Properties Of Proton Exchange Membranes Research Articles

Novel sulfonated poly(arylene ether sulfone) containing hydroxyl groups for enhanced proton exchange membrane properties

We here report a new sulfonated poly(arylene ether sulfone) modified with hydroxyl side groups for a proton exchange membrane fuel cell (PEMFC).

Sulfonated fluorinated-aromatic polymers as proton exchange membranes

AbstractIn recent years, extensive research on the preparation and properties of proton exchange membranes (PEMs) has been realized. This article focusses on the recent studies on new PEM materials based on aromatic hydrocarbon polymers with sulfonated groups as hydrophilic domains and fluorinated groups as hydrophobic domains as alternatives to conventional perfluorinated polymers. It is necessary to improve the proton conductivity especially under low-humidity conditions and at high operating temperatures to break through the current aromatic PEM system. Hence, there is a need to develop new high-conductivity fuel cell ionomers with improved thermal, chemical, and electrochemical stability by designing a suitable polymer structure for PEM application.

Synthesis of highly branched sulfonated polymers and the effects of degree of branching on properties of branched sulfonated polymers as proton exchange membranes

Branched sulfonated polymers exhibit excellent properties as proton exchange membranes (PEMs). However, very few highly branched sulfonated polymers are reported as PEMs. The highly branched polymer, including the method to increase degree of branching (DB) and the effects of DB on the properties of PEMs, should be further studied. In this work, novel branched sulfonated poly(fluorenyl ether ketone sulfone)s with different DB value are synthesized by direct polycondensation reactions from bisphenol fluorene (A2), sulfonated 4,4′-difluorobenzphenone, 1,3,5-tris(4-(4-fluorophenylsulfonyl)phenyl)benzene (B3-3) and 4,4′-difluorodiphenyl sulfone. The highest DB with 10% branching agent is obtained using the B3-3 monomer. The method to increase the DB is discussed. It is found that B3 scaffold with long and hard arms can effectively increase the DB value. The effects of DB on the properties, including oxidative stability, proton conductivity, water uptake, swelling ratio, thermal stability, mechanical property and microstructure, are investigated. With increasing DB value, oxidative stability and proton conductivity of the membranes increase remarkably, but swelling ratio and tensile strength decrease slowly. The membrane with the highest DB value (10%) exhibits high proton conductivity (0.42 S cm−1) and oxidative stability (327 min), as well as relatively low swelling ratio (16.2%) at 80 °C.

Proton exchange fuel cell membranes of poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles-grafted-multi-walled carbon nanotubes

In this study, we have reported poly(benzimidazole-amide) containing flexible moieties such as ether, fluoro, and siloxane. The poly(benzimidazole-amide) synthesis was carried out by the condensation of 4‐(3,4-diaminophenoxy)benzene-1,2-diamine, bis(carboxypropyl)-tetramethyldisiloxane, and 2,2-bis(4-aminophenyl)hexafluoropropane in the presence of polyphosphoric acid at 160℃. Afterwards, poly(benzimidazole-amide) was blended with sulfonated polystyrene and 0.1–2 wt.% titania nanoparticles- grafted-multi-walled carbon nanotubes for the formation of hybrid proton exchange membranes for fuel cell (poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles- grafted-multi-walled carbon nanotubes) for fuel cell. Inclusion of titania modified multi-walled carbon nanotubes influenced the membrane performance by modifying the microstructure, mechanical properties, as well as water retention and proton conductivity properties of proton exchange membranes (PEM). The hybrid membranes were doped with phosphoric acid before subjecting to various characterizations. Field emission scanning electron microscopic study depicted typical blend morphology in which titania nanoparticles- grafted-multi-walled carbon nanotubes were partially embedded in the matrix and seemed to be pulled out of the matrix surface during fracturing. The phase separated structure was accountable for the water retention and higher proton conductivity. The tensile stress and modulus of acid doped poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles- grafted-multi-walled carbon nanotubes nanocomposites increased from 65.1 to 72.5 MPa and 9.5 to 15.2 GPa, respectively, with the increasing titania nanoparticles- grafted-multi-walled carbon nanotubes loading (0.1–2 wt.%). The glass transition temperature of phosphoric acid doped poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles-grafted-multi-walled carbon nanotubes 0.1–2 membranes increased from 227 to 236 C. They also had higher ion exchange capacity of 2.5–3.7 mmol/g and proton conductivity of 2.3–3.1 S/cm at 80℃ (higher than perfluorinated Nafion®117 membrane 1.1 × 10−1S/cm). A H2/O2 fuel cell using poly(benzimidazole-amide)/sulfonated polystyrene/titania nanoparticles- grafted-multi-walled carbon nanotubes 2 (ion exchange capacity 3.7 mmol/g) showed better performance than that of Nafion® 117 at 40℃ and 30% relative humidity.

