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Perfluorosulfonic acid (PFSA) ionomer membranes degrade under accelerated testing conditions such as open circuit voltage (OCV). Analyzing effluent water for fluoride ion release rate is a primary method for evaluating the membrane degradation rate. However, many proposed degradation reaction pathways should result in the release of small molecule polymer fragments. Liquid chromatography/mass spectrometry (LC/MS) methods are well suited to analyze for these fragments and provide insight into the degradation reactions. Accelerated OCV durability tests were conducted on membrane electrode assemblies made with 3M Ionomer or Nafion™ membranes. Effluent water was analyzed for fluoride, sulfate, and trifluoroacetic acid by ion chromatography (IC) and other polymer fragments by LC/MS. The results of this study imply significant degradation occurs at the tertiary fluoride on the polymer backbone or the ether oxygen connecting the side chain to the backbone. In Nafion™, these structural features also exist in the ionomer sidechain.

Commercial proton exchange membrane heavy-duty fuel cell vehicles will require a five-fold increase in durability compared to current state-of-the art light-duty fuel cell vehicles. We describe a new composite membrane that incorporates silicotungstic heteroply acid (HPA), α-K8SiW11O40▪13H2O, a radical decomposition catalyst and when acid-exchanged can potentially conduct protons. The HPA was covalently bound to a terpolymer of tetrafluoroethylene, vinylidene fluoride, and sulfonyl fluoride containing monomer (1,1,2,2,3,3,4,4-octafluoro-4-((1,2,2-trifluorovinyl)oxy)butane-1-sulfonyl fluoride) by dehydrofluorination followed by addition of diethyl (4-hydroxyphenyl) phosphonate, giving a perfluorosulfonic acid-vinylidene fluoride-heteropoly acid (PFSA-VDF-HPA). A composite membrane was fabricated using a blend of the PFSA-VDF-HPA and the 800EW 3M perfluoro sulfonic acid polymer. The bottom liner-side of the membrane tended to have a higher proportion of HPA moieties compared to the air-side as gravity caused the higher mass density PFSA-VDF-HPA to settle. The composite membrane was shown to have less swelling, more hydrophobic properties, and higher crystallinity than the pure PFSA membrane. The proton conductivity of the membrane was 0.130 ± 0.03 S cm−1 at 80 °C and 95% RH. Impressively, when the membrane with HPA-rich side was facing the anode, the membrane survived more than 800 h under accelerated stress test conditions of open-circuit voltage, 90 °C and 30% RH.

Unraveling ion conduction limitations in nanothin ionomer films is crucial for designing efficient ionomer–catalyst interfaces and improving redox efficiency in electrochemical devices. This work t...

There is a critical need for higher performing proton exchange membranes for electrochemical energy conversion devices that would enable higher temperature and drier operating conditions to be util...

Perfluorosulfonic acid ionomers (PFSAs) are the benchmark materials used as proton exchange membranes in fuel cells due to their exceptional proton conductivity and chemical and mechanical stability. Typical PFSAs consist of a PTFE backbone with perfluoroether sidechains bearing pendant sulfonic acid groups. PFSAs of different ion contents and sidechain compositions have been developed in an effort to optimize functionality while maintaining mechanical stability. One such ionomer is 3M’s perfluoroimide acid (PFIA) containing a sulfonimide acid group in addition to the sulfonic acid on each sidechain to increase proton conductivity while preserving TFE backbone segments to maintain sufficient crystallinity. In this study, we investigate the effect of this additional acidic site on the thermomechanical properties and morphology of PFIA compared to the single acid PFSA. Utilizing alkylammonium counterions we are able to systematically manipulate the strength of the physically crosslinked network in the perfluorinated ionomers to probe the origins of thermomechanical transitions. The addition of the sulfonimide in PFIA leads to interesting differences in the morphology and thermomechanical properties including the appearance of an anomalous glass transition that is not observed in single acid PFSAs. Techniques including dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and variable temperature small angle x-ray scattering (SAXS) are utilized to probe the anomalous glass transition and assign it to the true glass transition temperature for PFIA. Figure 1

Bromoalkyl-functionalized poly(olefin)s were synthesized by copolymerization of 4-(4-methylphenyl)-1-butene with 11-bromo-1-undecene using Ziegler–Natta polymerization. The resulting bromoalkyl-fun...

