A comparative study of phosphoric acid‐doped m‐PBI membranes

Phosphoric acid (PA)‐doped m‐polybenzimidazole (PBI) membranes used in high temperature fuel cells and hydrogen pumps were prepared by a conventional imbibing process and a sol–gel fabrication process. A comparative study was conducted to investigate the critical properties of PA doping levels, ionic conductivities, mechanical properties, and molecular ordering. This systematic study found that sol–gel PA‐doped m‐PBI membranes were able to absorb higher acid doping levels and to achieve higher ionic conductivities than conventionally imbibed membranes when treated in an equivalent manner. Even at similar acid loadings, the sol–gel membranes exhibited higher ionic conductivities. Heat treatment of conventionally imbibed membranes with ≤29 wt % solids caused a significant reduction in mechanical properties; conversely, sol–gel membranes exhibited an enhancement in mechanical properties. From X‐ray structural studies and atomistic simulations, both conventionally imbibed and sol–gel membranes exhibited d‐spacings of 3.5 and 4.6 Å, which were tentatively attributed to parallel ring stacking and staggered side‐to‐side packing, respectively, of the imidazole rings in these aromatic heterocyclic polymers. An anisotropic staggered side‐to‐side chain packing present in the conventional membranes may be related to the reduction in mechanical properties. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Polym. Phys. 2014 , 52, 26–35     

Chemically modified proton‐conducting membranes based on sulfonated polyimides: Improved water stability and fuel‐cell performance

A series of branched/crosslinked sulfonated polyimide (B/C‐SPI) membranes were prepared and evaluated as proton‐conducting ionomers based on the new concept of in situ crosslinking from sulfonated polyimide (SPI) oligomers and triamine monomers. Chemical branching and crosslinking in SPI oligomers with 1,3,5‐tris(4‐aminophenoxy)benzene as a crosslinker gave the polymer membranes very good water stability and mechanical properties under an accelerated aging treatment in water at 130 °C, despite their high ion‐exchange capacity (2.2–2.6 mequiv g^?1). The resulting polymer electrolytes displayed high proton conductivities of 0.2–0.3 S cm^?1 at 120 °C in water and reasonably high conductivities of 0.02–0.03 S cm^?1 at 50% relative humidity. In a single H₂/O₂ fuel‐cell system at 90 °C, they exhibited high fuel‐cell performances comparable to those of Nafion 112. The B/C‐SPI membranes also displayed good performances in a direct methanol fuel cell with methanol concentrations as high as 50 wt % that were superior to those of Nafion 112. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3751–3762, 2006     

Proton‐conducting polymer membrane based on sulfonated polystyrene microspheres and an amphiphilic polymer blend

A new type of amphiphic polymer blend comprising polystyrene (PS), polyethylene oxide (PEO) and microspheres of crosslinked polystyrene sulfonic acid (PSSA) was prepared by solution blending and followed by casting. Besides providing protons, PSSA plays a role in enhancing the miscibility of polystyrene (PS) and polyethylene oxide (PEO) according to the IR and the DSC studies. The resulting polymer blend is a proton electrolyte. The influence of the mixing extent between PS and PEO on the proton conductivity has been studied. It is also found that for those samples in which PEO and PS mix well, the hydrophobic PS component can effectively prevent water evaporation from the hydrophilic components at elevated temperatures, and therefore preserve the proton conductivity (10⁻⁴ S/cm) at the temperature as high as 80 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1530–1538, 2000     

Proton‐conducting electrolytes based on silylated and sulfonated polyetheretherketone: Synthesis and characterization

A derivative of polyetheretherketone (PEEK) having sulfonic acid groups and silicon‐containing substituents covalently bound to the aromatic backbone has been prepared as proton‐exchange membrane material. The polymer 4 (PhSiSPEEK) has been synthesized via (i) sulfonation of PEEK up to 0.9 degree of sulfonation (DS, the number of sulfonic groups per repeat unit), (ii) conversion of sulfonated PEEK 1 (SPEEK09) into sulfonyl chlorinated derivative 2 (PEEKSO₂Cl), (iii) lithiation of 2 and subsequent addition of PhSiCl₃, followed by hydrolysis. The chemical structure of the synthesized polymers has been investigated by ¹H NMR and ¹³C NMR and ATR/FTIR spectroscopy and their thermal stability has been evaluated by thermogravimetric analysis. The presence of inorganic moieties increases the thermal stability of 4 with respect to the sulfonated and not silylated product. Despite its very high DS, PhSiSPEEK is insoluble in water but does not possess the plastic properties needed to be used as an electrolyte membrane. Blend membranes made of SPEEK05 (DS = 0.5) and containing 10 and 25 wt % of 4 (DS = 0.9, degree of silylation DSi = 0.1) have been prepared and characterized by water uptake measurements and electrochemical impedance spectroscopy. The combination of the two functionalized polymers having different properties allows to obtain proton‐conducting electrolytes that are potential candidates for fuel cells applications. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2178–2186, 2010     

