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.     

Preparation and characterization of fluorinated poly(aryl ether oxadiazole)s anion exchange membranes based on imidazolium salts

A series of fluorinated poly(aryl ether oxadiazole)s ionomers based on imidazolium salts (FPAEO-xMIM) were synthesized by quaternization of bromomethylated poly(aryl ether oxadiazole)s (FPAEO-xBrTM) with 1-methyl imidazole as aminating reagent. The anion exchange membranes (AEMs) were prepared by casting method and then immerged in aqueous sodium hydroxide for hydroxide ion exchanging. The structure of the obtained ionomers was characterized by ¹H-NMR and FT-IR measurements. The physical and electrochemical properties of the membranes were also investigated. The hydroxide conductivity of FPAEO-xMIM membranes was higher than 10⁻² S cm⁻¹ at room temperature, while the water uptake and swelling ratio was moderate even at elevated temperature. TGA analysis revealed that the membranes based on imidazolium salts had good thermal stability.     

Constructing an internally cross-linked structure for polysulfone to improve dimensional stability and alkaline stability of high performance anion exchange membranes

The preparation of quaternary ammonium polysulfone anion exchange membranes (AEMs) with good dimensional stability and alkaline stability is an urgent problem to solve. In response, a series of cross-linked based on polysulfone and 4, 4′-trimethylenedipiperidine (TMDP) as crosslinkers with different degrees AEMs were developed in this work through a simple process. Among the fabricated AEMs, CAPSF-5 exhibits superb alkaline stability in a 1 M KOH aqueous solution at 60 °C for 15 days, whereas the non-crosslinked APSF membrane became tremendously brittle within 24 h and could not be further studied under the same conditions. In addition, even at 60 °C, CAPSF-5 demonstrates a superior dimensional stability compared to the non-crosslinked APSF membrane due to the formation of a dense internal network structure. These observations demonstrate that crosslinked CAPSF membranes can be a viable strategy to improve the deficiency of the polysulfone backbone, especially in terms of alkaline stability.     

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.     

A review of anion exchange membranes prepared via Friedel-Crafts reaction for fuel cell and water electrolysis

Hydrogen is considered a potential, clean, and renewable energy for the future. Anion exchange membranes (AEMs) are significant components in AEM fuel cells and water electrolysis, crucial devices in the hydrogen industry. Friedel-Crafts (F–C) reaction has been widely used in preparing AEMs due to its versatility, high catalytic efficiency, relatively mild reaction conditions, etc. This review article provides a comprehensive literature survey for AEMs prepared via Friedel-Crafts reaction. Firstly, the fundamentals of the F–C reaction were introduced in detail, including the category, mechanism, catalyst and chloromethylating agent. Different types of AEMs, including polysulfones (PSUs), poly(arylene ether)s (PAEs), poly(ether ketones) (PEKs), and poly(2,6- dimethyl-1,4-phenylene oxide) (PPO), etc. were discussed. The cell performance of fuel cells and water electrolysis was investigated and analyzed. Finally, this review addresses the current challenges facing the development of AEM and proposed research implications for future investigations.     

Development of crosslinked SEBS-based anion exchange membranes for water electrolysis: Investigation of the crosslinker effect

SEBS (styrene-b-(ethylene-co-butylene)-b-styrene)) is a non-aryl-ether-type tri-block copolymer widely used as an anion exchange membrane (AEM) material due to its excellent alkaline stability and phase separation properties. However, low tensile strength due to the aliphatic chains and the poor physical properties of the SEBS-based membranes limit their practical application for AEM water electrolysis (AEMWE) or AEM fuel cell (AEMFC). In this study, three types of crosslinked AEMs were prepared using bromohexyl pentafluorobenzyl SEBS as a polymer backbone, and three different crosslinkers, dimethyl amine (DMA), tetramethyl diaminohexane (TMHA), and tris(dimethyl aminomethyl) phenol (TDMAP). Once introduced, these crosslinking agents were converted into the corresponding conducting head groups. The thermal, chemical, physical, and electrical properties of the obtained crosslinked membranes were then investigated for use in AEMWE. In particular, the TDMAP-50x-SEBS membrane with 50% degree of crosslinking experienced hydrogen bonding with water and OH⁻ due to the presence of OH groups in the structure of the crosslinking agent (TDMAP). Because of this, the membrane showed an improved morphology and high conductivity (20 °C: 31.8 mS cm⁻¹, 80 °C: 109.9 mS cm⁻¹). In addition, TDMAP induced physical crosslinking by hydrogen bonding between molecules so that the corresponding membrane (TDMAP-50x-SEBS) exhibited high alkaline and oxidative stability and good mechanical properties. This SEBS-based membrane has a tensile strength of 18.0 MPa and Young's modulus of 165.14 MPa. The WE single-cell test (1 M KOH solution at 70 °C) using TDMAP-50x-SEBS also showed a cell performance of 1190 mA cm⁻² at 2.0 V. This is 126% higher than the cell performance measured for FAA-3-50, a commercialized AEM material, under the same conditions.     

