Ionomer Dispersions Research Articles

Ionomer Aggregation in Dispersions: Revealing the Role of Solvent By a Fully Atomistic MD Study

Ionomer dispersions are a key ingredient for fabrication of fuel cell catalyst layers. The structural characteristics of ionomer (e.g. Nafion) in dispersions and thereby in the colloidal catalyst ink (comprising Pt/C catalyst and ionomer in a solvent/media) has been speculated to play an influential role in the structure, property and performance of CLs made thereof [1-5]. How or why the chains of the sulphonic acid containing perfluorinated ionomers interact with each other in organic solvents or water or water-alcohol mixtures to form aggregates is not understood. The shape and size of aggregates are still under debate [6]. Furthermore, the commonly deployed x-ray, neutron and light scattering techniques for characterization of ionomer dispersion are limited in their capabilities to shed light on the internal structure of the aggregates, i.e. chain entanglement and coiling, solvent induced swelling, as well as the clustering or dissociation of sulphonic acid. Our all-atom Molecular Dynamics (MD) study aims to reveal the role of solvents on the aforementioned aggregation behavior and aggregate characteristics. Nafion ionomer in common lab solvents such as toluene, IPA, ethanol, glycerol, formic acid, formamide and water was simulated. The results reveal that depending on the solvent ionomers aggregate either due to ionic interactions between protonated sulphonate ion pairs (Figure 1a) or due to hydrophobic effects (Figure 1b). Interesting structure is observed for mixture of alcohol and water (Figure 1c), consistent with the structure proposed by Los Alamos researchers (Welch et al)[6]. The presentation will discuss the results in further details including aggregate shape/size, chain conformation, ionic cluster shape/size, and acid deprotonation for various solvents. A new aggregation phase diagram will be introduced.

Understanding Polymer/Particle Interactions in Fuel-Cell Inks Using Model Systems

The formation process of hydrogen fuel-cell catalyst layers (CLs) is poorly understood, and represents a major area of effort with regard to increasing overall cell efficiency and durability. CLs are made from precursor inks that are dispersions of catalyst nanoparticles, ion-conducting polymer, and solvent (typically water and propanol mixtures). The polymer is generally a perfluorinated sulfonic-acid polymer, the prototypical example of which is Nafion. The duality between the strongly acidic sidechains and hydrophobic backbone of the polymer cause it to adopt unique conformations in solution, which are a strong function of the amount of water present in the ink.1 Indeed, tuning the ratio of water to propanol has recently been demonstrated to change the effective pH of these polymer solutions, possibly by changing the amount of exposed sidechains.2 The complex interplay between particle/solvent/polymer interactions govern CL ink and layer properties.3-4 However, much remains unknown about the fundamental forces controlling these interactions. In this study, we explore the polymer/particle interaction, and how it is affected by solvent quality. To do so, we use quartz crystal microbalance with dissipation (QCM-D) with model functionalized substrates that probe specific relevant forces (electrostatic, hydrophobic, etc.) in order to elucidate interactions present on the comparably more-complex carbon-platinum particle surface. Different ionomer dispersions in a variety of solvents are studied to understand ionomer adsorption and desorption behavior. QCM-D data is coupled with calorimetry measurements to extract thermodynamic binding information. Results reveal that hydrophobic interactions are a major driving force for ionomer adsorption. Acknowledgements This study was mainly funded under the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02- 05CH11231. S.B. also acknowledges support from the National Science Foundation Graduate Research Fellowship under grant number DGE 1752814. References Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S., ACS Macro. Lett. 2012, 1 (12), 1403-1407.Berlinger, S. A.; McCloskey, B. D.; Weber, A. Z., J. Phys. Chem. B. 2018, 122 (31), 7790-7796.Dixit, M. B.; Harkey, B. A.; Shen, F.; Hatzell, K. B., J. Electrochem. Soc. 2018, 165 (5), F264-F271.Hatzell, K. B.; Dixit, M. B.; Berlinger, S. A.; Weber, A. Z., J. Mater. Chem. A 2017, 5 (39), 20527-20533.