Open circuit voltage tests were conducted on sixteen 3M Ionomer and eight Nafion™ NR211 membranes. Lifetime distributions were determined, and ion chromatography (IC) techniques were used to measure fluoride, sulfate, trifluoro acetate, and oxalate ions. Combustion Ion Chromatography (CIC) was used to determine the total organic fluoride (TOF) associated with water soluble membrane degradation fragments. The ratio of these products relative to the fluoride release rates were used to infer the likely degradation mechanism for each membrane. Peroxide attack at the sulfonic acid side chain was determined to be the least relevant reaction pathway while the long-proposed polymer chain end unzipping appears to be the dominant mechanism. Abstraction of the tertiary fluorine in the NR211 backbone and side chain is evident by organic fluoride release rates higher than can be explained by unzipping alone.

Proton exchange membranes based on perfluorosulfonic acid (PFSA) ionomers are the industry standard for proton exchange membrane fuel cells. These ionomers offer remarkable proton conductivity and chemical stability, especially when fully hydrated with water. Performance, however, rapidly decreases as the membranes dry out, thereby requiring external humidification equipment. Because of this, there exists a need for new ionomers that can maintain high proton conductivity at low humidification levels. Perfluoro bissulfonimides have acid strengths similar to those of sulfonic acids and can be used to extend the polymer side chain with an additional protogenic group. These polymers have been designated PerfluoroSulfonImide Acid (PFIA) or PerfluoroIonene Chain Extended (PFICE) ionomers. Compared to PFSAs, these ionomers have improved conductivities under all humidity conditions, however, the benefit of these improvements are most significant under the driest conditions.

Perfluoroalkylsulfonic acid (PFSA) ionomer membranes degrade under accelerated testing conditions such as open circuit voltage (OCV). Fluoride release rate is commonly used for evaluating the membrane degradation rate; however, many proposed degradation mechanisms should result in the release of small molecule polymer fragments. Liquid chromatography/mass spectrometry (LC/MS) methods are well suited to analyze for these fragments and provide insight into the degradation reactions. Accelerated OCV durability tests were conducted on membrane electrode assemblies made with 3M Ionomer ™ or Nafion™ XL membranes. Effluent water was analyzed for fluoride, sulfate, and trifluoroacetic acid by ion chromatography (IC) and other polymer fragments by LC/MS. The detection of partially hydrogenated side chain fragments and long chain dicarboxylic acids suggest hydrogen atoms play a significant role in these reactions. The results of this study show the possibility that more than one reaction may occur at the tertiary fluoride on the polymer backbone. The presence of a tertiary fluoride on the backbone and side chain of the Nafion™ XL membranes allows for these reactions in more than one location on this polymer. Performance loss for the Nafion™ XL samples during these tests is consistent with adsorption of ionomer fragments on the catalyst surface.

Proton exchange membranes (PEMs) are considered as the most promising material for fuel cell automobile applications due to their high proton conductivity and power density. The radical attacks with peroxide formation cause lower durability, which is a major challenge to further commercialize fuel cell vehicles. Cerium and manganese cations and oxides have been implemented as radical scavenging agents to overcome the challenge from radical attacks. However, the additives tend to not stay on the initial location but move during the operation due to migration and diffusion. The migration results in cluster formation and eventually the additives may dissolve and leave the membrane system. Our previous work modifying commercial fluroelastomers with chemically bonded polyoxometalate resulted in a membrane with radical scavenging properties while inhibiting additive clustering. Advancement in PEM durability with additives has implications not only for the light-duty fuel cell vehicles but also for medium and heavy-duty vehicles applications, which would aid the widespread of commercialization of fuel cell vehicles. This work focuses on characterizing the blended perfluorinated sulfonic acid (PFSA) ionomer membrane with the chemically bound radical scavenger to enhance the durability of the membrane. Proton conductivity of the membrane was characterized under controlled relative humidity and temperature with electrochemical impedance spectroscopy (EIS). Ionic Exchange capacity of two or more proton conductive sites was measured using titration method. Correlation between water uptake and membrane properties is discussed and measuring using dynamic vapor sorption (DVS). Thermal stability was investigated through thermalgravimetric analysis (TGA).

