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Ion-Conducting Membrane for Vanadium Redox Flow Batteries

Feiran Wang , Fengjing Jiang

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (3) : 462 -470.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (3) : 462-470 DOI: 10.7536/PC200567
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Ion-Conducting Membrane for Vanadium Redox Flow Batteries
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Abstract

With the continuous development of renewable energy technology, vanadium redox flow battery, as a large-scale energy storage device with great development prospects, has widely drawn attention of domestic and foreign scholars. Ion-conducting membrane is one of the important components of vanadium redox flow battery, which has a key influence on the performance, life time and cost of the battery. Based on the research work reported domestically and abroad, this review reports in detail the research and application progress as well as the technical problems of ion-conducting membrane for vanadium redox flow battery. In addition, some new developments of ion-conducting membrane for vanadium redox flow battery are introduced.

Contents

1 Introduction

2 Performance requirements of ion⁃conducting membrane

3 Perfluorosulfonic acid ion exchange membrane

3.1 Modification of inorganic particles

3.2 Polymer blending

3.3 Porous matrix filling

4 Partial fluorination ion exchange membrane

5 Non⁃fluoride ion exchange membrane

6 Porous ion⁃conducting membrane

7 Conclusion and prospect

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Keywords

vanadium redox flow battery / ion exchange membrane / porous membrane / large-scale energy storage technology

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Feiran Wang, Fengjing Jiang. Ion-Conducting Membrane for Vanadium Redox Flow Batteries. Progress in Chemistry, 2021, 33(3): 462-470 DOI:10.7536/PC200567

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1 Introduction

The excessive use of fossil energy has brought many problems to human sustainable development, such as insufficient energy reserves, environmental pollution and so on. New energy technology has attracted wide attention because of its clean, renewable and other advantages. The development of renewable energy can not only improve the energy structure, but also solve the problems of global warming. However, solar energy, wind energy, tidal energy and other new energy power generation technologies often face the problem of imbalance between electricity demand and power supply due to the influence of natural random factors, while the instability of power generation capacity will cause a large number of "light curtailment" and "wind curtailment"[1]. In order to build a clean, efficient and renewable new energy system, it is of great significance to establish a large-scale electrochemical energy storage system with high stability and low cost.
As a large-scale energy storage device, flow battery has the function of peak shaving and valley filling, and maintaining the stability of the power grid[2]. Because of its high safety, long life and high energy conversion efficiency, flow batteries have been developed and applied in many commercial applications. In addition to flow batteries such as all-vanadium flow batteries, zinc-bromine batteries and lithium-ion flow batteries, some research teams have reported new flow battery systems such as aqueous organic flow batteries in recent years, which provide new ideas for the research and development of flow batteries[3⇓~5]. Vanadium redox flow battery is more suitable for large-scale energy storage because of its low environmental pollution and long service life[6⇓~8].
The all-vanadium redox flow battery is a secondary battery in which the positive and negative electrolytes flow circularly in the positive and negative electrodes respectively for charge and discharge reactions. The positive and negative electrolytes in the battery are separated by an ionic conductive membrane. The single cell structure is shown in Figure 1. Ion conducting membrane (ICM) is one of the most important components of vanadium redox flow battery (VRB). However, its chemical stability, ion selectivity and cost limit the commercial application of VRB[9]. The most commercialized ion-conducting membrane currently used in all-vanadium flow batteries is the Nafion ® membrane. Nafion ® membrane has good chemical stability and high ionic conductivity, but it also has the problems of high water absorption and swelling rate, low ion selectivity and high price. Therefore, it is very important to research and develop new ionic conductive membranes with high chemical stability, high ionic conductivity, high ion selectivity and low cost. In this paper, the research work of ion conducting membrane in vanadium redox flow battery is summarized and prospected, which provides a reference for the research and development of ion conducting membrane.

