Tafel slope determination

Determine from this plot the Tafel slopes 6, and 6 by curve fitting using the theoretical curves calculated for various values of 6 and 6,.. Calculate from equation 19.14 using the Rp, value evaluated in Step 1 and the Tafel slopes determined in Step 3. 

The slope of the plot of logarithm of current density versus potential, which characteristically is linear for an activation-controlled reaction, is defined as the Tafel slope. The Tafel slope determined in the exponential region of an i-V curve in HE solutions is about 60 mV/decade for p types and heavily doped n types of silicon samples as shown in Table 5.3. For lowly doped n-Si, since illumination is required... 

In studies of the O2 evolution reaction at Pt from aqueous H2SO4, it is found that the Tafel slopes determined for the steady-state kinetics of this reaction at several temperatures do not correspond to the conventionally expected behavior according to Eq. 

This follows theTafei relationship w h a Tafel slope Of,2/3 RT/F = 0- 4 V. hce direct transfer of two electrons aS shown in equation 1.92 is highly unlikely, it Would, appear that the mechanism might involve a twO Step prOr cess in which one Step is rale determining, say... 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

These are the coefficients that determine the Tafel slope of the log / against q curve of a multistep reaction, and they are of fundamental importance in providing information on the mechanism of the reaction. Equations 20.86 and 20.87 are of the same form as equations 20.59 and 20.58 that were derived for a simple one-step reaction involving a symmetrical energy barrier, and under these circumstances equations 20.90 and 20.91 simplify to... 

The slope of the Tafel curve drj/d log / is only one of the criteria that are required to determine the mechanism of the h.e.r., since different mechanisms, involving different r.d.s. often have the same Tafel slope. Parameters that are diagnostic of mechanism are the transfer coefficient, the reaction order, the stoichiometric number, the hydrogen coverage, the exchange current density, the heat adsorption, etc. 

The ability to use the Tafel slope as a diagnostic criterion can be exemplified by considering a discharge-chemical desorption mechanism for the h.e.r. in which either discharge or chemical desorption may be rate determining. ... 

The exact calculation of icorr for a given time requires simultaneous measurements of Rp and anodic and cathodic Tafel slopes (/> and be). Computer programs have been developed for the determination of precise values of /corr according to Eqs. (2) and (3). Experimental values of Rp (2p contain a contribution from the uncompensated solution resistance... 

A qualitative measure of the corrosion rate can be obtained from the slope of the curves in Fig. 2. Z INT is given in units of s ohm" . Owing to the presence of the uncompensated ohmic resistance and lack of values for Tafel slopes [Eq. (2)], data in Fig. 2 should be viewed as indicative of significant changes in corrosion rates. Corrosion loss remained low during the first 2 months, followed by a large increase for both flushed samples and controls. The corrosion rate increased when the surface pH reached values of 1 or less. Total corrosion loss as determined from integrated Rp data was less for the control than for the flushed sample. 

The ORR at Pt metal sites has a rate dependence on potential described by a Tafel slope of 120 mV/decade (as determined for Pt model systems at high cathodic overpotential, where Pt is practically surface oxide-free [Parthasarathy et al., 1992a, b]). 

The method permits the simultaneous determination of reaction order, m, and reaction rate constant, k, from the slope and the intercept of the straight line. The procedure can be repeated for various potential values below the limiting current plateau to yield k as a function of electrode potential. The exchange current density and the Tafel slope of the electrode reaction can be then evaluated from the k vs. potential curves. 

The first step [Eq. (5)] was postulated to be rate determining because of the Tafel slope of 107 mV/ decade and the first-order dependence of the reduction current on the C02 concentration. The second-order rate constant of Eq. (6) was estimated to be 7.5 x 103 M-1 s-1. 

The overpotential A( A< 0/s) could not be experimentally determined. However, taking only the first term in Eq. (24) (which is a reasonable assumption at any real anodic dissolution current density), one could derive the ratio of the Tafel slopes of the two currents as... 

Figure 7.12 Schematic Tafel plot of log / (as y ) against overpotential rj (as x ). The linear regions yield the Tafel slopes, from which the transfer coefficients a can be determined. The intersection between the two Tafel regions occurs on the y-axis at log /o. ...

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. 

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