Vanadium Ion Permeability Research Articles

Sulfonated poly(ether ether ketone)/sulfonated graphene oxide hybrid membrane for vanadium redox flow battery

In this paper, a series of sulfonated poly(ether ether ketone) (SPEEK) hybrid membrane doped by the different amount of sulfonated graphene oxide (SGO) nanosheets are prepared by solution-casting method. The membrane structure, physicochemical property and cell performance of vanadium redox flow battery (VRB) have been characterized through FT-IR, SEM, XPS, and single-cell test system, etc. The incorporation of SGO in SPEEK improves the water uptake, ion exchange capacity and proton conductivity of the hybrid membrane, and effectively reduces the swelling degree and the vanadium ion permeability. In the single cell performance test, the energy efficiency of the SPEEK/SGO-3 hybrid membrane is 81.1%, which is higher than that of SPEEK (76.0%) and Nafion 117 membrane (73.8%). And, a longer self-discharge time (56.6 h) against Nafion 117 (23.5 h) and SPEEK (32.8 h) membrane is shown. In addition, the incorporated SGO nanofillers don't improve the chemical stability of SPEEK membrane soaked by 1.5 M V5+ solution although they block the vanadium ion permeation. Then, the durability for SPEEK/SGO hybrid membranes should be further explored for VRB system.

Preparation of sulfonated poly(phthalazinone ether sulfone)/poly(vinylidene fluoride) composite membrane for vanadium redox flow battery

A sulfonated poly(phthalazinone ether sulfone) (SPPES)/poly(vinylidene fluoride) (PVDF) composite membrane was prepared through a solution-casting method for vanadium redox flow battery (denoted as VRB). The composite membrane exhibits dramatically lower vanadium ions permeability and better cell performance compared to the pristine SPPES membrane and Nafion 115 membrane. The vanadium ion permeability of SPPES/PVDF membrane is one order of magnitude lower than that of Nafion 115 membrane. In the further work, the VRB single cell with SPPES/PVDF composite membrane shows higher columbic efficiency (92.80%) and energy efficiency (84.1%) at the current density 36 mA·cm−2 compared with the single cell with Nafion 115 membrane. In the self-discharge test, SPPES/PVDF membrane showed 1.7 times longer duration in the open circuit decay than Nafion 115 membrane. With all the good properties and low cost, this new kind of composite membrane is of excellent commercial prospects as an ion exchange membrane for VRB systems.

Pore-Size-Tuned Graphene Oxide Frameworks as Ion-Selective and Protective Layers on Hydrocarbon Membranes for Vanadium Redox-Flow Batteries.

The laminated structure of graphene oxide (GO) membranes provides exceptional ion-separation properties due to the regular interlayer spacing ( d) between laminate layers. However, a larger effective pore size of the laminate immersed in water (∼11.1 Å) than the hydrated diameter of vanadium ions (>6.0 Å) prevents its use in vanadium redox-flow batteries (VRFB). In this work, we report an ion-selective graphene oxide framework (GOF) with a d tuned by cross-linking the GO nanosheets. Its effective pore size (∼5.9 Å) excludes vanadium ions by size but allows proton conduction. The GOF membrane is employed as a protective layer to address the poor chemical stability of sulfonated poly(arylene ether sulfone) (SPAES) membranes against VO2+ in VRFB. By effectively blocking vanadium ions, the GOF/SPAES membrane exhibits vanadium-ion permeability 4.2 times lower and a durability 5 times longer than that of the pristine SPAES membrane. Moreover, the VRFB with the GOF/SPAES membrane achieves an energy efficiency of 89% at 80 mA cm-2 and a capacity retention of 88% even after 400 cycles, far exceeding results for Nafion 115 and demonstrating its practical applicability for VRFB.

