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.     

Studies of sulfonated polyphenylene membranes containing benzophenone moiety for PEMFC

The sulfonated polyphenylenes containing benzophenone structure (sulfonated Parmax 1200, S-Parmax) were prepared by sulfonation reaction of Parmax 1200 with 30% fuming sulfuric acid, and degree of sulfonation was controlled by sulfonation reaction time. These polymers have all carbon structure without ether linkages that have the possibility of attack by nucleophiles (made by PEMFC operating system). The polyphenylene structure of Parmax provides a stiff and resistant backbone, whereas the pendant benzoyl group enables the solubility of the material, and also provides sites for chemical modifications. The structure properties of the synthesized polymers were investigated by ¹H NMR spectroscopy. The membranes were studied by ion exchange capacity (IEC), water uptake, and proton conductivity. These membranes deterioration test was performed by Fenton reagent, and compared with normal sulfonated poly(ether sulfone)s and Nafion. The power densities of membranes were performed by single cell.     

Styrene-co-butyl acrylate copolymers with potential application as membranes in PEM fuel cell

Styrene-butyl acrylate (St-BuA) copolymers were obtained by mass free radical copolymerization reactions, using BPO as initiator. A 70:30 (St-BuA) comonomer composition was selected. Three different molar concentrations of sulfonating agent (50, 100 and 150%) were considered to obtain the sulfonated copolymer (sSt-BuA) and membranes were casted from sSt-BuA copolymers. Copolymers were characterized with FTIR, TGA and NMR ¹H and mechanically by TMA. The sSt-BuA sulfonation degree (SD) effect on the ion-exchange capacity (IEC) was evaluated by titration and water uptake (WU) by gravimetry. Microstructure was also observed using scanning electron microscopy (SEM), and its electrochemical properties were evaluated by electrochemical impedance spectroscopy (EIS). The chemical composition of St-BuA copolymer was confirmed by proton nuclear magnetic resonance (NMR ¹H) spectroscopy. Fourier transformed infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA) analysis also confirmed the existence of both comonomers and a successful sulfonation up to an actual 30% level, as well as a good thermal stability over 300 °C for the molecular structure and over 150 °C for sulfonated membranes. TMA indicate an increase in flexure modulus along degree of sulfonation. SEM images show a highly dense material with low pore size for the sulfonated copolymer. The IEC values obtained varied from 0.83 to 1.18 meq g⁻¹, depending on sulfonation degree. The proton conductivity for St-BuA 50% membrane (9.77 × 10⁻⁵ S cm⁻¹) was a two order of magnitude lower than the commercial Nafion membrane (4.53 × 10⁻³ S cm⁻¹). Thermal, mechanical and electrochemical results demonstrate that these sSt:BuA copolymers are promising materials to be used as membranes in fuel cells applications.     

Investigation on sulfuric acid sulfonation of in-situ sol-gel derived Nafion/SiO2 composite membrane

In this paper, an effective method to cover the conductivity loss of Nafion/SiO₂ through sulfonation and its mechanism are studied. Nafion/SiO₂ composite membranes are prepared via an in-situ sol-gel route, and then sulfonated with concentrated sulfuric acid (marked as Nafion/S-SiO₂). The effects of the sulfonation on properties of the Nafion/SiO₂ composite membrane are investigated. The results show that sulfonation can improve the proton conductivity of the Nafion/SiO₂ effectively, though it brings water-uptake loss to the composite membrane to some extent simultaneously. According to the results of FT-IR, UV resonance Raman spectroscopy, ²⁹Si solid-state MAS NMR and XRD, it’s proved that higher conductivity of Nafion/S-SiO₂ should be relevant to hydrogen bonds & chemical bonds between SiO₂ nano particles and sulfuric acid molecules. While, lower water uptake & swelling ratio should be caused by hydroxyl-elimination on the surface of SiO₂ nano-particles during sulfonation. Single cell tests show that the performance of Nafion/S-SiO₂ composite membranes substantially exceeds Nafion/SiO₂ at 110 °C and 59%RH, and in the initial testing stage no performance reduction is observed.     

