Effect of PEFC Rib/Channel Width on Liquid Water Accumulation and Discharge by In-situ X-Ray Imaging

Abstract

Water management within a polymer electrolyte fuel cell (PEFC) is one of the most important factor for improving the performance. Experimental studies have shown that liquid water tend to accumulate within catalyst layer (CL), gas diffusion layer (GDL) and on channel wall [1][2]. In this study, effects of PEFC rib/channel width on liquid water distribution and performance has been investigated by using in-situ X-ray imaging technique. For analyzing effects of rib/channel width, we used three flow channels with different rib/channel width (rib/channel: 0.2/0.4, 0.3/0/3, 0.4/0.2 mm), and in-situ X-ray visualization of liquid water was carried out. In addition, oxygen transport resistance was calculated by using a limiting current measurement, established by Baker et al [3]. In this study, we used the visualization cell with 4.5 mm2 active area and 1-path serpentine flow field. The cell was operated at cell temperature of 60 degC, relative humidity of 80 %, and various oxygen concentration (Oxygen concentration:2, 4, 6, 10, 15, 20 %, diluted by Nitrogen). As a result of performance test, the cell with wider channel width showed higher performance. Because liquid water was mainly accumulated in GDL under rib area, it suggested that the CL under rib area is not effectively used. To understand the effects of rib/channel with on liquid water distribution, oxygen transport resistance measurement was carried out. At low oxygen concentration condition (2, 4 and 6%), no liquid water was observed within PEFC, and oxygen diffusion resistance was approximately constant. On the other hand, liquid water accumulation within GDL under rib area, on channel wall and interface between micro porous layer (MPL) and substrate layer was observed at high oxygen concentration condition (10, 15 and 20%). As current increased, more liquid water accumulated, and it incur increase of oxygen diffusion resistance. In addition, fluctuation of cell voltage during liquid water accumulation and discharge behavior was observed at high current density. Figure 1 shows the result of liquid water visualization at cell voltage of 0.25 V. Liquid water is discharged from the GDL under rib area to the channel as a droplet (Figure 1 (b)). When growing liquid water droplet reached the bottom of channel, current density decreased immediately. At this moment, X-ray image showed that the rapidly growing water on another channel wall (Figure 1 (c)). Therefore, it can be concluded that water droplet continues to block gas flow until pressure difference before and after water drop grows up. So there is a possibility that flooding at one part induces another flooding at upper stream of the channel. These results show the importance to discharge a lot of liquid water from GDL to channel without flooding the channel. For improving PEFC performance while satisfying these conflicting factors, an appropriate combination of rib/channel width and channel wall wettability is essential.