Molecular Weight Effects on Poly(arylene ether sulfone)-Based Random and Multiblock Copolymers Characteristics for Fuel Cells

Perfluorosulfonic acid (PFSA) polymers are the most promising state-of-the-art materials as proton exchange membranes (PEM) for fuel cells, however, there are some drawbacks such as high production cost and hydrothermal stability. Aromatic membranes based on hydrocarbon polymers have been studied as alternatives. Among them, multi-block copolymers seem to be promising to compete with PFSA resulting high proton conductivity even under low RH conditions.[1, 2] In this report, novel sulfonated poly(arylene ether sulfone)s multi-block copolymer membranes containing highly sulfonated hydrophilic blocks were synthesized.[3] Different local concentration of sulfonic acid in their hydrophilic blocks affected chemical and physical properties of the SPAES. To investigate the effects of chemical composition on their membrane properties, different hydrophilic oligomers sharing same hydrophobic blocks gave us exact comparison of effect of hydrophilic blocks. The higher concentration of sulfonic acid groups resulted in higher proton conductivity under certain relative humidity conditions than that of the state-of-the-art perfluorinated sulfonic acid membrane and showed that the well-developed phase separation of SPAES. Moreover, physical properties of theses SPAES including water behavior, humidity dependence of proton conductivity were investigated along with morphology characterizations by transmission electron microscopy (TEM) for PEMFC application. Two types of oligomers, F-terminated and OH-terminated telechelic oligomers, were synthesized by controlling the feed ratio of dihydroxyl- and difluoro-monomers. Their number of repeating unit (X and Y) was analyzed by GPC and 1H NMR. Copolymerization with F-terminated and OH-terminated telechelic oligomers via nucleophilic aromatic substitution, gave high-molecular-weight multi-block PESs. Each block length was controlled to have different values with X5Y10, X10Y10, X20Y10 and X20Y20. The SPAES X10Y10 membrane showed highest proton conductivity than that of our previous random and block copolymers at wide range of humidity. The best balanced membrane was SPAES X10Y5 membrane with high proton conductivity and lower water uptake. Consequently, the SPAES X10Y5 membrane showed high cell performance under various conditions (510 mA/cm2, 290 mA/cm2 @ 0.6V under 80% and 50 % RH conditions at 80℃, respectively). Systematic approaches for developing alternative block SPES membrane for low RH condition will be discussed.

Disulfonated poly(arylene ether sulfone) random copolymers have shown promise as potential candidates for proton exchange membranes. The chemical structure of these copolymers has served as a basis for the molecular design of new random and multiblock copolymers. In random copolymers the sulfonic acid is distributed statistically, while in the multiblock copolymers the chemical structure is designed to have co continuous hydrophilic and hydrophobic domains. The effect of these variables on the transport properties of the membranes was investigated. For the random copolymers, high proton conductivity was observed at higher IEC along with high methanol permeability. But for the multiblock copolymers, it was possible to combine high proton conductivity with low methanol permeability. Improved proton conductivity was observed under partially hydrated conditions for samples with higher block lengths. Self diffusion coefficient of water was also measured by NMR which provided better understanding of the transport process.

Fuel cells for Heavy Duty Vehicles (HDVs) offer easy scalability and the potential for critical reduction in CO2 emissions. During the drive cycles of HDVs, the fuel cells will operate over a range of temperatures, with increases while climbing up steep gradients. Operating temperatures above 90°C will be necessary, along with low relative humidity (RH) due to the need to avoid pressurization of gases. Therefore, Proton Exchange Membranes (PEMs) for HDV applications need to be suitable over this range of conditions, including functioning at high temperatures and low relative humidity. Moreover, the membranes should not swell extensively when more water is present.In this study, sulfonated poly (arylene ether sulfone) multiblock copolymers were modified with the incorporation of clusters of modified cerium oxide nanoparticles to form composites. These synthesized novel membranes approach operational proton conductivity requirements for HDV vehicles, with conductivity of 15mS/cm at 120°C and 25% relative humidity, remarkably avoiding the typical large decrease in conductivity observed below 70% RH in non-fluorinated PEMs. One purpose of this study is to understand the water, proton, and polymer interactions within these membranes. The cerium nanoparticles and sulfonamide clusters play a critical role in the membrane, affecting conductivity, and water transport, especially at high temperatures and low relative humidity, while also suppressing swelling of the membrane. Membrane proton conductivity is also strongly influenced by physical properties such as water uptake and dimensional swelling behavior, which is further impacted by polymer morphology. To probe the dynamics of the components, the diffusion coefficient and relaxation time of water in the membrane and the protonic conductivity of the membrane as functions of membrane water content are analyzed using nuclear magnetic resonance (NMR) measurements and water uptake measurements. Film property and morphology changes are evaluated using Transmission Electron Microscopy (TEM) and Small-Angle X-ray Scattering (SAXS). This study demonstrates the advantageous and non-linear benefits of closely packed acid clusters with cerium oxide composites on membrane functionality under HDV operating conditions.