(Invited) High Performance Commercial Nafion(TM) PFSA Materials for Fuel Cells and Water Electrolyzers

Perfluorosulfonic acid (PFSA) materials are perhaps the key enabling component for many polymer electrolyte membrane devices such as fuel cells and water electrolyzers. PFSA materials are by far the most proven class of polymer in this field, and are therefore uniquely capable to support the expected commercial expansion over the next few years. PFSA suppliers will be relied upon to deliver quality products at scale to the marketplace. At the same time, increasing performance demands across the industry require novel materials research and product optimizations to support diverse OEM growth plans. Chemours™, a recent spinoff from the DuPont® corporation in 2015, assumes all expertise and production capacity associated with the Nafion™ brand of PFSA materials. Chemours™, leveraging this historical PFSA leadership position, is excellently positioned to support electrochemical device proliferation. Significant R&D investments are being made to improve product offerings in membranes and ionomer dispersions to support the hydrogen economy. This presentation will summarize Chemours™’ recent efforts and future product direction for PEM fuel cell and water electrolyzers.

In-Situ Electrochemical Techniques to Determine Ionomer Coverage in PEFC Electrodes

Many electrochemical devices such as low temperature fuels utilize ionomers to improve efficiency and overall performance. In polymer electrolyte fuel cells (PEFCs), perflurosulfonic acid ionomers improve ionic (protonic) conductivity and Pt utilization within the catalyst layer. However, the strong interaction between sulfonate (-SO3 -) functional groups and Pt nanoparticles have been linked lower catalytic rates and higher O2 transport resistances.1,2 There have been numerous strategies implemented to control the local distribution of ionomer and Pt/C catalyst particles, but limitations in the resolution of state-of-the-art characterization techniques make it difficult to assess how ink formulations and/or electrode fabrication techniques control ionomer-catalyst interactions and its impact device performance.3–5 Herein, the development and utilization of in situ electrochemical techniques designed to probe the local ionomer interactions will be utilized in conjunction with established testing protocols to relate ionomer coverage (on Pt and/or C support) with kinetic performance and O2 transport resistance of fully-conditioned PEFC electrodes. Specifically, CO displacement chronoamperometry and electrochemical impedance spectroscopy techniques will be described to determine local ionomer coverages across a series of Pt/C MEAs. These results will promote a deeper understanding of ionomer-catalyst interactions which in turn will inform future materials development and integration necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) electrocatalyst-based electrodes. Schuler, T. et al. Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements. J. Electrochem. Soc. 166, F3020–F3031 (2019).Subbaraman, R., Strmcnik, D., Paulikas, A. P., Stamenkovic, V. R. & Markovic, N. M. Oxygen Reduction Reaction at Three-Phase Interfaces. ChemPhysChem 11, 2825–2833 (2010).Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation. J. Phys. Chem. B 122, 7790–7796 (2018).Orfanidi, A., Rheinländer, P. J., Schulte, N. & Gasteiger, H. A. Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. J. Electrochem. Soc. (2018). doi:10.1149/2.1251814jesKhandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039

Effect of Dispersion Solvents in Catalyst Inks on the Performance and Durability of Catalyst Layers in Proton Exchange Membrane Fuel Cells

