•Highly sulfonated polybenzimidazoel copolymers have been synthesized.•The covalently cross-linked sulfonated polybenzimidazole membranes exhibit 3-4 orders of magnitude lower VO2+ permeability and 6–30 times higher H/V selectivity than Nafion 117.•The redox flow battery assembled with a cross-linked membrane exhibits high energy efficiency (85%) at 60 mA cm−2.•The redox flow battery displays little performance decay after 300 charge-discharge cycles.A series of polybenzimidazole copolymers with varied content of pendant amino groups have been synthesized by condensation polymerization of 4,4′-dicarboxydiphenyl ether (DCDPE), 5-aminoisophthalic acid (APTA) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid at 190 °C for 20 h. The resulting copolymers undergo post-sulfonatation in fuming sulfuric acid at 100 °C yielded the highly sulfonated polybenzimidazoles (SOPBI-NH2(x/y), ‘x/y’ refers to the monomer molar ratio of DCDPE to APTA). A series of covalently cross-linked membranes (CSOPBI-NH2(x/y)) with good mechanical properties are fabricated by solution cast technique using bisphenol A epoxy resin as a cross-linker. The CSOPBI membranes show 3–4 orders of magnitude lower VO2+ permeability and 6–30 times higher ion diffusion selectivity (proton vs. VO2+) than Nafion117. The charge-discharge behaviors of the vanadium redox flow batteries (VRBs) assembled with the CSOPBI-NH2(x/y) membranes and Nafion 117 are investigated and compared. The VRBs assembled with the CSOPBI membranes exhibit significantly higher columbic efficiency and lower self-discharge rate than that assembled with Nafion 117 owing to the extremely lower vanadium cations crossover of the former. The VRB assembled with the CSOPBI-NH2(9/1) membrane exhibits fairly high energy efficiency (~85% at 60 mA cm−2) and little decay in performance is observed after 300 charge-discharge cycles.

•New diamine monomer with pendant benzimidazole groups synthesized.•Sulfonated copolyimides with the benzimidazole containing diamine prepared.•Copolymer membranes showed super radical oxidative and hydrolytic stabilities.•Copolymer membranes showed high conductivity and enhanced mechanical properties.•Preliminary fuel cell demonstration performed.Sulfonated polyimides are among the most interesting proton exchange membrane materials with high proton conductivity and good mechanical characteristics. As a major challenge the hydrolytic instability of the polymer backbone is addressed by introducing basic moieties in the polymer main chain. A series of sulfonated copolyimides (SPI) are prepared via random copolymerizatio of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) with a new diamine monomer with pendant benzimidazole groups, 2,2′-bis(4-(1H-benzo[d]imidazol-2-yl)phenoxy)benzidine (BIPOB), and a sulfonated diamine monomer 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid (BAPBDS) at different diamine molar ratios (BAPBDS/BIPOB, 4/1, 6/1, 9/1 and 12/1). With ion exchange capacities in the range of 1.60–2.24 meq g−1, transparent and ductile membranes are obtained by solution casting. The incorporation of benzimidazole pendant groups significantly improves the hydrolytic stability as well as the radical oxidative stability of the membranes. In addition, the SPI membranes exhibit high proton conductivities of 0.1 S cm−1 in the fully hydrated state at 60 °C and high elastic modulus and tensile strength. Preliminary fuel cell tests demonstrate the technical feasibility and stability of the materials.

