Perfluorosulfonic Acid Ionomer Research Articles

Influence of casting substrate on bulk morphology and vanadium ion transport in ionomer nanocomposites

Perfluorosulfonic acid (PFSA) ionomer nanocomposites are a promising solution to address the poor ion selectivity of current membranes utilized in vanadium redox flow batteries. Herein, we investigate the impact of a casting substrate on the nanostructure and vanadium ion transport in bulk ionomer and ionomer nanocomposite membranes (i.e., films with thicknesses of ∼100 μm). Specifically, solution-cast ionomer nanocomposite membranes, containing either unfunctionalized (hydroxyl groups), amine-functionalized, or sulfonic acid-functionalized silica nanoparticles (SiNPs), were fabricated by casting on either a polished quartz or polytetrafluoroethylene (PTFE) substrates. Surprisingly, the choice of the casting substrate was seen to affect the bulk morphology of the PFSA ionomers, resulting in substrate-specific vanadium ion transport, where suppressed ion transport was observed for membranes cast on the polished quartz, when compared to their PTFE-cast counterparts. Additionally, the chemical composition of the substrate-adjacent surface was a function of both the substrate and the surface functionality of the SiNPs. Moreover, it was observed that both the chemical composition of the membrane surface and the substrate-induced changes to the bulk ionomer morphology governed vanadyl ion transport through the PFSA ionomers. Results from this work have direct implications for the design of next-generation ionomer nanocomposites, as the casting substrate used to fabricate these materials, and the orientation of these membranes inside the operating flow battery, can significantly influence transport of vanadium ions.

Effect of Carbon Structure and Ink Composition on 3M Ionomer Adsorption

Conventional catalyst layers (CLs) in Proton Exchange Membrane Fuel Cells (PEMFCs) involve an ionomeric component to provide a proton-conducting pathway to the catalyst sites. Such CLs are commonly formed by a mixture of the components, also known as catalyst ink, which comprise the carbon-supported catalyst (e.g. Pt/C) and perfluorosulfonic acid (PFSA) ionomer, dispersed in an aqueous solvent. The resulting CL structure is influenced by the dispersion of the components in solution which, in turn, depend on the dispersion medium.Different CL fabrication techniques, however, require dispersion media with specific characteristics. Fabrication techniques such as hand-painting, screen-printing or roll-to-roll coating require a viscous ink with a high solid content (>5 wt. %) and high-boiling-point additives.(1) In contrast, the spray-coating technique requires a lower solid content (<2 wt. %) and a volatile medium, typically using a water or alcohol solution. The early work of Aldebert et al. found a significant influence of the nature of the solvent on the morphology of Nafion in solution.(2) The Small-Angle Neutron Scattering (SANS) spectra of Nafion solutions in water and ethanol evidenced significant differences in the polymer morphology.Here, the interactions of standard carbon support structures and carbon-supported Pt materials with 3M ionomer were studied with adsorption isotherm in aqueous solution based on 19F NMR and ICP-OES. We provide an analysis of the adsorption considering the results of applying several physical characterization methods to explore CL structure and interactions with 3M 825 EW ionomer.Figure 1 shows the adsorption behavior of the carbon samples: Vulcan XC-72 (V72), Black Pearls 2000 (BP), acetylene black (AB), and Ketjenblack (KB). A Langmuir approach was used to obtained information on the adsorption behavior of the carbon samples. These materials exhibit degrees of graphitization as analyzed through Raman spectroscopy. The intensity ratio of the D band over the graphitic band intensities (ID/IG) was used to evaluate the degree of disorder of the carbon samples.The highest equilibrium constant and area-specific maximum surface coverage correspond to the BP and V72 samples, respectively. These samples exhibited intermediate values of the ID/IG ratio. In contrast, both the AB and KJB samples, which exhibited the lowest and highest ID/IG ratios, respectively, showed lower and values. This trend suggests that the carbon preferentially adsorbs on the amorphous/graphitic boundaries present in the turbostratic structure of carbon. These sites of the microcrystalline carbon domains areFurthermore, the effect of the solvent structure on the 3M ionomer distribution can also be explored through this methodology. The dissolution of the 3M ionomer in a series of solvents (isopropanol, ethylene glycol, methanol, glycerin, propylene glycol) can provide information on the impact of the solvent nature on the adsorption and coverage of the ionomer on the various substrates.

