Research progress on optimized membranes for vanadium redox flow batteries

Abstract

Energy storage systems are considered one of the key components for the large-scale utilization of renewable energy, which usually has an intermittent nature for production. In this case, vanadium redox flow batteries (VRFBs) have emerged as one of the most promising electrochemical energy storage systems for large-scale application, attracting significant attention in recent years. To achieve a high efficiency in VRFBs, the polymer electrolyte membrane between the positive and negative electrodes is expected to effectively transfer protons for internal circuits, and also prevent cross-over of the catholyte and anolyte. However, the high cost of membrane materials is currently a crucial factor restricting the large-scale application of VRFBs. In this review, key aspects related to the polymer electrolyte membranes in VRFBs are summarized, including their functional requirements, characterization methods, transport mechanisms, and classification. According to its classification, the latest research progress on the polymer electrolyte membrane in VRFBs is discussed in each section. Finally, the research directions and development of next-generation membrane materials for VRFBs are proposed, aiming to present a future perspective of this component in full batteries and inspire the ongoing efforts for building high-efficiency VRFBs in the power grid.

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1. Introduction

With the increasing energy consumption, it is imperative to develop efficient methods for the production of energy to avoid the depletion of fossil fuels and reduce greenhouse gas emission. In this case, renewable energy sources such as solar and wind energy are abundant and have much lower greenhouse gas emissions and high sustainability compared to fossil fuels. However, their inherent intermittent nature and volatility affect their large-scale integration into the power grid.^1–3 Among the currently developed large-scale energy storage systems, electrochemical energy storage systems have attracted significant attention owing to their advantages such as short construction period and flexible usage scenarios.^4,5 The most widely used lithium-ion batteries (LIBs) have an excellent specific energy and small volume, making them suitable for portable applications; however, their thermal runaway issue poses serious safety risks particularly for large-scale applications.^6,7 Therefore, redox flow batteries have received considerable attention because of their high reliability, fast response time, long lifespan and performance decoupling of capacity and power.^8–10

As shown in Fig. 1a, a typical redox flow battery system is mainly composed of a core stack, two external storage tanks for storing electrolytes (catholyte and anolyte), pumps for transporting the electrolytes, and liquid flow pipelines. The core stack consists of several repeating units including five main components, i.e., electrodes, membrane, bipolar plates, current collectors, and end plates, as shown in Fig. 1b. In this stack, the polymer electrolyte membrane is one of the critical components because this membrane separates the catholyte from the anolyte during the operation of the flow battery and functions to conduct charge carriers. Moreover, another function of the membrane is to minimize the cross-over of the active materials and prevent short circuits.^9,11,12 Furthermore, the polymer electrolyte membrane should have excellent mechanical and chemical stability to ensure the long-term durability of the battery.¹³ Finally, it must be affordable for the economic feasibility and market penetration potential of redox flow battery systems.^11,14

Fig. 1 Schematic diagram of (a) a vanadium redox flow battery (VRFB) system and (b) single cell.

Among the various flow battery chemistries, the vanadium redox flow battery (VRFB) designed by Skyllas-Kazacos et al. presents effective suppression of cross-over of the positive and negative electrolytes because it employs different valence vanadium elements in the positive and negative cells to realize redox reactions.¹⁵ Nowadays, VRFB has received continuous and extensive research interest and has been configured in the power grid for output regulation.^16–18 However, VRFB face the same challenge with the polymer electrolyte membrane as other redox flow batteries, including iron-based,^19,20 zinc-based,^21,22 organic-based^23,24 and other novel flow batteries.^25–27 Specifically, only perfluorosulfonic acid proton exchange membranes (Nafion) exhibit excellent stability and proton conductivity at room temperature, but they are expensive and exhibit poor ion selectivity and mechanical properties simultaneously.^8,28–30 Thus, in recent years, tremendous effort has been devoted to the continuous development of polymer electrolyte membranes due to their selectivity, conductivity, stability and reduced cost.

