Neutron Reflectometry Measurements of the Depth Profile of Water in a Fuel Cell Membrane.

Specular neutron reflectometry (NR) measurements have been performed on a prototypical polymer electrolyte membrane (PEM) for fuel cell application, e.g., Nafion. The samples were formed by spin coating onto a variety of substrates, and annealing in vacuum. The measurements are designed to reveal the water distribution across the thickness of the membrane material, as a function of relative humidity and for the case in which the film is in contact with a liquid water reservoir. Data from several samples indicates how the water profile for a given humidity changes with annealing temperature of the polymer film. Finally, we consider the application of phase-sensitive methods to eliminate potential ambiguity in the scattering length density profile of the membrane obtained from NR.