Selective Growth of MoS2 for Proton Exchange Membranes with Extremely High Selectivity

Proton conductivity and methanol permeability are the most important transport properties of proton exchange membranes (PEMs). The ratio of proton conductivity to methanol permeability is usually called selectivity. Herein, a novel strategy of in situ growth of MoS2 is employed to prepare MoS2/Nafion composite membranes for highly selective PEM. The strong interactions between the Mo precursor ((NH4)2MoS4) and Nafion's sulfonic groups in a suitable solvent environment (DMF) probably lead to a selective growth of MoS2 flakes mainly around the ionic clusters of the resultant MoS2/Nafion composite membrane. Therefore, it would significantly promote the aggregation and hence lead to a better connectivity of these ionic clusters, which favors the increase in proton conductivity. Meanwhile, the existence of MoS2 in the ionic channels effectively prevents methanol transporting through the PEM, contributing to the dramatic decrease in the methanol permeability. Consequently, the MoS2/Nafion composite membranes exhibit greatly increased selectivity. Under some severe conditions, such as 50 °C with 80 v/v% of methanol concentration, an increase in the membrane selectivity by nearly 2 orders of magnitude compared with that of the recast Nafion membrane could be achieved here, proving our method as a very promising way to prepare high-performance PEMs. All these conclusions are confirmed by various characterizations, such as (FE-) SEM, TEM, AFM, IR, Raman, TGA, XRD, etc.

Imidoaryl biphenol based new fluorinated sulfonated poly(arylene ether sulfone) copolymers and their proton exchange membrane properties

Abstract A series of new partially fluorinated sulfonated poly(arylene ether sulfone) (IBQSH-XX) copolymers based on imidoaryl biphenol (IB) such as 3,8-bis(4-hydroxyphenyl)-N-phenyl-1,2-naphthalimide were synthesized by potassium carbonate mediated aromatic nucleophilic substitution reaction. The content of the sulfonated comonomer 3,3′-disodiumsulfonyl-4,4′-dichlorodiphenylsulfone was varied to obtain a degree of sulfonation from 0.2 to 0.6. The copolymers showed number average molar masses as high as 60 kDa and good solubility in various aprotic polar solvents. The copolymer structures were confirmed by FTIR and NMR spectroscopy. The IB moiety in the copolymers contributed decisively for very high thermal stability, oxidative stability and mechanical properties. Transmission electron microscopy (TEM) showed good phase separated morphology with ionic cluster size in the range of 3–60 nm for the copolymer membranes. The copolymer membranes showed good dimensional stability with very low water uptake and swelling ratios due to the rigid aromatic backbone moieties. Ion exchange capacity (IEC W ) of the copolymer films was calculated from the monomer composition obtained by 1 H NMR spectroscopy and was further determined by titration methods. The proton conductivities of the copolymer membranes increase with degree of sulfonation and temperature maximum up to 49 mS/cm at 80 °C.

