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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]

A novel long-side-chain sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (S-L-PPO) membrane was successfully prepared for VRFB applications. The long side chains were introduced onto the PPO backbones by a simple and controllable acylation with 4-fluorobenzoyl chloride and a subsequent condensation with sodium 4-hydroxybenzenesulfonate. The introduction of long side chains drives the formation of a good hydrophilic/hydrophobic micro-phase separation structure, which is evidenced by AFM. The S-L-PPO membrane with a degree of sulfonation (DS) of 51% showed an ultralow vanadium permeability (4.3 × 10 −9 cm 2 s −1 ) and a decent proton conductivity (44 mS cm −1 ). As a result, the energy efficiency of VRFB with S-L-PPO-51% membrane was up to 81.8% that was higher than that with Nafion 212 (78.0%) at a current density of 120 mA cm −2 . In addition, the self-discharge duration of the cell with S-L-PPO-51% membrane was 222 h that was nine times longer than that of Nafion 212 (23 h). All these indicate that the membrane prepared here is promising for the application in VRFB. [ABSTRACT FROM AUTHOR]

A long-side-chain sulfonated polybenzimidazole (PBI-PS) membrane was successfully prepared by the reaction of PBI with 1, 3-propanesultone for improving the efficiencies of vanadium redox flow battery. The interaction between sulfonic acid and imidazole groups provides the PBI-PS membrane with excellent swelling resistance, leading to a swelling ratio of only 13% for the membrane with IEC of as high as 3.18 mmol g−1. This results in an ultra-low vanadium permeability (6.5 × 10−9 cm2 s−1). The introduction of sulfonic acid-terminated flexible side chain in the membrane also constructs good microphase separation structure, leading to a low area resistance of 0.51 Ω cm2, which is close to Nafion 212 (0.41 Ω cm2). This membrane exhibits excellent cell performance. The voltage and energy efficiencies are up to 90.73% and 89.13% at 80 mA cm−2, respectively. Furthermore, the efficiencies remain constant values for 200 cycles, and no changes in chemical structure and cell efficiencies are observed after 720 h immersion in electrolyte solution. • Long-side-chain sulfonated polybenzimidazole membrane was prepared for RFB. • High conductivity and vanadium resistance were achieved. • The cell with this membrane exhibited high efficiencies and good stability. [ABSTRACT FROM AUTHOR]

The trade-off between proton conduction and vanadium permeability is a great challenge for ion conductive membrane to achieve high vanadium redox flow battery (VRFB) performance. Herein, the quinone convertible phenolic hydroxyl group side chain is proposed to endow Donnan effect and continuously narrow ion conductive channel to improve H+/Vn + selectivity. Uniquely, phenol can be chemically converted into quinone of lower electronegativity in-situ in the strong acidic VRFB environment, which greatly increases the Zata potential (from 2.6 to 10.2 mV) and Donnan effect to repel vanadium ions. A high phenol hydroxyl group capacity with 5.43 mmol g−1 can be achieved owing to the swelling restriction by the hydrogen bonding crosslinking between quinone and benzimidazole, which constructs small but densely distributed ion clusters and continuously narrow ion conductive channels for selective proton conduction. The quinone type polybenzimidazole membrane (PBIPhQ-5.43) presents low area resistance and vanadium permeability (0.25 Ω cm2 and 1.3 × 10−9 cm2 s−1, 13.8 % and 99.6 % decreased, respectively, compared with that of Nafion 212). The VRFB exhibits excellent balance between energy efficiency (83.1 %) and discharge capacity decay (0.29 %/cycle) at 100 mA cm−2, far superior to that of Nafion 212 membrane (75.1 %, 0.71 %/cycle). [Display omitted] • Phenol group can be chemically converted to quinone of lower electronegativity. • Quinone conversion greatly increases Donnan effect to repel vanadium ions. • Superior VRFB energy efficiency and capacity retention to Nafion 212 membrane. [ABSTRACT FROM AUTHOR]

The ion sieving effect of the porous two-dimensional covalent organic frameworks (COFs) in vanadium redox flow battery (VRFB) was greatly restricted by the brittleness and easy agglomeration of COFs in ion conductive membranes. Herein, the strategy of free-standing COF nanofiber as continuous ion conductive pathways is proposed to maximize the selective ion conduction of COFs. The free-standing TFP-TAPA electrospun COF nanofiber mat was fabricated by an in-situ growth-template etching method, densified with sulfonated polybenzimidazole (SPBI) and continuously dispersed in the membrane. Fibrillization gives rigid COF materials excellent flexibility to be freely bent, induces the orientated aggregation of sulfonic acid groups via hydrogen bonding for proton hopping, as well as the long-range vanadium ions repelling barrier via the sieving pores (6 Å) and Donnan effect of the imine groups in COF. The TFP-TAPA COF nanofiber continuous membrane shows 3 folds of H+/Vn+ selectivity over Nafion212, achieves excellent VRFB energy efficiency of about 80.5 % at 200 mA cm−2, which surpasses most state-of-the-art COFs based ion conductive membranes for VRFBs. [Display omitted] • Free-standing COF nanofiber continuous ion conductive membrane is first reported. • Fibrillization gives rigid COF excellent flexibility and long-range ion conduction. • VRFB performance surpasses most recently reported COF based ion conductive membranes. [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]