Nafion/polyaniline/silica composite membranes for direct methanol fuel cell application

This work investigates the characterization and performance of polyaniline and silica modified Nafion membranes. The aniline monomers are synthesized in situ to form a polyaniline film, whilst silica is embedded into the Nafion matrix by the polycondensation of tetraethylorthosilicate. The physicochemical properties are studied by means of X-ray diffraction and Fourier transform infrared techniques and show that the polyaniline layer is formed on the Nafion surface and improves the structural properties of Nafion in methanol solution. Nafion loses its crystallinity once exposed to water and ethanol, whilst the polyaniline modification allows crystallinity to be maintained under similar conditions. By contrast, the proton conductivities of polyaniline modified membranes are 3–5-fold lower than that of Nafion. On a positive note, methanol crossover is reduced by over two orders of magnitude, as verified by crossover limiting current analysis. The polyaniline modification allows the membrane to become less hydrophilic, which explains the lower proton conductivity. No major advantages are observed by embedding silica into the Nafion matrix. The performance of a membrane electrode assembly (MEA) using commercial catalysts and polyaniline modified membranes in a cell gives a peak power of 8 mW cm⁻² at 20 °C with 2 M methanol and air feeding. This performance correlates to half that of MEAs using Nafion, though the membrane modification leads to a robust material that may allow operation at high methanol concentration.     

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.     

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%.     

Synthesis and characterization of proton exchange membranes based on sulfonated poly(fluorenyl ether nitrile oxynaphthalate) for direct methanol fuel cells

A series of sulfonated poly(fluorenyl ether nitrile oxynaphthalate) (SPFENO) copolymers with different degree of sulfonation (DS) are synthesized via nucleophilic polycondensation reactions with commercially available monomers. Incorporation of the naphthalanesulfonate group into the copolymers and their copolymer structures are confirmed by ¹H NMR spectroscopy. Thermal stability, mechanical properties, water uptake, swelling behavior, proton conductivity and methanol permeability of the SPFENO membranes are investigated with respect to their structures. The electrochemical performance of a direct methanol fuel cell (DMFC) assembled with the SPFENO membrane was evaluated and compared to a DMFC with a Nafion 117 membrane. The DMFC assembled with the SPFENO membrane of proper DS exhibits better electrochemical performance compared to the Nafion 117-based cell.     

Preparation and characterization of sulfonated poly(tetra phenyl ether ketone sulfone)s for proton exchange membrane fuel cell

Sulfonated poly(tetra phenyl ether ketone sulfone)s SPTPEKS were successfully synthesized for proton exchange membrane. Poly(tetra phenyl ether ketone sulfone)s PTPEKS were prepared by the 4,4′-dihydroxydiphenylsulfone with 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenylbenzene (BFBTPB) and 4,4′-difluorodiphenylsulfone, respectively, at 210 °C using potassium carbonate in sulfolane. PTPEKS were followed by sulfonation using chlorosulfonic acid and concentrated sulfuric acid at two stage reactions. Different contents of sulfonated unit of SPTPEKS (17, 20, 23 mol% of BFBTPB) were studied by FT-IR, ¹H NMR spectroscopy, and thermo gravimetric analysis (TGA). Sorption experiments were conducted to observe the interaction of sulfonated polymers with water. The ion exchange capacity (IEC) and proton conductivity of SPTPEKS were evaluated with increase of degree of sulfonation. The water uptake of synthesized SPTPEKS membranes exhibit 25–61% compared with 28% of Nafion 211^®. The SPTPEKS membranes exhibit proton conductivities (25 °C) of 11.7–25.3 × 10⁻³ S/cm compared with 33.7 × 10⁻³ S/cm of Nafion 211^®.     