A series of novel multiblock copolymers consisting of benzimidazole-groups-containing sulfonated polyimide hydrophilic blocks (averaged block length = 20) and non-sulfonated polyimide hydrophobic blocks (averaged block length = 5 or 10) have been synthesized via two-pot synthetic procedures. The anhydride-terminated hydrophilic oligomer is synthesized by copolymerization of excess 1,4,5,8-naphthalelnetetracarboxylic dianhydride (NTDA) with amine-terminated polybenzimidazole (PBI-NH2) and 4,4′-bis(4-aminophenoxy) biphenyl-3,3′-disulfonic acid (BAPBDS), while the amine-terminated hydrophobic oligomers are synthesized by polymerization of excess non-sulfonated diamines with NTDA or a fluorinated dianhydride. The resulting multiblock copolymers can be cast into tough membranes indicating that reasonably high molecular weights block copolymers have been obtained. The block SPIs exhibit microphase-separated structure, whereas the random one is amorphous. Fenton’s test reveals that the block copolymer membranes, in particular, those consisting of fluorinated hydrophobic blocks, are fairly stable toward radical oxidation. Preliminary fuel cell tests are performed to evaluate the fuel cell performance of the block copolymer membranes. The single cell equipped with the block copolymer membrane of which hydrophobic block is prepared from NTDA and a fluorinated diamine (averaged block length = 5) exhibits a peak output power density of 0.70 W/cm2 at 90 °C and 92% relative humidity for H2/air which is comparable to that of Nafion 112.

•Novel copolyimides with varied content of benzimidazole groups have been synthesized from a naphthalenic dianhydride monomer.•Post-sulfonation of the copolyimides in concentrated sulfuric acid significant degradation.•Covalent cross-linking has been achieved by treating the sulfonated copolyimide membranes in polyphosphoric acid.•Membranes with excellent hydrolytic stability, high radical oxidative stability and high proton conductivity are prepared.A series of benzimidazole-groups-containing copolyimides (PIs) have been synthesized by random condensation copolymerization of 4,4′-(biphenyl-4,4′-diyldi(oxo))bis(1,8-naphthalic anhydride) (BPNDA), 1,3-bis(4-aminophenoxy)benzene (BAPBz), and 2-(4-Aminophenyl)-5-aminobenzimidazole (APABI) in m-cresol in the presence of benzoic acid and isoquinoline at 180 °C for 20 h. The resultant PIs were subsequently post-sulfonated in concentrated sulfuric acid at 50 °C for 24 h to yield the desired sulfonated copolyimides (SPIs). No significant polymer degradation occurred during the process of sulfonation and tough membranes (tensile stress: 50–83 MPa) of the SPIs were obtained by solution casting. To reduce the swelling ratio, the SPI membranes were further covalently cross-linked by immersing in polyphosphoric acid at 180 °C for 14 h. The resulting covalently cross-linked membranes (CSPIs) displayed significantly reduced swelling ratio, while high ion exchange capacities (IECs) were maintained. The CSPI membranes exhibited comparable proton conductivities to that of Nafion112® in their fully hydrated state. The Fenton's test results suggest good radical oxidative stability of the CSPI membranes due to the synergic action of the covalent cross-linking and the presence of benzimidazole groups in polymer main chains. The CSPI membranes also showed excellent hydrolytic stability due to the covalent cross-linking as well as the low electron affinity of the BPNDA moiety making them good candidate for fuel cell applications.

Chemical stability of polymer electrolyte membranes (PEMs) is the key factor affecting the lifetime of fuel cells. It is greatly desirable to develop the PEMs with both high proton conductivity and excellent chemical stability. In this study, a series of sulfonated polyimide–polybenzimidazole copolymers (SPI-co-PBIs) are synthesized via random condensation polymerization of 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid, and an amine-terminated polybenzimidazole oligomer. The ion exchange capacities of the resulting SPI-co-PBIs are in the range 1.90–2.47 meq g−1. Under fully hydrated condition, the SPI-co-PBI membranes show higher proton conductivities than Nafion112. It is found that the incorporation of a small fraction of PBI moiety into the polyimide structure resulted in significant improvement in radical oxidative stability. For example, the SPI-co-PBI-19/1 containing 5 mol % PBI moiety shows only 0.6 wt% weight loss after being soaked in the Fenton’s reagent (3 % H2O2 + 3 ppm FeSO4) at 80 °C for 150 min, whereas the corresponding benzimidazole group-free sulfonated polyimide is completely dissolved in the Fenton’s reagent at 80 °C for 140 min. The SPI-co-PBI membranes also show excellent hydrolytic stability due to the highly stable ladder structure of the benzimidazobenzisoquinolinone linkages.

