Poly(arylene ether sulfone) with tetra(quaternary ammonium) moiety in the polymer repeating unit for application in solid alkaline exchange membrane fuel cells

Poly(arylene ether sulfone)s with tetra(quaternary ammonium) hydroxide groups in the repeating unit of the polymer main chain were prepared for solid alkaline exchange membrane fuel cells. We synthesized a novel monomer with four amine groups which were derivatized to ammonium functional groups. Using this monomer, we easily synthesized poly(arylene ether sulfone)s which conduct hydroxyl ions. This direct synthetic method can control the exact amount of quaternary ammonium hydroxide groups in the polymer structure. The polymers were characterized by NMR, thermogravimetric analysis, water uptake, conductivity, and cell performance. In a solid alkaline exchange membrane fuel cell test, a maximum power output of 77 mW cm⁻² was achieved using hydrogen and oxygen.     

N-spirocyclic ammonium-functionalized graphene oxide-based anion exchange membrane for fuel cells

Graphene oxide (GO) is a potential material in the electrode and membrane of polymer electrolyte membrane fuel cells due to its unique structure and various oxygen-containing functional groups. A class of three-layered GO/poly (phenylene oxide) for AEMs was prepared in this work. GO was functionalized with highly stable 6-azonia-spiro [5.5]undecane groups and used as a fast hydroxide conductor, named ASU-GO. Functionalized by N-spirocyclic cations, poly (phenylene oxide) (PIPPO) was then combined with ASU-GO and GO to fabricate the ASU-GO/PIPPO and GO/PIPPO. Notably, the maximum hydroxide conductivity of the ASU-GO/PIPPO was 73.7 mS cm⁻¹ at 80 °C, which was 3 times higher than that of the GO/PIPPO. The enhancement in hydroxide conductivity was due to the changes in the hydroxide transport mechanism and the poor stacked structure of the ASU-GO layer. Only 10.8% drops in hydroxide conductivity of ASU-GP/PIPPO after the alkaline test (1 M KOH at 80 °C for 700 h). Furthermore, the ASU-GO/PIPPO-50 membrane showed a maximum peak power density of 102 mW cm⁻², demonstrating the prepared membrane was promising in the AEM applications.     

Homologous flexible multi-cationic cross-linkers modified poly(aryl ether sulfone) anion exchange membranes for fuel cell applications

Issues of unsatisfactory ionic conductivity and chemical stability should be solved for anion exchange membranes (AEMs) in practical fuel cell applications. A series of flexible spacers, homologous multi-cationic cross-linkers with different lengths, were designed and synthesized from 1,4-diazabicyclo(2.2.2)octane (DABCO) and 1,6-dibromohexane, and subsequently used to fabricate a series of flexible multi-cationic cross-linked poly(aryl ether sulfone) (CQPAES) AEMs. The CQPAES membranes fabricated by simultaneous cross-linking and membrane formation are tough and pliable. The length and number of cations of the cross-linkers show noticeable effects on the comprehensive membrane performance. The CQPAES membranes display more distinct nano-phase separation morphology and well-developed ion transfer channels due to the higher mobility and hydrophilicity of the flexible long-chain multi-cationic segments. As a result, the CQPAES membranes exhibit gradually increased water uptake and ionic conductivity with the increase of cross-linker length. Furthermore, DABCO and hexyl segments in the cross-linker greatly enhance the steric hindrance and electron cloud density of the quaternary ammonium groups, which inhibit the Hoffmann elimination reaction. Consequently, the CQPAES membranes display a significant improvement in alkali stability than conventional benzyl-substituted quaternary ammonium group type AEMs.     

