image
Home Journals Progress in Chemistry
Progress in Chemistry

Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

About  /  Aim & scope  /  Editorial board  /  Indexed  /  Contact  / 
Online first  /  Current issue  /  All issues  /  Top access  /  RSS

Progress in Chemistry >

2023 , Vol. 35 >Issue 11: 1595 - 1612

DOI: https://doi.org/10.7536/PC230323

Review

Modified Nafion Membrane in Vanadium Redox Flow Battery

  • Yang Haoling 1 ,
  • Xu Kunyu 1 ,
  • Zhang Qi 1 ,
  • Tao Liang 1 ,
  • Yang Zihao , 1, * ,
  • Dong Zhaoxia 1, 2
Expand
  • 1 Unconventional Petroleum Research Institute, China University of Petroleum (Beijing),Beijing 102249, China
  • 2 School of Energy Resources, China University of Geosciences (Beijing),Beijing 100083, China
*Corresponding author e-mail:

Received date: 2023-03-22

  Revised date: 2023-08-02

  Online published: 2023-09-10

Supported by

National Natural Science Foundation of China(51774302)

National Natural Science Foundation of China(52074320)

Fold

Abstract

Vanadium redox flow battery (VRB) is the most promising large-scale energy storage system due to its flexibility, high efficiency and being pollution-free, which has attracted wide attention from researchers. The separator is a key component of VRB, which plays a role in isolating vanadium ions from cross-penetration and providing proton transmembrane transfer channels. Nafion membranes produced by DuPont are the most commonly used ion exchange membranes for VRB due to their good chemical stability and high proton conductivity. However, they have problems such as poor vanadium resistance and high cost. Therefore, the key point of current research is to control the ion exchange capacity of the Nafion membrane reasonably, improve the vanadium resistance capacity of the Nafion membrane while retaining the excellent performance of the Nafion membrane through modification methods, and reduce the cost of the Nafion membrane. In this paper, the working principle of VRB and the performance characteristics of Nafion membrane are discussed. The current trend and future direction of Nafion membrane modification methods are also discussed in detail. This is of great significance for understanding the structure-activity relationship between modified Nafion membrane structure and battery performance, and guiding the future modification and design of Nafion membrane.

Contents

1 Introduction

2 Principle of VRB

3 Performance evaluation of VRB

4 Functional modification method of Nafion membrane

4.1 In situ sol-gel method

4.2 Functional material blending

4.3 Spin-coating method

4.4 Deposition method

4.5 Polymer grafting

4.6 Construction of sandwich structure

5 Conclusion and prospect

Key words： vanadium redox flow battery (VRB); Nafion; ion exchange; proton conduction

Cite this article

Yang Haoling , Xu Kunyu , Zhang Qi , Tao Liang , Yang Zihao , Dong Zhaoxia . Modified Nafion Membrane in Vanadium Redox Flow Battery[J]. Progress in Chemistry, 2023 , 35(11) : 1595 -1612 . DOI: 10.7536/PC230323

1 Introduction

With the increasing demand for energy, it is of great significance to develop safe, reliable, stable and efficient energy storage systems. Electrochemical energy storage system is widely used because of its short construction period, flexible configuration of power and energy according to different application requirements, fast response of charging and discharging, and stable power supply[1]. Because the electrolyte of the flow battery is stored in the liquid storage tank and separated from the stack, the charging and discharging power and capacity can be designed independently, which is more flexible than other electrochemical energy storage systems.At the same time, the system can be fully automatic closed operation, pollution-free, is the first choice of large-scale energy storage system for large-scale power stations, and has broad application prospects in peak shaving power supply system, large-scale wind and solar power system energy storage, emergency power supply system and other fields[2][3].
Flow batteries can be divided into vanadium flow batteries, iron-chromium flow batteries, all-iron flow batteries, aqueous organic flow batteries and solid-liquid hybrid flow batteries according to their different reactive substances[4][5][6⇓⇓~9][10⇓~12]. The vanadium redox flow battery (VRB) is a redox flow battery system based on a variety of different valence vanadium ions. It was proposed by Skyllas-Kazacos et al. Of the University of New South Wales (UNSW) in Australia in 1986. Because only one reactive substance, vanadium, is used as the redox substance in the battery, the problem of electrolyte pollution caused by ion penetration of the positive and negative electrolytes is fundamentally solved[13]. After multiple charge-discharge cycles, the electrolyte can be restored to its initial state by simple chemical treatment, so that the raw materials can be recycled[14]. This has become the most mature chemical energy storage technology and has successfully entered the stage of commercial development[15].
As the core part of VRB, the separator plays a key role in separating the positive and negative electrolytes. For VRB, the ideal separator should have good ion exchange capacity, high proton conductivity, low water absorption, low expansion rate, low area resistance, low permeability of polyhalide ions such as vanadium, low cost, good chemical and thermal stability[16]. Compared with anion exchange membranes and amphoteric ion exchange membranes, cation exchange membranes are widely used because of their good commercialization, chemical stability and proton transport ability[17]. Among them, Nafion series membrane produced by DuPont Company is the most commonly used cation exchange membrane because of its good chemical stability and high proton conductivity[18]. However, the proton transport channel size of Nafion membrane is 1. 5 ~ 5 nm, which is larger than the size of hydrated polyvalent vanadium ions (~ 0.6 nm), resulting in the cross penetration of vanadium ions, which reduces the capacity of positive and negative electrolytes, resulting in serious self-discharge phenomenon and poor cycle stability[19,20]. At the same time, although Nafion membrane has a better degree of commercialization than other types of ion exchange membranes, its cost still accounts for 30% ~ 40% of the total production cost of VRB, which seriously hinders the further commercial development of VRB. Therefore, while reducing the production cost of Nafion membrane, improving the permeability of vanadium ions and improving the ion selectivity of Nafion membrane are the key to the modification design of Nafion membrane[21].
At present, there are many literatures about the modification methods of Nafion membrane, but there is still no relevant review to systematically sort out the modification methods of Nafion membrane for VRB. In view of this, the working mechanism of vanadium battery was introduced firstly, and the reasons why the film affects the performance of vanadium battery were analyzed systematically. After that, the characteristics of Nafion membrane were introduced, and the performance parameters of commonly used commercial Nafion were summarized. Finally, several common modification methods of Nafion membrane in recent years were summarized, and the relationship between the structure change and membrane performance after modification was analyzed, and the advantages and disadvantages of each modification method, research status and future development were systematically sorted out. It is of great significance to understand the relationship between the structure of Nafion membrane and the performance of battery, and to guide the modification and design of Nafion membrane in the future.

2 VRB principle

Compared with the traditional closed system secondary battery, the VRB presses the electrolyte into the battery stack through an external pump, so that it circulates in the closed loop of different liquid storage tanks and half batteries, thus forming a circulation loop, as shown in Figure 1[22]. The solutions stored in the positive half cell of the VRB are V O 2 + and VO2+ ionic acid solutions, and the solutions stored in the negative half cell are V3+ and V2+ ionic acid solutions. To avoid short circuit between the positive and negative electrodes, the positive and negative electrodes are separated by an ion exchange membrane, which only allows specific ions to pass through the membrane, and the cell stack and the electrolyte reservoir are independent of each other.
图1 VRB示意图

Fig.1 VRB diagram

When the battery is charged, the VO2+ of the positive electrode loses electrons to form a V O 2 +, the V3+ of the negative electrode gains electrons to form a V2+, the electrons pass through an external circuit from the positive electrode to the negative electrode to form current, and the H+ transfers charges from the positive electrode to the negative electrode through an ion conducting membrane to form a closed loop. The change of ions during discharge is just the opposite. The potential difference between the two redox couples is used as the driving force for the redox reaction to promote the battery to complete the charge and discharge. The chemical reaction equation of VRB is shown as follows:
Positive reaction:
$\mathrm{VO}_{2}^{+}+2 \mathrm{H}^{+}+\mathrm{e}^{-} \stackrel{\text { discharge }}{\rightleftharpoons} \mathrm{VO}^{2+}+\mathrm{H}_{2} \mathrm{O}$
Negative reaction:
$\mathrm{V}^{2+}-\mathrm{e}^{-} \underset{\text { charge }}{\stackrel{\text { discharge }}{\rightleftharpoons}} \mathrm{V}^{3+}$
The overall reaction equation:
$\mathrm{VO}_{2}^{+}+2 \mathrm{H}^{+}+\mathrm{V}^{2+} \underset{\text { charge }}{\stackrel{\text { discharge }}{\rightleftharpoons}} \mathrm{VO}^{2+}+\mathrm{H}_{2} \mathrm{O}+\mathrm{V}^{3+}$