2 Performance requirements for ionically conductive membrane

In the battery, the ionic conductive membrane should not only separate the positive and negative electrodes and the electrolyte of the positive and negative electrodes to prevent short circuit and self-discharge of the battery, but also allow charge carriers to pass through, so as to ensure the charge balance between the negative and positive electrodes and form a battery circuit[2]. It can be seen that the ion conducting membrane not only has the function of "separating medium", but also has the function of conducting ions quickly between the positive and negative electrodes. An ideal ion exchange membrane should have the following performance characteristics[10].
(1) High ionic conductivity
During the operation of vanadium redox flow battery, the internal current of the battery can be formed only by the directional movement of ions in the solution. This requires the ion conductive membrane to have good ion conductivity. The high ionic conductivity of the ionic conductive film can reduce the internal resistance of the battery, thereby improving the voltage efficiency of the charging and discharging process.
(2) high ion selectivity
One of the important functions of ion conducting membrane in vanadium redox flow battery is to allow the rapid passage of hydrogen ions and effectively block the penetration of vanadium ions with different valences, that is, selective permeability[10]. The high ion selectivity can reduce the cross contamination of the positive and negative electrolytes, reduce the self-discharge of the battery, and improve the coulombic efficiency of the battery.
(3) High chemical stability
In the vanadium redox flow battery, the electrolyte generally contains 2 to 3 M of sulfuric acid, and the pentavalent vanadium in the positive electrolyte has strong oxidizability, so the ionic conductive film needs to have outstanding chemical corrosion resistance and electrochemical oxidation resistance to ensure a longer cycle life of the battery when it works in a strong acidic and oxidizing environment for a long time.
(4) High mechanical strength
The ion-conducting membrane also needs to meet certain mechanical strength requirements. In the working environment of the battery, the erosion of the electrolyte solution, the extrusion between the battery plate frames and the burr of the carbon felt may lead to the damage of the ion conducting membrane and the internal leakage of the battery. Therefore, the ion-conducting membrane should have a high mechanical strength.
In addition, the availability of raw materials, the operability of membrane preparation process and the rationality of cost are the key factors for the commercial use of ion conducting membranes for all-vanadium redox flow batteries, and also the important aspects to be considered in the future research and development of ion conducting membranes for all-vanadium redox flow batteries.

3 Perfluorosulfonic acid type ion exchange membrane

So far, the most widely commercialized ionic separator in all-vanadium redox flow batteries is Nafion ® membrane of DuPont Company in the United States, which is copolymerized by perfluorovinyl ether comonomer and tetrafluoroethylene. Because the perfluoro ether bond with sulfonic acid group is connected to the perfluoro main chain, the structure of the hydrophilic side chain and the hydrophobic main chain endows the polymer with unique microscopic structure and special performance[11]. The perfluorosulfonic acid membrane has good electrochemical performance and chemical stability, can work stably for a long time in the strong oxidizing environment of the vanadium redox flow battery, has high ionic conductivity, and can achieve high voltage efficiency (more than 90%), so the perfluorosulfonic acid membrane becomes the most widely used ion exchange membrane for the vanadium redox flow battery[12⇓~14]. However, the disadvantages of perfluorosulfonic acid membrane, such as low ion selectivity and high price (500~1000$/m2), have seriously hindered its large-scale application in all-vanadium redox flow batteries. Therefore, the development of high-performance and low-cost ion-conducting membranes to replace perfluorosulfonic acid membranes is of great significance to accelerate the industrialization of flow batteries and the utilization of clean and renewable energy.
The high vanadium ion permeability and low ion selectivity of perfluorosulfonic acid membrane are the main bottlenecks for its large-scale application in vanadium redox flow batteries. Researchers have modified perfluorosulfonic acid membrane by inorganic particle modification, polymer blending and porous matrix filling to improve its ion selectivity.

3.1 Inorganic particle modified

Researchers have found that adding inorganic particles to fuel cell proton exchange membranes can effectively reduce methanol permeability[15]. Similarly, researchers used inorganic particles to modify perfluorosulfonic acid membranes and applied them to all-vanadium redox flow batteries to reduce ion permeability by modifying hydrophilic ion channels. Qiu et al prepared Nafion/SiO2 hybrid membrane by sol-gel method, and found that the time of maintaining open circuit voltage of the hybrid membrane was twice that of the original Nafion membrane, and the hybrid membrane could still maintain good performance after more than 100 cycles at a current density of 60 mA/cm2[16]. On this basis, Teng et al continued to improve the modified SiO2 particles and prepared a new type of Nafion/ORMOSIL hybrid membrane by in-situ sol-gel reaction of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEDMS), which could achieve higher energy efficiency in the all-vanadium redox flow battery test[17]. In addition, the perfluorosulfonic acid membranes modified by organic silicon modified TiO2, ZrP or graphene oxide inorganic particles can reduce the vanadium ion permeability of perfluorosulfonic acid membranes to a certain extent, and have good chemical stability[18⇓~20]. Hossain et al. added neodymium zirconia nanotubes (NdZr) to Nafion to prepare a membrane, and then modified it with polycationic polydiallyldimethylammonium chloride (PDDA) and polyanionic polystyrene sulfonate (PSS) to form a multilayer composite membrane[21]. The ion selectivity of the prepared Nafion-NdZr(1%)/[P-S]2 composite membrane is 6.9 times higher than that of the Nafion membrane. It is worth noting that the new composite film has a longer self-discharge time of 513. 7 H (Nafion is 32. 7 H) and better stability.

