We have previously shown1,2 that there is a suppression in both swelling and water diffusivity when Nafion is confined to thin and ultrathin films. We have also shown that the modulus of ultrathin Nafion films increases upon confinement3,4, as well as upon thermal annealing. Interestingly, we also observe a decrease in swelling and an increase in modulus for ultrathin films aged in high humidity environments. Similar behavior has been shown to occur in bulk Nafion films.5,6 Some researchers suggest that the root cause of such a decrease in swelling and increase in modulus is due to the formation of sulfonic anhydrides, while others suggest that it is due to internal hydrogen bonding of dehydrated sulfonic acids. In this talk, we will present our measurements results on thin and ultrathin Nafion films. We apply polarization-modulation infrared reflection-absorption spectroscopy to study the kinetics of dehydration and formation of sulfonate species in thin Nafion films. We also employ cantilever bending during humidity-induced swelling to probe the mechanical properties of these films. Grazing-incidence small-angle x-ray scattering is used to elucidate changes in morphology upon annealing and/or aging. By combing the results from these techniques, we can paint a more holistic picture of the origins of suppressed swelling and increased mechanical properties in thin and ultrathin Nafion films. S. A. Eastman, S. Kim, K. A. Page, B. W. Rowe, S. Kang, C. L. Soles, and K. G. Yager. Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films, Macromolecules, 45, 7920-7930 (2016). E. M. Davis, C. M. Stafford, and K. A. Page. Elucidating Water Transport Mechanisms in Nafion Thin Films, ACS Macro Letters, 3, 1029-1035 (2014). K. A. Page, J. W. Shin, S. A. Eastman, B. W. Rowe, S. Kim, A. Kusoglu, K. G. Yager, and G. R. Stafford. In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles, ACS Applied Materials and Interfaces, 7, 17874-17883 (2015). K. A. Page, A. Kusoglu, C. M. Stafford, S. Kim, R. J. Kline, and A. Z. Weber. Confinement-Driven Increase in Ionomer Thin-Film Modulus, Nano Letters, 14, 2299-2304 (2014). F. M. Collette, C. Lorentz, G. Gebel, F. Thominette. Hygrothermal aging of Nafion, Journal of Membrane Science, 330, 21-29 (2009). S. Shi, T. J. Dursch, C. Blake, R. Mukundan, R. L. Borup, A. Z. Weber, and A. Kusoglu. Impact of Hygrothermal Aging on Structure/Function Relationship of Perfluorosulfonic-Acid Membranes, Journal of Polymer Science B: Polymer Physics, 54, 570-581 (2016).

Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Young's modulus of thin Nafion films as a function of relative humidity is obtained. The Young's modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell.

Polyelectrolyte membranes (PEMs) have been employed as solid electrolytes in fuel-cell technologies as early as the 1950s, when they were used in NASA's Gemini program. However, PEM materials have only gained wide-spread attention in the last two decades due to advancements in membrane electrode-assembly (MEA) formation and the synthesis of new and interesting materials. Over the past several decades, various neutron techniques have played an instrumental role in measuring the structure and transport properties of PEMs in order to develop a deeper understanding of structure-property and performance relationships in PEM materials for fuel-cell applications.

ABSTRACT Neutron spin‐echo spectroscopy has been used to measure the time‐scale of fluctuations associated with the inter‐chain correlation peak observed in the diffraction pattern of the perfluorosulfonate ionomer, Nafion ® . We have successfully measured the chain dynamics as a function of water content and temperature and have demonstrated that the chain dynamics become faster with increasing temperature. The addition of water also results in shorter relaxation times due to a plasticization of the chain motions. At water contents above a λ ≈ 6, the chain dynamics seem to approach a plateau and the relaxation times of the chains are no longer being plasticized by the presence of additional water molecules. The increased mobility in molecular relaxations induced by the presence of water points to the molecular origins of the temperature‐ and humidity‐dependent softening mechanisms in Nafion and other perfluorinated sulfonic acid membrane materials. Moreover, these results show that there is a strong inter‐dependence of the dynamics of water and the polymer chains in Nafion membranes. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. 2014 , 52 , 624–632

