Branched poly(ether ether ketone) based anion exchange membrane for H2/O2 fuel cell

High-performance anion exchange membranes (AEMs) are in need for practical application of AEM fuel cells. Novel branched poly(ether ether ketone) (BPEEK) based AEMs were prepared by the copolymerization of phloroglucinol, methylhydroquinone and 4,4′-difluorobenzophenone and following functionalization. The effects of the branched polymer structures and functional groups on the membrane's properties were investigated. The swelling ratios of all the membranes were kept below 15% at room temperature and had good dimensional stability at elevated temperatures. The branching degree has almost no effect on the dimensional change, but plays a great role in tuning the nanophase separation structure. The cyclic ammonium functionalized membrane showed a lower conductivity but a much better stability than imidazolium one. The BPEEK-3-Pip-53 membrane with the branching degree of 3% and piperidine functionalization degree of 53% showed the best performances. The ionic conductivity was 43 mS cm⁻¹ at 60 °C. The ionic conductivity in 1 M KOH at 60 °C after 336 h was 75% of its initial value (25% loss of conductivity), and the IEC was 83% of its initial value (17% loss of IEC), suggesting good alkaline stability. The peak energy density (60 °C) of the single H₂/O₂ fuel cell with BPEEK-3-Pip-53 membrane reached 133 mW cm⁻² at 260 mA cm⁻².     

High alkaline stability and long-term durability of imidazole functionalized poly(ether ether ketone) by incorporating graphene oxide/metal-organic framework complex

The poly(ether ether ketone) (PEEK) was prepared as organic matrix. ZIF-8 and GO/ZIF-8 were used as fillers. A series of novel new anion exchange membranes (AEMs) were fabricated with imidazole functionalized PEEK and GO/ZIF-8. The structure of ZIF-8, GO/ZIF-8 and polymers are verified by ¹H NMR, FT-IR and SEM. This series of hybrid membranes showed good thermal stability, mechanical properties and alkaline stability. The ionic conductivities of hybrid membranes are in the range of 39.38 mS cm^?1–43.64 mS cm^?1 at 30 °C, 100% RH and 59.21 mS cm^?1–86.87 mS cm^?1 at 80 °C, 100% RH, respectively. Im-PEEK/GO/ZIF-8-1% which means the mass percent of GO/ZIF-8 compound in Im-PEEK polymers is 1%, showed the higher ionic conductivity of 86.87 mS cm^?1 at 80 °C and tensile strength (38.21 MPa) than that of pure membrane (59.21 mS cm^?1 at 80 °C and 19.47 MPa). After alkaline treatment (in 2 M NaOH solution at 60 °C for 400 h), the ionic conductivity of Im-PEEK/GO/ZIF-8-1% could also maintain 92.01% of the original ionic conductivity. The results show that hybrid membranes possess the ability to coordinate trade-off effect between ionic conductivity and alkaline stability of anion exchange membranes. The excellent performances make this series of hybrid membranes become good candidate for application as AEMs in fuel cells.     

Preparation and characterization of a polyvinyl alcohol grafted bis-crown ether anion exchange membrane with high conductivity and strong alkali stability

A new type of symmetrical bis-crown ether is prepared by connecting dibenzo-18-crown-6 ether on both sides of the chromotropic acid, and then grafting the aforementioned bis-crown ether onto polyvinyl alcohol matrix to prepare a series of anion exchange membranes (AEMs), which their have high conductivity and strong alkali stability. These synthesized membranes were named B-C_X%-P AEMs (x is the mass percentage of the symmetrical bis-crown ether (B–C)). Then, the chemical structure of aforementioned AEMs were verified by means of ¹H NMR, FT-IR and UV. Meanwhile, the OH⁻ conductivity, alkaline stability and single cell performance of the synthesized membrane were also investigated. The results revealed that the conductivity of B–C_30%-P membrane is the highest at 80 °C (235 mS cm⁻¹), and the power density is also the highest (197 mW cm⁻²), and the alkali stability of the membrane synthesized in this paper was also improved. The conductivity at 80 °C was only reduced by 4%, which was obtained by immersing the B–C_30%-P membrane immersed in 6 mol L⁻¹ KOH solution for 168 h, which the aforementioned results proved that the synthesized membrane in this research had excellent OH⁻ conductivity and alkaline stability.     

