Vanadium Redox Flow Battery Application Research Articles

Novel ether-free membranes based on poly(p-terphenylene methylimidazole) for vanadium redox flow battery applications

The fabrication of low-cost and high-performance membranes is one of the most critical issues for vanadium redox flow batteries (VRFBs). Herein, a novel ether-free poly(p-terphenylene methylimidazole) (PTMIm) is synthesized by a facile one-pot Friedel-Crafts polyhydroxyalkylation of p-terphenyl and 1-methyl-2-imidazolecarboxaldehyde. The PTMIm has excellent organic solubility and good chemical stability towards high valence vanadium ions. Meanwhile, due to the present pendant imidazole groups, the PTMIm membrane exhibits a high sulfonic acid absorption capability, achieving a suitable area resistance (AR). In order to further enhance the performance of the PTMIm membrane, side-chain quaternization is carried out by using 1-bromopropane, 3-bromo-N,N,N-trimethylpropan-1-aminium bromide and 3-chloro-2-hydroxypropyltrimethyl ammonium chloride as quaternized reagents. Comparing with pure PTMIm membrane and propyl group grafted membrane (i.e. PTMIm-C3), terminal quaternary ammonium side chain grafted membranes of PTM-C3-QA and PTMIm-C3-OH-QA possess lower AR and better vanadium ion resistance simultaneously. Consequently, the ion selectivity value of the PTMIm-C3-QA membrane is 8.7 × 105 S min cm−3, which is approximately 130 times higher than that of Nafion 115 (6.6 × 103 S min cm−3), mainly resulting from its low AR (0.68 Ω cm2) and ultralow vanadium ion permeability (9.5 × 10−9 cm2 min−1). Meanwhile, both PTMIm and its grafted membranes exhibit excellent chemical stabilities within 400 h chemical stability test. Owing to the superior restriction on vanadium migration, the single cell with PTMIm-C3-QA displays an excellent self-discharge duration of 500 h, which is greatly longer than that of Nafion 115 (58 h). In the range of 40–160 mA cm−2, the PTMIm-C3-QA based cell expresses higher coulombic efficiencies (>99.5%) and larger energy efficiencies than the cell with Nafion 115. For the cycling test of VRFB during 500 cycles, the PTMIm-C3-QA based cell displays excellent cycle stability and good discharge capacity retention, indicating a great application potentiality of the PTMIm-C3-QA membrane for VRFBs.

Versatility of sulfonated poly (vinylidene fluoride‐co‐hexafluoropropylene) membranes incorporated with sulfonated octaphenyl polyhedral oligomeric silsesquioxane for vanadium redox flow battery applications

AbstractHybrid membranes are fabricated using the blends of sulfonated poly (vinylidene fluoride‐co‐hexafluoropropylene) (SPVDF‐co‐HFP) and sulfonated octaphenyl polyhedral oligomeric silsesquioxane (SOPS) as an alternate for expensive Nafion in vanadium redox flow battery (VRFB) applications. The FT‐IR and TEM investigation indicates the successful sulfonation of OPS and uniform distribution of SOPS. The morphological study of hybrid membranes by SEM–EDX and FESEM revealed the excellent dispersing ability of SOPS on a SPVDF‐co‐HFP matrix. The physicochemical analysis of SPVDF‐co‐HFP/SOPS membranes demonstrated an increase in hydrophilic ion‐exchangeable sites. The SPVDF‐co‐HFP/SOPS (0.5 wt.%) membrane displayed a maximum proton conductivity of 6.27 × 10−3 Scm−1 at 80°C because of strong hydrogen bond formation between SPVDF‐co‐HFP and SOPS facilitates proton migration. Besides, the cage‐like structure of SOPS effectively shields vanadium‐ion permeation to 1.21 × 10−8 cm2 min−1 and thus, showed a maximum membrane selectivity of 43.05 × 104 Scm−3 min. Overall, the hybrid membranes of SPVDF‐co‐HFP/SOPS were considered to be a better substitute for Nafion in VRFBs.

TiO2 Containing Hybrid Composite Polymer Membranes for Vanadium Redox Flow Batteries.