Well-designed mono- and di-functionalized comb-shaped poly(2,6-dimethylphenylene oxide) based alkaline stable anion exchange membrane for fuel cells

To mitigate the membrane stabilities (dimensional and mechanical) and ionic properties, we report well functionalized hydrophilic and hydrophobic (responsible for stabilities) phase separated quaternized anion exchange membrane (AEM). The N,N,N,N-Tetramethyl-1,3-propanediamine quaternized AEM spliced with alkyl chain (DQCP-36) formed self-amassed morphology with larger ionic clusters. The most suitable optimized AEM (DQCP-36) demonstrated enhanced hydroxide ion conductivity (4.66 × 10^?2 S cm^?1), ion-exchange capacity (1.35 meq./g) and lower activation energy (11.52 kJ/mol). These AEMs showed self-amassed morphology (well balanced hydrophilic and hydrophobic domain) and excellent stabilities (thermal, alkaline and dimensional). Under harsh alkaline medium (2 M NaOH) at 60 °C, DQCP-36 AEM showed about 9% reduction in conductivity after 700 h treatment, and assessment to be a suitable candidate for alkaline fuel cells.     

Novel poly(carbazole-butanedione) anion exchange membranes constructed by obvious microphase separation for fuel cells

To develop anion exchange membranes with excellent chemical stability and high performance. A series of quaternary ammonium functionalized (hydrophilic) hydrophobic rigid poly (carbazole-butanedione) (HOCB-TMA-x) anion exchange membranes were prepared, where x represents the percentage content of hydrophobic unit octylcarbazole (OCB). Due to the introduction of hydrophobic rigid unit octylcarbazole and hexyl flexible side chain, the hydrophilic-hydrophobic microstructure of AEMs was developed. The AEMs exhibit excellent overall performance, specifically the low swelling ratio HOCB-TMA-30 membrane exhibits the highest OH^? conductivity of 152.9 mS/cm at 80 °C. Furthermore, the ionic conductivity of AEM decreased by only 9.5% after 2250 h of immersion in 1 M NaOH. The maximum peak power density of a single cell with a current density of 4.38 A/cm² at 80 °C was 1.85 W/cm².     

Sulfonated poly(arylene ether)s based proton exchange membranes for fuel cells

During the past decade proton exchange membrane fuel cells (PEMFCs) as one kind of the potential clean energy sources for electric vehicles and portable electronic devices are attracting more and more attentions. Although Nafion® membranes are considered as the benchmark of proton exchange membranes (PEMs), the drawbacks of Nafion® membranes restrict the commercialization in the practical application of PEMFCs. As of today, the attention is to focus on developing both high-performance and low-cost PEMs to replace Nafion® membranes. In all of these PEMs, sulfonated poly(arylene ether ketone)s (SPAEKs) and sulfonated poly(arylene ether sulfone)s (SPAESs) are the most promising candidates due to their excellent performance and low price. In this review, the efforts of SPAEK and SPAES membranes are classified and introduced according to the chemical compositions, the microstructures and configurations, as well as the composites with polymers and/or inorganic fillers. Specifically, several perspectives related to the modification and composition of SPAESs and SPAEKs are proposed, aiming to provide the development progress and the promising research directions in this field.     

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.     

Anion conductive poly(tetraphenyl phthalazine ether sulfone) containing tetra quaternary ammonium hydroxide for alkaline fuel cell application

The poly(tetraphenyl ether ketone sulfone)s (PTPEKSs) were synthesized from 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenyl benzene (BFBTPB) and bis(4-fluorophenyl) sulfone with bis(4-hydroxydiphenyl) sulfone in sulfolane. The synthesis of poly(tetraphenyl phthalazine ether sulfone)s (PTPPESs) was carried out via an intramolecular ring-closure reaction of dibenzoylbenzene moiety with hydrazine monohydrate. The PTPPES-QAHs [poly(tetraphenyl phthalazine ether sulfone-quaternary ammonium hydroxide)]s were synthesized via chloromethylation of PTPPES, quaternization with trimethylamine, and followed by an anion exchange of tetra-quaternary ammonium chloride polymers with KOH. Different contents of quaternized unit in PTPPES-QAH (15, 20, 25 mol% of BFBTPB) were studied by FT-IR, ¹H NMR spectroscopy, and thermogravimetric analysis (TGA). Sorption experiments were conducted to observe the interaction of quaternized polymers with water. The ion exchange capacity (IEC), ion conductivity and cell performance of PTPPES-QAH were evaluated with increasing the degree of quaternization.     