The influence of chemical compositions on the properties of sulfonated poly(arylene ether sulfone)‐based proton‐exchange membranes was studied. First, we synthesized three different series of random SPAES copolymers using three kinds of hydrophobic monomers, including 4,4′‐dihydroxyldiphenylether, 2,6‐dihydroxynaphthalene (DHN), and 4,4′‐hexafluoroisopropylidenediphenol (6F‐BPA) to investigate effects of hydrophobic components on the properties of SPAES membranes as proton‐exchange membranes. Random SPAES copolymers with 6F‐BPA showed the highest proton conductivity while random SPAES copolymers with DHN displayed the lowest methanol permeability among the three random copolymers. Subsequently, we synthesized multiblock SPAES using the DHN as a hydrophobic monomer and studied the effect of the length of hydrophilic segments in the multiblock SPAES copolymers on membrane performance. The results indicated that longer hydrophilic segments in the copolymers led to higher water uptake, proton conductivity, and proton/methanol selectivity of membranes even at low humidity. In addition, the morphology studies (AFM and SAXS measurements) of membranes suggested that multiblock copolymers with long hydrophilic segments resulted in developed phase separation in membranes, and ionic clusters formed more easily, thus improving the membrane performance. Therefore, both the kinds of hydrophobic monomers and the length of hydrophilic segments in SPAES copolymers would influence the membranes performance as proton‐exchange membranes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Novel proton exchange membranes (PEMs) based on graft copoly(arylene ether sulfone)s with enhanced phase-separated morphology were prepared using atom transfer radical polymerization (ATRP). A series of PEMs with different graft lengths and sulfonation degrees were prepared. The phase-separated morphologies were confirmed by transmission electron microscopy. Among the membranes prepared and evaluated, PAESPS18S2 exhibited considerably high proton conductivity (0.151 S/cm, 85 °C), benefitting from the graft polymer architecture and phase-separated morphology. The membranes also possessed excellent thermal and chemical stabilities. Highly conductive and stable copoly(arylene ether sulfone)-based membranes would be promising candidates as polymer electrolytes for fuel cell applications.

Mesoscale modeling of sulfonated polyimides copolymer membranes: Effect of sequence distributions

If the necessary infrastructure can be provided, a practical future electric vehicle will be an efficient hydrogen-powered hybrid operating on a polymer fuel cell. Light weight, lowest cost and high efficiency imply an atmospheric pressure, rather than a pressurized, system. Polymer fuel cells require humidification for ionic conductivity, which means that product water is normally rejected as liquid. Atmospheric pressure systems of this type are difficult to operate. A self-humidified atmospheric pressure system operating at higher temperature (about 75-80 °C) on oxygen-enriched air rejecting product water in the vapour phase is described. It combines the advantages of unpressurized operation with the higher power density of a pressurized system, but with much lower parasitic power requirements.

A new series of poly(arylene ether sulfone) multiblock copolymers were synthesized by polycondensation of linear and rigid hydrophobic and densely sulfonated hydrophilic oligomers. Thin membranes were prepared therefrom by solution casting and their properties were compared with those of Nafion and other poly(arylene ether sulfone) multiblock copolymers containing perfluorinated biphenylene moieties. The membranes showed good water affinity, high proton conductivity, low hydrogen and oxygen permeability, high mechanical strength, and reasonable oxidative stability. These properties are attractive as an electrolyte for polymer electrolyte membrane fuel cells. It was found that the perfluorinated biphenylene moieties as connecting groups for hydrophilic and hydrophobic blocks affected only slightly the properties of poly(arylene ether sulfone) multiblock copolymer membranes.