Five different ionomer dispersions using water–isopropanol (IPA) and N-methylpyrrolidone (NMP) were investigated as ionomer binders for catalyst layers in proton exchange membrane fuel cells. The distribution of ionomer plays an important role in the design of high-performance porous electrode catalyst layers since the transport of species, such as oxygen and protons, is controlled by the thickness of the ionomer on the catalyst surface and the continuity of the ionomer and gas networks in the catalyst layer, with the transport of electrons being related to the continuity of the carbon particle network. In this study, the effect of solvents in ionomer dispersions on the performance and durability of catalyst layers (CLs) is investigated. Five different types of catalyst inks were used: (i) ionomer dispersed in NMP; (ii) ionomer dispersed in water–IPA; (iii) ionomer dispersed in NMP, followed by adding water–IPA; (iv) ionomer dispersed in water–IPA, followed by adding NMP; and (v) a mixture of ionomer dispersed in NMP and ionomer dispersed in water–IPA. Dynamic light scattering of the five dispersions showed different average particles sizes: ~0.40 μm for NMP, 0.91–1.75 μm for the mixture, and ~2.02 μm for water–IPA. The membrane-electrode assembly prepared from an ionomer dispersion with a larger particle size (i.e., water–IPA) showed better performance, while that prepared from a dispersion with a smaller particle size (i.e., NMP) showed better durability.

Characterization of Catalyst Inks By Rheology and Microscopic Particle Properties

Fundamental understanding of the state of catalyst inks and fabrication process is important to fabricate well-established catalyst layers of proton exchange membrane fuel cells (PEMFCs). The catalyst ink is a slurry which contains platinum-supported carbon and ionomer in a solvent. The state of the catalyst ink affects dynamics of materials during the fabrication process of the catalyst layer(1), and then the resultant structure(2). Additionally, the state of the catalyst ink can have time dependence, which means the procedure of particle agglomeration and sedimentation inside the slurry. Therefore, quantitative evaluation of the catalyst ink is required. In this study, a rheological measurement was conducted to characterize catalyst inks used to fabricate PEMFC catalyst layers. The rheological measurement can apply to opaque and high viscosity slurries like catalyst inks. Ionomer dispersion, pseudo-catalyst ink and catalyst ink with various ionomer content were measured to clarify the relationship between the rheological behavior and the types and amount of particles and interparticle interactions in the slurry. The rheological measurement was conducted by using a rheometer with a cone and plate system. Shear rate range was 200 to 3000 /s. The measurement temperature was 25°C. Downward measurement (from upper to lower shear rate) following the upward measurement was conducted. Ionomer dispersion (DE2020, Sigma-Aldrich), carbon black (Ketjenblack, Lion)–ionomer dispersion (pseudo-catalyst ink), and platinum-supported carbon (Pt/C, TEC10E50E, Tanaka Kikinzoku Kogyo)–ionomer dispersion (catalyst ink) were prepared. The volume fraction of carbon to the solvent (45wt.% 1-propanol aqueous solution) was 2% and ionomer to carbon ratio (I/C) was a parameter from 0.3 to 2.0. The viscosities of the slurries as a function of the shear rate are shown in Figure 1. The three types of the slurries showed quite different rheological properties with each other. The ionomer dispersions are Newtonian fluid in all I/C conditions from 0.3 to 2.0 (Fig. 1 (a)). On the other hand, pseudo-catalyst ink (Fig. 1 (b)) and catalyst ink (Fig. 1 (c)) were non-Newtonian fluid when I/C was low. The pseudo-catalyst ink showed a transition from non-Newtonian to Newtonian as I/C increased from 0.3 to 0.5. The ink also showed the transition from Newtonian to non-Newtonian as I/C increased from 1.0 to 2.0. The catalyst ink slightly showed non-Newtonian properties even in I/C 0.5 and 1.0. Additionally, the catalyst ink with I/C 0.3 and 0.5 showed relatively large hysteresis between the upward and downward measurements. The rheological property of the slurries can be the results of the state difference of the particles in the slurries such as the particle agglomeration and/or ionomer adsorption. The measurements of particle size distribution by dynamic light scattering and zeta potential were applied to the slurries (The results are not shown here). The comprehensive discussion to understand the state of the catalyst ink will be presented. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 16K18028. References T. Suzuki, M. Kobayashi, H. Tanaka, M. Hayase and S. Tsushima, ECS Trans., 69, 465 (2015).W. K. Epting, J. Gelb and S. Litster, Adv. Funct. Mater., 22, 555 (2012). Figure 1

