A facile,effective thermal crosslinking to balance stability and proton conduction for proton exchange membranes based on blend sulfonated poly(ether ether ketone)/sulfonated poly(arylene ether sulfone)

The development of hydrocarbon polymer electrolyte membranes with high proton conductivities and good stability as alternatives to perfluorosulfonic acid membranes is an ongoing research effort. A facile and effective thermal crosslinking method was carried out on the blended sulfonated poly (ether ether ketone)/poly (aryl ether sulfone) (SPEEK/SPAES) system. Two SPEEK polymers with ion exchange capacities (IECs) of 1.6 and 2.0 mmol g^?1 and one SPAES polymer (2.0 mmol g^?1) were selected to create different blends. The effect of thermal crosslinking on the fundamental properties of the membranes, especially their physicochemical stability and electrochemical performance, were investigated in detail. The homogeneous and flexible thermally-crosslinked SPEEK/SPAES membranes displayed excellent mechanical toughness (27–46 Mpa), suitable water uptake (<60%), high dimensional stability (swelling ratio < 15%) and large proton conductivity (>120 mS cm^?1) at 80 °C. The thermal crosslinking membranes also show significantly enhanced hydrolytic (<2.5%) and oxidative stability (<2%). Fuel cell with t-SPEEK/SPAES (1:2:2) membrane achieves a power density of 665 mW cm^?2 at 80 °C.     

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.     

Synthesis and characterization of sulfonated poly(arylene ether ketone ketone sulfone) membranes for application in proton exchange membrane fuel cells

A series of sulfonated poly(arylene ether ketone ketone sulfone) (SPAEKKS) copolymers were synthesized by nucleophilic polycondensation. The copolymers exhibit good thermal and oxidative stabilities, all the SPAEKKS copolymers can be cast into tough membranes. Ionic exchange capacities (IEC), water uptake properties, thermal stabilities, methanol diffusion coefficients and proton conductivities were thoroughly studied. Also the microstructures of the membranes were investigated by TEM. The proton conductivity of the SPAEKKS-4 membrane is close to that of Nafion-117 at 80 °C. The methanol diffusion coefficient of the membrane is much lower than that of Nafion-117 under the same testing conditions. The SPAEKKS membranes are promising in proton exchange membranes fuel cell (PEMFC) application.     

Hydrophilic treatment poly(tetrafluoroethylene) reinforced sulfonated poly(ether ether ketone) composite membrane for proton exchange membrane fuel cell application

A reinforced composite membrane based on SPEEK (sulfonated poly ether ether ketone) and porous PTFE substrate (polytetrafluoroethylene) is fabricated and investigated for proton exchange membrane fuel cell application. In order to improve the combination between SPEEK polymer and PTFE matrix, PTFE substrate is hydrophilically pretreated by naphthalene sodium solution. The experimental results indicate that SPEEK can impregnate into treated PTFE substrate (abbreviated as trPTFE) more easily. The variation of PTFE surface property before and after treatment is characterized by water contact angle experiment and ATR-FTIR technique. The impregnated status of SPEEK polymer in PTFE matrix is also characterized by ATR-FTIR. According to the appearance photo of two composite membranes, it is showed that SPEEK/trPTFE composite membrane has more uniform and homogeneous morphology. Moreover, the mechanical property of SPEEK/trPTFE composite membrane also has an advantage over pristine SPEEK membrane. Due to the reinforced effect of trPTFE substrate, thinner composite membrane can be applied in single cell evolution and achieves better performance as a result.     

Layer-by-layer self-assembly of polyaniline on sulfonated poly(arylene ether ketone) membrane with high proton conductivity and low methanol crossover

Sulfonated poly(arylene ether ketone) bearing carboxyl groups (SPAEK-C) membranes were first modified by alternating deposition of oppositely charged polyaniline (PANI) and phosphotungstic acid (PWA) via the layer-by-layer method in order to prevent the crossover of methanol in a direct methanol fuel cell. The methanol permeability of SPAEK-C–(PANI/PWA)₅ is 2 orders of magnitude less than those of Nafion 117 and pristine SPAEK-C. Furthermore, the modified membrane shows a proton conductivity of 0.093 Scm⁻¹ at 25 °C and 0.24 Scm⁻¹ at 80 °C, which are superior to those of Nafion 117 and pristine SPAEK-C. Fourier transform infrared spectroscopy confirms that PANI and PWA are assembled in the multilayers. The SEM images show the presence of thin PANI/PWA layers coated on the SPAEK-C membrane. Thermal stability, water uptake, water swelling, proton and electron conductivity at different temperature of the SPAEK-C and SPAEK-C-(PANI/PWA)_n membranes are also investigated.     

Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell

The proton exchange membrane (PEM) was synthesized using polyethersulfone (PES), sulfonated poly (ether ether ketone) (SPEEK) and nanoparticles. The metal oxide nanoparticles such as Fe₃O₄, TiO₂ and MoO₃ were added individually to the polymer blend (PES and SPEEK). The polymer composite membranes exhibit excellent features regarding water uptake, ion exchange capacity and proton conductivity than the pristine PES membrane. Since the presence of sulfonic acid groups provides by added SPEEK and the unique properties of inorganic nanoparticles (Fe₃O₄, TiO₂ and MoO₃) helps to interconnect the ionic domain by the absorption of more water molecules thereby enhance the conductivity value. The proton conductivity of PES, SPEEK, PES/SPEEK/Fe₃O₄, PES/SPEEK/TiO₂ and PES/SPEEK/MoO₃ membranes were 0.22 × 10^?4 S/cm, 5.18 × 10^?4 S/cm, 3.57 × 10^?4 S/cm, 4.57 × 10^?4 S/cm and 2.67 × 10^?4 S/cm respectively. Even though the blending of PES with SPEEK has reduced the conductivity value to a lesser extent, hydrophobic PES has vital role in reducing the solvent uptake, swelling ratio and improves hydrolytic stability. Glass transition temperature (T_g) of the membranes were determined from DSC thermogram and it satisfies the operating condition of fuel cell system which guarantees the thermal stability of the membrane for fuel cell application.     

Synthesis of the sulfonated poly(ether ether ketone ketone) proton exchange membrane with butane-1,4-diylbis(oxy) spacers and sulfopropyloxy pendants through Friedel-Crafts polyacylation

A novel proton exchange membrane composed of the sulfonated poly(ether ether ketone ketone) (SPEEKK) with butane-1,4-diylbis(oxy) spacers in the backbone and sulfopropyloxy pendants is synthesized through the Friedel-Crafts polyacylation of 1,4-diphenoxybutane (DPB), diethyl-3,3’-([1,1′-biphenyl]-2,2′-diylbis(oxy))bis(propane-1-sulfonate) (SBP–OEt), and terephthaloyl chloride (TPC) under mild conditions. The monomer DPB can introduce the flexible aliphatic butane-1,4-diylbis(oxy) spacers into the polymeric backbone and thus promote the formation of proton transport channels. The as-synthesized SPEEKK exhibits a high proton conductivity of 152.5 mS cm^?1 at 95 °C and 90% relative humidity (RH), a moderate water uptake of 53.2% at 100 °C and also a high ion exchange capacity of 1.74 mmol g^?1. These characteristics are attributed to the enhanced nanophase separation induced by the flexible butane-1,4-diylbis(oxy) spacers and hydrophilic sulfopropyloxy sidechains.     

Cross-linked membranes based on sulfonated poly (ether ether ketone) (SPEEK)/Nafion for direct methanol fuel cells (DMFCs)

A series of cross-linked membranes based on SPEEK/Nafion have been prepared to improve methanol resistance and dimension stability of SPEEK membrane for the usage in the direct methanol fuel cells (DMFCs). Sulfonated diamine monomer is synthesized and used as cross-linker to improve the dispersion of Nafion in the composite membranes and decrease the negative effect of cross-linking on proton conductivity of membranes. FT-IR analysis shows that the cross-linking reaction is performed successfully. The effects of different contents of Nafion on the properties of cross-linked membranes are investigated in detail. All the cross-linked membranes show lower methanol permeability and better dimensional stability compared with the pristine SPEEK membrane. SPEEK-N30 with the 30 wt % Nafion shows a methanol permeability of 0.73 × 10⁻⁶ cm² s⁻¹ and a water uptake of 24.4% at 25 °C, which are lower than those of the pristine membrane. Meanwhile, the proton conductivity of SPEEK-N30 still remains at 0.041 S cm⁻¹ at 25 °C, which is comparable to that of the pristine SPEEK membrane. All the results indicate that these cross-linked membranes based on SPEEK/Nafion show good prospect for the use as proton exchange membranes.     