In recent years, vanadium redox flow batteries (VRFB) have captured immense attraction in electrochemical energy storage systems due to their long cycle life, flexibility, high-energy efficiency, time, and reliability. In VRFB, polymer membranes play a significant role in transporting protons for current transmission and act as barriers between positive and negative electrodes/electrolytes. Commercial polymer membranes (such as Nafion) are the widely used IEM in VRFBs due to their outstanding chemical stability and proton conductivity. However, the membrane cost and increased vanadium ions permeability limit its commercial application. Therefore, various modified perfluorinated and non-perfluorinated membranes have been developed. This comprehensive review primarily focuses on recent developments of hybrid polymer composite membranes with inorganic TiO2 nanofillers for VRFB applications. Hence, various fabrications are performed in the membrane with TiO2 to alter their physicochemical properties for attaining perfect IEM. Additionally, embedding the -SO3H groups by sulfonation on the nanofiller surface enhances membrane proton conductivity and mechanical strength. Incorporating TiO2 and modified TiO2 (sTiO2, and organic silica modified TiO2) into Nafion and other non-perfluorinated membranes (sPEEK and sPI) has effectively influenced the polymer membrane properties for better VRFB performances. This review provides an overall spotlight on the impact of TiO2-based nanofillers in polymer matrix for VRFB applications.

Facile strategy for preparing a novel reinforced blend membrane with high cycling stability for vanadium redox flow batteries

• The ether-free backbone and acid-base interaction greatly improve the structural stability of the acid-base blend membrane. • The acid-base interaction and Donnan effect contribute to the superb ion selectivity of the blend membrane. • The membrane exhibits a remarkable cycling stability with high performance in VRFB. Herein, to resolve issues regarding the long-term stability of cost-effective non-fluorinated membranes for Vanadium redox flow batteries (VRFBs), we design a facile strategy for fabricating a composite membrane based on long-side-chain Sulfonated poly(terphenylene) (SPTP) blended with Polybenzimidazole (PBI). The introduction of PBI in the ether-free SPTP matrix effectively mitigates the transportation of vanadium ions owing to the Donnan repelling effect and the low swelling ratio of the membranes caused by the acid-base interactions between the imidazolium and sulfonic groups. 1 H NMR and XPS spectra confirm the chemical structure of the prepared membranes. The optimized membrane achieves a lower area resistance, of 0.34 Ω cm 2 , as well as a low vanadium permeability, of 1.62 × 10 −8 cm 2 min −1 , in comparison to that of Nafion117 (0.4 Ω cm 2 and 4.2 × 10 −7 cm 2 min −1 ). Consequently, VRFBs that utilize such membranes exhibit superior coulombic and energy efficiencies—of 99.5% and 86.2% at 100 mA cm −2 , respectively—which are an improvement on those of Nafion117 (at 96.1% and 80.9%, respectively). It is important to note that the ether-free backbone, electrostatic repulsion, and reinforcement of PBI greatly improve the mechanical and chemical stability of the blend membranes. Therefore, the C8 membrane presents excellent cycling stability, of up to 1500 charge–discharge cycles, with no significant attenuation in the CE/EE under strong acidic and oxidizing conditions, demonstrating its high physicochemical stability. The results of this study indicate that the SPTP/PBI blend membranes exhibit promising prospects for application in VRFBs.

Electron‐Deficient Sites for Improving V2+/V3+ Redox Kinetics in Vanadium Redox Flow Batteries

AbstractFundamental kinetics of the V2+/V3+ of vanadium redox flow battery (VRFB) are still not well understood despite tremendous efforts in improving the sluggish kinetics of V2+/V3+. This article first reveals the rate‐determining step in the electrochemical oxidation of V2+ to V3+ by exploring the reaction kinetics. Thereafter, TiB2 with abundant electron‐deficient sites, which possesses a strong electron‐accepting ability, is demonstrated to improve the rate‐determining step of V2+/V3+ by enhancing the electron transfer from V2+ to the electrode. The mechanism of TiB2 for boosting V2+/V3+ kinetics is also unraveled by analyzing the reaction order. VRFB with the electron‐deficient TiB2 demonstrates a remarkable enhancement in the electrochemical performance, exhibiting an excellent rate performance from 70 to 300 mA cm−2. The energy efficiency is improved by 14.06% compared to the cell with the pristine electrode at 150 mA cm−2 for 300 cycles. This study is critical for not only proposing the promising electron‐deficient catalyst in VRFB application but also promoting fundamental understanding and offering a design strategy for achieving superior performance electrocatalysts in VRFB.

