David A. Condit

Degradation of Polymer-Electrolyte Membranes in Fuel Cells I. Experimental

cRussian Academy of Sciences, Moscow, Russia In this work, chemical degradation is studied using highly controlled measurements of the fluoride ion release from subscale cells in degrading environments using perfluorosulfonic-acid-based membrane electrode assemblies, primarily with cast, 25 m 1m il thick membranes. Effects of key variables, such as oxygen concentration, relative humidity RH, temperature, and membrane thickness on the fluoride ion emission rate FER are described under open-circuit decay conditions. Some of the observed trends are expected or consistent with previous observations, such as decreasing FER with decreasing temperature and increasing RH. Other trends observed are not expected, such as a logarithmic decrease of FER with oxygen concentration and increasing FER with increasing membrane thickness. Cross-sectional transmission electron microscopy analysis of decayed membranes indicates a surprisingly homogeneous distribution of small Pt particles 3 to 20 nm in diameter, presumably from dissolution and migration from the cathode. The experimental results are consistent with radical generation at these Pt particles from crossover hydrogen and oxygen, subsequent radical migration, and polymer attack. The response of the FER to new experimental conditions in this study suggests that the attack can exist at any plane within the membrane, not just the “Xo” plane of maximum Pt precipitation.

Degradation of Polymer-Electrolyte Membranes in Fuel Cells II. Theoretical model

A physics-based theoretical model that predicts the chemical degradation of the perfluorosulfonic acid polymer electrolyte membrane during fuel cell operation is developed. The model includes the transport and reaction of crossover gases, hydrogen and oxygen, to produce radicals in the membrane that subsequently react with the polymer to release hydrogen fluoride. The model assumes that a uniform distribution of nanometer-sized platinum deposits in the membrane (as a model input) originating from cathode dissolution provides the sites for radical generation. The degradation rate, measured by the release of hydrogen fluoride, depends on the net radical generation sites in the membrane, the concentration of the crossover gases, the hydration level of the membrane, the operating temperature, the operating voltage, and the thickness of the membrane. The model-predicted trends agree well with those reported and with our experimental results reported in the first article of this series by Madden et al. [J. Electrochem. Soc., 156, B657 (2009)]. Furthermore, the model provides insight to the factors that affect radical generation vs radical quenching, which aids in explaining the experimentally observed nonlinear trends of fluoride emission with reactant concentration and membrane thickness.

Mechanical Response of 3-Layered MEA During RH and Temperature Variation Based on Mechanical Properties Measured Under Controlled T and RH

Mechanical fracture of Nafion® membrane limits the life of PEM FC stacks. This is likely a result of gradual strength degradation and mechanical stress/strain transients induced by the cycling relative humidity (RH). Mechanical properties of Nafion® membrane strongly depend on water content. The objectives of the authors’ work are (1) to understand the fundamental mechanical behavior of an ionomer membrane, i.e., Nafion® , as a function of RH and (2) to develop physically meaningful models to perform stress/strain analysis of membrane electrode assemblies under RH and temperature variations. To characterize the mechanical response of an ionomer as a function of temperature and relative humidity, an environment chamber capable of generating temperatures from 25 to 100 degrees Centigrade and relative humidities from 5 to 85 percent was designed and built. An electromechanical membrane test (load) frame was mounted as an integral part of the system. An optical strain measurement device was used to record axial extension and lateral contraction of the membrane specimens without contact. Extensive mechanical tests on a commercial ionomer membrane were conducted under carefully controlled hydration and temperature. Fully nonlinear, fully anisotropic elasto-plastic constitutive representation of this ionomer material was obtained as function of temperature and RH. Water content significantly affects the elastic modulus of the membranes. Experimental data show that the elastic modulus of the membrane continuously increases up to about twice the original value during dry out. Such has been taken into account in order to accurately model the stress/strain history of the membrane during dry-out. The collected experiment data were represented in material constitutive models for use in a finite element code, ABAQUS. A 3-layer membrane electrode assembly (MEA) structure has been modeled to observe stress/strain distribution during RH and T cycling. Non-uniform electrode/membrane interfaces have been modeled as well as uniform sections to see the effects of geometric irregularities on the extreme values of stress and strain.Copyright