3 Performance evaluation of VRB.

Separator is the core component of battery, and the performance of battery is closely related to the ion selectivity of separator. In order to meet the needs of complex operating environment, ion exchange membrane should have excellent ion selectivity, good chemical stability and mechanical stability in the actual use of battery. Table 1 summarizes the important performance tests of the membrane and the evaluation methods of the VRB charge-discharge test.
表1 膜的重要性能以及钒电池充放电测试评估方法

Table 1 The important performance of the membrane and the evaluation method of charge and discharge test of vanadium battery

Properties method equation ref
Cell performance Coulombic efficiency (CE) Charge
discharge test		 C E = ∑ 0 m - 1 1 2 ( I j + I j + 1 ) ( t j + 1 - t j ) ∑ 0 n - 1 1 2 ( I k + I k + 1 ) ( t k + 1 - t k ) × 100 % 23
Voltage efficiency
(VE)		 V E = ∑ 0 m - 1 1 2 ( V j + V j + 1 ) ( t j + 1 - t j ) ∑ 0 n - 1 1 2 ( V k + V k + 1 ) ( t k + 1 - t k ) × 100 %
Energy efficiency
(EE)		 E E = C E × V E = ∫ V d I d d t ∫ V c I c d t × 100 %
Membrane
properties		 Ion exchange capacity (IEC) back-titration method I E C = C H C L V H C L - C N a O H V N a O H m d r y 24,25
Water uptake (WU) — W U = W W - W d W d × 100 % 26
Swelling ratio (SR) — S R = L w - L d L d × 100 % 26
Water transport water transport
experiment.		 — 27
Proton conductivity (R) — R = ( r 1 - r 2 ) × A 28
Permeabilit(σ) — σ = σ ⊥ = l A r 29
Permeability(P0) UV-Visible
spectroscopy		 P o = - V B l 2 A t I n 1 1 - 2 χ B 30,31
Ion selectivity(S) — S = σ P O 32
Chemical stability — 33
For VRB, the electrolyte is an acidic solution, which requires higher chemical stability of the separator than other batteries, and the C-H matrix membrane commonly used in the market is obviously not suitable for VRB[34]. Nafion series membrane is the most used ion exchange membrane in VRB at present because of its excellent chemical stability. Fig. 2 shows the chemical structure of Nafion. Structurally, it combines a hydrophobic polytetrafluoroethylene (PTFE) main chain and a hydrophilic ionic side group, so that its performance is similar to that of PTFE polymers, and it has strong chemical corrosion resistance, and only alkali metals can react with it, so its chemical properties are very stable[35]. At the same time, due to the existence of sulfonic acid group (-SO3H) in the polymer backbone, it is easily hydrolyzed into — S O 3 - and H3O+ when in contact with water, which makes the proton transfer easily in solution, which endows Nafion membrane with excellent chemical stability and excellent electrochemical performance. At the same time, due to the three-dimensional structure generated by crosslinking, the Nafion membrane has high mechanical strength and high thermal stability.
图2 Nafion的化学结构

Fig.2 Chemical structure of Nafion

Typical commercial extruded Nafion series include N115, N117, N112, as well as N1135 and N1100, while the dispersion cast series are coded as NR211 and NR212. Table 2 lists the important parameters of commercial Nafion commonly used at present.
表2 目前商业常用Nafion膜性能汇总

Table 2 Performance summary of Nafion membranes commonly used in commercial applications

membrane Casting procedure Water uptake (WU) Swelling ratio(SR) Thickness
(μm)		 IEC(mmol/g) Conductivity (mS/cm) ref
N115 Extruded 32.1±2.2 69.0±1.5 127 0.87±0.02 100 34,36,37
N117 34.1±1.1 63.0±3.0 183 0.88±0.01 96 34,36,38,39
N112 44.1±1.8 73.9±1.6 50 0.84±0.02 102 34,36
N1135 32.5±2.0 67.6±1.4 88 0.86±0.01 98 34,36
NR211 Dispersion cast 40 — 25 — — 37
NR212 47 — 50 0.88 70 37⇓~39
Fig. 3 demonstrates the effect of VRB separator performance on vanadium battery performance. Although Nafion membrane has excellent chemical stability and high proton conductivity, the cost of Nafion membrane is high, and the size of its proton transport channel is 1. 5 ~ 5 nm, which is larger than that of hydrated multivalent vanadium ions (~ 0.6 nm), resulting in the cross penetration of vanadium ions, resulting in serious self-discharge phenomenon, which reduces the coulombic efficiency (CE) of the battery[40,41]. Although the solution casting method is used to introduce nanoparticles to block the ion transport pore size, it also reduces the proton transport capacity of Nafion membrane to a certain extent, resulting in a higher area resistance and a lower voltage efficiency (VE) of the cell[42]. Therefore, balancing the vanadium ion permeability and proton conductivity and reducing the cost of Nafion are the key to the design of Nafion series VRB separators. At the same time, the water absorption and swelling rate of Nafion membrane are also important properties of VRB, which determine the cost, ion exchange capacity and cycle life of Nafion to a certain extent. Therefore, the development of VRB separators with excellent performance and low cost is still a key challenge for academia and industry, and the optimization and development of new Nafion membrane modification methods are necessary for the further commercialization of VRB.
图3 目前商业常用Nafion膜在不同电流密度下的能量效率[34,3~39]

Fig.3 The energy efficiency of commercial Nafion membrane at different current densities[34,36~39]

4 Nafion membrane functional modification method

Modification of Nafion membrane is a simple and efficient method to improve the performance of VRB and reduce the cost of membrane. A variety of Nafion membrane modification methods have been reported, including in-situ sol-gel method, nanoparticle solution casting, layer-by-layer self-assembly polyelectrolyte, polymer grafting, and polymer blending. The main purpose is to reduce the vanadium ion permeability while maintaining the excellent chemical stability of the Nafion membrane, or to intentionally reduce the amount of Nafion by blending other polymers, so as to reduce the cost of the membrane. Fig. 4 is a schematic diagram of several commonly used methods at present, which will be described in detail below.
图4 Nafion膜改性方法示意图

Fig.4 Schematic diagram of Nafion membrane modification method

4.1 In-situ sol-gel method

The in situ sol-gel method is to use the nanoparticle precursor to react in situ in the formed (extruded) Nafion membrane through condensation reaction to produce nanoparticles, so as to achieve the purpose of blocking the penetration of vanadium ions. Because of the good compatibility between nanoparticles and Nafion membrane matrix, the modified Nafion membrane prepared by this method has better dispersion than that prepared by solution casting method. Therefore, this method is the most common method for Nafion membrane modification in the early stage. Fig. 5 summarizes the Nafion membrane modified by the in-situ sol-gel method and the change of VRB performance parameters.
图5 (a)原位溶胶-凝胶法改性后Nafion膜的厚度及吸水率,(b) 原位溶胶-凝胶法改性后Nafion膜的离子交换能力及质子电导率,(c) 原位溶胶-凝胶法改性后Nafion膜在不同电流密度下的能量效率[43~51]

Fig.5 (a) The thickness and water absorption of Nafion membrane modified by in-situ-gel method, (b) The ion exchange capacity and proton conductivity of Nafion membrane modified by in-situ-gel method, (c) The energy efficiency of Nafion membrane modified by in-situ-gel method at different current densities[43~51]

