Mechanics of Persulfonated Polytetrafluorethylene Proton Exchange Membranes

Abstract

Fuel cells enable direct chemical to electrical conversion of fuel to electricity, providing an efficient and clean process. Proton Exchange Membrane Fuel Cells (PEMFC), in which protons from hydrogen or methane cross a membrane to react with oxygen producing electricity, are the preferred transportable fuel cell. Nafion, a phase separated perfluorosulfonated ionomer, is the current benchmark membrane but still exhibits limited lifetime due to stress encountered during constrained cyclic hygro-thermal loading. In previous work [1] the viscoplastic nature of Nafion under uniaxial extension was experimentally characterized. The experimental results were used to develop a three-dimensional constitutive model which was then implemented within a finite element analysis software package. These results will be briefly reviewed. Here, the model is used to simulate the rate dependence of the stress and strain evolution in Nafion under the loading typically encountered during fuel cell operation. This loading consists of hygro-thermal cycling within partially constrained boundary conditions defined by other components of the fuel cell. This information could be used to either change the startup/shutdown procedure for a fuel cell or to guide the procedures used for accelerated membrane lifetime testing.