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Anion exchange membranes with star main chains or side chains were prepared by grafting the 1-butyl-2-methylimidazole and/or 1-aminopropyl-2-methyl-3-butylimidazolium hydroxide onto branched or linear poly(aryl ether ketone) polymers. In order to know the influence of the star structures on the physicochemical properties of the prepared membranes, comprehensive characterizations were made including ion exchange capacity, conductivity, mechanical property and alkaline stability. All the membranes exhibit conductivities of about 40 mS cm−1 at 60 °C and about 60 mS cm−1 at 80 °C, respectively. The durability of the membranes in alkaline medium was detected by monitoring the changes in conductivity, ion exchange capacity and contact angle with water. The results indicate that the introduced star structures of both main chains and side chains are effective to enhance the tensile strength and alkaline stability of the membranes. The membranes with star main chains exhibited a tensile stress at break of 45 MPa and retained a conductivity of 36.1 mS cm−1 at 60 °C after exposed to 1 M KOH at 80 °C for 300 h.

Novel high temperature proton exchange membranes, quaternized poly(aryl ether sulfone) (QPAES), were prepared using triethylamine (TEA) as quaternization reagent and (N,N-diethyl-3-aminopropyl) trimethoxysilane (EPMS) as the crosslinking agent, respectively. To improve the mechanical strength and dimensional stability of the membranes, the crosslinked structure of Si O Si network was formed by hydrolyzing siloxane groups of EPMS. Compared with the non-crosslinked membrane, the crosslinked PAES membranes displayed significantly enhanced mechanical properties, improved dimensional stabilities as well as high conductivities. The QPAES-10EPMS-90TEA membrane with a phosphoric acid doping level of 12.1 exhibited a tensile strength of 9.16 MPa at room temperature, and a conductivity of 74.8 mS cm−1 at 180 °C under no humidification conditions.

Two N3-substituted imidazoles 1,2-dimethylimidazole and 1-butyl-2-methylimidazole were chosen to functionalize poly(aryl ether ketone), respectively. The generated imidazolium cations could electrostatically react with sulfonate ions of the sulfonated poly(ether ether ketone) forming the ionic crosslinking structure of the membranes. The changes in crosslinking degree and the alkyl chain-length on N3 site of the imidazoliums could highly affect the properties of the anion exchange membranes (AEMs). The AEMs functionalized by 1-butyl-2-methylimidazole exhibited superior properties compared to those functionalized by 1,2-dimethylimidazole according to the tolerance tests of the AEMs towards hot alkaline solutions. After exposed to 1M KOH at 80°C for 200h, the 1-butyl-2-methylimidazole modified AEMs maintained the ion exchange capacity of above 85%, the conductivity of about 70%, and the tensile stress at break of around 80%, respectively. The hydrophile-lipophile balance of the polymer membranes was calculated and proposed to better understand the correlation between structures and properties of the AEMs. The degradation of the imidazolium functional groups of the AEMs under the attack of hydroxide ions was evidenced by FT-IR analysis.

It is found that free radicals of OH would be generated in KOH solutions at elevated temperatures according to the results of fluorescence analysis. Five kinds of radical inhibitors (tert-butylphenol-containing reagents, 4-amino-tempo, benzophenone, phenothiazine and N-phenyl-1-naphthylamine) were employed as the dopant, separately, to prepare anion exchange membranes (AEMs) based on poly(4-vinylbenzylchloride-styrene). The investigaions on the properties of the prepared AEMs indicate that more radicals might be produced during the degradation of the AEMs via chain propagation reactions. Moreover, the influences of the radical inhibitors on the degradation of the AEMs in 8 M KOH solutions at 80 °C are quite different although all these inhibitors could react with OH radicals. Therein the radical inhibitors o-, m-, p-tert-butylphenols could effectively enhance the chemical stability of the AEMs. However, when the phenol-containing inhibitors have a large substituent group in its benzene ring, the membranes tend to be brittle during the stability test.