Novel UV‐induced photografting process for preparing poly(tetrafluoroethylene)‐based proton‐conducting membranes

A novel process comprising the UV‐induced photografting of styrene into poly(tetrafluoroethylene) (PTFE) films and subsequent sulfonation has been developed for preparing proton‐conducting membranes. Although under UV irradiation the initial radicals were mainly generated on the surface of the PTFE films by the action of photosensitizers such as xanthone and benzoyl peroxide, the graft chains were readily propagated into the PTFE films. The sulfonation of the grafted films was performed in a chlorosulfonic acid solution. Fourier transform infrared and scanning electron microscopy were used to characterize the grafted and sulfonated membranes. With a view to use in fuel cells, the proton conductivity, water uptake, and mechanical properties of the prepared membranes were measured. Even through the degree of grafting was lower than 10%, the proton conductivity in the thickness direction of the newly prepared membranes could reach a value similar to that of a Nafion membrane. In comparison with γ‐ray radiation grafting, UV‐induced photografting is very simple and safe and is less damaging to the membranes because significant degradation of the PTFE main chains can be avoided. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2624–2637, 2007     

Proton conducting polymers containing 1H‐1,2,3‐triazole moieties

Polyacrylates containing a different number of 1H‐1,2,3‐triazole groups per repeat unit have been synthesized via conventional free radical polymerization. These polymers were characterized by nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Proton conductivity measurements were made using impedance spectroscopy. Introduction of more than one triazole per repeat unit did not result in an increase in conductivity as there was an accompanying increase in glass transition temperature (T_g). A maximum conductivity of 17.5 μS/cm was obtained at 200 °C under anhydrous condition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 188–196, 2009     

Proton‐conducting poly(vinylpyrrolidon)–polyphosphoric acid blends

Anhydrous, proton‐conducting polymer electrolytes of poly(vinylpyrrolidon) (PVP) with polyphosphoric acid (PPA) were prepared. PVP‐x‐PPA blends were obtained for 0.5 ≤ x ≤ 3, where x was the number of moles of PO per polymer repeat unit. Fourier transform infrared studies indicated protonation of the carbonyl group in the five‐member ring. Thermogravimetric analysis showed that these materials were stable up to about 180 °C. Differential scanning calorimetry data demonstrated that the addition of the acid plasticized the material, shifting the glass‐transition temperature from 180 °C for the pure polymer to ?23 °C for x = 3. The temperature dependence of the mechanical properties was investigated with shear experiments. The direct‐current conductivity increased with x and reached about 10^?5 S/cm at ambient temperature. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1987–1994, 2001     

Crosslinking effects on hybrid organic–inorganic proton conducting membranes based on sulfonated polystyrene and polysiloxane

New hybrid semi‐interpenetrating proton‐conducting membranes were obtained using sulfonated polystyrene (SPS) and inorganic–organic polysiloxane phases with the aim of improving the mechanical and thermal characteristics of the pristine polymer and to study the effects of crosslinking in the latter phase in several of their properties, mainly proton conductivity. Siloxane phases were prepared using poly(dimethylsiloxane) (PDMS) and PDMS with tetraethoxysilane (TEOS) or phenyltrimethoxysilane (PTMS) as crosslinking agents. To study the crosslinking effect, membranes were prepared with different TEOS:PDMS and PTMS:PDMS mole ratios. The films obtained were characterized by FTIR, ²⁹Si‐HPDEC MAS‐NMR, ¹³C‐CP‐MAS NMR, elemental and thermal analyses. Certain properties, such as water uptake (WU), ion exchange capacity (IEC) and the state of the water, were determined. The proton conductivity was measured at different temperatures (30°C and 80°C) and relative humidities (50–95%). The water content of the hybrid membranes declined significantly, compared with the SPS membranes, depending on the nature and amount of siloxane phase added. Nonetheless, the conductivity values remained relatively high (>100 mS cm^?1 at 80°C and 95% RH) when compared to Nafion®117 presumably because of the formation of well developed proton channels, which makes them potentially promising as proton exchange membranes for fuel cells. These membranes proved to be thermally stable up to 350°C. Scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM) were used to characterize the hybrid membranes microstructures; the latter provided contrast for the conductive domains. Copyright © 2015 John Wiley & Sons, Ltd.     