Novel branched sulfonated polyimide/molybdenum disulfide nanosheets composite membrane for vanadium redox flow battery application

A novel branched sulfonated polyimide/molybdenum disulfide nanosheets (bSPI/MoS2-ns) composite membrane was fabricated for vanadium redox flow battery (VRFB) application. Scanning probe microscope (SPM) and field emission-scanning electron microscope (FE-SEM) images verify the successful exfoliation of monolayer and a few layers of MoS2 nanosheets. X-ray diffraction (XRD), attenuated total reflection-fourier transform infrared spectrometer (ATR-FTIR) and FE-SEM results demonstrate that bSPI, bSPI/MoS2-ns and bSPI/MoS2 nanopowders (bSPI/MoS2-np) membranes are successfully prepared. Both thermogravimetric analysis/differential thermogravimetry (TGA/DTG) and ex-situ chemical stability testing results confirm bSPI/MoS2-ns membrane has improved thermal and chemical stabilities in contrast with pristine bSPI membrane. The bSPI/MoS2-ns membrane owns high proton conductivity (5.04 × 10−2 S cm−1) and extremely low vanadium ion permeability (0.65 × 10−7 cm2 min−1), leading to its highest proton selectivity (7.75 × 105 S min cm−3) among all the membranes tested. Besides, during 500 times cycle charge–discharge test, the coulombic efficiency, energy efficiency and discharge capacity retention of VRFB using bSPI/MoS2-ns membrane are higher than or comparable to those of VRFB using Nafion 117 membrane. And the surfacial morphological integrity of bSPI/MoS2-ns membrane maintains better than reported pristine bSPI membrane after long-time VRFB usage. Above results indicate that the as-prepared bSPI/MoS2-ns membrane has great potential for VRFB application.

Toward Cheaper Vanadium Flow Batteries: Porous Polyethylene Reinforced Membrane with Superior Durability

Developing cheap and durable proton exchange membrane is crucial to promote the practical application of vanadium flow batteries (VFB). Here we report a simple and scalable method to fabricate a reinforced sulfonated poly(ether ether ketone) (SPEEK) membrane using a lithium-ion battery separator, ceramic-coated porous polyethylene (CCP), as a robust scaffold. With the confinement effect of the extremely stable CCP substrate, the reinforced SPEEK membrane (S@CCP) shows significantly improved chemical/mechanical stability and reduced vanadium ion permeability compared to the control SPEEK membrane. Accordingly, the S@CCP membrane demonstrates excellent rate performance and cycling stability than those of the benchmark Nafion 212 membrane. It exhibits stable performance over 1500 cycles at 160 mA cm–2 with 99% of CE, 76% of EE and 0.126% of capacity decay per cycle. Meanwhile, the S@CCP membrane is highly resistant to temperature fluctuations over a wide range of −20–60 °C. The superior durability, wide temp...

Tuning the ion selectivity of porous poly(2,5-benzimidazole) membranes by phase separation for all vanadium redox flow batteries

Porous polybenzimidazole membranes have been suggested by far as the most promising candidate to replace perfluorinated ionomer membranes in all vanadium redox flow batteries (VRFB). These porous membranes can simply be prepared from a phase separation process induced by water vapor. In this work, the influence of the material properties of poly(2,5-benzimidazole) (ABPBI) on the membrane structure formation and ion transport characteristics is systematically investigated. Two batches of ABPBI polymers (intrinsic viscosity 2.0 and 1.4 dL g−1) are used; concentrations of polymer solution, the water vapor activity (temperature and relative humidity), and the exposure time of polymer solution to the water vapor are varied. It is found that high viscosity of ABPBI solution, among other factors, is essential in obtaining membranes with very low interconnectivity between macropores in the membrane bulk, therefore very low vanadium ion permeability suitable for VRFB applications. The porous ABPBI membranes in dry state have a low amount of pores ranging from 1 to ~ 200 nm. Due to the benzimidazole moieties in ABPBI polymers, the acid in vanadium electrolytes has two positive effects on ABPBI membranes: fixed positive charges, and absorption of acid in the membranes. The ion exchange capacity of ABPBI membranes increases with the increase in solution acid concentration, and reaches striking values as high as 8 mequ g−1 in 4.0 M H2SO4. This makes the Donnan exclusion towards positively charged vanadium ions very profound. The macropores in the porous membranes increase the absorption amount of free acid, therefore significantly increase the membrane ionic conductivity. These properties make porous ABPBI membranes very efficient separators for VRFB applications.

