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.     

A novel synthesis approach to partially fluorinated sulfonimide based poly (arylene ether sulfone) s for proton exchange membrane

Innovation of novel low cost proton conductive materials with super acidity has been the ever-increasing thirst for PEMFCs. Sulfonimide groups have the strongest gas-phase super-acidity with excellent thermal and chemical stability. Therefore, a series of partially fluorinated sulfonimide functionalized poly(arylene ether sulfone)s (SIPAES-xx) were successfully synthesized by chemical modification of sulfonated polyarylethersulfone (SPAES). The SPAESs were synthesized first by the direct polymerization of 4,4′ -dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS), and bisphenol. Thereafter, all arylsulfonic acid groups were converted into more acidic sulfonimide acid groups using partial fluorinated fluorosulfonyl imide monomer. The effect of the conversion of arylsulfonic acid function into sulfonimide was evaluated through thermal and chemical analysis. ¹H-NMR, FTIR, TGA, FE SEM, and AFM were used to illustrate the structure, thermal and chemical properties of (SIPAES-xx) membranes. The membranes showed IEC values of 0.78–1.41 mequiv./g with 6.7–40.6% water uptake for 20–40% ionic groups. The synthesized SIPAES-40 membranes showed comparable proton conductivity to Nafion^® 117 at same conditions. Nevertheless, the aromatic sulfonimide remained stable up to 370 °C. Furthermore, the presence of fluorine within the sulfonimide polymer provided high dimensional stability and oxidative durability by protecting the polymer chain from oxidizing radical species. Therefore, the synthesized SIPAES-xx membranes have the potential features as a proton exchange membrane (PEM) materials in the fuel cell.     

Synthesis and properties of novel HMS-based sulfonated poly(arylene ether sulfone)/silica nano-composite membranes for DMFC applications

Novel 4,4′-dihydroxy-α-methylstilbene (HMS)-based sulfonated poly(arylene ether sulfone) with sulfonic acid composition ranging from 10 to 40 mol% was synthesized via nucleophilic step polymerization of 4,4′-dihydroxy-α-methylstilbene, 4,4′-dichloro-3,3′-disulfonic acid diphenylsulfone and 4,4′-dichlorodiphenylsulfone and blended with silica sol to form organic/inorganic nano-composite membranes. The organic/inorganic nano-composite copolymers produced show a high glass transition temperature and thermal decomposition temperatures from 318 to 451 °C. The copolymers present appropriate toughness during the membrane process. The equilibrium water uptake and proton conductivity of the obtained organic/inorganic nano-composite membranes were measured as functions of temperature, degree of sulfonation and silica content. In general, the water uptake increased from 8 to 37 wt.%, and the proton conductivity of the organic/inorganic nano-composite membranes increased from 0.003 to 0.110 S cm⁻¹ as the degree of sulfonation increased from 10 to 40 mol%, the silica content increased from 3 to 10 wt.%, and the temperature increased from 30 to 80 °C. The single cell performance of the 40 mol% organic/inorganic nano-composite membrane with various silica contents ranged from 11 to 13 mW cm⁻² at 80 °C, and the power density was higher than Nafion^® 117. Including the thermal properties, swelling, conductivity and single cell performance, the nano-composite membranes are able to satisfy the requirements of proton exchange membranes for direct methanol fuel cells (DMFC).     

Properties and fuel cell performance of proton exchange membranes prepared from disulfonated poly(sulfide sulfone)

A series of disulfonated poly(sulfide sulfone)s (SPSSF)s copolymers are synthesized via direct aromatic nucleophilic substitution polycondensation of 4,4′-dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS) and 4,4′-thiobisbenzenethiol at various molar ratios. Tough and flexible membranes with 30 mol% (SPSSF30) to 50 mol% (SPSSF50) SDCDPS monomers are obtained by casting from DMAc solution. Their physicochemical properties including thermal properties, mechanical properties, water uptake, swelling ratio and oxidative stability are fully investigated. And the fuel cell performance of SPSSF membranes at different temperature and relative humidity is evaluated comprehensively for the first time. It is found that the SPSSF40 membrane exhibited low dimensional change in the temperature range of 20–100 °C, good mechanical properties, high oxidative stability and comparable fuel cell performance to Nafion 212 membrane. Besides, the H₂ crossover density of the SPSSF40 membrane is only 50% of that of Nafion 212 membrane. Consequently, SPSSF40 membranes prove to be promising candidates as new polymeric electrolyte materials for proton exchange membrane (PEM) fuel cells operated at medium temperatures.     