3.2 Polymer blending

In addition to the inorganic particles modified perfluorosulfonic acid membrane, polymer blending modification is also an important method to improve the performance of perfluorosulfonic acid membrane. Organic polymer blends fall into two main categories: adherent barrier and doped blends.
In the first method, a barrier layer is sprayed or grafted on the surface of the perfluorosulfonic acid membrane, and because the perfluorosulfonic acid membrane has a better mechanical structure and ionic conductivity, the barrier layer can endow the perfluorinated sulfonic acid membrane with better ionic selectivity without excessively reducing the conductivity. Austing et al. Obtained a multilayer (Layer-by-Layer) composite ion exchange membrane of polyethyleneimine (PEI) and Nafion by alternating impregnation in a polymer solution[22]. Compared with the original perfluorosulfonic acid membrane, the ion exchange membrane prepared by the method reduces the vanadium ion permeation by about 70%. It is also reported that the Nafion membrane modified by the cationic charged layer synthesized by radiation-induced graft copolymerization of N, N-dimethylaminoethyl methacrylate (DMAEMA) also shows good vanadium battery performance[23].
In the second method, the organic polymer is directly blended with the perfluorosulfonic acid membrane solution. Zhang et al. Blended polyvinylidene fluoride (PVDF) with Nafion membrane solution to limit the swelling behavior of Nafion by using highly crystalline and hydrophobic PVDF[24]. It was found that the energy efficiency of the battery could reach 85% when the current density of the N0.8P0.2 blend membrane was 80 mA/cm2.

3.3 Porous matrix filling

The porous matrix filling is to fill the ion exchange resin or the polymer electrolyte together with the crosslinking agent into the inert porous membrane matrix, the original porous matrix endows the ionic membrane with better mechanical stability, and the filling of the ion exchange resin ensures the ionic conductivity of the composite membrane[25,26]. Xie et al. Used highly composite porous polyolefin ion-conducting membrane with ultra-thin Nafion layer in zinc-iodine flow single cell (ZISFB), which effectively improved the selectivity of ion-exchange membrane[27]. When the current density is 40 mA/cm2, the current efficiency of the zinc-iodine battery is 97%, the energy efficiency is 81%, and the zinc-iodine battery has a long service life of more than 500 cycles of continuous operation. This provides an important scientific basis for the application of ion exchange membranes prepared by this method in all-vanadium redox flow batteries.
Through the above methods, the ion selectivity of the perfluorosulfonic acid membrane is improved to different degrees, but the perfluorosulfonic acid membrane has high cost and technical barriers, which hinders its further large-scale promotion and use in all-vanadium redox flow batteries[28].

