 

A polarization curve, i.e., an 1-V curve of a fuel cell stack under certain operating conditions, is frequently used to describe fuel cell performance and determine performance decay with time. Polarization curve analysis can provide the most important kinetic parameters, such as the Tafel slope, exchange current density, cell resistance, limiting current density, and specific activity of Pt. A single fuel cell potential can be described by ... 

Figure 8.8 Normalized Tafel plots obtained from chronopotentiometric data at cathodic current densities, as indicated. Determined from this NTP kinetic parameters are aiso listed.

The kinetic parameters are slightly dilferent for iron N4-macrocyclic complexes, compared to cobalt complexes. In previous investigation of the electrooxidation of hydrazine catalyzed by FeN4 macrocyclics, the proposed mechanism involved adduct formation between Fe and the hydrazine molecule, prior to the rate determining step [46]. It is evident that the formation of a bond between the metal active site and the hydrazine molecule is a crucial step in electrocatalysis phenomena [47-50]. The electrooxidation of hydrazine on iron N4 macrocyclic complexes results in a Tafel plot with slope of around 0.040 V/decade, instead of 0.060 V/decade. The order in hydrazine is still one, but the order with respect to OH is two, so a reaction mechanism was proposed as follows [44, 45] ... 

Potentiodynamic polarization (intrusive). This method is best known for its fundamental role in electrochemistry in the measurement of Evans diagrams. A three-electrode corrosion probe is used to polarize the electrode of interest. The current response is measured as the potential is shifted away from the free corrosion potential. The basic difference from the LPR technique is that the apphed potentials for polarization are normally stepped up to levels of several hundred millivolts. These polarization levels facihtate the determination of kinetic parameters, such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk. 

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