We quantify the interfacial nanostructure and corresponding water transport kinetics in thin films of Nafion which are known to show nonbulk like transport properties using neutron reflectivity (NR) and quartz-crystal microbalance (QCM) measurements integrated with in-situ, controlled relative humidity environments. Rigorous fitting of the NR data under humidified conditions reveals that a hydrophilic organosilicate substrate induces an interfacial layering of the water transport domains parallel to the substrate whereas the hydrophobic organosilicate analogue does not trigger this interfacial ordering. The interfacial layering on the hydrophilic substrate is accompanied by an excess in the total mass of water absorption as verified by QCM measurements. The excess water in the thin Nafion films is quantitatively consistent with the segregation amounts and length scales quantified by NR. However, we do not observe strong differences in the water transport kinetics in thin Nafion films where the volume fraction of the materials with the water transport oriented parallel substrate, orthogonal to the primary direction of transport, is on the order of approximate to 7 vol %; to a first approximation the majority of the transport kinetics are similar on the hydrophilic (oriented) and hydrophobic (disordered) surfaces.

Surface deposition of polydopamine, PD, using facile aqueous-based chemistry at mild reaction conditions, was accomplished on reverse osmosis, nanofiltration, ultrafiltration, and microfiltration membranes. This surface treatment not only retained much of the membranes’ intrinsic pure water permeability, but also improved the fouling resistance of polypropylene microfiltration (MF), poly(tetrafluoroethylene) MF, poly(vinylidene fluoride) MF, poly(arylene ether sulfone) ultrafiltration (UF), polysulfone UF, polyamide (PA) nanofiltration, and PA reverse osmosis membranes, as measured using oil/water emulsion filtration. To demonstrate scalability of this approach, PD was applied to, and improved the fouling resistance of, membrane modules. Following PD deposition, membranes could be further modified by grafting fouling-resistant macromolecules, such as poly(ethylene glycol), to further improve fouling resistance of MF membranes.

While fuel cells have received considerable attention over the last 10 years to 20 years, the history of hydrogen fuel cells dates back to 1838. It would take more than a century before polymers would be implemented as an electrolyte for proton transport (1955) and another 40 years after that before a real renaissance would be sparked that would finally make polymer electrolyte membrane (PEM) fuel cells a conceivable means of electrochemical energy conversion. This chapter covers a brief history of the fuel cell and the use of polymer electrolytes as an ion-transport medium. In addition to an overview of the materials challenges, the various types of polymeric materials being pursued as potential fuel cell membranes are presented. Although this chapter is not an exhaustive review of the literature, it is our hope that it will give the reader an appreciation for the history of PEM fuel cells and the approaches that polymer chemists are taking in order to address the major impediments for wide-spread commercialization of PEM fuel cells.

