Structure Of Ionomers Research Articles

Molecular Simulation of Nafion Swelling in the Cathode Catalyst Layer of Polymer Electrolyte Fuel Cells

As the world moves towards carbon neutrality, polymer electrolyte fuel cells (PEFCs) have emerged as a vital technology in recent years. The cathode catalyst layer of PEFCs is characterized by its porous structure, which promotes the production of water molecules from oxygen and protons through chemical reactions. The ionomer thin films, predominantly composed of Nafion, are critical in regulating the transport and mobility of molecules and ions in the cathode catalyst layer. Sulfonic groups in the side chains of Nafion enable accumulation of water molecules, causing the ionomer thin films to swell.Previous studies have examined the swelling of Nafion under wet conditions[1][2]. However, precise swelling mechanisms remain unclear. While privious research has analyzed the relationship between relative humidity and the amount of water within bulk Nafion samples[3], the swelling behavior in films with thickness of several nanometers differs significantly, necessitating further investigation[4]. This study therefore investigates the swelling mechanisms and the properties of ionomer thin films used in the cathode catalyst layer of PEFCs. This study aims to establish guidelines for designing the molecular structures of ionomers by elucidating the mechanisms underlying ionomer swelling in the cathode catalyst layer. Grand canonical Monte Carlo (GCMC) simulations and molecular dynamics (MD) simulations were employed to reproduce swelling in Nafion thin films and examine the impact of Nafion's structure on water content and swelling. These molecular simulation techniques enable the elucidation of the influence exerted by polymer structure on water content and swelling at the molecular scale.In this study, we firstly created a simulation model of a Nafion system using LAMMPS[5], the DREIDING Force Field[6], and Mulliken charges[7] in which parameter settings were referred from literature values[8]. The Nafion system contains 10 Nafion molecules with a degree of polymerization of 10, water molecules, and 100 oxonium ions for electrical neutrality. The density of 1.9 g/cm3 achieved during simulations closely agrees with literature values[8].To reproduce Nafion swelling under humid conditions, GCMC simulations and MD simulations were employed. The rationale behind utilizing GCMC simulations lies in the fact that they enable the insertion of water molecules into the Nafion system under constant chemical potential conditions, thereby inducing Nafion swelling. Subsequent to the GCMC simulations, MD simulations were performed under the NPT ensemble, which facilitated the system to reach equilibrium. This alternating cycle of GCMC and MD simulations is repeated to realize the final swelling state. Preliminary results indicate lower water content values than those experimentally measured under similar conditions. Possible causes include inadequate force field parameters that lead to polymer morphology different from the actual Nafion structure.Future work will explore the causes of these discrepancies and examine the effects of Nafion's structure on water content and swelling under various conditions. This will involve testing alternative force field parameters and initial structures to better represent the molecular-level structure of ionomer thin films on carbon support particles in the cathode catalyst. Additionally, we will investigate how different relative humidity conditions, temperatures, and ionomer compositions influence swelling behavior, which could provide insights into optimizing PEFCs performance under various operating environments.[1] G. C. Abuin et al., J. Membr. Sci. 428, 507 (2013).[2] J. Catalano et al., Int. J. Hydrog. Energy 37, 6308 (2012).[3] T. A. Zawadzinski et al., J. Electrochem. Soc. 140, 1981 (1993).[4] M. A. Modestino et al., Macromolecules 46, 867 (2013).[5] A. P. Thompson et al., Comp. Phys. Comm. 271, 10817 (2022).[6] S. L. Mayo et al., J. Phys. Chem. 94, 8897 (1990).[7] S. S. Jang et al., J. Phys. Chem. B 108, 3149 (2004).[8] Y. Kurihara et al., J. Power Sources 414, 263 (2019).Fig. 1 An ionomer model composed of Nafion molecules, oxonium ions, and water molecules. The red and white spheres represent oxygen and hydrogen atoms making up water molecules. The size of the water molecules is emphasized for clarity. Figure 1