A new bisphenol monomer, 9,9-bis(3,5-dimethoxy-4-hydroxyphenyl) fluorene, was synthesized and polymerized to form fluorene-based poly(arylene ether sulfone) copolymers containing tetra-methoxy groups (MPAES). After converting the methoxy group to the reactive hydroxyl group, the respective side-chain type sulfonated copolymers (SPAES) were obtained by sulfobutylation. The polymers were characterized by 1H NMR, thermogravimetric analysis (TGA), water uptake, and proton and methanol transport for fuel cell applications. These SPAES copolymers had good overall properties as polymer electrolyte membrane (PEM) materials, having high proton conductivity in the range of 0.061–0.209 and 0.146–0.365 S/cm at 30 and 80 °C (under hydrated conditions), respectively. SPAES-39 (IEC = 1.93 mequiv/g) showed higher or comparable proton conductivity than that of Nafion 117 at 50–95% RH (relative humidity). The methanol permeabilities of these membranes were in the range of 3.22 to 13.1 × 10–7 cm2/s, which is lower than Nafion (15.5 × 10–7 cm2/s). In comparison with some reported sulfonated poly(arylene ether sulfone)s containing pendent sulfophenyl groups, the present fluorene-based SPAES containing clustered flexible pendent aliphatic sulfonic acid groups displayed better properties, such as lower water uptake and higher proton conductivities. A combination of high proton conductivities, low water uptake, and low methanol permeabilities for selected SPAES indicates that they are good candidate proton exchange membrane materials for evaluation in fuel cell applications.

Multiblock copoly(arylene ether sulfone)s with different block lengths and ionic contents are tailored for durable and proton‐conducting electrolyte membranes. Two series of fully aromatic copolymers are prepared by coupling reactions between non‐sulfonated hydrophobic precursor blocks and highly sulfonated hydrophilic precursor blocks containing either fully disulfonated diarylsulfone or fully tetrasulfonated tetraaryldisulfone segments. The sulfonic acid groups are exclusively introduced in ortho positions to the sulfone bridges to impede desulfonation reactions and give the blocks ion exchange capacities (IECs) of 4.1 and 4.6 meq. g−1, respectively. Solvent cast block copolymer membranes show well‐connected hydrophilic nanophase domains for proton transport and high decomposition temperatures above 310 °C under air. Despite higher IEC values, membranes containing tetrasulfonated tetraaryldisulfone segments display a markedly lower water uptake than the corresponding ones with disulfonated diarylsulfone segments when immersed in water at 100 °C, presumably because of the much higher chain stiffness and glass transition temperature of the former segments. The former membranes have proton conductivities in level of a perfluorosulfonic acid membrane (NRE212) under fully humidified conditions. A membrane with an IEC of 1.83 meq. g−1 reaches above 6 mS cm−1 under 30% relative humidity at 80 °C, to be compared with 10 mS cm−1 for NRE212 under the same conditions.

In this study, the low-humidity proton conductance and degradation mechanisms are theoretically investigated for Nafion®, which is the most frequently used proton exchange membrane. The proton conductance mechanism of Nafion membrane was first explored to reveal the cause for its high proton conductance under low-humidity conditions (Ref. 1). Nafion membrane gets an advantage in proton exchange membranes in fuel cells mainly due to its high low-humidity proton conductance. The actual proton conductance of Nafion at low humidity was explored by comparing to that of hydrocarbon multiblock sulfonated poly(arylene ether sulfone)s (SPE) membrane. Using long-range corrected density functional theory (LC-DFT) that one of the authors (TT) developed (Ref. 2), we calculated the proton dissociation energies from the sulfonic acid groups and investigated the subsequent proton conductance mechanism at low humidity. As a result, we found that these membranes have negligible difference in the proton dissociation ability inside the hydrated water clusters but show a remarkable difference in the conductivity of protonated water clusters in disconnected hydrogen bond networks. Note that low-humidity membranes are known to contain the regions of disconnected hydrogen bond networks (Ref. 3), in which Grotthuss mechanism assuming connected hydrogen bond networks does not proceed. Though vehicle mechanism assuming the diffusion of protonated water molecules (or clusters) seems to explain the proton conductance in the disconnected networks, we found that protonated water molecules and clusters need very large dissociation energies (about 400 kJ/mol). As an alternative, we suggested the relay mechanism (Fig. 1), in which protonated water clusters are relayed between the sulfonic acid groups through the doubly-hydrated structures. The calculated results showed that protonated water clusters require only about 100kJ/mol to dissociate in this relay mechanism. The vibrational spectra of hydrated Nafion membrane was also investigated to reveal the dependence of the proton conductance on humidity in further detail by a collaborative experimental and theoretical infrared (IR) spectroscopy study (Ref. 4). The experimental time-resolved attenuated total reflection Fourier transform IR spectroscopic results showed a unique IR peak intensity dependence on the hydration number. Based on the experimental results, we theoretically explored the dependence of the vibrational spectra of Nafion membrane on the hydration number by using LC-DFT. As a result, we found that this unique IR peak intensity dependence is correctly reproduced for the singly-hydrated but for the doubly-hydrated Nafion, in which protons are initially detached from the sulfonic acid groups in the geometry optimizations. This strongly supports the above-mentioned relay mechanism of the proton conductance. The degradation mechanism of Nafion membrane was also explored focusing on the H2O2-induced dissociation of the ether-linkages of the side chains. Many conventional studies on the degradation of membranes in fuel cells have assumed the principal mode of the degradation involves OH radicals. However, recent experimental studies have raised questions concerning this assumption. For example, a Fenton test, which is supposed to provide OH radicals, have revealed that the ratio of the degradation fragment molecules is considerably different from that of actual operating conditions (Ref. 5). Using LC-DFT, we found an H2O2-induced degradation mechanism, which is consistent with experimental results. In this mechanism, the ether-linkage of the side chains of Nafion is decomposed with a relatively low activation energy to produce (H2O)lHO3S-CF2-CF2-O-OH (l is the hydration number) as a key fragment. We also elucidated the subsequent decomposition reaction mechanisms, though these mechanism need metal ions to proceed. The vibrational spectra for the decomposed Nafion are found to be consistent with the experimental IR spectra. This mechanism is supposed to proceed together with OH radical-induced degradation mechanism. This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. T. Tsuneda, R. K. Singh, K. Miyatake, and M. Watanabe, Chem. Phys. Lett., 608, 11 (2014).H. Iikura, T. Tsuneda, T. Yanai, and K. Hirao, J. Chem. Phys., 115, 3540 (2001).G. Gebel, Polymer, 41, 5829 (2000); Macromolecules, 46, 6057 (2013).R. K. Singh, K. Kunimatsu, K. Miyatake, and T. Tsuneda, Macromolecules, 49, 6621 (2016).T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y. Iriyama, T. Abe, and Z. Ogumi, J. Pow. Sour., 158, 1222 (2006). Figure 1. Schematic diagram for the proton conductance in Nafion membrane in fuel cells. Figure 1