Constructing micro-phase separation structure by multi-arm side chains to improve the property of anion exchange membrane

In order to improve the performance of anion exchange membrane (AEMs) as the core component of alkaline fuel cell, a novel pentamethyl-contained phenolphthalein multi-arm monomer is synthesized. The highly imidazolium-functionalized poly (arylene ether ketone) membrane (Im-PEK-x) are prepared by introducing 1,2-dimethylimidazole as hydrophilic segments. The monomer, polymer and anion exchange membranes are confirmed by ¹H NMR spectra. The well-defined micro-phase separated structure of membranes is conducive to ion transport and the structure is investigated by TEM and SAXS. The imidazolium-functionalized membranes (Im-PEK-0.8) exhibits high ionic conductivity (0.148 S/cm at 80 °C). The tensile strength of Im-PEK-0.8 membrane is 30.06 MPa. Furthermore, after immersing in 60 °C, 2 M NaOH solution for 240 h, the ionic conductivity remains 0.092 S/cm for Im-PEK-0.8. The 1,2-dimethylimidazole enhance alkaline stability by steric effect of the substituent group at the C2 position. All these results indicate that this is a new method to enhance conductivity and stability performance of AEMs.     

Fluorene-containing poly(arylene ether sulfone nitrile)s multiblock copolymers as anion exchange membranes

A series of novel fluorene-containing poly(arylene ether sulfone nitrile)s (FPESN-m/n) multiblock copolymers bearing 1,2-dimethylimidazole groups (ImFPESN-m/n) were synthesized for preparing anion exchange membranes (AEMs). Bromination rather than chloromethylation was used in this work. The bulky and rigid fluorene groups were introduced to force each chain apart to create large interchain spacing. Strong polar nitrile groups were introduced into the hydrophobic segments with the intention of enhancing the anti-swelling property of the AEMs. The length of fluorene–containing hydrophobic segment was varied to study the structure–property of the AEMs. With the ion groups anchored selectively and densely on the hydrophilic segments, all the AEMs exhibited well-defined hydrophilic/hydrophobic microphase-separated structures. As a result, the AEMs showed high hydroxide conductivities in the range of 35.2–118.3 mS cm⁻¹ from 30 to 80 °C and superb ratios of ionic conductivity to swelling at 80 °C. Furthermore, the AEMs also exhibited good mechanical properties, thermal and alkaline stabilities.     

Anion exchange membranes based on poly (ether ether ketone) containing N-spirocyclic quaternary ammonium cations in phenyl side chain

It was reported that the existence of N-spirocyclic quaternary ammonium (QA) cation could improve alkaline stability of anion exchange membrane materials (AEM). Therefore, the cyclo-quaternization reaction with pyrrolidine (Pyr) and piperidine (Pip) was carried out to prepare quaternized poly (ether ether ketone)s bearing five-membered and six-membered N-spirocyclic quaternary ammonium (QA) groups in the phenyl side chains (QPEEK-spiro-pyr and QPEEK-spiro-pip), respectively. From the transmission electron microscope, the hydrophilic-hydrophobic phase-separated morphology was formed in QPEEK-spiro membranes after incorporating N-spirocyclic QA cations and bulky spacer simultaneously in the phenyl side chain. The effect of N-spirocyclic QA groups on performance of resulted AEMs was then studied in detail. The anion conductivities of QPEEK-spiro-pyr and QPEEK-spiro-pip in OH^? form at 80 °C were 49.6 and 30.9 S cm^?1, respectively. The remaining proportions of hydroxide conductivity for QPEEK-spiro-pyr and QPEEK-spiro-pip membranes after immersing in 1 M NaOH at 60 °C were 81.0% and 74.7%, respectively, which were higher than that of 62.3% for QPEEK-TMA containing conventional QA groups in the phenyl side chain. Fuel cell assembled with QPEEK-spiro-pyr achieves a peak power density of 90 mW cm^?2. These results indicate the strategy of simultaneously introducing N-spirocyclic QA cations and bulky spacers can improve the performance of AEM to a certain extent. There are some other factors that influence the alkaline stability of the prepared AEMs, such as the existence of ether bonds in the main chain. However, this work still provides a valuable reference towards the molecular design of AEMs with improved performance.     