Studies of proton transport in novel materials are important to enable a large array of electrochemical devices. In this study, we show that heteropoly acids (HPAs) when immobilized in polymer matrixes have highly mobile protons. Divinyl-11-silicotungstic acid, an HPA, was copolymerized with butyl acrylate and hexanediol diacrylate at various weight percentage loadings from 25% to 85% using UV initiated polymerizations. The resultant films were tan colored flexible sheets of ca. 120 μm thickness. The morphology of these films varied with loading, showing phase separation into clustered HPA above a 50 wt % loading and lamella morphologies above an 80 wt % loading. Water uptake was strongly associated with the HPA clusters, which facilitated transport of protons. This was realized by proton conductivities as high as 0.4 S cm–1 at 95 °C and 95% RH and 0.1 S cm–1 at 85 °C and 50% RH. Pulse field gradient spin echo NMR measurements indicated that water self-diffusion was fast (1.4 × 10–5 and 4.4 × 10–5 cm2 s–1...

An extensive SAXS investigation of the 3M perfluorinated sulfonic acid ionomer was performed to investigate the morphological changes that occur during and after annealing at temperatures above the Tα. The effect of film thickness in the range studied, 11–45 μm, was found to be negligible. These properties were studied as a function of equivalent weight from 700 to 1100 and correlated with the water uptake as measured by dynamic vapor sorption. Isoscattering points were observed in dynamic annealing experiments of the unboiled annealed films at q = 0.023, 0.096 A–1. On initial water uptake these films also showed isoscattering points at q = 0.024, 0.220 A–1; q = 0.029, 0.223 A–1; and q = 0.030, 0.211 A–1 at 50, 80, or 95 °C, respectively, indicating a decrease in the symmetry of the scattering objects in these size regimes. Isoscattering points were absent in similar water uptake experiment for the films after boiling.

During the pulsed-electron beam direct grafting of neat styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) substrate, the radiolytically-produced styryl and carbon-centered FEP radicals undergo various desired and undesired competing reactions. In this study, a high-dose rate is used to impede the undesired free radical homopolymerization of styrene and ensure uniform covalent grafting through 125-µm FEP films. This outweighs the enhancement of the undesired crosslinking reactions of carbon-centered FEP radicals and the dimerization of the styryl radicals. The degree of uniform grafting through 125- µm FEP films increases from ≈8%, immediately after pulsed electron irradiation to 33% with the subsequent thermal treatment exceeding the glass transition temperature of FEP of 39°C. On the contrary, steady-state radiolysis using (60)Co gamma radiolysis, shows that the undesired homopolymerization of the styrene has become the predominant reaction with a negligible degree of grafting. Time-resolved fast kinetic measurements on pulsed neat styrene show that the styryl radicals undergo fast decays via propagation homopolymerization and termination reactions at an observed reaction rate constant of 5 × 10(8) l mol(−1) s(−1). The proton conductivity of 25- µm film at 80°C is 0.29 ± 0.01 s cm(−1) and 0.007 s cm(−1) at relative humidity of 92% and 28%, respectively. The aims of this work are: 1. electrolyte membranes are prepared via grafting initiated by a pulsed electron beam; 2. postirradiation heat-treated membranes are uniformly grafted, ideal for industry; 3. High dose rate is the primary parameter to promote the desired reactions; 4. measurement of kinetics of undesired radiation-induced styrene homopolymerization; and 5. The conductivity of prepared membranes is on par or higher than industry standards.

We have fabricated proton conducting films using monomers based on vinyl substituted silicotungstic acid heteropoly acids (HPAs) and acrylate co-monomers. In this work we probe the limits of this system based on increasing the weight loading of the HPA to 85 wt%. Although impressive proton conductivities can be achieved with these films under hotter and drier operating conditions than in conventional proton exchange membranes, the materials have mechanical limitations. We show that very different film morphologies can be prepared based on whether or not the film is polymerized thermally or by UV light. In general the UV cured films have superior proton conductivity, but have a linear morphology resulting in a brittle film. The thermally cured films have a clustered morphology with good mechanical attributes but have poor proton conductivity.

Using SAXS data, the microstructure of the ionomer formed by copolymerization of tetrafluoroethylene and CF2═CFO(CF2)4SO3H films has been approached by two methods: a numerical method (the unified ...

