Electrochemical characterization of sulfonated PEEK-WC membranes for PEM fuel cells

Sulfonated PEEK-WC polymer was obtained according to chloro-sulfonic acid procedure making possible the preparation of different membrane samples with a sulfonation degree from 48 to 90%. Dense membranes were prepared from casting solutions of S-PEEK-WC dissolved in DMF. Proton conductivity measurements were performed on such S-PEEK-WC membranes in a range of temperature from 30 to 120 °C and 100% of relative humidity, reaching 2.5 × 10⁻² S cm⁻¹ as the best value at 100 °C and with a sulfonation degree of 90%. In the meanwhile, the open circuit voltage of this S-PEEK-WC membrane (DS = 90%) varied from 0.963 V at 60 °C to 0.802 V at 100 °C, demonstrating that an increase of temperature negatively affects the membrane performance due to the mechanical properties degradation. In this work, a wide experimental campaign was carried out to investigate the electrochemical performances in terms of polarization curves, open circuit voltage and proton conductivity of S-PEEK-WC membranes as well as the fuel crossover and water uptake.     

Preparation, characterization and evaluation of proton-conducting hybrid membranes based on sulfonated hydrogenated styrene-butadiene and polysiloxanes for fuel cell applications

This paper describes the preparation of proton-conducting hybrid membranes (HMs) obtained by a solvent casting procedure using a solution containing sulfonated hydrogenated styrene-butadiene (HSBS-S) and an inorganic-organic mixture (polysiloxanes) previously prepared by a sol-gel route. HSBS-S copolymers with different sulfonation degrees were obtained and characterized by means of elemental analysis (EA), chemical titration and electrochemical impedance spectroscopy (EIS). HSBS-S with the best properties in terms of proton conductivity and solubility for the casting procedure was selected to prepare the HMs. The solvent casting procedure permitted the two phases to be homogeneously distributed while maintaining a relatively high proton conductivity in the membrane. HMs with different blend ratios were characterized using structural (Fourier transform infrared-attenuated total reflectance (FTIR-ATR), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC)), electrical (EIS), physicochemical (water uptake, ion-exchange capacity) and thermal (TGA-MS) methods. Finally, the optimized HSBS-S membrane and HMs were tested in hydrogen single fuel cells to obtain the polarization and power curves at different cell temperatures and gas pressures. Results indicate that HMs show a considerable improvement in performance compared to the optimized HSBS-S membrane denoting the benefit of incorporating the inorganic-organic network in the hydrogenated styrene-butadiene matrix. A Nafion membrane was used as reference material throughout this work.     

Synthesis and characterization of sulfonated poly(ether sulfone) copolymer membranes for fuel cell applications

Sulfonated poly(ether sulfone) copolymers (PESs) are synthesized using hydroquinone 2-potassium sulfonate (HPS) with other monomers (bisphenol A and 4-fluorophenyl sulfone). A series of PESs with different mol% of hydrophilic group is prepared by changing the mole ratio of HPS in the polymerization reaction. The chemical structure and thermal stability of the polymers are characterized by using ¹H NMR, FT-IR and TGA techniques. The PES 60 membrane, which has 60 mol% of HPS unit in the polymer backbone, has a proton conductivity of 0.091 S cm⁻¹ and good insolubility in boiling water. The TGA showed that PES 60 is stable up to 272 °C with a char yield of about 29% at 900 °C under a nitrogen atmosphere. To investigate single-cell performance, a catalyst-coated PES 60 membrane is used together with hydrogen and oxygen as the fuel and the oxidant, respectively. Cell performance is enhanced by increasing the temperature. A current density of 1400 mA cm⁻² at 0.60 V is obtained at 70 °C.     

