Effects of MPL Pore Size Distribution on Components of Oxygen Transport Resistance Increased Due to Water Accumulation in a PEFC

Abstract

To increase the power output of PEFC, management of generated water is extremely important. A gas diffusion layer (GDL) with a microporous layer (MPL) on the cathode side is thought to play important roles in drainage of product water and optimizing oxygen transport, but the details of the mechanism have not been elucidated. In this study, to reveal the water and oxygen transport phenomena, the components of oxygen transport resistance were evaluated using the limiting current analysis (1) for two types of MPL with different pore size distributions. Further, the water distributions in the cathode side MPL and catalyst layer (CL) were observed by a freezing method and a cryo-SEM (2). The active area of the cell used in this study was 1.8 cm2 (0.9 cm × 2.0 cm). The anode and cathode flow fields were parallel straight type with 1.0 mm wide ribs and channels. The anode side was supplied with hydrogen, and the cathode side was supplied with a mixed gas of oxygen and nitrogen. The gas flow rates were 500 sccm for the anode and 4000 sccm for the cathode. The relative humidity of the supplied gases was 90%. Experiments were conducted under two different cell temperature, 30°C (low temperature condition) and 80°C (high temperature condition). The catalyst coated membrane was produced by Japan Gore (PRIMEA). We used two types of MPL with different pore size distributions for the cathode GDL: MPLs with one peak (conventional type) and with two peaks. The MPLs are called MPL-1 and MPL-2, respectively. For the anode side, a GDL with an MPL (SGL, 28 BC) was used. This study used the limiting current analysis developed by the authors (1). The developed method extended the conventional method to the separation under flooded conditions, considering vapor transport and water distribution. Inside the CL, it was assumed that the water production rate dominates the water accumulation, which is proportional to current density. Outside the CL, it was assumed that the product of total pressure and current density determines the water accumulation (1). The experiments were conducted with different back pressures (110 kPa, 140 kPa, 180 kPa) and different oxygen concentrations (1%, 2%, 4%, 6%, 8%, 12%, 15%, 18%, 21%) for the cathode side. Figure 1 shows the total oxygen transport resistances at 110 kPa. There is a significant difference in the tendencies of the total oxygen transport resistance between the MPL-1 and the MPL-2 under the low temperature condition, while the tendencies are similar under the high temperature condition. This suggests that the effects of water accumulated in the cell are larger at lower temperature. Figure 2 shows the results separating the total oxygen transport resistance into two resistances. The I Lim is the limiting current density. The R P and R NP are the pressure-dependent and -independent oxygen transport resistances. These correspond to the oxygen transport resistances outside and inside the CL. Under the low temperature condition, the pressure-dependent oxygen transport resistances (corresponding to those outside the CL) are different between the MPL-1 and the MPL-2. The pressure-independent resistances (corresponding to those inside the CL) are similar. These mean that the smaller total oxygen transport resistances with the MPL-2 in Figure 1 are caused mainly by the smaller resistance outside the CL. Before the separation, we predicted that the increase in the total oxygen transport resistance is suppressed by the MPL-2 due to enhanced drainage of product water from the CL. However, Figures 2(a) and (c) revealed that the R P of the MPL-2 becomes smaller than that of the MPL-1 rather than the R NP. This may be considered because the larger pores in the MPL-2 form drainage pathway for product water at the low temperature condition, which significantly contributes to increasing the oxygen transport pathway through the smaller pores. As expected from Figure 1, there is no significant difference in the R P and the R NP between two types of the MPL under the high temperature condition (Figures 2(b) and (d)). The water distribution in the vicinity of MPL-2 was observed after discontinuing operation at 2.0 A/cm2 just before the limiting current condition. The experiment was conducted with 30°C, 180 kPa back pressure, and 21% oxygen concentration. Cryo-SEM images showed that there was ice at the MPL/CL interfaces. Additionally, ice crystals were observed at the larger pores in the MPL-2, supporting the above consideration, the drainage pathway for product water through the larger pores.Reference (1) Y. Iizuka, et al., ECS Transactions, 104(8), 83-92 (2021). (2) Y. Aoyama, et al., Electrochem. Commun., 41, 72-75 (2014). Figure 1