Influence of Sidechain Length in Perfluoro-Sulfonic Acid Ionomer on Electrode Slurry Interactions and Finalized Microstructure in the Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) have emerged as one of the most promising next-generation energy devices for both passenger-owned and heavy-duty vehicles due to their high energy efficiency, low emissions, and quiet operation. However, the performance of the fuel cell electrode is often limited by the strong specific adsorption of long sidechain (LSC) perfluoro-sulfonic acid (PFSA) ionomer onto the platinum (Pt) catalyst2, which results in a decrease in the electrocatalytic activity1 as well as increase oxygen mass transport resistenace at the MEA level. To address this issue, a short sidechain (SSC) ionomer has been developed, which has been found to exhibit excellent efficiency over LSC ionomer3 The reduced specific adsorption of SSC ionomer onto the Pt catalyst results in a higher available surface area for the electrochemical reaction, which improves the overall fuel cell performance. The sidechain length of ionomers plays a crucial role in determining the overall cell performance4 however, due to the differences between them, the interactions among slurry components are not yet fully understood.Unlike the previous literatures, we investigates the details of ionomer sidechain length and its impact on the interaction among slurry components and resulting electrode microstructure, through a comparative analysis of LSC and SSC ionomers.5 Our hypothesis propose that the physical interactions between components are considerably impacted by the sidechains with a specific emphasis on the mobility and ion-pair association in the slurry. To probe the influence of ionomer sidechains, we conducted a thorough investigation wherein incrementally varied its concentration from 0 to 1.5 mmolSO3-gC -1, at intervals of 0.3 mmolSO3-gC -1. The main purpose of this evaluation was to observe the evolution of particle aggregation according to the correlation between adsorbed ionomer and free ionomer in electrode slurry. Drawing from the U-shaped viscosity profile obtained through rheological measurements, we propose an optimized ionomer concentration of 0.6 mmolSO3-gC -1 for the SSC ionomer-based slurry and 0.9 mmolSO3-gC -1 for the LSC ionomer-based slurry. To characterize the microstructure of the as-prepared electrode, we employed scanning electron microscopy (SEM) and confocal microscopy, with a focus on particle agglomeration and the homogeneous ionomer distribution. Finally, the electrochemical analysis was performed on the fabricated membrane electrode assembly (MEA) in a 25 cm² single cell for a cathode Pt loading of 0.1 mgPtcm⁻². As a result, the optimized SSC ionomer-based electrode exhibited low local O₂ transport resistance and high Pt utilization, leading to over a 40 % enhancement in fuel cell performance at 0.6 V with a high proton accessibility value above 0.85 when compared to the optimized LSC ionomer-based electrode. This study highlights the crucial importance of understanding the effects of ionomer sidechain length on electrode slurry interactions and microstructure and establishes a significant correlation between them. Our findings contribute to the ongoing development of high-performance PEFCs, offering valuable insights into optimizing electrode composition and structure for enhanced fuel cell performance.(1) Ahn, C.-Y.; Park, J. E.; Kim, S.; Kim, O.-H.; Hwang, W.; Her, M.; Kang, S. Y.; Park, S.; Kwon, O. J.; Park, H. S. Differences in the electrochemical performance of Pt-based catalysts used for polymer electrolyte membrane fuel cells in liquid half-and full-cells. Chemical Reviews 2021, 121 (24), 15075-15140.(2)Kodama, K.; Motobayashi, K.; Shinohara, A.; Hasegawa, N.; Kudo, K.; Jinnouchi, R.; Osawa, M.; Morimoto, Y. Effect of the side-chain structure of perfluoro-sulfonic acid ionomers on the oxygen reduction reaction on the surface of Pt. ACS Catalysis 2018, 8 (1), 694-700. (2) Garsany, Y.; Atkinson, R. W.; Sassin, M. B.; Hjelm, R. M.; (3) Gould, B. D.; Swider-Lyons, K. E. Improving PEMFC Performance Using Short-Side-Chain Low-Equivalent-Weight PFSA Ionomer in the Cathode Catalyst Layer. Journal of The Electrochemical Society 2018, 165 (5), F381-F391.(4) Ramaswamy, N.; Kumaraguru, S.; Koestner, R.; Fuller, T.; Gu, W.; Kariuki, N.; Myers, D.; Dudenas, P. J.; Kusoglu, A. Editors’ Choice—Ionomer Side Chain Length and Equivalent Weight Impact on High Current Density Transport Resistances in PEMFC Cathodes. Journal of The Electrochemical Society 2021, 168 (2), 024518.(5) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chemical Reviews 2004, 104 (10), 4535-4586. DOI: 10.1021/cr0207123.