Poly(arylene ether sulfone) multi-block copolymers bearing perfluoroalkylsulfonic acid groups

Introduction A pre-irradiation grafting method is a fascinating technique for direct introduction of a new functional polymer phase as a grafting chain into polymer films serving as a substrate which allows those films to retain their characteristics such as thermal stability, mechanical strength, and electronic properties. The radiation technique has been widely applied to the preparation of high performance fuel cell polymer electrolyte membranes (PEMs) for vehicles and domestic co-generation systems [1]. The PEMs of aromatic hydrocarbon polymers, so-called “super engineering plastics” including poly (ether ether ketone) (PEEK), have useful characteristics such as high mechanical strength, gas barrier property, and radiation resistance. However, PEEK films also have high chemical resistance; thus, it is difficult to introduce grafting monomers into the films. Recently, we succeeded in radiation-induced grafting of Ethyl 4-styrenesulfonate (E4S) into a PEEK film with grafting degrees around 100% [2]. We report herein the synthesis, characterization, and electrolyte properties of graft-type PEEK PEM with thin PEEK films less than 50 mm thickness. Fuel cell performance of graft-type PEEK PEM was estimated at 80 °C under 30 and 100% RH conditions. Experiments Graft-type PEEK PEM was synthesized by pre-irradiation grafting of E4S into PEEK films (Scheme 1) [3]. The PEEK films were immersed in a 10 wt% DVB solution of 1,4-dioxane at 50°C for 24h. The PEEK-g-DVB films were pre-irradiated with a 60Co γ-ray source (JAEA Takasaki, Gunma, Japan) at room temperature in argon atmosphere to a absorbed dose of 160 kGy. Then, the samples were immersed in 15 ml of the E4S solution of 1,4-dioxane (1/1 v/v) at 80 °C. The obtained PEEK-g-DVB-g-E4S film was hydrolyzed in 95 °C hot water for 24h (PEEK-g-DVB-g-PSSA PEM). The ion exchange capacity (IEC (mmol/g)), proton conductivity (S/cm) in-plane direction at room temperature, and water uptake (WU) was measured according to the previous report [2]. Proton conductivity was estimated at 80 °C in 30%RH; mechanical strength was measured at 80 °C in 100% RH, which is severe conditions for the fuel cell operation, was measured as previously reported [4]. The membrane/electrodes assembly (MEA) (Pt loading 0.5 mg/cm2) prepared by hot-press set into a 5 cm2. The fuel cell test was carried out at 80°C in a RH range of 30-100%. Results and Discussion According to the previously reported methods, we could prepare the PEEK-based PEM with grafting degrees of 88-107% and ion exchange capacity (IEC) of 2.3 – 2.5 mmol/g using the original PEEK substrates with film thickness of 6, 12, 16, 25, and 50 mm (Table 1). The variation of film thickness does not affect the conductivity and water uptake of the PEM. This is one of the thinnest PEM possessing sufficient conductivity as well as high mechanical strength (60 MPa of tensile strength at break). Then, we compare the fuel cell performance of the single fuel cell devices consisting of the PEEK-based PEM with various thicknesses in the operation condition at 80 ºC and 100%RH. As expected from the similar ion conductivity around 0.045 S/cm, the thinner PEM showed higher cell voltages at the current density in the range from 0.2 to 1.0 A/cm2. Especially, compared with a Nafion212 film (thickness: 50 μm), higher cell voltages (namely, power densities (W/cm2)) can be achieved in the PEM with film thickness less than 12 mm (Figure 1). The above results imply the advantages of mechanically tough PEEK as an original polymer substrate to be able to decrease the film thickness, resulting in higher power density owing to the less cell resistance. 19 μm PEEK PEM has good proton conductivity (0.0068 S/cm) at 80°C and 30%RH. The elastic modulus of the grafted film showed twice higher than that of Nafion 212 film at 80 degree and 100%RH. Acknowledgments For support of this work, the authors acknowledge the financial support from a Grant-in-Aid for Scientific Research (KAKENHI, No. 24550263) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Fabrication and electrochemical properties of SPVdF-co-HFP/SPES blend proton exchange membranes for direct methanol fuel cells