Sulfonated poly(ether sulfone)/short-carboxylic multi-walled carbon nanotube composite membrane applied for vanadium redox flow battery

Sulfonated poly(ether sulfone) (SPES) is a potential polymer for replacing Nafion membranes in vanadium redox flow batteries (VRB); however, at a high degree of sulfonation, SPES membrane displays high swelling, poor mechanical stability, and high vanadium crossover. In this study, to improve membrane performance, composite membranes of SPES and short-carboxylic multi-walled carbon nanotube (denoted as SPES/SCCT membrane) were prepared and employed as the separator for VRB to evaluate the vanadium ions permeability and cell performance. The composite membrane showed the lowest vanadium ion permeability and highest selectivity compared with the Nafion221 and SPES membranes. The VRB with SPES/SCCT membrane also showed higher coulombic efficiency of about 91.7% and energy efficiency of about 84.1% but lower capacity loss and self-discharge than that of VRB with Nafion221 membrane during the cycling test, which further indicated the promising prospects of SPES/SCCT composite membrane application in VRB systems.

Commercial perfluorosulfonic acid membranes for vanadium redox flow battery: Effect of ion-exchange capacity and membrane internal structure

A series of perfluorosulfonic membranes is screened for application in vanadium redox flow batteries (VRFB): membranes of constant thickness 50 µm with different ion-exchange capacities ranging from 0.56 to 1.15 mol eq. g−1. Diffusion flux of each vanadium ion occurring in VRFB electrolytes through each examined membrane is measured by UV/Vis spectroscopy. Permeation of V2+ ions contributes most to the self-discharge losses and the mechanism of vanadium ions permeation is discussed for all oxidation states. The membranes are characterized in the single-cell by electrochemical impedance spectroscopy, load curve measurements with linearly increasing current and charge-discharge cycles at various current densities ranging from 50 to 200 mA cm−2. Generally, at lower current densities the permeation of vanadium ions decreases the battery efficiency and thus lower ion-exchange capacity membranes are more suitable. Concurrently, at higher current densities the battery efficiency is decreased by the membrane resistance and thus higher ion-exchange capacity membranes are optimal. However, membrane ion-exchange capacity is not the only characteristic that affects the VRFB performance. Small-angle X-ray scattering of membranes revealed the effect of polymer molecular architecture on the size of hydrophilic domains which affects the membrane transport properties.

Preparation and characterization of SPES/PVA (double-layer) membrane for vanadium redox flow battery

The double-layer membrane consisting of sulfonated poly(ether sulfone) (SPES) sub-layer and polyvinyl alcohol (PVA) sub-layer (denoted as SPES/PVA membrane) was prepared and employed as the separator for vanadium redox flow battery (VRB) system to evaluate the vanadium ions permeability and cell performance. The SPES/PVA membrane is a double-layer structure and exhibits dramatically lower vanadium ions permeability and better cell performance compared to the pristine SPES membrane, PVA membrane, and Nafion117 membrane. The vanadium ion permeability of SPES/PVA membrane is one order of magnitude lower than that of Nafion117 membrane. In further work, the single cell with SPES/PVA membrane showed significantly lower capacity loss, higher coulombic efficiency (>92.5%), and higher energy efficiency (>83.9%) than Nafion117 membrane. In the self-discharge test, SPES/PVA membrane showed 1.8 times longer duration in the open circuit decay than Nafion117 membrane. With all the good properties and low cost, this new kind of double-layer membrane is suggested to have excellent commercial prospects as an ion exchange membrane for VRB systems.

A microfluidic all-vanadium photoelectrochemical cell for solar energy storage

In this work, a microfluidic all-vanadium photoelectrochemical cell (μVPEC) was designed for the solar energy storage. The miniaturization design could enhance the photon and mass transport, reduce the internal cell resistance, and improve the uniformity of the light distribution. Because of these advantages, the developed μVPEC was able to yield good performance. Experimental results indicated that the developed μVPEC showed good photoresponse and operation stability. Besides, its performance was also evaluated under various operating conditions, including the membrane thickness, the light intensity and the vanadium ion concentrations. It was shown that although the vanadium ion permeation was small with thicker membrane, the increased proton transfer resistance decreased the photocurrent density. The increase of the light intensity could produce more photo-generated electron-hole pairs, which could improve the photoelectrochemical reaction rate and the conversion rate. It was also found that the photocurrent density was increased with the vanadium ion concentration as a result of the enhanced mass transfer. The results obtained in this work reveal that the developed all-vanadium photoelectrochemical cell shows the promising potential for the solar energy storage.