Impact of Ionomer Dispersion on PEM Fuel Cell Performance

Conventional polymer electrolyte membrane (PEM) fuel cell electrodes using aqueous ionomer dispersion suffer from high manufacturing costs and rapid performance degradation. We have investigated an innovative method of designing fuel cell electrodes to reduce manufacturing costs and extend the lifetime. Our approach to obtaining good fuel cell performance is to; 1) understand ionomer particle morphology in dispersion; 2) investigate the electrode morphology, catalyst, and ionomer binder distributions in the electrodes; 3) evaluate the electrode performance using various electrodes prepared from different dispersing agents. First, laser diffraction particle size analysis was used to understand the various solvents’ impacts on catalyst ink structure. It can be seen that nPA/H2O and ethylene glycol (EG) solvent systems provide better ink structure, implied by much smaller agglomeration sizes. IPA/H2O and pentanediol based solvent systems exhibit large agglomerations in the ink, which may account for their poor quality. The laser diffraction particle size will be correlated with electrode structure and fuel cell performance later. Low magnification transmission electron microscopy element mapping was then used to characterize the ionomer distribution in these cathode layers. The best ionomer distribution was found in the EG based sample, with ionomer aggregates <50 nm. The porosity and pore size distribution of the electrode layers were also obtained from transmission electron microscopy image analysis. The EG-based electrode shows the lowest porosity (i.e., 13%). It also contains the smallest pore sizes and the highest number of pores. The effect of the dispersing agents on initial fuel cell performance was investigated. In this experiment, four ionomer dispersions were used and compared with nPA/H2O and IPA/H2O. All MEAs had a Pt loading of ~0.20 mgPt/cm2. The IPA/H2O baseline shows very low performance especially in the mass transport region, which is probably attributed to its poor coating quality. The ionomer dispersions have shown significant influence on the fuel cell performance, with a general ranking as: nPA/H2O > ethylene glycol > butanediol > pentanediol). The ethylene glycol based sample displays the best performance of all the non-aqueous samples, and its performance is very comparable to the nPA/H2O baseline.

Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation.

Perfluorosulfonic acid (PFSA) dispersions are used as components in a variety of electrochemical technologies, particularly in fuel-cell catalyst-layer inks. In this study, we characterize dispersions of a common PFSA, Nafion, as well as inks of Nafion and carbon. It is shown that solvent choice affects a dispersion's measured pH, which is found to scale linearly with Nafion loading. Dispersions in water-rich solvents are more acidic than those in propanol-rich solvents: a 90% water versus 30% water dispersion can have up to a 55% measured proton deviation. Furthermore, because electrostatic interactions are a function of pH, these differences affect how particles aggregate in solution. Despite having different water contents, all inks studied demonstrate the same particle size and surface charge trends as a function of pH, thus providing insights into the relative influence of solvent and pH effects on these properties.

Coarse-Grained Molecular Dynamics Study of Ionomer Dispersions in Catalyst Ink

Coarse-Grained Molecular Dynamics Study of Ionomer Dispersions in Catalyst Ink

Ionomer Dispersions in Water/Alcohol Solutions by Coarse-Grained Molecular Dynamics

