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.     

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.     

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.     

Influence of hydrophobic block length and ionic liquid on the performance of multiblock poly (arylene ether) proton exchange membrane

The present article includes the synthesis of nanophase-separated poly (arylene ether) multiblock copolymers. A series of poly (arylene ether sulfone) hydrophobic oligomers consisting of bisphenol-A groups were reacted with a disulfonated poly (arylene ether ketone) hydrophilic oligomer containing 4, 4′- bis (4-hydroxyphenyl) valeric acid moieties to prepare multiblock copolymers. The synthesized oligomers and block copolymers were characterized by using FT-IR, ¹H NMR spectra and Gel Permeation Chromatography. The membranes obtained by solution casting method exhibited good dimensional and thermal stability. The increase in hydrophobic block length reduced the water uptake and methanol permeability of the membranes. The complexation of multiblock copolymer with ionic liquid (1-butyl-3-methyl-imidazolium tetrafluoroborate) resulted into novel hybrid membranes. These showed enhanced proton conductivity without affecting the mechanical stability. The Fenton's test revealed that the hybrid multiblock membranes were stable towards radical oxidation. The hydrophobic-hydrophilic phase separation was characterized by using tapping mode Atomic Force Microscopy (AFM). The hybrid membranes showed better fuel cell performance than that of pristine membrane.     

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.     

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.     

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.     

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.     

Cross-linked anion exchange membranes with flexible,long-chain,bis-imidazolium cation cross-linker

Herein, poly (phenylene) oxide (PPO)-based cross-linked anion exchange membranes (AEMs) with flexible, long-chain, bis-imidazolium cation cross-linkers are designed and synthesized. Although the cross-linked membranes possess high ion exchange capacity (IEC) values of up to 3.51–3.94 meq g⁻¹, they have a low swelling degree and good mechanical strength because of their cross-linked structure. Though the membranes with the longest flexible bis-imidazolium cation cross-linker (BMImH-PPO) possess the lowest IEC among these PPO-based AEMs, they show the highest conductivity (24.10 mS cm⁻¹ at 20 °C) and highest power density (325.7 mW cm⁻² at 60 °C) because of the wide hydrophilic/hydrophobic microphase separation in the membranes that promote the construction of ion transport channels, as confirmed by atom force microscope (AFM) images and the small angle X-ray scattering (SAXS) analyses. Furthermore, the BMImH-PPO samples exhibit good chemical stability (10% and 6% decrease in IEC and conductivity, respectively, in 2 M KOH at 80 °C for 480 h, and a 22% decrease in weight in Fenton's reagent at 60 °C for 120 h), making such cross-linked AEMs potentially applicable in alkaline anion exchange membrane fuel cells.     

Mechanically robust semi-interpenetrating polymer network via thiol-ene chemistry with enhanced conductivity for anion exchange membranes

Anion exchange membranes (AEMs) have emerged as crucial functional materials in various electrochemical device, such as fuel cell. Both the mechanical property and ionic conductivity play important roles in AEMs. Herein, a series of semi-interpenetrating polymer network AEMs are prepared by introducing flexible polyvinyl alcohol to the rigid photo-crosslinked poly (2,6-dimethyl-1,4-phenylene oxide) network. Such strategy endows AEM with tunable composition and mechanical property. Among these AEMs, membrane with an IEC of 1.46 mmol/g shows the highest mechanical strength of 30.8 MPa and a relatively lower swelling ratio, as well as the highest hydroxide conductivity. Importantly, the alkaline stability of these AEMs has been improved, 66.5% of the hydroxide conductivity is maintained after treatment in 1 M NaOH at 80 °C for 1000 h. Tentative assembly of H₂/O₂ fuel cell at 60 °C with this AEM displays a peak power density of 78 mW/cm². All the results demonstrate that sIPN structure is a promising way to enhance the mechanical property, ionic conductivity, and the alkaline stability of AEMs for the future application in AEMFCs.     

Proton exchange membranes based on hydrophilic-hydrophobic multiblock copolymers using different hydrophobic oligomer

Although sulfonated aromatic polymers are thought to be one of the promising materials for proton exchange membranes, most of them suffer from significant drop of proton conductivity with decrease in humidity, which causes poor cell performance under partially hydrated condition. In this study, to improve proton conduction at low hydration level, two different multiblock copolymer membranes based on different hydrophobic oligomers are prepared and their hydrophilic/hydrophobic phase-separated morphology is examined. Both multiblock copolymer membranes show better developed phase separation than the random copolymer membrane. Multiblock copolymer having more hydrophobic oligomer exhibits better interconnection between hydrophilic channels than that having less hydrophobic oligomer. This can lead to an improvement of both proton conductivity and cell performance under partially hydrated condition.     

