Cross-linked Proton Exchange Membrane Research Articles

Cross-linked proton exchange membrane covalently bonded with silicotungstic acid for enhanced proton conductivity

Proton exchange membranes with cross-linked structures often showed excellent mechanical and chemical stability, but their water absorption and proton conductivity were restricted. This study reported a convenient and efficient thermal cross-linking reaction of sulfonic acid groups and benzene rings using dimethyl sulfoxide and sulfolane mixed solvents to prepare the membrane. To further illustrate, phenoxyphenyl-attached silicotungstic acid (POP-SiWA) was incorporated in the sulfonated poly(ether ether ketone) membrane and covalently bonded with the polymer by thermal cross-linking reaction. The immobilization ratio for the membranes prepared with 10∼30 wt% POP-SiWA addition was 88.0–63.7%. The modified membranes exhibited high hydrophilicity and improved proton conductivity under hydrated or low relative humidity conditions due to the strong acidity and water-retaining properties of silicotungstic acid. Specifically, the proton conductivity of the modified membrane with 30 wt% POP-SiWA reached 212 mS/cm. It exhibited a fuel cell power density of 629 mW/cm2 at 80 °C and 100% relative humidity, which were 1.90 and 2.67 times higher than the pristine membrane.

Designing Advanced Cross-Linked Proton Exchange Membranes with Enhanced Structural Homogeneity and Proton Conductivity via Radiation-Induced RAFT Polymerization.

This study introduces an innovative approach to fabricate well-defined cross-linked proton exchange membranes (PEMs) using radiation-induced reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization on cost-effective ethylene tetrafluoroethylene (ETFE) films. The incorporation of the RAFT mechanism into the cross-linking process significantly enhanced structural homogeneity, providing uninterrupted proton conductivity. Thorough characterizations confirmed the successful grafting of polystyrene (PS) chains onto ETFE films and subsequent sulfonation. Despite a reduction in proton conductivity attributed to restricted chain movements, a notable improvement in chemical stability was observed after cross-linking reactions. Chemical stability of the cross-linked membranes increased approximately 4-fold compared to those synthesized without a cross-linker. The synthesized PEMs with degrees of grafting at 45% and 67% demonstrated superior proton conductivity, outperforming various alternatives, including commercial Nafion samples. Specifically, these cross-linked membranes exhibited promising proton conductivity values of 93.7 and 139.1 mS cm-1, respectively. This work highlights the potential of radiation-induced RAFT-mediated polymerization in carrying out cross-linking reactions as an efficient pathway for designing well-defined high-performance PEMs, offering enhanced homogeneity and conductivity compared to existing literature counterparts.

High Performance and Self-Humidifying of Novel Cross-Linked and Nanocomposite Proton Exchange Membranes Based on Sulfonated Polysulfone.

The introduction of inorganic additive or nanoparticles into fluorine-free proton exchange membranes (PEMs) can improve proton conductivity and have considerable effects on the performance of polymer electrolyte membrane fuel cells. Based on the sol–gel method and in situ polycondensation, novel cross-linked PEM and nanocomposite PEMs based on a sulfonated polysulfone (SPSU) matrix were prepared by introducing graphene oxide (GO) polymeric brushes and incorporating Pt-TiO2 nanoparticles into an SPSU matrix, respectively. The results showed that the incorporation of Pt-TiO2 nanoparticles could obviously enhance self-humidifying and thermal stability. In addition, GO polymer brushes fixed on polymeric PEM by forming a cross-linked network structure could not only solve the leakage of inorganic additives during use and compatibility problem with organic polymers, but also significantly improve proton conductivity and reduce methanol permeability of the nanocomposite PEM. Proton conductivity, water uptake and methanol permeability of the nanocomposite PEM can be up to 6.93 mS cm−1, 46.58% and be as low as 1.4157 × 10−6 cm2 s−1, respectively, which represent increases of about 70%, about 22% and a decrease of about 40%, respectively, compared with that of primary SPSU. Therefore, the synergic action of the covalent cross-linking, GO polymer brush and nanoparticles can significantly and simultaneously improve the overall performance of the composite PEM.