2. Evaluation criteria of membrane

The properties of polymer electrolyte membranes are strongly associated with the performance of full batteries, and thus systematic characterization using standardized evaluation criteria is essential for the comparison of various membranes. As shown in Fig. 2, the evaluation criteria of membranes mainly include water absorption, swelling ratio, ion exchange capacity, ionic selectivity, proton conductivity, mechanical properties, chemical stability, and cost. High-performance vanadium flow batteries with promising development prospects require membranes that exhibit high ionic conductivity, low cross-over of active substances, low solvent absorption, good mechanical and chemical stability and economic feasibility for large-scale applications. In this section, we summarize the key indicators and testing methods of the membrane. In addition, the utilization of polymer electrolyte membranes is discussed, and the electrochemical performance of the full battery is also discussed.

Fig. 2 Schematic of membrane classification, structure, and evaluation criteria of membranes for vanadium redox flow batteries.

2.1 Water uptake and swelling ratio

Hydrophilic polymer electrolyte membranes usually have a high swelling ratio, which greatly reduces their ionic selectivity. To ensure the reliability of membranes, it is necessary to evaluate their water uptake and swelling ratio. The water uptake rate (WU) of a membrane is defined as the rate of weight change of the wet membrane (immersed in electrolyte) relative to the dry membrane, while the swelling ratio (SR) is defined as the rate of change in length from the dry membrane to the wet membrane. WU and SR can be calculated using the following equations:^31,32where W_wet and W_dry are the weight of the wet membrane and dry membrane, respectively. L_wet and L_dry are the length of wet membrane and dry membrane, respectively.

2.2 Ion exchange capacity

During the electrochemical charge–discharge process, charge carriers need to easily pass through the membrane to reduce the Ohmic loss, enabling excellent voltage efficiency and overall battery performance.³³ However, excessive ion exchange capacity (IEC) may aggravate the unwanted crossover of the active materials through the membrane.¹² Therefore, it is necessary to adjust the IEC value to achieve a compromise between the ionic conductivity of the membrane and crossover of the active materials.

The ion exchange capacity of polymer electrolyte membranes can be calculated by back titration.^34–36 In the case of CEM, it is usually soaked in a certain concentration of hydrochloric acid solution to protonate its sulfonic acid groups. Then, it is thoroughly rinsed and soaked in deionized water to remove excess hydrochloric acid. For the titration process, the dried membrane is soaked in an NaCl solution to replace the protons in the sulfonic acid groups with sodium ions. Next, phenolphthalein is used as an indicator to titrate the exchanged H⁺ and the value of IEC is calculated using eqn (3), as follows:³⁶

where C_NaOH is the concentration of NaOH solution and V_NaOH is the volume of NaOH solution consumed in titration. W_d is the weight of the protonated dry membrane.

2.3 Ionic selectivity

During the operation of a flow battery, the osmotic pressure and electric field drive the migration of the active materials through ion-selective membranes, leading to self-discharge and capacity degradation in the battery.^37,38 In asymmetric flow battery systems, such as organic,³⁹ alkaline zinc iron,⁴⁰ and polysulfide^41,42 based redox flow batteries, high ion selectivity is particularly important given that it determines the lifetime of the battery system. In addition, low ion selectivity may also cause membrane fouling during the long-term operation. Taking the Nafion membrane as an example, vanadium ions with low diffusivity (such as VO₂⁺ and V³⁺) deposit on the surfaces of the membrane, while vanadium ions with high diffusivity (such as VO²⁺ and V²⁺) enter the channels of the membranes and accumulate inside, both of which reduce the ionic conductivity of the membrane and increase the resistance of the battery.³³

The ionic permeability of vanadium ions is usually measured using an H-shaped permeation cell separated by a membrane, as shown in Fig. 3.⁴³ Both sides of the tank are filled with an equal volume of VOSO₄ and MgSO₄ solutions, respectively. The MgSO₄ solution is sampled at regular intervals and the concentration of VO²⁺ is measured using a UV visible spectrophotometer. The VO²⁺ permeability (P) can be calculated using eqn (4), as follows:^44,45

where V is the volume of MgSO₄ solution, A is the effective area of the membrane, L is the thickness of the membrane, C₀ is the initial concentration of VOSO₄, and C_t is the concentration of VO²⁺ in the MgSO₄ sampling solution at time t.