Proton conductive group is essential to provide ion transport site and tune micro-phase separation in proton exchange membranes for vanadium redox flow battery (VRFB), while nearly all the fabrication methods are based on the benchmark sulfonic acid group. Herein, the sulfate ester group (-OSO 3 H) with superior acidity is proposed as a novel proton conductive group to achieve high proton transport at lower IECs, therefore disentangles the trade-off between vanadium crossover and conductive ion diffusion. The electron withdrawing ester oxygen atom endows sulfate ester group with 27 kJ mol−1 lower proton binding energy than the sulfonic acid group, leading to higher proton dissociation ability and better micro-phase separation. The 1,3-propanediol cyclosulfate grafted polybenzimidazole with IEC of 1.85 (OPBI-OSO 3 H-1.85) reaches comparable area resistance (0.19 Ω cm−2) to the sulfonic acid grafted polybenzimidazole with much higher IEC of 2.95 (OPBI-SO 3 H-2.95), but 19.1% decrease in vanadium permeability. The cell assembled with OPBI-OSO 3 H-1.85 achieves high energy efficiency of about 86.7% and low decay of discharge capacity (0.41% per cycle), superior to that of OPBI-SO 3 H-2.95 (0.51% per cycle) and Nafion 212 (0.71% per cycle). The sulfate ester group is proved to be chemically stable in H 2 SO 4 /vanadium environment. • Superior acidic sulfate ester group is proposed for high conductive membrane in VRFB. • High proton conduction at lower IECs leads to low swelling and vanadium crossover. • EE 86.7% and capacity decay rate 0.41% outperform the benchmark sulfonic acid group. • Sulfate ester group is stable in H 2 SO 4 /vanadium after 300 cycles at 100 mA cm−2. [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]

Anionic conductive group repels vanadium ions but is difficult to be incorporated into the selectively ionic conductive channels of the amphoteric ion conductive membrane with tunable ratio. Herein, the novel hydroxyl bromopropane anionic precursor side chain is proposed, in which the hydrophilic hydroxyl groups aggregate into ionic clusters with the sulfate ester cationic conductive side chains of polybenzimidazole via hydrogen bond during membrane casting. The subsequent conversion of the bromopropane precursor to quaternary ammonium can easily tune the anionic exchange capacity and integrate cations into the hydroxyl involved ionic conductive channels for selectively repelling vanadium ions. Compared with Nafion 212 membrane, the obtained amphoteric membrane exhibits much lower vanadium ion permeability (1.9 × 10−9 cm2s−1) and slightly lower area resistance (0.25 Ω cm2). Excellent performance of vanadium redox flow battery (coulombic efficiency, 99.2%, energy efficiency, 85.9% at 100 mA cm−2 and 0.31% discharge decay rate) is achieved, and efficiencies keep stable during the 500 cycles test. The performance of VRFB is surpass than that of most reported amphoteric ion conductive and Nafion 212 membranes. [Display omitted] • Amphoteric ion conductive membrane is fabricated with –OSO 3 - and –OH–N+ side chains. • Anionic precursor –OH(CH 2) 2 Br involves in ionic cluster via hydrogen bond as casting. • Converting -Br to N+ in membrane promotes Vn + blocking in ionic conductive channel. • Performance superior to that of Nafion 212 and most recently reported AIEMs. [ABSTRACT FROM AUTHOR]

