SPEEK Membrane Research Articles

Investigation the Effect of TiO2 Nanoparticles on Proton Exchange Membrane of sPEEK Used as a Fuel Cell Electrolyte Based on Phase Diagram

Sulfonated polyetheretherketone (sPEEK) membrane is a good alternative for Nafion membrane due to its low price and excellent chemical and physical properties. In this investigation, sPEEK and the composite of sPEEK with the TiO2 nanoparticles are prepared using the method of phase inversion. Different tests such as water uptake measurement, ion exchange capacity, degree of sulfonation, scanning electron microscopy images and electrochemical impedance spectroscopy are performed for characterization of the prepared membranes. Additionally, to evaluate thermodynamic instability of the prepared membranes in the presence of TiO2 nanoparticles as a hydrophilic additive, the Flory–Huggins theory is employed to theoretically draw the phase diagram. To this end, Hansen solubility parameters are calculated by the Van Krevelen method, and then the interaction parameters are determined. Using thermodynamic properties and phase diagram, the proton conductivity of sPEEK/TiO2 membrane are predicted. The results showed that with the addition of 0.5 wt% of TiO2 nanoparticles, the water uptake and proton conductivity significantly increased in comparison with pure sPEEK membrane. However, with increasing the concentration of the additive, the viscosity significantly increased, and the water uptake and proton conductivity decreased.

Enhanced performance of sulfonated poly (ether ether ketone) membranes by blending fully aromatic polyamide for practical application in direct methanol fuel cells (DMFCs)

The high dimensional swelling has been considered as one of the most important issues for obstructing sulfonated poly (ether ether ketone) (SPEEK) membrane with high IEC value as an excellent performance proton exchange membrane (PEM) for direct methanol fuel cell (DMFC) application. Here, we present a facile way to overcome the issue via blending the fully aromatic polyamide (fa-PA) into SPEEK membrane. The Influence of the fa-PA on the key properties of the fabricated fa-PA/SPEEK composite membranes (fa-PASPs) was systematically investigated. As the promising dimensional stability of the fa-PA, the fa-PASPs display excellent dimensional stability both in water and methanol aqueous solution. In addition, the fa-PASPs exhibit uniform and smooth morphology due to the same fully aromatic polyamide backbones as SPEEK and strong hydrogen bonding interactions between fa-PA and SPEEK with significantly reduced methanol crossover. The remarkable cell performance at high methanol concentration up to neat methanol was significantly obtained, suggesting that the prepared fa-PASPs are suitable for using as PEMs in direct methanol fuel cells application.

The influence of various ionic liquids on the properties of SPEEK membrane doped with mesoporous silica

Anhydrous proton conducting membranes made of different ionic liquids (ILs), sulfonated poly (ether ether ketone) (SPEEK), and mesoporous silica (SiO2) are successfully prepared. Influences of different ionic liquids on the properties of composite membranes doped with 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][Tfo]), 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), diethylmethylammonium trifluoromethanesulfonate ([dema][Tfo]), and 1-ethylimidazolium trifluoromethanesulfonate ([EIm][Tfo]) ionic liquids are described. Fourier transform infrared spectroscopy (FT-IR) analysis showed that there were interactions between ionic liquids, SPEEK and mesoporous silica. IL leaching studies demonstrated that IL loss decreased significantly after doping with SiO2. The proton conductivity study showed that both anions and cations of IL influenced the proton conductivity of the composite membranes. Proton conductivity of composite membranes displayed a positive correlation with the number of hopping sites within IL cation structures. Of all the conducting membranes studied, SP/SiO2-7.5/[BMIm][BF4]-50 showed the highest anhydrous proton conducting ability (15mScm−1 at 200°C). Therefore, it is expected to find applications in proton exchange membrane fuel cells under medium temperature, anhydrous conditions.

Two-Dimensional Zeolitic Imidazolate Framework/Carbon Nanotube Hybrid Networks Modified Proton Exchange Membranes for Improving Transport Properties.