Cross-linked proton-exchange membranes with strongly reduced fuel crossover and increased chemical stability for direct-isopropanol fuel cells

Ionic and covalent cross-linking of an acid–base blend is used to manufacture membranes with high stability against dissolution and reduced isopropanol and acetone crossover for direct-isopropanol fuel cell applications.

Crosslinked Proton Exchange Membranes with a Wider Working Temperature Based on Phosphonic Acid Functionalized Siloxane and PPO

A series of proton exchange membranes were prepared by incorporating phosphonic functionalized siloxane into sulfonic poly(2,6-dimethyl-1,4-phenyleneoxide) (SPPO) and imidazole functionalized poly(2,6-dimethyl-1,4-phenyleneoxide) grafted with siloxane (IPPO-Si). Phosphonic acid functionalized siloxane was synthesized from amino trimethyl phosphonic acid (ATMP) and (3-aminopropyl)triethoxysilane (APTES). FTIR results showed that 1-methylimidazole and APTES were successfully grafted onto polyphenylene oxide, and APTES successfully formed Si-O-Si crosslinked networks through hydrolytic crosslinking. The membranes were thermally stable up to 220 °C and exhibited excellent oxidative stability and mechanical performance. We also measured the proton conductivity of the membranes. The results showed that the proton conductivities of the composite membranes increased with the increasing of SPPO content at different degrees under high (100 °C–160 °C) and low (25 °C–80 °C) temperature conditions. Furthermore, the IPPO-SI-P/SPPO-30 has the best conductivity, reaching to 0.1131 S cm−1 at 80 °C, 100%RH and 0.1049 S cm−1 at 160 °C, 5%RH, respectively. Therefore, this novel membrane acts as a potential candidate for proton exchange membranes with a wider applicable temperature (25 °C–160 °C).

Highly stable ionic-covalent cross-linked sulfonated poly(ether ether ketone) for direct methanol fuel cells

Abstract A novel ionic cross-linked sulfonated poly(ether ether ketone) containing equal content of sulfonic acid and pendant tertiary amine groups (TA-SPEEK) has been initially synthesized for the application in direct methanol fuel cells (DMFCs). By adjusting the ratio of p-xylene dibromide to tertiary amine groups of TA-SPEEK, a series of ionic-covalent cross-linked membranes (C-SPEEK-x) with tunable degree of cross-linking are prepared. Compared with the pristine membrane, the ionic and ionic-covalent cross-linked proton exchange membranes (PEMs) exhibit reduced methanol permeability and improved mechanical properties, dimensional and oxidative stability. The proton conductivity and methanol selectivity of protonated TA-SPEEK and C-SPEEK-x at 25 °C is up to 0.109 S cm−1 and 3.88 × 105 S s cm−3, respectively, which are higher than that of Nafion 115. The DMFC incorporating C-SPEEK-25 exhibits a maximum power density as high as 35.3 mW cm−2 with 4 M MeOH at 25 °C (31.8 mW cm−2 for Nafion 115). Due to the highly oxidative stability of the membrane, no obvious performance degradation of the DMFC is observed after more than 400 h operation, indicating such cost-effective ionic-covalent cross-linked membranes have substantial potential as alternative PEMs for DMFC applications.

Preparation and evaluation of crosslinked sulfonated polyphosphazene with poly(aryloxy cyclotriphosphazene) for proton exchange membrane

Several crosslinked proton exchange membranes with high proton conductivities and low methanol permeability coefficients were prepared, based on the sulfonated poly[(4-fluorophenoxy)(phenoxy)] phosphazene (SPFPP) and newly synthesized water soluble sulfonated poly(cyclophosphazene) (SPCP) containing clustered flexible pendant sulfonic acids. The structure of SPCP was characterized by fourier transform infrared spectroscopy (FTIR) and 1H NMR spectra. The membranes showed moderate proton conductivities and much lower methanol permeability coefficients when compared to Nafion 117. Transmission electron microscopy (TEM) results indicated the well-defined phase separation between the locally and densely sulfonated units and hydrophobic units, which induced efficient proton conduction. In comparison with SPFPP membrane, the proton conductivities, oxidative stabilities and mechanical properties of crosslinked membranes remarkably were improved. The selectivity values of all the crosslinked membranes were also much higher than that of Nafion 117 (0.74×105S·s/cm3). These results suggested that the cSPFPP/SPCP membranes were promising candidate materials for proton exchange membrane in direct methanol fuel cells.

