4-Aminopyridine grafted sulfonated poly(arylene ether ketone sulfone) proton exchange membrane with high relative selectivity for fuel cells

A series of sulfonated poly(arylene ether ketone sulfone)s polymer having a degree of sulfonation of 80% and a carboxyl group in the side chain (C-SPAEKS) were prepared by polycondensation. The 4-aminopyridine grafted sulfonated poly(arylene ether ketone sulfone)s polymer membranes (SPPs) were prepared by amidation reaction with the carboxyl group to immobilize 4-aminopyridine on the side chain. The ¹H NMR results and Fourier transform infrared of SPP membranes demonstrated the successful grafting of the 4-aminopyridine. Proton conductivity, water absorption, swelling ratio, and thermal stability of different proportions of SPP membranes were investigated under the different conditions. With the increase of pyridine grafting content, the methanol permeability coefficient of the membrane decreased significantly from 8.17 × 10⁻⁷ cm²s⁻¹ to 8.92 × 10⁻⁸ cm²s⁻¹ at 25 °C. And, the proton conductivity and relative selectivity of the membrane were positively correlated with the grafted pyridine content. Among them, the SPP-4 membrane exhibited the highest proton conductivity of 0.088 Scm⁻¹ at 100 °C. The relative selectivity increased from 4.73 × 10⁴ S scm⁻³ to 9.84 × 10⁴ S scm⁻³.     

Highly branched poly(arylene ether)/surface functionalized fullerene‐based composite membrane electrolyte for DMFC applications

Sulfonated branched polymer membranes have been gaining immense attention as the separator in energy‐related applications especially in fuel cells and flow batteries. Utilization of this branched polymer membranes in direct methanol fuel cell (DMFC) is limited because of large free volume and high methanol permeation. In the present work, sulfonated fullerene is used to improve the methanol barrier property of the highly branched sulfonated poly(ether ether ketone sulfone)s membrane without sacrificing its high proton conductivity. The existence of sulfonated fullerene with larger size and the usage of small quantity in the branched polymer matrix effectively prevent the methanol transportation channel across the membrane. The composite membrane with an optimized loading of sulfonated fullerene displays the highest proton conductivity of 0.332 S cm^?1 at 80°C. Radical scavenging property of the fullerene improves the oxidative stability of the composite membrane. Composite membrane exhibits the peak power density of 74.38 mW cm^?2 at 60°C, which is 30% larger than the commercial Nafion 212 membrane (51.78 mW cm^?2) at the same condition. From these results, it clearly depicts that sulfonated fullerene‐incorporated branched polymer electrolyte membrane emerges as a promising candidate for DMFC applications.     

SPEEK/sulfonated cyclodextrin blend membranes for direct methanol fuel cell

In this paper, the blend membranes based on sulfonated poly(ether ether ketone) and sulfonated cyclodextrin as the proton conducting membranes for DMFCs usage are prepared and investigated. The incorporation of sulfonated cyclodextrin in SPEEK membranes is evaluated by the characteristic absorptions of FT-IR spectra. Thermal stability and micro-morphology of the blend membranes are determined by thermogravimetry analysis and scanning electron microscope tests. The properties of the blend membranes are investigated such as swelling behavior, methanol permeability and proton conduction as function of the fraction of sulfonated cyclodextrin. The methanol crossover could be suppressed by the incorporation of sulfonated cyclodextrin and the methanol permeability decreases when the methanol concentration increases from 2.5 M to 20 M. Proton conduction is also promoted by the introduction of sulfonated cyclodextrin and the proton conductivity increases with the increase of sulfonated cyclodextrin content. The calculated activation energy for proton conduction of the blend membranes is very low and the maximum value is 4.20 kJ mol⁻¹, which is much lower than that of Nafion 115 (9.15 kJ mol⁻¹, measured in our experiments). These data indicate that proton can transport easily through the blend membranes. The selectivity of the blend membranes, a compromise between proton conductivity and methanol permeability, is much higher than that of Nafion 115 at the sulfonated cyclodextrin content above 15 wt.%. The blend membranes with 15, 20, and 25 wt.% of sulfonated cyclodextrin are assembled in the practical DMFCs and their polarization curves with 2.5 M and 8.0 M methanol solution are determined, respectively. The membrane with 20 wt.% sulfonated cyclodextrin reaches the highest power density of 29.52 mW cm⁻² at 120 mA cm⁻² and 8.0 M methanol solution. These results suggest the potential usage of the SPEEK membranes incorporating with sulfonated cyclodextrin in DMFCs.     