Tuning the Ion Selectivity and Chemical Stability of a Biocellulose Membrane by PFSA Ionomer Reinforcement for Vanadium Redox Flow Battery Applications

A low-cost biocellulose (BC) membrane is incorporated and coated with the perfluorosulfonic acid (PFSA) ionomer through a new facile preparation route to develop an efficient membrane for vanadium ...

Perfluorosulfonyl Imide versus Perfluorosulfonic Acid Ionomers in Proton-Exchange Membrane Fuel Cells at Low Relative Humidity.

Designing highly conductive ionomers at high temperature and low relative humidity is challenging in proton-exchange membrane fuel cells. Perfluorosulfonyl imide ionomers were believed to achieve this goal, owing to their exceptional acidity and excellent thermal stability. Perfluorosulfonyl imide ionomers are less conductive than the analogous perfluorosulfonic acids despite similar membrane microstructure. In this study, the distinct behavior is rationalized by in situ synchrotron infrared spectroscopy during hydration. The protonation mechanism, formation of the protonic moiety and water clustering are totally different for the two different families of membranes. The ionization mediated by trans-to-cis conformational transition of the perfluorosulfonyl imide ionomer is not accompanied by the formation of hydronium ions. In contrast, Zundel-ion entities were identified as the elementary protonic complex, which is stable over the hydration range. The H-bond network of surrounding water molecules appears to be less connected and the protons remain highly localized and unavailable for efficient structural transport. The delocalization of protons and their mitigated interaction with the surrounding medium are prominent effects that negatively impact conductivity.

Vapor-fed photoelectrolysis of water at 0.3 V using gas-diffusion photoanodes of SrTiO3 layers

A proton exchange membrane (PEM) system using a SrTiO3 gas-diffusion photoanode achieves vapor-fed photoelectrolysis of water at an applied voltage of 0.3 V.

Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy

Perfluorosulfonic acids (PFSA) are the most widely used ionomers in Polymer electrolyte membrane fuel cells as both membrane and catalyst layer support. This study presents an electrochemical based analysis of using long-side-chain Nafion® and short-side-chain Aquivion® PFSA ionomers as binders in the catalyst-layers of PEMFC. Membrane electrode assemblies were designed with consistent components, varying only the catalyst ink's ionomer type from 28 wt% Aquivion® to 28 wt% Nafion®. The durability and performance profile of the resulting catalyst-coated-membranes made from using these PFSA ionomers as binders in the catalyst layers was investigated. Semi-empirical modeling shows that the Nafion® ionomer based binders give a catalyst-coated-membrane with a 46.6% higher ohmic resistance compared to using Aquivion® ionomers as binders. The power density analysis also showed that catalyst-coated-membranes made with Aquivion® ionomer based binder gives an output power that is 22.33% higher than catalyst-coated-membranes made with Nafion® ionomers as binders in the catalyst layers. The Aquivion® ionomer binder also shows higher catalyst utilization compared to the Nafion® ionomer binders. The Electrochemical Impedance Spectroscopy– Equivalent Circuit Model analysis showed that the membrane electrode assembly degradation is more pronounced in the Nafion® ionomer based binder compared to its Aquivion® counterpart. It is thus shown that Aquivion® ionomers are better catalyst binders compared to the Nafion® ionomers as they result in catalyst-coated-membranes that are better performing and more durable.

