The quantification procedure of oxygen-transport resistances for different fuel-cell layers and phenomena is described. The total transport resistance is obtained from limiting-current measurements under conditions where oxygen diffusion is dominant (i.e., high flow rates, small cell size, humidified but subsaturated feeds, and low feed oxygen partial pressure). By systematically varying the experimental conditions, the contributions of molecular and Knudsen diffusion and permeation through the ionomer film covering the catalyst-layer agglomerates are determined. It is found that the ionomer-film resistance is dominant, especially at lower temperatures and lower Pt loadings. The calculated film properties through the ionomer hint that it is much more resistive than the bulk membrane for state-of-the-art cells.
We have developed a new cyclic voltammetry (CV) technique to analyze the ionomer coverage of Pt/C surfaces. The ionomer coverage ratio was estimated by comparing the capacitance current under perfluorohydrocarbon solvent and under nitrogen. When the cathode electrode was immersed in perfluorohydrocarbon solvent, the proton supply from adsorbed water is inhibited by blocking water adsorption. Accordingly, proton supplies from the ionomer can be differentiated from that of adsorbed water. Using this technique, the effect of carbon species on the ionomer coverage ratio was investigated. Using Ketjen carbon in the catalyst layer resulted a low ionomer coverage ratio, which also influenced V-I performance.
Membrane electrode assembly (MEA) buckling tests in microscopic clearances under humidity cycles and numerical analyses by finite element method (FEM) were conducted. The NR211 (Dupont, 25-μm thickness, equivalent weight (EW) = 1100) sandwiched between catalyst layers (CLs) was used as the MEA. Based on tensile tests of the NR211 and NR211-CL and FEM simulation of tensile tests, the Young’s modulus and yield point of CL were estimated. While the CL had a higher Young’s modulus than the NR211 in water vapor, the CL indicated a lower Young’s modulus than the NR211 in liquid water at 80 °C. The buckling tests in microscopic diameter of 200 μm in polyimide film were carried out. The heights of bulge in the NR211 and NR211-CL after five humidity cycles were measured with a laser microscope. The height of the NR211-CL was lower than that of the NR211, due to the stiffer CL and the lower swelling ratio of the NR211-CL. Moreover, when the humidity cycles were repeated less than 1000 times, cracks were formed in the CL. The stress-strain behaviors of the NR211-CL buckling test under a humidity cycle were investigated by using the FEM. When the NR211-CL swelled, higher stress was developed at the topside of bulge and topside of bulge round. These portions corresponded to the CL crack-formed portions in the buckling test. When the NR211-CL deswelled, the tensile stress was induced in the entire NR211. The mechanical degradation mechanisms were considered as follows: Firstly, cracks initiate and propagate in the CL when the MEA swells in repeating humidity cycles. Moreover, the tensile stress is induced in the polymer electrolyte membrane (PEM) under deswelling and the CL cracks propagate into the PEM from the CL, which results in pinholes in the PEM.
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