The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long-term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain-to-failure of the chemically aged and RH-cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge-constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed.

Ethylenediaminetetraacetic acid (EDTA), a common industrial agent for complexing metal ions in water, frequently inhibits conventional metals-removal technologies used in water treatment. This study investigated the use of TiO2 photocatalysis for the aqueous-phase oxidation of EDTA and several metal complexes of EDTA. Reactions were performed at 0.1 wt % loading of Degussa P-25 TiO2, a solute concentration of 0.8 mM and at a constant pH. The different metal−EDTA complexes exhibited widely different photocatalytic oxidation rates under equivalent conditions of pH = 4 ± 0.1 in an aerobic system:  Cu(II)−EDTA > Pb(II)−EDTA >> EDTA > Ni(II)−EDTA ≈ Cd(II)−EDTA ≈ Zn(II)−EDTA >>> Cr(III)−EDTA. Photoefficiency based on the Cu(II)−EDTA initial rate is nearly 60%. The rates of total organic carbon (TOC) removal and formaldehyde generation during photocatalytic EDTA oxidation indicate similarities to electrochemical oxidations of EDTA. Several means were explored to enhance the oxidation of Ni(II)−EDTA, whose behavior was taken to represent that of the slowly oxidizing complexes. Continuous addition of H2O2 solution during the photocatalytic treatment increased the oxidation rate for Ni(II)−EDTA as did the presence of homogeneous Cu2+. The presence of Cu2+ led to rapid ligand exchange transforming the Ni(II)−EDTA into Cu(II)−EDTA, which is easily oxidized.

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.

Development of fuel-cell catalysts for elevated temperature applications requires electrochemical techniques that simulate these conditions. A pulsed reactant electrochemical flow cell (PREFC) technique has been developed, capable of independent half-cell measurements of methanol electro-oxidation current and poisoning adsorbate charge on Pt/C-Nafion catalyst layers over a wide range of temperatures (25-100°C) and pressures while maintaining potential control at all times. The cell was fabricated from Pyrex and silicon using standard microfabrication techniques and exhibits adequate corrosion resistance to the sulfuric acid electrolyte for accurate electrochemical measurements up to 100°C. Thin Pt/C-Nafion catalyst layers of 50­100 μnormalg Pt/cm2 were prepared at roughly 100% catalyst utilization, as determined independently by in situ hydrogen voltammetry and ex situ transmission electron microscopy analysis. Increasing the temperature from 50 to 100°C at 0.35 VnormalPdH resulted in a substantial increase in methanol electro-oxidation rates with an activation energy of 70 kJ/mol. This kinetic enhancement is not due to thermal desorption of the poisoning adlayer, as determined by experiments showing the adlayer to be stable in flowing electrolyte at 95°C for durations up to 15 min. These results also show that the technique is able to perform the methanol oxidation and poisoning adlayer determinations without any significant effects of oxygen or other homogeneous contamination. © 2002 The Electrochemical Society. All rights reserved.