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appropriate. In addition, other more recent data (37) suggest a value for k(T) of -2.12 at a temperature of 190 °C (374 °F) rather than -3.54. Table 5-3 Dependence of k(T) on Temperature T T (°C) (°F) 163 325 177 351 190 374 204 399 218 424 k(T)a (mV/ percent) -11.1 -6.14 -3.54 -2.05 -1.30 a -Based on electrode with 0.35 mg Pt/cm2, and at 269 mA/cm2 (35) The data in Figure 5-6 illustrate the influence of H2 partial pressure and CO content on the performance of Pt anodes (10 percent Pt supported on Vulcan XC-72, 0.5 mg Pt/cm2) in 100 percent H3PO4 at 180 °C (356 °F) (11). Diluting the H2 fuel gas with 30 percent CO2 produces an additional polarization of about 11 mV at 300 mA/cm2. The results show that the anode polarization with fuel gases of composition 70 percent H2/(30-x) percent CO2/x percent CO (x =0, 0.3, 1, 3 and 5) increases considerably as the CO content increases to 5 percent. Sulfur Containing Compounds: Hydrogen sulfide and carbonyl sulfide (COS) impurities17 in fuel gases from fuel processors and coal gasifiers can reduce the effectiveness of fuel cell catalysts. Concentrations of these compounds must also be limited in a power plant's fuel processing section, because the fuel reformer too has catalysts. As a result, sulfur must be removed prior to fuel reforming with the non-sulfur tolerant catalysts now in use in PAFC power plants. It is prudent to be concerned about sulfur effects in the cell, however, because the fuel processor catalyst's tolerance limits may be less than the fuel cell catalyst's or there could be an upset of the fuel processor sulfur guard with sulfur passing through to the cell. The concentration levels of H2S in an operating PAFC (190 to 210 °C (374 to 410 °F), 9.2 atm (120 psig), 80 percent H2 utilization, <325 mA/cm2) that can be tolerated by Pt anodes without suffering a destructive loss in performance are <50 ppm (H2S + COS) or <20 ppm (H2S) (42). Rapid cell failure occurs with fuel gas containing more than 50 ppm H2S. Sulfur does not affect the cathode, and the impact of sulfur on the anodes can be re-activated by polarization at high potentials (i.e., operating cathode potentials). A synergistic effect between H2S and CO negatively impacts cell performance. Figure 5-7 (37) shows the effect of H2S concentration on ∆V with and without 10 percent CO present in H2. The ∆V is referenced to performance on pure H2 in the case of H2S alone and to performance on H2 with 10 percent CO for H2S and CO. In both cases, at higher H2S concentrations, the ∆V rises abruptly. This drop in performance occurs above 240 ppm for H2S alone and above 160 ppm for H2S with 10 percent CO. 17. Anode gases from coal gasifiers may contain total sulfur of 100 to 200 ppm. 5-17PDF Image | Fuel Cell Handbook (Seventh Edition)
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