The polymer electrolyte membrane (PEM) is one of the principal components for polymer electrolyte fuel cells. As the phase separation is considered one of the primary factors affecting its performance, the morphological properties of PEM have been studied experimentally and theoretically. Under good solvent conditions for the polymer backbone, where solvents dissolve the backbone chains, ionomers exhibit polyelectrolyte-type behavior, forming loose ionic aggregates. In sufficiently polar solvents, ionomer backbones undergo aggregation and phase separation. This process results in the formation of a solid PEM at high ionomer concentration. Perfluorosulfonic acid (PFSA) ionomers such as Nafion (developed by DuPont) are currently the principal electrolyte and separator materials adopted for use as PEMs. Chemically, Nafion is a random copolymer that consists of hydrophilic sulfonate groups and a hydrophobic polytetrafluoroethylene backbone, which together form microphase-separated higher-order structures. Under sufficiently hydrated conditions, PEMs exhibit high mechanical robustness, warranted by hydrophobic backbone segments, as well as good water retention and high proton conductivity, conferred by the high volumetric ion density. The characterization of the ionomer structures has been extensively studied using experimental measurements such as small-angle neutron scattering (SANS), wide-angle neutron scattering (WANS), small-angle X-ray scattering (SAXS), and nuclear magnetic resonance (NMR). They observed that hydrophobic backbones of Nafion form the cylindrical-like core of self-assembled ionomer structures while ionic groups are located at the periphery of this aggregates. Based on a combination of scattering and microscopy, the rod-like aggregates in well-hydrated PEMs were observed. The proton conductivity and mechanical strength of solution-cast membranes are correlated with the properties of ionomer dispersions in solutions. Theoretically, while most of studies have primarily focused on the solid membrane state with water volume fraction significantly below 50%, ionomer aggregations in solutions can also be an important phenomenon to understand the self-assembly behaviors of ionomers as key structure-forming elements in ionomer films. In this study, ionomer aggregations in water/alcohol solutions have been investigated using coarse-grained (CG) molecular dynamics simulations to understand the dependence of aggregation behaviors on the water and alcohol fractions. The use of CG models is a valuable tool to prove the time and length scales of systems beyond what is feasible with traditional all atom models. Our model has been developed based on the MARTINI force fields, which is in close connection with more detailed atomistic models in terms of thermodynamic data, in particular hydration/partitioning free energies. The whole CG model of Nafion copolymer, as shown in Fig. 1, includes a part of the atomistic level configuration of a Nafion chain. A three-monomer unit of polytetrafluoroethylene (PTFE) (-(CF2)3-) are coarse-grained as a spherical bead of diameter 0.47 nm, and one side chain is coarse-grained as two spherical beads. This model is comparable to the inter-distance of ~2 nm between side chains and a side chain length of ~1 nm in the Nafion ionomer with EW = 1100. To include the effect of polarization in water model, which is significant at the interfaces between water and other phases and in the vicinity of charged particles. The initial configurations were generated by randomly placing 15 ionomer chains in a box with periodic boundary condition in all directions. Water and 1-propanol CG beads were then added based on the water/1-propanol molar fractions. All the solutions were fixed at an ionomer concentration of 5 wt%. The equilibration of the systems was first carried out with NPT MD simulations at T = 300 K and P = 1 atm for 100 ns. The additional NPT ensemble for 50 ns was then performed for the production. The dependence of NPA content on the ionomer structures was studied by systematically changing the NPA content in the system. It was found that ionomers form the bundle-like structures with cylindrical shape as shown in Fig. 2, which are in general agreement with cylindrical-like structures found in experimental scattering measurements of Nafion solutions. The ionomer aggregation size was found to tend to be larger and more dispersive at higher NPA contents, although the ionomers aggregated into one cluster at all NPA contents. Figure 1

Ionomer Dispersions in Water/Alcohol Solutions by Coarse-Grained Molecular Dynamics

Structural properties of ionomer aggregations in a mixture of 1-propanol (NPA) and water have been investigated using coarse-grained molecular dynamics simulations. To perform simulations of larger systems and longer time spans, the reduced spatial resolution of the bead representation was used as a coarse-grained model for the enhancement of the computational efficiency compared to all atomistic simulations. The dependence of NPA content on the ionomer structures was studied by systematically changing the NPA content in the system. The self-assembly behavior of ionomers into cylindrical bundle-like aggregates was observed for all NPA content solutions. The radius (thickness) of an ionomer bundle was also found to be in good agreement with experimental measurements. Formation of ionomer aggregations showed different behavior at each NPA content, although the ionomers aggregated into one cluster at all NPA contents.