Power enhancement of passive micro-direct methanol fuel cells with self-sulfonation of P(VDF-TrFE) copolymer during lamination on Nafion membrane

Among the various advances that have taken place in fuel cells, efforts to reduce the methanol crossover and thereby increase fuel cell performance are important. One method by which crossover can be reduced is through introduction of hydrophobic surface on membrane which reduces the entry of methanol into the membrane. Here we show that coating of poly(vinylidenefluoride-trifluoroethylene) on Nafion results in reduction in crossover due to the introduction of hydrophobicity on the surface of the composite membrane which, in turn, improves the fuel cell performance. Further, FTIR results have shown that sulfonic-acid groups diffuse from Nafion into the poly(vinylidenefluoride-trifluoroethylene) during the dip-coating process which introduces proton conductivity in the lamination without the sulfonation process of polymer. Passive micro-direct methanol fuel cells are used as a platform for our experiments. Results show for the first time that 10 μm thick coating of poly(vinylidenefluoride-trifluoroethylene) on Nafion results in enhancement of power density.     

40SiO2–40P2O5–20ZrO2 sol-gel infiltrated sSEBS membranes with improved methanol crossover and cell performance for direct methanol fuel cell applications

Membranes commonly used in direct methanol fuel cell (DMFC) are expensive and show a great permeability to methanol which reduces fuel utilization and leads to mixed potential at the cathode. In this work, sulfonated styrene-ethylene-butylene-styrene (sSEBS) modified membranes with zirconia silica phosphate sol-gel phase are developed and studied in order to evaluate their potential use in DMFC applications. The synthesized hybrid membranes and sSEBS are subjected to an exhaustive physicochemical characterization by liquid uptake, ion exchange capacity, atomic force microscopy, X-ray photoelectron spectroscopy and dynamic mechanical and thermogravimetric analyses. Likewise, the potential use of the prepared membranes in DMFC is evaluated by means of electrochemical characterizations in single cell, determining the limiting methanol crossover current densities, proton conductivities and DMFC performances. The hybrid membranes show lower water and methanol uptakes, higher stiffness, water retention capability, upper power density and lower methanol crossover than sSEBS and Nafion 112.     

Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell

This study examined methanol crossover through PtRu/Nafion composite membranes for the direct methanol fuel cell. For this purpose, 0.03, 0.05 and 0.10 wt% PtRu/Nafion composite membranes were fabricated using a solution impregnation method. The composite membrane was characterized by inductively coupled plasma-mass spectroscopy and thermo-gravimetric analysis. The methanol permeability and proton conductivity of the composite membranes were measured by gas chromatography and impedance spectroscopy, respectively. In addition, the composite membrane performance was evaluated using a single cell test. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in the pure Nafion membrane, while the level of methanol permeation was retarded. From the results of the single cell test, the maximum performance of the composite membrane was approximately 27% and 31% higher than that of the pure Nafion membrane at an operating temperature of 30 and 45 °C, respectively. The optimum loading of PtRu was determined to be 0.05 wt% PtRu/Nafion composite membrane.The PtRu particles embedded in the Nafion membrane act as a barrier against methanol crossover by the chemical oxidation of methanol on embedded PtRu particles and by reducing the proton conduction pathway.     

Evaluation of SPEEK/PBI blend membranes for possible direct borohydride fuel cell (DBFC) application

Direct Borohydride Fuel Cell (DBFC) is one of the most promising liquid fuel cell technologies. However, similar to the other classes of fuel cells, there are technical problems to be solved and new materials specific to the technology should be developed for each component. The electrolyte membrane is one of the key components for its success similar to the other FC types. Commercial perfluorosulfonic acid type membranes namely Nafion^® is still the first choice in relatively less number of DBFC studies. In this study, less costly blend membranes were fabricated and characterized for comparison of the key properties with Nafion^® especially for DBFC application. For this purpose, the selected base polymer poly ether-ether-ketone (PEEK) was sulfonated up to high degrees of sulfonation (DS) and blended with another base polymer polybenzimidazole (PBI) at various ratios. Key electrolyte membrane properties such as DS, water uptake, ionic conductivity, BH₄⁺ fuel crossover, mechanical strength and glass transition temperature (T_g) were investigated by proton nuclear magnetic resonance spectroscopy (H NMR), electrochemical impedance spectroscopy (EIS), voltammetry, universal testing machine and Differential Scanning Calorimetry (DSC) respectively. Finally single cell test performances were investigated in a DBFC test system. Results showed that the mechanical strength of SPEEK which has a good ionic conductivity value could be improved well beyond the value of Nafion 117 without sacrificing too much of the conductivity. It has been observed that there is a trade-off between the important properties such as ionic conductivity, fuel (borohydride) permeability and mechanical strength at the first sight. The peak power densities obtained for blend membranes are close to the value of the commercial Nafion^® 117 membrane. These results show that these blend membranes have a potential that can be improved for direct borohydride fuel cells.     