One critical issue to phosphoric acid (PA) doped high-temperature proton exchange membranes (HT-PEMs) is to balance the proton conductivity and mechanical properties for overall application performance in fuel cells. Addressing the issue, we prepare durable HT-PEMs having the dual crosslinking structure by employing poly(vinylimidazole-divinylbenzene-styrene) (poly(VIm-DVB-St)) copolymer as a crosslinker and using the poly (aromatic ether ketone) (PAEK) polymer containing four methyl groups as the host membrane matrix. The imidazole groups of poly(VIm-DVB-St) react with benzyl bromide groups of brominated PAEK for both the primary cross-linking network and high PA doping. The divinylbenzene crosslinked poly (styrene-co-vinylimidazole) network generates the secondary cross-linking structure. The formed reticular polymer chain structure brings on low swelling and high mechanical strength of the HT-PEMs. The fuel cell based on the acid doped PAEK41-85%VIm/233.0 PA shows a H2-air fuel cell peak power density of 306 mW cm−2 at 200 °C without back pressure, and a low degradation rate of 3.9 × 10−5 V h−1 during a period of 600 h under a constant current density of 200 mA cm−2 at 160 °C.

Imidazolium-type anion exchange membranes (AEMs) were prepared by functionalization of bromomethylated poly(2,6-dimethyl-1,4-phenyleneoxide) with six kinds of imidazole compounds, respectively, to investigate the correlation of the grafted imidazolium structure and the physicochemical properties of the membrane. The chemical structure of substituted groups in imidazolium cations would highly influence the ion exchange capacity, ionic conduction, mechanical strength, as well as stability in strong alkaline solutions of the AEMs. The membrane having C2-methyl and N3-butyl substituted groups in the imidazolium pendants exhibited superior properties among the prepared AEMs, i.e., an IEC of around 1.03 mmol g −1 , hydroxide ion conductivity of 42.5 mS cm −1 at 80 °C, and tensile strength at break of 15.0 MPa, respectively. In addition, this membrane showed high alkaline stability and no obvious decline in conductivity was observed after being exposed to 1 M KOH at 60 °C for 180 h.

Low cost poly(epichlorohydrin) (PECH) was modified with imidazolium groups to prepare high temperature proton exchange membranes. Chloromethyl groups in the structure of the PECH benefit its modification with no need for highly toxic and carcinogenic chloromethylation reagents. Both methylimidazole (MeIm) and triethoxysilylpropyldihydroimidazole (SiIm) were used to carry out the SN2 nucleophilic substitution for grafting the imidazolium groups onto the PECH. Meanwhile crosslinking of the modified PECH was achieved by forming a crosslinked silane network via the hydrolysis reaction of SiIm in an acid medium. Moreover, porous poly(tetrafluoroethylene) (PTFE) was used as the membrane matrix to enhance the mechanical strength of the fabricated membranes. The obtained PECH–SiIm–MeIm/PTFE membranes displayed phosphoric acid doping capacities of 110–170 wt% with low volume swelling ratios of less than 120%. Anhydrous proton conductivities of 0.010–0.063 S cm−1 were reached at elevated temperatures of 100–180 °C by the membranes with adequate mechanical strength. Fuel cell tests demonstrated the technical feasibility of acid doped PECH–SiIm–MeIm/PTFE membranes for high temperature proton exchange membrane fuel cells.

Quaternary imidazolium groups were grafted onto poly(aryl ether ketone) as both pendants and joints of crosslinking by using 1-methylimidazole and C2-substituted imidazole of 2-undecylimidazole in a certain mole ratio. Crosslinking of the imidazolium-functionalized poly(aryl ether ketone) was performed with three dibromoalkane crosslinkers having different alkyl chain-length in order to know the correlation of the structure and property of the crosslinked membranes meanwhile to enhance their mechanical strength. The obtained crosslinked membranes in hydroxide form showed a high tensile strength of about 20 MPa at room temperature with less swelling, and they were thermally stable up to around 230 °C according to the data of thermogravimetric analysis. All the crosslinked anion exchange membranes exhibited conductivities in purified water of higher than 0.010 S cm−1 at 25 °C, and over 0.040 S cm−1 at 80 °C, respectively. The durability of the membranes in alkaline medium was tested by monitoring the changes in conductivity, ion exchange capacity (IEC) and tensile strength at break, respectively. The results indicate that the anion exchange membrane crosslinked with a long alkyl spacer demonstrated high tolerance to the nucleophilic attack. In addition, the increase in the temperature could accelerate the degradation of the polymer membrane more than that increase in the concentration of the alkaline solution.