Sulfonated poly(arylene ether sulfone)s membranes with distinct microphase-separated morphology for PEMFCs

Copoly (arylene ether sulfone)s was employed for proton exchange membrane preparation via atom transfer radical polymerization followed by mild sulfonation, enhanced phase-separated morphology and favorable proton conductivity were achieved. The comprehensive ex-situ properties of a range of membranes with different ion exchange capacities were characterized alongside the fuel cell performances investigation. The membranes exhibit higher water uptake, which is beneficial to the proton conduction, compared to Nafion® 211 while maintaining similar swelling ratio. The prepared membranes exhibit reasonably high proton conductivity (0.16 S/cm at 85 °C) benefitting from the well-defined microstructure and high connectivity of the hydrophilic domains. Considering the comprehensive property, membrane with moderate ion exchange capacity (1.39 mmol/g) was employed to fabricate the membrane electrode assembly and peak power density of 0.65 W/cm² at 80 °C, 60% relative humidity was achieved for a H₂/O₂ fuel cell, these hydrocarbon membranes can therefore be implemented in PEMFCs.     

Block poly(arylene ether nitrile ketone)s with comb-shaped alkyl chains as anion exchange membranes

A new approach is presented here for constructing higher-performance anion exchange membranes (AEMs) by combining block-type and comb-shaped architectures. A series of quaternization fluorene-containing block poly (arylene ether nitrile ketone)s (QFPENK-m-n) were synthesized by varying the length of hydrophobic segment as AEMs. The well-designed architecture, which involved grafting comb-shaped C10 long alkyl side chains onto the block-type main chains, formed efficient ion-transport channels, as confirmed by atomic force microscopy. As a result, the AEMs showed high hydroxide conductivities in the range of 34.3–102.1 mS⋅cm⁻¹ from 30 to 80 °C at moderate ion exchange capacities (IECs). Moreover, the hydrophobic segment with nitrile groups also exhibited a profound anti-swelling property for the AEMs, resulting in ultralow swelling ratios ranging from 4.7% to 7.1% at 30 °C and 7.5%–9.8% at 80 °C, as well as superb conductivity-to-swelling ratios at 80 °C. In addition, the AEMs displayed good mechanical properties, thermal and oxidative stability, and optimizable alkaline stability.     

Highly alkaline stable fully-interpenetrating network poly(styrene-co-4-vinyl pyridine)/polyquaternium-10 anion exchange membrane without aryl ether linkages

To avoid the detrimental effect of aryl ethers on the alkali stability of anion exchange membrane (AEM), elaborately designed and synthesized poly(styrene-co-4-vinyl pyridine) (PS4VP) copolymer without aryl ether linkages is used to prepare AEMs. By introducing commercialized polyquaternium-10 (PQ-10) as the main ion-conducting molecule, the fully-interpenetrating polymer network quaternized PS4VP/PQ-10 (F-IPN QPS4VP/PQ-10) AEMs are prepared by cross-linking PS4VP and PQ-10, respectively. The as-prepared F-IPN QPS4VP/PQ-10 AEMs have obvious nanoscale microphase-separated morphologies, which ensure the membranes have good mechanical properties and dimensional stability. With optimized component ratios, F-IPN QPS4VP/PQ-10 AEM exhibits high ionic conductivity (74.29 mS/cm at 80 °C) and power density (111.83 mW/cm² at 60 °C), as well as excellent chemical stabilities (94.36% retaining of initial mass after immersion in Fenton reagent at 30 °C for 10 days, and 92.28% retaining of original ionic conductivity after immersion in 1 M NaOH solution at 60 °C for 30 days), which are greater than those of semi-interpenetrating polymer network QPS4VP/PQ-10 AEM. In summary, a combination of fully-interpenetrating polymer network and stable polymer chains and ion-conducting moieties is found to effectively overcome the trade-off between high ionic conductivity and good dimensional/chemical stabilities.     

