Effect of Wettability of Micro-Porous Layer on Microscopic Water Transport Phenomena in PEFC

Water transport mechanism in the micro-porous layer (MPL) is reported from various viewpoints. Recently, the wettability of the MPL is attracted as a key to improve the performance of a polymer electrolyte fuel cell (PEFC). According to the earlier study investigating the influence of MPL wettability on cell performances, the cell with hydrophilic MPL showed better cell performance in a wide range of humidity conditions compared with the cell with hydrophobic MPL (1). In contrast, how MPL wettability affects water transport mechanism is not fully understood. In this study, we observed the cross-sectional distribution of liquid water inside both the hydrophobic and hydrophilic MPLs at the cathode side by using freezing method and cryo-SEM (2). From the observed results, we discuss the effect of the wettability on water transport phenomena in the vicinity of the MPL.

A single cell with an active area of 1.8 cm²was used in the experiment. Both anode and cathode bipolar plates had straight flow channels. Hydrogen and air were used for the anode and the cathode side gases, respectively. In the experiments, the cell was set in a thermostatic chamber to keep the cell temperature constant. Two types of cathode side MPLs with different wettability were used in this study; the conventional hydrophobic MPL is composed of carbon black and PTFE, and the hydrophilic MPL is composed of carbon fiber and ionomer. The interface between the cathode side MPL and the catalyst layer (CL) were fabricated by the gas diffusion electrode (GDE) method (1). The GDE method improves interfacial contact between the MPL and the CL by directly coating the catalyst ink on the MPL. These membrane electrode assemblies (MEAs) were experimentally produced by Asahi Glass Co., Ltd.

In this study, the experiment consisted of cell performance measurement and the observation of liquid water in the vicinity of MPLs. The freezing method and cryo-SEM were used for the observation (2). The method immobilizes the liquid water in the cell immediately after discontinuing operation by freezing them, and the cryo-SEM observation can visualize the liquid water distribution in the vicinity of the MPL at high spatial resolution without ice melting.

Figure 1 is the cryo-SEM images of the cathode side MEA before operation.  Figures 1(a) and 1(b) are the images of the hydrophobic and hydrophilic MPLs respectively. The structure of the hydrophilic MPL with the carbon fibers is clearly different form the conventional hydrophobic MPL with carbon blacks. The pore size of the hydrophilic MPL is larger than that of the hydrophobic MPL. Figures 1(c) and 1(d) are images of the hydrophobic and hydrophilic MPL/CL interfaces, respectively. The interface between the hydrophobic MPL and the CL can be hardly distinguished because both the hydrophobic MPL and the CL are composed of carbon black. In contrast, the interface between the hydrophilic MPL and the CL can be clearly observed due to the hydrophilic MPL with carbon fiber.

Figure 2 is the cryo-SEM images of the cathode side MEA after the operation. Figures 2(a) and (b) are the hydrophobic and hydrophilic MPLs respectively. The cell was operated at 70 ºC, and constant current density was 0.7A/cm². Relative humidity was set at 81% (MEA with hydrophobic MPL) and 100% (MEA with hydrophilic MPL). The ice distributions in the hydrophobic and hydrophilic MPLs are clearly different. There is no ice in the hydrophobic MPL (Fig. 2(a)). In contrast, much ice is observed in the hydrophilic MPL (Fig. 2(b)). The difference in the ice distributions is due to the wettability of the MPL. Figures 2(c) and (d) are the hydrophobic and hydrophilic MPL/CL interfaces, respectively. Although the further investigation is required under other conditions, e.g. dry condition, ice in the CL of the MEA having the hydrophilic MPL appears to be little as compared with ice in the CL of the MEA having the hydrophobic MPL. There might be a possibility that the hydrophilic MPL drains liquid water from the CL.

References

1. T. Tanuma and S. Kinoshita, J. Electrochem. Soc., 161, F94 (2014).
2. Y. Aoyama, K. Suzuki, Y. Tabe, and T. Chikahisa, Electrochem. Commun., 41, 72 (2014).