A series of sulfonated polybenzimidazoles (SPBIs) with varied ion exchange capacities (IECs) have been synthesized by random condensation copolymerization of a new sulfonated dicarboxylic acid monomer 4,6-bis(4-carboxyphenoxy)benzene-1,3-disulfonate (BCPOBDS-Na), 4,4′-dicarboxydiphenyl ether (DCDPE) and 3,3′-diaminobenzidine (DAB) in Eaton's reagent at 140 °C. Most of the SPBIs show good solubility in polar aprotic organic solvents such as dimethylsulfoxide (DMSO) and N,N-dimethylacetamide (DMAc). Thermogravimetric analysis (TGA) reveals that the SPBIs have excellent thermal stability (desulfonation temperatures (on-set) > 370 °C). The SPBI membranes show good mechanical properties of which tensile strength, elongation at break, and storage modulus are in the range of 89–96 MPa, 12–42%, and 2.4–3.1 GPa, respectively. Moreover, the SPBI membranes exhibit phosphoric acid (PA) uptake in the range of 180–240% (w/w) in 85 wt% PA at 50 °C, while high mechanical properties (13–20 MPa) are maintained. The SPBI membrane with 240% (w/w) PA uptake displays fairly high proton conductivity (37.3 mS cm−1) at 0% relative humidity at 170 °C. The fuel cell fabricated with the PA-doped SPBI membrane (PA uptake = 240% (w/w)) displays good performance with the highest output power density of 0.58 W cm−2 at 170 °C with hydrogen–oxygen gases under ambient pressure without external humidification.Research highlights▶ A series of novel sulfonated polybenzimidazoles (SPBIs) have been synthesized from a new sulfonated dicarboxylic acid monomer, disodium 4,6-bis(4-carboxyphenoxy)benzene-1,3-disulfonate. ▶ The phosphoric acid (PA)-doped SPBI-11 membrane shows high mechanical strength (20 MPa) despite the high PA-doping level (240% (w/w)). ▶ The fuel cell fabricated with the PA-doped SPBI-11 membrane exhibits high performance (the highest output power density: 0.58 W cm-2) at 170 °C with hydrogen-oxygen gases under ambient pressure without any gas humidification.

This paper reports the synthesis, characterization, and thermal stability of cation-exchange membranes based on naphthalenic copolyimides. To improve the mechanical stability, the membranes were cross-linked through the reaction of sulfonic acid groups with hydrogen atoms of activated phenyl groups located in neighboring chains using appropriate catalysts. The ion-exchange capacity of the membranes is higher than 2 meq/(g of dry membrane), and the number of molecules of water per ion-exchange group lies in the vicinity of 20. The response of the membrane electrode assembly (MEA) to electrical perturbation fields in a wide range of frequencies was measured, and the equivalent electrical circuit governing the response is discussed. The conductivity of the MEAs at 75 °C is of the order of 10−2 S/cm. Hydrogen crossover in the membranes was determined by electrochemical measurements and the permeability coefficient of the gas in the membranes determined. Finally, the single cell exhibits a fair performance, the power of the cell at 75 °C being only 225 mW/cm2 or lower, at 0.5 V.