Polymer Design Strategies for Alkaline Membrane Water Electrolysis

Water electrolysis is a carbon-free hydrogen production technology, making it a promising candidate to mitigate aggravating climate change. Ion exchange membranes are an essential component of membrane-based water electrolysis, enabling high hydrogen production efficiency through a zero-gap configuration. The recent research efforts to develop anion exchange membranes and electrode ionomers have made significant improvement in durability of alkaline membrane water electrolyzers without sacrificing performance and efficiency. Still, there are several factors related to polymer electrolytes that contribute to the voltage loss for water splitting in the alkaline condition including electrode local pH, the interface between membrane and electrodes, and electrochemical oxidation of ionomers at high potentials. This presentation will provide the state-of-the-art performance of alkaline membrane water electrolyzers with respect to the structure of anion exchange membranes and ionomers. The material design strategies and future directions to develop advanced polymer electrolytes will be discussed for this green hydrogen production technology.Reference: Park, E.J., Arges, C.G., Xu, H., Kim, Y.S. ACS Energy Lett. 2022 , 7, 10, 3447–3457Motz, A.R., Li, D., Keane, A., Manriquez, L.D., Park, E.J., Maurya, S., Chung, H., Fujimoto, C., Jeon, J., Pagels, M.K., Bae, C., Ayers, K.E., Kim, Y.S. Mater. Chem. A 2021 , 9, 22670-22683Matanovic, I., Kim, Y.S. Current Opinion in Electrochemistry 2023 , 38, 101218Leonard, D.P., Lehmann, M., Klein, J.M., Matanovic, I., Fujimoto, C., Saito, T., Kim, Y.S. Advanced Energy Materials 2023 , 13, 2203488

Explicit Polarization in Coarse-Grained Simulations of Ionomer Melts

The structure and morphology of ionic aggregates in ionomer melts significantly influence their ion transport properties. Understanding the underlying mechanisms and relevant time scales of ion transport can facilitate design of viable ionomer materials as single-ion conductors for energy applications. Previous studies have characterized the ionic aggregate structure, morphology, and dynamics using non-polarizable coarse-grained molecular dynamics (CGMD) simulations. In this work, we examine the role of polarization in ionomer melts by explicitly incorporating Drude oscillators in CGMD simulations. We systematically study the structure and dynamics of pendant ionomers, focusing on the comparison to non-polarizable systems. Polarization within the ion clusters leads to less overall ion structuring. On the aggregate scale, less ion structuring yields smaller ionic aggregates. The extent to which the clusters are smaller delicately depends on the strength of the electrostatic interactions (the dielectric constant). Under certain conditions, we find that the time scale for free counterion diffusion does not depend on the morphology, unlike that in a non-polarizable model.

(Invited) Poly(aryl-co-aryl piperidinium) Copolymers for High-Performance Anion Exchange Membrane Fuel Cells

Poly(aryl piperidinium)s (PAPs) have attracted great attention as durable anion exchange membrane (AEMs) and ionomers for AEM fuel cells (AEMFCs). However, most of PAP AEMs display poor microphase morphology, and ionomer research on PAPs is still lacking to date. Here, we report a series of high-performance poly(aryl-co-aryl piperidinium) copolymers (c-PAPs) for AEMs and ionomers, along with improving AEMFC performance (Figure 1). Compared with PAPs, c-PAP AEMs possess higher ion conductivity (>200 mS cm-1), enhanced mechanical properties (tensile strength>80 MPa), lower hydrogen permeability (<10 Barrer) and optimized microphase morphology. On the other hand, c-PAP ionomers display lower ionomer adsorption on platinum-group-metal (PGM) catalysts and higher electrochemical performance than PAP ionomers. Based on poly(diphenyl-co-terphenyl piperidinium) AEMs (PDTP) and poly(fluorenyl-co-biphenyl piperidinium) (PFBP) ionomers, the peak power density (PPD) of presented AEMFCs reaches 2.67 W cm-2 in H2-O2 and 1.38 W cm-2 in H2-air (CO2 free) at 80 oC based on optimized c-PAP AEMs and ionomers. Moreover, c-PAP based AEMFCs reach >1000 h in-situ durability under 1 A cm-2 current density in H2-O2 at 60oC. The presented works provide insight into c-PAP AEMs and ionomers. Figure 1. Chemical structure of c-PAP AEMs and ionomers, along with high PPDs of AEMFCs over 2.6 W cm-2. Reference [1] N. J. Chen and Y. M. Lee. Progress in Polymer Science. 2021, 113, 101345.[2] N. J. Chen, C. Hu, Y. M. Lee, et al. Angew. Chem. Int. Ed. 2021, 60, 7710-7718.[3] N. J. Chen, H. H. Wang, Y. M. Lee, et al. Nat. Commun. 2021. DOI. Figure 1

