Measurement of internal wettability of gas diffusion porous media of proton exchange membrane fuel cells

Abstract

One of the major problems of current proton exchange membrane (PEM) fuel cells is water management. The gas diffusion layer (GDL) of the fuel cell plays an important role in water management since humidification and water removal are both achieved through the GDL. Various numerical models developed to illustrate the multiphase flow and transport in the fuel cell. The accuracy of these models depends on the accurate measurement of the GDL properties such as wettability, surface energy, and porosity. Most of the studies conducted for measuring the wettability of the GDL are based on the external contact angle measurements. However, the external contact angle does not describe adequately capillary forces acting on the water inside the GDL pores. In a recent study, the capillary penetration technique has been used to measure indirectly the wettability of the GDL based on the experimental weight increase due to penetration of the liquid into the porous sample. In essence, the mass penetration technique was used along with the Washburn's equation. The shortcoming of this method is that the external factors such as the mass of the meniscus formed outside the sample as well as evaporation occurring during the experiment were not considered. It was found that these factors affect the wettability measurements of the GDL, especially for a hydrophilic sample. In this paper, the experimental setup of the capillary penetration method has been modified to control the evaporation rate as the liquid is penetrated into the sample. Also, the capillary penetration technique which was initially used based on mass penetration has been modified to the height penetration method to eliminate the effect of the weight of the meniscus formed outside the sample. The experiments were performed for a time period of 10 s. For this time period, it was found that the Washburn's equation is not an accurate model since it does not include the frictional work effects that are significant at the first few seconds of the experiments. Therefore, the Washburn's equation was replaced by a more general form. Using the Levenberg–Marquardt optimization technique, the experimental data obtained from the height penetration technique is fitted to the theoretical curve to find the internal contact angles of a sample GDL. Finally, these contact angle results are used to determine the surface tension of the GDL using two approaches: the Owens–Wendt surface tension components and the equation-of-state models.