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.     

Proton conductivity enhancement by nanostructural control of sulphonated poly (ether ether ketone) membranes

A series of sulphonated poly (ether ether ketone) (sPEEK) membranes for direct methanol fuel cells are successfully prepared under different humidity degree conditions. These membranes exhibit enhanced proton conductivity at high humidity degree. It is proved that the different proton conductivity is ascribed to the orientation arrangement of sulphonic acid groups, which is caused by environmental water in the preparing process of membranes. A model is established by analyzing the dynamics of membrane formation and proved by Field emission scanning electron microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS).     

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.     

Sulfonated poly(ether ether ketone) as an ionomer for direct methanol fuel cell electrodes

Sulfonated poly(ether ether ketone) has been investigated as an ionomer in the catalyst layer for direct methanol fuel cells (DMFC). The performance in DMFC, electrochemical active area (by cyclic voltammetry), and limiting capacitance (by impedance spectroscopy) have been evaluated as a function of the ion exchange capacity (IEC) and content (wt.%) of the SPEEK ionomer in the catalyst layer. The optimum IEC value and SPEEK ionomer content in the electrodes are found to be, respectively, 1.33 meq. g⁻¹ and 20 wt.%. The membrane-electrode assemblies (MEA) fabricated with SPEEK membrane and SPEEK ionomer in the electrodes are found to exhibit superior performance in DMFC compared to that fabricated with Nafion ionomer due to lower interfacial resistance in the MEA as well as larger electrochemical active area. The MEAs with SPEEK membrane and SPEEK ionomer also exhibit better performance than that with Nafion 115 membrane and Nafion ionomer due to lower methanol crossover and better electrode kinetics.     

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.     

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.     

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 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.     

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.     

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.     

Branched poly(ether ether ketone) based anion exchange membrane for H2/O2 fuel cell

High-performance anion exchange membranes (AEMs) are in need for practical application of AEM fuel cells. Novel branched poly(ether ether ketone) (BPEEK) based AEMs were prepared by the copolymerization of phloroglucinol, methylhydroquinone and 4,4′-difluorobenzophenone and following functionalization. The effects of the branched polymer structures and functional groups on the membrane's properties were investigated. The swelling ratios of all the membranes were kept below 15% at room temperature and had good dimensional stability at elevated temperatures. The branching degree has almost no effect on the dimensional change, but plays a great role in tuning the nanophase separation structure. The cyclic ammonium functionalized membrane showed a lower conductivity but a much better stability than imidazolium one. The BPEEK-3-Pip-53 membrane with the branching degree of 3% and piperidine functionalization degree of 53% showed the best performances. The ionic conductivity was 43 mS cm⁻¹ at 60 °C. The ionic conductivity in 1 M KOH at 60 °C after 336 h was 75% of its initial value (25% loss of conductivity), and the IEC was 83% of its initial value (17% loss of IEC), suggesting good alkaline stability. The peak energy density (60 °C) of the single H₂/O₂ fuel cell with BPEEK-3-Pip-53 membrane reached 133 mW cm⁻² at 260 mA cm⁻².     

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.     

Preparation of main-chain-type and side-chain-type sulfonated poly(ether ether ketone) membranes for direct methanol fuel cell applications

Novel main-chain-type and side-chain-type sulphonated poly(ether ether ketone)s (MS-SPEEKs) are synthesised by reacting the sulphonic acid groups of pristine SPEEKs with 2-aminoethanesulphonic acid to improve the nano-phase separated morphology of the material. ¹H NMR and FT-IR spectroscopy are employed to determine the structure and composition of main-chain-type and side-chain-type sulphonated polymers. Flexible and tough membranes with reasonable thermal properties are obtained. The MS-SPEEKs show good hydrolytic stability, and water uptake values ranging from 15% to 30% are observed. Compared to Nafion 117^®, the methanol permeability of the MS-SPEEKs is dramatically reduced to 8.83 × 10⁻⁸ cm² s⁻¹ to 3.31 × 10⁻⁷ cm² s⁻¹. The proton conductivity increases with increasing temperature, reaching 0.013-0.182 S cm⁻¹. A maximum power density and open circuit voltage of 115 mW cm⁻² and 0.830 V are obtained at 80 °C, respectively, which is significantly greater than the values generated with Nafion 117^®. The introduction of pendent side-chain-type sulphonic acid groups increases the single-cell performance by more than approximately 20%; thus, the lower water diffusivity, methanol permeability, electro-osmotic drag coefficient and high cell performance indicated that MS-SPEEK is a promising candidate for DMFC applications.     

