In-Situ Analysis of Water Distribution within PEFC Catalyst Layer By Soft X-Ray Imaging

Abstract

The polymer electrolyte fuel cells (PEFCs) are regarded as a promising alternative power source for automobiles (fuel cell electric vehicle; FCEV). Cost reduction and increasing power density of PEFCs are the key challenges for commercialization of FCEVs. In PEFC, the chemical energy of fuel is directly converted into electricity through the electrochemical reactions on Platinum catalyst. Since Platinum is a noble metal, reduction of Platinum amount in the catalyst layer (CL) is essential to achieve cost reduction of FCEVs. In order to accomplish the reduction of Platinum loading without sacrificing PEFC performance under high current density operation, it is important to make more effective use of Pt used in the CL of the membrane electrode assembly (MEA), and the mass transport of reactants within the CL should be enhanced significantly. From this background, extensive number of diagnostic research for the fundamental understanding of the transport phenomena in operating PEFCs has been carried out. It is known that imaging technique is highly important to reveal dynamics and distribution of water within PEFCs, which include magnetic resonance imaging, neutron imaging, and synchrotron X-ray imaging [1]. However, because of the fine structure and the complexity of mas transport phenomena within the CL, water distribution and mass transport phenomena within the CL is not fully understand yet. In our group, water visualization technique by soft X-ray radiography that can be used in ordinary laboratory for PEFC research has been developed (Figure 1), and we have been succeeded to visualize water distribution within the CL, micro porous layers, and the gas diffusion layers [2-4]. In this study, the effect of Platinum loading on the liquid water accumulation and the mass transport phenomena within the cathode CL was investigated. Liquid water distribution within the CL was quantitatively visualized by soft X-ray imaging technique, and the CL with low Platinum loading showed higher water saturation. The electrochemical impedance spectroscopy (EIS) measurement obtained under the high current density condition indicated that the charge transfer resistance of the CL with low Platinum loading is higher than that of the CL with high Platinum loading. It might be due to the increased reaction rate per platinum particle within the CL with low Platinum loading [5]. It would exacerbate the mass transport limitation within the cathode CL. To accomplish the reduction of Platinum loading without sacrificing PEFC performance under high current density operation, further understanding on water distribution and mass transport phenomena within the CL is required. Soft X-ray imaging technique can visualize water distribution within the CL in-situ, and more detailed research will make it possible to optimize the structure of the CL. Acknowledgement: The authors acknowledged Dr. Kazuhiko Shinohara in Nissan Motor Co., Ltd. for his pioneering effort on soft X-ray application for PEFCs. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.