Fuel cells based on polymer electrolyte membranes (PEM) show promise as a means of energy conversion for a wide range of applications both in the transportation sector and for stationary power production due to their high charge density and low operating temperatures. While the structure and transport of bulk PEMs for fuel cell applications have been studied extensively, much less is known about these materials at interfaces and under confinement, conditions that are highly relevant in the membrane electrode assembly of a working PEM fuel cell. Using X-ray reflectivity, neutron reflectivity, grazingincidence small-angle X-ray scattering, quartz crystal microbalance, and polarization-modulation infrared reflection−absorption spectroscopy, we have studied the structure, swelling, water solubility, and water transport kinetics as a function of relative humidity for confined polyelectrolyte films thinner than 222 nm. While the humidity-dependent equilibrium swelling ratio, volumetric water fraction, and effective diffusivity are relatively constant for films thicker than ca. 60 nm, we observe measurable suppressions of these properties in films less than ca. 60 nm. These effects occur at length scales that are relevant to transport (ion and water) in the polyelectrolyte binders found in the catalyst layer of the membrane−electrode assembly (MEA) of a functional fuel cell. The thin film methodology and findings presented here provide a platform to quantify and validate models of interfacial impedance used within the fuel cell community and have the potential to lead to improvements in MEA materials, design, and optimization. ■ INTRODUCTION Proton exchange membranes (PEMs) are a critical component in the conversion of chemical energy into electrical energy in solid H2/O2 fuel cells. Perfluorosulfonate ionomers (PFSIs) are one of the most widely studied classes of materials for PEM fuel cells and also show great promise in many other applications including polymer actuation, ion transport, and sensor design and functionality. Nafion is the most widely studied PFSI and has become the industry standard for fuel cell membranes. Nafion consists of a perfluoroethylene backbone with flexible perfluorinated vinyl ether side chains terminated by a sulfonic acid group as shown below. The polar perfluoroether side chains have been shown to segregate from the more nonpolar backbone to form ionic aggregates. These interactions lead to a complex morphological structure comprised of hydrophilic, ionic domains dispersed in a semicrystalline, hydrophobic matrix. This complex structure plays an integral role in the performance properties (i.e., proton conductivity, mechanical properties, swellability, transport, etc.) observed for Nafion membranes. There have been many studies characterizing the structure and performance properties of Nafion under a variety of conditions. Of particular importance is the proton conductivity of Nafion, which is critical to fuel cell performance. Proton conductivity in Nafion has been shown to be directly connected to the level of hydration of the membrane. Nafion must have a minimal hydration level λ (mol of H2O/mol of SO3H) of 22 to achieve sufficient conductivity for fuel cell performance (i.e., σ = 0.1 S/cm). There are significant efforts in water management to maintain conditions for optimal fuel cell performance. As a result, the absorption, desorption, Received: June 25, 2012 Revised: September 7, 2012 Published: September 19, 2012 Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 7920 dx.doi.org/10.1021/ma301289v | Macromolecules 2012, 45, 7920−7930 and transport of water within Nafion has been extensively studied. A survey of this literature demonstrates that water transport in Nafion can be affected by membrane thickness, annealing conditions, processing methodologies, and water vapor concentration. However, the aforementioned body of work has largely focused on bulk membranes, and there is a general lack of knowledge on the response of Nafion at interfaces and confined to very thin films or layers. While bulk Nafion transport properties are relevant for the active membrane layer within the membrane electrode assembly (MEA) in a fuel cell, this complex polyelectrolyte is also used as an ionically conductive binder in the electrode and catalyst layer. It has been shown that in these composite electrodes Nafion is heterogeneously dispersed and often confined to films that are on the order of 2−10 nm thick. It is known that polymer confinement can affect both morphology and chain dynamics and, therefore, material properties such as the glass transition, solute sorption, and mechanical properties. The structure, transport, and performance properties of Nafion confined near an interface as it exists at within the catalyst layer of the MEAhave received little attention. This interfacial boundary region of the fuel cell is critical to the performance of the entire fuel cell as it mediates the transport of reactant gases, ions, electrons, and water. Simulations of water, proton, and reactant gas diffusion within a catalyst layer are powerful tools to predict and optimize fuel cell performance; however, all of these simulations are based on bulk membrane properties. A limited number of studies have demonstrated deviations in either the morphology, water (or solute) sorption, or proton conductivity from their bulk values in Nafion thin films. This raises the issue of whether it is appropriate to use the bulk properties of Nafion when modeling its behavior within the catalyst layer. However, these studies lack a systematic and comprehensive investigation of these properties to establish broad, fundamental insight into the structure− property relationships in these polyelectrolyte thin films. There exists a need to characterize the interdependence of the morphology, proton conductivity, mechanical properties, swellability, and water transport of Nafion under confinement near an interface in order to more accurately model the performance of the MEA in a working fuel cell. This work focuses on the water absorption and transport in Nafion thin films (20−222 nm) cast onto silicon substrates. While these properties have been studied in detail for bulk membranes, very little is known about their behavior in thin films or at interfaces. These planar films serve as a model system for the confined thin films of Nafion on the catalyst particles and carbon supports in the electrode layer of a MEA. Utilizing specular X-ray reflectivity (SXR), neutron reflectivity (NR), grazing-incidence small-angle X-ray scattering (GISAXS), polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and quartz-crystal microbalance (QCM) measurements, we have measured the structure, water content, extent of swelling, and effective diffusion coefficients of water in Nafion thin films exposed to water vapor. These studies are a step toward a comprehensive understanding of how the material properties of Nafion, germane to fuel cell performance, behave under confinement. This insight has the potential to provide a deeper understanding into the origins of fuel cell performance loss due to mass transport limitations within the catalyst layer. ■ EXPERIMENTAL METHODS Film Preparation for Specular X-ray Reflectometry (SXR). A series of Nafion dispersions were prepared for spin-coating films onto silicon wafers. Nafion stock solution (20% by mass in water, alcohol, and ether mixture purchased from Aldrich) was diluted to varied extents with anhydrous ethanol (1:20 to 1:4 by volume) to obtain concentrations appropriate for film thicknesses in the rage of approximately 20−222 nm. The solutions were mixed thoroughly before spin-coating. Silicon wafers were washed with toluene and acetone and then dried with a dry nitrogen jet. Wafers were then placed in an Anatech SP100-QTZ plasma etcher for 5 min at ∼60 W. The Nafion solutions were immediately cast onto the cleaned silicon wafers at a spinning rate of 2000−3500 rpm to obtain films of the desired thickness. Films were then stored under ambient conditions until needed, but for no more than 48 h. Film Preparation for Polarization-Modulation Infrared Reflection−Absorption Spectroscopy (PM-IRRAS). Phosphorusdoped, double-side-polished, silicon wafers, reported by the vendor to have a thickness of 500 ± 25 μm and resistivity of 15 ± 5 ohm·cm (for high infrared transparency), were purchased from Silicon Quest International and used for the supporting substrates in the PM-IRRAS diffusion studies. One side of the polished silicon wafer was coated with a 5 ± 0.5 nm chromium adhesion layer and a 100 ± 2 nm gold layer as an infrared reflective mirror. Each metal layer was deposited via metal evaporation under reduced pressure, and the thickness was measured using a quartz crystal microbalance. Nafion films were cast on the opposite side of the silicon wafer, i.e., the native oxide, per the procedures in the SXR film preparation. SXR Sample Analysis. A Philips X’pert X-ray diffractometer was used to characterize the equilibrium film thickness and water content of Nafion thin films exposed to a series of water vapor concentrations. Reflectivity data were collected in the specular condition with the grazing incident angle equal to the detector angle. Data were collected over a specular angle range of 0.1°−1.0° with a step size of 0.0003°. The tension and current of the X-ray source were set to 45 kV and 40 mA, respectively. The instrument was equipped with a temperaturecontrolled environmental chamber fitted with beryllium windows, a humidified air inlet, and an exhaust port with a humidity sensor. The vapor pressure of water within the sample chamber was controlled via an external mass flow device. Two mass flow controllers were connected in parallel and programmed to deliver a total flow rate of 500 ± 5 mL/min. Controller A controlled the flow rate of dry air (0% RH)