Effect of grafting of poly(styrenesulfonate) onto Nafion membrane on the performance of vanadium redox flow battery

A modified membrane for vanadium redox flow battery (VRFB) was prepared by grafting sodium 4-styrenesulfonate (NaSS) to the surface of Nafion 212 via the oxygen plasma induced grafting technique to reduce the crossover of vanadium ions of the membrane. By grafting with poly(styrenesulfonic acid) (PSS), the crossover of vanadium ions through the Nafion membrane is only 32% of that of the pristine Nafion membrane. Thus, comparing with pristine Nafion membrane, the resulting Nafion-g-PSS membrane exhibits higher ion exchange capacity (IEC) and through-plane conductivity. In addition, this Nafion-g-PSS membrane improves the voltage efficiency (VE), coulombic efficiency (CE), and energy efficiency (EE) of VRFB. Owing to lower vanadium ions permeability, the VRFB with Nafion-g-PSS membrane exhibited much slower self-discharge than the VRFB with pristine Nafion 212, and the VE, CE, and EE after 200cycles of charge-discharge test remain stable. In particular, the VRFB with Nafion-g-PSS membrane exhibits higher capacity retention than the VRFB with pristine Nafion 212 membrane.

Hydrocarbon membranes with high selectivity and enhanced stability for vanadium redox flow battery applications: Comparative study with sulfonated poly(ether sulfone)s and sulfonated poly(thioether ether sulfone)s

A series of sulfonated poly(ether sulfone) copolymers (SPES-Xs) with varying degrees of sulfonation were prepared and investigated as ion-exchange membranes for vanadium redox flow battery (VRFB) applications. Sulfonated poly(thioether ether sulfone) copolymers (SPTES-Xs) were initially synthesized via polycondensation, and the SPES-Xs were then obtained by oxidation of the corresponding SPTES-Xs. The SPES-X membranes showed reduced vanadium-ion permeability, low area resistance, and, thereby, much superior selectivity compared with the parent SPTES-X membranes and a Nafion115 membrane. In single-cell VRFB performance tests, a SPES-50 membrane with an ion-exchange capacity of 1.80 meq/g exhibited a higher coulombic efficiency (>99%) and energy efficiency (76–89%) than the Nafion115 membrane over a wide range of current densities from 40 to 100 mA/cm2 and a significantly larger capacity retention (>62%) during 200 charge-discharge cycles. The SPES-X materials, in which every benzene ring is deactivated by the presence of electron-withdrawing sulfone linkages, showed much better chemical stability during ex situ and in situ tests than the SPTES-X materials, which contain an electron-donating thioether linkage in their repeat unit.

A Low-Cost and High-Performance Sulfonated Polyimide Proton-Conductive Membrane for Vanadium Redox Flow/Static Batteries.

A novel side-chain-type fluorinated sulfonated polyimide (s-FSPI) membrane is synthesized for vanadium redox batteries (VRBs) by high-temperature polycondensation and grafting reactions. The s-FSPI membrane has a vanadium ion permeability that is over an order of magnitude lower and has a proton selectivity that is 6.8 times higher compared to those of the Nafion 115 membrane. The s-FSPI membrane possesses superior chemical stability compared to most of the linear sulfonated aromatic polymer membranes reported for VRBs. Also, the vanadium redox flow/static batteries (VRFB/VRSB) assembled with the s-FSPI membranes exhibit stable battery performance over 100- and 300-time charge-discharge cycling tests, respectively, with significantly higher battery efficiencies and lower self-discharge rates than those with the Nafion 115 membranes. The excellent physicochemical properties and VRB performance of the s-FSPI membrane could be attributed to the specifically designed molecular structure with the hydrophobic trifluoromethyl groups and flexible sulfoalkyl pendants being introduced on the main chains of the membrane. Moreover, the cost of the s-FSPI membrane is only one-fourth that of the commercial Nafion 115 membrane. This work opens up new possibilities for fabricating high-performance proton-conductive membranes at low costs for VRBs.

