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∆ VAnode (mV) = 55 log (PH2 )2 (5-10) (PH2 )1 where P H2 is the average partial pressure of H2. At 190 °C (374 °F), the presence of 10 percent CO2 in H2 should cause a voltage loss of about 2 mV. Thus, diluents in low concentrations are not expected to have a major effect on electrode performance; however, relative to the total anode polarization (i.e., 3 mV/100 mA/cm2), the effects are large. It has been reported (16) that with pure H2, the cell voltage at 215 mA/cm2 remains nearly constant at H2 utilizations up to 90 percent, and then it decreases sharply at H2 utilizations above this value. Low utilizations, particularly oxygen utilization, yield high performance. Low utilizations, however, result in poor fuel use. Optimization of this parameter is required. State-of-the-art utilizations are on the order of 85 percent and 50 percent for the fuel and oxidant, respectively. 5.2.4 EffectofImpurities The concentrations of impurities entering the PAFC are very low relative to diluents and reactant gases, but their impact on performance is significant. Some impurities (e.g., sulfur compounds) originate from fuel gas entering the fuel processor and are carried into the fuel cell with the reformed fuel, whereas others (e.g., CO) are produced in the fuel processor. Carbon Monoxide: The presence of CO in a H2-rich fuel has a significant effect on anode performance because CO affects Pt electrode catalysts. CO absorption is reported to arise from the dual site replacement of one H2 molecule by two CO molecules on the Pt surface (40, 41). According to this model, the anodic oxidation current at a fixed overpotential, with (iCO) and without (iH2) CO present, is given as a function of CO coverage (θCO) by Equation (5-11): iCO = (1 - θCO )2 iH2 For [CO]/[H2] = 0.025, θCO = 0.31 at 190°C (35); therefore, iCO is about 50 percent of iH2. (5-11) Both temperature and CO concentration have a major influence on the oxidation of H2 on Pt in CO containing fuel gases. Benjamin, et al. (35) derived Equation (5-12) for the voltage loss resulting from CO absorption as a function of temperature ∆VCO = k(T) ([CO]2 - [CO]1) (5-12) where k(T) is a function of temperature, and [CO]1 and [CO]2 are the mole fractions CO in the fuel gas. The values of k(T) at various temperatures are listed in Table 5-3. Using Equation (5-12) and the data in Table 5-3, it is apparent that for a given change in CO content, ∆VCO is about 8.5 times larger at 163 °C (325 °F) than at 218 °C (424 °F). The correlation provided by Equation (5-12) was obtained at 269 mA/cm2; thus, its use at significantly different current densities may not be 5-16PDF Image | Fuel Cell Handbook (Seventh Edition)
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