Fuel cells employing a polymer electrolyte membrane (PEM) such as Nafion show promise for a wide range of applications for electrical power production, particularly in the transportation sector. In hydrated Nafion, the hydrophobic fluorocarbon polymer backbone will phase separate from the water, with the hydrophilic sulfonic acid side chains residing at the phase boundary. The water/Nafion structures that result are of critical interest since ionic conduction occurs by proton transport along the sulfonic acid functional groups. Modifications of these structures at the interface between the Nafion and the catalyst / electrodes may play a key role in issues related to proton conductivity, gas diffusion, catalysis, and other phenomena that occur in the three-phase regions where the PEM, the catalyst/electrode, and gases (i.e., water vapor, fuel, and oxidizer) interact. Furthermore, certain modes of device failure, e.g. delamination, dissolution of the catalyst, irreversible changes to the reaction layer, and membrane degradation, can nucleate at the interface. To date, few studies have been able to target the interfacial structures in PEM fuel cells.
Recently, we have discovered that at the interface with the native oxide on Si, the water and Nafion nanophase segregate into lamellar regions extending over ~6nm from the substrate1. A similar lamellar structure, if present in fuel cells, could influence fuel cell efficiency and durability. On the other hand, even if lamellae do not occur in materials typically found in fuel cells, further knowledge of their properties is still of practical benefit since SiO2 nanoparticles can be added to Nafion to trap water in these membranes, improving their hydration even at elevated temperatures, or to fabricate water channels perpendicular to the membrane, thus increasing conductivity.
I will describe recent experiments undertaken to further elucidate the origins of this lamellar phase. Neutron Reflectometry with in-situ control of relative humidity has been used to characterize the composition depth profile of thin films of Nafion spin-coated onto a variety of surfaces. Studies have been performed that vary the surface hydrophobicity and chemical character of the substrate. In addition, we have examined the evolution of the lamellar structures with Nafion thickness.
1 Dura, J. A., Murthi, V. S., Hartman, M., Satija, S. K. & Majkrzak, C. F. Macromolecules 42, 4769 (2009).