Studies of sulfonated polyphenylene containing fluorine moiety via nickel catalyzed C–C coupling polymerization

The synthesis of polyphenylenes containing fluorine moiety (PPTF), their functionalization with sulfonic acid groups, and the measurement of apposite parameters for PEMs are described. The polymers were prepared by Ni-catalyzed carbon–carbon coupling reaction of -(2,2,2-trifluoro-1-phenylethylidene)-bis(4-chlorobenzene) (TFPECB) and 2,5-dichlorobenzophenone, followed by sulfonation reaction with chlorosulfuric acid. These polymers have all carbon–carbon linkages without any ether linkage on the polymer backbone, which was not attacked by nucleophiles (H₂O, hydrogen peroxide, hydroxide anion and radical), and sulfuric acid groups were selectively attached to the side phenyl rings of the TFPECB unit. A series of membranes was studied by ¹H NMR spectroscopy, ion exchange capacity (IEC), water uptake, and proton conductivity. The membranes' degradation was tested with Fenton reagent and compared with normal sulfonated poly(ether sulfone)s and Nafion.     

Characterization of polyethyleneterephthalate (PET) based proton exchange membranes prepared by UV-radiation-induced graft copolymerization of styrene

Polymer electrolyte membranes (PEMs) were successfully prepared by simultaneous ultraviolet (UV) radiation-induced graft copolymerization of styrene (35 vol.% concentration) onto poly(ethyleneterephthalate) (PET) film, followed by sulfonation on the styrene monomer units in the grafting chain using 0.05 M chlorosulfonic acid (ClSO₃H). The radiation grafting and the sulfonation have been confirmed by titrimetric and gravimetric analyses as well as Fourier Transform Infrared (FTIR) spectroscopy. The maximum ion-exchange capacity (IEC) of the PEM was measured to be 0.04385 mmol g⁻¹ at its highest level of grafting and sulfonation. They exhibited high thermal and mechanical properties as well as oxidative stability. They are highly stable in H₂SO₄ solutions and can be used in the acidic fuel cells. The membranes showed low water uptake as well as low proton conductivity than Nafion. In this study, the preparation of PEMs from commodity-type polymers is found to be very inexpensive and is a suitable candidate for applications in fuel cells.     

Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine

A role of metallophthalocyanine (MPc) as an anti-oxidizing agent of polymer membrane and an accelerating agent of proton conductivity was discussed. The poly(ether sulfone)s bearing MPc (Ni, Co and Fe), PMPc were prepared by two-step reaction from phenolphthalein and fumaronitrile and followed reaction with metal (II) chloride (Ni, Co and Fe) and 1,2-dicyanobenzene in quinoline. The sulfonated polymer was synthesized by condensation polymerization using 1,2-bis(4-hydroxyphenyl)-1,2-diphenyl ethylene, bis(4-fluorophenyl) sulfone and followed by sulfonation reaction with concentrated sulfuric acid. A series of hybrid membranes (H–Ni, H–Co and H–Fe) were prepared from a mixture of the sulfonated copolymer and PMPcs in dimethylacetamide (DMAc). The structural properties of the synthesized polymers were studied by ¹H-NMR spectroscopy and FT-IR. The membrane properties were investigated by measurements of ion exchange capacity (IEC), water uptake, and proton conductivity, chemical degradation test, and atomic force microscopy (AFM) analysis. The cell performance of the membranes was compared with those of normal sulfonated poly(ether sulfone)s and Nafion.     