In order to improve the vanadium and water barrier properties of Nafion membrane, the SiO2 nanoparticles precursor tetraethyl orthosilicate (TEOS) and dimethyldiethoxysilane (DEDMS) were introduced into Nafion membrane by in-situ sol-gel method, and the Nafion/SiO2 composite membrane was successfully synthesized[43,44]. However, the vanadium inhibition ability of single SiO2 nanoparticle precursor is limited, Deng et al. And Youang et al. Found that blending TEOS and DEDMS can affect the nanostructure of organic modified silicate (ORMOSIL), so Teng et al modified the Nafion membrane by the mixture of TEOS and DEDMS, which not only improved the vanadium inhibition ability of Nafion membrane, but also endowed the membrane with higher chemical stability during long-term charge-discharge in strong acid solution[43][45]. Drillkens et al. Also synthesized TEOS/DEDMS/NH3 modified Nafion membranes by a similar method to improve long-term stability[46].
Although the modification of Nafion membrane by SiO2 nanoparticle precursor in-situ sol-gel method has achieved some results, the ion selectivity and vanadium inhibition ability of Nafion membrane are limited, and even the proton conductivity of the modified Nafion membranes is reduced, so it is the trend of in-situ sol-gel method to improve the performance of Nafion membranes by introducing additional groups. Because the SiO2 nanoparticles will occupy the sulfonic acid group of Nafion membrane during the reaction between the precursor of SiO2 nanoparticles and Nafion membrane, resulting in the decrease of proton conductivity, the researchers improved the proton conductivity of Nafion membra ne by introducing sulfonic acid group into (3-mercaptopropyl) trimethoxysilane (MPTMS) and sulfonated diphenyldimethoxysilane (sDDS).As shown in Fig. 5B, the ion exchange capacity of the Nafion membrane NM-1H modified by MPTMS is significantly increased, while the ion exchange capacity of the Nafion-sDDS modified by sDDS is almost the same as that of Nafion/ORMOSIL at a lower membrane thickness (Fig. 5B), and the energy efficiency is significantly increased (Fig. 5C)[47,48]. In order to enhance the vanadium resistance of Nafion membrane, researchers modified commercial Nafion membrane by [N- (2-aminoethyl) -3-aminopropyl] trimethoxysilane (AATMS) and N- (2-aminoethyl) -3-aminopropylmethyldimethoxysilane (APPS), respectively.Aminated silica-modified Nafion membranes were prepared, and the Donnan effect of the modified Nafion membranes was enhanced by the introduction of cations, especially the Nafion membrane modified by AATMS showed higher energy efficiency in the 20~80 mA·cm-2 current density range (Figure 5C)[49,50]. In order to reduce the water absorption of the ion exchange membrane, the researchers improved the hydrophobicity of the Nafion membrane by 1 H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFTOS), which improved the long-term stability of Nafion[51]. Although the in-situ sol-gel method has achieved some results in improving the performance of Nafion, the filler size in the modification process is not easy to control, and the nanoparticles are easy to aggregate and fall off, which leads to the loss of vanadium resistance of Nafion membrane. At the same time, the occupied sulfonic acid groups in the reaction process will also reduce the proton conductivity of Nafion membrane, which hinders its further application in VRB.

4.2 Functional material blending

The solution casting method for modifying Nafion membrane is to directly cast the polymer solution in a flat mold after blending nanoparticles or organic polymers with Nafion, and then volatilize the solvent at a certain temperature to form the membrane. Compared with the in-situ sol-gel method, the solution casting method can better control the size of the nanoparticles, solve the problem of agglomeration and caking in the in-situ condensation growth process of the in-situ sol-gel method nanoparticles, and further improve the ion selectivity of the Nafion membrane. At the same time, the polymer with lower price is used as the polymer filler to be blended with the Nafion, which can also improve the performance of the Nafion membrane and reduce the preparation cost of the diaphragm. It is the most common method for Nafion membrane modification at present.

4.2.1 SiO2 modification

Compared with the SiO2 in situ sol-gel method, the SiO2 solution casting method is more convenient in the process of controlling nanoparticle size and nanoparticle surface functional modification, as shown in fig. 6B for TA-SiNPs and fig. 6a for PS-SiNPs.The vanadium resistance and ion exchange capacity of the Nafion membrane can be greatly improved by aminated silica and sulfonated silica nanoparticles, and meanwhile, the uniform dispersion of the filler in the polymer matrix helps to hinder the cross penetration of vanadium ions,By controlling the transportation of vanadium ions in the polymer nanoparticle composite membrane, the N/ST-6 membrane obtained by surface grafting modification of SiO2 by TEMPO can improve the compatibility between SiNPs and Nafion substrate, and the N — O · radicals of Si-TEMPO can be converted into positively charged N+=O to enhance the Donnan effect of Nafion membrane when VRB is electrified, thus effectively reducing the transmembrane transportation of vanadium ions[52,53][54][55]. Although the improved dispersion of SiNPs has a good effect on inhibiting the cross permeation of vanadium ions, the introduction of SiNPs still reduces the proton conductivity of Nafion membranes, so it is often necessary to add additives to optimize the proton conductivity.Phosphotungstic acid (PWA) is used to improve the proton conductivity of Nafion composite membranes modified by SiO2 due to its excellent acidity and thermodynamic stability[56]. As shown in fig. 6B and fig. 6C, by introducing nanohybrids composed of SiO2 and PWA into Nafion ionomers, VRB can exhibit ultra-high energy efficiency while improving proton conductivity.
图6 (a) SiO2溶液浇铸法改性后Nafion膜的厚度及吸水率,(b) SiO2溶液浇铸法改性后Nafion膜的质子电导率及钒离子渗透率,(c) SiO2溶液浇铸法改性后Nafion膜在不同电流密度下的能量效率[52~56]

Fig.6 (a) The thickness and water absorption of Nafion membrane modified by SiO2 solution casting, (b) The proton conductivity and vanadium ion permeability of Nafion membrane modified by SiO2 solution casting, (c) The energy efficiency of Nafion membrane modified by SiO2 solution casting at different current densities[52~56]

4.2.2 WO3 modification

WO3 has attracted much attention due to its excellent properties in electrochemical devices, solar cells, gas sensors, and catalysts. At the same time, it is simple to prepare, more stable in sulfuric acid compared with other metal oxides, low cost, and has been well applied in VRB electrode materials and membrane fillers[57]. Sun et al. Introduced WO3 nanoparticles into Nafion matrix by solution casting method to obtain different loading levels of [Nafion/(WO3)x] membrane (X = 0, 0.024, 0.329, 0.587), and examined its performance in VRB, revealing its unique bilayer structure, which can significantly reduce the penetration of vanadium ions[58].

4.2.3 TiO2 modification

TiO2 is widely used as an efficient additive for polymer membranes because of its low cost, high utilization, and good chemical stability. Compared with other inorganic nanomaterials, TiO2 has a certain hygroscopicity because of its rich hydroxyl groups on the surface, which is helpful to improve the proton conductivity of the blend membrane after blending with Nafion membrane, and is widely used in proton exchange membrane fuel cell system. However, it is found that TiO2 modified Nafion membrane is not suitable for VRB, because TiO2 can be dissolved in VRB electrolyte. At the same time, the interaction between TiO2 nanoparticles and polar clusters in the ion channel of Nafion membrane is weak, which makes the TiO2 easy to fall off in VRB electrolyte solution, resulting in the loss of vanadium rejection ability of Nafion membrane modified by TiO2. The surface modification of TiO2 by silane coupling agent DEDMS can effectively improve the chemical stability and vanadium resistance of TiO2 in VRB electrolyte, but the area resistance of Nafion/Si/Ti composite membrane increases due to the poor proton conductivity of DEDMS (Figure 7B)[59].
图7 (a) TiO2以及氧化锆溶液浇铸法改性后Nafion膜的厚度及吸水率,(b) TiO2以及氧化锆溶液浇铸法改性后Nafion膜的区域电阻、质子电导率及离子交换能力,(c) TiO2以及氧化锆溶液浇铸法改性后Nafion膜的钒离子渗透率及离子选择性,(d) TiO2以及氧化锆溶液浇铸法改性后Nafion膜在不同电流密度下的能量效率[59~64]

Fig.7 ( a ) The thickness and water absorption of Nafion membranes modified by TiO2 and zirconia solution casting, ( b ) The area resistance, proton conductivity and ion exchange capacity of Nafion membranes modified by TiO2 and zirconia solution casting, ( c ) The vanadium ion permeability and ion selectivity of Nafion membranes modified by TiO2 and zirconia solution casting, ( d ) The energy efficiency of Nafion membranes modified by TiO2 and zirconia solution casting at different current densities[59~64]

Therefore, it is still necessary to reduce the ion channel size of Nafion membrane by improving the interaction between TiO2 and polar clusters. The researchers prepared the Nafion/TiO2 composite membrane by hydrothermal method. The polar clusters of the Nafion membrane were expanded by MeOH/H2O solution and the Ti4+ and urea molecules were introduced into the polar clusters, and then TiO2 was generated by hydrothermal reaction, which improved the stability of the modified Nafion membrane and reduced the area resistance of the modified Nafion film (Fig. 7 B)[60]. Ye et al. Used super-hydrophilic TiO2 nanotubes and Nafion to prepare Nafion/TiO2 composite membrane by solution casting method. Because the dispersed nanotubes blocked and prolonged the ion diffusion pathway,At the same time, the combination of sulfonic acid groups in the polymer matrix and TiO2 improves the dispersion of TiO2 in the Nafion membrane and the binding ability with ion clusters, which greatly reduces the permeability of the ion exchange membrane, making VRB perform the best energy efficiency in a wide range of current densities (Fig. 7d)[61].