Hydrogel Ionic Diodes toward Harvesting Ultralow‐Frequency Mechanical Energy (Adv. Mater. 36/2021)

Energy Harvesting Energy harvesting from human motion is regarded as a promising protocol for powering portable electronics. In article number 2103056, Chang Kyu Jeong, Wen Chen, Qing Wang, and co-workers demonstrate a hydrogel ionic diode based on the stacked structures of anionic and cationic ionomers embedded with conductive nanomaterials networks for tactile sensing, pressure imaging, and touchpad applications.

Hydrogel Ionic Diodes toward Harvesting Ultralow‐Frequency Mechanical Energy

AbstractEnergy harvesting from human motion is regarded as a promising protocol for powering portable electronics, biomedical devices, and smart objects of the Internet of things. However, state‐of‐the‐art mechanical‐energy‐harvesting devices generally operate at frequencies (&gt;10 Hz) well beyond human activity frequencies. Here, a hydrogel ionic diode formed by the layered structures of anionic and cationic ionomers in hydrogels is presented. As confirmed by finite element analysis, the underlying mechanism of the hydrogel ionic diode involves the formation of the depletion region by mobile cations and anions and the subsequent increase of the built‐in potential across the depletion region in response to mechanical pressure. Owing to the enhanced ionic rectification ratio by the embedded carbon nanotube and silver nanowire electrodes, the hydrogel ionic diode exhibits a power density of ≈5 mW cm−2 and a charge density of ≈4 mC cm−2 at 0.01 Hz, outperforming the current energy‐harvesting devices by several orders of magnitude. The applications of the self‐powered hydrogel ionic diode to tactile sensing, pressure imaging, and touchpads are demonstrated, with sensing limitation is as low as 0.01 kPa. This work is expected to open up new opportunities for ionic‐current‐based ionotronics in electronics and energy devices.

Modifying the Structure and Dynamics of Ionomers through Counterion Sterics

The role of counterion sterics on the structure and dynamics of a low glass transition temperature, amorphous poly(isoprene-ran-styrenesulfonate) copolymer was investigated using a series of symmetric, tetraalkylammonium counterions with methyl (TMA), ethyl (TEA), propyl (TPA), and butyl (TBA) pendent groups, in addition to a sodium (Na) control. A detailed analysis of the aggregate structure was achieved by fitting the copolymers’ X-ray scattering profiles with a modified hard sphere model. Increasing the counterion sterics from Na to TEA resulted in slight changes to the aggregates with some ionic groups present in the isoprene matrix. For the more hindered TPA and TBA counterions, considerable disruption of the structure occurred. Using solid-state NMR, dynamic mechanical analysis, and rheology, the effect of the counterion sterics on the copolymer’s dynamics was determined. The T1ρ relaxation of the copolymers revealed a rigid isoprene fraction, associated with the ion clusters, and a mobile isoprene ...

Influence of two‐phase behavior of ethylene ionomers on diffusion of water

ABSTRACTIn this study, diffusion of water into ethylene ionomers neutralized with sodium or zinc cations (Na‐ionomer and Zn‐ionomer, respectively) was studied by in situ Fourier transform near infrared (NIR) spectroscopy. Water ingress was measured by NIR from the edge of the laminated glass toward the center and the measured water concentration profiles were analyzed by Fick's second law or the modified Jacobs–Jones model, which describes diffusion into a two‐phase system of different density and polarity. It was found that the modified Jacobs–Jones model agrees well with the measured water concentration profiles of ionomers, while the Fickian model fits well to the poly(vinyl‐butyral) (PVB) results. The modified Jacobs–Jones model was also confirmed to apply to water vapor transmission rate (WVTR) data of another sodium ionomer (Na‐ionomer‐HTM) with a higher melting temperature than Na‐ionomer. The water diffusion coefficient for Na‐ionomer, Zn‐ionomer, and PVB are compared, and the diffusion parameters are discussed considering the higher‐ordered structures of ionomers studied by small‐angle X‐ray scattering (SAXS). Simulating conditions that PV modules encounter in field applications, model projections show that Zn‐ionomer and Na‐ionomer‐HTM give significantly lower water permeation than PVB, and due to its low diffusion coefficient, Na‐ionomer will have significantly less water ingress than PVB. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48929.

