Overpotential Characteristic and Flow Field Structure for High Power Output of HT-PEFC

Polymer electrolyte fuel cells (PEFCs) are used for automobiles and other applications due to their excellent startup ability and load followability. From now on, PEFCs are expected to be widely applied to commercial applications. However, when conventional PEFC needs high-load operation, insufficient cooling capacity becomes a major issue. This is due to the small temperature difference between the FC stack and the outside air. The conventional electrolyte membrane uses the water as a proton-conducting path, so PEFC’s operation temperature must be controlled below 100°C. Therefore, applying high-temperature PEFC (HT-PEFC) is effective to improve cooling capacity. HT-PEFC can operate at high-temperature (above 100°C) by doping the acid as a proton conducting path in the polymer electrolyte membrane. In this study, we evaluated the effects of the temperature, the gas pressure, and the gas flow field structure on the power output at high-temperature. Figure 1 shows the structure and picture of HT-PEFC single cell used in this study. Polybenzimidazole (PBI) -based MEA APM STD25 (reaction area: 5 x 5 cm 2 ) manufactured by Advent Technologies was used. The performance was compared by using the straight flow channels (pitch: 2.0 mm, 1.0 mm) and the porous flow field (porosity: 91%) for the separator. All experiments are conducted with non-humidified air and hydrogen. Figure 2 shows the I-V characteristics at various cell temperature and gas pressure with the 2.0 mm pitch straight flow channel. As the cell temperature increases, the I-V characteristic improves, especially in the low-medium current density region. In addition, increasing the gas pressure improves the performance in the entire current density range. The overpotential was separated to discuss these results. The activation overpotential was calculated by Tafel equation, and the concentration overpotential was calculated by subtracting the activation overpotential and the resistance overpotential (= IR) from the total overpotential. Figure 3 shows the results of the activation overpotential and the concentration overpotential at the current density of 2.0 A/cm 2 . The activation overpotential is reduced at high-temperature and high-pressure. These trends agree with the previous studies. The exchange current density of PBI-MEA increases at high-temperature (1) . The oxygen molar concentration become high and the exchange current density is increased at high-pressure (2) . On the other hand, the concen