Invited: Advanced Visualization Tools to Investigate PEM Fuel Cell Materials

Abstract

The commercialization of polymer electrolyte membrane fuel cells (PEMFCs) has been hindered by durability and cost issues that could be overcome with an improved understanding of the water transport mechanism in PEMFC operations. To better understand this phenomenon, various techniques have been employed as powerful diagnostic tools to investigate PEMFC components in relations to their role in water management issues.A key component of interest for visualization is the gas diffusion layer (GDL), which provides passages for electron conduction, fuel transport, heat conduction, and water removal. X-ray imaging, due to its high sensitivity to carbon and non-destructive nature, is well suited for the study of the GDL microstructures. Using conventional desktop micro-computed tomography (micro-CT), with a spatial resolution of approximated 5 µm, ex-situ analyses of the heterogeneous porosity distributions of various GDLs were performed in both the through-plane and in-plane directions [1]. GDL materials show a linear transitional region near the outer surfaces which led to high overall bulk porosities. GDLs treated with micro-porous layers (MPLs) were also visualized to determine the porosity distributions of the GDL microstructure and the MPL coating independently. In general, it was found that MPL penetration into the GDL highly depended on local through-plane GDL porosity [2]. Other investigations utilizing the micro-CT examined the effect of rib and channel compression on the GDL porosity, and enabled the measurement of water content in the GDL microstructure at various current densities [3].Synchrotron X-ray radiography provides another effective visualization tool. In particular, due to the high intensities that the parallel monochromatic beam the synchrotron can provide, this technique suitably lends itself to the typically challenging task of visualizing the dynamic fuel cell operations. In-situ studies of water management in the microstructure of PEM fuel cells were previously performed at the Biomedical Imaging and Therapy Beamline (BMIT-BM) at the Canadian Light Source (Saskatoon, Canada). The facility provided image acquisitions with an effective spatial resolution of 10 µm and a temporal resolution of 3 seconds per frame. Applying the principle of the Beer-Lambert law, raw images were processed to measure the water thickness distributions within the fuel cell in the in-plane and through-plane directions [4]. The effect of MPL thickness and channel wettability on the overall performance and the liquid water saturation within the microstructures were examined [5].Additional visualization tools which have shown to provide invaluable insight in the microstructure of PEM fuel components include atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and nano-computed tomography (nano-CT). AFM has been used to examine the surface morphology of GDL fibres, in order to determine the effective GDL thermal conductivity. Measurement of heterogeneous through-plane distribution of polytetrafluoroethylene (PTFE) within the GDL can be achieved through SEM and EDS imaging. Finally, nano-CT provides the means to visualize the sub-micron pores within the MPL, which are undetectable with traditional micro-CT scanners.Z. Fishman, J. Hinebaugh, and A. Bazylak. Microscale tomography investigations of heterogeneous porosity distributions of PEMFC GDLs. Journal of the Electrochemical Society, 157 (11) B1643-B1650 (2010).Z. Fishman and A. Bazylak. Heterogeneous through-plane porosity distributions for treated PEMFC GDLs. II Effect of MPL cracks. Journal of the Electrochemical Society, 158 (8), B846-B851 (2011).R. Yip and A. Bazylak. Investigation of liquid water content of a compressed PEMFC GDL using micro-computed tomography. Proceedings of the ASME 2012 6th International Conference on Energy Sustainability & 10th Fuel Cell Science, Engineering and Technology Conference, FuelCell2012-91446, 473-477 (2012).J. Lee, J. Hinebaugh, and A. Bazylak. Synchrotron X-ray radiographic investigations of liquid water transport behavior in a PEMFC with MPL-coated GDLs. Journal of Power Sources, 227, 123-130 (2013).J. Lee, P. Antonacci, N. Ge, R Yip, T. Kotaka, Y. Tabuchi, and A. Bazylak. Impact of MPL thickness on water management of PEMFC by synchrotron X-ray radiography. ECS Orlando.