4.2.4 Zirconia modification

Compared with TiO2, zirconium dioxide has stronger hygroscopicity, and the Nafion film modified by zirconium dioxide nanotubes shows excellent proton conductivity while improving the vanadium resistance[62]. To further improve the proton conductivity, inspired by the fact that the incorporation of zirconia into ceria would stimulate the structural change of ceria, Aziz et al. Fabricated TiZrO4 nanotubes and prepared a high ion-exchange capacity Nafion/TiZrO4 composite membrane for high-performance VRB (Figure 7B)[63]. Since the rare earth lanthanide neodymium oxide (Nd2O3) is also hygroscopic, the Nd2Zr2O7 filler was introduced into the Nafion membrane according to its characteristics to improve its proton conductivity, and the selectivity of the membrane was further improved by the self-assembly technique of [P-S] layer (Fig. 7 C)[64].

4.2.5 Two-dimensional carbon nanomaterials modified

Although the introduction of inorganic nanoparticles into Nafion membrane by solution casting method can effectively improve the vanadium resistance of Nafion membrane, the spherical morphology of nanoparticles and the unshed block structure still seriously limit the barrier effect of the separator. To further improve the vanadium rejection ability of Nafion membrane, the design of new H+/V ion-selective membranes with ion or molecular sieving effect is the focus of future research[65]. Due to their high specific surface area and inherent mechanical stability, two-dimensional layered materials are easier to form nano-and sub-nano-sized pores with narrow size distribution than nanoparticles, and are the best choice for the design of new H+/V ion-selective membranes with ionic or molecular sieving effects.
There are very small pores on the surface of graphene, which have a good corresponding relationship with the size of protons, effectively inhibiting the diffusion and migration of vanadium ions in the Nafion membrane, while improving the proton transport rate and ion selectivity of the composite membrane[66]. g-C3N4 nanosheets, similar to the graphene structure, have excellent electrochemical properties, and the preparation of Re-N/C3N4(x) composite membrane by casting can not only improve the energy efficiency of Nafion membrane during VRB operation (Fig. 8D),At the same time, the layered structure of g-C3N4 nanosheets in the Nafion membrane and the existence of N atoms significantly improve the vanadium resistance of the modified Nafion membrane, as shown in Fig. 8 C Re-N/C3N4(x)) composite membrane has the lowest vanadium ion permeability compared with other membranes[67]. Compared with graphene and g-C3N4 nanosheets, graphene oxide (GO) nanosheets are rich in hydroxyl groups, which makes Nafion membrane have excellent vanadium resistance and electrochemical properties through barrier effect when GO nanosheets are randomly embedded in Nafion membrane. Meanwhile, the hydroxyl groups in GO nanosheets can form hydrogen bonds with sulfonic acid groups in Nafion matrix, which endows the separator with more excellent mechanical stability[68][69]. Therefore, compared with other modification methods, the rN212/GO membrane prepared by blending GO and Nafion solution is thinner than the thinnest commercial membrane Nafion212, which provides a new idea for reducing the cost of Nafion membrane while maintaining the vanadium resistance of Nafion membrane. At the same time, the rich hydroxyl groups in GO nanosheets also provide abundant active sites for the functional modification of GO. Kim et al. Prepared sulfonated GO by the reaction of sulfonation reagent with the surface hydroxyl groups of GO, and improved the dispersion of GO by phenyl isocyanate treatment, which greatly reduced the penetration of vanadium ions on the premise of maintaining good conductivity and electrochemical performance[70]. Cui et al. Successfully synthesized tertiary amino-modified graphene oxide nanosheets (TAGO) by grafting dimethylaminoethyl methacrylate onto the surface of GO through γ-ray irradiation, which improved the good compatibility with Nafion matrix[71]. At the same time, the hydrogen bond network between the tertiary amino group, the carboxylic acid group and hydroxyl group in TAGO, and the sulfonic acid group in Nafion forms an acid-base pair, which forms an ion channel for proton transport in the composite membrane. The Nafion/TAGO composite membrane exhibits excellent thermal stability, high proton conductivity, low VO2+ permeability, and high ion selectivity (Fig. 8C).
图8 (a) GO溶液浇铸法改性后Nafion膜的厚度及吸水率,(b) GO溶液浇铸法改性后Nafion膜的质子电导率及离子交换能力,(c) GO溶液浇铸法改性后Nafion膜的钒离子渗透率及离子选择性,(d) GO溶液浇铸法改性后Nafion膜在不同电流密度下的能量效率[65~71]

Fig.8 (a) The thickness and water absorption of Nafion membrane modified by GO solution casting. (b) The proton conductivity and ion exchange capacity of Nafion membrane modified by GO solution casting. (c) The vanadium ion permeability and ion selectivity of Nafion membrane modified by GO solution casting. (d) The energy efficiency of Nafion membrane modified by GO solution casting at different current densities[65~71]

4.2.6 MOFs material modification

As a kind of porous material, metal-organic frameworks (MOFs) have a rigid structure formed by the combination of inorganic and organic units. Compared with two-dimensional planar materials, MOFs can adjust the pore size by controlling the organic ligand, design MOFs fillers with unique pore/pore structure, and provide unique proton transfer channels. The selectivity and conductivity of MOFs were improved by introducing functional groups into organic ligands. Therefore, the incorporation of MOFs into Nafion ionomers can be used as a prospective approach to reduce vanadium ion permeability.
Zr-based MOF material UiO-66 and Al-based MOF material CAU-10 provide excellent H+/V ion selective channels for Nafion membrane as solution casting fillers due to their appropriate pore structure and good chemical stability. As PWA and sulfonic acid groups have excellent proton conduction ability (Fig. 9 B), to further compensate for the decrease of sulfonic acid group density of Nafion membrane caused by the filler, the researchers synthesized Nafion-(UiO-66-NH2@PWA) and Nf/S-U66-3 composite membranes with excellent proton conductivity and vanadium battery performance by combining PWA on amino-functionalized UiO-66(UiO-66-NH2) and designing UiO-66-SO3H with sulfonic acid groups, respectively[72][73]. Choi et Al. Used a similar method to attach functional groups to Al-based MOF material CAU-10. Due to the excellent proton conductivity of sulfonic acid groups, the N/CAU-10-OS1 and N/CAU-10-OS2 composite membranes with hydrophilic group combination have better proton conductivity compared with hydrophobic N/CAU-10-CH3 (Fig. 9 B), but due to the presence of —CH3, the MOFs membrane N/CAU-10-CH3 with hydrophobic group or functional group combination has lower water absorption, which helps to improve the service life of VRB (Fig. 9 a)[74]. Fig. 9 shows the performance parameters of Nafion membrane and VRB modified by MOFs solution casting method.
图9 (a) MOFs溶液浇铸法改性后Nafion膜的厚度及吸水率,(b) MOFs溶液浇铸法改性后Nafion膜的质子电导率及离子交换能力,(c) MOFs溶液浇铸法改性后Nafion膜的钒离子渗透率及离子选择性,(d) MOFs溶液浇铸法改性后Nafion膜在不同电流密度下的能量效率[72~74]

Fig.9 (a) The thickness and water absorption of Nafion membrane modified by MOFs solution casting, (b) The proton conductivity and ion exchange capacity of Nafion membrane modified by MOFs solution casting, (c) The vanadium ion permeability and ion selectivity of Nafion membrane modified by MOFs solution casting, (d) The energy efficiency of Nafion membrane modified by MOFs solution casting at different current densities[72~74]

4.2.7 Polymer blending

Compared with the inorganic nanoparticle filler, the organic polymer has better compatibility with the Nafion substrate, which makes the Nafion membrane modified by the polymer blend solution casting method have good vanadium resistance and excellent mechanical stability. Conductive polymers with nitrogen-rich structure (polyaniline and polypyrrole) are commonly used as polymer blend fillers at present. By introducing polyaniline and polypyrrole into Nafion membrane, the vanadium inhibition ability of Nafion membrane can be effectively improved. At the same time,Polyaniline with stronger basicity can be tightly combined with sulfonic acid groups along both sides of the pore and the surface of the Nafion membrane, which makes the Nafion/polyaniline composite membrane more effective in reducing the passage of vanadium ions than Nafion/polypyrrole composite membrane[75]. Polybenzimidazole (PBI) is also rich in nitrogen structure, and the PBI/Nafion composite film prepared by solution spraying can improve the vanadium resistance of Nafion film, as shown in Figure 10 C, the vanadium resistance of B20N10 composite film is better than that of other films[76,77]. However, the chemical stability of the modified membrane and the battery performance are greatly reduced due to the poor connection between the PBI layer and the Nafion layer. In order to solve the adverse effects of loose coating and low conductivity on VRB, the researchers introduced PBI into Nafion by blending solution casting to improve the chemical stability of the composite membrane. At the same time, there is an "acid-base pair effect" between PBI and Nafion, which is conducive to the ion transport of protons in the composite membrane. As shown in Figure 10d, Nafion/PBI-1.0 shows higher energy efficiency than other types of membranes[78].
图10 (a) 聚合物溶液浇铸法改性后Nafion膜的厚度及吸水率,(b) 聚合物溶液浇铸法改性后Nafion膜的质子电导率及离子交换能力,(c) 聚合物溶液浇铸法改性后Nafion膜的钒离子渗透率及离子选择性,(d) 聚合物溶液浇铸法改性后Nafion膜在不同电流密度下的能量效率[75~88]

