Interfacial Structure of Nafion

While ion-conducting materials are primarily considered for their use as electrolytes (which depend upon their bulk properties, i.e. conductivity, crossover rejection, and material strength) their interfacial properties are becoming increasingly relevant for optimizing electrochemical devices.  Ionomers are often applied as binders, which coat fuel cell catalyst electrode particles with layers that are several nanometers thick.  Here confinement effects or the interfacial properties of the ionomer may directly influence the efficiency of the electrochemical device.  Furthermore various fillers are increasingly used to modify the properties of ionomers to improve the bulk electrolyte characteristics.  For example silica particles are added to Nafion to increase water retention especially at high temperatures in fuel cells and to reduce V crossover in flow batteries. 

Neutron reflectometry, NR, is capable of determining structure with sub-Angstrom precision in idealized planar interfaces.   Because of isotopic contrast variations it is particularly sensitive to certain elements such as H (and thus water) as well as Li, V, and numerous others.  Herein, NR has been applied to investigate finite-size effects on the lamellar ordering of water domains in Nafion at interfaces with hydrophilic materials.  Moreover, contrast variation was employed to quantify the segregation of the sulfonate groups from the fluorocarbon chains in Nafion.

These results confirm the lamellar structure previously observed on hydrophilic substrates, and demonstrate that for hydrated films they can accurately be described as layers rich in both water and sulfonate groups, alternating with water-poor layers containing an excess of the fluorocarbon backbone. The thickness of these layers increases slightly and the amplitude of the oscillation that defines the water volume fraction exponentially decreases away from the hydrophilic interface.  A partial monolayer of sulfonate groups is determined to be bonded to the SiO₂, accompanied by a large excess of water (compared to that expected by the water-to-sulfonate ratio, λ, of the remainder of the film). Ultrathin (6nm) Nafion coatings that were limited in extent enough to truncate this lamellar region showed a depth profile nearly identical to thicker films, indicating that there are no confinement or surface effects altering the structure.  For films dehydrated for 2 hours at 60 °C, the composition oscillations are decreased in amplitude and extent from the interface.  Comparing the SLD profile measured for dried films to modeled composition profiles derived by removing water from the hydrated lamellae suggests incomplete remixing of the polymer groups upon dehydration, indicated either limited polymer mobility or retained equilibrium structures at in the Nafion/SiO₂ interface.