Characterization of Catalyst Layer Ionomer Degradation in PEM Fuel Cells

The catalyst layer ionomer unavoidably gets involved in other components degradation processes since it is subjected to exposure to different operating effects, including the presence of the catalyst, catalyst support, and the porous nature of the electrode layer which includes 2-phase flow. PEMFC durability issues cannot be fully resolved without understanding the contribution of ionomer degradation in electrode to the performance decay in life time. However, catalyst layer ionomer is essentially chemically identical to the membrane ionomeric material, and is composed of low atomic number elements, making characterization difficult. In the present work, MEAs with different Nafion ionomer types: stabilized (S) and non-stablized (NS) ionomer in the electrode layer (Type I) and mixed membrane / ionomer MEAs (Type II) were designed to separate ionomer degradation from membrane degradation respectively. Stabilized and non stabilized ionomers were 5% Nafion ®ҏ solutions. The non-stabilized version is the typical Nafion chemical structure with carboxylic acid (-COOH) end groups; these end groups are thought to be a susceptible point of degradative peroxide attack Type I MEA fabrication follows the same procedure described as Ref Commercial Nafion ® 212 membrane was applied in Type I MEAs initially. The MEAs failed fast (< 200h) during the AST mainly due to membrane degradation. The short life time of the AST test provided limited time resolved ECSA loss data, with limited impedance characterization due to membrane thinning, which makes it difficult to unambiguously conclude much about the different ionomers' effect on the degradation process. Similar catalyst layer degradation analysis by XPS as Zhang et al The ratio of CF x /QRQ-fluorinated C decreased from 0.59 to 0.52 at the surface of NS electrodes, while that of S electrodes decreased from 0.53 to 0.49. The ratio of CF x to total carbon decreased from 0.373 to 0.345 and 0.346 to 0.331 for NS and S electrodes respectively. The higher decrease in the NS electrodes probably was due to the longer life time (~240h) compared to that of S electrodes (~192 h). The biggest change was observed for the ratio of IOXRULQDWHG &JUDSKLF LQ 16 HOHFWURGHV IURP WR 3.25, ~ 42% decrease, while the drop of S electrodes was from 3.88 to 3.63, only ~7%. This difference is not consistent with the testing time and longer durability testing may be required for better understanding of this difference. For a longer membrane life time during AST testing to obtain better time resolved analysis, MEAs were made using stabilized Nafion ® XL 100 membrane supplied by Ion Power. The MEA durability during the OCV AST test was extended over 400hr

Synthesis of novel fluoropolymers

grantor: University of TorontoFluoropolymers; have high chemical and thermal stabilities, making them desirable for a number of applications, yet limit their broader applicability. To enhance processability and solubility in organic solvents, new fluoropolymers incorporating partial hydrocarbon functionality were synthesized. In one approach, three new trifluorovinyl ether monomers (TFVEs), having hydrocarbon pendant groups were polymerized: 1-[2-(2-ethoxy-ethoxy)-ethoxy]-1,2,2-trifluoroethene (Et-TFVE), 1-[2-(2-'tert'-butoxy-ethoxy)-ethoxy]-1,2,2-trifluoroethene ('tert'-Bu-TFVE) and 1-(2-phenoxy-ethoxy)-1,2,2-trifluoroethene (Ph-TFVE). The TFVEs were homo- and copolymerized with ethyl vinyl ether (EVE) and vinyl acetate (VAc) by redox-initiated aqueous emulsion polymerization. Et-TFVE was also homo- and copolymerized with tetrafluoroethylene (TFE) in carbon dioxide (CO2) As a result of TFVE structure and low reactivity, relatively low molar mass homopolymers; and TFE copolymers resulted (on the order of 10,000 g mol -1). The polymerization mechanism was complicated because of ß-scission termination/chain transfer for all TFVE homopolymers and copolymers with TFE. incorporating Et-TFVE and tert-Bu-TFVE but not poly(Ph-TFVE). Interestingly, from the pendant group adjacent to the propagating macro-radical. TFVE copolymerization with EVE and VAc resulted in significantly higher molar mass polymers (typically >=100,000-g mol-1) because more significant for copolymers having a greater fraction of (Et-TFVE), in the monomer feed. Reactivity ratios were estimated for a series of bulk copolymerizations of Ph-TFVE with EVE or VAc and confirmed azeotropic copolymerization. In a second approach to processable fluoropolymers, TFE and VAc were copolymerized in supercritical CO2. High molar mass poly(TFE-co-VAc)s incorporating up to 71 mo1% TFE were synthesized and were organic solvent soluble. Previous syntheses in aqueous emulsions resulted in a branched structure of the resulting macro-radical. Consequently, at least a 10-fold decrease in molar mass was observed following hydrolysis. Only a small decrease in molar mass was observed after hydrolysis for poly(TFE-'co'-VAc)s in carbon dioxide relative to propagation, thereby yielding predominantly linear polymers.Ph.D

Synthesis of trifluorovinyl ethers

grantor: University of TorontoNovel trifluorovinyl ethers (TFVEs, ROCF=CF\sb2), where R is a functionalized oligo-ether, were synthesized by two methods. In the first method, thermolysis of trimethylsilyl 2-alkoxy-2,3,3,3-tetrafluoropropionate esters in the gas phase yielded TFVEs. Variable temperature \sp{19}F NMR spectroscopy of trimethylsilyl 2-alkoxy-2,3,3,3-tetrafluoropropionates suggests that these esters exist in an equilibrium between two structural conformations. In one conformation, there is intramolecular "interaction" of silicon with fluorine. The second is a random conformation. To some degree, this interaction may influence the outcome of the thermolysis reaction which is ultimately successful because of favorable reaction kinetics. In the second method, the TFVEs were synthesized by the reaction of a sodium alkoxide with tetrafluoroethylene (TFE). For alkoxides of limited solubility, the observed rate of reaction was determined to be rate-limited by the forward and reverse rates of solubility and the rate of reaction with TFE. Addition of 18-crown-6 was shown to significantly increase the rate of reaction.M.Sc

Photopolymerized Low-Surface-Energy Coatings Based on a Novel Fluorinated Ether Acrylate

The synthesis, characterization, and polymerization of a perfluoroalkyl ether substituted methacrylic acid (C8F7) were investigated. C8F7 was photopolymerized at different temperatures, higher double-bond conversions being achieved at higher polymerization temperatures. The polymerization rates were fast in comparison with those of typical methacrylate esters. Thermogravimetric analysis of the obtained polymers showed thermal stabilities up to 270–290°C. The initial degradation at 200°C involved the loss of water and the partial loss of the perfluoroalcohol via the intramolecular formation of anhydride and lactone groups. The surface properties of coatings obtained with C8F7 coatings on various substrates were evaluated with water and hexadecane contact-angle measurements, which confirmed that low-surface-energy polymeric coatings were obtained. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3301–3314, 200