Highly conducive to high conductivity: Polyoxometalates were incorporated in the backbone of a hydrocarbon polymer to produce proton-conducting films. These first-generation materials contain large, dispersed clusters of polyoxometalates. Although the morphology of these films is not yet optimal, they already demonstrate practical proton conductivities and proton diffusion within the clusters appears to be very high.

A perfluorosulfonic acid (PFSA) polymer with pendant side chain –O(CF 2 ) 4 SO 3 H was doped with the heteropoly acids (HPAs), H 3 PW 12 O 40 and H 4 SiW 12 O 40 . Infrared spectroscopy revealed a strong interaction between the HPA and the PFSA ionomer. Modes associated with the peripheral bonds of the HPA were shifted to lower wave numbers when doped into PFSA membranes. Small-angle X-ray scattering (SAXS) measurements showed the presence of large crystallites of HPA in the membrane with d spacings of ca. 10 A, close to the lattice spacing observed in bulk HPA crystals. Under wet conditions the HPA was more dispersed and constrained the size of the sulfonic acid clusters to 20 A at a 5 wt% HPA doping level, the same as in the vacuum treated ionomer samples. Under conditions of minimum hydration the HPA decreased the E a for the self-diffusion of water from 27 to 15 kJ mol −1 . The reverse trend was seen under 100% RH conditions. Proton conductivity measurements showed improved proton conductivity of the HPA doped PFSAs at a constant dew point of 80 °C for all temperatures up to 120 °C and at all relative hummidities up to 80%. The activation energy for proton conduction generally was lower than for the undoped materials at RH ≤80%. Significantly the E a was 1/2 that of the undoped material at RHs of 40 and 60%. A practical proton conductivity of 113 mS cm −1 was observed at 100 °C and 80% RH.

Recent work at 3M has focused on the development of solvent cast proton exchange membranes (PEM's) for use in PEM fuel cells. These new membranes are a perfluorinated sulfonic acids based on a low molecular weight perfluorinated monomer and they exhibit excellent mechanical properties and chemical stability and high ionic conductivity. The low molecular weight of the monomer allows membranes with equivalent weight as low as 800 g/mole to have good mechanical properties when hydrated. Stabilizing additives in these membranes have been shown to improve the oxidative stability in Fenton's tests. Physical property, conductivity and fuel cell tests have been performed. When incorporated into membrane electrode assemblies, these new membranes have provided excellent performance and a greater than 15-fold increase in durability under accelerated fuel cell test conditions, compared with similar commercial PEM's.

X-ray scattering is used to determine levels of crystallinity in both 3M® ionomer and Nafion®, both perfluorosulfonic acid (PFSA) based polymers. Weight loss and measured fluoride ion release both suggest breakdown of the polymer. X-ray scattering as well as mechanical measurements suggest that the crystalline domain size and spacing are unaffected by this degradation.

Proton exchange membranes were cast from three perfluorosulfonic acid (PFSA) ionomers, Nafion®, the 3M PFSA polymer, and the Dow membrane, with side chains -O-(CF2CF(CF3)- O-(CF2)2SO3H, -O-(CF2)4-SO3H, and -O-(CF2)2-SO3H respectively. The materials were heated above the Tg and the reorganization of the polymer morphology was studied in situ on the nanoscale by SAXS using synchrotron radiation. It was shown that both the 3M and the Dow membranes are much more crystalline and featured than the Nafion® membrane. The membranes were then hydrated at 80, 100, and 120ºC, with a humidified gas stream (80ºC dew point), and the kinetics of the morphological changes studied. In general the SAXS features increased in intensity as the ionomer peak grew in and swelled. Various heteropoly acid additives were also added to the membrane which was found to suppress the ionomer peak.

This paper presents an overview of the key requirements for the proton exchange membranes (PEM) used in fuel cell applications, along with a description of the membrane materials currently being used and their ability to meet these requirements. Also discussed are some of the new materials, technologies, and research directions being pursued to try to meet the demanding performance and durability needs of the PEM fuel cell industry.

