Effect of functionality of polyhedral oligomeric silsesquioxane [POSS] on the properties of sulfonated poly(ether ether ketone) [SPEEK] based hybrid nanocomposite proton exchange membranes for fuel cell applications

Organic–inorganic hybrid nanocomposite membranes were prepared using three different types of POSS i.e., PEG POSS^® cage mixture (PPOSS), trisilanol phenyl POSS^® (TSP POSS) and trisulfonic acid isobutyl POSS^® (SPOSS) at a fixed loading of 2% (w/w) as filler and SPEEK with degree of sulfonation (DS) 55% as polymer matrix. The influence of POSS functionality on hybrid membrane's thermo-mechanical properties, morphology, water uptake and proton conductivity was investigated. Thermal and mechanical stability of hybrid membranes increased upon incorporation of POSS. The size and distribution of POSS particles into SPEEK matrix was studied using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) and it was found that TSP POSS and PPOSS based membranes showed smaller particle size and uniform distribution as compared to SPOSS based membranes which consequently affect the water uptake and proton conductivity of these hybrid membranes. The water uptake studies were carried out at three different temperatures i.e. 30, 80, 100 °C for 24 h and POSS based composite membranes showed higher water uptake and proton conductivity compared to neat SPEEK membranes. The highest proton conductivity (64.6 mS/cm) was observed for TSP POSS containing membrane which is more than double of neat SPEEK (31.3 mS/cm) membrane. The composite membrane containing TSP POSS can be considered as suitable membrane for PEMFCs applications.     

Constructing a long-range proton conduction bridge in sulfonated polyetheretherketone membranes with low DS by incorporating acid-base bi-functionalized metal organic frameworks

Functionalized metal-organic frameworks (MOFs) are being extensively developed as viable fillers to enhance the proton conductivity of proton exchange membranes. Herein, an amino-pendant sulfonic acid bi-functionalized MOFs material (UNCS)-doped SPEEK membrane with low degree of sulfonation (DS) can improve the proton conductivity as well as maintain the membrane dimensional stability. UNCS can act as bridges of proton donors and acceptors to reduce the activation energy barrier and shorten the distance of long-range proton conduction. Among all as-prepared membranes, SPEEK/UNCS-3 exhibited the highest proton conductivity of 186.4 mS·cm^?1 at 75 °C and 100% relative humidity (RH), which is much greater than that of pristine SPEEK and Nafion 117. Benefiting from the acid-base pair interaction between the amino groups of UNCS and the sulfonic acid groups of SPEEK, the dimensional stability and mechanical properties of the composite membranes were enhanced. More interestingly, STEM-HAADF and SAXS characterization consistently revealed that UNCS served as bridges among proton channels in the composite membranes for continuous proton transport.     

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.     

Effects of sulfonated polyether-etherketone (SPEEK) and composite membranes on the proton exchange membrane fuel cell (PEMFC) performance

Sulfonated polyether-etherketone (SPEEK) has a potential for proton exchange fuel cell applications. However, its conductivity and thermohydrolytic stability should be improved. In this study the proton conductivity was improved by addition of an aluminosilicate, zeolite beta. Moreover, thermohydrolytic stability was improved by blending poly-ether-sulfone (PES). Sulfonated polymers were characterized by H-NMR. Composite membranes prepared were characterized by Electrochemical Impedance Spectroscopy (EIS) for their proton conductivity. Degree of sulfonation (DS) values calculated from H-NMR results, and both proton conductivity and thermohydrolytic stability was found to strongly depend on DS. Therefore, DS values were controlled time in the range of 55–75% by controlling the reaction time. Zeolite beta fillers at different SiO₂/Al₂O₃ ratios (20, 30, 40, 50) were synthesized and characterized by XRD, EDX, TGA, and SEM. The proton conductivity of plain SPEEK membrane (DS = 68%) was 0.06 S/cm at 60 °C and the conductivity of the composite membrane containing of zeolite beta filled SPEEK was found to increase to 0.13 S/cm. Among the zeolite Beta/SPEEK composite membranes the best conductivity results were achieved with zeolite beta having a SiO₂/Al₂O₃ ratio of 50 at 10 wt% loading.     