Orientated graphene oxide/Nafion ultra-thin layer coated composite membranes for vanadium redox flow battery

Graphene oxide (GO)/Nafion composite membranes, with orientated GO nanosheets in parallel to the surface of the ultra-thin coating layer (400–440 nm), are prepared by spin coating method and evidenced by electron microscopy analysis. Orientation of GO maximizes the vanadium ions barrier effect of GO. The GO/Nafion membrane (M-2) achieves lower vanadium ion permeability (8.2 × 10−8 cm2 min−1, only 2.64% of the pristine Nafion membrane), and higher coulombic efficiency and energy efficiency (92.9–98.8% and 81.5–88.4%, respectively) comparing with the pristine Nafion membrane (73.3–90.5% and 68.9–79.1%, respectively) at current densities of 20–100 mA cm−2. With the design of orientated GO nanosheets and ultra-thin GO/Nafion coating layer, good balance between vanadium crossover suppression and protons conduction retention is achieved. M-2 exhibits excellent battery performances over 200 charge-discharge cycles. The capacity decay rate is about 0.23% per cycle, much lower than those assembled with Nafion 212 (0.40% per cycle) and the recast Nafion membrane (0.44% per cycle). Spin coating with water suspensor leads to uniform dispersion of GO and good binding between GO/Nafion coating layer and substrate Nafion membrane. Therefore, the composite membrane could be reinforced by GO and keep integration even with 200 cycles operation.

Chitosan–silica anion exchange membrane for the vanadium redox flow energy storage battery applications

In this study, chitosan–silica anion exchange membranes were synthesized using a sol-gel process and nucleophilic substitution reaction. The basic properties of the synthesized ion exchange membranes were assessed, including water uptake, contact angle, ion exchange capacity, vanadium ion permeability, impedance, and conductivity. Furthermore, we used the chitosan–silica membranes as a separator in a single all‑vanadium redox flow battery system to investigate their charge–discharge characteristics. We then compared the performance of the chitosan–silica membranes with that of commercially available membranes in terms of impedance as a function of thickness and efficiency. In addition, we studied the relationships between the basic properties and the charge–discharge efficiency of the membranes by varying the concentration of (3-glycidyloxypropyl)trimethoxysilane (GPTMS). The results demonstrate that the mechanical strength and chemical stability of the synthesized chitosan–silica membranes were enhanced with the increase in GPTMS concentration. The chitosan–silica anion exchange membranes exhibited lower vanadium ion permeability and higher coulombic efficiency, and have a lower cost than their commercial counterparts. The highest measured coulombic efficiency reached 91%.

Crosslinked anion exchange membranes with primary diamine-based crosslinkers for vanadium redox flow battery application

A series of polysulfone-based crosslinked anion exchange membranes (AEMs) with primary diamine-based crosslinkers has been prepared via simple a crosslinking process as low-cost and durable membranes for vanadium redox flow batteries (VRFBs). Chloromethylated polysulfone is used as a precursor polymer for crosslinked AEMs (CAPSU-x) with different degrees of crosslinking. Among the developed AEMs, CAPSU-2.5 shows outstanding dimensional stability and anion (Cl−, SO42−, and OH−) conductivity. Moreover, CAPSU-2.5 exhibits much lower vanadium ion permeability (2.72 × 10−8 cm2 min−1) than Nafion 115 (2.88 × 10−6 cm2 min−1), which results in an excellent coulombic efficiency of 100%. The chemical and operational stabilities of the membranes have been investigated via ex situ soaking tests in 0.1 M VO2+ solution and in situ operation tests for 100 cycles, respectively. The excellent chemical, physical, and electrochemical properties of the CAPSU-2.5 membrane make it suitable for use in VRFBs.

Enhanced membrane ion selectivity by incorporating graphene oxide nanosheet for vanadium redox flow battery application

High ion selectivity with high proton conductivity and low vanadium ionic permeability is one of the critical issues for an ideal ion conductive membrane (ICM) in vanadium redox flow battery (VRFB). In this work, a novel ICM is fabricated by-polyvinylpyrrolidone (PSF-PVP) membrane to enhance its membrane ion selectivity. Both the proton conductivity and vanadium ion permeability of the PSF-PVP/GO composite membranes are decreased with the loading of GO. However, the ion selectivity of the composite membranes shows volcano shape against the GO loading. After adding 0.05wt% GO, the ion selectivity of the composite membrane reaches the highest point of 2.2×107Scm3/min, which is 47% higher than that of the pristine PSF-PVP membrane. Incorporating graphene oxide nanosheet framework is of help to improve the membrane ion selectivity by size sieving effect, tortuous pathways and electrochemical exclusive behavior on vanadium ions. Moreover, a single battery with the optimized membrane exhibits coulumbic efficiency of 98% and energy efficiency of 87%, while the high efficiencies are also maintained during continuous running of 100 charge-discharge cycles. These convincing results indicate that the introduction of GO to the PSF-PVP membrane is promising and environmental friendly in VRFB application.

