Finite Thickness Effects on Nafion Properties in the PEMFC Catalyst Layer, Probed By in Situ Neutron Reflectometry
Through the application of multiple NR measurements, elemental conservation, and Bayesian analysis, results presented will document varying water sorption and interfacial structures in thin-film Nafion for varying thickness, substrates, and humidity levels. Results provide an unambiguous confirmation of the reduced water sorption with reducing thickness below 60 nm, a detailed analysis of the sheet-like lamellar phase segregation on SiO2 substrates, as well as an illustration of how substrate modification can affect Nafion properties. Figure 1 shows the variation in water sorption with varying film thickness. As previously observed, for layers that approximate the phase-segregation of bulk Nafion films (“bulk-like”) water uptake decreases steadily for film thicknesses below 60 nm. However, within multi-lamellar polymer structures found at hydrophillic substrate interfaces, the water content reaches a minimum for films roughly 20 nm thick, and increases for films thinner than 15 nm. Furthermore, the average water content within the multi-lamellar structures is slightly higher than in the bulk-like outer layers. Simple numerical simulation results will be presented to discuss the implications of Nafion structural and composition variations in the CL for PEMFC performance.