A novel anion exchange membrane based on poly (2,6-dimethyl-1,4-phenylene oxide) with excellent alkaline stability for AEMFC

We report a novel comb-shaped anion exchange membrane based on poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) and 4-(dimethylamino)butyraldehyde diethyl acetal (DABDA). The Menshutkin reaction successfully introduced DABDA into the brominated PPO backbone, which was proved through FT-IR and ¹H NMR. The distinct hydrophilic/hydrophobic microphase separation structure observed by transmission electron microscope (TEM). As the increase of grafting degree, so does the water uptake, swelling ratio, hydroxide conductivity and alkaline stability. The membranes also possess good mechanical property with tensile strength from 14.2 to 38.11 MPa. This is due to the increasing number of hydroxide and unique steric hindrance effect caused the higher water uptake and higher dimensional stability. Simultaneously, especially PPO/DABDA-60 in comb-shaped membranes demonstrate an excellent long-term alkali resistance stability. In a 576-h alkali resistance stability test, the retained ionic conductivity of the PPO/DABDA-60 membrane is 96% of the initial value. The PPO/DABDA-60 is a potential candidate material for AEM.     

Comparative study of anion exchange membranes for low-cost water electrolysis

Various anion-exchange membranes (AEMs) were studied in the electrolysis cell using the non-precious metal-based catalysts showing the good potential of selected AEMs in low-cost water electrolysis application. The 0.1–1 M potassium hydroxide electrolyte is applied for high performance electrolysis process, whereas the usage of pure water leads to the significant AEM resistance increase. The post mortem MEA analysis, using SEM is performed to study the structure and morphology of catalyst layers transferred from the electrodes prepared by catalyst coated substrate approach. The importance of the catalyst layer–membrane interface and the binder used to bond the catalyst layer is discussed. AEM electrolysis safety aspect in terms of the hydrogen crossover through the 28 μm thin A-201 membrane is studied. The linear dependency of the permeated hydrogen flux on current density is shown. Hydrogen content in the anode outlet gas is less enough to ensure high safety of the AEM electrolysis technology in the operating currents range.     

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.     

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.     

Operational parameters correlated with the long-term stability of anion exchange membrane water electrolyzers

In this study, we investigated the long-term stability of anion exchange membrane water electrolyzers (AEMWEs) under various bias conditions. The cell performance was relatively stable under conditions of voltage cycling in a narrow range, constant voltage and constant current. On the other hand, a relatively dynamic condition, voltage cycling, in a wide range detrimentally affected the cell stability. Abnormally high negative and positive currents were observed when the cell voltage was switched between 2.1 and 0 V. Impedance results and post-material analyses indicated that the performance degradation was mainly due to anode catalyst detachments, which increased non-ohmic resistance in the wide range voltage cycling. An increase in ohmic resistance was also observed, which was due to the membrane dehydration that occurred in the frequent rest times. Thus, it can be said that the voltage cycling range as well as the frequency of rest times are critical operational parameters in determining the long-term stability of AEMWEs.     

