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Covalent organic frameworks (COFs) have permanent, nanometer‐scale pores endowed with high porosity and thermal and chemical stability, which is widely used as composite membranes and applied in the field of separation. Conventional methods for preparing composite membranes, such as spray coating, spin coating, and pressure‐assisted or vacuum‐assisted filtration, have problems such as the selective layer can easily fall off. Here, an integrated casting–precipitation–evaporation method is proposed to prepare flexible COF composite membranes (DPBI/HTpBD). The DPBI/HTpBD composite membranes constructed with hollow spherical shell COF selective layer (HTpBD) and porous supporting layer are designed and applied in a vanadium flow battery. The excellent coulombic efficiency (CE, 99.5%) and voltage efficiency (VE, 81.1%) of batteries (180 mA cm−2) are achieved by the nanometre‐scale pores of the hollow spherical shell COF selective layer. In addition, the rigid pore structure inside COF bonded by covalent bonds endows the DPBI/HTpBD composite membranes with outstandingly high temperature stability. Compared to that of the Nafion 212 and DPBI pristine membranes (CE reduces by 5.2%, 3.3%), the CE of DPBI/HTpBD‐9 composite membrane‐based cell only reduces by 0.9% as the temperature increases from 25 to 55 °C. This work provides a new preparation method for high ion‐selective and high temperature stable battery membranes. [ABSTRACT FROM AUTHOR]

Hydroxide exchange membrane fuel cells (HEMFCs) are attracting growing interest owing to their advantages such as low cost and high power density. However, their applications are hindered due to the poor stability of the membrane. Here, we proposed a novel strategy to improve the alkaline stability of hydroxide exchange membranes (HEMs) using the interactions between strongly polar nitrile groups and side-chain functional cations. A poly(ether nitrile) (PEN) was synthesized by the polycondensation of bisphenol A and 2,6-difluorobenzonitrile, and then imidazolium and morpholinium functional groups were integrated by the typical chloromethylation-functionalization method. The existence of the interactions between nitrile groups and cation groups i.e. imidazolium and morpholinium, was proved by the density functional theory calculations. The interactions have two positive effects that contribute to the improvement of the alkaline stability of the PEN based membranes. On the one hand, they increase the LUMO energies of the functional groups; and on the other hand they reduce the free volume around hydrated cationic groups. As a result, PEN based membranes showed much better alkaline stabilities compared to the membrane based on commercial polysulfone that has a similar chemical structure. In addition, given similar swelling ratios, PEN based membranes exhibited higher hydroxide conductivities than simple polysulfone based ones. [ABSTRACT FROM AUTHOR]

Abstract: In order to increase both of the water uptake and conductivity, the proton exchange membranes were fabricated by sulfonated poly(ether ether ketone) (SPEEK) doped with varied contents of silica sulfuric acid (SSA) which is obtained by treating SiO2 nanoparticles with more volatile SO2Cl2. SEM images of the composite membranes show that SSA nanoparticles are dispersed within the membranes uniformly, indicating the good organic compatibility of SSA particles. TEM images show that the composite membranes have improved ionic clusters distribution. The water uptakes of the composite membranes in water and under low relative humidities are all higher than that of the pristine SPEEK membrane. The addition of SSA enhances the conductivity obviously. The composite membrane containing 5wt.% SSA exhibits the highest conductivity of 0.13 Scm−1 at 80 °C, approximately 18.6% higher than that of the pristine SPEEK membrane and 8.6% higher than that of Nafion117. The composite membranes also show good thermal stability. These results imply the potential application of the SPEEK/SSA composite membranes as improved PEMs in PEMFC. [Copyright &y& Elsevier]

