Pt dissolution and further precipitation within the membrane of proton exchange membrane fuel cells (PEMFCs) was investigated at open-circuit voltage conditions. It was found that the location of Pt precipitation is affected by both normalH2 and normalO2 permeability through the membrane. A simple model is developed that can predict the location of the Pt precipitation band as a function of normalH2 and normalO2 partial pressure, which agrees well with measurements. The implications of Pt deposition on membrane chemical degradation are briefly discussed.

Postmortem thickness and morphology studies on a degraded membrane electrode assembly ͑MEA͒ caused by carbon corrosion under a local H 2 starvation operation in a proton exchange membrane ͑PEM͒ fuel cell were carried out using optical, scanning, and transmission electron microscopy. Samples used for the postmortem studies were selected and indexed with the aid of a limiting current density distribution map that was premeasured from the degraded MEA using an electrochemical diagnostic technique. An explicit correlation was found between the postmortem studies and the diagnostics: where the electrode structure damage ͑i.e., thickness reduction and porosity collapse͒ is significant, the limiting current density is low. The structure damage and current density drop were found to be in the H 2 -starved region where the carbon corrosion is severe. The results presented here are of significance in understanding the fundamentals of carbon corrosion mechanism and related structural origin of fuel cell performance degradation.

Recoverable voltage loss was observed for a PEM fuel cell due to membrane chemical degradation under open circuit voltage (OCV) conditions. The anion analysis of the fuel cell effluent water, collected during both the OCV hold and voltage recovery stages, indicates that sulfate release rate is much higher during recovery than that during the OCV hold. The surge in sulfate anion release occurs simultaneously with the recovery of fuel cell voltage. It was found that the reversible voltage loss, and concurrent sulfate release rate during voltage recovery, is lower for Ce-mitigated NRE211 membrane compared to as-received membrane. The recoverable voltage loss is proposed to be mainly due to the sulfate anion generated by membrane chemical degradation adsorbing onto the platinum catalyst surface.

Membrane failures at catalyst layer edges in proton exchange membrane fuel cell ͑PEMFC͒ membrane electrode assemblies ͑MEAs͒ were investigated using MEAs with segmented electrodes. A mathematical model was developed to predict the potential distribution at the edge of the MEA with misaligned electrodes. Control experiments were performed using an accelerated membrane durability test protocol and significant membrane degradation was observed in the region where the cathode overlaps the anode. The model-experiment comparisons suggest that a high cathode potential contributes to the membrane failure. A dependence of membrane degradation on relative humidity ͑RH͒ was observed in the experiments, regardless of the electrode overlap. The observed membrane degradation in the overlap region of MEAs with an anode catalyst overlap, run at low RH, is not explained by the model and needs further investigation.

Impact of reactant gas partial pressure on membrane chemical degradation is presented in this paper. The relationship between degradation rate and p H2 / p O2 is elucidated using fluoride release rate (FRR) as the assessment criterion. The effect of p H2 / p O2 on the location of the Pt redeposition band is clarified. Experimental results and model predictions are compared. The mechanism of FRR dependency on relative humidity (i.e., relative water partial pressure) is also examined by H 2 O 2 flow cell study.

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Recoverable voltage loss was observed for a PEM fuel cell due to membrane chemical degradation under open circuit voltage (OCV) conditions. The anion analysis of the fuel cell effluent water, collected during both the OCV hold and voltage recovery stages, indicates that sulfate release rate is much higher during recovery than that during the OCV hold. The surge in sulfate anion release occurs simultaneously with the recovery of fuel cell voltage. It was found that the reversible voltage loss, and concurrent sulfate release rate during voltage recovery, is lower for Ce-mitigated NRE211 membrane compared to as-received membrane. The recoverable voltage loss is proposed to be mainly due to the sulfate anion generated by membrane chemical degradation adsorbing onto the platinum catalyst surface. IntroductionIt has been well known that operating at an open circuit voltage (OCV) condition can accelerate polymer electrolyte membrane chemical degradation in PEM fuel cells (1-3). This degradation leads to thinning of the membrane, eventually forming pin holes as characterized by a significant increase of hydrogen and oxygen gas crossover.Membrane decomposition generates various compounds including fluoride, sulfate, and other low-molecular-weight organic acids (3-5). Using direct gas mass spectroscopy, Teranishi et al reported the detection of a series of fragments of sulfuric acid in addition to HF, H 2 O 2 , and CO 2 in the cathode outlet gas produced during OCV operation (3). No additional high molecular weight products were observed. In the water extracts of severely degraded fuel cell MEAs, Healy et al found the existence of perfluoro(3-oxapentane)-1-sulfonic-4-carboxylic diacid through F19 NMR and mass spectroscopy analysis, which is believed to be a degradation product from the side chain of the PFSA membrane (5). Kabasawa et al investigated the impact of potential membrane chemical degradation products on PEM fuel cell performance using model compounds. Of the several compounds evaluated, including sulfuric acid, perflurocarboxylic acids and perfluorosufonic acids, sulfuric acid poses the most poisoning effect on the cathode catalyst, leading to a large mass activity reduction and performance drop (4). The impact of these chemical degradation products on fuel cell performance at practical operating conditions has rarely been systematically studied (4, 6).Kundu et al observed that the open circuit voltage loss is partially recoverable due to unintentional interruption of the OCV tests (7). The authors did not provide further mechanistic analysis for this reversible voltage decay phenomenon. However, they highlighted the necessity of understanding the difference between reversible and irreversible voltage decay modes. Sugawara et al conducted a detailed analysis on the performance decay under an open circuit voltage condition (6). Using exhaust water analysis, the loss in performance is tentatively attributed to the contamination of the catalyst surface by membrane chemical degradation products, such as sulfate anio...

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