Fuel cell Tafel plot

It is often claimed that electrocatalysts in fuel cells are dependent on the exchange current density, i0 of the slowest reaction in the cell, (a) Make Tafel plots for i0 = 10 I,10 6, and 10-3 A cm-2 and bTaM = 0.12. (b) Then draw plots of the same type and the same i0, but with b values of 0.12, 0.05, 0.038, and 0.029 (T = 298 K). (c) Write out your conclusions concerning the interplay of /0 and b in the Tafel relation (B = RT/aF). (d) How does this relate to the choice of electrocatalytic surfaces for optimal fuel cell performance (Bockris)... 

FIGU RE 21.52 Tafel plots for Oj reduction at 25°C on oxidized Pt bare electrodes and Pt electrodes with PBI or Nafion film in 0.1 M acid solutions as indicated in the diagram. Scan rate 50 mV s". Film thickness indicated in the diagram. (From Savinell, R.F. et al.. Presentation High Temperature Polymer Electrolyte for PEM Fuel Cells, 2001.)... 

Figure 13.1. Steady-state IR-corrected Tafel plots for the cathodic ORR catalyzed by several binary Pt alloy electrocatalysts at 90 °C and 5 atm pressure. Performance of a Pt/C electrocatalyst is shown for comparison. The electrodes had a metal loading of 0.3 mg cm and the loading of the metal on the carbon-snpported catal5rst was 20 wt%. The humidification temperature for the anode and cathode gas streams was kept at 10 and 5 °C above the cell temperature, respectively [6]. (From Mukeijee S, Srinivasan S. O2 reduction and structure-related parameters for supported catalysts. In Vielstich W, Gasteiger HA, Lamm A, editors. Handbook of fuel cells - fundamentals, technology and appheations. New York John Wiley Sons, 2003. Reproduced with permission.)...

Where b is referred to as the Tafel slope and can be determined from the plot of overpotential as a function of log(i). Using the exchange current density, the Tafel equation for the cathodic reaction in the fuel cell is provided by... 

Fuel cell engineers refer to jo/mpt as the mass activity. From an electrochemical perspective. Figure 1.6a represents a Tafel plot. However, the voltage Eceii (jo) does not only account for a single plain electrified interface, but also depends on electron flux, ion flux, and mass transport in all components and across all interfaces of the MEA. 

TABLE 12.2 Kinetic Parameters at Different Temperatures, Estimated From the Tafel Plots of Methanol Oxidation on a Fuel Cell Anode Catalyzed by Carbon-Supported Pt/Ru (40%, 1 1 atomic ratio) Catalyst, 1.0 mol dm Methanol Solution with 1.0 mol dm H2SO4 as Supporting Electrolyte, and at Ambient Pressure... 

The j-E curve can be used not only to quantitatively describe the overall fuel cell performance but also to identify and quantify the activation loss, ohmic loss, and the mass transfer limited current density. At low current density, the ohmic loss is negligible and hence the activation loss can be directly obtained from the j-E curve at low current density. The semi-log plot of the/-E curve is linear for low current density and it can be fit to a Tafel equation (Equation 5.83) as shown in Figure 8.1 at low current density. Using the line fit to the Tafel equation. 

FIGURE 12.15. Tafel plots of the anodic overvoltage for three kinds of electrode Ni-YSZ prepared by powder mixing process Ni-YSZ prepared by drip- pyrolysis process and (Ni gMg.,2) O-YSZ prepared by the drip-pyrolysis process. (From Okumura, K., Yamamoto, Y, Fukui, X, Hanyu, S., Kubo, Y, Esaki, Y, Hattori, M., Kusunoki, A., and Takeuchi, S., Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, Singhal, S.C. and Iwahara, H., Eds., The Electrochemical Society, Pennington, NJ, 1993, 444-453. With permission.)... 

Figure 4.23 Experimental Tafel plot of cell voltage versus current, corrected for fuel cell ohmic and other losses, so that only cathode polarization losses are remaining. The results are normalized to platinum loading. Results with open circles are with humidified oxygen, and closed circles are with humidified air. The dashed line represents the Tafel slope behavior. Note that for all loadings the Tafel slope for oxygen reduction on platinum is the same but deviates from this behavior under mass-limiting behavior. Also note that the vertical axis is ohmic corrected fuel cell voltage, not electrode overpotential, so the voltage falls with increasing current density. (Reproduced with permission from [9].)...

With the previous numerical values the Tafel slope is 0.066V/decade, which is a typical value for hydrogen/oxygen fuel cells. For a = 0.5 and a = 1.5 the Tafel slope is 0.132 and 0.44 V/decade. Figure 3-12 shows the polarization curves for three different values of the Tafel slope. For convenience, the curves are plotted in the log scale, with the cell potential corrected for resistive losses, and concentration polarization losses have been neglected. In that case the polarization curve becomes a straight line. Higher Tafel slopes result in lower performance. 

The experimental plots of iR-free voltage vs. current density obtained for O2 or air and hydrogen as a fuel have been used for the estimation of the factors, which determine the cell polarization losses, namely activation potential, Tafel slope, and mass transport limitations. 

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