Amphoteric Membrane Research Articles

The effect of polymer with tunable alkyl spacers on the microstructure of amphoteric membrane for vanadium redox flow battery (VRB)

Amphoteric membranes, as promising candidates for VRBs, have attracted more attention. The side-chain-type polymers go far towards improving ion exchange capacity and further enhancing ion conductivity. A kind of Amphoteric ion exchange membranes (AIEMs) with different alkyl chain lengths (C2, C6, and C12 spacers) were synthesized by using ring-opening metathesis polymerization (ROMP). In addition, the interactions between N-containing groups with positively charged and -SO3- groups prevent swelling and vanadium crossover. The microphase separation structure through side chain induction was regulated, to respond to the ionic mobility of the membrane. The effect of the spacer lengths on the ion selectivity of AIEMs is investigated. In addition, the size and matrix of the constructed ion channel were calculated by using coarse-grained molecular dynamics (CGMD) simulation, which further verified the experiment.

Review of SPEEK Amphoteric Proton Exchange Membranes in All Vanadium Flow Batteries

Sulfonated polyether ether ketone (SPEEK) membranes have been widely used in the field of all vanadium flow batteries (VFRB) due to their simple structure, convenient preparation, good thermal and mechanical stability, low cost, and easy modification. However, its membrane performance largely depends on the degree of sulfonation. As the degree of sulfonation increases, the proton conductivity increases, but it also increases water uptake, leading to excessive swelling and vanadium ion penetration, thereby reducing the stability of the membrane and the performance of the battery. The introduction of alkaline functional groups can serve as proton acceptors to promote proton transport through the Grotthus mechanism, and on the other hand, they can form acid-base pairs with sulfonic acid groups. The resulting hydrogen bonds, acid-base interactions, ion bonds, and other interface interactions are beneficial for reducing the swelling rate of SPEEK membranes, and can also adjust the size of proton transport channels, constructing efficient proton transport channels that are both conducive to proton transport and can hinder the passage of vanadium ions, Improve the ion selectivity of membranes. Therefore, this article reviews the basic research and practical development status of SPEEK amphoteric membranes in VRFB, including the latest progress in various modification strategies. And evaluated the challenges and potential future research directions faced by the development of SPEEK membranes.

Anionic conductive group tunable amphoteric polybenzimidazole ion conductive membrane for vanadium redox flow battery

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.

Amphoteric Membrane Loaded with a Noble Metal-Free Hollow Spherical NiCoP@rGO Bifunctional Electrocatalyst for Alkaline Water Electrolyzers

Amphoteric Membrane Loaded with a Noble Metal-Free Hollow Spherical NiCoP@rGO Bifunctional Electrocatalyst for Alkaline Water Electrolyzers

Active material crossover suppression with bi-ionic transportability by an amphoteric membrane for Zinc–Bromine redox flow battery

Zinc-bromine redox flow batteries (Zn/Br2 RFBs) are gaining attention as a next-generation energy storage system with the advantages of a cost-effective redox couple material price, high output, and high energy density. However, bromine (Br2) crossover through a commercial porous membrane causes self-discharge to lower the capacity retention. Nafion, a commercial ion exchange membrane, can lower the crossover but has low voltage efficiency due to high membrane resistance. To address this trade-off, the amphoteric functionalized silica (Am-SiO2) is introduced into the Nafion membrane (Nafion/Am-SiO2). It suppresses the crossover of active materials such as Br2 and polybromide (Brn−) and possesses high ionic conductivity due to the quaternary ammonium and sulfonic groups on the Am-SiO2 surface. In addition, increasing the water content in the membrane prevents the expansion of the water cluster size, which could help balance bi-ionic transport. As a result, the composite membrane showed 83.3 and 19.0 times higher ion selectivity than the commercial porous membrane (SF600) and ion-exchange membrane (NRE-212). Compared to SF600 and NRE-212, the energy efficiency of Nafion/Am-SiO2 was improved by 4.2 and 6.4%, respectively. Balancing anion and cation transport can be successfully applied in Zn/Br2 RFBs by introducing an amphoteric group into an ion-exchange membrane.