Interfacial shear rheology of perfluorosulfonic acid ionomer monolayers at the air/water interface

We report a systematic rheological analysis of perfluorosulfonic acid (PFSA) ionomer monolayers at the air/water interface. Equipped with a custom-designed double wall knife-edge interfacial rheometer, we measure both the linear and nonlinear viscoelasticity of various PFSA ionomer monolayers. Based on rheological measurements along with other static measurements, we find that the rheological properties of the PFSA ionomer monolayers mainly originate from the electrostatic interaction of negatively charged groups (SO3−) rather than direct interactions between backbones, as is typical of other polymeric monolayers. Although the rheological properties mainly come from the SO3− groups, equivalent weight (EW) and length of the side chain affect the rheological properties as well: (1) surface activity increases with EW, thereby a larger EW tends to have stiffer interfaces; (2) PFSA ionomer with longer side chains also has larger shear moduli at the same surface pressure (Π), presumably because the longer side chain has larger configurational entropy, thus leading to better closed packing. Moreover, it is found from Π dependent measurements that both the elastic and viscous moduli exhibit unusually weak dependence on Π compared with typical polymeric and surfactant monolayers. The weak Π dependence can be explained in terms of charged colloids with long-range repulsive interactions, as they are susceptible to shear stress but resistive to compressional stress. This rheology of ionomers and their underlying morphology at the interface are expected to provide useful information for various membrane applications where ultrathin membranes can be helpful, such as ion exchange membranes and fuel cell/flow battery membranes.

Anomalous Appearance of a Distinct Glass Transition in Perfluoroimide Acid Ionomers

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

(Invited) The Glass Transition of Perfluorosulfonic Acid Membranes in Consideration of Thermal Processing-Property Relationships

Perfluorosulfonic acid ionomers (PFSAs) are the benchmark materials used as proton-exchange membranes (PEMs) in fuel cells. Conventional PFSA membranes have a chemical structure consisting of a PTFE backbone with randomly distributed perfluoroether side chains that terminate with a sulfonic acid group. These polar -SO3H groups, distributed within a matrix of non-polar PTFE segments, tend to aggregate forming a complex nanostructured network of hydrophilic domains. Further percolation of these domains (generally observed with increasing water content) provides pathways for efficient transport of protons (or other ions) and water in a variety of membrane-based electrochemical energy conversion and storage applications. To enhance or optimize the transport properties of a given PFSA, a variety of solvent casting and post-processing thermal treatments have been developed to facilitate molecular mobility during morphological development. In this presentation, we will focus on molecular-level considerations of the glass transition, T g, of PFSAs with respect to thermal drying and annealing protocols used in conventional PFSA membrane processing. Using dynamic mechanical analysis (DMA), we have previously assigned the true T g of H+-form Nafion® to be associated with a weak β-relaxation observed near -20 °C. Recently, we have found that the β-relaxations in other PFSAs (e.g., 3M PFSA and Solvay’s Aquivion®) can also be assigned to the glass transitions of these polymers. Nevertheless, the β-relaxations of all these PFSAs are weak in comparison to the dominant α-relaxation, associated with a transition from a physically crosslinked, H-bonded network to a dynamic network that allows for significant reorganization of the polar functionalities during thermal processing.

Chemical and Mechanical Reinforcement of PFSA Membranes with Functional Nanofibers

Fibre webs of polybenzimidazole modified with radical scavenger nanomaterials have been prepared and embedded in perfluorosulfonic acid ionomers.

Temperature Dependent Mechanical Behavior of Perfluorosolfonic Acid Membranes for Electrochemical Applications