The development of high-performance and low-cost membrane is of significance towards vanadium flow battery (VFB). The membrane usually suffer from a trade-off challenge between conductivity and selectivity especially in long-term operation of flow battery. Herein, using a proton sponge drived by the proton absorption capacity of extended pyridine groups, a highly ion-selective membrane is fabricated. During acid-assisted doping behaviors, transport nanochannels with highly proton-selective conductivity are constructed based on well-defined nanophase separation structure. The protonated nanochannels display effectively acidic absorption during hydration process for enabling fast proton conduction (a low area resistance of 0.33 Ω cm2) and repelling vanadium ion (a low vanadium ion permeability of 3.04 × 10−10 cm2 s−1). Consequently, prepared membrane with proton sponge exhibits remarkable performance of high energy efficiency of 85.5%, together with excellent chemical durability over 1000 charge-discharge cycles (more than 1100 h) at 120 mA cm−2 in VFB application. These notable properties make prepared membrane as a potential alternative used in large-scale energy storage systems. A proton sponge drived by the proton absorption capacity of extended pyridine groups in membrane is constructed for highly ion-selective conductivity in flow battery. [Display omitted] • Proton sponge drived by extended pyridine groups is constructed in membrane. • The protonated transport nanochannels display highly proton-selective conductivity. • The prepared membrane has excellent durability over 1000 cycles in VFB application. [ABSTRACT FROM AUTHOR]

Sieving of H+/Vn + ions is essential to vanadium redox flow battery (VRFB). Herein, the interconnected hierarchical proton sieving-conductive channels are constructed through the graft of sulfated UIO-66 (UIO–66OSO3) along the polybenimidazole (PBI) backbone with the highly reactive and acidic sulfate ester functional group. Highly porous UIO-66OSO3 provides numerous angstrom scale sieving channels (about 5 Å) to transport proton and repel vanadium ion, and the sulfated groups ended flexible side chains are densely bonded to UIO-66 and aggregate into broad nanoscale ionic clusters (∼8 nm) for fast proton conduction between UIO-66OSO3 nanoparticles. The as-prepared membrane exhibits low vanadium permeability (7.88 × 10−9 cm2s−1) and area resistance (0.23 Ω cm−2, lower than that of Nafion 212). High energy efficiency (86.1%) and low discharge capacity decay (0.15% per cycle) are achieved even after 100 charge-discharge cycles at 100 mA cm−2, and the battery keeps running for 2000 cycles without morphological and chemical changes. The VRFB proprieties are far superior to Nafion 212 membrane VRFB proprieties (energy efficiency 74.9%, decay rate 0.71% per cycle), and also surpasses most recently reported porous, dense and hybrid ion conductive membranes. [Display omitted] • The hierarchical channels are constructed by sulfated UIO-66 and PBI backbone. • The hierarchical channels provide high H+/Vn + sieving and fast proton conduction. • High VRFB performance is achieved that surpass most of the recently reported ones. [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]

• COF based proton permselective membranes are designed for flow batteries. • Precise H+/Vn+ sieving is achieved by appropriate pore size (0.48 nm) of COF. • No vanadium permeation can be detected for the membrane. • Cell with the membrane shows improved efficiencies and long-term cycle stability. Covalent organic framework (COF) based proton permselective membranes are for the first time designed to break through the trade-off between proton permeability and selectivity for improved efficiencies of acid supporting redox flow batteries (the representative one is all vanadium redox flow battery). The precise hydrated proton (<0.24 nm)/vanadium (>0.6 nm) sieving is achieved originating from the appropriate pore size (0.48 nm) of COF prepared here. As a result, no vanadium permeation can be detected for the membrane. While, the presences of micropores and the amino groups in COF lead to a low area resistance. The cell assembled with this membrane presents very high coulombic efficiency (CE: 98.7–99.6%) and energy efficiency (EE: 93.5–76.4%) at 40–180 mA cm−2. In addition, the membrane is free of ion exchange groups, leading to excellent chemical stability. The membrane structure shows no changes after the cell steadily runs over 500 cycles (>1300 h) at 80 mA cm−2. This work provides a new orientation to design a membrane with an ultrahigh proton permselectivity for redox flow batteries. [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]

A novel hydrophilic/hydrophobic bi-comb-shaped amphoteric membrane was for the first time designed by the one-pot reactions of PBI with bromohexane and 1,3-propanesultone. The sulfonic acid chain promotes the formation of amphoteric ion clusters, while the alkyl side chain enhances the packing of hydrophobic segments. This unique structure design optimizes the segregated microphase morphology of the membrane, which is proved by TEM. Compared to the membrane without alkyl side chain, the bi-comb-shaped membrane shows a lowered area resistance (0.58 vs. 0.96 Ω cm2) as well as a reduced swelling ratio (11.7% vs. 14.3%). As a result, the cell with the bi-comb-shaped membrane exhibits improved voltage efficiency, thus achieving increased energy efficiency. In addition, the membrane of this structure occupies excellent cycle stability. The cell efficiencies maintain almost stable for 250 cycles, and the chemical structure has no change after immersing in the electrolyte solution for 28 days. • Hydrophilic/hydrophobic-bi-comb-shaped amphoteric membrane was prepared. • The membrane shows low area resistance and excellent alkaline stability. • The cell employing this membrane shows excellent performances. [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]