Fuel cells based on polymer electrolyte membranes (PEM) show promise as a means of energy conversion for a wide range of applications both in the transportation sector and for stationary power production due to their high charge density and low operating temperatures. While the structure and transport of bulk PEMs for fuel cell applications have been studied extensively, much less is known about these materials at interfaces and under confinement, conditions that are highly relevant in the membrane electrode assembly of a working PEM fuel cell. Using X-ray reflectivity, neutron reflectivity, grazing-incidence small-angle X-ray scattering, quartz crystal microbalance, and polarization-modulation infrared reflection-absorption spectroscopy, we have studied the structure, swelling, water solubility, and water transport kinetics as a function of relative humidity for confined polyelectrolyte films thinner than 222 nm. While the humidity-dependent equilibrium swelling ratio, volumetric water fraction, and effective diffusivity are relatively constant for films thicker than ca. 60 nm, we observe measurable suppressions of these properties in films less than ca. 60 nm. These effects occur at length scales that are relevant to transport (ion and water) in the polyelectrolyte binders found in the catalyst layer of the membrane-electrode assembly (MEA) of a functional fuel cell. The thin film methodology and findings presented here provide a platform to quantify and validate models of interfacial impedance used within the fuel cell community and have the potential to lead to improvements in MEA materials, design, and optimization.