4 Partially fluorinated ion-exchange membrane

In order to maintain the performance of the ion exchange membrane and reduce the cost, a partially fluorinated ion exchange membrane is obtained by reducing the fluorine content of the ion exchange membrane. The commonly used materials are polyethylene-polytetrafluoroethylene (ETFE) and polyvinylidene fluoride (PVDF). The grafting method is to realize the surface grafting by the chemical reaction between the reactive groups on the surface of the material and the grafted monomer or macromolecular chain. Luo et al. Modified polyvinylidene fluoride (PVDF) by grafting polystyrene sulfonic acid (PSSA) in solution to prepare PVDF-g-PSSA membrane[29]. After grafting, the ion conducting membrane has good barrier property for pentavalent vanadium ions, but the penetration of trivalent vanadium ions is relatively high. When the current density is 30 mA/cm2, the energy efficiency of the cell can reach 75.8%. Subsequently, Qiu et al used ETFE as a substrate to graft polymers containing cationic and anionic groups onto the substrate membrane in two steps, and the ETFE-g-PSSA membrane assembled on the vanadium redox flow battery maintained an open circuit voltage (OCV) higher than 1.3 V after 300 H, while Nafion117 maintained an open circuit voltage for only 14 H[30].
In addition to grafting, the modification of perfluorosulfonic acid membranes has also been applied to partially fluorinated ion exchange membranes. Ling et al. introduced sulfonic silica into porous polyvinylidene fluoride membrane by sol-gel method, and prepared polyvinylidene fluoride/sulfonic silica porous composite membrane with high vanadium ion selectivity[31]. The sulfonic silica filled in the pores can not only provide a proton transfer carrier, but also inhibit the penetration of vanadium ions. Compared with Nafion115 at room temperature, the prepared composite membrane has three times of ion selectivity, and the energy efficiency of the cell is 75. 6%. In addition to SiO2, the application of graphene in PVDF porous membranes has also been reported[32]. Lee et al. sprayed an ultra-thin polybenzimidazole (PBI) barrier layer on the polyvinylidene fluoride porous membrane by attaching the barrier layer. Based on the Donnan exclusion principle, the positive charge on the polymer skeleton repels vanadium ions, which can effectively reduce the capacity fading of the battery, and the energy efficiency can reach 79% under the condition of 80 mA/cm2[33].
Some fluoride ion exchange membranes have good chemical stability and mechanical strength, and their price is lower than that of perfluorosulfonic acid membranes. The cost of the ion-conducting membrane prepared from polyvinylidene fluoride reported by our group can be as low as 50$/m2, and the price is about 1/10 of that of Nafion ® membrane[34]. The ion selectivity of ion exchange membranes can be effectively improved by means of inorganic particle modification and organic polymer blending. This kind of ion exchange membrane has the advantages of fluorine-containing polymers and ionic polymers, and will have a better prospect in the future research of ion conducting membranes.