Studies on novel heat treated sulfonated poly(ether ether ketone) [SPEEK]/diol membranes for fuel cell applications

The paper describes the preparation of membranes based on sulfonated poly(ether ether ketone) [SPEEK] [degree of sulfonation ∼65%] in the presence of varying amounts of poly (ethylene glycol) (molecular weight 200) [PEG-200] and cyclohexane dimethanol [CDM] using water:ethanol (50:50) as solvent. After drying, the membranes were heat treated at 60 °C (2 h), 80 °C (2 h), 100 °C (2 h), 120 °C (2 h) and 135 °C for 16 h. After the heat treatment, samples were insoluble in water:ethanol (50:50) mixture. The membranes thus obtained were characterized by FTIR spectroscopy (structural), water uptake (hydrolytic stability), thermal stability (TG), mechanical stability and proton conductivity. A significant increase in the hydrolytic stability was observed, SPEEK became elastic and fragile whereas the heat-treated SPEEK/PEG and SPEEK/CDM remained stable even after 135 days of water immersion at 35 °C.     

Sulfonated poly(ether ether ketone)-based composite membrane for polymer electrolyte membrane fuel cells

The solid proton conductor zirconium phosphate sulfophenylenphosphonate of composition Zr(HPO₄)_0.65(SPP)_1.35 where SPP denotes metasulfophenylenphosphonate was prepared in the amorphous gel form in dimethyl formamide (DMF) and characterized by ³¹P NMR. The composite membranes of SPEEK up to 50 wt.% of zirconium phosphate sulfophenylenphosphonate content were prepared by introducing the solid proton conductor from the gel. The composite membranes were characterized using FT-IR, powder X-ray diffraction, SEM, DSC/TGA. The proton conductivity of the membranes was measured under 100% relative humidity up to 70 °C. The composite membranes had better thermal stability when compared with that of SPEEK. A three-fold increase in proton conductivity at 70 °C was observed for the composite membrane with 50 wt.% of solid proton conductor. Furthermore, the conductivity results imply that a critical percentage of proton conductor is needed to establish conduction pathways in the polymer matrix.     

Investigation of the effects of SPEEK and its clay composite membranes on the performance of Direct Borohydride Fuel Cell

In this study, composite cation exchange membranes (CEM) were developed. With the experience from widely studied proton exchange membrane fuel cells (PEMFC), sulfonated polyether ether ketone (SPEEK) was prepared to be a more effective and cheaper ionomer alternative to the industry standard Nafion ®. SPEEK polymer membrane can reach sufficient ionic conductivities but have some mechanical and chemical stability problems (at a high degree of sulfonations (DS)). Therefore, in order to optimize the membrane, composite mixing with a well-known organic/inorganic clays called Cloisite® 15A, Cloisite ® 30B and MMT were used. Test cells for both single-cell and conductivity were designed and constructed. The ionic conductivity cell was different than the ones used in most studies, measuring conductivity in-plane with 4 probes using EIS. The membranes were characterized for their proton conductivity with electrochemical impedance spectroscopy (EIS), for DS with H NMR, water uptake, and fuel cell performance tests. First results showed that the acidic sulfonic groups of SPEEK interacted with organic/inorganic clays and as a result of partial barrier the ionic conductivity was decreased but power densities were increased. SPEEK-Cloisite® 30B composite membrane has given 40 mW/cm² power density value which is higher than pure SPEEK membrane (35 mW/cm²). The proton conductivities of the final composite membranes were close to bare SPEEK membranes which are 0,065 and 0,075 S/cm for SPEEK-Cloisite ® 30B and pristine SPEEK, respectively.     

Comparison of the small angle X-ray scattering study of sulfonated poly(etheretherketone) and Nafion membranes for direct methanol fuel cells

The microstructural evolution and swelling behaviors of sulfonated poly(etheretherketone) (SPEEK) and Nafion polymer membranes have been investigated by small angle X-ray scattering (SAXS) after equilibrating them in 2 M methanol solution at various temperatures, which is relevant for their use in direct methanol fuel cells (DMFC). The relationships among Bragg distance, sulfonation levels of the membrane, equilibrating temperature and transport properties are discussed. The proton conduction properties of the SPEEK and Nafion membranes have been investigated by electrochemical impedance spectroscopy. The network cluster model is employed to retrieve the structural information from the scattering and proton conductivity data. While the SPEEK membranes have narrower pathways for methanol/water permeation at T < 70 °C, the Nafion membranes have a wider channel even at lower temperatures, resulting in a higher methanol permeability in the latter. Based on the differences in the structural/cluster evolutions, the advantages and limitations of the two polymer membranes for use in DMFC are discussed.     

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.     

Thermal crosslinked and nanodiamond reinforced SPEEK composite membrane for PEMFC

SPEEK has been widely considered as a promising alternative to Nafion^® membrane for PEMFC. However, a conflict between high degree of sulfonation (DS) and poor mechanical strength needs to be resolved prior to wide application. In this work, such an effort was made by double strategies: thermal crosslinking and adding nanodiamond into high DS SPEEK matrix. The obtained sample was characterized by XRD, high resolution TGA and dynamic mechanical analysis (DMA). Small part of –SO₃H groups within SPEEK matrix participated in the crosslinking process by forming –SO₂– bridge bond, while the rest large part of –SO₃H groups still contributed to the proton transport. Mechanical and thermal stability of SPEEK membrane were modified by crosslinking-induced three-dimensional (3D) networks and interactions of SPEEK with nanodiamond. In addition, the water uptakes of composite membranes were also slightly improved possibly due to the capillary condensation in nanodiamond particles.     