Structures, properties and applications of the polyurethane ionomers

On the basis of the reports from 2010 to 2018, the chemical structure, production methods and applications of polyurethane ionomers were reviewed. The paper presents ionogenic reagents and counterions responsible for the incorporation of anionic and cationic groups into polyurethane chains and the resulting physicochemical properties of these polymers. The most important applications of synthesized ionomers as a waterborne polyurethane, elastomer materials, biomaterials and materials for special applications (electronics, nanocomposites) were presented.

Optical Properties and Swelling of Thin Film Perfluorinated Sulfonic-Acid Ionomer

Perfluorinated sulfonic-acid (PFSA) ionomers are members of a class of ion-conductive polymers that are commonly used as a proton-exchange membrane or catalyst layers in polymer-electrolyte fuel cells. Numerous studies have been undertaken to understand how the structure of PFSA ionomers with thickness of 1 to 200 µm and their water uptake affect the material properties and the performance of fuel cells. Thin film confinement affects the hydration behavior of thin film ionomers; the deviation from transport properties are still unclear. Advanced tools have been used to study water uptake of sub-micron PSFA ionomers at different humidity levels; however, their temporal resolution are of the order of 1 minute. Label free, high-speed Surface Plasmon Resonance imaging (SPRi) is proposed to provide a 3D map of a thin film (<300 nm) at high thickness resolution (~ 0.1 nm), high lateral resolution (~ 4 μm), and high temporal resolution (up to 3000 frames per second). Nafion films with 28-250 nm and 3M films with 7-150 nm are used for a systematic study of the swelling and diffusion transport phenomena of water in thin film ionomers deposited on a 50 nm gold film on a glass substrate. SPRi with angular modulation (AM) and intensity modulation (IM) is employed to determine the swelling ratios of the thin film PSFA’s at different humidity levels. SPRi with IM is used to study the kinetics of diffusion transport of water in the thin film ionomers. The preliminary results show that the water uptake in the thin film ionomers locally decreases the refractive index of the material, whose changes are tracked by SPRi. This can provide the diffusion map during water transports through the thin film ionomer.

Adsorption Behavior of Perfluorosulfonic Acid Ionomers on a Pt(111) Surface Observed By Atomic Force Microscopy

Understanding of the adsorbed structure of ionomers on the Pt electrocatalyst and carbon support surfaces are very important to improve the cell performance of polymer electrolyte membrane fuel cells (PEMFCs). The effect of the solvent composition and temperature on the adsorbed structure of ionomers have been studied by using various techniques, such as 19F nuclear magnetic resonance (19F NMR) spectroscopy, cryogenic scanning electron microscopy (cryo-SEM), dynamic light scattering (DLS), small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and electron spin resonance (ESR) techniques. However, it has not been fully clarified yet, due to the complexity of the system composed of a variety of ingredients including the Pt electrocatalyst, carbon support, ionomers, solvent and other contaminants. Thus, we have investigated the adsorbed structure of ionomers on the Pt(111) surface as a model of an electrocatalyst by using atomic force microscopy (AFM) at various solvent compositions. A droplet of aqueous solutions of Nafion was placed on Pt (111) surfaces mounted on the AFM sample holder. AFM measurements were performed with Acoustic AC mode AFM using Agilent 5500 AFM microscope (Agilent Technologies, USA) with a silicon nitride cantilever. The images were recorded over a period of ~30 minutes with a scanning area of 1 µm × 1 µm. In a 2 wt% aqueous solution of Nafion, relatively small aggregates of Nafion ionomers with a height of ~2-3 nm were observed at the Pt(111) surface. Amount of adsorption increased with time and, eventually, the surface was entirely covered by randomly oriented aggregates with a thickness of ~4 nm. In a 0.2 wt% aqueous solution, aggregates larger than those observed in the 2 wt% aqueous solution were imaged, suggesting that larger aggregates were formed in the lower concentration solution. Such a concentration-dependent adsorption behavior is further discussed on the basis of solvent composition and conformation of Nafion ionomers in liquid phase. References Masuda, H. Naohara, S. Takakusagi, P. R. Singh, K. Uosaki: Chemistry Letters, 38, 884 (2009).Masuda, S. Faridah, P. Singh, H. Naohara, K. Uosaki, Journal of Physical Chemistry C, 117, 15704 (2013).Masuda, K. Ikeda, K. Uosaki, Langmuir, 29, 2420 (2013).