Poly(ether ketone)s bearing pendent sulfonate groups via copolyacylation of a sulfonated monomer and isomeric AB‐type comonomers

ABSTRACTPoly(ether ketone)s bearing pendent sulfonate groups (SPEK‐x/y/z) have been successfully synthesized via copolyacylation of a presulfonated monomer SBP and two isomeric AB‐type self‐condensable comonomers, that is, 4‐phenoxybenzoic acid (p‐POBA) and 3‐phenoxybenzoic acid (m‐POBA). Proton‐exchange membranes (PEMs) with precisely controlled ion‐exchange capacity (IEC) and high strength can be readily prepared from these ionomers. PEMs prepared from p‐POBA other than m‐POBA exhibit much higher dimensional stability and proton conductivity at elevated temperature above 60 °C, showing prominent isomeric (para vs. meta) effects of polymer structural units. Furthermore, properties of PEMs prepared from p‐POBA are optimized by tuning IEC. SPEK‐1.0/2.2/0 with an IEC of 1.84 mmol g−1 exhibits acceptable swelling, much higher proton conductivity, and lower methanol permeability than commercial Nafion 115, implying potential application in direct methanol fuel cells. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 200–207

Synthesis of branched sulfonated poly(aryl ether ketone) copolymers and their proton exchange membrane properties

Branched sulfonated poly(aryl ether ketone) copolymers (Br-SPAEKs) were synthesized based on 4,4′-(hexafluoroisopropylidene) diphenol (6F-BPA, A2 monomer), 4,4′-difluorobenzophenone (DFBP, unsulfonated B2 monomer), 3,3′-disodiumsulfonyl-4,4′-difluorobenzophenone (SDFBP, sulfonated B2 monomer) and 2,4′,6-trifluoro-benzophenone (TFBP, branched BB′2 monomer). Structures of Br-SPAEKs were confirmed by FT-IR and NMR. Compared to linear sulfonated poly(aryl ether ketone) copolymer (L-SPAEK) membrane with similar ion exchange capacity (IEC), these Br-SPAEK membranes showed a higher proton conductivity and a lower methanol permeability. This synergistic effect on proton conductivity and methanol permeability was mainly due to the characteristics of large free volume and similar to cross-linking architecture arising from the introduction of the branched monomers. Moreover, the Br-SPAEK membrane containing 20mol% branched monomer possesses the best selectivity, and the value of selectivity is almost 3 times higher than that of L-SPAEK. Proton conductivities of Br-SPAEK membranes are higher than 10−2S/cm and increase gradually with increasing temperature. At 80°C, the proton conductivity of Br-20-SPAEK-40 (branched monomer of 20mol% and sulfonated monomer of 40mol%) membrane reaches 0.121Scm−1, compare to that of Nafion 117 (0.139Scm−1). Additionally, the methanol permeability of Br-20-SPAEK-40 is one order magnitude lower than that of Nafion 117.

Highly Fluorinated Sulfonated Poly(arylene ether sulfone) Copolymers: Synthesis and Evaluation of Proton Exchange Membrane Properties

This article reports the synthesis and characterization of several new fluorinated sulfonated poly(arylene ether sulfone) (6FBPAQSH-XX). The structures of the prepared copolymers were confirmed by FTIR and NMR spectroscopy. The copolymers showed high molecular weight, good solubility, and film-forming capabilities. The copolymers exhibited high thermal and oxidative stability and good mechanical properties depending upon their chemical compositions. Transmission electron microscopy (TEM) showed cluster like microstructures for the copolymer membranes suggesting good phase separated morphology. The cluster size increased with increasing mole percentage of sulfonate comonomer and was in the range of 5–65 nm. The copolymers showed good dimensional stability with very low water uptake and swelling ratios. Ion exchange capacity (IECW) of the copolymer films was determined by titration methods and was also calculated from the monomer composition obtained by 1H NMR spectroscopy. The proton conductivities of acid form membrane increased with IECW value and temperature, with maximum to 108 mS/cm at 90 °C for 6FBPAQSH-60 copolymer.