Guanidinium cationic covalent organic nanosheets-based anion exchange composite membrane for fuel cells

Covalent organic frameworks (COFs) used for anion exchange membrane fuel cells (AEMFCs) are commonly endowed with ion conductivity by post-synthesis modification. However, this method usually results in uneven distribution of functional groups, low functionalization and severe ion capacity fade. Limited by hydrophobic skeleton and relatively large particle size of COFs, the COFs doping amount of the composite membrane is not high. Here we design and synthesize a series of guanidinium cationic covalent organic nanosheets-based anion exchange composite membranes. The positively charged guanidinium group as a building block can induce COF-DhaTG_Cl self-exfoliation into a few layered nanosheets through strong interlayer repulsion. Then, the nanosheets were introduced into quaternary ammonium-functionalized poly(2,6-dimethyl-1,4-phenyl ether) (QPPO). A series of COF-DhaTG_Cl/PPO composite AEMs was prepared with the highest doping amount of 30 wt% by casting method. The porous structure and repeat cationic guanidinium units on the skeleton will expose ion sites to the target ones, providing faster OH⁻ diffusion kinetics in one-dimensional channels. The OH⁻ conductivity of COF-DhaTG_Cl/PPO-20 composite membrane can reach 148.65 mS/cm at 80 °C. Meanwhile, the composite membrane also exhibits enhanced mechanical strength and alkaline stability with the maximum stress strength of 37.3 MPa and the residual conductivity of 96.29% after immersion in 2 M NaOH solution at 60 °C for two weeks.     

Application of 2D nanomaterial MXene in anion exchange membranes for alkaline fuel cells: Improving ionic conductivity and power density

Two types of new 2D MXenes (LiF–Ti₃C₂T_x and NH₄HF₂–Ti₃C₂T_x) were prepared by selective etching of Ti₃AlC₂ with LiF/HCl and NH₄HF₂ aqueous solutions, respectively, and they were introduced as nanofillers into quaternized polysulfone/polyquaternium-10 (QPSU/PQ-10) anion exchange membranes (AEMs) with semi-interpenetrating polymer network, thereby preparing two types of MXene-doped QPSU/PQ-10 AEMs. The resulting MXene-doped QPSU/PQ-10 AEMs have obvious nanoscale microphase-separated morphologies, and their ionic conductivity and power density are significantly improved compared with the pristine QPSU/PQ-10 AEM. Among them, the ionic conductivity and power density of NH₄HF₂–Ti₃C₂T_x MXene-doped AEM can reach 88.76 mS/cm at 80 °C and 106.28 mW/cm² at 60 °C, respectively, which are 26.3% and 37.5% higher than those of the pristine QPSU/PQ-10 AEM. Additionally, the MXene-doped QPSU/PQ-10 AEMs with semi-interpenetrating network have moderate water uptake and swelling ratio, excellent thermal and mechanical properties, as well as good oxidation resistance and alkaline stability, which can meet the application requirements of AEMs for fuel cells, exhibiting their bright application prospects in fuel cells.     

Improved conductivity and stability of anion exchange membranes by introducing steric hindrance and crosslinked structure

The alternating copolymer based on poly (ether ether ketone) (PEEK) is synthesized with ordered side chain. A series of novel anion exchange membranes grafte with the 1, 2-dimethylimidazole and 1-vinylimidazole are obtained. The copolymer was verified by ¹H NMR and the crosslinked membranes are further investigated by solvability test. The ordered hydrophilic side chains form well-defined microphase separation structure, which are proved by Transmission electron micrographs microscopy (TEM). The ionic conductivity is 0.075 S/cm at 80 °C of Im-PEEK-0 uncross-linked membrane. With the addition of 1-vinylimidazole, the maximum stress increases to 66.57 MPa, the water uptake drop to 17.1% and swelling ratio drop to 14.8% at 80 °C of Im-PEEK-0.3 membrane. The hydroxide conductivity remains 82.8% in 2 mol L⁻¹ NaOH solution at 60 °C for 400 h. Meanwhile, all the membranes exhibit excellent thermal stability. Overall, the ordered imidazolium-functionalized side chains provide a method to balance hydroxide conductivity and alkali stability of anion exchange membranes.     

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.     

High chemical stability anion exchange membrane based on poly(aryl piperidinium): Effect of monomer configuration on membrane properties