Highly ion-selective sulfonated polyimide membranes with covalent self-crosslinking and branching structures for vanadium redox flow battery

With an innovatively synthesized X-shaped tetramine monomer 4,4',4'',4'''-(1H,3'H-[5,5'-bibenzo[d]imidazole]) tetraaniline with imidazole groups, a series of novel sulfonated polyimide membranes with covalent self-crosslinking and branching structures (sc-bSPI-x) are designed and prepared for application in vanadium redox flow battery (VRFB). The synergistic effects of protonated imidazole and sulfonic acid are supposed to create proton transport channels and Donnan exclusion, leading to break the trade-off between proton conductance and vanadium ion resistance. Among all the prepared sc-bSPI-x membranes, the sc-bSPI-14 membrane with 14% theoretical cross-linking degree shows the highest ion selectivity (2.78 × 105 S min cm−3), which is about 6.5 times of commercial Nafion 212 membrane (0.43 × 105 S min cm−3). The VRFB performances including self-discharge time, coulombic and energy efficiencies at 80–200 mA cm−2 of sc-bSPI-14 membrane are superior to those of Nafion 212 membrane (sc-bSPI-14: self-discharge time 46 h, CE = 97.6–99.2% and EE = 82.9–63.2%; Nafion 212: self-discharge time 11 h, CE = 86.5–94.5% and EE = 78.4–61.4%). Meanwhile, the 1000-time cycling charge–discharge test of sc-bSPI-14 membrane at 140 mA cm−2 is performed, demonstrating its outstanding operational stability. The results indicate that the proposed sc-bSPI-14 membrane is a promising candidate for VRFB application.

Novel branched sulfonated polyimide membrane with remarkable vanadium permeability resistance and proton selectivity for vanadium redox flow battery application

A series of novel branched sulfonated polyimide (bSPI-x) membranes with 8% branched degree are developed for application in vanadium redox flow battery (VRFB). The sulfonation degrees of bSPI-x membranes are precisely regulated for obtaining excellent comprehensive performance. Among all bSPI-x membranes, the bSPI-50 membrane shows strong vanadium permeability resistance, which is as 8 times as that of commercial Nafion 212 membrane. At the same time, the bSPI-50 membrane has remarkable proton selectivity, which is four times as high as that of Nafion 212 membrane. The bSPI-50 membrane possesses slower self-discharge speed than Nafion 212 membrane. Furthermore, the bSPI-50 membrane achieves stable VRFB efficiencies during 200-time charge-discharge cycles at 120–180 mA cm−2. Simultaneously, the bSPI-50 membrane exhibits excellent capacity retention compared with Nafion 212 membrane. All results imply that the bSPI-50 membrane possesses good application prospect as a promising alternative separator of VRFB.

A coupled-layer ion-conducting membrane using composite ionomer and porous substrate for application to vanadium redox flow battery

A thin ionomer layer composed of perfluorosulfonic acid (PFSA) polymer in composite with functionalized alkoxysilane (FAS) is prepared and coated onto one side of a porous polyethylene (PE) substrate for vanadium redox flow battery (VRFB) to produce a coupled-layer membrane. The approaches are: (1) suppression of permeation through membrane using additive component, (2) the use of thin ionomer layer for lowering the amount of costly PFSA, and (3) strengthening the thin ionomer layer by coating onto an inexpensive PE substrate. The membrane is easily prepared using automatic control coater. The observation of morphology and infrared spectroscopy at both sides of the coupled-layer membrane confirm the thin ionomer layer on only one side of the PE substrate, plus the addition of FAS helps achieve lower diffusion coefficient. After the unit cell cycling, the coupled-layer membrane shows the good cycling performance closing to Nafion 212 membrane level. Additionally, the morphology after cycling of the coupled layers between PFSA ionomer and PE substrate can remain the same as in initial state, indicating a stable combination from both layers. Based on the easy production and the good performance, the developed membrane can be readily adapted by membrane manufacturer for the wide application in VRFB. • A thin ionomer layer is coated on one side of porous polyethylene substrate. • The thin layer reduces amount of costly perfluorinated sulfonic acid ionomer. • VRFB cell performance was good by closing to that of Nafion 212 membrane. • Unchanged morphology between two layer was found after VRFB unit cell operation.

A sulfonated polyethylene–styrene cation exchange membrane: a potential separator material in vanadium redox flow battery applications

Benchmarking of a polyethylene–styrene–divinylbenzene-based interpolymer cation exchange membrane for vanadium redox flow batteries.