Proton‐conductive membranes based on blends of polyimides

We prepared novel proton‐conductivity membranes based on blends of sulfonated polyimides. The blend membranes were prepared from a sulfonated homopolyimide and a sulfonated copolyimide with a solvent‐casting method. The proton conductivities of the blend membranes were measured as functions of the temperature with four‐point‐probe electrochemical impedance spectroscopy. The conductivity of the membranes strongly depended on the sulfonated homopolyimide content and increased with an increase in the content. The proton conductivity of all the blended membranes indicated a higher value than that determined in Nafion at 80 °C, and this may mean that the proton transfer in the blend membranes is responsible for the ionic channels induced by the hydrophobic and hydrophilic domains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1325–1332, 2007     

Polymer electrolyte membranes based on p‐toluenesulfonic acid doped poly(1‐vinyl‐1,2,4‐triazole): Synthesis,thermal and proton conductivity properties

Throughout this work, the synthesis, thermal as well as proton conducting properties of acid doped heterocyclic polymer were studied under anhydrous conditions. In this context, poly(1‐vinyl‐1,2,4‐triazole), PVTri was produced by free radical polymerization of 1‐vinyl‐1,2,4‐triazole with a high yield. The structure of the homopolymer was proved by FTIR and solid state ¹³C CP‐MAS NMR spectroscopy. The polymer was doped with p‐toluenesulfonic acid at various molar ratios, x = 0.5, 1, 1.5, 2, with respect to polymer repeating unit. The proton transfer from p‐toluenesulfonic acid to the triazole rings was proved with FTIR spectroscopy. Thermogravimetry analysis showed that the samples are thermally stable up to ～250 °C. Differential scanning calorimetry results illustrated that the materials are homogeneous and the dopant strongly affects the glass transition temperature of the host polymer. Cyclic voltammetry results showed that the electrochemical stability domain extends over 3 V. The proton conductivity of these materials increased with dopant concentration and the temperature. Charge transport relaxation times were derived via complex electrical modulus formalism (M*). The temperature dependence of conductivity relaxation times showed that the proton conductivity occurs via structure diffusion. In the anhydrous state, the proton conductivity of PVTri1PTSA and PVTri2PTSA was measured as 8 × 10^?4 S/cm at 150 °C and 0.012 S/cm at 110 °C, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1016–1021, 2010     

Synthesis and characterization of thermally stable sulfonated polybenzimidazoles obtained from 3,3′‐disulfonyl‐4,4′‐dicarboxyldiphenylsulfone

A novel sulfonated aromatic diacid, 3,3′‐disulfonyl‐4,4′‐dicarboxyldiphenylsulfone (DSDCDPS), was successfully synthesized from 4,4′‐dimethyldiphenylsulfone by sulfonation and further oxidation. A series of sulfonated polybenzimidazoles (sPBI‐SS) with various sulfonation degrees was prepared from DSDCDPS, 4,4′‐sulfonyldibenzoic acid and 3,3′‐diaminobenzidine by solution copolycondensation in poly(phosphoric acid). The chemical structure of the resulting sPBI‐SS was confirmed by FTIR and ¹H NMR. The DSDCDPS‐based sPBI‐SS with the number‐average molecular weights of 32,000–55,000 were easy to dissolve in polar aprotic solvents such as DMF, DMSO, and DMAc, and could be cast into transparent, tough, and flexible membranes. The membranes presented good thermal stabilities (5% weight loss temperatures higher than 430 °C), and the thermal degradation activation energies of the sulfonic group of sPBI‐SS40 evaluated under N₂ by both Ozawa and Kissinger methods were 266.06 and 264.79 kJ/mol, respectively. The membranes also exhibited high storage moduli, glass transition temperatures (above 238 °C) and tensile strengths (～80 MPa), in addition to water uptakes (22.3–25.2%) and low swelling degrees (<14.0%). © 2005 Wiley Periodicals, Inc. J Polym Sci A: Polym Chem 43: 4363–4372, 2005     

Synthesis and characterization of fluorine‐containing polybenzimidazole for proton conducting membranes in fuel cells