An alkaline direct ethylene glycol fuel cell with an alkali-doped polybenzimidazole membrane

An alkaline direct ethylene glycol fuel cell (DEGFC) with an alkali-doped polybenzimidazole membrane (APM) is developed and tested. It is demonstrated that the use of APMs enables the present fuel cell to operate at high temperatures. The fuel cell results in the peak power densities of 80 mW cm⁻² at 60 °C and 112 mW cm⁻² at 90 °C, respectively. The power output at 60 °C is found to be 67% higher than that by DEGFCs with proton exchange membranes, which is mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation and oxygen reduction reactions in alkaline media.     

Preparation and evaluation of a proton exchange membrane based on oxidation and water stable sulfonated polyimides

A series of novel oxidation and water stable sulfonated polyimides (SPIs) were synthesized from 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride (BTDA), and wholly aromatic diamine 2,2′-bis(3-sulfobenzoyl) benzidine (2,2′-BSBB) for proton exchange membrane fuel cells. These polyimides could be cast into flexible and tough membranes from m-cresol solutions. The copolymer membranes exhibited excellent oxidative stability and mechanical properties due to their fully aromatic structure extending through the backbone and pendant groups. Moreover, all BTDA-based SPI membranes exhibited much better water stability than those based on the conventional 1,4,5,8-naphthalenecarboxylic dianhydride. The improved water stability of BTDA-based polyimides was attributed to its unique binaphthalimide structure. The SPI membranes with ion exchange capacity (IEC) of 1.36–1.90 mequiv g⁻¹ had proton conductivity in the range of 0.41 × 10⁻¹ to 1.12 × 10⁻¹ S cm⁻¹ at 20 °C. The membrane with IEC value of 1.90 mequiv g⁻¹ displayed reasonably higher proton conductivity than Nafion^® 117 (0.9 × 10⁻¹ S cm⁻¹) under the same test condition and the high conductivity of 0.184 S cm⁻¹ was obtained at 80 °C. Microscopic analyses revealed that well-dispersed hydrophilic domains contribute to better proton conducting properties. These results showed that the synthesized materials might have the potential to be applied as the proton exchange membranes for PEMFCs.     

Chemical modification of Nafion membrane with 3,4-ethylenedioxythiophene for direct methanol fuel cell application

Nafion 117 membranes were modified by in situ chemical polymerization of 3,4-ethylenedioxythiophene using H₂O₂ as oxidant for direct methanol fuel cell application. Methanol permeability and proton conductivity of the poly(3,4-ethylenedioxythiophene)-modified Nafion membranes as a function of temperature were investigated. An Arrhenius-type dependency of methanol permeability and proton conductivity on temperature exists for all the modified membranes. Compared with Nafion 117 membrane at 60 °C, the methanol permeability of these modified membranes is reduced from 30% to 72%, while the proton conductivity is decreased from 4% to 58%, respectively. Because of low methanol permeability and adequate proton conductivity, the DMFC performances of these modified membranes were better than that of Nafion 117 membrane. A maximum power density of 48.4 mW cm⁻² was obtained for the modified membrane, while under same condition Nafion 117 membrane got 37 mW cm⁻².     

Incorporation of H3PO4 into three-dimensional polyacrylamide-graft-starch hydrogel frameworks for robust high-temperature proton exchange membrane fuel cells

To enhance the anhydrous proton conductivities of proton exchange membranes, we report here the incorporation of H₃PO₄ into three-dimensional (3D) framework of polyacrylamide-graft-starch (PAAm-g-starch) hydrogel materials using extraordinary absorption of hydrogels to H₃PO₄ aqueous solution. Intrinsic microporous structure can close to seal H₃PO₄ molecules in the interconnected 3D frameworks of PAAm-g-starch after suffering from dehydration. The hydrogel membranes are thoroughly characterized by morphology observation, thermal stability, swelling kinetics, proton-conducting performances as well as electrochemical behaviors. The results show that the H₃PO₄ loadings and therefore the proton conductivities of the hydrogel membranes are dramatically enhanced by employing PAAm-g-starch matrix. H₃PO₄ loading of 88.68 wt% and an anhydrous proton conductivity as high as 0.046 S cm⁻¹ at 180 °C are recorded. A fuel cell using a thick membrane shows a peak power density of 517 mW cm⁻² at 180 °C by feeding with H₂/O₂ streams. The high H₃PO₄ loading, reasonable proton conductivity in combination with simple preparation, low cost and scalable matrix demonstrates the potential use of PAAm-g-starch hydrogel membranes in high-temperature proton exchange membrane fuel cells.     