5 Non-fluoride ion exchange membrane

From the point of view of cost reduction, ion exchange membranes made of non-fluoropolymer are favored by researchers, and aromatic compounds have better chemical stability than aliphatic compounds. The non-fluorine materials include polyethersulfone, polyetheretherketone, polyimide, polybenzimidazole and the like, and ion exchange groups are introduced into polymer chains through sulfonation and grafting treatment modification to obtain ion conductivity[35][36][37][38]. Due to the different ion exchange groups, the performance of ion exchange membranes in battery applications is also very different.
Liu et al. prepared Nafion/h-BN/SPEEK sandwich membranes by chemical vapor deposition, and found that the sandwich structure did not affect proton transport[39]. Because the 2D h-BN layer selectively blocks vanadium ions, the ion selectivity is 3 times higher than that of the pure SPEEK membrane. Ye et al. First used the green biopolymer lignin as the additive of the original liquid membrane, and the composite membrane prepared replaced Nafion membrane for all-vanadium redox flow battery[40]. The abundant hydroxyl groups in lignin not only promote the dispersion of lignin in SPEEK matrix, but also significantly improve the ionic conductivity and ion selectivity of the ion exchange membrane. It has been reported that the chemical stability of a novel carboxyl-containing polyimide (PID) grafted sulfonated polyvinyl alcohol (SPVA) copolymer membrane is superior to that of the SPI membrane. Compared with Nafion117, PID30-g-SPVA membrane has higher coulombic efficiency, higher energy efficiency and lower self-discharge rate[41]. In addition, ionic liquids are also one of the methods to improve the ion selectivity of non-fluorinated ion exchange membranes[42]. A novel asymmetric porous sulfonated polyethersulfone (SPES) membrane with ultra-high ion selectivity was prepared by ionic liquid induced phase separation[43]. The preparation method is shown in Figure 2. The results showed that the vanadium ion permeability of SPES porous ion exchange membrane was reduced by 10 times compared with Nafion 212. At the current density of 100 mA/cm2, the coulombic efficiency and energy efficiency of the single cell equipped with SPES porous ion exchange membrane were 98. 8% and 84. 1%, respectively, and the membrane had good cycle stability.
In addition to cation-exchange membranes, the development of non-fluorinated anion-exchange membranes with high selectivity has also attracted the attention of researchers. The outstanding barrier effect of anion exchange membrane is mainly due to the mutual repulsion between the charged groups in the membrane and the specific ions in the solution[44]. At present, the reported methods of preparing anion exchange membranes are mainly chloromethylation of polymers with chloromethyl ether, and then quaternization with trimethylamine to obtain quaternary ammonium groups[45⇓~47]. The quaternary ammonium functionalized QA-OMPAEK anion-exchange membrane prepared by Chen et al., whose ion selectivity was significantly improved due to the interaction between the microscopic morphology of nano-phase separation and water absorption, maintained a long self-discharge time in the test of all-vanadium redox flow battery[48]. In addition, it has been reported that ion channels are constructed in anion exchange membranes to accelerate anion migration. By designing the structure of amphiphilic quaternary ammonium salt polysulfone with hydrophobic n-hexane side chains, good phase separation morphology is achieved and three-dimensional anion channels are constructed. The unique morphology enhances the conductivity of the membrane, reduces the water absorption, and has good cycle stability[49]. Recently, Ma et al. Proposed a method for the preparation of anion exchange membranes based on the β-cyclodextrin template method[50]. The β-cyclodextrin is uniformly dispersed in the hydroxyl-modified polysulfone precursor membrane, and due to the existence of hydrogen bonds between the β-cyclodextrin and the polymer, the selective passage of protons and anions in the battery is facilitated.
Based on the study of anion and cation exchange membranes, researchers have proposed a new type of amphoteric ion exchange membrane[51,52]. Chen et al. Designed and prepared high-performance acid-base blend ion exchange membranes suitable for all-vanadium redox flow batteries[53]. By transferring the H+-SPEEK to the Na+-SPEEK, the compatibility of the hydrophilic sulfonated SPEEK and the polybenzimidazole is improved, and the acid-base interaction caused by the internal crosslinking network is realized. The energy efficiency of the single cell reaches 89.8% at 80 mA/cm2 current density. Wang et al. Synthesized acid-base sulfonated polyimide (SPIs) membranes with different sulfonic acid groups and imidazole groups[54]. The VO2+ permeability of SPI50 membrane with 50% sulfonation degree is 2.2×10-7cm2·min-1, which is much lower than that of Nafion115 membrane (20.2×10-7cm2·min-1). In addition, the battery equipped with the SPI50 acid-base membrane exhibited stable battery performance over 400 charge-discharge cycles at 160 mA/cm2 current density.
However, the biggest challenge facing non-fluoropolymer ion exchange membranes is still chemical stability. Although many studies have reported that no significant decline in energy efficiency has been found in tens to thousands of charge-discharge cycles of flow batteries, it is still not enough to prove that aromatic compounds with ionic groups have sufficient service life in the actual working environment of all-vanadium flow batteries[35]. In particular, SPES and SPEEK membranes were found to degrade significantly in strong oxidizing and acidic battery environments. With the increase of the sulfonation degree of the polymer, unstable chemical groups are introduced, and the chemical stability of the ion exchange membrane is obviously reduced[55⇓~57]. However, it is worth noting that the introduction of ionic groups, compounds and molecular chains to improve the performance of ion exchange membranes will significantly affect the chemical stability of ion exchange membranes, which brings uncertainty to the practical application of non-fluorinated ion exchange membranes. Zirconium sulfide (S-ZrO2) was doped into polyethersulfone (SPES) to enhance the ionic conductivity and inhibit the permeation of vanadium ions[58]. The preparation process is shown in Figure 3. The vanadium ion permeability of the SPES/S-ZrO2 composite membrane is one order of magnitude lower than that of Nafion 212 through battery tests and other characteristic tests. The current efficiency and energy efficiency of the all-vanadium redox flow battery were 98. 89% and 86. 78%, respectively. After 70 charge-discharge cycles at a current density of 100 mA/cm2, the energy efficiency can still be maintained at more than 86%.
For the first time, Liu et al. Added metal-organic frameworks to non-fluorinated polymer membranes, which improved the ion selectivity of ion exchange membranes to a certain extent[59]. Our group recently reported that a novel sulfonated polyethersulfone composite membrane with excellent performance for all-vanadium redox flow batteries was prepared by using core-shell structured nanocellulose (CNC-SPES) as a reinforcing agent[60]. The results show that when the current density is 100 mA/cm2, the mechanical strength of the membrane reaches 54. 5 MPa, the energy efficiency reaches 82%, and the membrane remains stable in 200 charge-discharge cycles.