A low vanadium permeability sulfonated polybenzimidazole membrane with a metal-organic framework for vanadium redox flow batteries

A proton selective membrane based on a metal-organic framework (MOF) and polybenzimidazole (PBI) with a controllable sulfonation degree is designed for vanadium redox flow battery (VRFB) applications. The size of UiO-66 is between hydrated multivalent vanadium ions and stokes radius of protons, and its suitable pore size can obstruct the penetration of vanadium ions, resulting in a high vanadium resistance rate. Simultaneously, the imidazole groups and sulfonic acid groups in the sulfonated polybenzimidazole compose the acid-base pairs, which block the penetration of vanadium ions. Therefore, sulfonated PBI together with UiO-66 forms a denser network structure for as a vanadium ion barrier. Compare with the Nafion117 film, the vanadium transmittance of prepared composite film is lower (5.15 × 10−7 cm2 min−1 vs. 2.95 × 10−9 cm2 min−1). Compare with the Nafion117 membrane, VRFB with MOF polymer exhibits higher voltage efficiency (94.54%–74.93%) and energy efficiency (84.82%–73.98%) at 40-200 mA•cm−2.

Thermally cross-linked sulfonated poly(ether ether ketone) membranes containing a basic polymer-grafted graphene oxide for vanadium redox flow battery application

Sulfonated poly(ether ether ketone) (SPEEK) with a very high degree of sulfonation (DS) and poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) are prepared and used as a polymer matrix and a filler, respectively, to prepare thermally cross-linked membranes for vanadium redox flow battery (VRFB) application. The thermally cross-linked SPEEK membranes containing ABPBI-GO exhibit significantly enhanced physicochemical stabilities due to the formation of the well-developed cross-linked structures by the chemical bonds formed through the solvothermal process and the acid–base interactions between the benzimidazole groups in filler and sulfonic acid groups in polymer chain. In addition, ABPBI-GO in the polymer matrix is found to be effective in decreasing the vanadium ion permeability while retaining high proton conductivity producing a high-performance VRFB membrane. The ionic selectivity of the thermally cross-linked SPEEK membrane containing 0.5 wt% of ABPBI-GO (TC-SPEEK/0.5) is 13.15 × 107 mS min cm−3, which is approximately 8 times larger than that of Nafion 212 (1.57 × 107 mS min cm−3). Accordingly, the VRFB cell fabricated using TC-SPEEK/0.5 membrane exhibits higher coulombic efficiencies (CE: 98.1–99.4%), voltage efficiencies (VE: 92.5–85.2%), and energy efficiencies (EE: 90.7–84.7%) than that using Nafion 212 (CE: 89.2–93.3%, VE: 90.7–81.2%, EE: 80.9–75.7%) at current density of 40–100 mA cm−2. Furthermore, the long-term stability of the VRFB cell using TC-SPEEK/0.5 membrane evaluated by the cycle test under realistic VRFB operation conditions is also found to be much better than that using Nafion 212.

Celgard/PIM‐1 proton conducting composite membrane with reduced vanadium permeability

AbstractRenewable energy systems need efficient and cheap storage devices and vanadium redox flow batteries (VRB) may become one of them. However, better performance of VRB membranes yet should be achieved. Novel composite Celgard‐based films coated with PIM‐1 for aqueous electrolyte VRB applications are presented. Two types of the composites with different PIM‐1 loadings are obtained. Their properties are studied and compared with the original Celgard films. The deposited PIM‐1 forms a smooth layer on the outer Celgard surface and penetrates inside the porous matrix, thus significantly reducing the pores diameter and affecting the transport properties of the composite film. The nanoporous structure of PIM‐1 permits size‐screening of H3O+/hydrated vanadium ions when used in aqueous vanadium redox flow batteries applications, which allows to tailor the membrane permeability for the two types of ions and, therefore, to increase its selectivity from 4.3 × 106 to 1.3 × 107 mS min cm−3, while maintaining high proton conductivity.