Cross-linked polyelectrolyte for direct methanol fuel cells applications based on a novel sulfonated cross-linker

A novel type of cross-linked proton exchange membrane of lower methanol permeation and high proton conductivity is prepared, based on a newly synthesized sulfonated cross-linker: carboxyl terminated benzimidazole trimer bearing sulfonic acid groups (s-BI). Compared to membranes cross-linked with non-sulfonated cross-linker (BI), SPEEK/s-BI-n membranes show higher IEC values and proton conductivities. Meanwhile, oxidative stability and mechanical property of SPEEK/s-BI-n membranes are obviously improved. Among SPEEK/s-BI-n membranes, SPEEK/s-BI-2 exhibits high proton conductivity, low swelling ratio (0.122 S cm−1 and 15.2% at 60 °C, respectively) and low methanol permeability coefficient. These results imply that the cross-linked membranes prepared with the newly sulfonated cross-linker are promising for the direct methanol fuel cells (DMFCs) application.

Hybrid polyelectrolytes based on stable sulfonated polynorbornene with higher proton conductivity and lower methanol permeability

We report preparation of hybrid cross-linked proton exchange membranes (PEMs), which are synthesized from sulfonated copoly(norbornene)s bearing sultone pendant (SPBN) and a zwitterionic silica containing sulfonic acid and ammonium groups, 3-[[3-(triethoxysilyl)-propyl]amino]butane-1-sulfonic acid (TPABS), using a sol–gel process. As these membranes are well processed as self-supporting film, they show high stabilities, proton conductivity and low methanol permeability. Reported SPBN/TPABS-50 (50 wt % of TPABS to SPBN in the matrix) shows the best performance with proton conductivity of 6.34 × 10−2 S cm−1, methanol permeability of 3.64 × 10−7 cm2 s−1, ion-exchange capacity value of 1.21 mequiv g−1 and comparable selectivity parameter of 1.74 × 104 S cm−3 s. While the membrane electrode assembly (MEA) is fabricated using the SPBN/TPABS-50 as PEM, the open circuit voltage of SPBN/TPABS-50 at 1.0 M methanol (80 °C) and its power density of the devices are 0.563 V and 50.2 mW cm−2, respectively, though which is lower than that of Nafion117 (124.2 mW cm−2). The new designed cross-linked membranes can thus be a promising candidate to satisfy the requirements of PEMs for direct methanol fuel cells, especially the simple and low cost preparation of the membranes.

Preparation and Performance of Crosslinked Proton Exchange Membranes Containing Carboxyl Group

Preparation and Performance of Crosslinked Proton Exchange Membranes Containing Carboxyl Group

Hybrid polymers based on sulfonated polynorbornene with enhanced proton conductivity for direct methanol fuel cells

Sulfonated polynorbornene (SPNB) and 3-aminopropyltriethoxysilane (KH550) hybrid cross-linked proton exchange membranes doped with different weight ratio of phosphotungstic acid (PWA) were prepared by a simple sol-gel process. The cross-linked structures led to low methanol permeability and good stability of the nanocomposites. Incorporation of PWA has significantly improved the proton conductivity of the hybrid membrane due to an extra provided conductive proton-conduction pathway to facilitate proton transportation. In particular, the conductivity of SPNB/KH550/PWA25 reached the maximum of 0.02 S.cm−1 at 80°C under the 100% relative humidity condition, and this value is on the same order of magnitude as that of Nafion117. Furthermore, SPNB /KH550/PWA20 owns the lowest proton transport activation energy (8.39 kJ.mol−1).

Ternary Hybrid Proton Exchange Membranes Based on POSS

The sulfonated poly(ether ether ketone) (SPEEK)/bisphenol S diglycidyl ether (DEBS)/POSS ternary hybrids is designed and prepared to incorporate POSS into SPEEK to form a new cross-linked proton exchange membrane (PEM). Compared with the SPEEK, the ternary hybrids possesses lower water uptake (WU) and linear expansion (LE), but higher proton conductivity. The proton conductivity of SPEEK/DEBS/POSS can reach to 1.1×10-1 S/cm.