To prepare high-performance anion exchange membrane, a twisted ether-free polyarylene piperidinium (QMter-co-Mpi) has been successfully synthesized via simple and controllable acid-catalyzed polycondensation reaction for alkaline water electrolysis. The hydroxide conductivity of QMter-co-Mpi membrane reaches 37 mS cm−1 at 30 °C owing to the fact that an efficient ion-conducting channels are formed by means of this unique molecular structure. Furthermore, the IEC and hydroxide conductivity of the membrane can remain above 80% of the original values after alkali treatment for more than 1000 h, indicating exceptional alkaline stability. As a result, the QMter-co-Mpi based cell presents a high current density of 1064 mA cm−2 at 2.5 V under 1 M KOH and 50 °C and the high frequency resistance of the cell is only 0.165 Ω cm−2 at 1.8 V. The durability test is performed at current density of 200 mA cm−2 for more than 500 h and the voltage is basically remained at 2.1 V, showing excellent electrochemical performance. All these demonstrate a good application prospect for QMter-co-Mpi membrane in water electrolysis. • Twisted ether-free polymer based alkaline membrane has been successfully synthesized. • The membrane showed decent conductivity and exceptional alkaline stability. • The cell with this membrane exhibited excellent electrochemical performance. [ABSTRACT FROM AUTHOR]

A series of novel sulfonated polybenzimidazole (SPBI) membranes containing flexible double ether segments, having high ion exchange capacity (IEC), good solubility as well as high molecular weight, are designed for vanadium flow batteries (VFBs). The bis-ether structure of self-synthesized sulfonic acid monomer, 4,4'-((2-sulfo-1,4-phenylene)bis(oxy))dibenzoic acid, improves the solubility of produced SPBI and the microphase separation structure of prepared membrane by disrupting the regularity of the backbone through a non-planar structure and attenuating the interchain stacking effect of the polymer backbone. Such a highly reactive bis-ether monomer also leads to high molecular weights (2.37−2.82 dL g−1). The copolymerization using another sulfonic acid monomer, Monosodium-5-Sulfoisophthalate monomer, leads to a high IEC of up to 2.08 mmol g−1. As a result, the SPBI membrane shows a low area resistance of 0.21 Ω cm2 and a low vanadium ion permeability of 3.36 × 10−9 cm2 s−1. The VFB cell employing SPBI membrane achieves a very high energy efficiency exceeding 82 % at 200 mA cm−2. In addition, an ultra-long lifetime of over 3400 cycles is accomplished. [Display omitted] • Stabilized SPBI membranes containing flexible double ether segments are investigated. • The membranes exhibit high ion exchange capacity and good solubility. • Flexible and rigid chains synergistically optimize conductivity and chemical stability. • SPBI-MS-x membranes combine high energy efficiency with a long lifetime of cycle. [ABSTRACT FROM AUTHOR]

It is challenging for ion conductive membranes to achieve both high proton conduction and low vanadium ion permeation because both ions transport mainly through the hydrophilic domains in membranes. A novel strategy of electrospinning fiberization of the carbon nanotube hybrid sulfonated poly (ether ether ketone) is proposed. The improved conductivity is achieved through interconnective proton pathways induced by the electrospun nanofiber. Multiwall carbon nanotubes contain carboxyl groups, align and disperse well in the nanofibers under a strong electrostatic field of 1.3 kV cm−1 as evidenced by TEM and SAXS, therefore result in enhanced hydrogen bond networks for proton hopping. In contrast, vanadium ions cannot transport through hopping and, thus, are blocked by the carbon nanotubes in hydrophilic domains. Consequently, with the carbon nanotube content increasing from 0 to 0.5 wt %, the area resistance of the electrospun membrane remains unchanged, but vanadium ion permeability dramatically decreases by approximately 67.6%. A vanadium redox flow battery assembled with an electrospun membrane exhibits high chemical and capacity stability as well as an energy efficiency of 83.4%, even after 100 cycles at 100 mA cm−2. These parameters are substantially superior to those of a battery assembled with Nafion 211 with a similar membrane thickness. Image 1 • Electrospun SPEEK nanofiber networks induce long range proton conductive pathways. • MWCNTs align along fiber axis to improve proton hopping and vanadium ions blocking. • Much higher VRFB efficiency and capacity stability than that with Nafion211. [ABSTRACT FROM AUTHOR]