Cross-linked, ETFE-derived and radiation grafted membranes for anion exchange membrane fuel cell applications

To develop a series of cross-linked anion exchange membranes for application in fuel cells, poly(ethylene-co-tetrafluoroethylene) (ETFE) films was radiation grafted with vinyl benzyl chloride (VBC), followed by quaternization and crosslinking with 1,4-Diazabicyclo[2,2,2]octane (DABCO), alkylation with p-Xylylenedichloride (DCX), and quaternization again with trimethylamine (TMA). These anion exchange membranes were characterized in terms of water uptake, ion-exchange capacity, ionic conductivity as well as thermal stability. The chemical structures of the membranes were examined by FT-IR. The anion conductivity of the resulting alkaline anion exchange membrane is as high as 0.039 S cm⁻¹ at 30 °C in deionized water and the ionic conductivity increases with the increasing of temperature from 20 to 80 °C. The membrane is stable after being treated by 10 M potassium hydroxide solution at 60 °C for 120 h .The fuel cell performance with the final AAEM obtained in a H₂/O₂ single fuel cell at 40 °C with this AAEM was 48 mW cm⁻² at a current density of 69 mA cm⁻².     

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.     

Proton exchange membrane fuel cell of polybenzimidazole electrolyte doped with phosphoric acid and antimony chloride

The proton conductivity of phosphoric acid (PA) doped polybenzimidazole (PBI) membrane is improved through imbibing antimony pentachloride before doping PA. The antimony involvement diminishes the temperature dependence of proton conductivity in the targeted temperature range 160–260 °C. Among the compositions being prepared, the most conductive one exhibits 8.08 × 10⁻² S cm⁻¹ at 180 °C, with a small temperature sensitivity 6.3 × 10⁻⁵ S cm⁻¹ °C⁻¹. Several membrane-electrode assemblies (MEAs) have been prepared with the Pt/C catalyst layers to evaluate the practicality of co-doped PBI membrane. After proper activation, the hydrogen/air cell generates substantial electric power, denoted by its peak value over 500 mW cm⁻² at 180 °C. Impedance analysis indicates carbon monoxide poisoning affects overall MEA kinetics, as evidenced in the rising resistances of electrolyte, cathode, and anode. But the poisoned performance due to 3% CO/H₂ fuel can be rejuvenated after the fuel is switched back to pure hydrogen in one or two hours.     

Transport properties and fuel cell performance of sulfonated poly(imide) proton exchange membranes

Selective sulfonated poly(imide)s with high proton conductivity and low methanol permeability were tested for their performance as proton exchange membranes in direct methanol fuel cells (DMFC). The proton to methanol transport selectivity of the poly(imide) membranes correlated well with the self-diffusion coefficients of water in the membranes as determined by pulsed-field gradient nuclear magnetic resonance. The poly(imide) membranes showed improved fuel cell device performance, however high interfacial resistance between the membranes and electrodes decreased the membrane electrode assembly (MEA) conductivity to methanol crossover selectivity, likely due to the use of NAFION^®-based electrodes. The maximum power densities of SPI-50, SPI-75, and NR-212 based MEAs were 75, 72, and 67 mW cm⁻², respectively, with a methanol feed concentration of 2 M at a cell temperature of 60 °C.     

Fast proton-conducting glass membrane based on porous phosphosilicate and perfluorosulfonic acid polymer

A proton-conducting glass membrane based on porous phosphosilicate and perfluorosulfonic acid polymer was prepared via a modified sol-gel approach. The morphology, pore structure, water uptake property, proton conductivity and fuel cell performance of the membrane were investigated in this work. The hybrid glass membrane showed extremely high proton conductivity of 0.1 S cm⁻¹ in humid atmosphere. In the H₂/O₂ fuel cell measurement, an open circuit potential (OCV) of 0.94 V and a maximum output power density of 42.6 mW cm⁻² was obtained at 25 °C.     

