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.     

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.     

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.     

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.     

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.     

Degradation of guanidinium-functionalized anion exchange membrane during alkaline environment

A guanidinium-functionalized anion exchange membrane (AEM) was prepared and characterized. The AEM stability in an alkaline environment and transition structures during the degradation process were studied with DFT (density functional theory)/B3LYP method, 6-31 + G (d) basis set. Experimental results and theoretical analysis showed that the second step (Step 2′) of degradation reaction was the control procedure; guanidinium cation was unstable under alkaline condition. It had lower energy barriers, which decided it was easier for the degradation reaction to occur in high pH environment. The ionic conductivity of AEM was 2.4 × 10⁻² S cm⁻¹ at 80 °C. However, the AEM soaked in 1 M NaOH solution, fragments were found after 10 days. The FTIR analysis showed that the structure of the membrane had been changed due to the attack of hydroxide ion. A new substance, tetramethylurea, had produced.     

The effect of side chain length on the morphology and transport properties of fluorene-based anion exchange membranes

Poly[(fluorene alkylene)- co(biphenyl alkylene)] (PFBA) compounds with quaternary ammonium (QA) groups (PFBA-nC-QAs) that are linked with side chains of various lengths (n = 1～6 carbon atoms) are designed and synthesized by a superacid catalysis reaction, which has the advantages of low cost, easy synthesis and mild reaction conditions. The correlative properties of PFBA-nC-QAs, including water uptake, thermal stability, morphology, ion conductivity and alkaline stability, are discussed in detail. The side chain length is vital to the morphology and transport performance of PFBA-nC-QAs. As the side chain length increases, the alkaline stability and hydroxide ion conductivity of the prepared membranes improve with decreasing water uptake. Experimental results indicate that the hydroxide conductivity of PFBA-6C-QA is 154 mS cm^?1 at 80 °C. Moreover, no degradation of functional groups of PFBA-6C-QA is observed during 30 days of immersion in 2 M NaOH at 80 °C. The peak power density of PFBA-6C-QA is 278 mW cm^?2 at 60 °C with a hydrogen/air single fuel cell. By controlling the length of the polymer side chain, the method is simple and effective for building anion exchange membranes with high performance.     

Quaternized cardo polyetherketone anion exchange membrane for direct methanol alkaline fuel cells

Quaternized cardo polyetherketone (QPEK-C) membranes for alkaline fuel cells were prepared via chloromethylation, quaternization and alkalization of cardo polyetherketone (PEK-C). The chemical reaction for PEK-C modification was confirmed by nuclear magnetic resonance (¹H NMR) and energy-dispersive X-ray spectroscopy (EDAX). The QPEK-C membrane was characterized by X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis (TG). The ion-exchange content (IEC), water and methanol uptakes, methanol permeability and conductivity of the QPEK-C membranes were measured to evaluate their applicability in alkaline methanol fuel cells. The ionic conductivity of the QPEK-C membrane varied from (1.6 to 5.1) × 10⁻³ S cm⁻² over the temperature range 20-60 °C. The QPEK-C membrane showed excellent methanol resistance. When the concentration of methanol was 4 M, the methanol permeability was less than 10⁻⁹ cm² s⁻¹ at 30 °C.     

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.     

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.     

Synthesis and characterization of anion exchange membranes for alkaline direct methanol fuel cells

Alkaline fuel cells suggest solution for the problems of low methanol oxidation kinetics and methanol crossover, which are limiting the development of direct methanol fuel cells. In this work, a novel anion exchange membrane, quaternized poly(aryl ether oxadiazole), was prepared through polycondensation, grafting and quaternization. The ionic conductivity of as-synthesized anion exchange membrane can reach up to 2.79 × 10⁻² S/cm at 70 °C. The physical and chemical stability of the anion exchange membranes could also meet the requirement for alkaline direct methanol fuel cells.     

Facile construction of poly(arylene ether)s-based anion exchange membranes bearing pendent N-spirocyclic quaternary ammonium for fuel cells

Anion exchange membrane (AEM) fuel cells have received significant attention due to their low fuel permeability and the use of non-platinum catalysts. However, the development of AEMs with robust chemical stability and high conductivity is still a great challenge. Herein, we prepare a new type of partially fluorinated backbone bearing pendent N-spirocyclic quaternary ammonium (QA) cations via a facile Williamson reaction, which displays great potential for fuel cells. The integration of the two substructures (a fluorinated moiety into a polymer backbone and a pendent cation structure) is beneficial for the fabrication of a well-defined micro-phase separation structure, thereby facilitating the construction of a highly-efficient ion transporting pathway. Correspondingly, the resulting AEM (PAENQA-1.0), despite its a relatively low ionic exchange capacity (0.93 meq g⁻¹) demonstrates a conductivity of 63.1 mS cm⁻¹ (80 °C). Meanwhile, the constrained ring conformation of N-spirocyclic QA results in improved stability of the AEMs.     

Cross-linked, ETFE-derived and radiation grafted membranes for anion exchange membrane fuel cell applications

To develop a series of cross-linked anion exchange membranes for application in fuel cells, poly(ethylene-co-tetrafluoroethylene) (ETFE) films was radiation grafted with vinyl benzyl chloride (VBC), followed by quaternization and crosslinking with 1,4-Diazabicyclo[2,2,2]octane (DABCO), alkylation with p-Xylylenedichloride (DCX), and quaternization again with trimethylamine (TMA). These anion exchange membranes were characterized in terms of water uptake, ion-exchange capacity, ionic conductivity as well as thermal stability. The chemical structures of the membranes were examined by FT-IR. The anion conductivity of the resulting alkaline anion exchange membrane is as high as 0.039 S cm⁻¹ at 30 °C in deionized water and the ionic conductivity increases with the increasing of temperature from 20 to 80 °C. The membrane is stable after being treated by 10 M potassium hydroxide solution at 60 °C for 120 h .The fuel cell performance with the final AAEM obtained in a H₂/O₂ single fuel cell at 40 °C with this AAEM was 48 mW cm⁻² at a current density of 69 mA cm⁻².     