Synthesis and characterization of novel sulfonated poly(arylene ether ketone) copolymers with pendant carboxylic acid groups for proton exchange membranes

A series of novel side-chain-type sulfonated poly(arylene ether ketone)s with pendant carboxylic acid groups copolymers (C-SPAEKs) were synthesized by direct copolymerization of sodium 5,5′-carbonyl-bis(2-fluorobenzenesulfonate), 4,4′-difluorobenzophenone and 4,4′-bis(4-hydroxyphenyl) valeric acid (DPA). The expected structure of the sulfonated copolymers was confirmed by FT-IR and ¹H NMR. Membranes with good thermal and mechanical stability could be obtained by solvent cast process. It should be noted that the proton conductivity of these copolymers with high sulfonatation degree (DS > 0.6) was higher than 0.03 S cm⁻¹ and increased with increasing temperature. At 80 °C, the conductivity of C-SPAEK-3 (DS = 0.6) and C-SPAEK-4 (DS = 0.8) reached up to 0.12 and 0.16 S cm⁻¹, respectively, which were higher than that of Nafion 117 (0.10 S cm⁻¹). Moreover, their methanol permeability was much lower than that of Nafion 117. These results showed that the synthesized materials might have potential applications as the proton exchange membranes for DMFCs.     

Macromolecule sulfonated Poly(ether ether ketone) crosslinked poly(4,4′-diphenylether-5,5′-bibenzimidazole) proton exchange membranes: Broaden the temperature application range and enhanced mechanical properties

Proton exchange membranes with a wide application temperature range were fabricated to start high-temperature fuel cells under room temperature. The volume swelling stability, oxidative stability as well as mechanical properties of crosslinked membranes have been improved for covalently crosslinking poly(4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI) with fluorine-terminated sulfonated poly(ether ether ketone) (F-SPEEK) via N-substitution reactions. High proton conductivity was simultaneously realized at both high (80–160 °C) and low (40–80 °C) temperatures by crosslinking and jointly constructing hydrophilic-hydrophobic channels. The crosslinked membranes exhibited the highest proton conductivity of 191 mS cm⁻¹ at 80 °C under 98% relative humidity (RH) and 38 mS cm⁻¹ at 160 °C under anhydrous, respectively. Compared with OPBI membrane, the fuel cell performance of the crosslinked membranes showed higher peak power density at full temperature range (40–160 °C).     

Water-alcohol dispersible and self-cross-linkable sulfonated poly(ether ether ketone): Application as ionomer for direct methanol fuel cell catalyst layer

A sulfonated poly(ether ether ketone) containing hydroxyl groups (HO-SPEEK) has been synthesized for investigation as the ionomer in cathode of direct methanol fuel cells. Na salt-formed HO-SPEEK shows excellent solubility in some aqueous solutions of monohydric alcohol and can be successfully self-cross-linked in-situ during the hot-pressing process of membrane-electrode assembly (MEA) fabrication. The resulted cross-linked HO-SPEEK displays improved stability and mechanical strength. MEA incorporating the HO-SPEEK as both membrane and ionomer shows excellent peak power density of 29.0 mW cm⁻² at 25 °C with 4 M methanol, which is comparable to the Nafion reference MEA (31.8 mW cm⁻²) and 2.9-fold higher than that of the MEA prepared from catalyst ink that contained dimethyl sulfoxide (10.3 mW cm⁻²). Thanks to the avoidance of high-boiling point solvent, the resulted HO-SPEEK-based cathode is loosened with many large pores for reactant gas and product transportation. These results demonstrate that water-alcohol dispersible and cross-linkable sulfonated hydrocarbons hold technological promise as ionomer for electrode.     