Synthesizing spindle-shaped anion exchange membranes to improve conductivity and stability

High ionic conductivity and excellent alkaline stability are very important for solid electrolyte. Therefore, spindle-shaped anion exchange membranes (AEMs) based on poly (arylene ether ketone) and 1-Bromo-N,N,N-trimethylhexane-6-aminium bromide (Br-QA) have been prepared. The obtained Br-QA can be grafted with poly (arylene ether ketone) main chains to form micro-phase separation structure enhancing the ionic conductivity. Especially, the grafting quaternary ammonium (QA) cation groups are separated by alkyl bromine endows the AEMs with alkaline stability features. Simultaneously, the OH⁻ conductivity of the QA-PAEK-0.6 obtained membranes is 0.046 S/cm under fully hydrated conditions at 60 °C. After immersing into 1 M NaOH alkaline solution for 15 days at 60 °C, the anionic conductivity still high to 0.03 S/cm. Meanwhile, the poly (arylene ether ketone) backbones provide excellent mechanical properties and the Br-QA cation groups also possess good thermal stability, which satisfy the requirement of wide applications.     

Macromolecular crosslink of imidazole functionalized poly(vinyl alcohol) and brominated poly(phenylene oxide) for anion exchange membrane with enhanced alkaline stability and ionic conductivity

Anion exchange membranes (AEMs) are important energy conversion device for fuel cell applications, where the overall redox reaction happened. Both alkaline stability and ionic conductivity should be considered in the long-term use of fuel cells. In this work, imidazole functionalized polyvinyl alcohol was designed as the functional macromolecular crosslinking agent to fabricate crosslinked AEMs with brominated poly(phenylene oxide) matrix. Benefitting from the macromolecular crosslinked structure, the membranes displayed enhanced ionic conductivity and alkaline stability at elevated temperature. Moreover, membrane with ion exchange capacity of 1.54 mmol/g displayed ionic conductivity of 78.8 mS/cm at 80 °C, and the conductivity could maintain 75% of the initial value after immersion in 1 M NaOH solution at 80 °C for 1000 h. Moreover, a peak power density of 105 mW/cm² was achieved when the assembled single cell with c-91 was operated at 60 °C. These results indicated that the construction of macromolecular crosslinked AEMs have great potential in the practical application of anion exchange membranes fuel cells.     

Bis-pyridinium crosslinked poly(ether ether ketone) anion exchange membranes with enhancement of hydroxide conductivity and alkaline stability

A series of tunable bis-pyridinium crosslinked PEEK-BiPy-x anion exchange membranes (AEMs) are prepared successfully to improve the “trade-off” between ionic conductivity and alkaline stability. The crosslinking density of bis-pyridinium is optimized to promote microphase separation and guarantee the free volume. All the PEEK-BiPy-x membranes have a distinct microphase separation pattern observed by atomic force microscopy (AFM) and the PEEK-BiPy-x membranes also display adequate thermal, mechanical and dimensional stability. Impressively, the PEEK-BiPy-0.5 membrane exhibits maximum tensile strength (58.53 MPa) and highest IEC of 1.316 mmol·g^?1. Meanwhile, its hydroxide conductivity reaches up to 70.86 mS·cm^?1 at 80 °C. Besides, great alkaline stability of PEEK-BiPy-0.5 membrane is obtained with conductivity retention of 91.74% after 1440 h in 1 M NaOH solution, owing to the crosslinked structure of the AEMs and steric effect of bis-pyridinium cations. Overall, the PEEK-BiPy-x membranes possess potential applications in AEMs.     

Chemically modified poly(arylene ether ketone)s with pendant imidazolium groups: Anion exchange membranes for alkaline fuel cells

Anion exchange membranes (AEMs) with high stability are prepared for alkaline fuel cells using poly(arylene ether ketone)s (PAEKs) containing pendant imidazolium groups (via a direct step-growth polycondensation reaction). ¹H nuclear magnetic resonance spectroscopy (¹H NMR) and Fourier transform infrared (FT-IR) spectroscopy are used to analyze the chemical structure of the prepared PAEK membranes. The anion conductivity, water uptake and swelling ratio, thermal, mechanical, and chemical stability of these membranes are investigated for PAEK membranes with different 1-(3-aminopropyl)imidazole (API) molar ratios (PAEK-API-x) in details. The anion conductivity of PAEK-API-x membranes increases with increasing molar ratio of API. The membrane with API 1.5 equiv. displays the highest anion conductivity (0.0053–0.0531 S cm^?1 from 30 °C to 80 °C). All prepared membranes show good chemical and mechanical stability as well as thermal stability up to 250 °C. This high anion conductivity with good thermal, mechanical, and chemical stability of the membrane show potential advantage to meet the demands for AEMs.     

