Control of the Balance between Vapor and Heat Transfer for the Reduction of Oxygen Transport Resistance in High Current Density PEFC Operation

Abstract

To increase the power output of PEFC, management of generated water is extremely important. Gas diffusion layers (GDLs) and micro-porous layers (MPLs) play a role in promoting the discharge of generated water. It was reported that a hydrophilic MPL with carbon fiber (CF) and ionomer has the effect of wicking liquid water from the catalyst layer (CL) under flooding conditions, and its performance is improved compared to a hydrophobic MPL with carbon black (CB) (1). This study confirmed an unusual increase in oxygen transport resistance with the hydrophobic CB-MPL at higher current densities during high temperature operation: the increase becomes pronounced with the lower relative humidity of the supplied gas. To examine the cause, we carried out observation of water distribution in the cell after the unusual increase by a freezing method and a cryo-SEM (2). In addition, we tried to reduce the oxygen transport resistance at higher current densities by controlling the balance between the vapor diffusion and the heat conduction. In this study, a small cell with active area of 1.8cm2 (0.9cm × 2.0cm) was used. The separator was a parallel straight flow channel with rib and channel widths of 0.3mm respectively. 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 100sccm for the anode, and set to a high value of 4000sccm for the cathode. The narrow rib width and the high flow rate are expected to contribute to little liquid water accumulated under the rib and inside the channel. We used two kinds of GDLs with MPLs: the GDL with hydrophobic CB-MPL (made by Asahi Glass Co., Ltd. (1)) and 28 BC (made by SGL Co., Ltd.). The thermal conductivity of the GDL with CB-MPL is lower than that of GDL 28 BC. The catalyst coated membrane was produced by Japan Gore-Tex Inc. In this study, the experiments consisted of oxygen transport resistance measurements and observations of water distribution in the vicinity of MPL. The freezing method and the cryo-SEM were used for the observation (2). The oxygen transport resistances were measured by using the limiting current method. Figure 1 shows the behavior of the unusual increase in oxygen transport resistance, , with the CB-MPL under a high temperature condition. This experiment was conducted with different relative humidity (53, 66, 81%RH), and different oxygen concentration (1, 2, 4, 6, 8, 12, 16, 20, 24 %). The cell temperature was 80°C and the gas pressure was 100kPa. In Figure 1, the oxygen transport resistance, , increases drastically at higher limiting current densities, and the increase becomes pronounced with the lower relative humidity of the supplied gas. This result suggests that these increases at the limiting current densities aren’t caused by flooding. Figure 2 shows the water distribution in the vicinity of MPL after the unusual increase at 81 %RH with oxygen 24%. There is no ice both in the MPL and in the CL. These results suggest that the increase in oxygen transport resistance in Figure 2 is due to drying of the CL. It is known that the ionomer in the CL has poor oxygen permeability due to the decrease in water content (3). In this experiment, the heat conductivity of the GDL with CB-MPL was very low, so it was difficult to remove the heat of reaction to the separator. Therefore, the cause of this phenomenon is considered to be the ionomer drying due to the poor conduction of generated heat compared to the diffusion of generated vapor. The above results also suggest that oxygen transport resistance can be kept low even at higher current densities by controlling the balance between the vapor and heat transfer. In this study, using a heat conduction and water vapor diffusion model, appropriate operating conditions were investigated. The operating conditions are set such that the relative humidity in the CL does not increase or decrease in the high current density region. Figure 3 shows the results of oxygen transport resistance measurement under the determined operating conditions. We used GDL 28 BC with higher heat conductivity and the appropriate condition was 80°C and 100kPa. In Figure 3, the oxygen transport resistance increases very little, showing the effectiveness of controlling the balance between the vapor and heat transfer. Reference (1) Y. Aoyama, et al., J. Electrochem. Soc., 165(7) (2018), F484. (2) Y. Aoyama, et al., Electrochem. Commun., 41(2014), 72. H. F. M. Mohamed, et al., Polymer., 40 (2008), 3091. Figure 1