Fig.10 (a) The thickness and water absorption of Nafion membrane modified by polymer solution casting, (b) The proton conductivity and ion exchange capacity of Nafion membrane modified by polymer solution casting, (c) The vanadium ion permeability and ion selectivity of Nafion membrane modified by polymer solution casting, (d) The energy efficiency of Nafion membrane modified by polymer solution casting at different current densities[75~88]

In addition, Kim et al. Proposed a new method to adjust the morphology of water nanoclusters using propylene carbonate (PC) to expand the ionic clusters of Nafion membrane and improve the conductivity of Nafion-based pore-filling membrane[79]. Yang et al. constructed PWA self-immobilized fiber through self-assembly of nano-Kevlar fibers (NKFs) and PWA based on hydrogen bonding, which not only improved the vanadium resistance, but also improved the proton conductivity by using the strong acidity of PWA. As shown in Figure 10B, the modified Nafion-NKFs @ PWA has higher proton conductivity[80]. Lignin and cellulose are abundant and renewable, and the structure rich in polyhydroxy is beneficial to the improvement of proton conductivity, which is an ideal filler for the preparation of environment-friendly blend membranes.Palanisamy et al. And Ye et al. Blended cellulose and lignin with Nafion by solution casting method respectively, which not only improved the proton conductivity of Nafion membrane, but also reduced the dosage of Nafion, thus reducing the cost of Nafion membrane[81][82]. In order to improve the long-term stability of Nafion membrane, Mai et al. First used Nafion/polyvinylidene fluoride membrane (PVDF) blend to prepare ion exchange membrane, and the expansion behavior of Nafion membrane was limited by the high crystallization and hydrophobicity of PVDF[83]. Because the proton conductivity of Nafion/PVDF composites decreases rapidly with the increase of PVDF content, in order to effectively improve the proton conductivity of Nafion/PVDF blend membranes, the researchers controlled the transition from macroscopic phase separation to microscopic phase separation in the blending process of Nafion and PVDF by controlling the annealing temperature, and improved the proton conduction of Nafion/PVDF blend membranes[84].
Porous PTFE membrane and Nafion/PTFE membrane are widely used in fuel cells, and are used as substrates in VRB. Nafion/PTFE membrane was prepared by casting Nafion solution and PTFE solution to replace PTFE porous membrane, which can effectively inhibit the self-discharge phenomenon of VRB while reducing the cost of the membrane[85,86][87]. Compared with Nafion, sulfonated polyimide (SPI) polymer has lower vanadium ion permeability, better proton selectivity and thermal stability, excellent VRB performance and lower cost.However, the poor chemical stability of pure SPI membranes hinders their further commercial application in VRB. Li et al. Blended SPI with Nafion by solution casting method, which improved the electrochemical performance of the membrane and reduced the cost of the membrane[88].

4.3 Spin coating

The spin coating method relies on the centrifugal and gravitational effects of the film in the process of rotation to spread the coating droplets on the surface of the film. The aligned GO/Nafion composite film with ultrathin spin coating prepared by spin coating method has lower vanadium permeability and better chemical and mechanical stability compared with the unaligned GO/Nafion composite film[89]. In order to further enhance the durable adhesion between Nafion substrate and GO coating, the interaction between Nafion substrate and GO interface can be improved by constructing a crosslinked network structure. Zhang et al.A large number of amino groups were generated on the surface of Nafion 212, and then a thin layer of m-xylene diamino-crosslinked GO was formed by spin coating to enhance the adhesion of GO on the surface of Nafion film[90].

4.4 Sedimentation method

Deposition method is a common surface modification method of Nafion membrane at present, which can build a barrier on the surface of Nafion membrane by physical or chemical means to improve the ion exchange capacity of Nafion membrane. Fig. 11 shows the Nafion membrane modified by the deposition method and the VRB performance parameters. At present, the common deposition methods of Nafion films include physical vapor deposition and atomic layer deposition. The physical vapor deposition method includes reactive spray deposition method and radio frequency magnetron sputtering method. Compared with the solution casting method, the reactive spray deposition method can make the TAGO filler orderly arranged on the surface of Nafion membrane, form an effective shielding layer, and improve the vanadium resistance of Nafion membrane[91]. Although the increase of hydrophobicity is beneficial to the chemical stability of Nafion membrane, the introduction of hydrophobic polymer by blending will lead to a significant decrease in the proton transport capacity of Nafion membrane.Therefore, the formation of PTFE @ Nafion composite membranes by RF magnetron sputtering of ultrathin PTFE films on Nafion substrates can enhance the stability of Nafion membranes while maintaining excellent ion selectivity[92]. Although the ion exchange capacity and stability of the Nafion membrane modified by physical deposition method have been well improved, it also brings about the problems of increased resistance area and the shedding of polyelectrolyte components in high concentration of sulfuric acid electrolyte solvent, so this method is less reported than other studies.
图11 (a)沉积法改性后Nafion膜的质子电导率、钒离子渗透率及离子选择性,(b) 沉积法改性后Nafion膜在不同电流密度下的能量效率[91~100]

Fig.11 (a) Proton conductivity, vanadium ion permeability and ion selectivity of Nafion membrane modified by deposition method, (b) Energy efficiency of Nafion membrane modified by deposition method at different current densities[91~100]

Atomic layer deposition can control the thickness, composition and structure of the film well through the layer-by-layer growth of atoms on the film surface, and has a lower budget than vapor deposition technology. Zeng et al. Electrodeposited pyrrole on the surface of Nafion membrane to effectively improve the stability and vanadium resistance of the membrane[93]. Layer-by-layer self-assembly technique is a special atomic layer deposition modification method, which was first proposed by Decher and Hong in 1992[94]. The synthesis of the multilayer film is controlled on a nanometer scale, a cationic polymer is used for constructing a vanadium barrier, an anionic polymer is used for improving the proton conduction capability, and the construction of a barrier layer on the surface of the Nafion film is realized through the alternate adsorption of the cationic polymer and the anionic polymer. Table 3 summarizes the research status of layer-by-layer self-assembled membranes applied to VRB. Although layer-by-layer self-assembly has made great progress in the construction of long-term stable Nafion membranes with good ion exchange capacity, the interaction between layers is weak, which leads to the easy shedding of assembled layers in strong acidic and oxidizing environments. Therefore, the layer-by-layer self-assembled film can form a compact molecular structure by inducing crosslinking between layers, and the mechanical stability and chemical stability of the Nafion film are improved. For example, Yoo et al. Prepared a multilayer composite proton exchange membrane by alternately depositing highly sulfonated poly (cyclic phenylene) (sPPO) and cationic polystyrene electrolyte on the surface of Nafion 212 to improve the ion exchange capacity of Nafion, and induced the crosslinking of LbL membrane by the activation of azide groups to improve the proton transport capacity and vanadium resistance of Nafion membrane while making Nafion membrane have excellent chemical stability[95].
表3 层层自组装法制备膜研究现状汇总

Table 3 Summary of research status of membrane preparation by layer-by-layer method

Membrane Basis Positive electricity Negative electricity layer ref
Nafion-[PDDA-PSS]5 Nafion117 Poly(diallyldimethylammonium chloride) (PDDA) Polyanion poly(sodium styrene sulfonate) (PSS) 5 96
Nafion-[CS-PWA]3 Nafion212 Polycation chitosan (CS) PWA 3 97
N117-(PEI/Nafion)10 Nafion117 Polyethylenimine (PEI) Nafion 10 98
Nafion-[PDDA/ZrP]3 Nafion115 PDDA zirconium phosphate (ZrP) nanosheets 3 99
GN212C-[PDDA-PSS]3 GN212C PDDA PSS 3 100

