Rapid Freezing and Cryo CT Imaging of Liquid Water inside PEFCs

Abstract

Liquid water in the gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs) prevents the gas transport and reduces the cell performance. Therefore, it is important to understand the actual structure of the liquid water in order to improve the GDL structure and the cell performance. X-ray measurement is one of the methods to understand the state of the liquid water inside the fuel cell. In the past, transmission imaging and in-situ computed tomography have been performed, but those methods provide spatial or temporal averaged images, and do not observe the actual instant three-dimensional structure. In this study, we observed the three-dimensional structure of the liquid water inside PEFCs by flash freezing and X-ray CT measurement. The operation was performed at 40 cell temperature. The relative humidity of the anode (H2)/cathode (air) gas was set at 80%, and the gas pressure was set at 101kPa. After rapid freezing using liquid nitrogen, CT imaging was performed in a low-temperature environment of -50. Figure(left) shows the liquid water structure on the cathode side at 0.1 V and 1.7 A/cm2. Liquid water accumulates widely inside the substrate, and it was also found to have a large structure connected between each neighborhood pores. Also, numerical simulations of oxygen transport were performed and showed that the accumulation of liquid water directly reduced the concentration of oxygen in the catalyst layer (right figure). Figure 1 Figure 2 Figure 3