Polymer Electrolyte Membranes for Water Photo-Electrolysis.

Water-fed photo-electrolysis cells equipped with perfluorosulfonic acid (Nafion® 115) and quaternary ammonium-based (Fumatech® FAA3) ion exchange membranes as separator for hydrogen and oxygen evolution reactions were investigated. Protonic or anionic ionomer dispersions were deposited on the electrodes to extend the interface with the electrolyte. The photo-anode consisted of a large band-gap Ti-oxide semiconductor. The effect of membrane characteristics on the photo-electrochemical conversion of solar energy was investigated for photo-voltage-driven electrolysis cells. Photo-electrolysis cells were also studied for operation under electrical bias-assisted mode. The pH of the membrane/ionomer had a paramount effect on the photo-electrolytic conversion. The anionic membrane showed enhanced performance compared to the Nafion®-based cell when just TiO2 anatase was used as photo-anode. This was associated with better oxygen evolution kinetics in alkaline conditions compared to acidic environment. However, oxygen evolution kinetics in acidic conditions were significantly enhanced by using a Ti sub-oxide as surface promoter in order to facilitate the adsorption of OH species as precursors of oxygen evolution. However, the same surface promoter appeared to inhibit oxygen evolution in an alkaline environment probably as a consequence of the strong adsorption of OH species on the surface under such conditions. These results show that a proper combination of photo-anode and polymer electrolyte membrane is essential to maximize photo-electrolytic conversion.

Characterization of Ionomer Thin Film Distributions in PEM Fuel Cell Catalyst Layers Using Advanced Analytical Microscopy

Polymer electrolyte membrane (PEM) fuel cell performance degradation, particularly associated with the cathode catalyst layer, can be directly attributed to the stability and durability of individual material constituents comprising the membrane electrode assemblies (MEAs), including the electrocatalyst, catalyst support, and ionomer films. The structural and chemical changes of these MEA constituents are quantified via advanced electron microscopy methods to elucidate the materials-specific degradation mechanisms contributing to performance loss, with the ultimate goal being microstructural and compositional improvements that will enhance MEA durability. Research efforts at Oak Ridge National Laboratory are focused on the high-resolution microstructural and microchemical characterization of as-fabricated fuel cell materials (e.g., the individual materials constituents and the same materials incorporated in fresh MEAs), MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components, and field-aged MEAs. These studies are used to establish critical processing-microstructure-performance correlations and to elucidate the materials factors contributing to MEA degradation, performance loss, and failure. Recently, high-resolution analytical microscopy methods have been used to directly image/map the distribution and chemistry of the ionomer layers within catalyst electrodes, both on a mm-level for assessing “bulk” ionomer dispersions and at the nm-level to understand the continuity and uniformity of thin ionomer films. This presentation will focus on understanding ionomer distributions as a function of initial ionomer chemistry (e.g., solvents used), electrocatalyst content and dispersion, and type of carbon support used. Additionally, the stability of the ionomer films in cathode catalyst layers after ASTs designed for catalyst degradation and carbon corrosion will be evaluated. ________________________________________________________________________ Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility.

Structure of Membranes for Fuel Cells: SANS and SAXS Analyses of Sulfonated PEEK Membranes and Solutions