A series of sulfonated copolyimides containing benzimidazole groups (SPIs) were synthesized by random copolymerization of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 2-(4-aminophenyl)-5-aminobenzimidazole (APABI), 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODADS) and 9,9-bis(4-aminophenyl)fluorene (BAPF) in m-cresol in the presence of benzoic acid and triethylamine at 180 °C for 20 h. Membranes with good mechanical properties were prepared by solution cast method. Proton exchange treatment resulted in ionic cross-linking and the membranes were further covalently cross-linked by treating them in polyphosphoric acid (PPA) at 180 °C for 6 h. The covalently cross-linked membranes displayed slightly lower ion exchange capacities (IECs) and proton conductivities than the corresponding covalently uncross-linked ones because small part of the sulfonic acid groups had been consumed during the cross-linking process. Fenton’s test (3% H2O2 + 3 ppm FeSO4, 80 °C) revealed that benzimidazole groups played an important role in the radical oxidative stability of the membranes, while the cooperative effect of benzimidazole groups and covalent cross-linking led to much more significant enhancements in the radical oxidative stability of the membranes than each alone. The membrane 4 (ODADS/APABI/BAPF = 2/1/1, by mol), for example, after covalent cross-linking could maintain membrane form within 8 h measurement, which was much longer than that (3 h) before covalent cross-linking under the same conditions. The membrane 5 (ODADS/BAPF = 3/1, by mol) without benzimidazole groups, however, after covalent cross-linking started to break into pieces after 85 min measurement, which was only slightly longer than that (60 min) before cross-linking under the same conditions.

A series of poly(ethylene oxide) (PEO) segmented polysulfone copolymers with different PEO content has been successfully synthesized via condensation polymerization of 4,4′-dichlorodiphenylsulfone (DCDPS), chlorine-end-capped poly(ethylene oxide) (PEO-Cl2)) and 4,4′-dihydroxybiphenyl (DHBP) in N,N-dimethylacetamide (DMAc) in the presence of anhydrous potassium carbonate at 160 °C for 20 h. The resulting copolymers showed high molecular weights (Mn = 28,000–40,200 Da) and relatively narrow molecular weight distributions (Mw/Mn = 1.57–1.87). They are well soluble in common organic solvents such as chloroform and tetrahydrofuran. Films with high tensile strength (38–50 MPa) and good elasticity of elongation (elongation at break: 190–430%) were prepared by solution cast from the copolymer solutions in chloroform. Sirolimus and paclitaxel were used as drugs and their release kinetics were investigated. For both drugs, the release rate was strongly influenced by the PEO content of the polymer films, i.e., PSF–PEO films with higher PEO content showed larger drug release rate. Film swelling due to liquid sorption is mainly responsible for the drug release.

A facile approach has been successfully developed for the preparation of a series of cross-linked sulfonated polyimide (SPI) membranes via the condensation reaction between the sulfonic acid groups and the activated hydrogen atoms of SPIs in the presence of phosphorous pentoxide : methanesulfonic acid in the ratio of 1 : 10 by weight (PPMA, method 1) or phosphorous pentoxide only (method 2). The resulting sulfonyl linkages are very stable and the cross-linked SPI membranes showed greatly improved water stability in comparison with the uncross-linked ones while high proton conductivity was maintained.

A series of amine-terminated hyperbranched polybenzimidazoles (HBPBIs) were successfully synthesized by condensation polymerization of aromatic dicarboxylic acids and an in situ synthesized aromatic hexamine intermediate product from 1,3,5-benzenetricarboxylic acid (BTA) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid (PPA) at 190 °C for 20 h. HBPBI membranes were fabricated by solution cast method in the presence of cross-linkers (ethylene glycol diglycidyl ether (EGDE) and terephthaldehyde (TPA)). The resulting HBPBI membranes displayed good mechanical properties and good thermal stability. High proton conductivity was obtained with the phosphoric acid-doped and TPA-cross-linked HBPBI membranes at 0% relative humidity.

A facile approach has been successfully developed for the preparation of a series of cross-linked sulfonated polyimide (SPI) membranes via the condensation reaction between the sulfonic acid groups and the activated hydrogen atoms of SPIs in the presence of phosphorous pentoxide : methanesulfonic acid in the ratio of 1 : 10 by weight (PPMA, method 1) or phosphorous pentoxide only (method 2). The resulting sulfonyl linkages are very stable and the cross-linked SPI membranes showed greatly improved water stability in comparison with the uncross-linked ones while high proton conductivity was maintained.