Cross-linked proton exchange membranes for direct methanol fuel cells: Effects of the cross-linker structure on the performances

A highly sulfonated poly (ether ether ketone) with pendant amino groups (Am-SPEEK) has been synthesized. The resulting copolymer showed good solubility in common organic solvents. Then the pendant amino groups of the Am-SPEEK were utilized to react with two kinds of cross-linker, epoxy resin and dibromide, to yield various cross-linked membranes. After cross-linking, the membranes could not dissolve in common solvents, just became swollen. In generally, all the cross-linked membranes showed improved mechanical properties and high dimensional stabilities, whereas the uncross-linked membranes highly swollen or even dissolved in water at high temperature. The proton conductivity of the membranes increased with an increase in temperature. At 80 °C, all the cross-linked membranes showed high proton conductivity, in the range of 0.111–0.140 S cm−1. Especially, the TMBP cross-linked membrane showed a proton conductivity of 0.140 S cm−1, which was higher than that of Nafion 117 (0.125 S cm−1). The membranes with high proton conductivity, good oxidative stability, and improved methanol residence have been successfully developed. Furthermore, the influence of different main chain of the cross-linker on the performance of the cross-linked membranes was also investigated.

High performance sulfonated poly(arylene ether phosphine oxide) membranes by self-protected cross-linking for fuel cells

This work investigated the thermal treatment of sulfonated poly(arylene ether phosphine oxide) membranes under vacuum without using any cross-linking agents or additives. During the thermal treatment, the sulfonic acid groups of the membranes acted as cross-linkable groups and reacted with the activated benzene rings in the membranes, resulting in self-cross-linking. Interestingly, the available degree of cross-linking of the membranes almost no longer increased with increasing time and temperature after the required thermal treatment because many sulfonic acid groups of the membranes could not be cross-linked due to steric hindrance and only functioned as a conducting group of protons. Meanwhile, this approach facilitated the cross-linked membranes to exhibit excellent dimensional stability, as well as high proton conductivity. This is a self-protected cross-linking to some extent. Through such a cross-linking, phosphine oxide-containing membranes with excellent overall properties were achieved. In addition, this effect of steric hindrance could be employed to design cross-linked proton exchange membranes with high performance.

Synthesis and Properties of Sulfonated Polyimide/Polybenzimidazole Cross-Linked Membranes for Fuel Cell Applications

A series of cross-linked proton exchange membranes with the ion exchange capacities (IECs) of 0.70 - 1.52 meq/g have been prepared via the reaction of the anhydride-terminated sulfonated polyimide oligomers (SPI-3, SPI-5 and SPI-7, here the figure refers to the averaged block length) and the polybenzimidazole with pendant amino groups (H2N-PBI) in dimethylsulfoxide (DMSO) during the membrane cast process. The prepared cross-linked membranes showed high tensile strength (55 - 80 MPa) and good water stability (> 2 months in deionized water at 100 °C). Fenton’s test revealed that all the cross-linked membranes displayed significantly better radical oxidative stability than the corresponding pure SPIs. This is attributed to the presence of the highly oxidative-stable PBI component in the cross-linked membranes. The proton conductivities of the cross-linked membranes increased with increasing temperature and relative humidity. The cross-linked membrane prepared from the longest oligomer (SPI-7) displayed the highest proton conductivity which is comparable to that of Nafion 117.

A facile approach to prepare self-cross-linkable sulfonated poly(ether ether ketone) membranes for direct methanol fuel cells

Sulfonated poly(ether ether ketone) containing hydroxyl groups (SPEEK-OH) has been prepared for use as a proton exchange membrane (PEM) by reducing the carbonyl groups on the main chain of the polymers. With the goal of reducing water uptake and methanol permeability, a facile thermal-cross-linking process is used to obtain the cross-linked membranes. The properties of the cross-linked membranes with different cross-linked density are measured and compared with the pristine membrane. Notably, SPEEK-4 with the highest cross-linked density shows a water uptake of 39% and a methanol permeability of 2.52 × 10 −7 cm 2 s −1, which are much lower than those of the pristine membrane (63.2% and 5.37 × 10 −7 cm 2 s −1, respectively). These results indicate that this simple approach is very effective to prepare cross-linked proton exchange membranes for reducing water uptake and methanol permeability.