Perfluorosulfonic acid (PFSA) membranes are the prototypical solid-electrolyte used in electrochemical devices such as polymer-electrolyte membrane fuel cells (PEMFCs) and electrolyzers [1]. To be commercially viable, polymer-electrolyte membranes (PEMs) in PEMFCs and electrolyzers must perform over a long operational time in both dry and wet environments. The ionic conductivity of PFSA membranes can be optimized by changing thickness, material (ionomer chemistry), and pretreatment to increase the ionic conductivity, and reduce power losses, thus improving cell efficiency. However, any effort to improve the transport functionality tends to undermine the membrane’s mechanical stability. [2] Therefore, a key challenge in PEM design is to improve transport of ions without compromising mechanical integrity and characterize the factors influencing mechanical stability of these materials in controlled environments. Most past studies on the mechanical properties of PFSA membranes investigated only tension [2], although there are some exceptions [3][4]. Compression testing, however, is critical for understanding membrane behavior in electrochemical devices. For example, in fuel cells, hydration-related swelling of a constrained membrane generates partially reversed cyclic stresses, whereas in electrolyzers, PEMS undergo static compression as high as 40 MPa. Cyclic stresses combined with a harsh chemical environment make the PEMs susceptible to failure such as pinhole formation as well as crack initiation and propagation. High static stresses make creep response a great concern. To investigate how temperature and hydration state affect the membrane’s mechanical behavior and properties under compression in-situ, a compression setup with temperature control was used. PFSA membranes of varying chemistry, equivalent weight, thickness (from 50 to 200 µm), and thermal pretreatments were tested. Our results show that the mechanical behavior of PFSA membranes is different under compression than tension. Uniaxial compression and creep tests indicate that the PFSA membranes exhibit time-dependent nonlinear response over long periods of time. This work demonstrates the importance of studying the mechanical properties of these membranes under compression, which is more relevant for the various stress-states the membrane undergoes during operation. Odgaard, M., Advanced Fluoride-Based Materials for Energy Conversion, in Advanced Fluoride-Based Materials for Energy Conversion. 2015, Elsevier. p. 325-374.Kusoglu, A. and A.Z. Weber, New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chemical Reviews, 2017. 117(3): p. 987-1104.Kusoglu, A., et al., Effect of compression on PFSA-ionomer morphology and predicted conductivity changes. Journal of Membrane Science, 2012. 421(0): p. 283-291.Kusoglu, A., et al., Role of Mechanical Factors in Controlling the Structure–Function Relationship of PFSA Ionomers. Macromolecules, 2012. 45(18): p. 7467-7476.

Quantifying Water in PEM Fuel Cell Catalyst Layer with Small Angle Neutron Scattering

A polymer electrolyte membrane (PEM) fuel cell requires effective water management to achieve high performance. Balancing the water content and distribution is particularly important in a catalyst layer (CL), consisted of catalyst on carbon (typically Pt/C) covered by ionomer films. The accumulation of product water in the pores impedes reactant transport; however, simultaneously, the lack of water results in poor proton conductance in the ionomer. A diagnostic tool for measuring water content within the CL of an operating fuel cell is necessary for the development of the CL, especially for low Pt loading and non-precious metal group catalyst [1]. The common method for measuring water content in operando fuel cells is imaging based on synchrotron X-ray and neutron sources [2]. By using imaging method, water in the membrane electrode assembly (MEA) can be visualized on the micron-scale pixel resolution. However, this is not an ideal technique for probing water content in the CL for two main reasons: (1) the water in the ionomer cannot be distinguished from that in the pores, and (2) the spatial resolution is often not sufficient (as it is several times of the pixel resolution). At the French Alternative Energies and Atomic Energy Commission (CEA), we have developed in situ small angle scattering (SAS) techniques for fuel cell application based on neutron and synchrotron X-ray sources (Institut Laue-Langevin and European Synchrotron Radiation Facility). These techniques utilize focused neutron or X-ray beam to scan the areas of the MEA, each scanned point producing a scattering pattern. By radial averaging, the scattering pattern is converted into Q-range. The scattering due to the perfluorosulfonic acid (PFSA) ionomer produce a peak in the range of 0.1-0.25 Å-1, which arises from the separation between hydrophobic and hydrophilic domain. The water uptake in ionomer can be deduced from the shift of the peak position resulting from the swelling upon hydration. On the other hands, the overall water content can be obtained by either direct transmission or analyzing the high Q-range (0.4-0.45 Å-1). We previously measured liquid water accumulation (in the channels and the GDL) and water uptake in the membrane by through-plane SAS (i.e. the fuel cell is aligned perpendicular to the beam) [3]. Recently, we have further refined the custom fuel cell and SAS technique to probe water content only in the CLs. This was accomplished by developing a cell with an active area of 0.1 cm2 (0.1 mm wide in the path of the beam) and narrowing the beam size to 3 and 15 µm (for X-ray and neutron, respectively). This work will provide an overview of in situ SAS technique for fuel cells and focus on the comparison of two types of SAS investigations (neutron- vs. synchrotron X-ray-based).