Sulfonated poly(ether ether ketone)/poly(ether sulfone) composite membranes as an alternative proton exchange membrane in microbial fuel cells

Custom-made proton exchange membranes (PEM) are synthesized by incorporating sulfonated poly(ether ether ketone) (SPEEK) in poly(ether sulfone) (PES) for electricity generation in microbial fuel cells (MFCs). The composite PES/SPEEK membranes at various composition of SPEEK are prepared by the phase inversion method. The membranes are characterized by measuring roughness, proton conductivity, oxygen diffusion, water crossover and electrochemical impedance. The conductivity of hydrophobic PES membrane increases when a small amount (3–5%) of hydrophilic SPEEK is added. The electrochemical impedance spectra shows that the conductivity and capacitance of PES/SPEEK composite membranes during MFC operation are reduced from 6.15 × 10⁻⁷ to 6.93 × 10⁻⁵ (3197 Ω–162 Ω) and from 3.00 × 10⁻⁷ to 1.56 × 10⁻³ F, respectively when 5% of SPEEK added into PES membrane. The PES/SPEEK 5% membrane has the highest performance compared to other membranes with a maximum power density of 170 mW m⁻² at the maximum current density of 340 mA m⁻². However, the interfacial reaction between the membrane and the cathode with Pt catalyst indicates moderate reaction efficiency compared to other membranes. The COD removal efficiency of MFCs with composite membrane PES/SPEEK 5% is nearly 26-fold and 2-fold higher than that of MFCs with Nafion 112 and Nafion 117 membranes respectively. The results suggest that the PES/SPEEK composite membrane is a promising alternative to the costly perfluorosulfonate membranes presently used as separators in MFC systems.     

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.     

Investigation of the effects of AMPS-modified nanoclay on fuel cell performance of sulfonated aromatic proton exchange membranes

Here we describe preparation and characterization of a series of nanocomposite polyelectrolytes based on partially sulfonated poly (ether ether ketone) (SPEEK) and organically modified montmorillonite (MMT). Optimum degree of sulfonation for SPEEK is selected based on its transport properties. MMT is modified via ion exchange reaction using a 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as a functional modifier. AMPS-MMT at different loadings is introduced into the SPEEK matrices via the solution intercalation technique. Also, the nanocomposite membranes are fabricated using SPEEK and commercially available nanoclays like Cloisite Na (Na-MMT) and Cloisite 15A. Transport properties, proton conductivity and methanol permeability of the fabricated composite membranes are evaluated. Presence of AMPS-MMT significantly decreases the activation energy needed for proton conductivity. A membrane based on SPEEK/AMPS-MMT-3 wt% is selected as an optimum formulation which exhibits a high selectivity and power density at the elevated methanol concentrations. Moreover, it is found that the optimum nanocomposite membrane not only provides higher power output compared to the neat SPEEK and Nafion^®117 membranes, but also exhibits a higher open circuit voltage (OCV) in comparison with pristine SPEEK and commercial Nafion^® 117 membranes. Owing to the desirable transport and electrochemical properties SPEEK/AMPS-MMT nanocomposite can be considered as an alternative membrane for direct methanol fuel cell 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 sulfonated polysulfone membranes for direct methanol fuel cells

Sulfonated polysulfones (SPSf) with different degree of sulfonation (DS) have been synthesized and evaluated as proton exchange membranes in direct methanol fuel cell (DMFC). The membranes have been characterized by ion exchange capacity (IEC), proton conductivity, liquid uptake, and single DMFC polarization measurements. The proton conductivities of the SPSf membranes increase with increasing sulfonation, but are lower than that of Nafion 115. Within the range of sulfonation of 50–70%, the SPSf membranes exhibit better performances in DMFC than Nafion 115 at lower methanol concentrations (1 M) despite lower proton conductivities due to suppressed methanol permeability and crossover. However, the performances of SPSf membranes at higher methanol concentrations (2 M) are inferior to that of Nafion 115 at current densities higher than about 50 mA cm⁻² as the suppression in methanol crossover could not quite compensate for the lower proton conductivities.     

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.     

