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direct proportion to the theoretical amount expected. The type of carbon support did not seem to be a major factor, based on using several typical supports during the tests. The anode of a phosphoric acid fuel cell is subject to a reduction in performance when even low amounts of contaminants are preferentially absorbed on the noble catalysts. Yet, hydrogen-rich fuel gases, other than pure hydrogen, are produced with contaminant levels well in excess of the anode's tolerance limit. Of particular concern are CO, COS, and H2S in the fuel gas. The fuel stream in a state-of-the-art PAFC anode, operating at approximately 200 °C (392 °F), must contain 1 vol percent or less of CO (12), less than 50 ppmv of COS plus H2S, and less than 20 ppmv of H2S (28). Current practice is to place COS and H2S cleanup systems and CO shift converters prior to the cell (normally in the fuel processor before reforming) to reduce the fuel stream contaminant levels to the required amounts. Giner, Inc. performed experiments to develop a contaminant- tolerant anode catalyst in order to reduce or eliminate the cleanup equipment (29). An anode catalyst, G87A-17-2, was identified that resulted in only a 24 mV loss from reference when exposed to a 75 percent H2, 1 percent CO, 24 percent CO2, 80 ppm H2S gas mixture at 190 °C (374 °F), 85 percent fuel utilization, and 200 mA/cm2. A baseline anode experienced a 36 mV loss from the reference at the same conditions. At 9.2 atm (120 psi) pressure, the anode loss was only 19 mV at 190 °C (374 °F) and 17 mV at 210 °C (410 °F) (compared with pure H2) with a gas of 71 percent H2, 5 percent CO, 24 percent CO2, and 200 ppm H2S. Economic studies comparing the tradeoff between decreased cell performance with increased savings in plant cost showed no advantage when the new anode catalyst was used with gas containing 1 percent CO/200 ppm H2S. A $7/kW increase resulted with the 5 percent CO gas (compared to a 1 percent CO gas) at a 50 MW size. Some savings would result by eliminating the low temperature shift converter. The real value of the catalyst may be its ability to tolerate excessive CO and H2S concentrations during fuel processor upsets, and to simplify the system by eliminating equipment. As previously mentioned, state-of-the-art gas diffusion electrodes are configured to provide an electrolyte network and a gas network formed with the mixture of carbon black and PTFE. In the electrodes, carbon black agglomerates, consisting of small primary particles 0.02 to 0.04 μm, are mixed with much larger PTFE particles of ~0.3 μm. The carbon black surface may not be covered completely by the PTFE because of the large size of conventional PTFE particles. The space in the agglomerates or the space between the agglomerates and PTFE may act as gas networks at the initial stage of operation, but fill with electrolyte eventually because of the small contact angle of carbon black, uncovered with PTFE, to electrolyte (<90°), resulting in the degradation of cell performance. Attempts to solve this flooding problem by increasing the PTFE content have not been successful because of the offset in performance resulting from the reduction of catalyst utilization. Higher performance and longer lifetime of electrodes are intrinsically at odds, and there is a limit to the improvement in performance over life by optimizing PTFE content in the state-of-the-art electrode structures. Watanabe, et al. (30) proposed preparing an electrode utilizing 100 percent of catalyst clusters, where the functions of gas diffusion electrodes were allotted completely to a hydrophilic, catalyzed carbon black and a wet-proofed carbon black. The former worked as a fine electrolyte network, and the latter worked as a gas-supplying network in a reaction layer. Higher utilization of catalyst clusters and longer life at the reaction layer were expected, compared to state-of-the-art electrodes consisting of the uniform mixture of catalyzed carbon black and PTFE particles. The iR-free electrode potentials for the reduction of oxygen and 5-10PDF Image | Fuel Cell Handbook (Seventh Edition)
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