Highly selective sulfonated poly(ether ether ketone)/titanium oxide composite membranes for vanadium redox flow batteries

To lower the cost and enhance the selectivity of membranes used in vanadium redox flow batteries (VRFBs), sulfonated poly(ether ether ketone) (SPEEK)/ titanium oxide (TiO2) composite membranes with different degrees of sulfonation (DS) (prepared at different time durations of 3h, 5h and 7h) and various TiO2 nanoparticles loadings (3%, 5% and 7%) were prepared. Optimum DS and TiO2 nanoparticles loading were confirmed by different characterization techniques including 1H NMR, scanning electron microscopy, water uptake, swelling ratio, proton conductivity, vanadium ion permeability and thermal stability. Results show that 5h SPEEK/5% TiO2 composite membrane possessed comprehensive merits, especially high ion selectivity (72.55 × 103Smincm−3) due to the blocking effect of TiO2 nanoparticles and good proton conductivity. TiO2 nanoparticles combined very well with SPEEK due to strong interaction, as deduced from flux tests. The chemical and oxidative stability were sufficiently good for applications of VRFB. The VRFB single cell with 5h SPEEK/5% TiO2 composite membrane exhibited excellent performance including high coulombic efficiency (98.3%), high energy efficiency (82.9%), long self-discharge time and low capacity loss compared to Nafion 117. Hence, SPEEK/TiO2 composite membranes can potentially be promising substitute for Nafion in VRFBs.

Novel sulfonated poly(ether ether ketone)/oxidized g-C3N4 composite membrane for vanadium redox flow battery applications

A novel oxidized g-C3N4 (OCN)-modified Sulfonated poly(ether ether ketone) (SPEEK) composite membrane were fabricated via a solution-casting method. The physicochemical properties of composite membranes with different oxidized-C3N4 content were characterized, including water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and ion selectivity. It was found that when the content of OCN incorporated into SPEEK matrix was 1%, this composite membrane showed a low vanadium ion permeability(9.09×10−7cm2min−1) and high ion selectivity(6.60×104Smincm−3). The vanadium redox flow battery (VRB) assembled with this membrane at 60mAcm−2 exhibited a higher coulombic efficiency (98%) than Nafion 117 membrane (95%). Moreover, in cycling tests VRB assembled with this composite membrane showed a more stable cycle performance compared with that of Nafion 117 membrane. Thus, the OCN-modified SPEEK composite membrane has a potential application for VRB system.

Effect of non-sulfonated diamine monomer on branched sulfonated polyimide membrane for vanadium redox flow battery application

Branched sulfonated polyimide (bSPI) membranes based on three different non-sulfonated diamine monomers were fabricated for vanadium redox flow battery (VRFB) usage. Both FT-IR and 1H NMR spectra confirm that bSPIs were successfully synthesized. TGA results show that bSPI membranes have good thermal stability. FESEM pictures indicate that the surface of membrane in contact with catholyte keeps better integrity than that in contact with anolyte after 500-time cycling charge-discharge test. The water uptake, ion exchange capacity, mechanical strength of bSPI membranes are larger than or close to those of Nafion 117 membrane, and bSPI membranes exhibit remarkably lower vanadium ion permeability compared with Nafion 117 membrane. In addition, bSPI membranes have outstanding chemical stability in contrast with linear SPI membrane in ex-situ degradation test. Further, the bSPI (BAPP) membrane displays the best chemical stability and highest proton selectivity (1.23×105Smincm−3) among all bSPI membranes, thus it is seen as an optimum one for further single VRFB test. During 500-time cyclic charge-discharge test, the coulomb efficiency (CE: 97.1∼99.7%) of VRFB containing bSPI (BAPP) membrane is higher than that containing Nafion 117 (CE: 95.2∼98.6%) at 30∼120mAcm−2. The VRFB using bSPI (BAPP) membrane presents higher or comparable energy efficiency (EE) and capacity retention compared with that using Nafion 117 membrane at low or high current density. Based on the excellent physico-chemical properties and single cell performance together with much lower cost compared with widely used Nafion 117 membrane, the as-prepared bSPI (BAPP) membrane has great potential to be applied in VRFB.