Long-term stability is desired for developing high temperature proton conducting membranes as electrolytes for clean energy conversion of fuel cells. To understand the correlation of the grafted imidazolium structure and the stability of the polymer, six kinds of imidazolium polysulfones were synthesized from various imidazole compounds and the chloromethylated polysulfone, as proved by 1H NMR and FT-IR spectra. Membranes with electron-withdrawing or long hydrophobic alkyl groups in the imidazolium pendants exhibited higher chemical stability than those with electron-donating short alkyl groups. After doping with phosphoric acid, the imidazolium polysulfone membranes showed acid doping levels of 8.2–13.0. The membrane with a long tail side-chain of decyl in the imidazolium pendant achieved the highest conductivity of 0.038 S cm−1 at 160 °C without humidifying. Based on this membrane, fuel cell tests demonstrated the technical feasibility as the high temperature proton exchange membrane electrolyte in fuel cells.

A high-strength anion exchange membrane with a full interpenetrating network (full-IPN) structure was prepared from quaternized chitosan (QCS), polyacrylamide (PAM) and polystyrene (PS). The influences of the component content of the membrane and crosslinking degree of the QCS on the mechanical properties, anionic conductivity as well as methanol permeability of the membranes were investigated. The results indicated that the full-IPN structure as well as the presence of the hydrophobic PS could improve the mechanical properties and decrease the methanol permeability of the membrane. Corresponding to the increase in the content of PAM/PS (the molar ratio of PAM/PS was 1:1) from 0 wt.% to 40 wt.% in the full-IPN membrane, the tensile stress was increased from 30.1 MPa to 43.9 MPa, the methanol permeability was decreased from 6.64 × 10 − 6 cm 2 s − 1 to 6.54 × 10 − 7 cm 2 s − 1 , respectively. Anionic conductivities of 6.00 × 10 − 3 –1.26 × 10 − 2 S cm − 1 were achieved at 80 °C for the obtained membranes. To the membranes with a QCS content of 60 wt.%, the conductivity and tensile stress of about 90% were maintained after soaking the membranes in 1 mol L − 1 KOH for 120 h, and a 10 mol L − 1 KOH for 50 h at room temperature, respectively.

Covalently cross-linked hexafluoropropylidene polybenzimidazole (F 6 PBI) was prepared and used to fabricate high temperature proton exchange membranes with enhanced mechanical strength against thermoplastic distortion. Three different epoxides, i.e. bisphenol A diglycidyl ether (R 1 ), bisphenol A propoxylate diglycidyl ether (R 2 ) and poly(ethylene glycol) diglycidyl ether (R 3 ), were chosen as the cross-linkers to investigate the influence of their structures on the properties of the cross-linked F 6 PBI membranes. All the cross-linked F 6 PBI membranes displayed excellent stability towards the radical oxidation. Comparing with the pure F 6 PBI membrane, the cross-linked F 6 PBI membranes showed high acid doping level but less swelling after doping phosphoric acid at elevated temperatures. The mechanical strength at 130 °C was improved from 0.4 MPa for F 6 PBI membrane to a range of 0.8–2.0 MPa for the cross-linked F 6 PBI membranes with an acid doping level as high as around 14, especially for that crosslinking with the epoxide (R 3 ), which has a long linear structure of alkyl ether. The proton conductivity of the cross-linked membranes was increased accordingly due to the high acid doping levels. Fuel cell tests demonstrated the technical feasibility of the acid doped cross-linked F 6 PBI membranes for high temperature proton exchange membrane fuel cells.

