Soft X-Ray Imaging of Polymer Electrolyte Fuel Cells Using Different Support Materials for Catalyst Layers

Abstract

The polymer electrolyte fuel cell (PEFC) is a highly efficient and environmentally friendly energy device that has been developed for fuel cell vehicles and other applications due to its low operating temperature and compact size. The PEFC is assembled by putting a membrane electrode assembly (MEA), which consists of the polymer electrolyte membrane with the catalyst layers and the gas diffusion layers on both sides, between separators. Pt nanoparticle catalysts supported on carbon black (Pt/C) are usually used for catalyst layers because of their large surface area and high electrical conductivity. However, carbon supports are oxidized and corroded during operation, resulting in performance degradation. Therefore, research has been conducted using Pt supported on tin oxide (Pt/Nb-SnO2), which has high durability and a porous structure similar to that of carbon supports, as an alternative material for the catalyst layers [1]. Tin oxide supports have material properties such as wettability that differ greatly from carbon supports, and the effect of these material properties on liquid water behavior during power generation has not been clarified. It is essential to understand the liquid water transport in PEFCs for the improvement of performance, because excessive liquid water accumulation (flooding) at the cathode hinders the transport of the reaction gas and causes concentration overvoltage.In this study, in order to clarify the effect of the catalyst layer support material on the liquid water transport, we made catalyst layers using carbon and tin oxide supports respectively, and investigated the liquid water behavior in the MEAs under fuel cell operation using high-resolution soft X-ray radiography.PEFCs with a reaction area of 5×8 mm2 were constructed using carbon support catalyst layers and tin oxide support catalyst layers, respectively. The same components were used except for the catalyst layer. The PEFCs were set in a soft X-ray imaging system (TUX-9000D, Mars-Tohken X-ray Inspection). This system can observe PEFCs and take images at a spatial resolution of about 1.0 µm. Hydrogen was supplied to the anode and air to the cathode, and the power generation tests were conducted in the high current density region where flooding is likely to occur. The voltage drop during power generation of the PEFC with Pt/SnO2 catalyst layer was larger than that of the PEFC with Pt/C catalyst layer. In-situ observation of the liquid water behavior in the PEFC was performed by taking images in the MEA cross-sectional direction at the same time as power generation. The images were taken at the position where both the area under the rib and the area under the channel could be observed. The soft X-ray radiograph of the MEA before power generation is shown in Figure 1 (a). The generated water during the power generation was identified by image processing that subtracted the image taken after operation from the image taken under open-circuit voltage conditions. Figure 1 (b) and (c) show the soft X-ray radiographs of liquid water after 20 minutes of power generation under the current density of 0.8 A/cm2. After the cell startup, the amount of water retention increases on the cathode side of the PEFC and the difference in the amount of water retention under the rib and under the channel was observed. It was also shown that the distribution of liquid water in the porous material was different between the carbon support and tin oxide support catalyst layers attributed to support materials and structures of the catalyst layers properties.AcknowledgmentsThe catalyst with Tin oxide support material used in this study was provided by NEDO ECCEED'30. The authors would like to acknowledge their assistance in assembling PEFCs for visualization study.[1] Y. Chino et al, J Electrochem Soc., 162-7 (2015), F736. Figure 1