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The adverse effect of H2S on the performance of MCFCs is illustrated in Figure 6-11. The cell voltage of a 10 cm x 10 cm cell at 650 °C decreases when 5 ppm H2S is added to the fuel gas (10 percent H2/5 percent CO2/10 percent H2O/75 percent He), and current is drawn from the cell. The measurements indicate that low concentrations of H2S do not affect the open circuit potential, but they have a major impact on the cell voltage as current density is progressively increased. The decrease in cell voltage is not permanent;34 when fuel gas without H2S is introduced into the cell, the cell voltage returns to the level for a cell with clean fuel. These results can be explained by the chemical and electrochemical reactions that occur involving H2S and S=. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide: H2S+CO3= →H2O+CO2 +S= followed by Ni + xS= → NiSx + 2xe- When the sulfided anode returns to open circuit, the NiSx is reduced by H2: NiSx + xH2 → Ni + xH2S (6-28) (6-29) (6-30) Similarly, when a fuel gas without H2S is introduced to a sulfided anode, reduction of NiSx to Ni can also occur. Detailed discussions on the effect of H2S on cell performance are presented by Vogel and co-workers (75, 76) and Remick (77, 78). The rapid equilibration of the water gas shift reaction in the anode compartment provides an indirect source of H2 by the reaction of CO and H2O. If H2S poisons the active sites for the shift reaction, this equilibrium might not be established in the cell, and a lower H2 content than predicted would be expected. Fortunately, evidence (77, 78) indicates that the shift reaction is not significantly poisoned by H2S. In fact, Cr used in stabilized-Ni anodes appears to act as a sulfur tolerant catalyst for the water gas shift reaction (78). The CO2 required for the cathode reaction is expected to be supplied by recycling the anode gas exhaust (after combustion of the residual H2) to the cathode. Therefore, any sulfur in the anode effluent will be present at the cathode inlet unless provisions are made for sulfur removal. In the absence of sulfur removal, sulfur enters the cathode inlet as SO2, which reacts quantitatively (equilibrium constant is 1015 to 1017) with carbonate ions to produce alkali sulfates. These sulfate 34. The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below that required to form nickel sulfide. 6-28PDF Image | Fuel Cell Handbook (Seventh Edition)
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