Scanning Force Microscopy Study of Phase Segregation in Fuel Cell Membrane Materials as a Function of Solvent Polarity a
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Scanning Force Microscopy Study of Phase Segregation in Fuel Cell Membrane Materials as a Function of Solvent Polarity and Relative Humidity M.E. Hawley1, Yu Seung Kim2, and R.P. Hjelm3 1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 2 Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3 Los Alamos Neutron Scattering Center, LANSCE-LC, Los Alamos National Laboratory, Los Alamos, NM 87545, USA ABSTRACT Scanning force microscopy (SFM) phase imaging provides a powerful method for directly studying and comparing phase segregation in fuel cell membrane materials due to different preparation and under different temperature and humidity exposures. In this work, we explored two parameters that can influence phase segregation: the properties of the solvents used in casting membrane films and how these solvents alter phase segregation after exposure to boiling water as a function of time. SFM was used under ambient conditions to image phase segregation in Nafion samples prepared using five different solvents. Samples were then subjected to water vapor maintained at 100°C for periods ranging from 30 minutes to three hours and re-imaged using the same phase imaging conditions. SFM shows what appears to be an increase in phase segregation as a function of solvent polarity that changes as a function of water exposure. INTRODUCTION Optimization of proton exchange membrane fuel cell (PEMFC) performance is an ongoing challenge. For solvent cast membranes, effects due to the solvent used to prepare the dispersion can have an influence on the properties that impact performance and durability for both the membrane and the electrodes that make up the fuel cell. Of particular interest in this study is whether the solvent used to prepare the dispersion and used to cast the membranes alters the phase segregation between the hydrophobic tetrafluoroethylene backbone and hydrophilic sulfonate terminated perfluorovinyl ether side chain groups in commercially available Nafion™ 212 (Figure 1). A recent investigation of solvent effects in solution cast Nafion focused on the solvent solubility parameter, δ, relative to that of the Nafion backbone and pendant groups [1]. The authors also focused on the dielectric properties of the solvent and their Figure 1. Schematic of Nafion structure with effect on the morphology and properties of the hydrophobic backbone (x and y repeat units) and membranes. The authors found significant side chain repeat unit with sulfonic acid group differences in both phase segregation and type termination. and degree of aggregation of the backbone between water-based and non water-based solvents. Since phase segregation between the ionomer pendant groups and the backbone plays an important role in the high ion exchange across the Nafion membrane, which is needed for high power output densities, solvent effects on phase segregation are extremely important. Solvent effects may also influence the interfacial
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