Novel amphoteric ion exchange membranes by blending sulfonated poly(ether ether ketone) with ammonium polyphosphate for vanadium redox flow battery applications

AbstractA novel amphoteric ion exchange membrane for vanadium redox flow battery (VRFB) was explored by blending sulfonated poly(ether ether ketone) (SPEEK) and ammonium polyphosphate (APP). The high‐stability flame retardant of cross‐linked APP with a large number of NH4+ groups was first introduced into SPEEK membrane. It was observed that the addition of APP with special structure could achieve a good balance between proton conductivity and vanadium ions permeability. The abundant NH4+ in APP could block the penetration of vanadium ions by Donnan/Manning exclusion effect and ionic crossing networks due to the ionic bonds between cation and anion groups, and specially a small amount of APP within 5% could remarkably improve the proton conductivity of pristine SPEEK membrane might be ascribed to the unique fast proton transport channels formed by hydrogen bond networks and particular micro‐phase separation as a result of interaction between SPEEK and APP. When 5% APP was blended, the SPEEK/APP‐5% (S/APP‐5%) amphoteric membrane showed a higher selectivity of 20.87 × 104 S min/cm3 (with a good proton conductivity of 0.075 S/cm and a lower VO2+ permeability of 3.45 × 10−7 cm2/ min) and presented better thermal and chemical stability compared to Nafion115 and SPEEK membranes. The VRFB single cell assembled with S/APP‐5% amphoteric membrane exhibited more excellent performance than that of Nafion115 and pristine SPEEK membranes, which revealed a higher coulombic efficiency of 96.3%–98.3%, comparable voltage efficiency of 88.4%–78.7% and higher energy efficiency of 85.1%–77.4% from 40 to 80 mA/cm2, respectively, and showed relatively good stability of the efficiency up to 50 cycles at 60 mA/cm2. The results demonstrated that the designed S/APP amphiprotic membrane of outstanding selectivity, high battery efficiency, and good durability is a prospected VRFB separator.

Bi-functional side chain architecture tuned amphoteric ion exchange membranes for high-performance vanadium redox flow batteries

The blended amphoteric membranes possessed a reducing cation exchange capacity with increasing the anion exchange capacity. Bi-functional side-chain-type polymers are conducive to have both high cation exchange capacity and high anion exchange capacity at the same time, and further enhance ion conductivity of amphoteric ion exchange membranes (AIEMs). Herein, novel side-chain type AIEMs are prepared via ring-opening metathesis polymerization of norbornene possessing the advantages of yielding easily modified molecular architectures, simple and controllable polymerization, and producing the membranes with high chemical stability. To avoid the occurrence of excess water swelling arising from the increased zwitterionic groups, hydrophobic spacers (alkyl chain-(CH2)6 -) were introduced to be situated between the aromatic main chain and ionic groups. This unique structure favors ion clusters to be aggregated and thus forms highly ordered water channels so that the protons can be efficiently transported along the center of channels and possess excellent proton conductivity. In addition, the ion crosslinking interactions enhance the mechanical stability of AIEMs. The resultant AIEM (1/2) exhibits a higher ion conductivity of 7.58 × 10-2 S cm-1 and vanadium ion permeability of 0.21 × 10-9 cm2 s-1 at room temperature. Based on these good properties, the vanadium redox flow batteries (VRBs) with prepared membranes and Nafion 212 were tested. The energy efficiency (EE) of VRBs with AIEM (1/2) still reaches 84.76% at the current density of 160 mA cm-2, which is better than that with Nafion 212 (76.26%). Moreover, the EE of VRBs with AIEM (1/2) is 82.11% after 200 cycles, indicating that the prepared membranes possess a good stability.