In recent years, ether-free polyaryl polymers prepared by superacid-catalyzed Friedel-Crafts polymerization have attracted great research interest in the development of anion exchange membranes(AEMs) due to their high alkali resistance and simple synthesis methods. However, the selection of monomers for high-performance polymer backbone and the relationship between polymer structure construction and properties need further investigated. Herein, a series of free-ether poly(aryl piperidinium) (PAP) with different polymer backbone steric construction were synthesized as stable anion exchange membranes. Meta-terphenyl, p-terphenyl and diphenyl-terphenyl copolymer were chosen as monomers to regulate the spatial arrangement of the polymer backbone, which tethered with stable piperidinium cation to improve the chemical stability. In addition, a multi-cation crosslinking strategy has been applied to improve ion conductivity and mechanical stability of AEMs, and further compared with the performance of uncrosslinked AEMs. The properties of the resulting AEMs were investigated and correlated with their polymer structure. In particular, m-terphenyl based AEMs exhibited better dimensional stability and the highest hydroxide conductivity of 144.2 mS/cm at 80 °C than other membranes, which can be attributed to their advantages of polymer backbone arrangement. Furthermore, the hydroxide conductivity of the prepared AEMs remains 80%–90% after treated by 2 M NaOH for 1600 h, exhibiting excellent alkaline stability. The single cell test of m-PTP-20Q4 exhibits a maximum power density of 239 mW/cm² at 80 °C. Hence, the results may guide the selection of polymer monomers to improve performance and alkaline durability for anion exchange membranes.     

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.     

Development of multiblock copolymers with novel hydroquinone-based hydrophilic blocks for proton exchange membrane (PEM) applications

Hydrophilic-hydrophobic sequenced multiblock copolymers were synthesized and evaluated for use as proton exchange membranes (PEMs). The multiblock copolymers were prepared by a coupling reaction between fully disulfonated hydroquinone-based hydrophilic oligomers (HQS100) and unsulfonated poly(arylene ether sulfone) hydrophobic oligomers (BPS0). The hydroquinone-based hydrophilic oligomers possess several advantages over previously utilized biphenol-based hydrophilic oligomers (BPS100), including higher hydrophilicity, enhanced nano-phase separation with hydrophobic segments, and lower cost. To maintain the hydrophilic-hydrophobic sequences in the system, the coupling reactions were conducted at low temperature (e.g., 105 °C) to avoid ether-ether exchange reactions. The coupling reaction was solvent sensitive due to a low reactivity of the hydroquinone-phenoxide end-group on the HQS100. All copolymers produced tough ductile films when cast from an NMP or DMF solution. Fundamental membrane parameters including water uptake, proton conductivity, and swelling ratio were investigated along with morphology characterizations by atomic force microscopy (AFM).     

Novel multi-channel anion exchange membrane based on poly ionic liquid-impregnated cationic metal-organic frameworks

The “trade-off” effect between hydroxide conductivity and dimensional stability is challenging issue for anion exchange membrane fuel cells (AEMFCs). In this study, the framework of UiO-66-NH₂ is for the first time applied to anion exchange membranes (AEMs). The robust pore walls of UiO-66-NH₂ with mechanical and structural durabilities protect the membrane from the excessive swelling effects (a swelling ratio of 7%). In addition, the framework of UiO-66-NH₂ is directly modified into (UiO-66-NH₂)⁺Cl⁻ as hydroxide conduction channels by anion stripping for the first time. And we construct well-organized ion nanochannels by the in-situ self-assembly of N,N,N′,N' -tetramethyl-1,6-hexanediamine (TMHDA) and allyl bromide within the highly ordered pores of (UiO-66-NH₂)⁺Cl⁻. The obtained QA@(UiO-66-NH₂)⁺Cl⁻ then incorporated into pristine membrane (QAPPO) to fabricate the novel multi-channel AEMs. The hydroxide conductivity of QA@(UiO-66-NH₂)⁺Cl⁻/PPO is up to 123 mS⋅cm⁻¹ at 80 °C, which is greatly improved compared to QAPPO pristine membrane.     

Novel anion exchange membrane with poly ionic liquid-confined hypercrosslinked polymer for enhanced anion conduction and stability

Hypercrosslinked polymer (HCP) holds great potential for utilization as novel anion exchange membrane (AEM) material due to their rich microporous structure and high thermal/chemical stability while remaining challenges due to lack of hydroxide carriers. Herein we report a novel strategy of fabricating poly ionic liquid (PIL)-confined HCP for ion transfer. PIL precursors are loaded into the pores of HCP and in-situ polymerized to prepare PIL@HCP, which is then incorporated into quaternized poly (2,6-dimethyl-1,4-phenylene oxide) (QAPPO) to fabricate composite membrane. The introduction of PIL provides high concentration of quaternary ammonium (QA) groups in the porous networks of HCP. And the organic components impart outstanding compatibility between PIL@HCP and QAPPO matrix. All these permit the formation of interconnected hydroxide transfer channels through the membranes. Especially, the rigid and hydrophobic HCP functions with steric hindrance effectively impedes the attack of hydroxide ions on QA groups and maintains structure stability. Accordingly, the PIL@HCP/QAPPO composite membrane with high ion exchange capacity (IEC) of 2.33 mmol g^?1 achieves a hydroxide conductivity of 98 mS cm^?1 (80 °C, 100% RH), 92% higher than that of QAPPO. Meanwhile, the area swelling degree of PIL@HCP/QAPPO reduces to 13.6% in comparison to QAPPO (25.7%) and its conductivity retains 88% after alkaline treatment.