Sulfonated titania submicrospheres-doped sulfonated poly(ether ether ketone) hybrid membranes with enhanced proton conductivity and reduced methanol permeability

Sulfonated titania submicrospheres (TiO₂-SO₃H) prepared through a facile chelation method are incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate organic-inorganic hybrid membranes with enhanced proton conductivity and reduced methanol permeability for potential use in direct methanol fuel cells (DMFCs). The pristine titania submicrospheres (TiO₂) with a uniform particle size are synthesized through a modified sol-gel method and sulfonated using 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt as the sulfonation reagent. The sulfonation process is confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectra (XPS). The hybrid membranes are systematically characterized in terms of thermal property, mechanical property, ionic exchange capacity (IEC), swelling behavior, and microstructural features. The methanol barrier property and the proton conductivity of the SPEEK/TiO₂-SO₃H hybrid membranes are evaluated. The presence of the fillers reduces methanol crossover through the membrane. Compared with the unsulfonated TiO₂-doped membranes, the TiO₂-SO₃H-doped ones exhibit higher proton conductivity due to the additional sulfonic acid groups on the surface of TiO₂. The hybrid membrane doped with 15 wt.% TiO₂-SO₃H submicrospheres exhibits an acceptable proton conductivity of 0.053 S cm⁻¹ and a reduced methanol permeability of 4.19 × 10⁻⁷ cm² s⁻¹.     

Molecular structure and transport dynamics in Nafion and sulfonated poly(ether ether ketone ketone) membranes

An atomistic simulation technique is performed to investigate the molecular structure and transport dynamics inside a hydrated Nafion membrane and a hydrated sulfonated poly(ether ether ketone ketone) (SPEEKK) membrane. The simulation system consists of the representative fragments of the polymer electrolytes, hydronium ions and solvent molecules, such as water plus methanol molecules. Simulation results show that the hydrated SPEEKK has less phase separation among hydrophobic and hydrophilic regions in comparison with the Nafion. Those water channels formed in the SPEEKK are much narrower compared to those in the Nafion. These characteristics lead to a lower mobility of hydronium ions and water molecules and hence relatively lower diffusion coefficient of methanol in the SPEEKK. It results in the reduction of the methanol permeation problem in direct methanol fuel cells.     

An effective strategy to enhance the performance of the proton exchange membranes based on sulfonated poly(ether ether ketone)s

We report an effective and facile approach to enhance the dimensional and chemical stability of sulfonated poly(ether ether ketone) (SPEEK) type proton exchange membranes through simple polymer blending for fuel cell applications, using commercial available materials. The polymeric blends with sulfonated poly(aryl ether sulfone)s (SPAES) were simply fabricated by a three-component system, which contained SPEEK (10–50 wt%, 1.83 mmol/g), and SPAES-40 (1.72 mmol/g)/SPAES-50 (2.04 mmol/g) at 1:1 in weight. The SPAES-40 was selected for mechanical and dimensional stability reinforcing, while SPAES-50 for the good polymer compatibility. The obtained SPEEK/SPAES blend membranes showed depressed water uptake, better dimensional and oxidative stability, together with higher proton conductivity beyond 70 °C than the pristine SPEEK membrane. The apparent improvements in membrane properties were associated with the homogeneous dispersion of SPEEK and both SPAES copolymers inside the membranes as well as the rearrangements of the polymeric chains. The SPEEK content should be properly controlled in the range of 10–40% (B10 to B40). In a H₂/O₂ fuel cell test, B30 showed a maximum power density of 700 mW/cm², which was 1.6 times as high as that of B40 at 80 °C under 100% RH. The further cross-linking treatment produced more ductile and enduring blend membranes, indicating an appreciable prospective for fuel cell applications.     

Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

Sulfonated organosilane functionalized graphene oxides (SSi-GO) synthesized through the grafting of graphene oxide (GO) with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated poly(ether ether ketone) (SPEEK) membranes. The incorporation of SSi-GOs greatly increases the ion-exchange capacity (IEC), water uptake, and proton conductivity of the membrane. With well-controlled contents of SSi-GOs, the composite membranes exhibit higher proton conductivity and lower methanol permeability than Nafion^® 112 and Nafion^® 115, making them particularly attractive as proton exchange membranes (PEMs) for direct methanol fuel cells (DMFC). The composite membrane with optimal SSi-GOs content exhibit over 38 and 17% higher power densities, respectively, than Nafion^® 112 and Nafion^® 115 membranes in DMFCs, offering the possibilities to reduce the DMFC membrane cost significantly while keeping high-performance.     

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.     

