Measurement of Contact Angles at Carbon Fiber-Water-Air Triple Phase Boundaries inside Gas Diffusion Media of Polymer Electrolyte Membrane Fuel Cells from Xray Computed Tomography

Abstract

Gas diffusion layers (GDLs) are porous fiber layers that are used for various electrochemical applications, including fuel cells, electrolyzers, CO2 reduction reaction electrolyzers, and redox flow batteries. Depending on its application, a GDL’s wettability is controlled by the addition of polytetrafluoroethylene (PTFE). The PTFE addition is highly stochastic, resulting in inhomogeneous distribution. Furthermore, the fibers, carbonaceous binder, and PTFE all have different surface roughness and surface properties that dictate GDL wettability. To determine the contact angle of the GDL, goniometer measurements of external contact angle are used, where a droplet of water is placed on the top of the GDL and a contact angle is projected onto a detector. However, this form of measurement has limitations that need to be accounted for as the GDL is highly porous and its surface and internal contact angles might be different due to inhomogeneous binder and PTFE distribution. For the first time, we have applied a method to calculate the internal contact angle distribution of water inside various commercial GDLs. Lattice Boltzmann water transport simulations using these contact angle distributions yielded results much closer to experimental values than simulations that used goniometer-measured constant contact angles. XPS was used to correlate surface oxide groups to measured internal contact angles.AlRatrout et al. developed an open-source C++ code to compute local contact angles at triple phase boundary points (comprised of a solid and two fluids) from micro Xray computed tomography (XCT) images of porous media [1]. We applied it to segmented micro XCT images of water filled commercial GDL samples and computed local contact angles at air-GDL-water triple phase contacts points (Figure 1A). We obtained a state of mixed wettability inside all GDL samples, with contact angles ranging from 0° to 180° inside the pores (Figure 1B). Several works have calculated average contact angles similar to our results using other methods [2, 3]. Hydrophilicity is due to the presence of surface oxide groups on GDL fibers while hydrophobicity is due to the chemical nature of carbon and the roughness of the GDL fibers. Lattice Boltzmann water transport simulations were performed with the spatially distributed, mapped contact angles and constant contact angles measured with a goniometer (Figure 1C-F) [4]. When our mapped, mixed-wettability contact angle distributions were used, the simulation revealed several regions of high liquid saturation that the constant contact angle simulations did not capture. These results agreed with experimental observations. Thus, for robust water transport simulations, we suggest that distributed contact angles should be used instead of goniometer-measured constant contact angles.