A facile,effective thermal crosslinking to balance stability and proton conduction for proton exchange membranes based on blend sulfonated poly(ether ether ketone)/sulfonated poly(arylene ether sulfone)

The development of hydrocarbon polymer electrolyte membranes with high proton conductivities and good stability as alternatives to perfluorosulfonic acid membranes is an ongoing research effort. A facile and effective thermal crosslinking method was carried out on the blended sulfonated poly (ether ether ketone)/poly (aryl ether sulfone) (SPEEK/SPAES) system. Two SPEEK polymers with ion exchange capacities (IECs) of 1.6 and 2.0 mmol g^?1 and one SPAES polymer (2.0 mmol g^?1) were selected to create different blends. The effect of thermal crosslinking on the fundamental properties of the membranes, especially their physicochemical stability and electrochemical performance, were investigated in detail. The homogeneous and flexible thermally-crosslinked SPEEK/SPAES membranes displayed excellent mechanical toughness (27–46 Mpa), suitable water uptake (<60%), high dimensional stability (swelling ratio < 15%) and large proton conductivity (>120 mS cm^?1) at 80 °C. The thermal crosslinking membranes also show significantly enhanced hydrolytic (<2.5%) and oxidative stability (<2%). Fuel cell with t-SPEEK/SPAES (1:2:2) membrane achieves a power density of 665 mW cm^?2 at 80 °C.     

Profile of extended chemical stability and mechanical integrity and high hydroxide ion conductivity of poly(ether imide) based membranes for anion exchange membrane fuel cells

We designed and synthesized a poly(ether imide) (PEI) membrane that has good chemical and mechanical stabilities. Alkalized PEI (A-PEI) membrane was fabricated by solution casting of chloromethylated PEI (CM-PEI) followed by quaternization and alkalization. The chemical structure of the synthesized polymers was verified by proton nuclear magnetic resonance (¹H NMR) and Fourier-transform infrared spectroscopy (FT-IR). Physiochemical properties of the membrane such as ion exchange capacity, water uptake, and swelling ratio were investigated. The membranes with a high degree of chloromethylation (DC) exhibited elevated hydroxide ion conductivity in range of 6.7–44.2 mS/cm at 90 °C under 100% relative humidity (RH). The hydrophilic-hydrophobic phase separation was verified by atomic force microscope (AFM) and small angle X-ray scattering (SAXS) measurements. Chemical stability was evaluated by measuring the durability of membranes while they were soaked in oxidative and alkaline solutions at 60 °C for 200 h.     

Crosslinked anion exchange membranes with regional intensive ion clusters prepared from quaternized branched polyethyleneimine/quaternized polysulfone

A series of anion exchange membranes (AEMs) with regionally dense ion clusters are prepared by crosslinking quaternized polysulfone (QPSU) with quaternized branched polyethyleneimine (QBPEI). For the as-prepared QPSU/QBPEI AEMs, the hydrophilic QBPEI forms locally aggregated ion clusters in the QPSU matrix, which can promote the formation of an obvious microphase separation structure in the membrane. The QPSU/QBPEI-3 AEM with an ion exchange capacity of 1.88 meq/g exhibits the best performance, achieving a reasonably high ionic conductivity of 66.14 mS/cm at 80 °C and showing good oxidation stability and alkali resistance. Finally, the maximum power density of a single H₂/O₂ fuel cell with QPSU/QBPEI-3 AEM reaches 75.34 mW/cm² at 80 °C. The above results indicate that QBPEI with a dendritic structure and abundant anionic conductive groups has a good application prospect in the preparation of AEMs with locally aggregated ion clusters and microphase separation structures.     

Comb-shaped fluorene-based poly(arylene ether sulfone nitrile) as anion exchange membrane

A series of comb-shaped fluorene-based poly (arylene ether sulfone nitrile) (CFPESN–x) was synthesized as anion exchange membranes (AEMs). The well-designed architecture of fluorene-based main chains and comb-shaped C8 long alkyl side chains containing quaternary ammonium groups was responsible for the clear microphase-separated morphologies, as confirmed by small angle X-ray scattering and atomic force microscopy. Moreover, nitrile groups on main chains also showed a profound influence on membrane morphology and properties. CFPESN–x exhibited more interconnected ionic domains with increasing the nitrile group content resulting in higher conductivities and anti-swelling property. Then CFPESN–x exhibited high ionic conductivities in the range of 27.1–91.5 mS cm⁻¹ from 30 to 80 °C and superior ratios of conductivity to swelling ratio at 80 °C at moderate IECs. Moreover, CFPESN–x also showed good mechanical properties and thermal stability, and optimizable alkaline stability and single cell performance.     