Role of post-sulfonation of poly(ether ether sulfone) in proton conductivity and chemical stability of its proton exchange membranes for fuel cell

Commercially available poly(ether ether sulfone), PEES, was directly sulfonated using concentrated sulfuric acid at low temperatures by minimizing degradation during sulfonation. The sulfonation reaction was performed in the temperature range of 5–25 °C. Sulfonated polymers were characterized by FTIR, ¹H NMR spectroscopy and ion exchange capacity (IEC) measurements. Degradation during sulfonation was investigated by measuring intrinsic viscosity, glass transition temperature and thermal decomposition temperature of sulfonated polymers. Sulfonated PEES, SPEES, membranes were prepared by solvent casting method and characterized in terms of IEC, proton conductivity and water uptake. The effect of sulfonation conditions on chemical stability of membranes was also investigated via Fenton test. Optimum sulfonation condition was determined to be 10 °C with conc. H₂SO₄ based on the characteristics of sulfonated polymers and also the chemical stability of their membranes. SPEES membranes exhibited proton conductivity up to 185.8 mS cm⁻¹ which is higher than that of Nafion 117 (133.3 mS cm⁻¹) measured at 80 °C and relative humidity 100%.     

In-situ molecular-level hybridization enabling high-sulfonation-degree sulfonated poly(ether ether ketone) membrane with excellent anti-swelling ability and proton conduction

Sulfonated poly(ether ether ketone) (SPEEK) membrane with high sulfonation degree (SD) is a promising substitute of Nafion as proton exchange membrane (PEM), due to the excellent proton conductivity and low cost. However, its widespread application is limited by the inferior structural stability. Here, we report the fabrication of high SD SPEEK membrane with outstanding structural stability through an in-situ molecular-level hybridization method. Concretely, the ionic nanophase of SPEEK membrane is filled with precursors, which are then in-situ converted into polymer quantum dots (PQDs) by a microwave-assisted polycondensation process. In this manner, the micro-phase separation structure of SPEEK membrane is well maintained. PQDs with abundant hydrophilic functional groups together with the inherent –SO₃H groups impart hybrid membrane highly enhanced proton conductivity of 138.2 mS cm⁻¹ at 80 °C, which is comparable to Nafion. This then offers a 116.3% enhancement in device output power. Meanwhile, PQDs act as cross-linkers via generated electrostatic interactions with SPEEK, affording hybrid membrane with SD of 94.1% an ultralow swelling ratio of 1.35% at 25 °C, about 35 times lower than control membrane. More importantly, the in-situ molecular-level hybridization method is versatile, which can also boost the performances of chitosan (CS)-based membranes.     

Novel modification method to prepare crosslinked sulfonated poly(ether ether ketone)/silica hybrid membranes for fuel cells