4.5 Polymer graft modification

Grafting polymer to improve the performance of Nafion membrane is a simple and effective method for surface modification of Nafion membrane, which can comprehensively improve the performance of Nafion membrane without affecting the overall advantages of the polymer, and modify the Nafion membrane by combining the functional polymer into the main chain through covalent connection at the molecular level, so as to improve the vanadium resistance and ion exchange capacity of the Nafion film. Fig. 12 shows the performance parameters of Nafion membrane and VRB modified by polymer grafting method and sandwich structure.
图12 (a) 聚合物接枝法以及构建夹层结构改性后的Nafion膜的厚度及吸水率,(b) 聚合物接枝法以及构建夹层结构改性后的Nafion膜的质子电导率及离子交换能力,(c) 聚合物接枝法以及构建夹层结构改性后的Nafion膜的钒离子渗透率及离子选择性,(d) 聚合物接枝法以及构建夹层结构改性后的Nafion膜在不同电流密度下的能量效率[101~112]

Fig.12 (a) Thickness and water absorption of Nafion membranes modified by polymer grafting and construction of sandwich structure ; (b) Proton conductivity and ion exchange capacity of Nafion membranes modified by polymer grafting and construction of sandwich structure ; (c) Vanadium ion permeability and ion selectivity of Nafion membranes modified by polymer grafting and construction of sandwich structure ; (d) Energy efficiency of Nafion membranes modified by polymer grafting and construction of sandwich structure at different current densities[101~112]

The introduction of anionic groups can provide a proton transport channel to enhance the proton conduction ability of Nafion membrane, and the introduction of cationic groups can construct a vanadium barrier to hinder the cross penetration of vanadium ions. Nibel et al. Proposed a potential and universal bifunctional membrane design concept through radiation grafting method.The sulfonic group and the amide group are introduced into the Nafion membrane, the proton conductivity is enhanced through the sulfonic group, the vanadium resistance is enhanced through the amide group, the charge-discharge efficiency of the Nafion membrane in the VRB is improved, and the capacity loss is reduced[101]. In order to reduce the cross penetration of vanadium ions, Yang et al. Used oxygen plasma-induced grafting technology to graft sodium 4-phenyldienesulfonate (NaSS) onto the surface of Nafion 212, and blocked the pore channels of Nafion surface by NaSS. Because NaSS is rich in sulfonic acid groups, it can improve the vanadium resistance of Nafion membrane while maintaining the excellent proton conductivity of Nafion membrane[102]. Inspired by the related research on atom transfer radical addition, Dai et al. Grafted zwitterionic SBMA (sulfonamide methacrylate) onto Nafion 115 by SI-ATRP (surface-initiated atom transfer radical polymerization), and successfully prepared a new N115-g-PSB MA membrane containing zwitterionic groups for the first time.The N R 4 + group was used to construct the vanadium barrier, and the —SO3 group in SBMA was used to improve the proton conductivity of the Nafion membrane, as shown in Figure 12b, N115-g-PSBMA had the highest proton conductivity compared with other grafted membranes[103,104][105]. However, the number of cationic and anionic groups in SBMA is fixed, and it is difficult to adjust the ratio of cationic and anionic groups in amphoteric Nafion membrane with SBMA as monomer. Therefore, the polymers of [2- (methacryloyloxy) ethyl] trimethyl ammonium chloride (METAC) and NaSS were selected as cationic monomer and anionic monomer, respectively.Amphoteric Nafion membranes with tunable anion-cation ratio were prepared by ATRP and solution casting. By adjusting the ratio of METAC to NaSS in the membrane, the vanadium permeability of Nafion was reduced, while its high stability and high ionic conductivity were maintained[106]. In addition, reducing the cluster space of the Nafion membrane helps to improve the vanadium rejection ability of the Nafion membrane, and on this basis, activating the —SO3H group on the surface of the Nafion membrane by CDI (1,1-carbonyldiimidazole), and grafting aminopropyl isobutyl polyhedral oligomeric silsesquioxane (NH2-POSS) onto the —SO3H group of the Nafion film can reduce the cluster of Nafion, effectively hindering the penetration of V O 2 + (shown in Figure 12C N112-0.2% -POSS composite film)[107].

4.6 Construction of sandwich structure modification

The construction of modified Nafion membrane with sandwich structure is to combine different types of membranes by chemical methods, which endows the sandwich membrane with the advantages of various membranes and reduces the cost. In order to reduce the cost of Nafion membrane and improve the ion exchange capacity while retaining its chemical stability, Luo et al. Prepared Nafion/SPEEK layered composite membrane composed of recast Nafion membrane and SPEEK membrane[108]. In order to further improve the vanadium rejection of the ion exchange membrane, Jia et al. Prepared a sandwich-type S/P/P composite membrane consisting of a PFSA layer, a transition center layer, and a SPEEK layer by solution casting[109]. To further enhance the proton transport rate, Yu et al. Synthesized a Nafion XL membrane with a novel sandwich structure consisting of a middle microporous PTFE layer and two dense Nafion outer layers[110]. However, it was found that the adhesion between the layered composite film and the layer was weak, and the proton conductivity decreased when the interaction between the layers was improved by crosslinking. Achieving strong interfacial adhesion between the two layers without losing proton conductivity is still a difficult problem that limits the layered composite modification of Nafion membranes. Kim et al. Firmly combined sulfonated poly (aryl ether sulfone) (SPAES) with Nafion through 3-D IIL with ball-and-socket connection structure.Combining the properties of the two chemically different membranes while maintaining their good proton conductivity, as shown in fig. 12b S/N, and the energy density is between 40~200 mA·cm-2, which is higher than that of the other membranes, as shown in fig. 12d[111]. In addition, compared with other methods, the Nafion membrane modified by the layered composite membrane is easier to arrange directionally and form a two-dimensional layer plane when the two-dimensional structure material is introduced. Two-dimensional (2D) hexagonal boron nitride (hBN) has attracted considerable attention due to its excellent chemical and thermal stability, electrical insulating properties, high proton conductivity, and good flexibility. Because its large area two-dimensional (2D) hexagonal boron nitride film with high homogeneity is difficult to grow, and the two-dimensional (2D) hexagonal boron nitride film is easily damaged during transfer, Liu et al.Wafer-scale uniform h-BN monolayer film growth was realized, and a Nafion functional layer assisted transfer method was also developed, which can effectively transfer the grown hBN monolayer film from Cu roll to SPEEK film[112]. The prepared Nafion/h-BN/SPEEK sandwich structure was used as the membrane and compared with the pure SPEEK membrane of the flow cell. The results showed that the ion selectivity of the sandwich membrane was 3 times higher than that of the pure SPEEK membrane (Figure 12C).

5 Conclusion and prospect

As the core component of VRB, the membrane plays a key role in providing proton transport channels and blocking the cross penetration of vanadium ions. Nafion membrane produced by DuPont has excellent stability and high proton conductivity, which is the most commonly used ion exchange membrane in VRB at present. However, its poor vanadium resistance limits its further application in VRB. Therefore, the functional modification of Nafion membrane has developed into an effective strategy to construct VRB with high efficiency, flexibility and excellent cycle performance, and is also an important means to further accelerate the commercialization of VRB. The Nafion membrane with high vanadium ion permeability was used as the cation exchange membrane, and the ion transmission channel was narrowed by doping fillers, and the positively charged groups were introduced to construct the vanadium barrier, which effectively hindered the cross penetration of vanadium ions, but the energy efficiency of VRB was still limited by the reduction of the number of sulfonic groups in the modification process. Therefore, it is still a great challenge to coordinate the balance between vanadium ion permeability and proton conductivity, rationally design and prepare advanced modified Nafion membranes with excellent electrochemical performance, and comprehensively and deeply understand the structure-activity relationship of modified Nafion membranes. At the same time, the high cost is also a bottleneck for the wide application of Nafion membranes in the near future. The sustainable development of membrane applications is also of great significance for large-scale membrane production processes, as most membrane production processes require toxic solvents, such as N, N-dimethylformamide and N, N-dimethylacetamide, which are still potentially harmful to the natural environment. In addition, the preparation process needs to be cleaned with corrosive inorganic solutions (such as H2SO4), and other polymers for blend casting are almost all petroleum derivatives. Therefore, the use of biopolymers and environmentally friendly solvents to reduce the impact on the environment is the trend of future development. In general, in the near future, the in-depth understanding of the structure-activity relationship of modified Nafion membranes and the in-depth study of environmentally friendly membrane preparation methods are expected to promote the further development of VRB ion exchange membranes with high chemical stability, high ionic conductivity, low vanadium permeability and low cost.At the same time, it is of great significance to accelerate the commercialization of VRB and the use of clean and renewable energy.

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]
He Q, Chen Z B, Niu X Y, Han X R, Kang T, Chen J Y, Ma Y W, Zhao J. Nano Res., 2023, 16(7): 9195.