PEMs with high proton conductivity and excellent methanol resistance based on sulfonated poly (aryl ether ketone sulfone) containing comb-shaped structures for DMFCs applications

Sulfonated poly (aryl ether ketone sulfone) polymers (SPAEKS-PSA X) with the comb-shaped structure were prepared by grafting with 1, 3-propanesultone (PSA). The successful synthesis of SPAEKS-PSA X was confirmed by the FT-IR spectra. The morphology of the membrane was investigated by the small angle X-ray scattering and the transmission electron microscopy. The nanophase separation was brought in a comb-shaped structure between the main chains and the propyl sulfonic acid groups. The SPAEKS-PSA 15 membrane exhibited the highest conductivity of 101 mS cm⁻¹ under 80 °C and 100% relative humidity conditions (100% RH). In addition, the methanol permeability coefficients of SPAEKS-PSA X membranes (7.56 × 10⁻⁷ to 9.44 × 10⁻⁷ cm² s⁻¹) were much lower compared to Nafion®117 (22.37 × 10⁻⁷ cm² s⁻¹). Meanwhile, the SPAEKS-PSA X membranes also displayed excellent mechanical properties (tensile modulus >30 MPa), thermal, oxidative stabilities and fuel cell performance. Considering the parameters above, especially the elevated proton conductivities as well as high methanol resistance, the comb-shaped structure is a promising design for DMFCs applications.     

Hybrid proton conducting membranes based on sulfonated cross-linked polysiloxane network for direct methanol fuel cell

A series of novel hybrid membranes based on sulfonated poly(arylene ether ketone)s (SNPAEKs), polysiloxane (KH-560) and sulfonated curing agent (BDSA) has been prepared by sol-gel and cross-linking reaction for direct methanol fuel cells (DMFCs). All the hybrid membranes (SKB-xx) show high thermal properties and improved oxidative stability compared with the pristine SNPAEK membrane. The sulfonated cross-linked polysiloxanes networks in the hybrid membranes enhance the mechanical properties and reduce the swelling ratio. The swelling ratio of SKB-20 is 22%, which is much lower than that of the pristine SNPAEK (37%) at 80 °C. Meanwhile, SKB-xx membranes with greatly reduced methanol permeabilities show comparative proton conductivities to pristine SNPAEK membranes. Notably, the proton conductivities of SKB-5 and SKB-10 reach to 0.192 S cm⁻¹ and 0.179 S cm⁻¹ at 80 °C, respectively, which are even higher than the 0.175 S cm⁻¹ of SNPAEK.     

Fluorene-containing poly(arylene ether sulfone)s with imidazolium on flexible side chains for anion exchange membranes

Anion exchange membranes with high ionic conductivity and dimensional stability attract a lot of research interests. In present study, a series of fluorene-containing poly(arylene ether sulfone)s containing imidazolium on the flexible long side-chain are synthesized via copolycondensation, Friedel-Crafts reaction, ketone reduction, and Menshutkin reaction sequentially. The membranes used for characterization and membrane electrode assembly are obtained by solution casting and ion exchange thereafter. The morphology of the membranes is studied via transmission electron microscopy, and the microphase separation is observed. The long side-chain structure is responsible for the distinct hydrophilic-hydrophobic microphase separation, which facilitates the transport of hydroxide ions in the membranes. The incorporation of imidazolium on the flexible long side-chain is favorable for the ionic aggregation and transport in the membranes. The resulted membranes exhibit high hydroxide conductivities in the range of 48.5–83.1 mS cm⁻¹ at 80 °C. All these membranes show good dimensional stability and thermal stability. The single cell performance shows a power density of 102.3 mW cm⁻² at 60 °C using membrane electrode assembly based-on one of the synthesized polymers.     