Comparison of homogeneously and heterogeneously sulfonated polyetheretherketone membranes in preparation,properties and cell performance

For application in direct methanol fuel cells, polyetheretherketone (PEEK) membranes with different degrees of sulfonation (40–80%) are prepared by both homogeneous and heterogeneous methods. Sulfonation and its degree are identified by means of Fourier transform infrared (FT-IR) spectroscopy and a back-titration method, respectively. Thermal analysis shows that an increase in the sulfonation degree increases the glass-transition temperature and enhances the thermal stability. The room-temperature ion conductivity of the homogeneously sulfonated PEEK (sPEEK) membrane with a 68% degree of sulfonation is higher than that of Nafion^® 117, while its methanol permeability is lower. The tensile strength of the sPEEK membrane is comparable with that of Nafion^® 117 at the equilibrium water swollen state. Various properties of sPEEK membranes prepared by two methods are compared. Overall, the ion conductivity of the homogeneous system is higher than that of the heterogeneous counterpart, but there is little difference in methanol permeability. The cell performance of the homogeneously sulfonated PEEK (homo-sPEEK) membrane is much better than that of the heterogeneously sulfonated PEEK (hetero-sPEEK) and Naifion^® 117 membranes.     

Synthesis and characterization of polymer electrolyte membrane containing methylisatin moiety by polyhydroalkylation for fuel cell

High molecular weight polymer containing N-methylisatin was synthesized by superacid-catalyzed polyhydroxyalkylation reactions. Their functionality with sulfonic acid groups and the measurement of apposite parameters for proton exchange membranes (PEMs) were described. Sulfonic acid groups were introduced into the polymer through sulfonation reaction with chlorosulfonic acid. The membranes were casted from the solution of sulfonated polymer in dimethylsulfoxide (DMSO). The structural properties of the synthesized polymers were investigated by ¹H NMR spectroscopy. The membranes were studied by thermogravimetric analysis (TGA), ion exchange capacity (IEC), water uptake, dimensional stability and proton conductivity assessment. Different levels of sulfonation and ion exchange were tested; the resulting membranes exhibited high proton conductivities of up to 88.63 mS/cm. The sulfonated membranes showed good dimensional stability owing to having all carbon-carbon linkages on polymers' backbone.     

Sulfonated PEEK-based polymers in PEMFC and DMFC applications: A review

Nowadays, polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are devices known for using proton conducting membranes. From a conceptual point of view, DMFC and PEMFC systems are very similar, except for being supplied by different fuels such as methanol and hydrogen, respectively. Commonly, PEMFCs are performed at temperature lower than 100 °C owing to the failure of the electrochemical performances of Nafion. Nevertheless, taking into account the poisoning effect of CO on the fuel cell catalyst (conventionally based on Pt), the ideal working temperature of the PEMFCs should be above 100 °C, where CO poisoning could be drastically reduced or avoided. Today, Nafion is recognized as the most used proton exchange membrane in the market, useful for both PEMFC and DMFC applications. It is based on a perfluorinated polymer and shows good thermal stability and high proton conductivity as main benefits. On the contrary, Nafion is an expensive material and suffers high fuel crossover (particularly, methanol crossover in DMFC applications) besides the proton conductivity loss above 100 °C. Therefore, in the last decades many scientists paid special attention on the development of new materials based on non-fluorinated polymers as an alternative to Nafion. One of the most promising class of is represented by the polyetheretherketone (PEEK). According to the specialized literature, interesting performances in terms of proton conductivity and thermo-chemical properties as well as low fuel crossover and costs are noticeable for sulfonated PEEK-based polymers. Indeed, many scientific applications are devoted to modify PEEK polymer for manufacturing membranes alternative to Nafion for both PEMFC and DMFC applications. Among them, important methods are exploited for preparing electrolyte membranes from PEEK such as: a) PEEK electrophilic sulfonation (S-PEEK); b) S-PEEK and non-functional polymers blending; c) S-PEEK, heteropolycompounds and poly-ether-imide doping with inorganic acids, etc.     

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.