Dense 1,2,4,5-tetramethylimidazolium-functionlized anion exchange membranes based on poly(aryl ether sulfone)s with high alkaline stability for water electrolysis

Anion exchange membranes with enough alkaline stability and ionic conductivity are essential for water electrolysis. In this work, a class of anion exchange membranes (PAES-TMI-x) with dense 1,2,4,5-tetramethylimidazolium side chains based on poly(aryl ether sulfone)s are prepared by aromatic nucleophilic polycondensation, radical substitution and Menshutkin reaction. Their chemical structure and hydrophilic/hydrophobic phase morphology are characterized by hydrogen nuclear magnetic resonance (¹H NMR) and atomic force microscope (AFM), respectively. The water uptake, swelling ratio and ionic conductivity for PAES-TMI-x are in the range of 23.8%–48.3%, 8.3%–14.3% and 18.22–96.31 mS/cm, respectively. These AEMs exhibit high alkaline stability, and the ionic conductivity for PAES-TMI-0.25 remains 86.8% after soaking in 2 M NaOH solution at 80 °C for 480 h. The current density of 1205 mA/cm² is obtained for the water electrolyzer equipped with PAES-TMI-0.25 in 2 M NaOH solution at 2.0 V and 80 °C, and the electrolyzer also has good operation stability at current density of 500 mA/cm². This work is expected to provide a valuable reference for the selection and design of cations in high-performance AEMs for water electrolysis.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

End-group crosslinked hexafluorobenzene contained anion exchange membranes

A kind of anion exchange membranes (AEMs) with CC bond end-group crosslinked structure was synthesized successfully. Unlike the traditional aliphatic AEMs, the AEMs were prepared in this work by a strategy to realize the CC bond thermal end-group crosslinking reaction, exhibiting an obvious microphase separation structure and a suitable dimensional stability. The well-defined ion channels constructed in the AEMs guarantee the fast OH⁻ conduction, as confirmed via physical and chemical characterization. The conductivity was dramatically enhanced due to the effective ion channels and increased ion exchange capacity. Among the as-prepared AEMs, the PHFB-VBC-DQ-80% AEM has a conductivity of 135.80 mS cm⁻¹ at 80 °C. The single cell based on PHFB-VBC-DQ-80% can achieve a power density of 141.7 mW cm⁻² at a current density of 260 mA cm⁻² at 80 °C. The AEMs show good thermal stability verified by a thermogravimetric analyzer (TGA). Furthermore, the ionic conductivity of PHFB-VBC-DQ-80% only decreased by 7.1% after being soaked in a 2 M NaOH solution at 80 °C for 500 h.     

Imidazolium functionalized block copolymer anion exchange membrane with enhanced hydroxide conductivity and alkaline stability via tailoring side chains

To develop polymer electrolyte membrane with both high hydroxide conductivity and good alkaline stability, series of poly(arylene ether sulfone)s block copolymers bearing varied imidazolium functionalized aromatic pendants are synthesized, and the relationship between ionic pendants and the membrane properties are investigated and discussed. Atomic force microscopy (AFM) results suggest that, the well-controlled block copolymers and pendent aromatic chain structures are responsible for the formation of the well-defined microphase-separated morphology which is benefit to construct highly conductive ionic transport channels in membrane. The membranes tethering longer imidazolium functionalized aromatic pendants (Im-DFDM-bPES) exhibit large hydroxide conductivity than those bearing shorter ones (Im-DFDB-bPES) in spite of their comparable IEC values, this is in accordance with their sizes of hydrophilic domains in membrane. Among the membranes, Im-DFDM-bPES-x7y32 with IEC of 1.30 mequiv g^?1 gives the highest hydroxide conductivity (34.2 and 98.7 mS cm^?1 at 25 and 80 °C, respectively). Besides, both Im-DFDM-bPES and Im-DFDB-bPES membranes exhibit high alkaline stability after aging under severe conditions (4 M NaOH at 80 °C) for 144 h, where the aged Im-DFDM-bPES and Im-DFDB-bPES give hydroxide conductivity remaining by 74.8%–77.2% and 64.5%–66.4%, mechanical properties with maximum stress of 47.36–51.30 MPa and 60.03–62.28 MPa, respectively, indicating good chemical stability of both imidazolium moiety and block copolymer backbone.     