Highly selective sulfonated poly(ether ether ketone)/polyvinylpyrrolidone hybrid membranes for vanadium redox flow batteries

A novel amphoteric membrane was designed by blending polyvinylpyrrolidone (PVP) with sulfonated poly(ether ether ketone) (SPEEK) to fabricate a vanadium redox flow battery. Acid–base pairs were formed by sulfonic acid and heterocyclic nitrogen, and the dense network structures were interwoven by the hydrogen bonds between the acid–base pairs. Vanadium ions were blocked by these network structures such that their penetration was weakened. Compared with Nafion115 and SPEEK membranes, the ion selectivity of SPEEK/PVP (S/P) hybrid membranes of various proportions was improved, of which the S/P-30% membrane performed best (106.1*104 S min cm−3). At the same time, these hybrid membranes showed far superior performances than the single-component membranes in a single-cell system. Most notably, the coulombic efficiency and energy efficiency achieved by the battery with a S/P-30% hybrid membrane were 97.5% and 84.5% at 60 mA cm−2, respectively, with notable stability after 50 cycles. The corresponding open-circuit voltage of the battery was maintained above 0.8 V for more than 73 h. This acid–base hybrid membrane has reached a high level of overall performance in binary non-fluorine proton exchange membranes in recent years.

Improved chemical stability and proton selectivity of semi‐interpenetrating polymer network amphoteric membrane for vanadium redox flow battery application

AbstractIn this work, semiinterpenetrating polymer network (semi‐IPN), consisting of sulfonated poly (arylene ether sulfone) (SPAES) and crosslinked vinyl imidazole grafted polysulfone (VMPSU), is prepared and characterized. FTIR, EDS, and solubility test indicate the successful preparation of amphoteric membranes. The semi‐IPN amphoteric membranes exhibit better stability than pure SPAES membrane, as demonstrated by thermogravimetric analysis and ex situ immersion testing results. More importantly, it is shown that the amphoteric membrane can effectively hinder vanadium ion crossover through the membrane, which is attributed to the semi‐IPN structure and Donnan exclusion. As expected, the amphoteric membrane containing 20% VMPSU exhibits the highest proton selectivity (6.86 × 104 S min cm−3), comparing to pristine SPAES (1.90 × 104 S min cm−3) as well as Nafion117 (1.31 × 104 S min cm−3).

Covalent/ionic co-crosslinking constructing ultra-densely functionalized ether-free poly(biphenylene piperidinium) amphoteric membranes for vanadium redox flow batteries

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.

Hydrophilic/hydrophobic-bi-comb-shaped amphoteric membrane for vanadium redox flow battery

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.

A new long-side-chain sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) /polybenzimidazole (PBI) amphoteric membrane for vanadium redox flow battery

A new amphoteric membrane was prepared by blending long-side-chain sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (S-L-PPO) and polybenzimidazole (PBI) for vanadium redox flow battery (VRFB) application. An acid–base pair structure formed between the imidazole of PBI and sulfonic acid of S-L-PPO resulted in lowered swelling ratio. It favors to reduce the vanadium permeation. While, the increased sulfonic acid concentration ensured that proton conductivity was still at a high level. As a result, a better balance between the vanadium ion permeation (6.1 × 10−9 cm2·s−1) and proton conductivity (50.8 mS·cm−1) in the S-L-PPO/PBI-10% membrane was achieved. The VRFB performance with S-L-PPO/PBI-10% membrane exhibited an EE of 82.7%, which was higher than those of pristine S-L-PPO (81.8%) and Nafion 212 (78.0%) at 120 mA·cm−2. In addition, the S-L-PPO/PBI-10% membrane had a much longer self-discharge duration time (142 h) than that of Nafion 212 (23 h). Open Science Identity:

Cross-linked amphoteric membrane: Sulphonated poly(ether ether ketone) grafted with 2,4,6-tris(dimethylaminomethyl)phenol using functionalized side chain spacers for vanadium redox flow battery

