Effect of Impurities 1
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 electrodes catalysts. The poisoning 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 (0co) by Equation (5-11):
For [CO]/[H2] = 0.025, 0co = 0.31 at 190°C (35); therefore, iro is about 50% of iH2.
As discussed previously, 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 poisoning as a function of temperature
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, AVcq is about 8.5 times larger at 163°C (325°F) than at 218°C (424°). The correlation provided by Equation (5-12) was obtained at 269 mA/cm2; thus, its use at significantly different current densities may not be appropriate. In addition, other more recent data (37) suggest a value for k(T) of -2.12 at a temperature of 190°C (374°) rather than -3.54.
|
T |
T |
k(T)a |
|
(°C) |
(°F) |
(mV/%) |
|
163 |
325 |
-11.1 |
|
177 |
351 |
-6.14 |
|
190 |
374 |
-3.54 |
|
204 |
399 |
-2.05 |
|
218 |
424 |
-1.30 |
a - Based on electrode with 0.35 mg Pt/cm2, and at 269 mA/cm2 (35)
The data in Figure 5-5 illustrate the influence of H2 partial pressure and CO content on the performance of Pt anodes (10% Pt supported on Vulcan XC-72, 0.5 mg Pt/cm2) in 100% H3PO4 at 180°C (356°F) (11). Diluting the H2 fuel gas with 30% 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% H2/(30-x)% CO2/x% CO (x =0, 0.3, 1, 3 and 5) increases considerably as the CO content increases to 5%.
Sulfur Containing Compounds: Hydrogen sulfide and carbonyl sulfide (COS) are impurities17 in fuel gases from fuel processors and coal gasifiers in PAFC power plants. The concentration levels of H2S in an operating PAFC (190 to 210°C (374 to 410°F), 9.2 atm (120 psig), 80% 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 poisoning does not affect the cathode, and poisoned anodes can be re-activated by polarization at high potentials (i.e., operating cathode potentials). As mentioned previously, there is a synergistic effect between H2S and CO that can negatively impact cell performance. Figure 5-6 (37) shows the effect of H2S concentration on AV with and without 10% CO present in H2. The AV is referenced to performance on pure H2 in the case of H2S alone and to performance on H2 with 10% CO for H2S and CO. In both cases, at higher H2S
17. Anode gases from coal gasifiers may contain total sulfur of 100 to 200 ppm.
concentrations, the AV rises abruptly. This drop in performance occurs above 240 ppm for H2S alone and above 160 ppm for H2S with 10% CO.
Experimental studies by Chin and Howard (43) indicate that H2S adsorbs on Pt and blocks the active sites for H2 oxidation. The following electrochemical reactions, Equations (5-13), (5-14), and (5-15) involving H2S are postulated to occur on Pt electrodes:
- Figure 5-5 Influence of CO and Fuel Gas Composition on the Performance of Pt Anodes in 100% H3PO4 at 180°C. 10% Pt Supported on Vulcan XC-72, 0.5 mg Pt/cm2. Dew Point, 57°. Curve 1, 100% H2; Curves 2-6, 70% H2 and CO2/CO Contents (mol%) Specified (21)
ppm H2S in H2
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