Construction of ion transport channels by grafting flexible alkyl imidazolium chain into functional poly(arylene ether ketone sulfone) as anion exchange membranes

A new series of imidazolium-functionalized anion exchange membranes (AEMs), based on poly (arylene ether ketone sulfone) containing pendant amino groups (Am-PAEKS) have been prepared. The structure of the copolymers is characterized by FT-IR and ¹H NMR spectra. The properties of the imidazolium-functionalized Am-PAEKS (Im-Am-PAEKS) including ionic conductivity, dimensional stability, thermal stability, fuel cell performance and mechanical property are investigated thoroughly. The hydroxide conductivities of the prepared membranes are in the range of 1.1 × 10^?2–13.9 × 10^?2 S cm^?1 (20–80 °C). The membranes exhibit excellent alkaline stability including high thermal stability and mechanical property after soaking in 2 M NaOH aqueous solution for 300 h. This study indicates that the imidazolium-functionalized membranes containing pedant amino groups have the potential to be applied in alkaline 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.     

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.     

Effect of hydroxide and carbonate alkaline media on anion exchange membranes

The effect of hydroxide and carbonate alkaline environments on the chemical stability and ionic conductivity of five commercially available anion exchange membranes was investigated. Exposure of the membranes to concentrated hydroxide environments (1 M) had a detrimental effect on ionic conductivity with time. Over a 30-day period, decreases in conductivity ranged from 27% to 6%, depending on the membrane. The decrease in ionic conductivity is attributed to the loss of stationary cationic sites due to the Hofmann elimination and nucleophilic displacement mechanisms. Exposure of the membranes to low concentration hydroxide (10⁻⁴ M) or carbonate/bicarbonate (0.5 M Na₂CO₃/0.5 M NaHCO₃) environments had no measurable effect on the ionic conductivity over a 30-day period. ATR-FTIR spectroscopy confirmed degradation of membranes soaked in 1 M KOH. Apparition of a doublet peak in the region between 1600 cm⁻¹ and 1675 cm⁻¹ confirms formation of carbon-carbon double bonds due to Hofmann elimination. Membranes soaked in mild alkaline environments did not show formation of carbon-carbon double bonds.     

Preparation and alkaline stability of polyethylene composite hydroxide exchange membranes with different cations

Hydroxide exchange membrane fuel cells (HEMFCs) are a cost-effective alternative to proton exchange membrane fuel cells due to the advantages on fast oxygen reduction reaction kinetics, low cost and weak corrosion. Hydroxide exchange membrane (HEM) is the core component for HEMFC. At present, lacking highly robust and chemically stable HEMs is the main challenge to realizing durable HEMFCs. Polyethylene composite HEMs are considered as the-state-of-the-art HEM candidates due to their excellent mechanical properties, thus allowing for the use of ultrathin HEMs in HEMFCs to achieve a high cell and stack performance. Yet, the chemical stability of polyethylene composite HEM still needs to be improved for more durable HEMFC. In this study, we report that the preparation and chemical stability understanding of three polyethylene composite HEMs (named AEH-TMA, AEH-DMP and AEH-TMI) functionalized with trimethylamine (TMA), N-methylpiperidine (DMP) and 1, 2, 4, 5-tetramethylimidazole (TMI), respectively. We find that AEH-TMI exhibits a low conductivity and inferior chemical stability in comparison to AEH-TMA and AEH-DMP. To gain insight into chemical stability behavior, we have also prepared three organic chemicals (named Bn-TMA, Bn-DMP and Bn-TMI) as cations model by reacting benzyl chloride with TMA, DMP and TMI cations and elaborately studied their chemical stability in various water content systems. The results show that the overall chemical stability of cations can be strongly improved with increasing the surrounding water, and Bn-TMI suggests a lower chemical stability than Bn-TMA and Bn-DMP at high water content condition.     

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.