Since the 21st century, energy and environmental issues have become the focus of global attention. Traditional fossil energy resources such as oil and natural gas are drying up. Meanwhile, a large number of harmful gases and other pollutants would be produced in the use of traditional energy, which has a serious damage to the environment and climate. With the rapid increase of energy consumption and the need of environmental protection, the demand for clean and efficient energy is growing. Fuel cell (FC) is considered as one of the most promising energy conversion technologies that can replace conventional fossil fuels, and it is also an important choice in the use of hydrogen, methanol, ethanol and other renewable energy. As a new energy with great potential, FC can reduce the energy crisis and environmental pollution effectively by directly converting the chemical energy of the external input fuel into electrical energy and continuously supplying power to the outside. Ion exchange membrane (IEM) is an important component of FC, which is responsible for transferring ions and forming a complete FC circuit. According to the types of conducting ions, IEM can be divided into proton exchange membrane (PEM) and anion exchange membrane (AEM), which are applied to proton exchange membrane fuel cell (PEMFC) and alkaline anion exchange membrane fuel cell (AAEMFC), respectively. Compared with PEMFC, AAEMFC has the advantages of using non noble metal catalyst, high electrode reaction reactivity, and low fuel purity requirements and so on. However, the commercial AEM with organic cations often has low ionic conductivity and poor alkali and dimensional stability, which seriously hinders its development and application in AAEMFC. Therefore, it is urgent to design and develop new high-performance AEM materials. In order to improve the conductivity, alkali stability and dimensional stability of AEMs, a novel reactive bisphenol monomer with pendant dimethylphenoxybenzene group, 1,1-bis (4-hydroxyphenyl)-1-(4-(3,5-dimethylphenoxy) phenyl) phenyl-2,2,2-trifluoroethane ( 2 ), was synthesized based on molecular design using 4-fluoro-2,2,2-trifluoroacetophenone, 3,5-dimethylphenol and phenol as starting materials by a two-step organic reaction. Then, two series of poly(aryl ether sulfone) anion exchange membranes (2-PAES-QA- xx and 2-PAES-Im- xx ) with comb structure were prepared through aromatic nucleophilic copolycondensation, bromination and quaternization reaction based on monomer 2 . The structures of the active bisphenol monomer, the poly(aryl sulfone) containing methyl groups, the brominated poly(aryl ether sulfone)s and the ionized polymers were characterized systematically by nuclear magnetic resonance spectroscopy, respectively. The water uptake, swelling ratio, ionic conductivity, thermal stability, mechanical properties, and alkaline resistance stability of the prepared 2-PAES-QA- xx and 2-PAES-Im- xx membranes were studied in detail. The relationship between the structure and properties of different types of AEMs is further compared. The obtained AEMs showed good overall performance, and the OH– conductivity of these membranes at 60°C is 35.9–51.6 and 32.6–44.9 mS/cm, respectively. The IEC values of the representative 2-PAES-QA-60 and 2-PAES-Im-60 maintained at 79.7% and 88.5% of the initial values after immersion in 4 mol/L NaOH at 80°C for 336 h, respectively. This work provides a new strategy for the design and preparation of comb-type AEM materials.