The microstructure of sulfonated poly(ether ether ketone) (sPEEK) membranes was investigated by combining small-angle neutron and X-ray scattering techniques (SANS and SAXS) for large and low water contents, respectively. The ion-exchange capacity, the water content and the nature of the counterion were varied for a better understanding of the membrane microstructure. SAXS and SANS contrast variation experiments reveal a significantly more complex structure than it is commonly believed with a delicate balance of the contrasts. At low water content, the structure can be depicted by the presence of small ionic clusters and larger more or less connected core–shell domains between crystallites for low values of the sulfonation degree. The first step of the swelling process corresponds to the filling without significant structural changes of the porosity created by the solvent evaporation during the casting process. The second step is associated with a major structural reorganization induced by a large increase of the membrane water content over a small range of temperature. This reorganization is attributed to an ionic domain percolation on a large scale. The third step corresponds to the swelling of lamellar ionic domains around the crystallites as revealed by the study of the dilution laws and of the structure of sPEEK ionomer dispersions. The sizes of the ribbon-like polymer particles were determined. They do not depend linearly with the membrane ion content suggesting a nonhomogeneous distribution of the ionic groups along the polymer chain during the sulfonation process associated with the semicrystalline nature of the polymers. Finally, the effect of the degradation in oxidative media on the membrane structure is shown to correspond to an increase of the membrane water content.

Synthesis, characterization and electrorheological effect of sulfosalt‐type liquid‐crystalline ionomers containing polyaniline units

ABSTRACTA series of main‐chain liquid‐crystalline polymers (LCPs) with pendant sulfonic acid groups have been synthesized by use of biphenyl‐4,4′‐diol, 6,7‐dihydroxynaphthalene‐2‐sulfonic acid, and bis(4‐(chlorocarbonyl)phenyl) decanedioate in a one‐step esterification reaction. Emeraldine base form of polyaniline (PAN) is doped by the synthesized sulfonic acid‐containing LCPs to obtain PAN–LCP ionomers. A series of electrorheological (ER) fluids are prepared using the synthesized PAN–LCP ionomers and silicone oil. The chemical structure, liquid‐crystalline behavior, dielectric property of LCPs, and PAN–LCP ionomers, and ER effect of the ER fluids are characterized by use of various experimental techniques. The synthesized sulfonic acid‐containing LCPs and PAN–LCP ionomers display nematic mesophase. The PAN–LCP ionomers show a slight elevation of glass transition temperatures and decrease of enthalpy changes of nematic–isotropic phase transition compared with corresponding sulfonic acid‐containing LCPs. The relative permittivity of the PAN–LCP ionomers is much higher than that of the corresponding sulfonic acid‐containing LCPs. The ER effect of the PAN–LCP ionomer dispersions is better than PAN dispersions, suggesting a synergistic reaction should be occurred among liquid crystalline component, and PAN part under electric fields. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3395–3403, 2013

Preparation of Anion Conducting Ionomer Dispersions

not Available.

Effect of side‐chain length on the electrospinning of perfluorosulfonic acid ionomers

AbstractThe effect of the side‐chain length (short side chain and long side chain, SSC and LSC, respectively) of perfluorosulfonic acid (PFSA) ionomers on the properties of nanofibers obtained by electrospinning ionomer dispersions in high dielectric constant liquids has been investigated with a view to obtaining electrospun webs as components of fuel cell membranes. Ranges of experimental conditions for electrospinning LSC and SSC PFSAs have been explored, with a scoping of solvents, carrier polymer and PFSA ionomer concentrations, and carrier polymer molecular weight. Under optimal conditions, the electrospun mats derived from SSC and from LSC PFSA show distinct fiber dimensions that arise from the different chain lengths of the respective ionomers. Enhanced interchain interactions in SSC PFSA with low equivalent weight compared to LSC PFSA result in a considerably lower average fiber diameter and a markedly narrower fiber size distribution. The proton conductivity of nanofiber mats of SSC and LSC PFSA with equivalent weights of 830 and 900 g mol−1, respectively, are 102 and 58 mS cm−1 at 80°C and 95% relative humidity. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Impact of Solvent on Ionomer Structure and Fuel Cell Durability

Electrode structure within PEFCs, including the Pt-ionomer interface, is created while making electrodes from catalyst inks based on ionomer dispersed in solvent. The relationship between final electrode structure and processing conditions is poorly understood. We have varied the solvent used in cathode catalyst inks, and then subjected the resulting MEAs to hydrogen-air performance and durability testing. Specifically, cathodes cast from inks based on inonomer dispersions in water-propanol-isopropanol (W/P cathode) initially perform better than cathodes cast from glycerol-based dispersions (Gly cathode), but are far less durable. After 10,000 potential cycles from 0.60 V to 1.0 V in N2, the performance on air of the W/P cathode falls significantly below that of the Gly cathode. NMR and neutron scattering measurements of ionomer dispersions, as well as AFM and TEM data from cast ionomer films, offer insight into how the effect of solvent choice on the ionomer structure may impact durability.