Abstract not Available.

The influence of polydopamine (PDOPA) deposition and poly(ethylene glycol) (PEG) grafting on pure water flux and bovine serum albumin (BSA) adhesion of two polysulfone ultrafiltration (UF) membranes, a poly(vinylidene fluoride) microfiltration (MF) membrane, and a polyamide reverse osmosis (RO) membrane is reported. When modified with PDOPA, all membranes exhibited a systematic reduction in protein adhesion. For example, 90 min of PDOPA deposition led to at least 96% reduction in BSA adhesion to these membranes at neutral pH. BSA adhesion was further reduced by subsequent PEG grafting to PDOPA (PDOPA-g-PEG). The membranes' pure water flux values (i.e., with no foulants present) were influenced to different extents by PDOPA and PDOPA-g-PEG modifications. In the porous membranes (i.e., the UF and MF membranes), the pure water flux reduction due to these modifications correlated with membrane pore size, with the smallest flux reductions observed in the MF membrane (e.g., <1% flux reduction for all PDOPA modification times considered), which have the largest pores, and the largest flux reductions occurring in UF membranes (e.g., a 40% flux reduction after 90 min of PDOPA deposition), which have pore sizes on the order of the PDOPA deposition thickness. The RO membranes, which are essentially non-porous, exhibited a flux reduction of 25% after 90 min of PDOPA deposition. (C) 2010 Elsevier Ltd. All rights reserved.

A model describing the influence of temperature on the permeability/selectivity tradeoff of polymeric membranes has been developed from fundamental theoretical considerations. This model was used to predict the influence of temperature on upper bound behavior for several gas pairs including O2/N2, H2/N2, CO2/CH4, CO2/N2, H2/CO2, and CO2/H2. The predicted upper bound shifts vertically with temperature, and the direction and magnitude of the shift depend on the sizes and condensabilities of the gases considered. Fair agreement between the model predictions and experimental behavior are realized by allowing three model parameters, two related to diffusivity and one to solubility, to vary with polymer chemistry. The utility of this model is to provide a better fundamental understanding of the influence of penetrant properties and polymer structure on the temperature dependence of transport and properties in membrane gas separation processes and to provide a method to benchmark the gas separation performance of materials, even if the data used for comparison were measured at temperatures far from ambient.

The physical aging behavior of thin glassy polysulfone (PSF) films (∼125nm) with different previous histories was tracked using gas permeability measurements. The initial states of these materials were modulated by thermal annealing at fixed temperatures below the glass transition or by exposure to high pressure (800psig (56.2bara)) CO2 for various times. Regardless of the previous history, the nature of the aging response in these samples was consistent with the aging behavior of an untreated film that was freshly quenched from above Tg, i.e., permeability decreased and pure gas selectivity increased with aging time. However, the extent of aging-induced changes in transport properties of these materials depended strongly on previous history. The aging behavior was described using Struik’s aging model by allowing the initial conditions to depend on each sample’s previous history.

Membrane-based separations play a key role in energy conservation and reducing greenhouse gas emissions by providing low energy routes for a wide variety of industrially-important separations. For reasons not completely understood, membrane permeability changes with time, due to physical aging, and the rate of permeability change can become orders of magnitude faster in films thinner than one micron. The gas transport properties and physical aging behavior of free-standing glassy polysulfone and Matrimid® films as thin as 18nm are presented. Physical aging persists in glassy films approaching the length scale of individual polymer coils. The films studied ranged from 18–550nm thick. They exhibited reductions in gas permeability, some more than 50%, after ∼1000h of aging at 35°C, and increases in selectivity. The properties of these ultrathin films deviate dramatically from bulk behavior, and the nature of these deviations is consistent with enhanced mobility and reduced Tg in ultrathin films. The Struik physical aging model was extended to account for the influence of film thickness on aging rate, and it was shown to adequately describe the aging data.