Preparation and characterization of sulfonated polysulfone/titanium dioxide composite membranes for proton exchange membrane fuel cells

In the present study, sulfonated polysulfone (sPS)/titanium dioxide (TiO₂) composite membranes for use in proton exchange membrane fuel cells (PEMFCs) were investigated. Polysulfone (PS) was sulfonated with trimethylsilyl chlorosulfonate in 1,2 dichloroethane at ambient temperatures. It was shown that the degree of sulfonation is increased with the molar ratio of the sulfonating agent to PS repeat unit. The degree of sulfonation was determined by elemental analysis and ¹H NMR was performed to verify the sulfonation reaction on the PS. Sulfonation levels from 15 to 40% were easily achieved by varying the content of the sulfonating agent. Composite membranes were prepared by blending TiO₂ with sPS solution in DMAC (5 wt.%) by the solution casting procedure. The membranes have been characterized by thermal analysis, water uptake, proton conductivity measurements and single cell performance. The addition of TiO₂ increased the thermal stability but high filler concentrations decreased the miscibility of the composite component, and resulted in brittle membranes. The conductivity values in the range of 10⁻³–10⁻² S/cm were obtained for composite membranes. The conductivities of the membranes show similar increasing trend as a function of operating temperature. The membranes were tested in a single cell operating at 60–85 °C in humidified H₂/O₂. Single fuel cell tests performed at different operating temperatures indicated that sPS/TiO₂ composite membrane is more hydrodynamically stable and also performed better than sPS membranes. The highest performance of 300 mA/cm² was obtained for sPS/TiO₂ membrane at 0.6 V for an H₂–O₂/PEMFC working at 1 atm and 85 °C. The results show that sPS/TiO₂ is a promising membrane material for possible use in proton exchange membrane fuel cells.     

Proton-conducting composite membranes of chitosan and sulfonated polysulfone for fuel cell application

Crosslinked composite membrane having a thin chitosan (CS) layer on microporous polysulfone (PSF) substrate was synthesized and assessed for its applicability in fuel cells. This composite (PSF/CS) was extensively characterized for morphology, intermolecular interactions, thermal stability, and physico-mechanical properties using SEM, XRD, FTIR, TGA and sorption studies, respectively. Since studies indicated a lack of interaction between the two polymers, efforts were made to enhance the interaction between polysulfone support and chitosan through surface modification of polysulfone. This rendered a novel type of composite (SPSF/CS) derived by surface modification of polysulfone substrate prior to casting of chitosan layer on it. Proton conductivity of both the composites, PSF/CS and SPSF/CS, was determined and compared with respective values of commercial Nafion 117. These composites exhibited an increase in conductivity with an increase in temperature. The composite, SPSF/CS exhibited high Ion Exchange Capacity (IEC) and proton conductivity higher than Nafion 117 at temperature above 100 °C. The membrane also showed adequate geometrical and thermal stabilities and can therefore be considered as a potential alternative fuel cell membrane especially for high temperature operations.     

Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications

New anion exchange membranes (AEMs) with high conductivity, good dimensional and alkaline stability are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investigated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal stability, sufficient mechanical strength, the membrane which containing 30% mass fraction of [DMVIm][Cl] shows conductivity up to 1.0 × 10^?2 S cm^?1 at room temperature and good long-term alkaline stability in 1 M KOH solution at 80 °C. The results of this study suggest that this type of AEMs have good perspectives for alkaline anion exchange membrane fuel cell applications.     

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.     

Novel anhydrous composite membranes based on sulfonated poly (ether ketone) and aprotic ionic liquids for high temperature polymer electrolyte membranes for fuel cell applications

Novel composite membranes were prepared using imidazolium type aprotic ionic liquids and sulfonated poly (ether ketone) (SPEK) as polymer matrix by solution casting process. All the prepared membranes were characterized for their thermal stability, mechanical properties, ion exchange capacity, proton conductivity and leaching out of ionic liquids in presence of water. Ionic liquid based membranes were more flexible than neat SPEK membrane due to the plasticization effect of ionic liquids. The interactions and compatibility occurring among components were investigated by vibration spectroscopy (FTIR ATR) and scanning electron microscopy respectively. The thermal stability of composite membranes was higher than unmodified membranes. The ion conductivity of composite membranes under anhydrous conditions was found to be dependent on temperature, type and concentration of ionic liquid in SPEK matrix. Ion conductivities of composite membranes under anhydrous condition were found to be up to two orders (∼100 times) higher than neat SPEK membrane and it was found to be ∼5 mS/cm at 140 °C for SPEK/OTf-70. These composite membranes can be successfully operated at temperatures ranging from 40 °C to 140 °C under anhydrous conditions.     