[Display omitted] • The poly(aryl ether piperidinium) membranes are prepared by super acid catalysis with less amount of catalyst. • The membrane shows the highest conductivity of 140 mS cm−1 and excellent stability of greater than 2000 h. • The fuel cell exhibits the highest peak power density of 1.2 W cm−2. • The polymers can be easily prepared into larger widths of membranes. Poly(aryl piperidinium) is considered a promising material for the preparation of anion exchange membranes (AEM) with considerable conductivity and excellent chemical stability for anion exchange membrane fuel cells (AEMFC). In this research, we report a range of biphenyl ether-containing poly(aryl ether piperidinium) unlike previous ones to increase the conductivity of AEMs and the peak power density (PPD) of fuel cells, and the alkaline stability of ether bonds in the polymer backbone is demonstrated. All of the poly(aryl ether piperidinium) membranes possess comparable mechanical properties and inherent viscosity derived from the high activity of biphenyl ethers in superacid-catalyzed reactions, which means that large-width AEMs can be prepared. The data demonstrated that the poly (ether p-terphenyl piperidinium) (QPEPTpi-35) exhibited a conductivity of 140 mS cm−1 at 80 ℃ compared to the original poly (p-terphenyl piperidinium) (QPTPpi) membrane owning to optimized micro-phase separation structure and higher water uptake. Meanwhile, QPEPTpi-35 membrane conductivity retention is greater than 85% after immersion in 1 M KOH for 2000 h at 80 ℃. Moreover, the PPD of the QPEPTpi-35 membrane reaches 1.2 W cm−2 in H 2 /O 2 fuel cells, which is much higher than that of the QPTPpi membrane (0.51 W cm−2). The durability of MEA prepared by the catalyst-coated substrate (CCS) method manifests over 20 h under 0.2 A cm−2 at 60 °C. [ABSTRACT FROM AUTHOR]

Neutral aqueous organic redox flow battery (NAORFB) stands out as one of the most prospective mass energy storage options. The main limitations of large-scale storage regarding the NAORFB lie in the low Na+/Cl− conductivity of membrane in pH7 systems, which leads to a lower voltage efficiency. Herein, quaternary ammonium functionalized fluorinated poly (arylene ether) (QAFPAE-x) anion exchange membrane with hydrophilic and hydrophobic blocks was designed successfully. The results demonstrated that the grafting of quaternary ammonium and fluorine-containing block enables QAFPAE-x membrane low swelling, high Cl− conductivity, and promising chemical stability characteristics. The voltage efficiency of QAFPAE-100 membrane (93.8%–60.7%) is much higher than the AMVN membrane (88.4%–37.8%) at 20–160 mA cm−2. Furthermore, the NAORFB with the QAFPAE-100 membrane steadily runs at 60 mA cm−2 over 1500 cycles. Therefore, the ionic highway induced excellent performance of QAFPAE-100 membrane to render this material a promising candidate for practical NAORFB applications. [Display omitted] • A membrane with hydrophilic groups and hydrophobic block was designed. • The membrane has low swelling, high conductivity and chemical stability. • The membrane support over 1500 cycles of cells. [ABSTRACT FROM AUTHOR]

• A high-strength hydrophobic backbone and a hydrophilic side chains were combined. • Membrane with low swelling, permeability and high stability were achieved. • The VE of NAORFB with the membrane is advantages over other best-performed batteries. Neutral aqueous organic redox flow battery (NAORFB) has been considered as a promising energy-storage device in the stationary storage field. Yet the insufficient Cl− conductivity of anion exchange membrane (AEM) has become a crucial obstacle for neutral energy systems compared to their counterparts, such as H+ and OH– in acidic and alkaline conditions, respectively. Here, by proper microstructure design in AEM, a membrane with high Cl− conductivity is constructed by coupling the hydrophobic fluorinated poly(arylene ether) backbone and hydrophilic piperazinium groups, which exhibits excellent Cl− conductivity up to 55.9 mS cm−1 at room temperature. Moreover, the developed piperazinium functional fluorinated poly(arylene ether) (QPFPAE) membrane exhibits excellent low swelling, low organic redox-active species permeability, and high chemical stability. The constructed NAORFB using QPFPAE membrane shows a remarkable energy efficiency (85.6% at 80 mA cm−2), which has outperformed the other best-performing membranes. In addition, the membrane demonstrates outstanding stability and state-of-the-art performance over 1000 charge–discharge cycles. This work provides a rational design strategy toward high anion-selective conductivity in the membrane for large-scale NAORFB development. [ABSTRACT FROM AUTHOR]