Non-precious transition metal based electrocatalysts for vanadium redox flow batteries: Rational design and perspectives

The vanadium redox flow battery (VRFB) is promising for large-scale energy storage, but commercial electrodes, such as graphite felt (GF), suffer from poor electrochemical activity caused by sluggish kinetics and high polarization, leading to a need for high performance and cost-effective electrocatalysts. Non-precious transition metal based materials show great promise as electrocatalysts for VRFB due to their activity and low cost. However, limited exposed active sites and their low intrinsic conductivity, in addition to the unsettled relationship between the structure and performance, remain to be solved. To clarify rational design principles and strategies for obtaining high-performance non-precious transition metal based electrocatalysts in VRFB, this review summarizes their recent advances, primarily focusing on defect engineering, engineering of hybrid materials, and hierarchical porous structure designing. In addition, we propose the challenges and prospects of non-precious transition metal based materials in VRFB, which should inspire insight into designing and optimizing non-precious transition metal based electrocatalysts in VRFB, as well as promoting the commercial application of VRFB. • Insightful review on non-precious transition metal based electrocatalysts for VRFB. • Strategies to improve the electrochemical performance are summarized. • The relationship between designing strategies and performance is analyzed. • Challenges and perspectives of this kind of electrocatalysts in VRFB are proposed.

A Sulfonated Polyimide/Nafion Blend Membrane with High Proton Selectivity and Remarkable Stability for Vanadium Redox Flow Battery.

A sulfonated polyimide (SPI)/Nafion blend membrane composed of a designed and synthesized SPI polymer and the commercial Nafion polymer is prepared by a facile solution casting method for vanadium redox flow battery (VRFB). Similar molecular structures of both SPI and Nafion provide good compatibility and complementarity of the blend membrane. ATR-FTIR, 1H-NMR, AFM, and SEM are used to gain insights on the chemical structure and morphology of the blend membrane. Fortunately, the chemical stability of the SPI/Nafion blend membrane is effectively improved compared with reported SPI-based membranes for VRFB applications. In cycling charge-discharge tests, the VRFB with the as-prepared SPI/Nafion blend membrane shows excellent battery efficiencies and operational stability. Above results indicate that the SPI/Nafion blend membrane is a promising candidate for VRFB application. This work opens up a new possibility for fabricating high-performance SPI-based blend membrane by introduction of a polymer with a similar molecular structure and special functional groups into the SPI polymer.

Poly(fluorenyl ether ketone)s with densely distributed multi-cationic side chains for anion exchange membranes

The anion conductivity is one of the key properties of anion exchange membranes (AEMs), which is affected by the content and distribution of ionic groups in the ionomers. Herein, a series of poly(fluorenyl ether ketone)s with multiple phenylmethyl groups were synthesized. After bromination, the phenylmethyl groups were attached with linear tri- and di-quaternary ammonium side chains. As a result, the quaternary ammonium groups were tethered closely at the aliphatic side chains which emanate from the specific segments of the aromatic backbone. The ion exchange capacity, water affinity, Br− conductivity, morphology, thermal property, mechanical property, oxidative stability, and VO2+ permeability of the obtained AEMs were studied. It has been found that both types of AEMs have well-developed hydrophilic-hydrophobic phase separation. When comparing at similar ion exchange capacity (IEC), the AEMs bearing tri-quaternary ammonium side chains have higher Br− conductivity than the AEMs bearing di-quaternary ammonium side chains, presumably because of the better phase separation in the former. All of the AEMs have significantly reduced VO2+ permeability than Nafion 212, owing to the positive effect of Donnan repulsion. Along with the other characterizations, the obtained AEMs were proven to be promising candidates for potential vanadium redox flow battery applications.

In Situ Grown Tungsten Trioxide Nanoparticles on Graphene Oxide Nanosheet to Regulate Ion Selectivity of Membrane for High Performance Vanadium Redox Flow Battery

AbstractThe application of vanadium redox flow batteries (VRFBs) has encountered challenges because the most commonly used commercial membrane (perfluorinated sulfonic acid, PFSA) has severe vanadium ion permeation, which yields poor stability of the battery. Herein, a PFSA‐based hybrid membrane with a sandwich structure created using a reinforced polytetrafluoroethylene thin layer with hydrophilic nanohybrid fillers is developed. The tungsten trioxide (WO3) nanoparticles are in situ grown on the surface of graphene oxide (GO) nanosheets to overcome the electrostatic effect, and to enhance the hydrophilicity and dispersibility of GO nanosheets, which is embedded in the PFSA matrix to act as a “barrier” to reduce vanadium ions permeation. In addition, these hydrophilic WO3 nanoparticles on GO nanosheets surface serve as proton active sites to facilitate proton transportation. As a result, at 120 cm−2, the cell of the hybrid membrane displays high Coulombic efficiency (over 98.1%) and high energy efficiency (up to 88.9%), better than the commercial Nafion 212 membrane at the same condition. These performances indicate the proposed hybrid membrane is applicable for VRFB application. Also, this design method of the membrane can be extended to other fields including water treatments and fuel cells.