[2]
Páez T, Zhang F F,Muñoz M Á, Lubian L, Xi S B, Sanz R, Wang Q, Palma J, Ventosa E. Adv. Energy Mater., 2022, 12(1): 2102866.

[3]
Ra N, Ghosh A, Bhattacharjee A. Energy Convers. Manag., 2023, 281: 116851.

[4]
Zeng Y K, Zhao T S, An L, Zhou X L, Wei L. J. Power Sources, 2015, 300: 438.

[5]
Gong K, Xu F, Grunewald J B, Ma X Y, Zhao Y, Gu S, Yan Y S. ACS Energy Lett., 2016, 1(1): 89.

[6]
Pan M G, Shao M H, Jin Z. SmartMat, 2023, 4(4): e1198.

[7]
Wang C X, Yu B, Liu Y Z, Wang H Z, Zhang Z W, Xie C X, Li X F, Zhang H M, Jin Z. Energy Storage Mater., 2021, 36: 417.

[8]
Pan M G, Gao L Z, Liang J C, Zhang P B, Lu S Y, Lu Y, Ma J, Jin Z. Adv. Energy Mater., 2022, 12(13): 2103478.

[9]
Pan M G, Lu Y, Lu S Y, Yu B, Wei J, Liu Y Z, Jin Z. ACS Appl. Mater. Interfaces, 2021, 13(37): 44174.

[10]
Nan M J, Qiao L, Liu Y Q, Zhang H M, Ma X K. J. Power Sources, 2022, 522: 230995.

[11]
Yan W, Wang C X, Tian J Q, Zhu G Y, Ma L B, Wang Y R, Chen R P, Hu Y, Wang L, Chen T, Ma J, Jin Z. Nat. Commun., 2019, 10: 2513.

[12]
Liu Y Z, Wen G H, Liang J C, Bao S S, Wei J, Wang H Z, Zhang P B, Zhu M F, Jia Q Q, Ma J, Zheng L M, Jin Z. ACS Energy Lett., 2023, 8(1): 387.

[13]
Skyllas-Kazacos M, Rychcik M, Robins R G, Fane A G, Green M A. J. Electrochem. Soc., 1986, 133(5): 1057.

[14]
Chen W S, Chen Y A, Lee C H, Chen Y J. Materials, 2022, 15(11): 3749.

[15]
Huang Z B, Mu A L, Wu L X, Yang B, Qian Y, Wang J H. ACS Sustain. Chem. Eng., 2022, 10(24): 7786.

[16]
Schwenzer B, Zhang J L, Kim S, Li L Y, Liu J, Yang Z G. ChemSusChem, 2011, 4(10): 1388.

[17]
Hwang G J, Kim S W,In D M, Lee D Y, Ryu C H. J. Ind. Eng. Chem., 2018, 60: 360.

[18]
Shi Y, Eze C K, Xiong B Y, He W D, Zhang H, Lim T M, Ukil A, Zhao J Y. Appl. Energy, 2019, 238: 202.

[19]
Düerkop D, Widdecke H, Kunz U, Schilde C, Schmiemann A. Chem. Ingenieur Tech., 2021, 93(9): 1445.

[20]
Mara Ikhsan M, Abbas S, Do X H, Choi S Y, Azizi K, Hjuler H A, Jang J H, Ha H Y, Henkensmeier D. Chem. Eng. J., 2022, 435: 134902.

[21]
Minke C, Turek T. J. Power Sources, 2015, 286: 247.

[22]
Ye R J, Henkensmeier D, Yoon S J, Huang Z F. J. Electrochem. Energy Convers. Storage, 2018, 15(1): 010801.

[23]
Wei Z B, Zhao J Y, Xiong B Y. Appl. Energy, 2014, 135: 1.

[24]
Chieng S C, Kazacos M, Skyllas-Kazacos M. J. Power Sources, 1992, 39(1): 11.

[25]
He S S, Frank C W. J. Mater. Chem. A, 2014, 2(39): 16489.

[26]
Li Z H, Dai W J, Yu L H, Liu L, Xi J Y, Qiu X P, Chen L Q. ACS Appl. Mater. Interfaces, 2014, 6(21): 18885.

[27]
Skyllas-Kazacos M, Menictas C, Kazacos M. J. Electrochem. Soc., 1996, 143(4): L86.

[28]
Vafiadis H, Skyllas-Kazacos M. J. Membr. Sci., 2006, 279(1/2): 394.

[29]
Chen S W, Hara R, Chen K C, Zhang X, Endo N, Higa M, Okamoto K I, Wang L J. J. Mater. Chem. A, 2013, 1(28): 8178.

[30]
Teng X G, Dai J C, Bi F Y, Jiang X M, Song Y Q, Yin G P. Solid State Ion., 2015, 280: 30.

[31]
Mohammadi T, Skyllas-Kazacos M. J. Power Sources, 1995, 56(1): 91.

[32]
Lei Y, Zhang B W, Zhang Z H, Bai B F, Zhao T S. Appl. Energy, 2018, 215: 591.

[33]
Park Y, Kim D. J. Membr. Sci., 2018, 566: 1.

[34]
Jiang B, Wu L T, Yu L H, Qiu X P, Xi J Y. J. Membr. Sci., 2016, 510: 18.

[35]
Qin C C, Wang D, Liu Y M, Yang P K, Xie T, Huang L, Zou H Y, Li G W, Wu Y P. Nat. Commun., 2021, 12: 7184.

[36]
Slade S, Campbell S A, Ralph T R, Walsh F C. J. Electrochem. Soc., 2002, 149(12): A1556.

[37]
Reed D, Thomsen E, Wang W, Nie Z M, Li B, Wei X L, Koeppel B, Sprenkle V. J. Power Sources, 2015, 285: 425.

[38]
Teng X G, Dai J C, Su J, Zhu Y M, Liu H P, Song Z G. J. Power Sources, 2013, 240: 131.

[39]
Jeong S, Kim L H, Kwon Y, Kim S. Korean J. Chem. Eng., 2014, 31(11): 2081.

[40]
Oh K, Moazzam M, Gwak G, Ju H. Electrochim. Acta, 2019, 297: 101.

[41]
Kim D K, Yoon S J, Kim S. Int. J. Heat Mass Transf., 2020, 148: 119040.

[42]
Balwani A, Faraone A, Davis E M. Macromolecules, 2019, 52(5): 2120.

[43]
Teng X G, Zhao Y T, Xi J Y, Wu Z H, Qiu X P, Chen L Q. Acta Chim. Sinica, 2009, 67(6): 471.

[44]
Xi J Y, Wu Z H, Qiu X P, Chen L Q. J. Power Sources, 2007, 166(2): 531.

[45]
Teng X G, Zhao Y T, Xi J Y, Wu Z H, Qiu X P, Chen L Q. J. Power Sources, 2009, 189(2): 1240.

[46]
Drillkens J, Schulte D, Sauer D U.Batteries and Energy Technology (General) - 217th ECS Meeting, 2010, 167.

[47]
Huang S L, Yu H F, Lin Y S. J. Chem., 2017, 2017.

[48]
Teng X G, Lei J, Gu X C, Dai J C, Zhu Y M, Li F Q. Ionics, 2012, 18(5): 513.

[49]
Kondratenko M S, Karpushkin E A, Gvozdik N A, Gallyamov M O, Stevenson K J, Sergeyev V G. J. Power Sources, 2017, 340: 32.

[50]
Lin C H, Yang M C, Wei H J. J. Power Sources, 2015, 282: 562.

[51]
Domhoff A, Martin T B, Silva M S, Saberi M, Creager S, Davis E M. Macromolecules, 2021, 54(1): 440.

[52]
Trogadas P, Pinot E, Fuller T F. Electrochem. Solid-State Lett., 2012, 15(1): A5.

[53]
Domhoff A, Wang X T, Silva M S, Creager S, Martin T B, Davis E M. Soft Matter, 2022, 18(17): 3342.

[54]
Jansto A, Davis E M. ACS Appl. Mater. Interfaces, 2018, 10(42): 36385.

[55]
Yang S H, Yang D S, Yoon S J, So S, Hong S K, Yu D M, Hong Y T. Energy Fuels, 2020, 34(6): 7631.

[56]
Yang X B, Zhao L, Goh K, Sui X L, Meng L H, Wang Z B. J. Energy Chem., 2020, 41: 177.

[57]
Sun C Y, Zlotorowicz A, Nawn G, Negro E, Bertasi F, Pagot G, Vezzù K, Pace G, Guarnieri M, Di Noto V. Solid State Ion., 2018, 319: 110.