Study on the conductivity of recast Nafion ®/montmorillonite and Nafion ®/TiO 2 composite membranes

Nafion ® composite membranes were formed from a recast procedure already applied by the authors. Montmorillonite (MMT) and titanium dioxide (TiO 2) were used separately as fillers in the recast process and dimethylformamide (DMF) was used as the casting solvent. Addition of 1 wt.% MMT or 1 wt.% TiO 2 to the ionomer dispersions prior to the heat treatment demonstrated that there is an increase in water content of the recast membranes. For both the samples, it was verified that the tangential conductivity increases with increasing relative humidity (RH) of the environment. Fuel cell tests carried out with recast membranes showed that the best performances are seen when the anode and cathode humidification temperatures are low. With a Δ T of −35 °C between cell temperature and anode humidification, the conductivity of additive-containing samples is 10% higher than that of additive-free membranes.

Effect of solvent and crystal size on the selectivity of ZSM-5/Nafion composite membranes fabricated by solution-casting method

Zeolite/Nafion composite membranes with high proton selectivity were successfully fabricated using the solution-casting method. The types of zeolites are nano-sized and large sized Na-ZSM-5, H-ZSM-5, and their ball-milled ones. Two different schemes of experiments were conducted depending on the type of solvent. In case of using as-received Nafion® ionomer dispersions, the experimental results clearly show that the proton conductivity of zeolite composite membrane using either H-type or Na-type ZSM-5 depends on the type of solvent. It is thought that when propanol and water as the solvents were used, more hydrophilic H-type ZSM-5 seems to have been more randomly dispersed into hydrophobic region rather than hydrophilic ionic clustered channels within Nafion. Therefore, H-type ZSM-5 existing near hydrophobic region seems to provide additional path for proton migration but weakening the mechanical strength. These composite membranes show higher water uptake than commercial Nafion® 115, strongly suggesting better water retention ability of zeolite. The most interesting result is that the methanol permeability has decreased with increasing zeolite contents even when the proton conductivity increased, and the proton selectivities of these composite membranes expressed as characteristic factor were higher than that of Nafion® 115. In case of using a mixture of high boiling point DMF and ethanol as the solvent, unlike the previous case where no DMF was used, the proton conductivity slightly dropped with increasing zeolite contents. These results should have been attributed to a blocking effect of zeolite particles surrounded by inversely oriented hydrophilic micelles of Nafion. However, the values of proton conductivity of most composite membranes were significantly higher than that of Nafion® 115, and methanol permeability also decreased with increasing zeolite contents. The significantly lower methanol permeability of the composite membrane fabricated with DMF as the solvent is probably due to the more effective blocking effect of H-ZSM-5 for ionic clustered channels as well as difficult transport of methanol through zeolite pores. In case of the composite membranes containing ZSM-5 with large crystal size, it is found that the methanol permeability has increased considerably with the increasing of zeolite contents due to void fractions between polymer phases and zeolite particles. In case of using ball-milled ZSM-5 with small crystal size, however, the value of characteristic factor tends to increase with increasing zeolite contents. Consequently, it is seen that the characteristic factor of Zeolite/Nafion composite membranes was much higher than Nafion® 115. The results obtained throughout this study strongly suggest that zeolites with small crystal size and high hydrophilicity are very prospective for composite membrane for direct methanol fuel cells in the future.