Metal-organic framework (MOF)/polymer composite proton exchange membranes (PEMs) are being intensively investigated due to their potentials for the systematic design of proton-conducting properties. However, the development of MOF/polymer composite PEMs possessing high selectivity remains exceedingly desirable and challenging for practical application. Herein, two-dimensional (2D) zeolitic imidazolate framework (ZIF-8)/carbon nanotube (CNT) hybrid cross-linked networks (ZCN) were synthesized via the rational design of the physical form of ZIF-8, and then a series of composite PEMs were prepared by hybridizing ZCN with sulfonated poly(ether ether ketone) (SPEEK) matrix. The effect of the incorporation of zero-dimensional (0D) raw ZIF-8 nanoparticles and 2D ZCN on the proton conduction and methanol permeability of the composite membranes was systemically studied. Benefiting from the morphological and compositional advantages of ZCN, the SPEEK/ZCN composite membranes displayed a significant enhancement in proton conductivity under various conditions. In particular, the proton conductivity of SPEEK/ZCN-2.5 membrane was up to 50.24 mS cm-1 at 120 °C-30% RH, which was 11.2 times that of the recast SPEEK membrane (4.50 mS cm-1) and 2.1 times that of SPEEK/ZIF membrane (24.1 mS cm-1) under the same condition. Meanwhile, the methanol permeability of the SPEEK/ZCN composite membranes was greatly reduced. Therefore, novel MOF/polymer composite PEMs with high selectivity were obtained. Our investigation results reveal that the proton conductivity and methanol permeability of the MOF/polymer composite membranes can be effectively tailored via creating more elaborate superstructures of MOFs rather than altering the chemical component. This effective strategy may provide a useful guideline to integrate with other interesting MOFs to design MOF/polymer composite membranes.

Poly(2,5-benzimidazole)-Grafted Graphene Oxide as an Effective Proton Conductor for Construction of Nanocomposite Proton Exchange Membrane.

To improve proton conduction properties of conventional sulfonated poly(ether ether ketone) (SPEEK), poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) was prepared to fabricate nanocomposite membranes, which then were further doped with phosphoric acid (PA). The ABPBI-GO was synthesized through the reaction of 3,4-diaminobenzoic acid with the carboxyl acid groups present on the GO surface. The simultaneous incorporation of ABPBI-GO and PA into SPEEK did not only improve the physicochemical performance of the membranes in terms of thermal stability, water uptake, dimensional stability, proton conductivity, and methanol permeation resistance but also relieve PA leaching from the membranes though acid-base interactions. The resulting composite membranes exhibited enhanced proton conductivities in extended humidity ranges thanks to the hygroscopic character of PA and the increased water uptake. Moreover, the unique self-ionization, self-dehydration, and nonvolatile properties of PA improved the high-temperature proton conductivities (σ) of PA-doped membranes. The PA-doped SPEEK/ABPBI-GO-3.0 delivered a σ of 7.5 mS cm-1 at 140 °C/0% RH. This value was fourfold higher than that of pristine SPEEK membranes. The PA-doped SPEEK/ABPBI-GO-3.0 based fuel cell membranes delivered power densities of 831.06 and 72.25 mW cm-2 at 80 °C/95% RH and 120 °C/0% RH, respectively. By contrast, the PA-doped SPEEK membrane generated only 655.63 and 44.58 mW cm-2 under the same testing conditions.

Facile synthesis of metal oxide nanofibers and construction of continuous proton-conducting pathways in SPEEK composite membranes

In this study, a facile approach was used to prepare hydrophilic TiO2 nanofibers (TiNFs) by calcining electrospun polyacrylonitrile nanofibers embedded with titanium precursors. A high-performance proton exchange membrane (PEM) was achieved by incorporating the TiNFs into a sulfonated poly(ether ether ketone) (SPEEK) matrix. The high aspect ratios of the inorganic nanofibers permitted interconnecting of ion-conducting channels, thereby significantly improving proton conductivity in the membrane. In addition, the TiNFs provided tortuous pathways for methanol transport, suppressing the methanol permeability coefficient. The maximum power density of the SPEEK/TiNFs-1.0 composite membrane was 1.4 times higher than that of the pristine SPEEK membrane at 100% relative humidity and 60 °C. Furthermore, the stable fiber skeleton improved the mechanical stability of the composite membrane. The SPEEK/TiNFs-1.0 membrane exhibited superior mechanical properties with the tensile strength of 40.4 MPa and maximum elongation at break of 76%.