Herein, we report a novel synthesis of sulphonated poly(ether ether ketone) (SPK) grafted with 2,4,6-tris(dimethylaminomethyl)phenol (TAP) for preparation of amphoteric ion-exchange membrane (AIEM). Cross-linking has been achieved by using di-bromo alkane with different number of carbon chains (C6 & C10), and different AIEMs are assessed for vanadium redox flow battery (VRFB). These AIEMs are architected to contain –SO3H groups maintained conductivity, while –NH3+ groups for reduction in vanadium permeability with improve charge-discharge time via acid-base interactions. Structural and surface morphology of AIEMs have been elucidated by different spectral techniques (FTIR, 1H NMR, SEM, and AFM). Cross-linking and acid-base interactions effectively reduced water uptake, swelling ratio, VO+2 permeability and improved ion-exchange capacity, conductivity, along with membrane stabilities. Due to good synergy between conductivity and VO+2 permeability, cross-linked AIEM with six number of carbon chains (CQSPK-6) exhibited higher Coulombic (CE: 98.4% vs 91.2%) and energy efficiency (81.4% vs~76%) than Nafion 117 membrane at 60 mA cm−2 in single cell VRFB. Further, CQSPK-6 AIEM maintained stable performance during in-situ cycle test (100 cycles) at 60 mA cm−2. Hence, AIEM based on cross-linked SPK grafted with high molality of functional groups via side chain spacer is a promising candidate for an efficient VRFB system.

Amphoteric-Side-Chain-Functionalized "Ether-Free" Poly(arylene piperidinium) Membrane for Advanced Redox Flow Battery.

To solve the stability issue of cost-effective nonfluorinated membranes, an ether-free poly(arylene piperidinium) (PBPip)-based membrane is first applied in redox flow batteries (RFBs). For improved efficiencies of RFB, amphoteric side chains are introduced onto the PBPip. Without an ether bond in the polymer backbone, the membrane shows a good stability in a strong oxidation environment. The Fourier transform infrared (FTIR) spectra exhibit no obvious changes over 30 days of oxidation test. Different from traditional blended amphoteric membranes, the amphoteric side chain allows both cation- and anion-exchange capacities to increase with grafting degree, which leads to a very high total ion-exchange capacity (IEC) (4.19 mmol g-1). Outstanding ion-conduction ability (area resistance: 0.22 Ω cm2) comparable to Nafion 212 (0.24 Ω cm2) is consequently achieved. Ionic cross-linking structure between cationic and anionic groups results in a low swelling rate (13.9%). Combined with the repelling effect of positively charged piperidinium, a low VO2+ permeability (1.31 × 10-8 cm2 s-1) is accomplished. On the basis of these good properties, the membrane exhibits excellent vanadium battery performances, especially at high current densities. The VE and EE both exceed 80% even at 200 mA cm-2. The battery performances have no obvious reductions after 500 cycles. These results indicate that this work provides a new orientation to design the membrane for RFB.

Amphoteric Membranes Based on Sulfonated Polyether Ether Ketone and Imidazolium‐Functionalized Polyphenylene Oxide for Vanadium Redox Flow Battery Applications

AbstractAmphoteric membranes were obtained from imidazolium‐functionalized polyphenylene oxide (ImPPO) and sulfonated poly(ether ether ketone) (SPEEK) for an all‐vanadium redox flow battery (VRFB) application. The SPEEK/ImPPO amphoteric membranes showed a considerably lower swelling degree and water uptake compared to those of pure SPEEK membranes, owing to the formation of ionic bonds between the cationic imidazolium groups of ImPPO and the anionic sulfonate groups of SPEEK. The cationic imidazolium groups of the ImPPO exhibited a Donnan exclusion effect on vanadium ions; therefore, the amphoteric membranes showed a much lower vanadium permeability than the pure SPEEK membrane. In addition, the vanadium permeability of the amphoteric membranes deceased with an increase in the ImPPO content. SPEEK‐ImPPO20 with 20 wt% ImPPO showed a vanadium permeability of 0.43×10−9 cm2 s−1 at room temperature, which is 17 times lower than that of Nafion 212 (8×10−9 cm2 s−1). The VRFB assembled with SPEEK‐ImPPO20 showed a coulombic efficiency of 97.2 % and a voltage efficiency of 85.8 % at a current density of 60 mA cm−2, whereas the values for Nafion 212 were 92.4 % and 86.7 % under the same conditions. In addition, the VRFB assembled with SPEEK‐ImPPO20 exhibited much better capacity retention than that assembled with Nafion 212. These results indicate that the SPEEK‐ImPPO amphoteric membrane is a promising material for VRFB applications.