Sol-Gel Assisted Synthesis of Encased Polymer Electrolyte Fuel Cell Cathode Catalysts

State-of-the-art polymer electrolyte fuel cell (PEFC) cathode catalyst layers (CCLs) are composites of proton-conducting perfluorosulfonic acid (PFSA) ionomer and platinum alloy catalyst particles. The PFSA is in the form of agglomerates dispersed throughout the CCL and also as a thin film coating the carbon support and platinum alloy particle surface.1 The thin PFSA film on the catalyst surface has been shown to both decrease oxygen reduction reaction (ORR) activity and cause high oxygen transport resistance.2-4 Such ionomer-catalyst surface interactions can negatively affect both PEFC performance and durability. This talk will describe an approach to addressing the issues of ionomer poisoning of ORR activity, local oxygen transport resistance, and the lack of PEFC cathode performance durability by encasing Pt and Pt alloy catalyst particles in the cavity of a 3D porous inorganic framework. Platinum (Pt) and platinum alloys (PtxMy) encased in 3D porous inorganic nanocrystal catalysts have been prepared by a sol-gel process combined with chemical reduction. Catalysts with 5, 10 and 20 Pt/PtxMy wt% have been prepared and their ORR activity has been evaluated using the thin-film rotating disk electrode (TF-RDE) technique. The platinum content in the encased catalysts has been determined using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF). The morphology, crystal structure, and spatial distributed of the metal nanoparticles have been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectroscopy, respectively. References C. Cetinbas, R. K. Ahluwalia, N. N. Kariuki, and D.J. Myers, J. Electrochem. Soc., 165(13), F1051 (2018).Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 7, 1127 (2016).Subbaraman, D. Strmcnik, A. P. Paulikas, V. R. Stamenkovic, and N. M. Markovic, Chemphyschem, 11, 2825 (2010).Subbaraman, D. Strmcnik, V. Stamenkovic, and N. M. Markovic, J. Phys. Chem. C, 114 8414 (2010). Acknowledgements This work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.

Cyclic Hygral Swelling and Shrinkage Behavior of Fuel Cell Membranes

Mechanical membrane durability is a key factor for the overall lifetime of PEM fuel cells. It is therefore important to understand the impact of the dynamic membrane hygral swelling and shrinkage characteristics on the in-situ mechanical stresses and the associated mechanical durability. In this work, an in-depth experimental investigation was carried out to determine the cyclic hygral swelling and shrinkage properties of four commonly used types of fuel cell membranes: Nafion® NRE211, two different expanded polytetrafluoroethylene (ePTFE) reinforced perfluorosulfonic acid ionomer membranes, and one hydrocarbon membrane. In addition to the bare membranes, the effect of coating the membranes with catalyst layers on the hygral swelling/shrinkage behavior was also investigated. Tensile specimens were subjected to ex-situ hydration and dehydration cycles using a dynamic mechanical analyzer (TA Instruments Q800 DMA) equipped with an environmental chamber, wherein the dynamic swelling and shrinkage characteristics as well as the residual stresses were measured for each membrane by displacement sensor and load cell, respectively. Hydrocarbon and reinforced PFSA ionomer membranes revealed higher shrinkage than swelling at each RH cycle, resulting in an overall shrinkage. Inside a fuel cell stack where membrane swelling and shrinkage is constrained, the observed shrinkage would result in internal tensile stress in the membrane and adjacent components, and hence reduce the overall membrane mechanical durability. The accumulation of residual stress during successive RH cycles was therefore measured explicitly using a custom-designed constrained shrinkage test and compared to the tensile properties of each individual membrane. For example, the hydrocarbon membranes analyzed in this work showed large residual stresses (severe contraction) during confined shrinkage at 80oC that exceeded their yield stress, and are therefore deemed prone to failure after repeated RH cycles. In addition, the hygral swelling and shrinkage behavior of each membrane was compared to the corresponding catalyst-coated membranes and the reinforcement offered by the catalyst layers was determined. The identified swelling and shrinkage properties, generalized into three different categories of overall cyclic behavior, have vital implications for the development of durable fuel cell membranes and membrane electrode assemblies. In addition to providing good mechanical strength, the membranes are also expected to accommodate or preferably eliminate progressive shrinkage stresses during dynamic fuel cell operation.