A superhydrophobic bromomethylated poly(phenylene oxide) as a multifunctional polymer filler in SPEEK membrane towards neat methanol operation of direct methanol fuel cells

Development of novel proton exchange membrane (PEM) with the aim toward neat methanol operation of direct methanol fuel cell (DMFC) has attracted increasing attention. Proton exchange membrane technology with remarkably low methanol permeability has been considered as one of the most facile ways to approach this target. Here, sulfonated poly(ether ether ketone) (SPEEK) membrane was modified by blending with superhydrophobic bromomethylated poly(phenylene oxide) (BPPO). The fabricated SPEEK/BPPO membranes possess obviously low methanol permeability, which is attributed to the formed interrupted methanol transport channels arising from the combined properties of the hydrophobic BPPO polymeric backbone, the formed spherical BPPO obstacles and the cross-linking networks derived from the Friedel-Craft reaction between bromobenzyl groups and nucleophilic phenyl rings. Therefore, the SPEEK/BPPO membrane DMFC demonstrates a much higher cell performance both at low methanol concentration and high methanol concentration than the Nafion 117 DMFC. In particular, the actual value of the maximum power density at high methanol concentration of 20M suggests that the SPEEK/BPPO membranes can be considered as a desirable candidate for potential application in high methanol concentration DMFCs.

Sulfonated Ti3C2Tx to construct proton transfer pathways in polymer electrolyte membrane for enhanced conduction

Here, Ti3C2Tx sheets, a representative of MXenes, with lamellar structure and hydrophilic surface are prepared, and then functionalized with sulfonated polyelectrolyte brushes (Ti3C2Tx-SO3H) through a facile surface-initiated precipitation-polymerization. The Ti3C2Tx-SO3H is then utilized as a new nanofiller to fabricate hybrid proton exchange membrane, where both acidic sulfonated poly (ether ether ketone) (SPEEK) and basic chitosan (CS) are employed as polymer matrixes. The resultant hybrid membranes are systematically characterized and measured including their microstructures, water uptake and proton conduction properties. The results demonstrate that using the sulfonated polyelectrolyte brushes, Ti3C2Tx-SO3H sheets construct efficient proton transfer pathways and connect the inherent conduction channels/paths in polymer phase. This significantly enhances the proton conduction of polymer membrane including SPEEK membrane and CS membrane, under both hydrated condition and anhydrous condition. Particularly, the incorporation of 10 wt.% Ti3C2Tx-SO3H readily offers 144% and 66% increase in proton conductivity, respectively, to SPEEK membrane and CS membrane under hydrated condition. Furthermore, the hybrid membranes achieve improved thermal and mechanical stabilities. These results herald further advances to preparing functionalized MXenes and their relevant hybrid materials with enhanced performances.

Sulfonated holey graphene oxide paper with SPEEK membranes on its both sides: a sandwiched membrane with high performance for semi-passive direct methanol fuel cells

The sandwiched membrane which consists of the sulfonated holey graphene oxide paper with the deposition of the sulfonated poly(ether ether ketone) (SPEEK/SHGO/SPEEK) membrane on its both sides has been prepared. This membrane is reported to have higher proton conductivity (155.6±1.6mScm−1), but lower methanol permeability (7.05±0.21×10−6cm2s−1) than Nafion® 112 at 80°C, whose respective values are 138.6±1.4mScm−1 and 15.40±0.46×10−6cm2s−1, respectively. The specific structure of the SHGO comprising the sulfonated and holey graphitic plane plays an important role in the high performance of the SPEEK/SHGO/SPEEK membrane. First, the sulfonated structure could increase the overall density of the sulfonic acid groups in the membranes, which increases the number of proton exchange groups participating in the proton transport. Second, the holey structure of the SHGO provides an additional path for the proton transport, which well avoids the blockage of the proton transport by the high aspect ratio graphitic planes. Third, the sulfonated and holey structure could also increase the interlayer spacing of the graphitic planes in the SHGO paper, which would promote the proton transport as well. Finally, the presence of the SHGO paper with the layer-by-layer stacked graphitic plane would effectively retard the diffusion of methanol through the membrane. When evaluated in the semi-passive direct methanol fuel cell (DMFC), the cell with the SPEEK/SHGO/SPEEK membrane shows higher stability and exhibits a maximum power density 61.1% higher than with Nafion® 112, which strongly suggests that the SPEEK/SHGO/SPEEK membrane could be used as the proton exchange membrane for DMFCs.