Quaternary ammonium-functionalized poly(ether sulfone ketone) anion exchange membranes: The effect of block ratios

Anion exchange membranes based on quaternary ammonium-functionalized poly(ether sulfone ketone) block copolymers (QA-PESK) with various hydrophilic–hydrophobic oligomer block ratios (10:7, 10:18, and 10:26) were synthesized, and the block length effect on the membranes' physicochemical and electrical properties were systematically investigated. The QA-PESK-10-18 membrane, prepared using a hydrophilic and hydrophobic block ratio of 10:18, displayed well-balanced hydrophilic/hydrophobic phase separation, the highest conductivity of 23.19 mS cm⁻¹ at 20 °C and 57.84 mS cm⁻¹ at 80 °C, and the highest alkaline stability among the three block ratios tested, indicating that the membranes' properties were closely related to their morphologies, which were determined by the hydrophilic/hydrophobic ratio of the block copolymer. The H₂/O₂ single cell performance using the QA-PESK-10-18 revealed a maximum power density of 235 mW cm⁻².     

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.     

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.     

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.     

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.     

Self-crosslinked anion exchange membranes by bromination of benzylmethyl-containing poly(sulfone)s for direct methanol fuel cells

A series of novel anion exchange membranes based on poly(arylene ether sulfone) were fabricated. And the synthesized 1, 1, 2, 3, 3-pentamethylguanidine was used as a hydrophilic group. Bromination reaction rather than chloromethylation was used for the preparation of target conductive polymers. Fourier transform infrared spectroscopy (FTIR), ¹H NMR and mass spectrometry (MS) were used to characterize the as-synthesized polymers. The ratio of hydrophilic to hydrophobic monomers was varied to study the structure-property of the membranes. The performance of the membrane with both hydrophilic/hydrophobic segments was improved over the membrane with sole hydrophilic segments. The self-crosslinking structure of the as-prepared membranes is partly responsible for their very low methanol permeability with the minimum of 1.02 × 10⁻⁹ cm⁻²⋅S⁻¹ at 30 °C and insolubility in organic solvents considered. The structural dependence of water uptake is in the range of 25–87%. The as-prepared membranes did not suffer from serious membrane swelling. The ionic exchange capacity (IEC) reached a maximum of 1.21 mmol⋅g⁻¹. The ionic conductivity of the membrane in deionized water is 6.00 and 13.00 × 10⁻² S⋅cm⁻¹ at 30 and 80 °C respectively.     

Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers

This paper reports the performance of a graphene oxide modified non noble metal based electrode in alkaline anion exchange water electrolyzer. The electrolytic cell was fabricated using a polystyrene based anion exchange membrane and a ternary alloy electrode of Ni as cathode and oxidized Ni electrode coated with graphene oxide as anode. The electrochemical activity of the graphene oxide modified electrode was higher than the uncoated electrode. The anion exchange membrane water electrolyzer (AEMWE) with the modified electrode gave 50% higher current density at 30 °C with deionised water compared to that of an uncoated electrode at 2 V. Performance was found to increase with increase in temperature and with the use of alkaline solutions. The results of the solid state water electrolysis cell are promising method of producing low cost hydrogen.     

Anion exchange membranes of bis-imidazolium cation crosslinked poly(2,6-dimethyl-1,4- phenylene oxide) with enhanced alkaline stability

Partially crosslinked anion exchange membranes (AEMs) with imidazolium-based cationic functionalities were fabricated based on a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) matrix. The PPO was activated by bromomethylation and functionalized with methylimidazole and 1,4-bis(imidazolyl)butane at different ratios through a gentle and facile heat curing method. The use of 1,4-bis(imidazolyl)butane resulted in a membrane with cationic functionalities incorporated in covalent crosslinks, which allowed for high ion exchange capacities (IECs) without compromising on mechanical robustness. Comprehensive characterizations were performed in terms of thermal stability, water uptake, IEC, swelling, conductivity, mechanical properties and alkaline stability to investigate the correlation of the structure and physicochemical properties. Comparing with the un-crosslinked imidazolium PPO membrane, crosslinked membranes exhibited improved mechanical robustness and alkaline stabilities. The membrane with a crosslinking degree of 10% displayed an IEC of around 1.5 mmol g⁻¹, tensile strength of 4.1 MPa, hydroxide ion conductivity of 40.5 mS cm⁻¹, and a retained ratio in conductivity of 40% after tolerance test of nearly 150 h in 1 mol L⁻¹ KOH (aq.) at 60 °C.     

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.