SCTF nanosheets@sulfonated poly (p-phenylene-co-aryl ether ketone) composite proton exchange membranes for passive direct methanol fuel cells

The development of a simple and efficient methanol-resistant membrane strategy is of great significance for improvement of the performance of fuel cells, making it an attractive and challenging topic. In this work, sulfonated covalent triazine framework (SCTF) nanosheets are prepared by a micro-interface method and post-sulfonation, which show excellent dispersion in polar solutions, such as water and N, N-Dimethylacetamide (DMAc). Then a series of composite proton exchange membranes (SCTF-x@SPP-co-PAEKs) are prepared by blending these SCTF nanosheets with sulfonated micro-block copolymers (SPP-co-PAEKs) resin. The results show that the appropriate addition of SCTF can significantly improve the proton conductivity (PC), methanol resistance and fuel cell performance of the prepared composite membrane, which can be attributed to the good interfacial compatibility between the SCTF nanosheets and the sulfonated micro-block copolymer matrix. The passive direct methanol fuel cells (DMFCs) with SCTF-x@SPP-3 membrane exhibit power density in the range of 28.0–33.3 mW cm⁻² at 25 °C, which is superior to the related values of the pristine membrane and the commercial Nafion® series membranes.     

Amelioration in physicochemical properties and single cell performance of sulfonated poly(ether ether ketone) block copolymer composite membrane using sulfonated carbon nanotubes for intermediate humidity fuel cells

A composite membrane composed of a sulfonated diblock copolymer (SDBC) based on poly(ether ether ketone) blocks copolymerized with partially fluorinated poly(arylene ether sulfone) and sulfonated carbon nanotubes (SCNTs) was fabricated by simple solution casting. Addition of the SCNT filler enhanced the water absorption and proton conductivity of membranes because of the increased per‐cluster volume of sulfonic acid groups, at the same time reinforced the membranes' thermal and mechanical properties. The SDBC/SCNT‐1.5 membrane exhibited the most improved physicochemical properties among all materials. It obtained a proton conductivity of 10.1 mS/cm at 120°C under 20% relative humidity (RH) which was 2.6 times more improved than the pristine membrane (3.9 mS/cm). Moreover, the single cell performance of the SDBC/SCNT‐1.5 membrane at 60°C and 60% RH at ambient pressure exhibited a peak power density of 171 mW/cm² at a load current density of 378 mA/cm², while the pristine membrane exhibited 119 mW/cm² at a load current density of 294 mA/cm². Overall, the composite membrane exhibited very promising characteristics to be used as polymer electrolyte membrane in fuel cells operated at intermediate RH.     

Fabrication of a highly efficient polymer electrolyte membrane by embedding nanofibrous metal oxide in cell components

In this work, a high-performance polymer electrolyte membrane based on sulfonated polyether ether ketone (SPEEK) loaded with zirconium oxide nanofibres (ZrN) was fabricated. ZrN with an average diameter of 182 nm were prepared by pyrolysing electrospun polyacrylonitrile fibres embedded with a zirconium precursor. The weight percentage of added ZrN and the relevant performance of the hybrid membrane (e.g., proton conductivity, methanol permeability and fuel cell performance) were experimentally determined and verified. It was found that the SPEEK-ZrN hybrid membrane exhibited sufficient proton conductivity (25.9 mS cm⁻¹), low methanol permeability (1.64 × 10⁻⁷ cm² s⁻¹) and superior selectivity (15.8 × 10⁴ S s cm⁻³) at room temperature. Compared to a pure SPEEK membrane, the SPEEK-ZrNT-1.5 membrane showed 1.3 and 1.7 times higher current density and power density, respectively.     

Construction of a new continuous proton transport channel through a covalent crosslinking reaction between carboxyl and amino groups