Study on Nafion/Nafion-g-poly (sulfobetaine methacrylate)-blended amphoteric membranes for vanadium redox flow battery

In order to reduce the vanadium ion permeability while still keep its high stability, atom radical polymerization (ATRP) method was used to introduce zwitterionic sulfobetaine methacrylate (SBMA) onto the Nafion matrix in homogenous solution. The Nafion-grafted poly SBMA (N-g-PSBMA) solution was then used to blend with common Nafion solution to prepare Nafion/Nafion-g-PSBMA (N/N-g-PSBMA) amphoteric membranes by solution casting method. The grafted resins and membranes were characterized by attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR), energy dispersive X-Ray spectroscopy (EDX), scanning electron microscopy (SEM), and water-contact angle analysis. The results prove that the SBMA has been successfully grafted onto the Nafion backbones. Furthermore, the N/N-g-PSBMA membranes have shown higher ion selectivity, coulombic efficiency, and energy efficiency than that of pure recast Nafion (r-Nafion) membrane for VRB application. At the current density of 40–80 mA cm−2, the average energy efficiency of the VRB with N/N-g-PSBMA-20% membrane has reached up to 84.9%, which is 4.2% higher than that of the VRB with the r-Nafion membrane at the same current densities. Moreover, the 100-cycles charge-discharge test proves that the N/N-g-PSBMA-20% membrane possesses higher stability and capacity retention ability than that of r-Nafion membrane, confirming the great potential of such membranes for VRB application.

Novel Triple Tertiary Amine Polymer-Based Hydrogen Bond Network Inducing Highly Efficient Proton-Conducting Channels of Amphoteric Membranes for High-Performance Vanadium Redox Flow Battery.

A novel amphoteric membrane was designed by blending triple tertiary amine-grafted poly(2,6-dimethyl-1,4-phenylene oxide) (PPO-TTA) with sulfonated poly(ether ether ketone) (SPEEK) for vanadium redox flow batteries. An "acid-base pair" effect is formed by the combination of the tertiary amine group and sulfonic group, and extra nonbonding amine groups could be protonated. Both of them constitute a hydrogen bond network, which facilitates proton conduction and also hinders vanadium permeability because of the lowered swelling ratio and Donnan effect. All these contribute to improve the ion selectivity of the membrane while maintaining ionic conductivity. Compared with other amphoteric and SPEEK-based membranes, the membrane exhibits an excellent performance. The amphoteric membrane containing 15% PPO-TTA exhibits an ultralow vanadium permeability of 3.4 × 10-9 cm2 s-1 and a low area resistance of 0.39 Ω cm-2. Consequently, the cell assembled with this membrane shows excellent performances far superior to SPEEK and Nafion 212. The Coulombic efficiency and energy efficiency of the cell are 94.3-98.3 and 90.3-77.1% at 40-200 mA cm-2, respectively, and have no significant reductions after 200 cycles. This performance is at a high level among the amphoteric and SPEEK-based membranes reported in recent years. The cell's open circuit voltage is maintained for up to 165 h. In addition, the membrane's chemical stability is improved by the effective barrier to the vanadium ion.

Treatment of Methylene Blue and Methyl Orange Dyes in Wastewater by Grafted Titania Pillared Clay Membranes.