Crosslinked organic-inorganic hybrid membranes are prepared from hydroxyl-functionalized sulfonated poly(ether ether ketone) (SPEEK) and various amounts of silica with the aims to improve dimensional stability and methanol resistance. The partially hydroxyl-functionalized SPEEK is prepared by the reduction of some benzophenone moieties of SPEEK into the corresponding benzhydrol moieties which is then reacted with (3-isocyanatopropyl)triethoxysilane (ICPTES) to get a side chained polymer bearing triethoxysilyl groups. These groups are subsequently co-hydrolyzed with tetraethoxysilane (TEOS) and allow the membrane to form a crosslinked network via a sol-gel process. The obtained hybrid membranes with covalent bonds between organic and inorganic phases exhibit much lower methanol swelling ratio and water uptake. With the increase of silica content, the methanol permeability coefficient of the hybrid membranes decreases at first and then increased. At silica content of about 6 wt.%, the methanol permeability coefficient reaches a minimum of 7.15 × 10⁻⁷ cm² s⁻¹, a 5-fold decrease compared with that of the pristine SPEEK. Despite the fact that the proton conductivity is decreased to some extent as a result of introduction of the silica, the hybrid membranes with silica content of 4-8 wt.% shows higher selectivity than Nafion117.     

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.     

Synthesis and characterization of long-side-chain type quaternary ammonium-functionalized poly (ether ether ketone) anion exchange membranes

A series of quaternary ammonium salt poly(ether ether ketone) AEMs containing long ether substituents are successfully prepared, and their chemical structure is confirmed by ¹H NMR and FT-IR. The distinct microphase separation morphology of AEMs is observed by TEM. As the content of methylhydroquinone increases, the ion conductivity of AEMs gradually increases. When the content of methylhydroquinone increases to 80%, the hydroxide conductivity of PEEK-DABDA-80 membrane reaches 0.052 S/cm at 80 °C. Meanwhile, it exhibits excellent mechanical properties and anti-swelling ability, with tensile strength of 25 MPa, elongation at break of 8.12% and swelling ratio is only 17.4% at 80 °C. And AEMs also display the better thermal stability. After soaked in 1 M NaOH at 60 °C for 30 days, PEEK-DABDA-80 membrane shows acceptable ion conductivity of 0.021 S/cm at 60 °C. In view of these properties, PEEK-DABDA-x AEMs may display potential application as alkaline AEMs.     

Effects of casting solvent on microstructrue and ionic conductivity of anhydrous sulfonated poly(ether ether ketone)-inoic liquid composite membranes

As the key component of polymer electrolyte membrane fuel cells, the membrane has significant effect on the performance of fuel cells. The commonly used approach for preparation of membrane is solvent casting. In this paper, high temperature polymer electrolyte membranes consisting of sulfonated poly(ether ether ketone) and 1-butyl-3-methylimidazolium tetrafluoroborate were prepared using solvent casting process from N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone solutions to understand the solvent effect on the nature of formed membranes. It was found that solvents used for casting process strongly affect the microstructure and ionic conductivity of formed membranes. The composite membrane cast from DMF solution has clearly inter-connected ionic clusters with diameters of several hundreds nanometers to about 1.5 μm and exhibits the highest ionic conductivity, reaching 1.04 × 10⁻² S cm⁻¹ at 170 °C under anhydrous conditions.     

Nafion/nitrated sulfonated poly(ether ether ketone) membranes for direct methanol fuel cells

Sulfonated poly(ether ether ketone)s (SPEEKs) are substituted on the main chain of the polymer by nitro groups and blended with Nafion^® to attain composite membranes. The sulfonation, nitration and blending are achieved with a simple, inexpensive process, and the blended membranes containing the nitrated SPEEKs reveal a liquid-liquid phase separation. The blended membranes have a lower water uptake compared to recast Nafion^®, and the methanol permeability is reduced significantly to 4.29 × 10⁻⁷-5.34 × 10⁻⁷ cm² s⁻¹ for various contents of nitrated SPEEK for S63N17, and 4.72 × 10⁻⁷-7.11 × 10⁻⁷ cm² s⁻¹ for S63N38, with a maximum proton conductivity of ∼0.085 S cm⁻¹. This study examines the single-cell performance at 80 °C of Nafion^®/nitrated SPEEK membranes with various contents of nitrated SPEEK and a degree of nitration of 23-25 mW cm⁻² for S63N17 and 24-29 mW cm⁻² for S63N38. Both the power density and open circuit voltage are higher than those of Nafion^® 115 and recast Nafion^®.