Preparation and characterization of imidazolium-functionalized poly (ether sulfone) as anion exchange membrane and ionomer for fuel cell application

Imidazolium-functionalized anion exchange membranes (AEMs) for anion exchange membrane fuel cells (AEMFCs) were synthesized by functionalization of chloromethylated poly (ether sulfone) (PES) with 1-alkylimidazole. The properties of AEMs can be controlled by the degree of chloromethylation of PES. Moreover, with the increment of the alkyl line length on the imidazolium group, the water uptake, swelling ratio and solubility of AEMs increased, whereas the hydroxide conductivity declined. By dissolving AEMs in the mixture of ethanol and water, IM-based anion exchange ionomers (AEIs) can be obtained. Electrochemical studies revealed that the catalytic activities of Pt/C towards oxygen reduction and hydrogen oxidation in the presence of imidazolium-functionalized AEIs were almost the same with that of commercial quaternary ammonium-based ionomers. The fabricated AEM and AEI were utilized to assemble H₂/O₂ AEMFC, yielding a peak power density of ∼30 mW cm⁻² with open circuit potential larger than 1.0 V. The results obtained indicate that imidazolium-functionalized AEMs and AEIs may be candidates which are worth further investigation for the application in the AEMFCs.     

Facile preparation of poly (2,6-dimethyl-1,4-phenylene oxide)-based anion exchange membranes with improved alkaline stability

Traditional quaternary ammonium (QA)-type anion exchange membranes (AEMs) usually exhibit insufficient alkaline stability, which impede their practical application in fuel cells. To address this issue, a facile method for the simple and accessible preparation of QA-type AEMs with improved alkaline stability was developed in this study. A series of novel AEMs (QPPO-xx-OH) were prepared from commercially available poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) and 3-(dimethylamino)-2,2-dimethyl-1-propanol, via a three-step procedure that included bromination, quaternization, and ion exchange. Scanning electron microscopy revealed that dense, uniform membranes were formed. These QPPO-xx-OH membranes exhibited moderate hydroxide conductivities with ranges of 4–15 mS/cm at 30 °C and 12–36 mS/cm at 80 °C. When tested at similar ion exchange capacity (IEC) levels, the IEC retentions of QPPO-xx-OH membranes were 16–22% higher than that of traditional PPO membranes containing benzyltriethylammonium ions, after immersed in 2 M NaOH aqueous solution at 60 °C for 480 h, and the QPPO-47-OH membrane displayed excellent alkaline stability with the IEC retention of 92% after 480 h. In addition, the QPPO-xx-OH membranes also exhibited robust mechanical properties (tensile strength up to 37.6 MPa) and good thermal stability (onset decomposition temperature up to 150 °C). This study provides a new and scalable method for the facile preparation of AEMs with improved alkaline stability.     

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.     

Anionic quaternary ammonium fluorous copolymers bearing thermo-responsive grafts for fuel cells

Fluorous copolymers are synthesized by grafting quaternary ammonium-functionalized 4-vinylbenzyl chloride (QVBC) and N-Isopropyl acrylamide (NIPAAM) from poly(vinylidene fluoride) (PVDF) via atom transfer radical polymerization (ATRP). Ionic PQVBC and thermo-responsive PNIPAAM grafts are characterized by proton nuclear magnetic resonance (¹H NMR) spectroscopy. Polarity difference between flexible grafts and rigid fluorous backbones allows convenient formation of ionic clusters, which impart resultant membranes excellent hydroxide conductivities in the range of 21–52 mS cm⁻¹ at 20 °C, and up to 98 mS cm⁻¹ at 70 °C. Moreover, amide–amide interactions between thermo-responsive grafts at high temperature can mitigate water swelling of hydrophilic clusters. This study suggests that the fluorous copolymer bearing both ionic and thermo-responsive grafts holds a promising selectivity as novel materials for anion exchange membrane with enhanced hydroxide conductivity and controlled water swelling.     