Preparation and characterization of high-durability zwitterionic crosslinked proton exchange membranes

The present paper describes the development of a novel proton exchange membrane comprising a poly (styrene sulfonic acid-co-4-vinylpyridine) copolymer that was crosslinked with a haloalkyl crosslinker through the formation of ionic bonding linkages. The nucleophilic substitution of these crosslinked membranes as well as a model reaction between pyridine and 1-bromobutane was confirmed by nuclear magnetic resonance (NMR). Tough and flexible membranes of a high mechanical strength were prepared. They demonstrated very elevated thermal, hydrolytic and oxidative stabilities as compared to other sulfonated polymers. The crosslinked membrane composed of zwitterionic molecules with a crosslinking fraction of 90.3, denoted SP-1, exhibited a proton conductivity of ca. 7.1 × 10 −2 S cm −1 at 30 °C under 90% relative humidity; a value comparable to that of Nafion 117. Moreover, the crosslinked SP-1 membrane possessed the highest selectivity for methanol fuel cells (3.38 × 10 5 S cm −3 s), approximately five times that of Nafion 117, thus implying its potential for practical use in high-energy-density devices.

Covalently cross-linked proton exchange membranes based on sulfonated poly(arylene ether ketone) and polybenzimidazole oligomer

To prepare a novel covalently cross-linked proton exchange membrane with high proton conductivity, sulfonated poly(arylene ether ketone)s bearing carboxylic acid groups (SPAEK-C) and o-diamino functional polybenzimidazole oligomers (PBI) were consequently synthesized. Then the SPAEK-C/PBI blend membranes were dried at 110 °C for 3 h to induce cross-links between amino groups of PBI oligomer and carboxylic acid groups of SPAEK-C. 1H NMR measurements and Fourier transform infrared spectroscopy were used to characterize and confirm the structures of SPAEK-C, PBI oligomers and the cross-linked membranes. The cross-linked membranes were immersed in various concentrations of phosphoric acid solution to enhance proton conductivity. These H3PO4 doped SPAEK-C/PBI membranes displayed comparable or even higher proton conductivity than that of Nafion 117, especially under low humidity conditions. Moreover, the water uptake, acid uptake, mechanical and thermal stability were also investigated in detail.

Novel cross-linked sulfonated poly (arylene ether ketone) membranes for direct methanol fuel cell

To prepare a cross-linked proton exchange membrane with low methanol permeability and high proton conductivity, poly (vinyl alcohol) is first blended with sulfonated poly (arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) and then heated to induce a cross-linking reaction between the carboxyl groups in SPAEK-C and the hydroxyl groups in PVA. Fourier transform infrared spectroscopy is used to characterize and confirm the structure of SPAEK-C and the cross-linked membranes. The proton conductivity of the cross-linked membrane with 15% PVA in weight reaches up to 0.18 S cm −1 at 80 °C (100% relative humidity), which is higher than that of Nafion membrane, while the methanol permeability is nearly five times lower than Nafion. The ion-exchange capacity, water uptake and thermal stability are investigated to confirm their applicability in fuel cells.

The effect of sulfonic acid groups within a polyhedral oligomeric silsesquioxane containing cross-linked proton exchange membrane

In this study, polyhedral oligomeric silsesquioxane (POSS) moieties were incorporated into sulfonated poly(ether ether ketone) (SPEEK) to form a new cross-linked proton exchange membrane (PEM). The distribution of the POSS containing cross-linkers with or without sulfonic acid groups dictates the water behavior and connectivity of hydrophilic domains of the PEM. A PEM formed by incorporating 17.5wt% of the cross-linker (containing POSS macromer and sulfonic acid groups) into SPEEK exhibits high proton conductivity (0.0153S/cm), low methanol permeability (1.34×10−7cm2/s), and high selectivity (1.14×105Ss/cm3).