Composite membranes based on a novel benzimidazole grafted PEEK and SPEEK for fuel cells

Poly(ether ether ketone) (PEEK) and sulfonated poly(ether ether ketone) (SPEEK, IEC = 2.07 mequiv.g⁻¹) have been synthesized via nucleophilic aromatic substitution reaction. Bromomethylated poly(ether ether ketone) (PEEK-Br) is then prepared and reacted with 2-benzimidazolethiol to obtain the benzimidazole grafted poly(ether ether ketone) (PEEK-BI). The structures of PEEK-Br and PEEK-BI are characterized by ¹H NMR spectra. Composite membranes based on SPEEK and PEEK-BI are prepared and their properties used for fuel cells are studied in detail. The results show that the composite membranes exhibit greatly improved mechanical properties as well as reduced water uptake and methanol permeability compared with the pristine SPEEK membrane. The increased oxidative stability and selectivity indicate that the composite membranes are promising to be used as proton exchange membranes.     

Electrochemical investigation of sulfonated poly(ether ether ketone)/clay nanocomposite membranes for moderate temperature fuel cell applications

In the present study, polyelectrolyte membranes based on partially sulfonated poly(ether ether ketone) (sPEEK) with various degrees of sulfonation are prepared. The optimum degree of sulfonation is determined according to the transport properties and hydrolytic stability of the membranes. Subsequently, various amounts of the organically modified montmorillonite (MMT) are introduced into the sPEEK matrices via the solution intercalation technique. The proton conductivity and methanol permeability measurements of the fabricated composite membranes reveal a high proton to methanol selectivity, even at elevated temperatures. Membrane based on sPEEK and 1 wt% of MMT, as the optimum nanoclay composition, exhibits a high selectivity and power density at the concentrated methanol feed. Moreover, it is found that the optimum nanocomposite membrane not only provides higher performance compared to the neat sPEEK and Nafion^®117 membranes, but also exhibits a high open circuit voltage (OCV) at the elevated methanol concentration. Owing to the high proton conductivity, reduced methanol permeability, high power density, convenient processability and low cost, sPEEK/MMT nanocomposite membranes could be considered as the alternative membranes for moderate temperature direct methanol fuel cell applications.     

SPEEK proton exchange membranes modified with silica sulfuric acid nanoparticles

In order to increase both of the water uptake and conductivity, the proton exchange membranes were fabricated by sulfonated poly(ether ether ketone) (SPEEK) doped with varied contents of silica sulfuric acid (SSA) which is obtained by treating SiO₂ nanoparticles with more volatile SO₂Cl₂. SEM images of the composite membranes show that SSA nanoparticles are dispersed within the membranes uniformly, indicating the good organic compatibility of SSA particles. TEM images show that the composite membranes have improved ionic clusters distribution. The water uptakes of the composite membranes in water and under low relative humidities are all higher than that of the pristine SPEEK membrane. The addition of SSA enhances the conductivity obviously. The composite membrane containing 5wt.% SSA exhibits the highest conductivity of 0.13 Scm⁻¹ at 80 °C, approximately 18.6% higher than that of the pristine SPEEK membrane and 8.6% higher than that of Nafion117. The composite membranes also show good thermal stability. These results imply the potential application of the SPEEK/SSA composite membranes as improved PEMs in PEMFC.     

Preparation of organic/inorganic composite membranes using two types of polymer matrix via a sol–gel process

Organic/inorganic composite membranes were prepared using two different polymers. BPO₄ particles were introduced into polymers via an in situ sol–gel process. Pre-/post-sulfonated polymers were used to prepare composite membranes as matrix. Pre-sulfonated poly(aryl ether ketone) (SPAEK-6F) copolymer was synthesized via nucleophilic aromatic substitution. Degree of sulfonation was adjusted by the percentage of sulfonated monomer. Post-sulfonated poly(ether ether ketone) (SPEEK) was prepared using concentrated sulfuric acid as sulfonation agent. The membranes were characterized in terms of the ion-exchange capacity (IEC), proton conductivity, water uptake, AFM, SEM and their thermal properties. The SPAEK-6F plain membranes showed higher proton conductivity than that of the SPEEK plain membranes at similar water uptake or IEC due to their structural difference. SEM images of the composite membranes showed that the BPO₄ particles were homogenously dispersed in the polymer matrices and BPO₄ particle size was greatly influenced by polymer matrix. The SPAEK-6F/BPO₄ composite membranes had much smaller BPO₄ particle size than the SPEEK/BPO₄ composite membranes due to well dispersion of BPO₄ sol-like particulates in SPAEK-6F polymer solutions forming more hydrophobic/hydrophilic nanophase than SPEEK polymer solutions. The latter containing a few micrometer-scale BPO₄ particles showed higher proton conductivity than the former containing hundreds nanometer-scale BPO₄ particles at similar water uptake due to the increase in freezable water and effect of particle size.