Enhanced durability of sulfonated poly (ether ether ketones)-based polymer electrolyte membranes by a multi-layer composite technology

Multi-layer composite membrane with low-cost and high durability is prepared via hot-spraying methods for proton exchange membrane fuel cell (PEMFC) applications, which consists of a mid layer of sulfonated poly(ether ether ketones) (SPEEK), two transition layers (TLs) and two PFSA outer layers (PLs). Here, PLs can protect SPEEK membrane against the chemical degradation from reactive oxygen species and endow the composite membrane with high chemical stability. TLs improve the interfacial compatibility between SPEEK membrane and PLs, avoiding the delamination problem. This multi-layer composite membrane exhibits a proper ion-exchange capacity (IEC) and higher proton conductivity compared with that of SPEEK membrane, and it also keeps a balance between water uptake and swelling ratio owing to the presence of the multi-layer structure. Besides, the lower cost of the composite membrane is confirmed due to the small content of Nafion. Single cell tests indicate that the multi-layer composite membrane has better cell performance than that of SPEEK membrane. The higher durability for the membrane under accelerated open circuit voltage (OCV) tests reveals the effectiveness of this multi-layer composite strategy in improving the chemical durability of the SPEEK membrane.

Sulfonated Holey Graphene Oxide (SHGO) Filled Sulfonated Poly(ether ether ketone) Membrane: The Role of Holes in the SHGO in Improving Its Performance as Proton Exchange Membrane for Direct Methanol Fuel Cells.

Sulfonated holey graphene oxides (SHGOs) have been synthesized by the etching of sulfonated graphene oxides with concentrated HNO3 under the assistance of ultrasonication. These SHGOs could be used as fillers for the sulfonated aromatic poly(ether ether ketone) (SPEEK) membrane. The obtained SHGO-incorporated SPEEK membrane has a uniform and dense structure, exhibiting higher performance as proton exchange membranes (PEMs), for instance, higher proton conductivity, lower activation energy for proton conduction, and comparable methanol permeability, as compared to Nafion 112. The sulfonated graphitic structure of the SHGOs is believed to be one of the crucial factors resulting in the higher performance of the SPEEK/SHGO membrane, since it could increase the local density of the -SO3H groups in the membrane and induce a strong interfacial interaction between SHGO and the SPEEK matrix, which improve the proton conductivity and lower the swelling ratio of the membrane, respectively. Additionally, the proton conductivity of the membrane could be further enhanced by the presence of the holes in the graphitic planes of the SHGOs, since it provides an additional channel for transport of the protons. When used, direct methanol fuel cell with the SPEEK/SHGO membrane is found to exhibit much higher performance than that with Nafion 112, suggesting potential use of the SPEEK/SHGO membrane as the PEMs.

Molecular Structuring and Percolation Transition in Hydrated Sulfonated Poly(ether ether ketone) Membranes.

The extent of phase separation and water percolation in sulfonated membranes are the key to their performance in fuel cells. Toward this, the effect of hydration on the morphology and transport characteristics of sulfonated poly(ether ether ketone), sPEEK, membrane is investigated using atomistic molecular dynamics simulation at various hydration levels(λ: number of water molecules per sulfonate group). The evolution of local morphology is investigated using structural correlations and minimum pair distances. Transport properties are probed using mean squared displacements and diffusion coefficients. The water-sulfonate interaction in sPEEK is found to be stronger than that in Nafion, as observed in experiments. Analyses indicate the presence of narrow connected path of water and hydronium at λ = 4 and large domains, spanning half the simulation box, at λ = 15. The behavior of membrane water remains far from bulk as indicated by its diffusion coefficient. The persistence of small isolated water clusters demonstrates the extent of phase separation in sPEEK to be lesser than that in Nafion. Analyses at molecular and collective levels suggest the occurrence of a percolation transition between λ = 8 and 10, which leads to a connected network of water channels in the membrane, thereby boosting the hydronium mobility.

Sulfonate poly(ether ether ketone) incorporated with ammonium ionic liquids for proton exchange membrane fuel cell

Sulfonate poly(ether ether ketone) (SPEEK)/ammonium ionic liquids (AIL) composite membranes were fabricated using the solution casting method. AIL is a triethyl ammonium dihydrogen phosphate ([(CH3CH2)3NXH+] [H2PO4−], TEAP) or triethyl ammonium hydrogen sulfate ([(CH3CH2)3NH+] [HSO4−], TEAS). It was found that SPEEK/AIL composite membranes have glossy exteriors from SEM images. Compared to the SPEEK membrane, the thermal durability and proton conductivity of SPEEK/AIL composite membranes decreases due to the incorporation of the AIL which affects the original proton conduct chain and polymer structure. The proton conductivity of the SPEEK96-15S membrane and SPEEK75-35S membrane goes to 7.06 and 6.94 mS/cm respectively at 393 K. The performance of the proton exchange membrane fuel cell (PEMFC) with the SPEEK96-15S membrane and SPEEK75-35S membrane is better than that with the Nafion212 membrane when H2/O2 was humidified at 333 K.