The alkaline membrane fuel cell (AMFC) field continues to expand in interest. Membrane chemical stability is approaching the level required for commercial application (1), and membrane-electrode assembly (MEA) power output has reached and even surpassed 1 W/cm2 (2). In this presentation, work at NREL in the AMFC field regarding the synthesis of PFAEM and its implementation into MEAs will be summarized. For example, our ‘Generation 2’ PFAEM, based on sulfonamide-linked hexyl trimethyl ammonium, retains greater than 90% of its ion exchange capacity (IEC) after 14 days of exposure to 2 M KOH at 80°C. Well over 200g of this material has been synthesized and is readily processed into solutions (for membrane fabrication) and dispersions (for electrode ionomer use). MEAs employing PFAEM membranes and ionomers outperform commercial AEM materials from Tokuyama with equivalent catalyst loadings (see figure). Further improvements have been attained with MEAs comprising PFAEM membranes and advanced electrodes where ~1 W/cm2 peak power and >500 hr durability in H2/O2 at 60°C were demonstrated. Performance and durability across different electrode architectures and compositions were studied via impedance, RH dependence, and cyclic voltammetry. Comparisons will be presented along with insights gained into the relative contributions of electrodes and membranes to durability and performance of AMFCs. References 1. W.-H. Lee, Y. S. Kim and C. Bae, ACS Macro Lett., 4, 814 (2015). 2. L. Wang, E. Magliocca, E. L. Cunningham, W. E. Mustain, S. D. Poynton, R. Escudero-Cid, M. M. Nasef, J. Ponce-Gonzalez, R. Bance-Souahli, R. C. T. Slade, D. K. Whelligan and J. R. Varcoe, Green Chemistry, 19, 831 (2017). Figure 1. Comparison between all-PFAEM and all-Tokuyama AMFC MEAs. Cell conditions comprised H2/O2 gas flows at 0.2 slpm, 60°C, 121 kpa absolute, and 100RH. Catalysts were 46% Pt/HSC for each electrode and loadings for all were 0.4 mg/cm2. Figure 1

Original research was carried out at the CSM and the 3M Company from March 2007 through September 2011. The research was aimed at developing new to the world proton electrolyte materials for use in hydrogen fuel cells, in particular with high proton conductivity under hot and dry conditions (>100mS/cm at 120°C and 50%RH). Broadly stated, the research at 3M and between 3M and CSM that led to new materials took place in two phases: In the first phase, hydrocarbon membranes that could be formed by photopolymerization of monomer mixtures were developed for the purpose of determining the technical feasibility of achieving the program's Go/No-Go decision conductivity target of >100mS/cm at 120°C and 50%RH. In the second phase, attempts were made to extend the achieved conductivity level to fluorinated material systems with the expectation that durability and stability would be improved (over the hydrocarbon material). Highlights included: Multiple lots of an HPA-immobilized photocurable terpolymer derived from di-vinyl-silicotungstic acid (85%), n-butyl acrylate, and hexanediol diacrylate were prepared at 3M and characterized at 3M to exhibit an initial conductivity of 107mS/cm at 120°C and 47%RH (PolyPOM85v) using a Bekktech LLC sample fixture and TestEquity oven. Later independent testing by Bekktech LLC, using amore » different preheating protocol, on the same material, yielded a conductivity value of approximately 20mS/cm at 120°C and 50%RH. The difference in measured values is likely to have been the result of an instability of properties for the material or a difference in the measurement method. A dispersed catalyst fuel cell was fabricated and tested using a 150¼m thick HPA-based photocurable membrane (above, PolyPOM75v), exhibiting a current density of greater than 300mA/cm2 at 0.5V (H2/Air 800/1800sccm 70°C/75%RH ambient outlet pressure). Multiple lots of a co-polymer based on poly-trifluorovinylether (TFVE) derived HPA were synthesized and fabricated into films, Generation II films. These materials showed proton conductivities as high as 1 S/cm under high RH conditions. However, the materials suffered from compromised properties due to impure monomers and low molecular weights. Multiple lots of an HPA-immobilized fluoropolymer derived from preformed PVDF-HFP (Generation III films) were synthesized and formed into membranes at 3M and characterized at 3M to exhibit conductivity reaching approximately 75mS/cm at 120°C/40%RH using a Bekktech sample fixture and TestEquity oven (optimized membrane, at close of program). Initial fuel cell fabrication and testing for this new class of membrane yielded negative results (no measureable proton conductivity); however, the specific early membrane that was used for the two 5cm2 MEAs was later determined to have