Sulfonation of PEEK-WC polymer via chloro-sulfonic acid for potential PEM fuel cell applications

The preparation and characterization of thin dense sulfonated poly-ether-ether-ketone with cardo group (PEEK-WC) membranes for proton exchange membrane fuel cell (PEMFC) applications are described. The sulfonation of PEEK-WC polymer was realized via chloro-sulfonic acid and different kinds of membrane samples were prepared with a sulfonation degree ranging from 67 to 99%. The degree of sulfonation, homogeneity and thickness significantly affect both the membrane transport properties and the electrochemical performances. The dense character of the membranes was confirmed by SEM analysis. Proton conductivity measurements were carried out in a temperature range from 30 to 80 °C and at 100% of relative humidity, reaching 5.40×10⁻³ S/cm⁻¹ as best value at 80 °C and with a sulfonation degree (DS) of 99%. At the same conditions, a water uptake of 17% was achieved. DSC and TGA characterizations were used in order to determine the thermal stability of the membranes, confirming a T_g ranging between 206 and 216 °C depending on the DS, whereas FT-IR yielded indication about intermolecular interactions and water uptake at various sulfonation degrees.     

Cross-linked miscible blend membranes of sulfonated poly(arylene ether sulfone) and sulfonated polyimide for polymer electrolyte fuel cell applications

Cross-linked miscible blend (CMB) membranes were prepared from sulfonated poly(arylene ether sulfone) (SPAES) and sulfonated polynaphthalimide (SPI). They were transparent and insoluble in solvents. They showed the intermediate properties between SPAES and SPI concerning mechanical strength, water uptake, membrane swelling and proton conductivity. As for membrane swelling and proton conductivity, SPAES was almost isotropic, whereas SPI was highly anisotropic. CMB membranes were moderately anisotropic and had the advantages of the smaller in-plane membrane swelling and the larger through-plane conductivity compared to SPAES and SPI, respectively. Polymer electrolyte fuel cell performance of CMB2 membrane with an equal weight ratio of SPAES/SPI and an ion exchange capacity (IEC) of 1.74 meq g⁻¹ was investigated, compared to SPI membrane (R1) with a slightly higher IEC of 1.86 meq g⁻¹. At 90 °C, 0.1 MPa and relatively high humidification of 82/68% RH or 0.2 MPa and low humidification of 50-30% RH, CMB2 showed the reasonably high cell performances. At 110 °C and 50-33% RH, the cell performance was fairly high only at a high pressure of 0.3 MPa, but low at 0.2-0.15 MPa. At these conditions, the cell performance was better for CMB2 than for R1 due to the more effective back-diffusion of water formed at cathode into membrane. CMB2 showed the fairly high PEFC durability at 110 °C.     

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.     

Quinoxaline-based crosslinked membranes of sulfonated poly(arylene ether sulfone)s for fuel cell applications