Anion exchange membranes (AEMs) are an important component in anion exchange membrane fuel cells, and their side chain structures have a significant impact on membrane performance. Here, we construct a series of poly(phenylene oxide) (PPO)-based AEMs with varied side chain structures and investigate the effects of alkyl or alkoxyl extenders (or "tail" of the cation) and spacers (or the link between backbone and cation head group) on AEM performance. Our investigations demonstrate that the simultaneous introduction of alkoxyl extender and spacer can significantly improve water absorption and ion conductivity of membrane; this is attributed to the good hydrophilicity of alkoxyl group and flexibility of spacer that benefit the formation hydrophilic domains. At an IEC (ion exchange capacity) of 1.13 mmol/g, the AEM containing both spacer and alkoxyl extender exhibits a hydroxide conductivity of 87.3 mS/cm (80 °C), which is much higher than that of other spacer-containing membranes at similar IECs. The H 2 /O 2 single cell constructed with this membrane exhibits a good peak power density of 292 mW/cm2 at 60 °C. In addition, the steric hindrance of the extender gives rise to higher alkali stability of the AEM than that of the membrane which does not have the extender. Our work provides a meaningful direction of side chain design which can effectively improve conductivity and alkaline stability of AEMs. Image 1 • Alkoxy extender and alkyl spacer were co-introduced into anion exchange membrane. • Alkoxy extender and alkyl spacer can synergistically improve ion conductivity and chemical stability of membrane. • The highest conductivity of 87.3 mS/cm at 80 °C was obtained at a low IEC of 1.13 mmol/g. • The H 2 –O 2 fuel cell employing the above AEM yielded a power density of 292 mW/cm2 at 60 °C. [ABSTRACT FROM AUTHOR]

A novel double-layer composite membrane with a MoS 2 -rich layer and a polymer-rich layer is prepared by the method of one-step slow solvent evaporation for vanadium redox flow batteries. The composite membrane is composed of MoS 2 nanosheets as protecting layer, providing the membrane with lower VO2+ permeability and excellent chemical stability, and sulfonated modified PBI as substrate, providing high proton conductivity. The introduction of MoS 2 nanosheets greatly reduces the area resistance of the PBI-PS/MoS 2 membrane (0.42 Ω cm2), which is comparable to that of Nafion 212 (0.41 Ω cm2). The cell equipped with PBI-PS/MoS 2 membrane presents improved coulombic efficiencies and energy efficiencies. At 200 mA cm−2, a high EE of about 80% is achieved. The cell maintains stable performances for nearly 800 cycles, showing impressive durability. The cell also shows a self-discharge rate of as low as 0.0021 V h−1. The MoS 2 nanosheets layer based composite membrane is promising for VRBs. Image 1 • The composite membrane composed of MoS 2 -rich layer and polymer-rich layer is designed for flow battery. • The MoS 2 nanosheets with two-dimensional structure achieve high proton-selective conduction. • Cell with the membrane shows high and stable performances during 800 cycles. [ABSTRACT FROM AUTHOR]

