Microstructures and electrical conductivity properties of compressed gas diffusion layers using X-ray tomography

Bulk resistance of the gas diffusion layer (GDL) plays an important role in the ohmic loss of proton exchange membrane fuel cells (PEMFCs). This work represents the first direct experimental study and comparison of the bulk resistances during the compression process of commonly used commercial GDLs, carbon paper and carbon felt, from the perspective of the microstructure mechanisms. The in-situ X-ray tomography and a fiber tracing algorithm are used to obtain the microstructure characteristics of compressed GDLs. The through-plane (T-P) and in-plane (I-P) bulk resistances of GDLs are measured by the microelectrode probe measurement method and the four-point probe method, respectively. Then, their resistivities are calculated. For the T-P direction, the arrangement of fibers gradually becomes compact during the compression, arousing the nearly linear increase of fiber contact points. Thus, more and more conductive paths are constructed, inducing a rapid decline of the T-P bulk resistivity. Since the fibers in carbon felt are entangled together, the fibers in the edge areas are arranged loosely at low compression. This uneven distribution of contact points in carbon felt results in much larger T-P bulk resistivity than that of carbon paper at low compressive strain. For carbon paper, binders contribute significantly to the smaller T-P resistivity because they tend to gather at the fiber intersections, which forms more conductive paths with better conductivity. For the I-P direction, the bulk resistivities of GDLs decrease linearly during compression. The nearly horizontal fibers are the key to the I-P bulk resistivity, and both smaller fiber density and higher fiber tortuosity lead to the increase of the I-P bulk resistivity. This study helps to understand the microstructure mechanisms of the bulk resistances of various GDLs during the compression process, which can guide the design and fabrication process of GDLs.