Background: Amphoteric membranes such as Titania Pillared Clay (TiPILC) membranes exhibit different charges at different pH ranges, subsequently show efficient separation for cationic and anionic organic pollutants in wastewater based on solution pH. The structural properties of such membranes determine their permeability and selectivity to a large extent. The present work is initiated to explore the possibility of enhancing the surface area of TiPILC membranes by grafting with 3-aminopropyltriethoxysilane.Method: Grafted TiPILC membranes were fabricated by colloidal sol- gel route followed by dip coating, drying and sintering the membranes at 300oC. X-ray diffraction (XRD) spectra of the pillared photocatalyst membranes show the presence of only anatase phase, which exhibits better photocatalytic activity. Fourier Transform Infrared (FT-IR) spectra reveal the formation of delaminated TiPILC material. The specific surface area of TiPILC membrane obtained at pH3.5 is 183m2/g.Results: Literature survey of 47 papers was carried out for the work. 10 papers were based on patents being dealt on degradation of organic dyes in wastewater by conventional methods, though reports of membrane technology are limited in literature. The rest 37 papers outlined the importance of widespread use of titania photocatalyst powder for degradation studies.Conclusion: The membrane material thus developed is efficient in complete decolouration of both Methylene Blue and Methyl Orange dyes on account of the high surface charge and high surface area in just one cycle of filtration.

Tackling capacity fading in vanadium flow batteries with amphoteric membranes

Capacity fading and poor electrolyte utilization caused by electrolyte imbalance effects are major drawbacks for the commercialization of vanadium flow batteries (VFB). The influence of membrane type (cationic, anionic, amphoteric) on these effects is studied by determining the excess and net flux of each vanadium ion in an operating VFB assembled with a cation exchange membrane (CEM), Nafion® NR212, an anion exchange membrane (AEM), Fumatech FAP-450, and an amphoteric ion exchange membrane (AIEM) synthesized in-house. It is shown that the net vanadium flux, accompanied by water transport, is directed towards the positive side for the CEM and towards the negative side for the AEM. The content of cation and anion exchange groups in the AIEM is adjusted via radiation grafting to balance the vanadium flux between the two electrolyte sides. With the AIEM the net vanadium flux is significantly reduced and capacity fading due to electrolyte imbalances can be largely eliminated. The membrane's influence on electrolyte imbalance effects is characterized and quantified in one single charge-discharge cycle by analyzing the content of the four different vanadium species in the two electrolytes. The experimental data recorded herewith conclusively explains the electrolyte composition after 80 cycles.

A novel imidazolium-based amphoteric membrane for high-performance vanadium redox flow battery

A novel amphoteric membrane was prepared using imidazolium-functionalized polysulfone (ImPSf) as the base polymer and sulfonated poly (ether ether ketone) (SPEEK) as the acid polymer for vanadium flow battery applications. The introduction of the ImPSf effectively resists the migration of vanadium ions owing to the Donnan repelling effect of the imidazolium cations on the vanadium ions and the low swelling ratio of the amphoteric membranes due to the ionic cross-linked interactions between the imidazolium and sulfonic groups. The amphoteric membrane with an ImPSf mass ratio of 17% shows a vanadium permeability of 1.5 × 10−8cm2s−1 that is much lower than that of Nafion212 (21 × 10−8cm2s−1). Having a high IEC of 2.04mmolg−1, it also exhibits a low area resistance of 0.48Ωcm−2 that is comparable to that of Nafion212 membrane (0.41Ωcm−2). As a result, the VFB cell with the amphoteric membrane exhibits high performances even at high current densities, i.e. at a current density of 200mAcm−2, and the amphoteric membrane achieves a coulombic efficiency of 97.5% and an energy efficiency of 77.3%, both of which are higher than those of Nafion212 (92.4% and 73.4%). These results indicate that the ImPSf/SPEEK amphoteric membrane is promising for VFB applications.