Development of Water Vapor Transport Resistance Protocol and Analysis

Abstract

W. L. Gore and Associates, Inc. (Gore) supports the lowest $/kW fuel-cell systems through development of thin, reinforced membranes, ca. 10 µm or less. Thin membranes decrease cost by reducing material content and facilitate higher performance through increased proton conductance and faster water transport. However, it has been postulated that an interfacial water transport resistance, significantly different than the bulk resistance, will dominate for thin membranes. For example, in a previous study, the interfacial effect accounted for half of the total transport resistance for a 10 µm membrane at 30% RH1. Understanding water transport through the membrane is necessary for system-level models to design optimal operation strategies. Gore implemented a protocol and analysis method for characterizing water transport properties of cation exchange membranes. Water flux is driven across the membrane by a chemical-potential gradient at different water activity levels. The water balance is tracked by monitoring the dew point at cell inlet and outlet and closes within 10%. Fick’s law is used to normalize flux data by the activity gradient across cell and inverted to calculate a total transport resistance. Cell hardware with 5 cm2 active area and straight-channel flow field configuration was used. Further details will be discussed in this presentation. Using this method, the bulk permeability of extrusion cast Nafion 11x films is 0.16 µmol/cm/s at 30% relative humidity (RH) and 95 °C, which is in agreement with literature at similar conditions1. An Evaluating the Measurement Process (EMP) study was used to quantify the amount of variation arising from measurement device and operator. With a sample size of 4, the protocol is able to detect a difference in total water transport resistance greater than 0.3 m2-s/mol. The protocol was used to study a series of 700 EW cast film thicknesses from 7 – 70 µm to separate individual contributions to the total water transport resistance, RTotal = RGas + RBulk + RInterfacial. The gas-phase transport resistance, RGas, was separated using hydrogen and nitrogen carrier gasses. The bulk water transport resistance, RBulk, is equal to thickness divided by bulk permeability. Figure 1a shows bulk permeability increases with higher water content, as expected due to the percolating network of water conducting channels2. Typically, any residual resistance is attributed to an interfacial effect, which has been rationalized by the orientation of the hydrophobic backbone toward surface2. In this study, the interfacial resistance is indistinguishable from zero. The equivalent thickness of membrane interface is shown in Figure 1b as a function of RH. ACKNOWLEDGMENTS The authors gratefully acknowledge the contribution of the Electrochemical Products team of W. L. Gore and Associates, Inc. REFERENCES Kienitz, B., Yamada, H., Nonoyama, N. & Weber, A. Z. Interfacial Water Transport Effects in Proton-Exchange Membranes. J. Fuel Cell Sci. Technol. 8, 011013-1-011013-7 (2011).Kusoglu, A. & Weber, A. Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 117, 987–1104 (2017). Figure 1