A series of thermotropic polyethers synthesized from 1-(4-hydroxyphenyl) - 2 - (2 - methyl - 4 - hydroxyphenyl) - ethane and α,ω-dibromo-n-alkanes with odd numbers of methylene units (MBPE-n = odd) shows monotropic mesophase behavior. In isothermal differential scanning calorimetry (DSC) experiments, two—sometimes even three—exothermic transition processes can be observed when the crystallization temperature is below the mesophase transition temperature, while only one exothermic process is present above the mesophase transition temperature. The melting behavior of the crystals grown from the mesophase and from the isotropic melt states is different. The crystals grown from the mesophase state exhibit a larger overall heat of transition and a higher transition temperature compared with those grown from the isotropic melt. This may be attributed to the molecular interfacial connections between the crystal and amorphous regions when MBPEs crystallize from the mesophase state. The difference in morphology between the crystals grown from the different states has also been studied with polarized light microscopy (PLM) and transmission electronic microscopy (TEM). The structures of the crystals grown from the different states are, however, the same, as evidenced through wide-angle X-ray diffraction (WAXD) measurements. From the banded morphology of MBPE samples observed from PLM, the defect textures observed through TEM and the results of WAXD experiments, this mesophase can be identified as a nematic liquid crystal state.

Heat capacities of a series of thermotropic polyethers consisting of a semiflexible mesogen [1-(4-hydroxyphenyl) - 2 - (2 - methyl - 4 - hydroxyphenyl)ethane], with n methylene flexible spacers (n = 4–12) (MBPE-n) have been measured by DSC and fitted at low temperature to an approximate frequency spectrum, as well as at high temperature to a general equation for the liquid MBPE-n. The latter equation is: Cp = n(17.33 + 0.04551T) + (280.9 + 0.3839T) where n is the number of methylenes in the polyether spacer. The calculated vibration-only heat capacities start to show deviations from the measured heat capacities below the melting temperature, reflecting contributions from conformational disorder and motion in methylene spacers. It is suggested that part of this increase in heat capacity can be looked upon as a glass transition of the partially conformationally disordered crystals. Solid state 13C–NMR studies showed similarly that over the range of temperature some of the CC bonds in the spacer are in a rotational state similar to that in the melt. The equilibrium heats of fusion (ΔH) and the changes of heat capacity (ΔCp) for the amorphous polymer at the glass transition temperature, Tg, were determined by WAXD and DSC. Based on the discrepancy of ΔCp it is concluded that these phenylene containing polyethers have a certain amount of rigid amorphous polymer. Thermodynamic functions H, S and G for all of the polyethers have been established.

A series of poly(enamine ketone)s (PEAKs) which exhibit liquid crystalline phases has been synthesized. The enamine ketone groups are able to exist in the cis and trans conformations. Molecular modelling results show that the cis conformation is energetically more favorable due to intramolecular hydrogen bonding. Model compound studies indicate that the formation of the mesogen requires the cis conformation in the enamine ketones to stabilize the liquid crystalline phase. This liquid crystalline phase is nematic for the model compound and the polymers. The heats of liquid crystal transition in the model compound and in PEAKs are very close (4.7 kJ/mol in model compound and 4.0 kJ/mol of repeating units in polymers). This is not only an indication of a similar percentage of the cis conformation in both cases, but also an example that the flexible ethyleneoxy spacers in the polymers do not contribute to the orientational order of this liquid crystalline phase. PEAKs can crystallize through either quenching from the isotropic melt or from the liquid crystalline phase. Crystallization kinetics exhibit the effect of pre-ordering when these two different kinetics are compared.

Polymers [OPh(Me)CH 2 CH 2 PhO(CH 2 ) m ] n (m=4-12) (MBPE-n) are studied. A mesophase is observed for MBPE-n=odd polyethers on cooling from the isotropic melt and found to be monotropic. Crystallization will follow either on further cooling or on isothermal annealing. MBPE-N=even polyethers exhibit rapid crystallization during cooling, with little cooling-rate dependence. Contributions to the enthalpy and the entropy of mesophase transition from both the semiflexible mesogen and methylene units can be determined. The polymers with n=odd show that the flexible spacers mainly contribute to the thermal transitions properties

A thermotropic polyether (MBPE-5) which was synthesized by coupling of 1-(4-hydroxyphenyl)-2-(2-methyl-4-hydroxyphenyl)ethane α , ω -dibromopentane shows complicated phase transition behaviour. Our thermal analysis, polarized light microscopy and wide angle X-ray diffraction results lead to a recognition of a liquid crystalline phase in this polymer, and it is monotropic. In addition, crystals with two different metastabilities are identified. The more stable crystal grows from its melt in a high isothermal temperature region, characterized by a normal spherulitic texture with banding pattern. The less stable crystal develops from its parent liquid crystalline phase in a low temperature region. The schlieren texture is still kept after this crystallization. Isothermally, in an intermediate temperature region, mixed crystals, formed by both different stabilities, can be observed.