Optimizing membrane thickness for vanadium redox flow batteries

Two important intrinsic properties of proton exchange membranes for vanadium redox flow battery (VRFB) operation are proton conductivity and vanadium permeability. These characteristics are thickness-normalized quantities and depend on fundamental material parameters. However, the operational criteria of proton exchange membranes in these devices are the membrane resistance and vanadium crossover flux, both of which depend on membrane thickness. Herein, we explore the influence of the thickness of ion exchange capacity (IEC)-optimized sulfonated fluorinated poly(arylene ether) (SFPAE) membranes on their VRFB performance including charge/discharge behavior, charge depth, coulombic efficiency, voltage efficiency, energy efficiency and cell polarization. IEC-optimized SFPAE membranes with three different thicknesses (28μm, 45μm and 80μm) were prepared and tested in this study. It was found that the combined effects of the ohmic loss and electrolyte crossover loss in the VRFB, which were governed by membrane thickness, resulted in an optimal membrane thickness of 45μm for SFPAE under the conditions tested. Thicker membranes were observed to cause higher cell resistance while thinner membranes yielded larger vanadium crossover flux, both of which had negative impacts on the cell performance. The maximum power densities of the VRFBs assembled with 28μm, 45μm and 80μm SFPAE membranes were 267mWcm−2, 311mWcm−2 and 253mWcm−2 respectively, much higher than that of the VRFB assembled with N212 membrane, which was 204mWcm−2. These results supported our previous observation that SFPAE was superior to N212 with regard to VRFB performance. The data also indicated that there is an optimum membrane thickness for a given set of properties through which the cell performance can be significantly improved while keeping the membrane material constant.

Ion Motion in Anion and Proton-Conducting Triblock Copolymers

To learn more about anion transport in functionalized polymers and improve anion exchange membrane (AEM) performance, we have directly compared the properties of proton exchange membranes (PEMs) and AEMs. A single batch of poly(hexyl methacrylate)-b-poly(styrene)-b-poly(hexyl methacrylate), PHMA-b-PS-b-PHMA, triblock copolymer was synthesized and postfunctionalized in the PS block with sulfonate or quaternary ammonium moieties. The conductivity, water uptake, and morphology of low, middle, and high IEC anionic and cationic PHMA-b-PS-b-PHMA triblock copolymers were investigated, and the metrics of ion diffusivity and membrane sensitivity to relative humidity are introduced. We observed that these block copolymer AEMs and PEMs had similar values for the ratio of calculated ion diffusion coefficient to the mobile species dilute ion diffusivity (D/D0) as a function of hydration. This observation points to similar morphological barriers to transport and similar hydration dependence in each material and provides a basis for understanding ion transport in AEMs compared to dilute solution. As the ion concentration in the functionalized block was increased, the sensitivity of conductivity to relative humidity decreased due to closer proximity of the ionic groups. These guidelines for AEM design will help to more deeply understand the similarities and differences between different types of water-absorbing, ion-conducting membranes.

Sulfonated copoly(norbornene)s bearing sultone pendant groups and application as proton exchange membranes candidates

Novel copolynorbornenes bearing pendant sultone groups (designated as P(BN/SulNBOH) and P(BN/SulNBOMe)) have been successfully synthesized via copolymerization of functionalized norbornenes bearing sultones (designated as SulNBOH and SulNBOMe) with 2-butoxymethylene norbornene (BN). The catalyst system showed high catalyst activity (104 gpolymer/molNi·h) and the obtained copolymers have high molecular weight and a narrow molecular weight distribution. Furthermore, the achieved copolymers P(BN/SulNBOH) and P(BN/SulNBOMe) were converted into sulfonated copolymers sP(BN/NBOH) and sP(BN/NBOMe). Both sP(BN/NBOH) and sP(BN/NBOMe) membranes displayed low water uptake, high thermal properties, good mechanical properties, and better proton exchange membranes properties. The proton conductivities measured in the hydrated state at 80 °C ranged from 10−5 to 7.19 × 10−3 S·cm−1.

Developments of highly proton-conductive sulfonated polymers for proton exchange membrane fuel cells

This review paper covers recent publications on the developments of highly proton-conductive sulfonated polymers for proton exchange membrane (PEM) fuel cells. The PEMs presenting proton conductivities exceeding that of Nafion membranes are especially focused on. The proton conductivities of PEMs have been correlated to their ion exchange values and morphological phase-separation. Formation of ionic channels is an effective approach to develop highly proton-conductive PEMs. The approaches could be grouped into (1) developments of microphase-separation with block copolymers, (2) side-chain and grafted sulfonated polymers, and (3) locally and densely sulfonated polymers. Formation of nanocomposites of PEMs with nanofillers has also shown great ability to increase the proton conductivities of the existed PEMs. Besides proton conductivities, the formation of nanocomposites also improves some PEM properties including mechanical strength, dimensional stability, and oxidative stability. The discussed publications have provided promising PEMs for the development of high performance fuel cells.

