Membrane failure results from a complex interplay of mechanical stress and chemical attack; each have multiple origins. A picture of degradation is emerging where the polymer is attacked by hydroxyl-radicals generated on Pt particles within the membrane. We extend this picture to include several reactions of hydroxyl radicals with peroxide and the PFSA polymer. This extended picture allows us to explain three differences in behavior exhibited by membrane-degradation rates in Fenton's Reagent tests and in-cell tests regarding correlation of rate with end groups, spatial distribution of polymer loss and existence of induction periods. We postulate a 3-step degradation scenario: 1) hydroxyl radicals attack polymer end groups and side chains, 2) hydroxyl radicals are converted to peroxyl radicals which only attack polymer end groups, and 3) loss of polymer volume and membrane embrittlement leading to mechanical failure depending on the extent of degradation.

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.

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, 25μm (1mil) 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 ( ∼3to20nm 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.

Two important aspects for understanding membrane degradation in PEM fuel cells are Pt dissolution, migration and precipitation from the electrodes into the membrane and chemical attack of the membrane by hydroxyl radicals. Transmission electron microscopy was used to detect and quantify the distribution of Pt particles in Nafion membrane after accelerated cyclic testing. Quantum chemical calculations revealed that hydroxyl radicals that chemically degrade the membrane can be directly generated at low and high potentials as opposed to indirectly generated through peroxide intermediate.