Improving the proton conductivity of sulfonated poly (ether ether ketone) membranes by incorporating a crystalline nanoassembly of trimesic acid and melamine

A novel proton exchange membrane was synthesized by embedding a crystalline which was nano-assembled through trimesic acid and melamine (TMA·M) into the matrix of the sulfonated poly (ether ether ketone) (SPEEK) to enhance the proton conductivity of the SPEEK membrane. Fourier transform infrared indicated that hydrogen bonds existed between SPEEK and TMA·M. XRD and SEM indicated that TMA·M was uniformly distributed within the matrix of SPEEK, and no phase separation occurred. Thermogravimetric analysis showed that this membrane could be applied as high temperature proton exchange membrane until 250 °C. The dimensional stability and mechanical properties of the composite membranes showed that the performance of the composite membranes is superior to that of the pristine SPEEK. Since TMA·M had a highly ordered nanostructure, and contained lots of hydrogen bonds and water molecules, the proton conductivity of the SPEEK/TMA·M-20% reached 0.00513 S cm⁻¹ at 25 °C and relative humidity 100%, which was 3 times more than the pristine SPEEK membrane, and achieved 0.00994 S cm⁻¹ at 120 °C.     

4-Aminopyridine grafted sulfonated poly(arylene ether ketone sulfone) proton exchange membrane with high relative selectivity for fuel cells

A series of sulfonated poly(arylene ether ketone sulfone)s polymer having a degree of sulfonation of 80% and a carboxyl group in the side chain (C-SPAEKS) were prepared by polycondensation. The 4-aminopyridine grafted sulfonated poly(arylene ether ketone sulfone)s polymer membranes (SPPs) were prepared by amidation reaction with the carboxyl group to immobilize 4-aminopyridine on the side chain. The ¹H NMR results and Fourier transform infrared of SPP membranes demonstrated the successful grafting of the 4-aminopyridine. Proton conductivity, water absorption, swelling ratio, and thermal stability of different proportions of SPP membranes were investigated under the different conditions. With the increase of pyridine grafting content, the methanol permeability coefficient of the membrane decreased significantly from 8.17 × 10⁻⁷ cm²s⁻¹ to 8.92 × 10⁻⁸ cm²s⁻¹ at 25 °C. And, the proton conductivity and relative selectivity of the membrane were positively correlated with the grafted pyridine content. Among them, the SPP-4 membrane exhibited the highest proton conductivity of 0.088 Scm⁻¹ at 100 °C. The relative selectivity increased from 4.73 × 10⁴ S scm⁻³ to 9.84 × 10⁴ S scm⁻³.     

Novel quaternized carbon dots modified polysulfone-based anion exchange membranes with improved performance

At present, low conductivity and poor chemical stability are still the biggest challenges in the research on anion exchange membranes (AEMs). Herein, novel nanocomposite AEMs were first constructed by introducing quaternized carbon dots (QCDs) into the imidized polysulfone matrix (Im-PSU). QCDs were synthesized by quaternization of CDs derived from citric acid and ethylenediamine. The physicochemical properties and electrochemical properties of the nanocomposite AEMs were significantly improved due to the introduction of QCDs. It was found that the QCDs can improve the ion transport channel of the nanocomposite AEMs. Compared with pure Im-PSU AEM, the OH⁻ conductivity and physicochemical properties of the nanocomposite membranes were enhanced, and the OH⁻ conductivity of ImPSU-1.0%-QCDs composite membrane can reach 109.3 mS cm⁻¹ at 80 °C, and 61.2% initial OH⁻ conductivity was maintained in 1.0 M NaOH solution for 500 h at 60 °C. Our research proves that the nanofiller with a small size can better improve the performance of composite AEMs, and provide an efficient strategy for future research work in the design and preparation of AEMs.