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3.2 Performance A summary of the performance levels achieved with PEFCs since the mid-1960s is presented in Figure 3-7. Because of changes in operating conditions involving pressure, temperature, reactant gases, and other parameters, a wide range of performance levels can be obtained. The performance of the PEFC in the U.S. Gemini Space Program was 37 mA/cm2 at 0.78 V in a 32- cell stack that typically operated at 50 °C and 2 atmospheres (49). Current technology yields performance levels that are vastly superior. Results from Los Alamos National Laboratory show that 0.78 V at about 200 mA/cm2 (3 atmospheres H2 and 5 atmospheres air) can be obtained at 80 °C in PEFCs containing a Nafion membrane and electrodes with a platinum loading of 0.4 mg/cm2. Further details on PEFC performance with Nafion membranes are presented by Watkins, et al. (50). In recent years, the development effort has been focused on maintaining power density while reducing platinum loading, broadening temperature and humidity operating envelopes, and other improvements that will reduce cost (25,51,14 ,16 ,11). Operating temperature has a significant influence on PEFC performance. An increase in temperature decreases the ohmic resistance of the electrolyte and accelerates the kinetics of the electrode reactions. In addition, mass transport limitations are reduced at higher temperatures. The overall result is an improvement in cell performance. Experimental data (55, 52, 53) suggest a voltage gain in the range of 1.1 - 2.5 mV for each degree (°C) of temperature increase. Operating at higher temperatures also reduces the chemisorption of CO. Improving the cell performance through an increase in temperature, however, is limited by the vapor pressure of water in the ion exchange membrane due to the membrane’s susceptibility to dehydration and the subsequent loss of ionic conductivity. 1 0.9 0.8 0.7 0.6 ’86, b, 4 atm, 75°C ’84, c, 1 atm, 70°C ’75, a, 2 atm, 90°C ’01, b, 1 atm, 70°C ’01, c, 1 atm, 70C ’86, b, 4 atm, 75°C ‘91, d, 5 atm, 95°C ‘01, b, 3 atm, 70°C ‘01, b, 1 atm, 65°C ’’88, d, 4.4 atm, 70°C 0.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Current Density (A/cm2) Figure 3-7 Evolutionary Changes in PEFCs Performance [(a) H2/O2, (b) H2/Air, (c) Reformate Fuel/Air, (d) H2/unkown)] [24, 10, 12, 54, 55] 3-14 Cell Voltage (V)PDF Image | Fuel Cell Handbook (Seventh Edition)
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