Tafel slope current-voltage curves

Current-voltage curves of this MEA before and after long-term operation are shown in Fig. 3. The slope in the Tafel region seems to be unchanged and to slide downward by 50 mV. Since the Tafel slope is approximately -87 mV per decade, the voltage drop of 50 mV corresponds to the decrease in effective surface area by approximately 1/3.8. Various factors are considered as a reason for the decrease in effective surface area sintering of platinnm particles, dissolntion of platinum, electrical isolation, and isolation from reactants. In Fig. 3, the cnrrent density at which the... 

The current/voltage characteristic curves for the reduction of 2 on chelate catalysts mostly show a Tafel region with a slope of about 60 mV. This could... 

The numerical methods [32, 33, 34, 35, 36, 37] developed for the analysis of experimental polarization curves described by the current-voltage characteristic (2) with the aim of determining the electrochemical parameters a, /3 and h, are of considerable importance in the field of basic research as well as in corrosion rate monitoring. They permit, in fact, a more objective evaluation of these quantities with reference to a given potential difference interval AE, removing the degree of subjectivity that is inherent in the graphic determination of the Tafel slopes. 

Polarization resistance or Unear polarization measurements are used to determine the polarization resistance, a quantity related to the corrosion rate. Current is measured as a function of voltage in the vicinity of the corrosion potential. The two values are plotted as voltage versus current The polarization resistance is the sloj>e of the curve at the corrosion potential. The importance of this quantity Ues in the fact that it is (usually) inversely proportional to the corrosion current, the propmitionaUty constant being related to the Tafel slopes. ASTM G 59, Practice for Conducting Poten-tiodynamic Polarization Resistance Measurements, describes a method by which these measurements are made and the polarization resistance is calculated. Figure 1 shows a hypothetical plot as a function of current. A very extensive discussion of the historical development, theoiy and assump>-tions, and pitfalls provides an excellent background to ASTM G 59 [2]. Additional expjerimental artifacts have also been discussed [3]. 

A conceptual polarization curve that would be obtained in the absence of mass transport and ohmic resistances can be constructed by extrapolating the /R-free cell voltage obtained at low current densities (< 0.1 A/cm ) by means of Equation 23.10, indicated by the square symbols in Figure 23.3 [10]. In the Tafel equation, the Tafel slope b is... 

A substitution into the rate equation results in a perfect description of the current-potential function, the rest potential (when /cathodic = anodic)> and the Tafel equation at higher voltages (overpotentials). It has also been realized that c = a = 0.5 corresponds to the situation in which the potential curves of both the initial and final states have the same slope at their point of intersection. 

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