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

Ce Cation Migration and Diffusivity in Perfluorosulfonic Acid Fuel Cell Membranes

A hydrogen pump experiment was utilized to simultaneously determine the migration and diffusivity of Ce ions in perfluorosulfonic acid (PFSA) ionomer membranes over a range of temperatures and relative humidities. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). Competing transport phenomena were decoupled by fitting XRF profile data with our previously-developed one-dimensional model, which was updated with improved conductivity and water uptake relations. Measured transport values showed good agreement with the Einstein relation and Okada transport theory, implying that conductivity can be used to estimate migration and diffusivity of cations in other cation/PFSA systems. Results presented here may be used to populate device-level models in order to further understand the effects of cation transport on fuel cell performance and durability to determine the mitigation controls for cation stabilization.

Anomalous Appearance of a Distinct Glass Transition in Perfluoroimide Acid Ionomers

This work presents the first in-depth study on the thermal and dynamic mechanical properties of multi-acid perfluorinated ionomers developed by 3M. Perfluoroimide acid ionomer (PFIA) contains two acidic sites per sidechain as opposed to conventional perfluorosulfonic acid ionomers (PFSA) with only one. This additional acidic site leads to remarkable improvements in proton conductivity and presents a novel membrane for use as proton exchange membranes in fuel cells. Interestingly, the additional acidic site on PFIA leads to unexpected differences in thermal and mechanical properties, including the appearance of a distinct glass transition temperature otherwise not seen in PFSA ionomers. Here we probe the apparent glass transition and assign it to the true glass transition temperature of PFIA.

Effect of Pt Loading on the Adsorption of Perfluoro-sulfonic Acid Ionomer in Catalyst Ink for Polymer Electrolyte Fuel Cells

The interfacial structure of the perfluoro-sulfonic acid ionomer and Pt-loaded carbon in a catalyst ink significantly influences the properties of the catalyst layer in polymer electrolyte fuel cel...

(Invited) Transport-Stability Relationships in Composite Polymer-Electrolyte Membranes

