THE EFFECT OF FILM THICKNESS ON WATER UPTAKE AND INTERFACIAL HYDRATION IN ULTRATHIN POLYELECTROLYTE MEMBRANES

 

Steven C. DeCaluwe, Joseph A. Dura, Andrew M. Baker, Pavan Bhargava, and Charles F. Majkrzak

 

One key challenge that currently delays the commercialization of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) as a cost-effective and reliable technology for a range of applications is the management of membrane hydration across a variety of working conditions.  While much effort has focused on the bulk properties of Nafion membranes, these bulk properties are likely less relevant to the behavior within the catalyst layers of membrane-electrode assemblies (MEAs).  Here, Nafion behavior is likely dominated by interactions at key interfaces, such as with Pt catalysts, carbon supports, and gas-phase reactants. Characterizing and controlling the behavior of Nafion in these interfacial regions is thus necessary for efficient, reliable PEMFC operation.

Here, we use Neutron Reflectometry (NR) to study hydration in Nafion films with varying thickness. NR is inherently sensitive to water structures in Nafion, and the thin-film geometry approximates the morphology of Nafion in PEMFC MEAs.  Results from our group have identified multi-lamellar structures at the interface with SiO2 surfaces. 

This poster will describe the total water uptake and changes in lamellae for film thicknesses ranging from 6 nm to 220 nm, at 30 °C with a relative humidity of 90 %.  Results show three different regions of behavior as a function of film thickness, with only lamellae seen below 10 nm, an outer bulk-like Nafion layer appearing between 10 nm and 50 nm, and a sharp increase in bulk layer hydration for films thicker than 50 nm.  The implications of these results for fuel cell technology will be presented, both in terms of fabrication and control strategies, and for the formulation of accurate simulations of Nafion in operating PEMFC MEAs, where the behavior of the polyelectrolyte is strongly influenced by interfacial phenomena.