Anion exchange membranes based on long side-chain quaternary ammonium-functionalized poly(arylene piperidinium)s for vanadium redox flow batteries

A new series of poly(arylene piperidinium)-based anion exchange membranes (AEMs) are proposed for vanadium redox flow batteries (VRFBs). The AEMs are fabricated via the Menshutkin reaction between poly(arylene piperidine) without ether bonds in the backbone and various quaternizing agents, including iodomethane, 1-bromopentane, and (5-bromopentyl)-trimethylammonium bromide. The properties of the AEMs are investigated in terms of sulfuric acid doping content, swelling, vanadium permeability, ion selectivity, area-specific resistance, mechanical properties, VRFB performance, and cyclic testing. Particularly, a method of measuring the H+ permeability of the AEM is developed. It demonstrates that the poly(p-terphenyl-N-methylpiperidine)-quaternary ammonium (PTP-QA) membrane with a QA cation-tethered alkyl chain exhibits high H+ permeability, resulting in low area resistance. Combined with its low vanadium permeance, the PTP-QA membrane achieves nearly 370 times higher ion selectivity than Nafion 115. The VRFB based on PTP-QA-based AEM displays high Coulombic efficiencies above 99% at current densities of 80–160 mA cm−2. The higher energy efficiency of 89.8% is achieved at 100 mA cm−2 (vs. 73.6% for Nafion 115). Meanwhile, the PTP-QA-based AEM shows good cycling stability and capacity retention, proving great potential as the ion exchange membrane for VRFB applications.

A novel fluorinated acid-base sulfonated polyimide membrane with sulfoalkyl side-chain for vanadium redox flow battery

A novel fluorinated acid-base sulfonated polyimide membrane (s-f-SPI) with trifluoromethyl (-CF3), side-chain-type-SO3H and benzimidazole groups was synthesized by a 2-step method for vanadium redox flow battery applications. A series of physico-chemical properties of s-f-SPI membrane were researched systematacially. The acid-base pairs between the side-chain-type -SO3H and protonated imidazole groups provided more channels for proton-transport, enhancing the proton conductivity (0.51×10−1 S cm−1 at ∼25 °C), and Donnan effect of the imidazole groups impeded the penetration of vanadium-ion (3.95×10−7 cm2 min−1). In VRFB tests, s-f-SPI membrane exhibited a coulombic efficiency (CE) of 98.1% and an energy efficiency (EE) of 81.2% at a current density of 100 mA cm−2, higher than those of Nafion 212 (CE: 94.1%, EE: 76.5%). Furthermore, the VRFB with s-f-SPI membrane showed a stable performance in the 500 cycles charging-discharging test at 100 mA cm−2, and the morphology and structure remained unchanged after the cycle test. These results demonstrate that the s-f-SPI membrane has great potential for VRFB applications.

An overview on the progress and development of modified sulfonated polyether ether ketone membranes for vanadium redox flow battery applications

Vanadium redox flow batteries (VRFB) are among the most promising approaches to efficiently store renewable energies. In such battery type, Nafion is commonly used as membrane material but suffers from high vanadium crossover and cost. These drawbacks negatively influence the widespread commercial application of VRFBs. Alternative membrane materials with high performance and low cost are thus being developed to address these shortfalls. Among those, possible materials for the VRFB membrane is sulfonated polyether ether ketone (SPEEK), which recently attracted considerable attention due to its low cost, combined with mechanical and chemical stability, and ease of preparation. This review summarizes the research activities related to the development of SPEEK-based membranes for VRFB applications and gives an overview of the properties of PEEK and its sulfonated form. A critical analysis on the challenges of SPEEK-based membranes is also discussed.

Branched sulfonated polyimide/s-MWCNTs composite membranes for vanadium redox flow battery application

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10−7 cm2 min−1) and higher proton selectivity (1.06 × 105 S min cm−3) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies (CEs: 96.0–98.2%) and energy efficiencies (EEs: 79.7–69.5%) than that with Nafion 212 membrane (CEs: 86.5–92.5% and EEs: 78.5–67.6%) at 80–160 mA cm−2. The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm−2, whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.