Kinetic parameters density Tafel slope

This equation simplifies the kinetic of a charge-transfer-controlled process to two parameters the exchange current density jo and the Tafel slope b. Both values do not depend not only on the electrochemical reaction but also on the electrode material and on the electrolyte composition. 

Work described in Ref. 86 includes a detailed investigation of the temperature dependence of ORR kinetic parameters at the Pt/bulk Nafion interface in the range from 30 to 90 °C. Plots of the log of mass-transport-corrected current density versus the potential of the Pt microelectrode showed an increase of a factor of about five in the rate of ORR at 0.90 V and about three at 0.85 V as the temperature was increased from 30 to 90 °C. The apparent activation energies for the two regions (low and high Tafel slope) were calculated from the dependence of the apparent exchange current density on T according to... 

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 transition to more realistic conditions, near fuel cell environment, is achieved using a thin-film rotating disk electrode with a practical supported catalyst. It allows obtaining kinetic parameters such as the the exchange current density and Tafel slopes, avoiding the use of complex models. This is of fundamental importance, because small catalyst particles having a significant fraction of surface atoms could behave differently compared to bulk materials. 

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

The Tafel equation rj = />xexp(z/Zo) describes the relationship between overpotential and current density. Two parameters, i.e., exchange current density z and Tafel slope b, are the most important kinetic parameters to measure the electrochemical activity of an electrode material. Exchange current density z is analogous to the rate constant used in chemical kinetics, and a high value of i often translates into a fast electrochemical reaction. On the other hand, a smaller Tafel slope is desirable from the kinetic point of view to obtain a smaller overpotential. As for any electrochemical reaction, the Tafel slope h is a function of temperature in the form RT... 

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 ... 

A semiempirical model that considered only the effect of toluene on kinetics was constructed to describe cell voltage as a fimction of contamination time and current density. The parameters independent of the contamination process, i.e., open circuit voltage (E°), cell resistance (RJ, and Tafel slope (fc), were first estimated based on experimental data in the absence of toluene (giving a baseline). Then these parameters were used to empirically obtain the expressions of two other parameters, and Kcbc/ which accoimted for the effect of toluene contamination on transient and steady-state cell performance, using experimental data at various levels of toluene concentration imder four current densities. The model was validated by comparing the contamination testing results with model-predicted results. Several other definitions were also presented, based on the model, such as the threshold toluene concentration and the degradation rate. 

Further interpretation of the polarization curves can be extended using Pour-baix graphical work depicted in Figure 3.5 for pure iron (Fe). The resultant plots represent the functions E = f i) and E va. f [log(i)] for an electrolyte containing CFe+2 — 0.01 g/l = 1.79a 10 mol/l — 1.49a l0 mol/cm at pH = 0. Additionally, the reactions depicted in Figure 3.5 and some related kinetic parameters are listed in Table 3.2 for convenience. One important observation is that both anodic and cathodic Tafel slopes, 0 0ci respectively are equal numerically and consequently. Figure 3.5b has an inflection point at icorr,Ecorr)-This electrochemical situation is mathematically predicted and discussed in the next section using a current density function for a mixed-potential system. 

Electrochemical corrosion systems can be characterized using the kinetic parameters previously described as Tafel slopes, exchange and limiting current densities. However, the mixed potential theory requires a mixed electrode system. This is shown in Eigure 5.1 for the classical pure zinc (Zn) electrode immersed in hydrochloric (NCl)acid solution [1,8-9]. This type of graphical representation of electrode potential and current density is known as Evans Diagram for representing the electrode kinetics of pure zinc. 

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