Proton-conducting membranes based on benzimidazole-containing sulfonated poly(ether ether ketone) compared with their carboxyl acid form

A series of sulfonated poly(ether ether ketone) containing pendant carboxyl (C-SPEEKs) have been synthesized using a nucleophilic polycondesation reaction. A condensation reaction between 1,2-diaminobenzene and carboxyl resulted in a new series of copolymers containing benzimidazole groups (SPEEK-BIms). The expected structures of the sulfonated copolymers are confirmed by ¹H NMR. The dependence of ion exchange capacity, water uptake, proton conductivity and methanol diffusion coefficient of SPEEK-BIm membranes has been studied and compared with their carboxyl acid form. The results suggest that the introduction of benzimidazole groups may be responsible for many excellent properties of the membranes for fuel cell. It is noticeable that the markedly improved oxidative stability is benefit for the application of membrane.     

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.     

Dielectric behaviour of UV-crosslinked sulfonated poly (ether ether ketone) with methyl cellulose (SPEEK-MC) as proton exchange membrane

Proton exchange membrane materials based on sulfonated poly ether ether ketone (SPEEK) with Methyl Cellulose (MC) are developed by solution cast technique and exposed to UV radiation with Bezoin Ethyl Ether (BEE) as photoinitiator. The addition of MC into SPEEK polymer enhances the conductivity up to 8.7 × 10^?3 Scm^?1 at 30 °C temperature and 80% relative humidity. This new crosslinked hybrid membrane shows good prospect for the use as proton exchange membrane in fuel cell.     

Molecular dynamics simulation study of proton diffusion in polymer electrolyte membranes based on sulfonated poly (ether ether ketone)

Molecular dynamics (MD) simulation technique was employed to investigate the effect of degree of sulfonation (DS) on structural and dynamical characteristics of sulfonated poly (ether ether ketone) (SPEEK) membranes at different temperatures. MD Simulations were performed for the cell containing SPEEK chains, hydronium ions and water molecules under NVT and NPT conditions. By evaluating the pair correlation functions, it was observed that with increasing the DS of SPEEK, the distance between sulfur atoms increases, more water molecules solvate the sulfur atoms and hydronium ions, the average sulfur–hydronium ion separation distance increases and larger water clusters are formed. It was also found that with increasing DS and temperature, the diffusion coefficient and conductivity of hydronium ions enhance. It was also understood, the simulated ionic conductivities qualitatively follow the experimental data.     

Novel branched sulfonated poly(ether ether ketone)s membranes for direct methanol fuel cells

In this article, novel branched sulfonated poly(ether ether ketone)s (Br-SPEEK) containing various amounts of 1,3,5-tris(4-fluorobenzoyl)benzene as the branching agent have been successfully prepared. Compared with the traditional linear polymer membranes, the membranes prepared by Br-SPEEK showed improved mechanical strength, excellent dimensional stability and superior oxidative stability with similar proton conductivity. Notably, the Br-SPEEK-10 membrane began to break after 267 min in Fenton's reagent at 80 °C, which was 4 times longer than that of the L-SPEEK. Although the proton conductivity decreased with the addition of the branching agent, satisfying methanol permeability value was observed (down to 6.3 × 10⁻⁷ cm² s⁻¹), which was much lower than Nafion 117 (15.5 × 10⁻⁷ cm² s⁻¹). All the results indicated that the novel branched sulfonated poly(ether ether ketone)s membrane was potential candidate as proton conductive membranes for application in fuel cells.     

Intermolecular ionic cross-linked proton conducting electrolyte membranes derived from branched sulfonated poly(ether ether ketone)s with benzoxazole pendants

A series of novel branched sulfonated poly(ether ether ketone)s containing intermolecular ionic cross-linkable groups, benzoxazole groups, have been prepared for direct methanol fuel cells. The benzoxazole groups are grafted onto the polymer chain via a thiol-ene click chemistry reaction. The expected structures of the copolymers are confirmed by ¹H NMR and Fourier transform infrared spectroscopy. Compared with the unmodified polymer membrane, the ionic cross-linked membranes show enhanced thermal and mechanical properties. We also investigate the changes in water uptake, proton conductivity and chemical stability. The dense membrane structures formed by branching and the interactions between sulfonic acid and benzoxazole groups make a great contribution to the improvements of dimensional stability and methanol resistance property. Although the proton conductivities of the ionic cross-linked membranes are lower than the pristine membrane, the selectivities are much higher. The results show that the novel copolymers in this study are possible potential candidate materials for proton electrolyte membrane.     

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 cm² s⁻¹, which are much lower than those of the pristine membrane (63.2% and 5.37 × 10⁻⁷ cm² 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.