High temperature operation of proton exchange membrane fuel cells under ambient pressure has been achieved by using phosphoric acid doped polybenzimidazole (PBI) membranes. To optimize the membrane and fuel cells, high performance polymers were synthesized of molecular weights from 30 to 94 kDa with good solubility in organic solvents. Membranes fabricated from the polymers were systematically characterized in terms of oxidative stability, acid doping and swelling, conductivity, mechanical strength and fuel cell performance and durability. With increased molecular weights the polymer membranes showed enhanced chemical stability towards radical attacks under the Fenton test, reduced volume swelling upon the acid doping and improved mechanical strength at acid doping levels of as high as about 11 mol H3PO4 per molar repeat polymer unit. The PBI-78kDa/10.8PA membrane, for example, exhibited tensile strength of 30.3 MPa at room temperature or 7.3 MPa at 130 °C and a proton conductivity of 0.14 S cm–1 at 160 °C. Fuel cell tests with H2 and air at 160 °C showed high open circuit voltage, power density and a low degradation rate of 1.5 μV h–1 at a constant load of 300 mA cm–2.

A polybenzimidazole variant polymer containing hydroxyl pyridine groups, termed as OHPyPBI, was synthesized from 3,3′-diaminobenzidine tetrahydrochloride and 4-hydroxy-2,6-pyridinedicarboxylic acid. The thermal-oxidative stability of the OHPyPBI polymer was as high as that of poly[2,2′-( m -phenylene)-5,5′-bibenzimidazole] ( m PBI) according to the TGA data. The hydroxyl pyridine groups in the OHPyPBI structure resulted in high proton conductivities of the phosphoric acid doped OHPyPBI membranes. This is because the hydroxyl pyridine groups not only increased the acid doping level of the membranes, but also benefited the proton conduction, which was proved by the results of acid conductivities of the membranes with comparable acid doping levels. At an acid doping level of 8.6, i.e. 8.6 mol acids per molar repeat unit of the polymer, the OHPyPBI membrane exhibited a proton conductivity of 0.102 S cm −1 at 180 °C without humidifying. In addition, an improved tensile modulus at elevated temperatures was observed for acid doped OHPyPBI membranes. Fuel cell tests demonstrated the technical feasibility of acid doped OHPyPBI membranes for high temperature proton exchange membrane fuel cells.

Covalently cross-linked polymer membranes were fabricated from poly(aryl sulfone benzimidazole) (SO(2)PBI) and poly(vinylbenzyl chloride) (PVBCl) as electrolytes for high-temperature proton-exchange-membrane fuel cells. The cross-linking imparted organo insolubility and chemical stability against radical attack to the otherwise flexible SO(2)PBI membranes. Steady phosphoric acid doping of the cross-linked membranes was achieved at elevated temperatures with little swelling. The acid-doped membranes exhibited increased mechanical strength compared to both pristine SO(2)PBI and poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (mPBI). The superior characteristics of the cross-linked SO(2)PBI membranes allowed higher acid doping levels and, therefore, higher proton conductivity. Fuel-cell tests with the cross-linked membranes demonstrated a high open circuit voltage and improved power performance and durability.

Anion exchange membranes with semi-interpenetrating polymer network (semi-IPN) were prepared based on quaternized chitosan (QCS) and polystyrene (PS). The PS was synthesized by polymerization of styrene monomers in the emulsion of the QCS in an acetic acid aqueous solution under nitrogen atmosphere at elevated temperatures. The semi-IPN system was formed by post-cross-linking of the QCS. A hydroxyl ionic conductivity of 2.80 × 10 −2 S cm −1 at 80 °C and a tensile stress at break of 20.0 MPa at room temperature were reached, respectively, by the semi-IPN membrane containing 21 wt.% of the PS. The durability of the semi-IPN membrane in alkaline solutions was tested by monitoring the variation of the conductivity and the mechanical strength. The degradation of the conductivity at 80 °C was about 5% by immersing the membrane in a 1 mol L −1 KOH solution at room temperature for 72 h and at 60 °C for 50 h, respectively. The tensile stress at break at room temperature could maintain about 20.0 MPa for the membrane soaking in a 10 mol L −1 KOH solution at ambient temperature for more than 70 h. The water swelling of the semi-IPN membranes was discussed based on the stress relaxation model of polymer chains, and it obeyed the Schott’s second-order swelling kinetics.

