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Even though alloys of chromium or aluminum strengthened nickel provides a stable, non-sintering, creep-resistant anode, electrodes made with Ni are relatively high in cost. Alloys, such as Cu-Al and LiFeO2, have not demonstrated sufficient creep strength or performance. Because of this, present research is focused on reducing the manufacturing cost of the nickel alloy anodes (37). There is a need for better sulfur tolerance in MCFCs, especially when considering coal operation. The potential benefit for sulfur tolerant cells is to eliminate cleanup equipment that impacts system efficiency. This is especially true if low temperature cleanup is required, because the system efficiency and capital cost suffer when the fuel gas temperature is first reduced, then increased to the cell temperature level. Tests are being conducted on ceramic anodes to alleviate the problems, including sulfur poisoning, being experienced with anodes (31). Anodes are being tested with undoped LiFeO2 and LiFeO2 doped with Mn and Nb. Preliminary testing, where several parameters were not strictly controlled, showed that the alternative electrodes exhibited poor performance and would not operate over 80 mA/cm2. At the present time, no alternative anodes have been identified. Instead, future work will focus on tests to better understand material behavior and to develop alternative materials with emphasis on sulfur tolerance. Cathode: An acceptable material for cathodes must have adequate electrical conductivity, structural strength, and low dissolution rate in molten alkali carbonates to avoid precipitation of metal in the electrolyte structure. State-of-the art cathodes are made of lithiated NiO (33, 38) that have acceptable conductivity and structural strength. However, in early testing, a predecessor of UTC Fuel Cells found that the nickel dissolved, then precipitated and reformed as dendrites across the electrolyte matrix. This decreased performance and eventual short-circuting of the cell. Dissolution of the cathode has turned out to be the primary life-limiting constraint of MCFCs, particularly in pressurized operation (35). Developers are investigating approaches to resolve the NiO dissolution issue. For atmospheric cells, developers are looking at increasing the basicity of the electrolyte (using a more basic melt such as Li/NaCO3). Another approach is to lower CO2 (acidic) partial pressure. To operate at higher pressures (higher CO2 partial pressure), developers are investigating alternative materials for the cathodes and using additives in the electrolyte to increase its basicity (37). Initial work on LiFeO2 cathodes showed that electrodes made with this material were very stable chemically under the cathode environment; there was essentially no dissolution (31). However, these electrodes perform poorly compared to the state-of-the-art NiO cathode at atmospheric pressure because of slow kinetics. The electrode shows promise at pressurized operation, so it is still being investigated. Higher performance improvements are expected with Co-doped LiFeO2. It also has been shown that 5 mol lithium-doped NiO with a thickness of 0.02 cm provided a 43 mV overpotential (higher performance) at 160 mA/cm2 compared to the state-of-the-art NiO cathode. It is assumed that reconfiguring the structure, such as decreasing the agglomerate size, could improve performance. Another idea for resolving the cathode dissolution problem is to formulate a milder cell environment. This leads to the approach of using additives in the electrolyte to increase its basicity. Small amounts of additives provide similar voltages to those without additives, but larger amounts adversely affect performance (39). Table 6-2 quantifies the limiting amounts of additives. 6-10PDF Image | Fuel Cell Handbook (Seventh Edition)
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