Comparison of alkaline fuel cell membranes of random & block poly(arylene ether sulfone) copolymers containing tetra quaternary ammonium hydroxides

A series of modified anion conductive block poly(arylene ether sulfone) copolymer membranes containing a selective substituted unit, 15%, 20% and 25% 4,4′-(2,2-diphenylethenylidene) diphenol, were prepared for use in alkaline fuel cells. The anion exchange membranes were synthesized by first introducing chloromethyl groups. Quaternary ammonium groups could then be added to the tetra-phenyl ethylene units, followed by subsequent ion exchange. The tetra quaternary ammonium hydroxide polymers showed high molecular weights and exhibited high solubility in polar aprotic solvents. The block copolymer membrane showed higher ionic conductivity (21.37 mS cm⁻¹) than the random polymer membrane of similar composition (17.91 mS cm⁻¹). The membranes showed good chemical stability in 1.0 M KOH solution at 60 °C. They were characterized by ¹H NMR, FT-IR, TGA and measurements of ion exchange capacity, water uptake and ionic conductivity.     

Synthesis and characterization of long-side-chain type quaternary ammonium-functionalized poly (ether ether ketone) anion exchange membranes

A series of quaternary ammonium salt poly(ether ether ketone) AEMs containing long ether substituents are successfully prepared, and their chemical structure is confirmed by ¹H NMR and FT-IR. The distinct microphase separation morphology of AEMs is observed by TEM. As the content of methylhydroquinone increases, the ion conductivity of AEMs gradually increases. When the content of methylhydroquinone increases to 80%, the hydroxide conductivity of PEEK-DABDA-80 membrane reaches 0.052 S/cm at 80 °C. Meanwhile, it exhibits excellent mechanical properties and anti-swelling ability, with tensile strength of 25 MPa, elongation at break of 8.12% and swelling ratio is only 17.4% at 80 °C. And AEMs also display the better thermal stability. After soaked in 1 M NaOH at 60 °C for 30 days, PEEK-DABDA-80 membrane shows acceptable ion conductivity of 0.021 S/cm at 60 °C. In view of these properties, PEEK-DABDA-x AEMs may display potential application as alkaline AEMs.     

Nafion-assisted cross-linking of sulfonated poly(arylene ether ketone) bearing carboxylic acid groups and their composite membranes for fuel cells

In this study, a new type of cross-linked composite membrane is prepared and considered for its potential applications in direct methanol fuel cell. Nafion and sulfonated poly(arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) are blended and subsequently cross-linked by a Friedel-Craft reaction using the carboxylic acid groups in the SPAEK-C to achieve lower methanol permeability. The perfluoroalkyl sulfonic acid groups of Nafion act as a benign solid catalyst, which assist the cross-linking of SPAEK-C. The physical and chemical characterizations of the cross-linked composite membranes are performed by varying the contents of SPAEK-C. The c-Nafion-15% membrane exhibits appropriate water uptake (10.49-25.22%), low methanol permeability (2.57 × 10⁻⁷ cm² s⁻¹), and high proton conductivity (0.179 S cm⁻¹ at 80 °C). DSC and FTIR analyze suggest the cross-linking reaction. These results show that the self-cross-linking of SPAEK-C in the Nafion membrane can effectively reduce methanol permeability while maintaining high proton conductivity.     

Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications

New anion exchange membranes (AEMs) with high conductivity, good dimensional and alkaline stability are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investigated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal stability, sufficient mechanical strength, the membrane which containing 30% mass fraction of [DMVIm][Cl] shows conductivity up to 1.0 × 10^?2 S cm^?1 at room temperature and good long-term alkaline stability in 1 M KOH solution at 80 °C. The results of this study suggest that this type of AEMs have good perspectives for alkaline anion exchange membrane fuel cell applications.     

Synthesis of multi-block poly(arylene ether sulfone) copolymer membrane with pendant quaternary ammonium groups for alkaline fuel cell

A series of multi-block poly(arylene ether sulfone)s are synthesized via the copolymerization of bis(4-hydroxyphenol) sulfone, 3,3′, 5,5′-tetramethylbiphenol and 4,4′-difluorodiphenyl sulfone. The resulting multi-block copolymers are brominated by using N-bromosuccinmide (NBS) as bromination reagent. The bromomethylated copolymer is solution cast to form clear, creasable films, and subsequent soaking of these films in aqueous trimethylamine to give benzyltrimethylammonium groups. The anion exchange membranes obtained by the solution hydroxide exchange with aqueous sodium hydroxide show varying degrees of ionic conductivity depending on their ion exchange capacity. The highest hydroxide conductivity 0.029 S cm⁻¹ is achieved with the QBPES-40 membrane having IEC value of 1.62 mequiv g⁻¹ at room temperature and 100% RH. The obtained anion exchange membranes also have good mechanical properties and dimensional stability, which greatly facilitates the preparation of a MEA and the cell operation.     