Construction of a new continuous proton transport channel through a covalent crosslinking reaction between carboxyl and amino groups

Sulfonated poly(arylene ether ketone sulfone) bearing pendant carboxylic acid groups (C-SPAEKS) and sulfonated poly(arylene ether ketone sulfone) containing amino groups (Am-SPAEKS) were used to prepare C-SPAEKS/Am-SPAEKS crosslinked membranes. ¹H NMR and Fourier transform infrared spectra proved that C-SPAEKS and Am-SPAEKS copolymers, as well as C-SPAEKS/Am-SPAEKS crosslinked membrane, were successfully synthesized. TEM images showed that a continuous proton transport channel formed after crosslinking. Thermogravimetric analysis curves demonstrated that the thermal property of the crosslinked membranes improved. The crosslinked membranes exhibited suitable mechanical properties at 25 and 80 °C. The methanol permeability of C-SPAEKS/Am-SPAEKS-40 was 2.35 × 10⁻⁷ cm² s⁻¹ at 60 °C, which was lower than that of C-SPAEKS (24.12 × 10⁻⁷ cm² s⁻¹) and Am-SPAEKS (17.91 × 10⁻⁷ cm² s⁻¹). The proton conductivity of C-SPAEKS/Am-SPAEKS-40 was 0.089 S cm⁻¹, which was higher than that of C-SPAEKS and Am-SPAEKS at 80 °C. The results proved that C-SPAEKS/Am-SPAEKS crosslinked membranes were potential proton exchange membranes for direct methanol fuel cell applications.     

Synthesis and characterization of crosslinked sulfonated poly(arylene ether sulfone) membranes for high temperature PEMFC applications

Sulfonated poly(arylene ether sulfone) copolymers containing carboxyl groups are prepared by an aromatic substitution polymerization reaction using phenolphthalin, 3,3′-disulfonated-4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfone and 4,4′-bisphenol A as polymer electrolyte membranes for the development of high temperature polymer electrolyte membrane fuel cells. Thin, ductile films are fabricated by the solution casting method, which resulted in membranes with a thickness of approximately 50 μm. Hydroquinone is used to crosslink the prepared copolymer in the presence of the catalyst, sodium hypophosphite. The synthesized copolymers and membranes are characterized by ¹H NMR, FT-IR, TGA, ion exchange capacity, water uptake and proton conductivity measurements. The water uptake and proton conductivity of the membranes are decreased with increasing the degree of crosslinking which is determined by phenolphthalin content in the copolymer (0-15 mol%). The prepared membranes are tested in a 9 cm² commercial single cell at 80 °C and 120 °C in humidified H₂/air under different relative humidity conditions. The uncrosslinked membrane is found to perform better than the crosslinked membranes at 80 °C; however, the crosslinked membranes perform better at 120 °C. The crosslinked membrane containing 10 mol% of phenolphthalin (CPS-PP10) shows the best performance of 600 mA cm⁻² at 0.6 V and better performance than the commercial Nafion^® 112 (540 mA cm⁻² at 0.6 V) at 120 °C and 30 % RH.     

A novel phosphoric acid loaded quaternary 1,4-diazabicyclo-[2.2.2]-octane polysulfone membrane for intermediate temperature fuel cells

Phosphoric acid loaded quaternary 1,4-diazabicyclo-[2.2.2]-octane (DABCO) polysulfone was synthesised with different degrees of substitution (DS) and the membranes were characterized. The polymer structure was investigated using NMR and FT-IR spectra. The effect of DS on ionic conductivity and fuel cell performance are described. Conductivities of 0.12 and 0.064 S cm⁻¹ were achieved for the high degree of substitution membrane (DS106) and low degree of substitution membrane (DS58), respectively. Fuel cell tests gave a high power output of 400 mW cm⁻² using H₂ and O₂ at 150 °C and atmospheric pressure. Therefore, PA/QDPSU membrane has potential applications for intermediate temperature fuel cells (ITFCs).     

Application of phosphoric acid and phytic acid-doped bacterial cellulose as novel proton-conducting membranes to PEMFC

Novel proton-conducting polymer electrolyte membranes have been prepared from bacterial cellulose by incorporation of phosphoric acid (H₃PO₄/BC) and phytic acid (PA/BC). H₃PO₄ and PA were doped by immersing the BC membranes directly in the aqueous solution of H₃PO₄ and PA, respectively. Characterizations by FTIR, TG, TS and AC conductivity measurements were carried out on the membrane electrolytes consisting of different H₃PO₄ or PA doping level. The ionic conductivity showed a sensitive variation with the concentration of the acid in the doping solution through the changes in the contents of acid and water in the membranes. Maximum conductivities up to 0.08 S cm⁻¹ at 20 °C and 0.11 S cm⁻¹ at 80 °C were obtained for BC membranes doped from H₃PO₄ concentration of 6.0 mol L⁻¹ and, 0.05 S cm⁻¹ at 20 °C and 0.09 S cm ⁻¹ at 60 °C were obtained for BC membranes doped from PA concentration of 1.6 mol L⁻¹. These types of proton-conducting membranes share not only the good mechanical properties but also the thermal stability. The temperature dependences of the conductivity follows the Arrhenius relationship at a temperature range from 20 to 80 °C and, the apparent activation energies (E_a) for proton conduction were found to be 4.02 kJ mol⁻¹ for H₃PO₄/BC membrane and 11.29 kJ mol⁻¹ for PA/BC membrane, respectively. In particular, the membrane electrode assembly fabricated with H₃PO₄/BC and PA/BC membranes reached the initial power densities of 17.9 mW cm⁻² and 23.0 mW cm⁻², which are much higher than those reported in literature in a real H₂/O₂ fuel cell at 25 °C.     