A novel covalently/ionically co-crosslinked poly(biphenylene piperidinium) amphoteric membrane was designed for improved stability and efficiencies in vanadium redox flow batteries (VRFB). The amphoteric-side-chain structure provides an ultrahigh functionalization degree (4.93 mmol· g −1) and unobstructed ion channels, which endowed the membrane with an excellent ion conduction capacity comparable to Nafion 212 (area resistance: 0.23–0.29 Ω·cm2). Surpassing traditional sulfonated membranes and single ionic crosslinked membranes, even at high level functionalization, the covalently/ionically co-crosslinking structures guaranteed the membrane a low swelling ratio (10.1%) and VO2+ permeability of 0.79 × 10−8 cm2· s −1. Moreover, due to the lack of sensitive aryl ether bonds, these membranes showed very good chemical stability, with a slight mass loss of 3.43% (nearly 5.5 times lower than SPEEK) after soaking in vanadium oxygen species for 30 days. On this basis, the membrane exhibited excellent battery performance. The Coulombic, voltage, and energy efficiencies of a battery using the above membrane reached 98.12, 87.27, and 85.98% at 120 mA·cm−2 current density and no obvious decline occurred after 400 cycles. The self-discharge time of battery up to 136 h. These observations indicated that this membrane appeared very promising for use in VRFB applications. [ABSTRACT FROM AUTHOR]

Protonated imidazole-hydroxyl-quaternary ammonium co-functionalized polybenzimidazole (PBI) anion exchange membranes are designed for vanadium redox flow battery (VFB). The structure is achieved by grafting glycidyl trimethyl ammonium chloride onto the PBI on basic of open-ring reaction. The synergistic effect of the protonated imidazole-hydroxyl-quaternary ammonium is supposed to create proton transport channels with hydrogen bonding and Donnan exclusion effect, leading to high proton conductivity and low vanadium permeability of the membrane. The single cell employing this membrane reveals a high performance with a coulombic efficiency of 99.3% and an energy efficiency of more than 80% at 200 mA cm−2. A long-term life of over 500 cycles at 120 mA cm−2 could be attributed to the good chemical stability of the membrane. These results indicate the membrane proposed here is a promising choice for VFB application. • Protonated imidazole-hydroxyl-quaternary ammonium co-functionalized PBI AEMs are designed for VFB. • The synergistic effect created proton transport channels with hydrogen bonding and Donnan exclusion effect. • The single cell reveals a high performance with a CE of 99.3% and an EE of more than 80% at 200 mA cm−2. [ABSTRACT FROM AUTHOR]

An interface-strengthened cross-linked graphene oxide/Nafion212 (CLGO/Nafion212) composite membrane was successfully fabricated and served as ion conducting membrane for vanadium flow batteries (VFBs). Firstly, the surface of Nafion212 was endowed with plenty of amino groups via in situ sol-gel chemistry approach of 3-aminopropyltriethoxysilane. Subsequently, a thin layer of m -xylylenediamino-crosslinked GO was formed using the spin-coating method. The composite membrane demonstrated much lower vanadium ion permeability (0.30 × 10−6 cm2 min−1) than pristine Nafion212 (1.88 × 10−6 cm2 min−1), mainly due to the presence of a cross-linked GO layer with angstrom-scale channels. The cell assembled with CLGO/Nafion212 exhibited enhanced coulombic efficiencies (CE s, 93.0–97.2% vs. 68.0–91.6%) and energy efficiencies (EE s, 88.9–85.2% vs. 65.7–82.2%) compared with the one with Nafion212 at 20–100 mA cm−2. At a constant current density of 80 mA cm−2, the long-term cycling tests showed that the discharge capacity fading of the flow cell equipped with CLGO/Nafion212 reached 0.25% per cycle, which was much smaller than that of Nafion212 (0.47% per cycle). The above findings proved that the cell equipped with CLGO/Nafion212 displayed superior electrochemical performances than that with Nafion212, therefore CLGO/Nafion212 composite membrane showed a great potential in VFBs. • An interface-strengthened cross-linked GO/Nafion212 membrane was prepared. • The membrane exhibited dramatically reduced vanadium-ion permeability. • The membrane showed good mechanical and chemical stabilities. • The cell with the membrane demonstrated improved efficiencies and capacity retention. [ABSTRACT FROM AUTHOR]