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using a combination of coarse and fine YSZ powder, mechanical strength can be ensured while also achieving the desired contact between the Ni phase and the YSZ phase. In some modern cell designs, a graded anode is used to achieve coarse porosity and high mechanical strength in most of the anode, and fine micro-porosity in the anode zone immediately adjacent to the electrolyte. Despite the relative success of the Ni-YSZ anode, it has drawbacks: • Sensitivity to sulfur and other contaminants. Strong reversible poisoning of the anode occurs at feed concentrations ranging from about 1 ppm H2S when operating at 1000 °C down to less than 50 ppb when operating at 750 °C (See Figure 7-2a (16, 17)). These concentrations require desulfurization of the anode feed, even if it is produced from low-sulfur fuels such as natural gas or ultra-low sulfur diesel or gasoline (See Figure 7-2b). No data is available publicly on the impact of other species (water or hydrocarbons) or different sulfur species on sulfur tolerance, or on the effect after long periods of time (e.g. 40,000 hours or more). Another strong anode poison reported is HCl. Poisoning by these species is reversible after exposure at low concentrations, but irreversible after exposure at concentrations above about 200 ppm. • Oxidation reduction intolerance. Ni-YSZ anodes are made by mixing NiO with YSZ and then reducing the NiO to Ni. However, if the anode is subsequently exposed to air, especially at elevated temperatures, the Ni re-oxidizes readily. Because of the large volume change during the reduction/oxidation of the anode, the anode’s structure and strength are severely compromised. Effectively, the anodes must be kept under reducing conditions at all times. • The thermal expansion coefficient of the anode is substantially higher than the electrolyte and cathode. In anode-supported designs, this can lead to mechanical and dimensional stability problems, especially during thermal cycling. • Poor activity for direct oxidation of hydrocarbons and propensity for carbon formation when exposed to hydrocarbons. To improve the activity for direct oxidation and reduce the anode’s propensity for carbon formation, copper – ceria anodes are being developed. Even though these drawbacks can typically be mitigated by appropriate system design, many consider that better anodes will be needed. To improve the sulfur tolerance and reduction oxidation tolerance of the anodes, several groups are working on oxide-based anodes. Researchers at PNNL have demonstrated sulfur tolerance up to 100 ppm, at least for short times. In addition, as expected, the oxide-based anodes provide excellent oxidation/reduction stability. However, activity for hydrogen oxidation must still be improved to be competitive with Ni-YSZ anodes. In addition, though little experimental data exists, one would expect that these anodes must be modified to provide adequate activity for reforming and water gas shift reactions. 7-4PDF Image | Fuel Cell Handbook (Seventh Edition)
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