Proton exchange nanohybrid membranes with high phosphotungstic acid loading within metal-organic frameworks for PEMFC applications

A novel approach to in-situ synthesize and encapsulate phosphotungstic acid into the cavity of MIL-101(Cr) using Na2WO4·2H2O and Na2HPO4 as precursors is presented to increase the acid loading content (31.4wt.%). The phosphotungstic acid-encapsulating MIL-101(Cr) (HPW@MIL101) is introduced in sulfonated poly(ether ether ketone) (SPEEK) to prepare SPEEK/HPW@MIL101 nanohybrid membranes for PEMFC applications. Due to the introduction of HPW@MIL101, proton-conducting nanochannels are constructed both in the cavity of MIL101 and at the interface between HPW@MIL101 and SPEEK. Meanwhile, due to the hygroscopicity of phosphotungstic acid, the membrane dehydration at elevated temperatures is alleviated. The proton conductivity at low relative humidity is remarkably enhanced. The nanohybrid membrane with 9wt.% HPW@MIL101 exhibits proton conductivity of 272mScm−1 at 65°C, 100% RH and 6.51mScm−1 at 60°C, 40% RH, which are 45.5% and 7.25 times higher than those of pristine SPEEK membrane (187mScm−1 and 0.898mScm−1), respectively. The single H2/O2 fuel cell with SPEEK/HPW@MIL-9 membrane acquires the power density of 383mWcm−2 at 100% RH, which is 27.2% higher than that of pristine SPEEK membrane. The peak power density of SPEEK/HPW@MIL-9 membrane at 55% RH is 2.97 times higher than that of pristine SPEEK membrane (79mW/cm2).

Proton-exchange membranes with enhanced anhydrous proton conductivity by room temperature ionic liquid anchored to silica

A strategy of immobilizing room temperature ionic liquid (RTIL) in proton-exchange membranes (PEMs) is demonstrated for the first time. After synthesized by ionic exchange and extraction, RTIL is anchored to the surfaces of silica nanoparticles (SiO2-RTIL) via condensation reaction. Then composite PEMs consisting of sulfonated poly(ether ether ketone) (SPEEK) and silica with RTIL are prepared through the solution casting method. The incorporation of SiO2-RTIL markedly improves the physical properties of the composite PEMs as compared to the plain SPEEK membrane, which was probably related to the plasticization and additional proton conductivity of the SiO2-RTIL. The anhydrous proton conductivity of the resultant composite PEM with 30% SiO2-RTIL was 50 times that of the plain membrane at 150[Formula: see text]C.

Nanocomposite Based on Functionalized Gold Nanoparticles and Sulfonated Poly(ether ether ketone) Membranes: Synthesis and Characterization.

Gold nanoparticles, capped by 3-mercapto propane sulfonate (Au-3MPS), were synthesized inside a swollen sulfonated poly(ether ether ketone) membrane (sPEEK). The formation of the Au-3MPS nanoparticles in the swollen sPEEK membrane was observed by spectroscopic and microscopic techniques. The nanocomposite containing the gold nanoparticles grown in the sPEEK membrane, showed the plasmon resonance λmax at about 520 nm, which remained stable over a testing period of three months. The size distribution of the nanoparticles was assessed, and the sPEEK membrane roughness, both before and after the synthesis of nanoparticles, was studied by AFM. The XPS measurements confirm Au-3MPS formation in the sPEEK membrane. Moreover, AFM experiments recorded in fluid allowed the production of images of the Au-3MPS@sPEEK composite in water at different pH levels, achieving a better understanding of the membrane behavior in a water environment; the dynamic hydration process of the Au-3MPS@sPEEK membrane was investigated. These preliminary results suggest that the newly developed nanocomposite membranes could be promising materials for fuel cell applications.

