Overpotential Characteristic and Flow Field Structure for High Power Output of HT-PEFC
Authors: Ichikawa Ryosuke , Sakakibara Ryo , Omura Shion , Aoyama Yusuke , Uemura Suguru , Tabe Yutaka 
Published: December 2024

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 cm2) 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/cm2. 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 concentration overpotential increases at high-temperature. This is because the air expands at high-temperature and the oxygen’s molar concentration decreases. Although this phenomenon is a major issue in HT-PEFC, the concentration overpotential can be reduced even at high-temperature by increasing the gas pressure.

Figure 4 shows the results of I-V characteristics for the three gas flow fields. Here, IR corrected voltage is used to eliminate the influence of the contact resistance due to the flow field structure. Compared to the 2.0 mm pitch straight flow channel, the concentration overpotentials of the 1.0 mm pitch straight flow channel and the porous flow field are reduced. As shown in Figure 5, this is due to improving the oxygen supply under the flow channel ribs(3). Especially, this effect is significant at high-temperature where the oxygen molar concentration is lower. Additionally, in our previous study(4), when using the porous flow field in conventional PEFC (below 100°C), performance degradation due to dry-out was a major issue. However, the porous flow field is useful for HT-PEFC because the polymer electrolyte membrane has low durability against the liquid water and active drainage of generated water is required.

Acknowledgement

We would like to thank the Suzuki Foundation in Japan for supporting this study.

References

(1) A. R. Korsgaad, et al., Journal of Power Sources, 162 (2006).

(2) Y. Tabe, et al., Journal of Electrochemical Society, 158 (2011).

(3) Y. Iizuka, et al., Journal of Electrochemical Society, 169, (2022).

(4) Y. Tabe, et al., Journal of Power Sources, 238, (2013).

Figure 1