Preparation and properties of crosslinked sulphonated poly(arylene ether sulphone) blend s for direct methanol fuel cell applications

HMS-based sulphonated poly(arylene ether sulphone) (HMSSH) is synthesised using 4,4′-dihydroxy-α-methylstilbene (HMS) monomer to introduce an interesting stilbene core as crosslinkable group. Crosslinked blend membranes are obtained by blending the BPA-based sulphonated poly(arylene ether sulphone) (BPASH) with crosslinkable HMS-based sulphonated poly(arylene ether sulphone) by UV irradiation of the blend membrane. Compared to the native BPASH with crosslinked BPASH/HMSSH blend membranes, the crosslinked blend membranes greatly reduce the water uptake and methanol permeability with only a slight reduction in proton conductivity. The crosslinked blend membrane, which has a 6% HMSSH content, has a water uptake of 59%, methanol permeability of 0.75 × 10⁻⁶ cm² s⁻¹, and proton conductivity of 0.08 S cm⁻¹. A membrane-electrode assembly is used to investigate single-cell performance and durability test for DMFC applications. Both the power density and open circuit voltage are higher than those of Nafion^® 117. A maximum power density of 32 mW cm⁻² at 0.2 V is obtained at 80 °C, which is higher than that of Nafion^® 117 (25 mW cm⁻²).     

Synthesis and characterization of novel cardo poly(aryl ether sulfone) bearing zwitterionic side groups for proton exchange membranes

A novel series of sulfonated poly(aryl ether sulfone)s with zwitterionic groups ([-CH₂CH₂CH₂N⁺CH₃(CH₂CH₂SO₃⁻)₂]) have been prepared by the copolycondensation of a secondary amine-containing biphenol monomer with 4,4′-biphenol and 4,4′-dichlorodiphenylsulfone, and this was followed by the reaction with sodium 2-bromoethanesulfonate. All the resulting copolymers can form uniform and tough membranes by simple solution casting. The investigation of ion exchange capacity (IEC) values indicated that each ammonium group interacted with one sulfonate group. Because of strong intermolecular interaction, the increased packing density of chain formed that resulted in polymer membranes with lower water uptake and swelling ratio, and better oxidative stability compared with side-chain-type sulfonated poly(aryl ether sulfone)s with the close IEC values. The polymer membranes bearing zwitterionic groups kept intact in Fenton’s reagent at 80 °C for 20 h. Furthermore, these membranes demonstrated higher proton conductivity than the side-chain-type sulfonated polymer membranes at the same measurement conditions.     

Immobilized phosphotungstic acid for the construction of proton exchange nanocomposite membranes with excellent stability and fuel cell performance

Phosphotungstic acid (HPW) has a good potential as nanofillers in nanocomposite proton exchange membrane with the prerequisite of solving the leakage issue. It is immobilized onto mesoporous graphitic carbon nitride (mg-C₃N₄) nanosheets surface, and then incorporated into sulfonated poly (aryl ether sulfone) (SPAES) membrane. Structures of the HPW/mg-C₃N₄ nanocomposites and corresponding SPAES/HPW/mg-C₃N₄ membranes are characterized by spectroscopic techniques. Fundamental properties and fuel cell performance of the fabricated nanocomposite membranes, and the leakage of HPW are investigated. Along with the highly suppressed HPW leakage, the SPAES/HPW/mg-C₃N₄ membranes show improved dimensional stability, water affinity and physicochemical stability, as well as better proton conductivity and fuel cell performance. At 80 °C and 60–100% RH, the SPAES/HPW/mg–C₃N₄–1.5 membrane exhibits 2–3.6 times peak power densities (354.9–584.2 mW/cm²) of the pristine SPAES membrane, and proton conductivity of 203 mS/cm, dimensional change less than 7.5% and weight loss of 1.4% in Fenton oxidation test at 80 °C.