Two new kinds of fluorine‐containing polybenzimidazoles (PBI), poly(2,2′‐(tetrafluoro‐p‐phenylene)‐5,5′‐bibenzimidazole) and poly(2,2′‐tetradecafluoroheptylene‐5,5′‐bibenzimidazole), were synthesized by condensation polymerization of 3,3′‐diaminobenzidine and perfluoroterephthalic acid (or perfluoroazelaic acid), with polyphosphoric acid as solvent. Thermogravimetric analysis results show that the fluorine‐containing polymers synthesized exhibit promising thermal stability. The film‐forming properties of the fluorine‐containing polymers are improved over nonfluorinated PBI. The introduction of fluorine into the backbone of the polymers has significant positive affection on their chemical oxidation stability demonstrated by Fenton test. Compared with poly(2,2′‐(m‐phenylene)‐5,5′‐bibenzimidazole)/phosphoric acid (PA) composite membrane, the resulting fluorinated membranes with a same PA doping level exhibit better flexibility and higher proton conductivity. The maximum proton conductivity gained is 3.05 × 10^?2 S/cm at 150 °C with a PA doping level of 7. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2115–2122, 2010     

Aromatic and rigid rod polyelectrolytes based on sulfonated poly(benzobisthiazoles)

Aromatic polyelectrolytes based on sulfonated poly(benzobisthiazoles) (PBTs) have been synthesized by a polycondensation reaction of sulfo-containing aromatic dicarboxylic acids with 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) in freshly prepared polyphosphoric acid (PPA). Several sulfonated PBTs, poly[(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl)-2-sulfo-1,4-phenylene] sodium salt (p-sulfo PBT), poly[(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl)-5-sulfo-1,3-phenylene] sodium salt (m-sulfo PBT), their copolymers, and poly[(benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl)-4,6-disulfo-1,3-phenylene] potassium salt (m-disulfo PBT), have been targeted and the polymers obtained characterized by ¹³C-NMR, FT-IR, elemental analysis, thermal analysis, and solution viscosity measurements. Structural analyses confirm the structures of p-sulfo PBT and m-disulfo PBT, but suggest that the sulfonate is cleaved from the chain during synthesis of m-sulfo PBT. m-Disulfo PBT dissolves in water as well as strong acids, while p-sulfo PBT dissolves well in strong acids, certain solvent mixtures containing strong acids, and hot DMSO. TGA indicates that these sulfonated PBTs are thermally stable to over 500°C. Free-standing films of p-sulfo PBT, cast from dilute neutral DMSO solutions, are transparent, tough, and orange in color. Films cast from basic DMSO are also free standing, while being opaque and yellow-green. p-Sulfo PBT was incorporated as the dopant ion in polypyrrole, producing conductive films with conductivities as high as 3 S/cm and electrical anisotropies as high as 10. © 1996 John Wiley & Sons, Inc.     

Synthesis and characterization of thiazolothiazole‐based polymers and their applications in polymer solar cells

A novel series of thiazolothiazole (Tz)‐based copolymers, poly[9,9‐didecylfluorene‐2,7‐diyl‐alt‐2,5‐bis‐(3‐hexylthiophene‐2‐yl)thiazolo[5,4‐d]thiazole] (P1), poly[9,9‐dioctyldibenzosilole‐2,7‐diyl‐alt‐2,5‐bis‐(3‐hexylthiophene‐2‐yl)thiazolo[5,4‐d]thiazole] (P2), and poly[4,4′‐bis(2‐ethylhexyl)‐dithieno[3,2‐b:2′,3′‐d]silole‐alt‐2,5‐bis‐(3‐hexylthiophene‐2‐yl)thiazolo[5,4‐d]thiazole] (P3), were synthesized for the use as donor materials in polymer solar cells (PSCs). The field‐effect carrier mobilities and the optical, electrochemical, and photovoltaic properties of the copolymers were investigated. The results suggest that the donor units in the copolymers significantly influenced the band gap, electronic energy levels, carrier mobilities, and photovoltaic properties of the copolymers. The band gaps of the copolymers were in the range of 1.80–2.14 eV. Under optimized conditions, the Tz‐based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 2.23–2.75% under AM 1.5 illumination (100 mW/cm²). Among the three copolymers, P1, which contained a fluorene donor unit, showed a PCE of 2.75% with a short‐circuit current of 8.12 mA/cm², open circuit voltage of 0.86 V, and a fill factor (FF) of 0.39, under AM 1.5 illumination (100 mW/cm²). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of novel proton‐conducting highly sulfonated polybenzimidazoles for PEMFC and the effect of the type of bisphenyl bridge on polymer and membrane properties