[58]
Sun C Y, Negro E, Nale A, Pagot G, Vezzù K, Zawodzinski T A, Meda L, Gambaro C, Di Noto V. Electrochim. Acta, 2021, 378: 138133.

[59]
Teng X G, Zhao Y T, Xi J Y, Wu Z H, Qiu X P, Chen L Q. J. Membr. Sci., 2009, 341(1/2): 149.

[60]
Wang N F, Peng S, Lu D, Liu S Q, Liu Y N, Huang K L. J. Solid State Electrochem., 2012, 16(4): 1577.

[61]
Ye J Y, Zhao X L, Ma Y L, Su J, Xiang C J, Zhao K Q, Ding M, Jia C K, Sun L D. Adv. Energy Mater., 2020, 10(22): 1904041.

[62]
Aziz M A, Shanmugam S. J. Power Sources, 2017, 337: 36.

[63]
Aziz M A, Han D B, Shanmugam S. ACS Sustain. Chem. Eng., 2021, 9(33): 11041.

[64]
Hossain S I, Aziz M A, Shanmugam S. ACS Sustain. Chem. Eng., 2020, 8(4): 1998.

[65]
Park S C, Lee T H, Moon G H, Kim B S, Roh J M, Cho Y H, Kim H W, Jang J, Park H B, Kang Y S. ACS Appl. Energy Mater., 2019, 2(7): 4590.

[66]
Bukola S, Li Z D, Zack J, Antunes C, Korzeniewski C, Teeter G, Blackburn J, Pivovar B. J. Energy Chem., 2021, 59: 419.

[67]
Wu C X, Lu S F, Zhang J, Xiang Y. Phys. Chem. Chem. Phys., 2018, 20(11): 7694.

[68]
Lee K J, Chu Y H. Vacuum, 2014, 107: 269.

[69]
Yu L H, Lin F, Xu L, Xi J Y. RSC Adv., 2016, 6(5): 3756.

[70]
Kim B G, Han T H, Cho C G. J. Nanosci. Nanotechnol., 2014, 14(12): 9073.

[71]
Cui Y, Hu Y, Wang Y C, Wang Y, Peng J, Li J Q, Zhai M L. Radiat. Phys. Chem., 2022, 195: 110081.

[72]
Yang X B, Zhao L, Goh K, Sui X L, Meng L H, Wang Z B. ChemistrySelect, 2019, 4(15): 4633.

[73]
Zhang D Z, Xin L, Xia Y S, Dai L H, Qu K, Huang K, Fan Y Q, Xu Z. J. Membr. Sci., 2021, 624: 119047.

[74]
Choi H J, Youn C, Kim S C, Jeong D, Lim S N, Chang D R, Bae J W, Park J. Micropor. Mesopor. Mater., 2022, 341: 112054.

[75]
Schwenzer B, Kim S, Vijayakumar M, Yang Z G, Liu J. J. Membr. Sci., 2011, 372(1/2): 11.

[76]
Jung M, Lee W, Nambi Krishnan N, Kim S, Gupta G, Komsiyska L, Harms C, Kwon Y, Henkensmeier D. Appl. Surf. Sci., 2018, 450: 301.

[77]
Ahn S M, Jeong H Y, Jang J K, Lee J Y, So S, Kim Y J, Hong Y T, Kim T H. RSC Adv., 2018, 8(45): 25304.

[78]
Zhao Y Y, Zhang D H, Zhao L N, Wang S L, Liu J G, Yan C W. Electrochim. Acta, 2021, 394: 139144.

[79]
Kim H G, Kim R, Kim S, Choi C, Kim B, Guim H, Kim H T. J. Ind. Eng. Chem., 2018, 60: 401.

[80]
Yang X B, Zhao L, Sui X L, Meng L H, Wang Z B. J. Colloid Interface Sci., 2019, 542: 177.

[81]
Palanisamy G, Sadhasivam T, Park W S, Bae S T, Roh S H, Jung H Y. ACS Sustain. Chem. Eng., 2020, 8(4): 2040.

[82]
Ye J Y, Yuan D, Ding M, Long Y, Long T, Sun L D, Jia C K. J. Power Sources, 2021, 482: 229023.

[83]
Mai Z S, Zhang H M, Li X F, Xiao S H, Zhang H Z. J. Power Sources, 2011, 196(13): 5737.

[84]
Yang X Q, Zhu H J, Jiang F J, Zhou X J. J. Power Sources, 2020, 473: 228586.

[85]
Lin H L, Leon Yu T, Huang L N, Chen L C, Shen K S, Jung G B. J. Power Sources, 2005, 150: 11.

[86]
Zhang F X, Zhang H M, Ren J X, Qu C. J. Mater. Chem., 2010, 20(37): 8139.

[87]
Wei W P, Zhang H M, Li X F, Mai Z S, Zhang H Z. J. Power Sources, 2012, 208: 421.

[88]
Li J C, Liu J, Xu W J, Long J, Huang W H, He Z, Liu S Q, Zhang Y P. Membranes, 2021, 11(12): 946.

[89]
Su L, Zhang D S, Peng S S, Wu X M, Luo Y L, He G H. Int. J. Hydrog. Energy, 2017, 42(34): 21806.

[90]
Zhang D S, Wang Q, Peng S S, Yan X M, Wu X M, He G H. J. Membr. Sci., 2019, 587.

[91]
Cecchetti M, Allen Ebaugh T, Yu H R, Bonville L, Gambaro C, Meda L, Maric R, Casalegno A, Zago M. J. Electrochem. Soc., 2020, 167(13): 130535.

[92]
Su J, Ye J Y, Qin Z Y, Sun L D. Coatings, 2022, 12(3): 378.

[93]
Zeng J, Jiang C P, Wang Y H, Chen J W, Zhu S F, Zhao B J, Wang R L. Electrochem. Commun., 2008, 10(3): 372.

[94]
Decher G, Hong J D, Schmitt J. Thin Solid Films, 1992, 210/211: 831.

[95]
Yoo H Y, Heo A, Cho C G. J. Nanosci. Nanotechnol., 2016, 16(10): 10515.

[96]
Xi J Y, Wu Z H, Teng X G, Zhao Y T, Chen L Q, Qiu X P. J. Mater. Chem., 2008, 18(11): 1232.

[97]
Lu S F, Wu C X, Liang D W, Tan Q L, Xiang Y. RSC Adv., 2014, 4(47): 24831.

[98]
grosse Austing J, Nunes Kirchner C, Komsiyska L, Wittstock G. J. Membr. Sci., 2016, 510: 259.

[99]
Zhang L S, Ling L, Xiao M, Han D M, Wang S J, Meng Y Z. J. Power Sources, 2017, 352: 111.

[100]
Sha’rani S S, Abouzari-Lotf E, Nasef M M, Ahmad A, Ting T M, Ali R R. J. Power Sources, 2019, 413: 182.

[101]
Nibel O, Schmidt T J, Gubler L. J. Electrochem. Soc., 2016, 163(13): A2563.

[102]
Yang M C, Lin C H, Kuo J T, Wei H J. J. Electroanal. Chem., 2017, 807: 88.

[103]
Peng K J, Wang K H, Hsu K Y, Liu Y L. ACS Macro Lett., 2015, 4(2): 197.

[104]
Peng K J, Lai J Y, Liu Y L. RSC Adv., 2017, 7(59): 37255.

[105]
Dai J C, Dong Y C, Yu C, Liu Y X, Teng X G. J. Membr. Sci., 2018, 554: 324.

[106]
Dai J C, Ding T L, Dong Y C, Teng X G. Ionics, 2021, 27(5): 2127.

[107]
An H L, Zhang R, Li W H, Li P, Qian H D, Yang H. ACS Appl. Mater. Interfaces, 2022, 14(6): 7845.

[108]
Luo Q, Zhang H, Chen J, You D, Sun C, Zhang Y. J. Membr. Sci., 2008, 325(2): 553.

[109]
Jia C K, Liu J G, Yan C W. J. Power Sources, 2012, 203: 190.

[110]
Yu L H, Lin F, Xu L, Xi J Y. RSC Adv., 2017, 7(50): 31164.

[111]
Kim S, Yuk S, Kim H G, Choi C, Kim R, Lee J Y, Hong Y T, Kim H T. J. Mater. Chem. A, 2017, 5(33): 17279.

[112]
Liu J M, Yu L W, Cai X K, Khan U, Cai Z Y, Xi J Y, Liu B L, Kang F Y. ACS Nano, 2019, 13(2): 2094.

Options
Outlines

/

〈 〉