Fluorene-containing poly(arylene ether sulfone)s with imidazolium on flexible side chains for anion exchange membranes

Anion exchange membranes with high ionic conductivity and dimensional stability attract a lot of research interests. In present study, a series of fluorene-containing poly(arylene ether sulfone)s containing imidazolium on the flexible long side-chain are synthesized via copolycondensation, Friedel-Crafts reaction, ketone reduction, and Menshutkin reaction sequentially. The membranes used for characterization and membrane electrode assembly are obtained by solution casting and ion exchange thereafter. The morphology of the membranes is studied via transmission electron microscopy, and the microphase separation is observed. The long side-chain structure is responsible for the distinct hydrophilic-hydrophobic microphase separation, which facilitates the transport of hydroxide ions in the membranes. The incorporation of imidazolium on the flexible long side-chain is favorable for the ionic aggregation and transport in the membranes. The resulted membranes exhibit high hydroxide conductivities in the range of 48.5–83.1 mS cm⁻¹ at 80 °C. All these membranes show good dimensional stability and thermal stability. The single cell performance shows a power density of 102.3 mW cm⁻² at 60 °C using membrane electrode assembly based-on one of the synthesized polymers.     

Synthesis and characterization of cross-linked quaternized poly(vinyl alcohol)/chitosan composite anion exchange membranes for fuel cells

Novel cross-linked composite membranes were synthesized to investigate their applicability in anion exchange membrane fuel cells. These membranes consist of quaternized poly(vinyl alcohol) (QAPVA) and quaternized chitosan (2-hydroxypropyltrimethyl ammonium chloride chitosan, HACC) with glutaraldehyde as the cross-linking reagent. The membranes were characterized in term of their water content, ion exchange capacity (IEC), ion conductivity and methanol permeability. FTIR, X-ray diffraction and scanning electron microscopy (SEM) were also used to investigate the relation between the structure and performance of the composite membranes. The composite membranes have a high conductivity (10⁻³ to 10⁻² S cm⁻¹), and a low methanol permeability (from 5.68 × 10⁻⁷ to 4.42 × 10⁻⁶ cm² s⁻¹) at 30 °C. After reviewing all pertinent characteristics of the membranes, we find that the membrane structure is the principal factor affecting the conductivity and methanol permeability of these membranes.     

Novel sulfonated poly(arylene ether ketone) copolymers bearing carboxylic or benzimidazole pendant groups for proton exchange membranes

A novel strategy in which the benzimidazole group and sulfonic group are simultaneously attached to an aromatic polymer has been reported in this paper. For this purpose, sulfonated poly(arylene ether ketone) copolymers containing carboxylic acid groups (SPAEK-x-COOH, x refers to the molar percentage of sulfonated repeating units) are prepared by the aromatic nucleophilic polycondensation of sodium 5,5′-carbonyl-bis(2-fluobenzene-sulfonate) (SDFBP), 4,4′-difluorobenzophenone (DFBP) and phenolphthalin (PPL). Then the carboxylic acid groups attached to the SPAEK-x-COOH are transformed to benzimidazole units through condensation reactions (referred to as SPAEK-x-BI). Fourier transform infrared spectroscopy and ¹H NMR measurements are used to characterize and confirm the structures of these copolymers. SPAEK-x-COOH membranes exhibit superior mechanical properties with maximum elongations at break up to 133%, meanwhile SPAEK-x-BI also shows good thermal and mechanical stability. The proton conductivity, swelling ratio and methanol permeability of the polymers with benzimidazole are lower than those with carboxylic groups, which indicated that there is an acid-base complex between benzimidazole and sulfonic acid groups. A balance of proton conductivity, methanol permeability, thermal and mechanical stabilities can be designed by incorporation of functional groups to meet the requirements for the applications in direct methanol fuel cells.