Tetra-alkylsulfonate functionalized poly(aryl ether) membranes with nanosized hydrophilic channels for efficient proton conduction

Abstract The microstructure of polymer electrolyte membranes plays a key role in ion conductivity and water transport. Herein, fluorinated poly(aryl ether)s with tetra-alkylsulfonate side chains (SFPAEs) have been successfully synthesized from the copolymerization of a newly developed tetra-allyl-containing bisphenol (TABP) monomer, followed by the thiol-ene addition with sodium 3-mercapto-1-propanesulfonate to attach the ionic groups at the end of the flexible chains. Being the first of its kind, the densely distributed and lengthy alkylsulfonate group possesses the benefit of ease to self-assemble into hydrophilic domains during membrane preparation via solution casting. Indeed, the TEM characterizations revealed that distinct hydrophilic channels of ∼1 − 2 nm width had been formed, much larger than those of a home-made control sample where only di-alkylsulfonate side chains were attached. The SFPAE-4-45 with an IECw of 2.0 mmol g − 1 exhibited an enhanced proton conductivity of 143.7 mS cm−1 at room temperature, which was superior to that of Nafion 212 (91.0 mS cm−1). Furthermore, the oxidative stabilities of SFPAEs were significantly higher than those of non-fluorinated analogs in literature. This study offered a new route to engineering the pendent structure of ionomers for well-defined microscopic morphologies.

Structure and permeability of ionomers studied by atomistic molecular simulation combined with the theory of solutions in the energy representation.

Ionomers play a key role in forming the catalyst layer of polymer electrolyte fuel cells. In the present work, we performed atomistic molecular dynamics simulations and free-energy calculations with the energy-representation method for sulfonated polyethersulfone (SPES) and its derivatives toward the rational design of ionomers for carbon alloy catalysts. It was observed that H2O aggregates strongly in the branched SPES systems with fluorocarbons and is located homogeneously in the systems without fluorocarbons. The O2 permeability was then examined within the framework of the solubility-diffusion mechanism. The permeability was seen to be large for the branched SPES with fluorocarbons, indicating that the performance of ionomers as a permeation medium for O2 may be tuned by the flexibility and branching of the polymer chain.

Effect of added ionic liquids on the solubility and viscosity of dilute ionomer solutions in tetrahydrofuran

AbstractSolubility and viscosity behaviors of three ionomers with comparable ion contents of ca 6 mol%, i.e. poly(methyl methacrylate) sodium salt, sulfonated polystyrene calcium salt and sulfonated polystyrene sodium salt, were studied in a low‐polarity solvent, i.e. tetrahydrofuran (THF), containing an ionic liquid (IL). Upon addition to THF, the IL disrupts ionic aggregates to form a homogeneous solution and increases viscosity, because the IL disrupts intramolecular ionic aggregates at dilute concentration. Among eight ILs of the imidazolium family studied, 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate performs best. The effectiveness of ILs for disrupting ionic aggregates is discussed in terms of the structures of ionomers and ILs. © 2017 Society of Chemical Industry

Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers on the Oxygen Reduction Reaction on the Surface of Pt

The effect of side-chain structures in perfluoro-sulfonic acid ionomers on the adsorption of the terminal sulfonate moiety on the surface of Pt is investigated with voltammetry and surface-enhanced infrared absorption spectroscopy (SEIRAS). Analyses with low-molecular-weight model anions with and without an ether group in the perfluoro-alkyl chain indicate that the anions are adsorbed on Pt through one or two oxygen atom(s) of the terminal sulfonate group and that the oxygen atom of the ether group also interacts with the Pt surface, leading to stronger adsorption of the anions with an ether group. On the basis of the results obtained with the model anions, the adsorption of the terminal sulfonate moieties in perfluorinated sulfonic acid ionomers and its effect on oxygen reduction reaction (ORR) is discussed. It is shown that the ionomers having longer side chains more strongly block ORR due to the flexibility of the side chains.