6 Porous ionically conductive membrane

In addition to dense membranes, porous ion-conducting membranes are also used in all-vanadium redox flow batteries. The ion exchange membrane realizes the ion conduction through the ion exchange groups in the molecular chain, while the porous ion conductive membrane realizes the conduction and selective passage of ions in the electrolyte through the size exclusion effect. Therefore, the porous ion-conducting membrane is different from the ion-exchange membrane, and the non-ionic polymer can be used as the membrane material in principle, which provides a great space for further improving the chemical stability of the porous ion-conducting membrane and reducing the cost.
Phase inversion method is one of the common methods to prepare nano-porous ion-conducting membranes[61,62]. For the first time, the porous membrane used in the vanadium redox flow battery is a polyacrylonitrile nanofiltration membrane. The vanadium ion/hydrogen ion selectivity can be adjusted by adjusting the pore size of the conductive membrane. The energy efficiency of the battery can reach 71%, and it can run stably for 200 cycles[63]. This also proves that the porous ion conducting membrane can also have good stability in the working environment of the battery. In the subsequent research, Cao et al. Changed the crystallization degree and surface roughness of the porous membrane by adjusting the ratio of water and ethanol in the coagulation liquid by using the double coagulation bath method, and simply and effectively controlled the surface morphology of the porous ion conducting membrane, which provided a new idea for the preparation of porous ion conducting membrane[64]. In addition, Wei et al. measured the current efficiency of 95% and the energy efficiency of 78.6% for the hydrophobic asymmetric PVDF porous membrane prepared by phase inversion method[65]. Qiao et al. Designed and prepared a new PES/SPEEK composite membrane with a slot-like selective layer on the surface[66]. The thick and hydrophobic surface structure of the porous ion-conducting membrane blocks the penetration of vanadium ions, while the narrow gap and sponge-like support layer are conducive to proton conduction. The energy efficiency is 90.4% at a current density of 80 mA/cm2. However, the phase inversion method makes it difficult for porous ion-conducting membranes to achieve high ionic conductivity and high ion selectivity[67⇓~69]. Recently, Zhao et al. Constructed a continuous ion transport channel by introducing partially suspended hydrophilic trimethylamine (TMA) groups on the pore wall of a porous membrane with an internal cross-linked network, allowing more protons or hydrates to be transferred along the channel through the Grotthuss hopping-mechanism[70]. The optimized ion conducting membrane was used in the vanadium redox flow battery, and the single cell performed well at the current density of 80 mA/cm2. Even in the case of high current density of 160 mA/cm2, the energy efficiency of the battery can still be maintained at more than 80%, and it can be continuously charged and discharged for more than 1500 cycles. This also provides a new idea for the preparation of porous ion conducting membranes with high ion selectivity and high ion conductivity. Dai et al. Fixed a thin layer of zeolite composite on PES/SPEEK membrane by in-situ interfacial polymerization. When the current density was 80 mA/cm2, the energy efficiency of the battery exceeded 91%, which was one of the highest performance all-vanadium redox flow batteries reported so far[71].
Template methods have also been used to prepare porous ion-conducting membranes. Lu et al. Made the membrane with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) and polyvinylpyrrolidone (PVP) anion exchange resin, and the shell was composed of cage-like pores formed after VO2+ treatment[72]. The cage-like pore structure can screen ions with different radii through a multi-layer pore structure, and has a good blocking effect on vanadium ions. When the current density is 80 mA/cm2, the energy efficiency can reach 88.01%, and it can run stably for 160 cycles. Chen et al. Used the hard template method to remove the template (phenolphthalein) to prepare a layered porous membrane[73]. By optimizing the template content in the casting solution, the prepared hierarchical porous membrane also has high ion selectivity and ionic conductivity.
In addition, Li et al. studied the difference in crystallinity of polyvinylidene fluoride (PVDF) polymer at different temperatures and times[74]. A controllable crystallization method is use for forming a porou structure through random stacking in a polymer crystallization process, that temperature and the time of PVDF polymer crystallization are controlled to adjust the pore size and the spatial distribution of the porou ion conducting membrane, so that the ion conductivity and the ion selectivity of the membrane are improved, and the energy efficiency of a battery reaches 80%.
In addition to the above methods for preparing porous membranes, our group first reported a new method called "gel immobilization method". The principle of this method is shown in Figure 4. According to the principle of polymer swelling, a specific gel state is formed by drying a polymer solution to form a film, and then the polymer gel film is directly immersed into a regulator solution for fine adjustment of the molecular structure, thereby realizing continuous and precise regulation and control of the morphology of a liquid phase channel.O as to balance the ionic conductivity and the ionic selectivity; and finally, putting the adjusted and optimized gel membrane into water to fix molecular chains, and replacing a liquid phase substance with an electrolyte to obtain the ion conductive membrane with high ionic selectivity and high ionic conductivity. The method is simple and efficient, has small material dependence, and can be widely applied to the preparation of almost all ion conducting membranes of soluble polymers. The pure PVDF ion-conducting membrane prepared by the method has good performance and low cost[34]. The ion conducting membrane prepared by the gel immobilization method is different from the porous ion conducting membrane, and realizes the rapid conduction of specific ions and the selective passage of different substances through a continuous channel formed by gaps between polymer molecular chains,It has the advantages of both ion exchange membrane (molecular scale ion transmission channel, homogeneous membrane) and microporous membrane (volume exclusion effect, non-ionic high-stability polymer can be used), and has good stability, which provides a new idea for the preparation of high-performance, long-life and low-cost ion conductive membrane.
To sum up, the battery performance of some ion conducting membranes introduced in this paper is compared, as shown in Table 1. It can be seen from Table 1 that different separator materials and different membrane preparation methods have different effects on the battery performance of ion conducting membranes.
From the point of view of material type, perfluorosulfonic acid ion exchange membrane has higher ionic conductivity and better voltage efficiency in battery test. However, its low ion selectivity leads to low coulombic efficiency, which affects the energy efficiency of the whole battery. Partially fluorinated or non-fluorinated polymers were used as separator materials, which showed better coulombic efficiency than the former in battery tests while reducing costs. However, in the process of battery cycle testing, the chemical stability of some fluorine or non-fluorine ion exchange membranes is insufficient in the strong oxidizing working environment of all-vanadium redox flow batteries, which leads to the attenuation of battery capacity.
As far as the membrane preparation methods are concerned, the commonly used membrane preparation methods are phase inversion method, template method, controlled crystallization method and so on. Porous ion-conducting membranes (PCMs) have attracted much attention due to their different ion conduction modes from ion-exchange membranes and the fact that they are not limited by membrane materials in principle. In the battery test, the porous ion conducting membrane showed relatively stable performance. However, how to accurately control the pore size and uniform distribution of porous structure is an important direction for the future research of porous ion-conducting membranes.