Research activities and interests in electrochemical conversion devices (e.g., fuel cells, flow batteries, and electrolyzers, etc.) have been continuously increasing due to their great potential to provide clean and renewable energy technologies for stationary, transportation, and solid-state applications. At the heart of these devices is the polymer-electrolyte membrane (PEM) that functions as an electrolyte and a separator. Ion-conductive polymers (ionomers) are the ubiquitous choice for PEMs as they inherently conduct ions and therefore could provide key roles such as ion/solvent transport and gas/reactant separation.1, 2 Nevertheless, diverse operating environment and performance metrics in these electrochemical devices require optimized ionomer multi-functionalities with a mechanically robust matrix.2-4 An improvement in transport functionality in these ionomers, however, usually undermines their mechanical stability and separator functionality.1 Thus, a challenge in these devices is achieving sustainable performance in their solid-state electrolyte membranes, which requires a synergistic effort to meet the performance and durability demands, i.e., to improve transport without compromising mechanical integrity.2 Accomplishing this requires, first, an understanding of how the ionomer’s transport and mechanical properties are interrelated through the morphology, and then, altering this structure-transport relationship by incorporating performance additives and mechanical support in ionomers. Next-generation polymer-electrolyte membranes contain not only an ionomer, but also additional materials that provide complementary functionalities,5-7 which create an even more complex structure with varying heterogeneities across the lengthscales, i.e. from microscopic composite morphology to changes in local interactions at nanoscales. In such a structure, it becomes challenging to establish a structure-property relationships for these hybrid ionomers and explore the balance between their mechanical and chemical properties. In this talk, an overview of the structure/property relationship of perfluorosulfonic acid (PFSA)-based ionomers and their composites will be presented with a focus on comparison of their conductivity-hydration correlations and transport-mechanical properties. In particular, we will examine properties of Nafion XL composite membrane with a porous reinforcement layer,6 as well as an electro-spun Nafion/PVDF polymer membrane.5 Water uptake, conductivity and nanostructural features of these membranes will be analyzed to provide insights into how the presence of distinct secondary materials affects the membrane’s “composite” response, compared to the baseline PFSA membrane.2 In addition, the concept of newly formed phases in these membranes that already contain a phase-separated ionomer will be discussed along with the critical lengthscales for ionomers that could influence their structure-functionality. It will also be explored how hydration-conductivity correlations observed in PFSA ionomers could be extended to various composite structures by accounting for the macroscopic and microscopic changes in morphology. Lastly, the interplay between the transport properties and mechanical response will be elucidated to identify the key factors in this new paradigm for hybrid ionomers, which could help development of robust high-performance membranes that can meet the design requirements for various electrochemical energy devices. Acknowledgements We acknowledge Peter Pintauro of Vanderbilt University for providing us with the electrospun membranes. AK acknowledges the ECS-Toyota Young Investigator Fellowship. This work made use of facilities at the Advanced Light Source, which is a user facility supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy. Part of this work was funded through the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), of the office of the Energy Efficiency and Renewable Energy (EERE), of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References Kusoglu, A.; Weber, A. Z. Chem. Rev. 2017, 117, (3), 987-1104.Kusoglu, A.; Weber, A. Z. J Phys Chem Lett 2015, 6, (22), 4547-52.Mukundan, R.; Baker, A. M.; Kusoglu, A.; Beattie, P.; Knights, S.; Weber, A. Z.; Borup, R. L. J. Electrochem. Soc. 2018, 165, (6), F3085-F3093.Tang, Y. L.; Kusoglu, A.; Karlsson, A. M.; Santare, M. H.; Cleghorn, S.; Johnson, W. B. J. Power Sources 2008, 175, (2), 817-825.Woo Park, J.; Wycisk, R.; Lin, G.; Ying Chong, P.; Powers, D.; Van Nguyen, T.; Dowd Jr, R. P.; Pintauro, P. N. J. Membr. Sci. 2017, 541, (Supplement C), 85-92.Shi, S. W.; Weber, A. Z.; Kusoglu, A. J. Membr. Sci. 2016, 516, 123-134.Ballengee, J. B.; Pintauro, P. N. J. Electrochem. Soc. 2011, 158, (5), B568-B572.

(Invited) Optimizing the Structure and Composition of PFSA/PVDF Fuel Cell Membranes

In order to control water uptake and improve the mechanical properties of a highly charged cation or anion exchange membrane, an uncharged polymer is often added to the membrane in one of two ways: (1) the uncharged polymer is added to the membrane casting solution to create a blended polymer film or (2) a composite membrane is made where ionomer is impregnated into a porous uncharged polymer network/support. Improvements in both approaches can be achieved by use of nanofiber electrospinning. Thus, one can electrospin a blended polymer solution of mutually incompatible charged and uncharged polymers, where good shear mixing at the spinneret tip and fast fiber drying minimizes phase separation and where hot-pressing of the resultant fiber mat results in a dense and defect free film. Similarly, dual fiber electrospinning of separate ionomer and uncharged polymer solutions followed by hot pressing/densification circumvents problems associated with impregnating an ionomer solution into a pre-formed reinforcing porous mat. In this talk, three methods of membrane fabrication will be described and contrasted: (1) solution-cast films of a blended polymer solution, (2) membranes from blended electrospun fibers, and (3) membranes from dual fiber electrospun mats. Membranes 20-25 μm in thickness were prepared using a perfluorosulfonic acid (PFSA) ionomer from 3M Co. (660, 725, or 825 EW) and poly(vinylidene fluoride) (PVDF). The effects of membrane composition, i.e., the EW of the PFSA and the PFSA/PVDF weight ratio, and the membrane preparation procedure on proton conductivity, water swelling (gravimetric and in-plane), and membrane mechanical properties will be discussed. In general, it was found that membranes prepared by separately electrospinning PFSA and PVDF exhibited the best combination of high proton conductivity and low in-plane water swelling – two properties relevant to membrane use in a H2/air fuel cell. Acknowledgement: The authors thank the US DOE (EERE Cooperative Agreement No. DE-EE0006362) for funding this work, as well as Dr. Michael Yandrasits at 3M for providing the PFSA ionomers used in this study.