The Morphology Effect on the Selectivity of SPEEK/ENR Membranes for Direct Methanol Fuel Cell

The objective of this study is to investigate the morphology effect of ENR-50 content on the conductivity and methanol permeability of SPEEK/ENR-50 membranes for DMFC application. Three different ratios (9.5:0.5, 8.5:1.5 and 7.5:2.5) of blended SPEEK/ENR-50 membranes were prepared via solution intercalation method. The cross sectional images of the all SPEEK/ENR-50 membranes studied by SEM shows an existence of ENR-50 droplets in SPEEK matrix which decrease in size with an increasing of ENR-50 content from 1.5 wt% to 2.5 wt%. The study of the performances of the blended membranes shows that the proton conductivity and methanol permeability of the membranes decrease with an increasing of ENR-50 content. However, the proton conductivity of the blended membrane with 0.5 wt% ENR-50 content (9.66x10-3 S cm-1) proven higher than pristine SPEEK but lower than Nafion 117. The barrier performance of all SPEEK/ENR-50 membranes was enhanced compared to the expensive Nafion 117 membrane and pristine SPEEK. Consequently, by incorporating ENR-50 content into SPEEK membrane, the selectivity of the membranes has considerably increased.

Comprehensive investigation of physicochemical and electrochemical properties of sulfonated poly (ether ether ketone) membranes with different degrees of sulfonation for proton exchange membrane fuel cell applications

The investigation of the effect of various degrees of sulfonation (DS) is important for the selection and modification of hydrocarbon type membranes towards ion exchange applications. In this work, twelve kinds of sulfonated poly (ether ether ketone) membranes with different DS were prepared and their physicochemical and electrochemical properties were studied. The sulfonated polymers were prepared by sulfonation of PEEK and characterized by H NMR, FTIR, and XRD. Then, the different SPEEK membranes were prepared by the solvent casting method, and their morphology and homogeneity were investigated by FESEM. The effect of DS on the thermal (TGA and DSC), mechanical and oxidative stabilities, water uptake behavior, and ionic exchange capacity of the membranes was determined. Also, the proton conductivity of the membranes at different temperatures and relative humidities were measured. The results indicated that the SPEEK samples showed progressive deterioration of thermal stability, an increase of glass transition temperature, a decrease of the elastic modulus, and a decrease of the chemical stability with increasing of the DS. The proton conductivity of SPEEK membranes increased with DS, and for each membrane increased with increasing temperature and humidity. The results showed that moderate DS membranes (DS ∼ 65%) had acceptable mechanical, thermal, electrochemical, and chemical stability.

Rice Paper Reinforced Sulfonated Poly(ether ether ketone) as Low-Cost Membrane for Vanadium Flow Batteries

Low-cost, highly efficient, and durable membrane is essential for practical application of vanadium flow battery (VFB). Sulfonated poly(ether ether ketone) (SPEEK) membrane is considered as potential candidate to replace the expensive Nafion membrane because of its high proton to vanadium ion selectivity. To overcome the poor mechanical/chemical stability of SPEEK membrane in VFB, a new strategy by using commercial paper (copy paper, rice paper, and filter paper) as the scaffold of SPEEK, and subsequently self-cross-linking the SPEEK through thermal treatment was conducted. The optimized rice paper reinforced SPEEK membrane exhibits superior VFB performances to that of the benchmark Nafion 115 membrane, such as 98.3% vs 92.8% for Coulombic efficiency, 79.5% vs 75.8% for energy efficiency, and 0.13% vs 0.16% for capacity decay per cycle at a high current density of 120 mA cm–2. Besides, S@RP-C6 shows an extremely stable performance in the temperature range of 0–70 °C. Combining the outstanding battery perf...

Optimization of hydrophilic/hydrophobic phase separation in sPEEK membranes by hydrothermal treatments.

Via SAXS, herein, we studied how a sPEEK microstructure evolves when it is immersed in water at a wide range of temperatures (20-100 °C) and time scales (from a few hours to dozens of days). In particular, we scrutinized the behavior of sPEEK at the temperature and time associated with the appearance of a well-defined nanosegregated morphology. At 80 °C, we observed nanoscale swelling along with smoothing of the water/polymer interface over a long period of time (several days). Herein, two of the main membrane properties, i.e., water uptake and proton conductivity, were studied for different immersion times and temperatures. It was found that the abovementioned properties were remarkably correlated with the evolution of the membrane microstructure, which was partly conserved after drying. The present findings helped us to understand that the thermally activated evolutions observed at both the nanoscale and macroscale were associated with the sPEEK β-relaxation crossover. Therefore, the very different swelling behaviors of sPEEK and Nafion are correlated to the much higher β-relaxation of sPEEK vs. Nafion (75 °C vs. -20 °C - dry state). From a practical viewpoint, this study emphasizes, for membranes alternative to Nafion, the importance and impact of the membrane pretreatment on their functional properties.