Finite Thickness Effects on Nafion Properties in the PEMFC Catalyst Layer, Probed By in Situ Neutron Reflectometry

Abstract

A major technical challenge impeding the optimization of polymer electrolyte membrane fuel cells (PEMFCs) is a lack of understanding regarding Nafion properties inside the fuel cell catalyst layer (CL). Inside the CL, thin Nafion layers (typically on the order of 2 – 80 nm thick) cover solid electrode particles and agglomerates. In this thickness range, dimensional confinement and interfacial effects at solid, liquid, and vapor phases alter the properties of the films. While recent studies have explored water content and transport properties in this thickness range to demonstrate reduced water mobility for films below roughly 60 nm in thickness, the effects of thickness and substrate variation on water uptake and Nafion structure in this range are still debated. Thus far, an unambiguous documentation of Nafion composition and structure for thicknesses typical of the PEMFC CL has not been presented. We present here the use of neutron reflectivity (NR) as an in situ probe of structure and composition for Nafion films with thicknesses of 5 – 200 nm. NR measurements provide simultaneous thickness and composition information for thin-films under controlled conditions, via an SLD depth profile with sub-Ångström resolution for structural elements thicker than roughly 1.5 nm. Neutrons transmit easily through sample chamber walls and have an inherent sensitivity to hydrogen isotopes, with a large SLD variation between H and D isotopes, making NR ideal for in situ measurements of thin-film Nafion hydration. 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. Figure 1