Tafel mass transport correction

St.eadyr .t te Measurements. In order to obtain a more quantitative understanding of the process resulting in the inhibition of the 0 reduction reaction, Tafel measurements were made. The mass transport corrected Tafel equation [35] ... 

Fig. 4. Mass-transport-corrected Tafel piot for oxygen reduction at the Pt/Nafion interface measured with the system described in [4]. (Reprinted by permission of the Electrochemical Society).

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... 

Fig. 17.4 Tafel plot of the applied potential, E (in the range 0.7 V > > 0.5 V), vs. log of the mass transport corrected current for the ORR in [demaliCFsSOs]. On the t -axis, i is the current at potential E and i l is the limiting current measured from the ORR polarisation curve. Reprinted from Walsh, D A Ejigu, A Smith, J Licence, P (2013) Phys. Chem. Chem. Phys. 15 7548-7554 with permission of the PCCP Owner Societies...

Survila, A. and Stasiukaitis, P.V. (1997) Partial currents of consecutive charge transfer in the processes of metal electrodeposition. Chemija, 3, 31-35. Streeter, I. and Compton, R.G. (2007) Mass transport corrected Tafel analysis of voltammetric waves when can it be applied Electrochim. Acta, 52 (13), 4305 -4311. 

Tafel Analysis Mass Transport Correction Problem... 

It follows from Eq. (8.15) that a mass transport corrected Tafel plot of E versus log[/ — 7j ] will have a gradient of 2.303RT/FViper decade. 

The mass-transport correction factor, A, is plotted in Fig. 1 as a function of /corrA / and 2.3AJE/6. As shown, the correction can be considerable even at small values of icovrlh for large cathodic polarizations and for small cathodic Tafel slopes. The value of A, is also large for large anodic polarizations, but under these conditions the cathodic term in Eq. (12) is usually negligible. 

For a reversible electron-transfer process, the Tafel relationship corrected for mass transport holds in the central region of the voltammogram (Brett and Oliveira-Brett, 1993). Therefore, for a reversible one-electron process, a plot of - Euz versus logio(/ - Zi7m) will have a slope of 59/n mV per decade at 25°C. 

Fig. 17.2 Tafel plots for the (normalized, dimensionless) current, yjy, that accompanies hydrogen evolution in a solution containing 3.4 mM HCl + 1.0 M KCl, corrected for diffuse-double-layer effects, mass transport controlled kinetics and ohmic potential drop, measured at three temperatures (5, 45, 75°C all results fall on the same line of this reduced plot) at a dropping mercury electrode. The slope obtained from this plot is 0.52, independent of temperature. (Based on data from E. Kirowa-Eisner, M. Schwarz, M. Rosenblum, and E. Gileadi, J. Electroanal. Chem. 381, 29 (1995) and reproduced by the authors.)...

Thus, the true charge-transfer current can be calculated from the ordinate at the origin in the plot between the reciprocal of the measured current density, j"1, as a function of w. The slope (B 1) is the reciprocal value of the Levich constant, 0.620nFCJoj, because it is the only portion that strictly depends on the co value [107], where D, is the coefficient of diffusion of they-particle. With the currents corrected from the mass transport effects, we can depict the Tafel lines, from which the values of j0 and a can be calculated. 

It becomes clear when Equations 11.41 and 11.42 are compared that the empirical Equation 11.41 is basically the Tafel Equation 11.42 plus terms to account for the mass transport resistance loss and the iR loss. It is then fair to say that the Tafel equation is valid only in the absence of mass transport resistance and when the Ohmic resistance is corrected. 

On the right hand, the first term can be calculated using equilibrium electrochemistry of the fuel cell reaction as described in Chapter 4, the second term is actually a modified Tafel equation (Chapter 6), with a parasitic current density correction described earlier, the third term is related to the mass transport of chemicals when the limiting current is approached (Chapter 6), and the last term is simply Ohms law (Chapter 2). In this equation, Apc and Bpc are the semiempirical positive coefficients in V, and rpc is the fuel cell area-specific resistance in 2 cm. Current density is in A cm , and the fuel cell and equilibrium potentials are in V. 

In contrast, the errors of the polarization-resistance technique have been very thoroughly and quantitatively evaluated, and the reported errors are the smallest among the four techniques for all error categories. On the other hand, this technique has two more error possibilities (in linearization and Tafel-slope estimate) than the other techniques. Consequently, the overall error may be comparable to those of the three-point and curve-fitting techniques, and it has to be evaluated for each experimental situation. The systematic errors can be avoided by using the appropriately corrected polarization equations in the data evaluation however, that requires numerical values for the appropriate parameters, such as mass transport, double layer, solution resistance, equi-... 

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