Sulfonated poly(arylene ether ketone sulfone) bearing pendant carboxylic acid groups (C-SPAEKS) and sulfonated poly(arylene ether ketone sulfone) containing amino groups (Am-SPAEKS) were used to prepare C-SPAEKS/Am-SPAEKS crosslinked membranes. ¹H NMR and Fourier transform infrared spectra proved that C-SPAEKS and Am-SPAEKS copolymers, as well as C-SPAEKS/Am-SPAEKS crosslinked membrane, were successfully synthesized. TEM images showed that a continuous proton transport channel formed after crosslinking. Thermogravimetric analysis curves demonstrated that the thermal property of the crosslinked membranes improved. The crosslinked membranes exhibited suitable mechanical properties at 25 and 80 °C. The methanol permeability of C-SPAEKS/Am-SPAEKS-40 was 2.35 × 10⁻⁷ cm² s⁻¹ at 60 °C, which was lower than that of C-SPAEKS (24.12 × 10⁻⁷ cm² s⁻¹) and Am-SPAEKS (17.91 × 10⁻⁷ cm² s⁻¹). The proton conductivity of C-SPAEKS/Am-SPAEKS-40 was 0.089 S cm⁻¹, which was higher than that of C-SPAEKS and Am-SPAEKS at 80 °C. The results proved that C-SPAEKS/Am-SPAEKS crosslinked membranes were potential proton exchange membranes for direct methanol fuel cell applications.     

Improving the proton conductivity of sulfonated poly (ether ether ketone) membranes by incorporating a crystalline nanoassembly of trimesic acid and melamine

A novel proton exchange membrane was synthesized by embedding a crystalline which was nano-assembled through trimesic acid and melamine (TMA·M) into the matrix of the sulfonated poly (ether ether ketone) (SPEEK) to enhance the proton conductivity of the SPEEK membrane. Fourier transform infrared indicated that hydrogen bonds existed between SPEEK and TMA·M. XRD and SEM indicated that TMA·M was uniformly distributed within the matrix of SPEEK, and no phase separation occurred. Thermogravimetric analysis showed that this membrane could be applied as high temperature proton exchange membrane until 250 °C. The dimensional stability and mechanical properties of the composite membranes showed that the performance of the composite membranes is superior to that of the pristine SPEEK. Since TMA·M had a highly ordered nanostructure, and contained lots of hydrogen bonds and water molecules, the proton conductivity of the SPEEK/TMA·M-20% reached 0.00513 S cm⁻¹ at 25 °C and relative humidity 100%, which was 3 times more than the pristine SPEEK membrane, and achieved 0.00994 S cm⁻¹ at 120 °C.     

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.     

Novel proton exchange membranes based on proton conductive sulfonated PAMPS/PSSA-TiO2 hybrid nanoparticles and sulfonated poly (ether ether ketone) for PEMFC

Nanocomposite membranes based on sulfonated poly (ether ether ketone) (SPEEK) and sulfonated core-shell TiO₂ nanoparticles were prepared. TiO₂ nanoparticles were sulfonated by redox polymerization method by using sodium styrene sulfonate (SSA) and 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) monomers. The resultant hybrid nanoparticles (PAMPS-gTiO₂ and PSSA-g-TiO₂) were introduced to SPEEK with a sulfonation degree of 68%. Grafting of sulfonated polymers onto TiO₂ nanoparticles enhanced the content of proton transport sites in the membrane, leading to an increase in proton conductivity and power density. Besides, the mechanical and dimensional stabilities of the nanocomposite membranes were also improved compared with pure SPEEK membrane. The maximum power density for membranes containing 7.5 wt% of PAMPS-gTiO₂ and PSSA-g-TiO₂ nanoparticles at 80 °C obtained 283 mW cm⁻² and 245 mW cm⁻², respectively.     

The effective methanol-blocking and proton conductivity membranes based on sulfonated poly (ether ether ketone ketone) and polyorganosilicon with functional groups