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.     

Flexibly crosslinked and post-morpholinium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) anion exchange membranes

Morpholinium-functionalized cross-linked poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) anion exchange membranes were prepared using a crosslinking/post-functionalization method. 4,4'-(oxybis(ethane-2,1-diyl)) dimorpholine (OEDM) having a flexible long chain and hydrophilic ether bond was for the first time used as the crosslinker. The brominated PPO reacted with OEDM, forming a crosslinked structure and accomplishing partial functionalization. Further functionalization was carried out by immersion in N-methylmorpholine aqueous solution. The degree of crosslinking was controlled by varying the proportion of brominated PPO to OEDM because excessive bromomethyl groups ensured the occurrence of crosslinking reaction. The cross-linked structure promoted the formation of micro-phase separation and effectively limited swelling. The crosslinked membrane shows much higher conductivity and lower swelling ratio compared to non-crosslinked one at similar IEC. The highest hydroxide conductivity that the crosslinked membrane achieves is 35 mS cm⁻¹ at 20 °C. The power density for the H₂/O₂ single cell assembled with this membrane is up to 193 mW cm⁻². In addition, the crosslinked PPO-MmOH-Cr membrane exhibits good alkaline stability with conductivity loss of less than 15% after soaking in 1 M KOH for 96 h.     

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.     

A novel strategy for constructing a highly conductive and swelling-resistant semi-flexible aromatic polymer based anion exchange membranes

A novel strategy was proposed to construct a bicontinuous hydrophilic/hydrophobic micro-phase separation structure which is crucial for high hydroxide conductivity and good dimensional stability anion exchange membranes (AEMs). A semi-flexible poly (aryl ether sulfone) containing a flexible aliphatic chain in the polymer backbone with imidazolium cationic group was synthesized by the polycondensation of bis(4-fluorophenyl) sulfone and the self-synthesized 4,4′-[butane-1,4-diylbis(oxy)] diphenol followed by a two-step functionalization. The corresponding membranes were prepared by solution casting. More continuous hydroxide conducting channels were formed in the semi-flexible polymer membranes compared with the rigid based ones as demonstrated by TEM. As a result, given the same swelling ratio, hydroxide conductivity of the semi-flexible polymer membrane was about 2-fold higher than the one of the rigid polymer based membrane (e.g., 45 vs. 22 mS cm^?1 with the same swelling ratio of 24% at 20 °C). The highest achieved conductivity for the semi-flexible polymer membranes at 60 °C was 93 mS cm^?1, which was much higher those of other random poly (aryl ether sulfone) based imidazolium AEMs (27–81 mS cm^?1). The single cell employing the semi-flexible polymer membrane exhibited a maximum power density of 125 mW cm^?2 which was also higher than those for other random poly (aryl ether sulfone) based imidazolium AEMs (16–105.2 mW cm^?2).     

Simultaneously enhanced hydroxide conductivity and mechanical properties of quaternized chitosan/functionalized carbon nanotubes composite anion exchange membranes

Low-cost biopolymer chitosan has received considerable attention in the field of anion exchange membranes (AEMs) because it can be easily quaternized and avoids the carcinogenic chloromethylation step. Simultaneously increasing the ionic conductivity and improving mechanical properties of quaternized chitosan (QCS) is key for its high-performance application. In this study, new composite AEMs consisting of QCS and functionalized carbon nanotubes (CNTs) were prepared. CNTs were coated with a thick silica layer onto which high-density quaternary ammonium groups were then grafted. The insulator silica coating effectively prohibits electron conduction among nanotubes and the grafted –NR₃⁺ provides new OH⁻ conductive sites. Incorporating 5 wt% functionalized CNTs into the matrix enhanced ionic conductivity to 42.7 mS cm⁻¹ (80 °C) which was approximately 2 times higher than that of pure QCS. The effective dispersion of CNTs and appropriate interfacial bonding between nanofiller and QCS improved the mechanical properties of AEMs, including both the strength and toughness of the composite membranes. An alkaline direct methanol fuel cell equipped with the composite membrane (5% functionalized CNTs loading) produced an maximum power density of 80.8 mW cm⁻² (60 °C), which was 57% higher than that of pure QCS (51.5 mW cm⁻²). This study broadens the application of natural polymers and provides a new way to design and fabricate composite AEMs with both improved mechanical properties and electrochemical performance.     

Current status of cross-linking and blending approaches for durability improvement of hydrocarbon-based fuel cell membranes

Due to the demand for developing reliable and economical fuel cells, many researchers have focused on durability improvement of hydrocarbon-based proton exchange membranes (PEMs), without compromising performance. Among various techniques, cross-linking and blending show promising potentials by introducing physical and/or chemical bonds between polymer chains and creating a 3D network within their structure. Cross-linking is accomplished through thermal, solvothermal, radiation-assisted, and cross-linker assisted methods, whereas blending is categorized based upon the existing interactions between polymers, namely acid-acid, acid-base, and charge transfer network. This review article discusses the recent achievements of cross-linked and blend hydrocarbon-based PEMs in long-term stability tests and durability studies. Additionally, their salient dimensional, thermo-chemical, and transport properties are highlighted, including the in-situ fuel cell performance and electrochemical diagnostics. Accordingly, cross-linked and blend membranes have shown over 4000 h durability in hydrogen fuel cells and more than 100 times lower methanol cross-over compared to conventional fluorinated membranes, implying significant potential for commercialization.