A series of poly(arylene ether triazine) homopolymers were synthesized that contain pendant diphenylamine groups. The polymers had inherent viscosities from 0.66 to 1.01 dL/g in DMAc at 25 °C, thermal stabilities >500 °C in air, glass transition temperatures from 156 °C to 309 °C, and solubilities that depended upon the bis(4-fluorophenyl) monomer that was used for polymerization. Polymers could be sulfonated with chlorosulfonic acid exclusively at the para position of the diphenylamine rings, with ion exchange capacities from 1.88 to 2.12 meq/g. Transmission electron microscopy images show that the morphology of the sulfonated polymer films depends on the functional group in the polymer backbone. Polymers containing sulfone groups exhibited small ionic clusters within a uniform ion-containing background, while polymers with ketone and isophthaloyl groups exhibited phase separation with different sizes of spherical hydrophilic clusters. The uniform distribution of ionic groups within polymers that contain sulfone groups resulted in higher proton conductivity, 0.11 S/cm at 90 °C and 100% relative humidity, in spite of having a lower degree of sulfonation and water uptake. Small angle neutron scattering data also shows this film has a robust morphology that does not change as a function of temperature or by counterion exchange. Films cast from unsulfonated polymers that contained phosphine oxide groups in the polymer backbone, as well polymer blends with polybenzimidazole soaked in 85% phosphoric acid at 75 °C, had phosphoric acid uptakes above 350 wt%, and as high as 830 wt%. However, these films lost dimensional stability at elevated temperatures.

A novel copolymer of polybenzimidazoles was prepared by copolymerization of 3,3′-diaminobenzidine tetrahydrochloride, 3,4-diaminobenzoic acid and isophthalic acid in polyphosphoric acid at 200 °C. The polymerization could be performed within 90–110 min with the assistance of microwave irradiation. The solubility of the copolymer obtained in N,N-dimethylacetamide (DMAc) was improved compared with those of poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and poly(2,5-benzimidazole). Thus copolymer membranes could be readily prepared by dissolving the copolymer powders in DMAc with refluxing under ambient pressure. The decomposition temperature of the copolymer was about 520 °C in air according to thermogravimetric analysis data. The proton conductivity and mechanical strength of the phosphoric acid-doped copolymer membranes were investigated at elevated temperatures. A conductivity of 0.09 S cm−1 at 180 °C and a tensile stress at break of 5.9 MPa at 120 °C were achieved for the acid-doped copolymer membranes by doping acids in a 75 wt% H3PO4 solution. Copyright © 2010 Society of Chemical Industry

A novel ionic liquid of trifluoroacetic propylamine, i.e., [CH 3 CH 2 CH 2 NH 3 + ] [CF 3 COO − ] (TFAPA), was synthesized from trifluoroacetic acid and propylamine. The ionic liquid of TFAPA was used to prepare anhydrous, conducting membranes based on polymers of sulfonated poly (ether ether) ketone (SPEEK) or polyvinylidenefluoride (PVDF). The ionic conductivity and mechanical strength of the composite membranes were investigated at elevated temperatures and under anhydrous conditions. Conductivity of 0.030 S/cm was achieved with TFAPA at 180 °C, and of 0.019 S/cm with a membrane containing 70% (wt) TFAPA in SPEEK with a sulfonation degree of 86% at 160 °C. Increasing either ionic liquid content or temperature reduced the mechanical strength of the composite membrane. Efforts were made to improve the strength of TFAPA/SPEEK composite membranes by cross-linking the SPEEK, which led to some strength enhancement at 110 °C and 130 °C.

The state-of-the-art of polymer electrolyte membrane fuel cell (PEMFC) technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a typical temperature of 80 °C. Some of the key issues and shortcomings of the PFSA-based PEMFC technology are briefly discussed. These include water management, CO poisoning, hydrogen, reformate and methanol as fuels, cooling, and heat recovery. As a means to solve these shortcomings, high-temperature polymer electrolyte membranes for operation above 100 °C are under active development. This treatise is devoted to a review of the area encompassing modified PFSA membranes, alternative sulfonated polymer and their composite membranes, and acid−base complex membranes. PFSA membranes have been modified by swelling with nonvolatile solvents and preparing composites with hydrophilic oxides and solid proton conductors. DMFC and H2/O2(air) cells based on modified PFSA membranes have been successfully operated at temperatures up to 120 °C under ambient pressure...