7 Conclusion and prospect

With the further development and utilization of renewable energy, all-vanadium redox flow battery, as a large-scale energy storage device with great development prospects, has attracted wide attention. As one of the key components of all-vanadium redox flow battery (VRB), the ion conducting membrane (ICM) plays an important role in the operation of VRB. At present, the most commercialized ion exchange membrane on the market is Nafion ® membrane of DuPont Company in the United States. The ion selectivity of perfluorosulfonic acid membrane has been improved by inorganic particle modification, polymer blending, inert porous membrane matrix filling and other methods, but the problem of high price is still prominent.
In view of the high price and other problems, researchers have found that ion exchange membranes made of partially fluorinated or non-fluorinated polymers can be effectively improved. The performance of ion exchange membranes can be improved by grafting modification and organic blending, but the lack of chemical stability in the strong oxidizing working environment of vanadium redox flow batteries is still the main bottleneck for most non-perfluorinated ion conducting membranes to be used in vanadium redox flow batteries.
Different from the conduction principle of ion exchange membrane, porous ion conductive membrane is another research direction of ion conductive membrane for all-vanadium redox flow battery. The appearance of phase inversion method, controlled crystallization method, gel immobilization method and other methods provides more directions and ideas for the preparation of high-performance, low-cost and long-life ionic conductive membranes. In addition to the performance of ion conducting membrane itself, the development of new ion conducting membrane also needs to focus on practical issues, such as the difficulty of mass production technology, reliability and comprehensive cost.
To sum up, the application prospect of vanadium redox flow battery is very broad, and the development of new ionic conductive membranes for vanadium redox flow batteries with high chemical stability, high ionic conductivity, high ionic selectivity and low cost is of great significance to accelerate the industrialization of vanadium redox flow batteries and the utilization of clean and renewable energy.

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Funding

the Key Research and Develop Project of Hainan Province(ZDYF2019004)

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