Sulfonated poly(arylene ether)/heteropolyacids nanocomposite membranes for proton exchange membrane fuel cells

Heteropolyacids (HPAs) are nano-size metal–oxygen clusters with anion acids stronger than the conventional solid acids. Insoluble and high surface area solid acids, i.e. Cs 2.5H 0.5PW 12O 40 (CsPW) and H 3PW 12O 40/SiO 2 (PWS) are dispersed throughout End-group cross-linkable, Sulfonated Fluorinated Bi- Phenol (ESF-BP) copolymer to produce organic–inorganic nanocomposite proton exchange membranes (PEMs). The molecular structure of ESF-BP copolymer is identified by 1H NMR spectra. The FT-IR of prepared CsPW and PWS inorganic powders show that the primary Keggin structure remains almost unaltered. The PEMs properties including density, water uptake, water diffusivity, swelling, ion exchange capacity (IEC), oxidative stability and proton conductivity are investigated to characterize the influence of added HPAs on the composite PEMs. The intermolecular interaction between HPAs and the sulfonic group of the copolymer affects the physico-chemical characteristics of membranes. The addition of HPAs to the polymer matrix increases oxidative stability, water uptake and conductivity but decreases the IEC, the water diffusivity and the density of membrane. The cells with the nanocomposite membranes have better performance than the cell with the plain polymer membrane. The prepared composite membrane using dispersion of PWS through the ESF-BP has superior characteristics and voltage–current responses in the PEMFC rather than using CsPW.

Sulfonated poly(fluorenyl ether ketone) ionomers containing aliphatic functional segments for fuel cell applications

A series of Sulfonated Poly (fluorenyl ether ketone) ionomers containing aliphatic functional segments were synthesized and characterized. The monomer 4,4′-Dihydroxy-α, ω-diphenoxydecane with aliphatic group was conveniently prepared from hydroquinone and 1,10-dibromodecane. A series of sulfonated aliphatic functional groups containing poly(fluorenyl ether ketone)s with different aliphatic group content were successfully synthesized and characterized in detail, in particular with respect to properties relevant for their application as membrane materials in proton exchange membrane (PEM) fuel cells. Tough and transparent membranes were formed by casting from their solutions. The effects of alkyl groups were investigated by comparison of the PEM properties of the copolymers with different content of aliphatic component on the copolymer chain. The introduction of aliphatic segments can provide an enhanced water uptake, increased proton conductivities, but worse oxidative stability. Transmission electron microscope (TEM) images of j and f revealed an improved phase separation structure under the effect of aliphatic groups. The as-made membranes can also exhibit comparable or much better single cell performance than Nafion@ 117 at 75 °C–95 °C under full or partial relative humidification.

Synthesis and properties of highly branched sulfonated poly(arylene ether)s as proton exchange membranes

A series of branched sulfonated poly(arylene ether)s were successfully synthesized using 1,3,5-tris[4-(4-fluorobenzoyl)phenoxy]benzene (B3) as branching agent. The synthesized branched copolymers were soluble in polar organic solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc) and dimethylsulfoxide (DMSO), and could be cast to form tough and smooth films. The effect of degree of branching (DB) on the proton conductivity, swelling ratio and oxidative stability of the membranes was investigated. With increasing DB value, the proton conductivity and oxidative stability of the membranes increased. A maximum oxidative stability of the branched membrane with 4% of DB value was determined to be 3.4 times larger than that of the linear membrane. In addition, as the DB value increased, the swelling ratio of the membranes decreased from 13.51% to 9.09% at 80 °C. The results indicated that increasing DB value might be an effective way to improve the properties of proton exchange membranes.