Two silane compounds, i.e., ((chloromethyl)phenylethyl)trimethoxysilane (Ph-Si) and 3-chloropropyltriethoxysilane (Pr-Si), are employed as covalent crosslinkers to fabricate high temperature proton exchange membranes based on polybenzimidazole (PBI) in order to better understand the correlation between the structure and the property of the crosslinked membranes. All the crosslinked PBI membranes display longer morphology durability over the neat PBI membrane toward the radical oxidation. The crosslinked membranes with the crosslinker containing a rigid group of phenylene (PBI-Ph) show superior acid doping level, high conductivity, and excellent mechanical strength simultaneously, comparing to those with the crosslinker having a flexible alkyl chain (PBI-Pr) and the pristine PBI based membrane. The technical feasibility for using as high temperature proton exchange membranes is demonstrated by the acid doped crosslinked PBI-Ph membranes via fuel cell tests.

High molecular weight polybenzimidazole (PBI) was synthesized and grafted with benzimidazole pendant groups. The high molecular weight of PBI resulted in good film-forming properties and superior tensile strength. With a phosphoric acid doping level (ADL) of 13.1, a tensile strength of 16 MPa was achieved at room temperature. Grafting of benzimidazole moieties onto the PBI macromolecular chain introduced additional basic sites which allowed the membrane to achieve higher phosphoric acid uptakes. A molar acid conductivity, defined as the specific conductivity of each mole of doping acid, was proposed to evaluate the effective conductivity contributed from the doping acids. With a grafting degree of 5.3% and an ADL of 13.1, the PBI membranes exhibited a total conductivity of 0.15 S cm−1. A H2–air fuel cell based on this membrane showed a peak power density of 378 mW cm−2 at 180 °C without humidification.

Poly(aryl sulfone benzimidazole) (SO2PBI) and its copolymers with poly[2,2′-p-(phenylene)-5,5′-bibenzimidazole] (pPBI), termed as Co-SO2PBI, were synthesized with varied feeding ratios of 4,4′-sulfonyldibenzoic acid (SDBA) to terephthalic acid (TPA). Incorporation of the stiff para-phenylene and flexible aryl sulfone linkages in the macromolecular structures resulted in high molecular weight copolymers with good solubility. The chemical stability towards radical oxidation was improved for SO2PBI and its copolymer membranes due to the electron-withdrawing sulfone functional groups. Upon acid doping, the membrane swelling was reduced and the mechanical strength was improved, as compared with their meta structured analogues. At an acid doping level of 11 mol H3PO4 per average molar repeat unit, the Co-20%SO2PBI membrane exhibited a tensile strength of 16 MPa at room temperature and an H2-air fuel cell peak power density of 346 mW cm−2 at 180 °C at ambient pressure. Durability tests with the membrane under a constant current density of 300 mA cm−2 at 160 °C showed a degradation rate of 6.4 μV h−1 during a period of 2400 h, which was significantly lower than that for meta PBI membranes with a similar acid doping level.

Polybenzimidazole (PBI) membranes have been prepared with different molecular weights. The water and acid swelling, mechanical strength, gas permeability and proton conductivity were studied for the pristine and acid doped PBI membranes. When doped with 5 mol of phosphoric acid per mole repeat unit of the polymer, a level necessary to obtain high enough proton conductivity for fuel cell uses, the polymer membrane exhibits a volume swelling by 118%, resulting in separation of the polymer backbones. The separation in turn reduces the mechanical strength of the membrane especially at high temperatures. Another consequence is the increased H2 and O2 permeability through the membrane. In the temperature range from 120 to 180 °C, the hydrogen permeability was found to be 1.6–4.3 × 10−17 and 1.2–4.0 × 10−15 mol cm cm−2 s−1 Pa−1 for pristine and acid doped PBI membranes, respectively, while for oxygen it was 5.0–10 × 10−19 and 3.0–9.4 × 10−16 mol cm cm−2 s−1 Pa−1, respectively. High molecular weights of the polymers improve the mechanical strength but have little influence on the proton conductivity of the membranes.