Proton exchange membrane fuel cells are expected to change the landscape of power generation over the next ten years. For this to be realized one of the most significant challenges to be met for stationary systems is lifetime, where 40,000 hours of operation with less than 10% decay is desired. This project conducted fundamental studies on the durability of membrane electrode assemblies (MEAs) and fuel cell stack systems with the expectation that knowledge gained from this project will be applied toward the design and manufacture of MEAs and stack systems to meet DOE’s 2010 stationary fuel cell stack systems targets. The focus of this project was PEM fuel cell durability – understanding the issues that limit MEA and fuel cell system lifetime, developing mitigation strategies to address the lifetime issues and demonstration of the effectiveness of the mitigation strategies by system testing. To that end, several discoveries were made that contributed to the fundamental understanding of MEA degradation mechanisms. (1) The classically held belief that membrane degradation is solely due to end-group “unzipping” is incorrect; there are other functional groups present in the ionomer that are susceptible to chemical attack. (2) The rate of membrane degradation can be greatly slowed or possibly eliminated through the use of additives that scavenge peroxide or peroxyl radicals. (3) Characterization of GDL using dry gases is incorrect due to the fact that fuel cells operate utilizing humidified gases. The proper characterization method involves using wet gas streams and measuring capillary pressure as demonstrated in this project. (4) Not all Platinum on carbon catalysts are created equally – the major factor impacting catalyst durability is the type of carbon used as the support. (5) System operating conditions have a significant impact of lifetime – the lifetime was increased by an order of magnitude by changing the load profile while all other variables remain the same. (6) Through the use of statistical lifetime analysis methods, it is possible to develop new MEAs with predicted durability approaching the DOE 2010 targets. (7) A segmented cell was developed that extend the resolution from ~ 40 to 121 segments for a 50cm2 active area single cell which allowed for more precise investigation of the local phenomena in a operating fuel cell. (8) The single cell concept was extended to a fuel size stack to allow the first of its kind monitoring and mapping of an operational fuel cell stack. An internal check used during this project involved evaluating the manufacturability of any new MEA component. If a more durable MEA component was developed in the lab, but could not be scaled-up to ‘high speed, high volume manufacturing’, then that component was not selected for the final MEA-fuel cell system demonstration. It is the intent of the team to commercialize new products developed under this project, but commercialization can not occur if the manufacture of said new components is difficult or if the price is significantly greater than existing products as to make the new components not cost competitive. Thus, the end result of this project is the creation of MEA and fuel cell system technology that is capable of meeting the DOEs 2010 target of 40,000 hours for stationary fuel cell systems (although this lifetime has not been demonstrated in laboratory or field testing yet) at a cost that is economically viable for the developing fuel cell industry. We have demonstrated over 2,000 hours of run time for the MEA and system developed under this project.

The existence of nonintegral folding chain (NIF) crystals in a series of poly(ethylene oxide) (PEO) fractions with different molecular weights ranging between 3000 to 23,000 and different end groups has been observed through time-resolved synchrotron small angle X-ray scattering (SAXS), different scanning calorimetry (DSC), and transmission electron microscopy (TEM) experiments. The integral folding chain (IF) crystals are formed through thickening and thinning processes during and/or after the NIF crystallization. From our experimental observations, it is found that with increasing molecular weight, the thickening and thinning processes are increasingly hampered. At sufficiently high molecular weights, NIF crystals may be permanently retained. Both the thermodynamic and kinetic reasons for the processes which transfer the NIF crystals to IF crystals are proposed. Of additional interest, the fold length of initial NIF crystals increases with crystallization temperature (or decreasing supercooling) as commonly observed in polymer lamellar crystals. Further understanding of the chain diffusional motion along the c-axis of the crystals can be illustrated through the end group effect in the fractions with the same molecular weights. An introduction of increasingly bulky end groups can severely block the thickening and thinning processes.