To solve the conflict between high proton conductivity and low methanol crossover of pristine sulfonated aromatic polymer membranes, the polyorganosilicon doped sulfonated poly (ether ether ketone ketone) (SPEEKK) composite membranes were prepared by introducing polyorganosilicon additive with various functional groups into SPEEKK in this study. Scanning electron microcopy (SEM) images showed the obtained membranes were compact. No apparent agglomerations, cracks and pinholes were observed in the SEM images of composite membranes. The good compatibility between polymer and additive led to the interconnection, thus producing new materials with great characteristics and enhanced performance. Besides, the dual crosslinked structure could be formed in composite membranes through the condensation of silanols and the strong interaction between matrix and additive. The formation of dual crosslinked structure optimized the water absorption, enhanced the hydrolytic stability and oxidative stability of membranes. Especially, the incorporation of additive improved the strength and flexibility of composite membranes at the same time, meaning that the life of the composite membranes might be extended during the fuel cell operation. Meanwhile, the proton conductivity improved with increasing additive content due to the loading of more available acidic groups. It is noteworthy that at 25% additive loading, the proton conductivity reached a maximum value of 5.4 × 10⁻² S cm⁻¹ at 25 °C, which exceeded the corresponding value of Nafion^@ 117 (5.0 × 10⁻² S cm⁻¹) under same experimental conditions. The composite membrane with 20 wt% additive was found to produce the highest selectivity (1.22 × 10⁵ S cm⁻³) with proton conductivity of 4.70 × 10⁻² S cm⁻¹ and methanol diffusion coefficient of 3.85 × 10⁻⁷ cm² s⁻¹, suggesting its best potential as proton exchange membrane for direct methanol fuel cell application. The main novelty of our work is providing a feasible and environment-friendly way to prepare the self-made polyorganosilicon with various functional groups and introducing it into SPEEKK to fabricate the dual crosslinked membranes. This design produces new materials with outstanding performance.     

Synthesis and properties of sulfonated poly(arylene ether ketone sulfone) containing amino groups/functional titania inorganic particles hybrid membranes for fuel cells

In this work, the organic-inorganic hybrid membranes were prepared. The synthesis and properties of the hybrid membranes were investigated. The sulfonated poly(arylene ether ketone sulfone) containing amino groups (Am-SPAEKS) was synthesized by nucleophilic polycondensation. The sol-gel method was used to prepared functional titania inorganic particles (L-TiO₂). The ¹H NMR and FT-IR were performed to verified the structure of Am-SPAEKS and L-TiO₂. The organic-inorganic hybrid membranes showed both good thermal stabilities and mechanical properties than that of Am-SPAEKS. The L-Am-15% membrane exhibited the highest Young's modulus (2262.71 MPa) and Yield stress (62.09 MPa). The distribution of L-TiO₂ particles was revealed by SEM. Compared to Am-SPAEKS, the hybrid membranes showed higher proton conductivities. The L-Am-15% exhibited the highest proton conductivity of 0.0879 S cm⁻¹ at 90 °C. The results indicate that the organic-inorganic hybrid membranes have potential for application in proton exchange membrane fuel cells.     

Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations

The performances of the proton exchange membrane fuel cell (PEMFC), direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) with sulfonated poly(ether sulfone) membrane are reported. Pt/C was coated on the membrane directly to fabricate a MEA for PEMFC operation. A single cell test was carried out using H₂/air as the fuel and oxidant. A current density of 730 mA cm⁻² at 0.60 V was obtained at 70 °C. Pt–Ru (anode) and Pt (cathode) were coated on the membrane for DMFC operations. It produced 83 mW cm⁻² maximum power density. The sulfonated poly(ether sulfone) membrane was also used for DFAFC operation under several different conditions. It showed good cell performances for several different kinds of polymer electrolyte fuel cell applications.     

Synthesizing spindle-shaped anion exchange membranes to improve conductivity and stability

High ionic conductivity and excellent alkaline stability are very important for solid electrolyte. Therefore, spindle-shaped anion exchange membranes (AEMs) based on poly (arylene ether ketone) and 1-Bromo-N,N,N-trimethylhexane-6-aminium bromide (Br-QA) have been prepared. The obtained Br-QA can be grafted with poly (arylene ether ketone) main chains to form micro-phase separation structure enhancing the ionic conductivity. Especially, the grafting quaternary ammonium (QA) cation groups are separated by alkyl bromine endows the AEMs with alkaline stability features. Simultaneously, the OH⁻ conductivity of the QA-PAEK-0.6 obtained membranes is 0.046 S/cm under fully hydrated conditions at 60 °C. After immersing into 1 M NaOH alkaline solution for 15 days at 60 °C, the anionic conductivity still high to 0.03 S/cm. Meanwhile, the poly (arylene ether ketone) backbones provide excellent mechanical properties and the Br-QA cation groups also possess good thermal stability, which satisfy the requirement of wide applications.