Mechanical durability of proton exchange membranes with catalyst platinum dispersion

The durability of proton exchange membrane (PEM) fuel cells remains a challenging issue for their long term operational use. Degradation of the PEM related to dissolution of the adjacent catalyst and re-deposition into the PEM significantly reduces cell efficiency. We investigate the effects of platinum (Pt) dispersions intended to simulate the re-deposited catalyst on the mechanical durability of the PEM. The bulge technique was applied to characterize the mechanical properties of PEMs simulating pressure loading on fully hydrated membranes in fuel cells. The results showed that with increasing Pt dispersion concentration the stiffness of the PEMs increased, and the membranes became less ductile and inclined to fracture at lower stresses under pressure loading. We also used the out-of-plane tearing test to characterize membrane fracture behavior which revealed the harmful effects of Pt dispersion on the fracture resistance under different environmental conditions. Deterioration in fracture resistance was explained in terms of the Pt distribution and aggregation as defects inside the membranes as characterized by electron microscopy. Fracture was shown to initiate preferentially at the interface of Pt particles and the polymer matrix, and propagate through the defect regions in polymer with lower energy, thus reducing the overall fracture resistance of the PEM.

Experimental Measurement of Stress and Strain in Nafion Membrane during Hydration Cycles

Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigue stresses in response to changing operation conditions. Determination of these stresses is complicated as both constitutive properties of PEM and the hygrothermal strain in PEM are highly dependent on time, temperature and water absorption. In this study, by tracking the spherical bending curvature of a bimaterial strip composed of a 25-μm thick PEM (Nafion) and a 250-μm thick PEEK (polyether ether ketone) substrate using a Digital Image Correlation (DIC) system, the biaxial tensile and compressive stresses in the plane of the PEM during humidity cycles are recorded. Due to the relatively high bending stiffness ratio between PEEK and PEM, the hygral stresses in the PEM are determined from the bimaterial curvature without requiring knowledge of the constitutive properties of the PEM. As the strain in the PEM released due to bending of the bimaterial strip is limited, the stresses measured using the bimaterial strip are very close to that in a fully constrained membrane, mimicking those in a fuel cell. The hygral strain in a freestanding membrane placed next to the bimaterial structure is recorded at the same time using the DIC system. The full stress and strain histories during incremental relative humidity (RH) test between 10%RH and liquid wet as well as during larger RH cycles are presented. The hygral expansion of Nafion during the increasing and decreasing RH step changes exhibits an asymmetry that resembles the behavior of glassy polymers during the classic Kovacs temperature jump test. The stress history also reveals the RH-dependent viscoelasticity of Nafion.

Effect of cation contamination and hydrated pressure loading on the mechanical properties of proton exchange membranes

Perfluorosulfonic acid (PFSA) polymer membranes are widely used as electrolyte thin films to transport protons in proton exchange membrane (PEM) fuel cells. The mechanical degradation of the membrane represents a common failure mode that limits the operational life of the fuel cells. In the present work, effect of contamination related to cation exchange on the mechanical reliability of PEMs was investigated. We applied the bulge test technique to assess the mechanical properties of Nafion ® PFSA membranes simulating pressure loading on hydrated PEMs in fuel cells. The corresponding elastic moduli of Nafion ® before and after cation exchange were analyzed and compared with the results measured by uniaxial tension experiments at selected humidity conditions, showing increasing stiffness with the increase of cation radius. We also used the out-of-plane tearing test method to characterize the fracture behaviors of PEMs. The effects of cation exchange and water absorption on mechanical and fracture properties of PEMs at different temperatures are discussed in terms of cation and water interactions with the molecular structure of PFSA polymers.

Mesoscale modeling of the influence of morphology on the mechanical properties of proton exchange membranes

The nano-scale morphology of a proton exchange membrane (PEM) strongly influences its proton conductivity and mechanical performance. In this paper, a multi-scale modeling approach has been developed to obtain the morphologies of hydrated perfluorosulfonic acid (PFSA) membranes and then to predict their mechanical properties based on the simulated morphology. Two representative ionic domain morphologies were compared, spherical and cylindrical, to represent cast and extruded membranes. The calculated overall elastic Young’s moduli are very close for both morphologies and agree well with experiments. The nano-scale phase segregation in hydrated PFSA induces a non-uniform distribution of local stress. The peak stress is localized at the smeared water/PFSA interface. The cylindrical morphology develops much lower peak stress than the isotropic spherical morphology under the same level of strain. These results explain why the extruded membranes show more than 10 times longer life in durability tests than recast membranes, despite having similar bulk moduli.