Six series of novel highly sulfonated polybenzimidazoles (sPBIs) with high molecular weight were prepared by direct polycondensation between 3,3′‐diaminobenzidine and original multisulfonated dinuclear dicarboxylic acids containing bridging ether, sulfone, and hexafluoroisopropylidene moieties. All reactions were carried out in polyphosphoric acid, which acts as both solvent and catalyst. The degree of sulfonation was modulated in the final products by varying the proportion of sulfonated to nonsulfonated dicarboxylic acids used in the synthesis. The high purity of the disulfonated and tetrasulfonated monomers allows wholly sulfonated homopolymers to be obtained. Confirmation of the chemical structure and the degree of sulfonation were derived from ¹H nuclear magnetic resonance spectroscopy. Inherent viscosity was estimated as between 0.70 and 5.33 dL g^?1 for sPBIs with ion exchange capacity in the range 0.87–4.68 mequiv g^?1. Dynamic thermogravimetric analysis in air showed no weight loss below 350 °C (heating rate 5 °C min^?1). The nature of the bisphenyl bridge has clear influence on the water uptake and proton conduction properties of the resulting sPBI membranes, with hexafluoroisopropylidene links providing materials of highest conductivity as well as favoring film‐forming characteristics. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The effect of structural variations on aromatic polyethers for high‐temperature PEM fuel cells

Three series of new aromatic polyether sulfones bearing phenyl, p‐tolyl or carboxyl side groups, respectively, and polar pyridine main chain groups were developed. Most of the polymeric materials presented high molecular weights and excellent solubility in common organic solvents. More importantly, they formed stable, self‐standing membranes that were thoroughly characterized in respect to their thermal, mechanical and oxidative stability, their phosphoric acid doping ability and ionic conductivity. Particularly, the copolymers bearing side p‐tolyl or carboxyl groups fulfill all necessary requirements for application as proton electrolyte membranes in high temperature fuel cells, which are glass transition temperatures higher than 220 °C, thermal stability up to 400 °C, oxidative stability, high doping levels (DLs) and proton conductivities of about 0.02 S/cm. Initial single fuel cell results at high temperatures, 160 °C or 180 °C, using a copolymer bearing p‐tolyl side groups with a relatively low DLs around 200 wt % and dry H₂/Air feed gases, revealed efficient power generation with a current density of 0.5 A/cm² at 500 mV. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and properties of fluorene‐based fluorinated polymers

Fluorene‐based polymers containing various fluorinated benzene (fluorobenzene, p‐difluorobenzene, and tetrafluorobenzene) moieties were synthesized. In addition, perfluorooctylation of poly‐[(9,9‐dioctylfluorene‐2,7‐diyl)‐co‐(fluorene‐2,7‐diyl)] was carried out to afford fluorene‐based polymers with perfluorooctyl moiety at the 9‐position on the fluorene ring. To evaluate the effect of fluorine moiety, polymers containing nonfluorinated benzene moieties and nonfluorinated octyl groups were synthesized. The photoluminescence measurements indicated that all these polymers exhibited blue emission in solution, but a polymer containing a perfluorooctyl group did not emit in the film state. Polymers containing various fluorinated benzene moieties showed higher fluorescence quantum yields and thermal stability than those containing nonfluorinated benzene. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3143–3150, 2001     

Synthesis and characterization of sulfonated poly(benzoxazole ether ketone)s by direct copolymerization as novel polymers for proton-exchange membranes

A new series of sulfonated poly(benzoxazole ether ketone)s (SPAEKBO-X) were prepared by the aromatic nucleophilic polycondensation of 4,4′-(hexafluoroisopropylidene)-diphenol with 2,2′-bis[2-(4-fluorophenyl)benzoxazol-6-yl]hexafluoropropane and sodium 5,5′-carbonylbis-2-fluorobenzenesulfonate in various ratios. Fourier transform infrared and ¹H NMR were used to characterize the structures and sulfonic acid contents of the copolymers. The copolymers were soluble in N-methyl-2-pyrrolidinone, N,N-dimethylacetamide, and N,N-dimethylformamide and could form tough and flexible membranes. The protonated membranes were thermally stable up to 320 °C in air. The water uptake, hydrolytic and oxidative stability, and mechanical properties were evaluated. At 30–90 °C and 95% relative humidity, the proton conductivities of the membranes increased with the sulfonic acid content and temperature and almost reached that of Nafion 112. At 90–130 °C, without external humidification, the conductivities increased with the temperature and benzoxazole content and reached above 10⁻² S/cm. The SPAEKBO-X membranes, especially those with high benzoxazole compositions, possessed a large amount of strongly bound water (>50%). The experimental results indicate that SPAEKBO-X copolymers are promising for proton-exchange membranes in fuel cells, and their properties might be tailored by the adjustment of the copolymer composition for low temperatures and high humidity or for high temperatures and low humidity; they are especially promising for high-temperature applications. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2273–2286, 2007