A series of crosslinkable sulfonated poly(arylene ether sulfone)s (SPAESs) were synthesized by copolymerization of 4,4′-biphenol with 2,6-difluorobenzil and 3,3′-disulfonated-4,4′-difluorodiphenyl sulfone disodium salt. Quinoxaline-based crosslinked SPAESs were prepared via the cyclocondensation reaction of benzil moieties in polymer chain with 3,3′-diaminobenzidine to form quinoxaline groups acting as covalent and acid-base ionic crosslinking. The uncrosslinked and crosslinked SPAES membranes showed high mechanical properties and the isotropic membrane swelling, while the later became insoluble in tested polar aprotic solvents. The crosslinking significantly improved the membrane performance, i.e., the crosslinked membranes had the lower membrane dimensional change, lower methanol permeability and higher oxidative stability than the corresponding precursor membranes, with keeping the reasonably high proton conductivity. The crosslinked membrane (CS1-2) with measured ion exchange capacity of 1.53 mequiv. g⁻¹ showed a reasonably high proton conductivity of 107 mS/cm with water uptake of 48 wt.% at 80 °C, and exhibited a low methanol permeability of 2.3 × 10⁻⁷ cm² s⁻¹ for 32 wt.% methanol solution at 25 °C. The crosslinked SPAES membranes have potential for PEFC and DMFCs.     

Preparation of nitrated sulfonated poly(ether ether ketone) membranes for reducing methanol permeability in direct methanol fuel cell applications

Sulfonated poly(ether ether ketone)s (SPEEKs) were further substituted on the polymer main chain by nitration. All sulfonation and nitration were achieved with an inexpensive and simple post substitute reaction. The nitrated SPEEKs have a high glass transition temperature and thermal decomposition temperature, and a lower water uptake than SPEEK, which provides sufficient mechanical strength without swelling in the direct methanol fuel cell (DMFC) application. The methanol permeability of nitrated SPEEKS is reduced to 1.76 × 10⁻⁷ cm² s⁻¹ for S53N22 and 1.86 × 10⁻⁷ cm² s⁻¹ for S63N17 with no loss of conductivity in the DMFC application, and a proton conductivity that reached 0.026 S cm⁻¹. The nitrated SPEEK membranes satisfy the requirements of proton-exchange membranes for the DMFC.     

Degradation of sulfonated polyimide membranes in fuel cell conditions

The stability of sulfonated polyimide (sPI) membranes in fuel cell conditions was studied under stationary and cycling electric loading conditions. The lifetime in operating conditions was shown to vary from few tens to several hundreds of hours depending on both the membrane ion content and the fuel cell temperature. The membrane degradation is shown to be a thermoactivated process and activation energies have been extracted (70–110 kJ). The membrane post-mortem analysis by infrared revealed a significant chemical degradation which was identified as the imide function hydrolysis inducing polymer chain scissions and consequently the loss of the mechanical properties. In addition to this main degradation process, the infrared analysis of the membrane surface exposed to the anodic compartment suggests the occurrence an additional oxidative process. sPI membranes exhibit a lower sensitivity to operation under electric loading cycling compared to Nafion.     

Direct methanol fuel cell performance of sulfonated polyimide membranes

Sulfonated polyimides (SPIs) derived from 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4′-bis(4-aminophenoxy) biphenyl-3,3′-disulfonic acid and hydrophobic aromatic diamines showed the much lower methanol permeability and the lower proton conductivity than Nafion 112. The performance and the water and methanol crossover for direct methanol fuel cells (DMFCs) with the SPI membranes were investigated in comparison with Nafion membranes. The methanol and water fluxes increased significantly with increasing load current density for Nafion membranes but not for the SPI membranes, indicating that they were controlled by both the electro-osmotic drag and the molecular diffusion for the former but by only the molecular diffusion for the latter. These resulted in the much better DMFC performance for the SPIs than Nafion membranes especially at high methanol feed concentrations. The Faraday's efficiency and overall DMFC efficiency at 60 °C and 200 mA cm⁻² for SPI membrane with IEC of 1.51 meq g⁻¹ were 75% and 21%, respectively, at 5 wt.% methanol feed concentration, and 36% and 9.5%, respectively, at 20 wt.% methanol concentration. They were about two times and three times higher at 5 wt.% and 20 wt.% methanol concentrations, respectively, than those for Nafion 112. The short-term durability test for 300 h at 60 °C revealed no deterioration in the DMFC performance. The SPI membranes have high potential for DMFC applications at mediate temperatures (40–80 °C).     