Assembled Structures of Perfluorosulfonic Acid Ionomers Investigated by Anisotropic Modeling and Simulations.

Nafion, a classic of perfluorosulfonic acid ionomers, has broad applications in proton conduction, attributed from the unique structures. However, a satisfactory structure model from theoretical calculation and simulation that can match with the well-known experimental observations is still absent. We performed GPU-accelerated molecular dynamics simulations to investigate the assembled structures of Nafion at different water contents based on an anisotropic coarse-grained model equipped with Gay-Berne potential. Accurate parameters for the coarse-grained model are collected by matching energy profiles based on density functional theory calculations. The results show that the hydrophilic phase in Nafion assemblies undergoes a crossover from isolated spherical clusters to interconnected cluster/channel networks with the increase of water content. We found the crystalline domains in polymer matrix and they are suppressed at elevated water content. These microphase-separated structures achieve quantitative agreement with existing experimental observations, including morphologies from electron microscopy and intensity profiles from scattering experiments. This work suggests that accurate consideration of the anisotropy is a key to reveal the formation of unique assembled structures of perfluorosulfonic acid ionomers at different water contents.

Структура иономерных полиуретанов, содержащих природные соединения

Структура иономерных полиуретанов, содержащих природные соединения

Multiblock sulfonated poly(arylene ether sulfone)s with fluorenyl hydrophilic moieties for PEMFC applications

Multiblock sulfonated poly(arylene ether sulfone) (MB-SPAES) ionomers containing fluorenyl hydrophilic moieties were synthesized and converted into proton exchange membranes (PEMs) through solution casting, as well as other two types of MB-SPAES membranes containing hydrophilic biphenyl and hexafluoroisopropyl diphenyl moieties for the comparison. Chemical structures of the MB-SPAES ionomers were confirmed by 1H NMR spectrometer. Fundamental physical properties were characterized based on the ionic group content, the hydrophilic/hydrophobic block structure and length, including ion exchange capacity (IEC), water uptake, size change, mechanical property, proton conductivity, hydrolytic stability and fuel cell performance. All the obtained MB-SPAES membranes were transparent and mechanical ductile, suitable for PEM applications. Water uptake and size change results showed that the MB-SPAES membranes containing fluorenyl hydrophilic moieties absorbed less water and swelled smaller in water than the other two types at similar IECs, indicating their better dimensional stability. Proton conductivity and hydrolytic stability results indicated that the fluorenyl hydrophilic moieties were also favorable to gain better proton conductivity and hydrolytic stability.

Mechanical Response of Thin Nafion Films during Hydration Cycles

Proton exchange membrane (PEM)-based fuel cells are being aggressively targeted as an alternative energy source for transportation and stationary power due to their high charge density and low operating temperatures. While the structure, transport and mechanical properties of bulk PEMs have been extensively studied, there is increasing interest on the behavior of these materials at interfaces and under confinement, since there exist many heterogeneous interfaces within an actual fuel cell assembly. One could argue that the performance and ultimate failure of the membrane electrode assembly is greatly impacted by the structure and transport of ionomers confined to extremely small length-scales. At the heart of the PEM fuel cell is the polymer membrane that must transport protons but not electrons, while also acting as a barrier to the reactant gases (H2 and O2). The most widely studied material for this purpose is perfluorosulfonic acid (PFSA) ionomers such as Nafion. Over its service life, the polymer membrane undergoes hundreds if not thousands of cycles between wet and dry conditions, which leads to swelling and shrinking of the membrane material. This can ultimately lead to mechanical failure in the membrane. This mechanical fatigue is of particular importance in the catalyst layer where the PFSA ionomers, often used as an ionically conductive binder, are confined in domains on the order of tens of nanometers in thickness. A further understanding of the swelling-induced stresses experienced in both the catalyst layer as well as the PEM is necessary. In this talk, we will describe our efforts to develop and implement a cantilever bending technique1 in order to investigate the swelling induced stresses in Nafion thin films. By monitoring the deflection of a cantilever beam coated with a thin Nafion film as it is exposed to varying humidity environments, the swelling induced stresses of the polymer film can be measured. Using this technique, we have measured the mechanical response to humidity cycling of thin Nafion films as a function of film thickness and thermal annealing, as well as hygrothermal aging. 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).