Polybenzimidazole/zwitterion-coated polyamidoamine dendrimer composite membranes for direct methanol fuel cell applications

The zwitterion-coated polyamidoamine (ZC-PAMAM) dendrimer with ammonium and sulfonic acid groups has been synthesized and used as filler for the preparation of PBI-based composite membranes for direct methanol fuel cells. Polybenzimidazole (PBI)/ZC-PAMAM dendrimer composite membranes were prepared by casting a solution of PBI and ZC-PAMAM dendrimer, and then evaporating the solvent. The presence of ZC-PAMAM dendrimer was confirmed by FT-IR and energy-dispersive X-ray spectroscopy (EDS) mapping of sulfur and oxygen elements. The water uptake, swelling degree, proton conductivity, and methanol permeability of the membranes increased with the ZC-PAMAM dendrimer content. For the PBI/ZC-PAMAM-20 membrane with 20 wt% of ZC-PAMAM, it shows a proton conductivity of 1.83 × 10⁻² S/cm at 80 °C and a methanol permeability of 5.23 × 10⁻⁸ cm² s⁻¹. Consequently, the PBI/ZC-PAMAM-20 demonstrates a maximum power density of 26.64 mW cm⁻² in a single cell test, which was about 2-fold higher than Nafion-117 membrane under the same conditions.     

Crosslinked sulfonated poly(ether ether ketone) proton exchange membranes for direct methanol fuel cell applications

In the present study, a series of the crosslinked sulfonated poly(ether ether ketone) (SPEEK) proton exchange membranes were prepared. The photochemical crosslinking of the SPEEK membranes was carried out by dissolving benzophenone and triethylamine photo-initiator system in the membrane casting solution and then exposing the resulting membranes after solvent evaporation to UV light. The physical and transport properties of crosslinked membranes were investigated. The membrane performance can be controlled by adjusting the photoirradiation time. The experimental results showed that the crosslinked SPEEK membranes with photoirradiation 10 min had the optimum performance for proton exchange membranes (PEMs). Compared with the non-crosslinked SPEEK membranes, the crosslinked SPEEK membranes with photoirradiation 10 min markedly improved thermal stabilities and mechanical properties as well as hydrolytic and oxidative stabilities, greatly reduced water uptake and methanol diffusion coefficients with only slight sacrifice in proton conductivities. Therefore, the crosslinked SPEEK membranes with photoirradiation 10 min were particularly promising as proton exchange membranes for direct methanol fuel cell (DMFC) applications.     

SPEEK/sulfonated cyclodextrin blend membranes for direct methanol fuel cell

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

Interpenetrating network-forming sulfonated poly(vinyl alcohol) proton exchange membranes for direct methanol fuel cell applications

Poly (vinyl alcohol) was sulfonated and subsequently cross-linked by a thermal curing reaction with dual cross-linkers to prepare membranes for direct methanol fuel cells. Sulfonated poly (vinyl) alcohol (SPVA) with a high degree of sulfonation was synthesized from 4-Formylbenzene-1,3-disulfonic acid disodium salt hydrate via an acetalization reaction with PVA. Various masses of the cross-linking agents 1,3-bis(3-glycidyloxypropyl) tetramethyldisiloxane and 4,4′-oxydiphthalic anhydride were polymerized with SPVA to facilitate manipulation of the properties of the membranes. Notably, the SPVA3 showed excellent proton conductivity (cf. σ  = 0.218 S cm⁻¹ at 70 °C and Nafion 117 = 0.127 S cm⁻¹), and low methanol permeability (around one half of that Nafion 117). These results suggest that the cross-linked SPVA membranes hold promise as potential proton exchange membranes and given their high proton conductivity and low methanol permeability they may offer advantages when used in direct methanol fuel cells (DMFCs) applications.