Fig. 3 Schematic diagram of H-type osmotic cell for measuring ion permeability.

There are critical steps in the procedure for the measurement of ionic permeability. (i) The concentrations of the two solutes and the supporting electrolyte must be consistent to maintain similar initial osmotic pressures. (ii) Magnetic stirring should be applied during testing to prevent concentration polarization on the surface of the membrane.⁴⁶ (iii) The device must be well sealed to ensure the accuracy of the test results. (iv) The tested solution needs to be refilled to its initial position after each test and it is important to maintain a constant volume of liquid on both sides.

2.4 Proton conductivity

During the cycling of a redox flow battery, the energy barriers at the interface between the electrolyte and polymer membrane, as well as inside the membrane increases the Ohmic polarization and energy loss. As the key objectives of redox flow batteries, high power density and high voltage efficiency require a low internal resistance, which is dependent on the proton conductivity of the polymer membrane.⁴⁷

Electrochemical impedance spectroscopy (EIS) is the common method employed to evaluate the proton conductivity (σ) of membranes.^48,49 The use of the same cell device and electrolyte composition as that in the conventional redox flow battery provides a more realistic evaluation.^50,51 The resistance of the cell device is tested with and without a membrane and the proton conductivity (σ) of the membrane can be calculated using eqn (5), as follows:⁵²

where L is the thickness of the membrane, A is the effective area of the membrane, and R₁ and R₂ are the resistance with and without the membrane, respectively.

2.5 Mechanical properties

During the long-term operation of a redox flow battery, the membrane is required to have excellent mechanical strength to resist changes in the battery pressure, which is especially important for solid mixed redox flow batteries. For instance, the membrane of a zinc-based redox flow battery needs sufficient mechanical strength to reduce the risk of penetration by zinc dendrites and further short circuit of the battery.^53,54

The characterization of the mechanical properties is mainly carried out using a tensile testing machine. In the case of Nafion membranes, it is essential to test both wet and dry samples.^55,56 The tensile strength can be calculated using eqn (6), as follows:⁵⁷

where T refers to the tensile strength of the membrane, F_m is the maximum tensile stress, W is the width of the sample, and L is the thickness of the membrane.

2.6 Chemical stability

Redox flow battery systems generally use strong acid or alkali as the supporting electrolyte, and their internal components are also immersed in an environment of strong oxidizing or reducing active materials during their operation.^58–61 Therefore, the membrane need to possess good chemical stability to suppress its corrosion and degradation in harsh environments, ensuring the stable cycling of the battery.

The chemical stability of membranes is usually tested by ex situ methods.^62,63 For example, the membrane is soaked in a sulfuric acid solution containing VO²⁺ for 1–3 months after cutting into an equal size shape. Then, it is taken out for comprehensive inspection including macroscopic and microscopic morphological changes, chemical structure changes, and ion concentration changes in the soaking solution.⁴⁵

2.7 Cost effectiveness

In VRFB, the cost of the ion selective membrane accounts for about 20% of the total system cost,⁶⁴ while the cost of the membrane increases to about 38% in iron chromium redox flow batteries.⁶⁵ Through the cost analysis of redox flow battery systems, it is considered that the cost of the membrane and electrolyte is an important factor hindering their wide application, and thus developing cost-effective and sustainable ion-selective membranes is crucial.^19,66,67

2.8 Electrochemical battery performance

The influence of the membrane on the performance of VRFB can be evaluated using the typical electrochemical testing methods for battery systems. Usually, a constant current charge–discharge procedure is used to obtain the cycling performance and voltage profile of the battery. In a flow battery, an ideal polymer electrolyte membrane is expected to provide a high and stable charge–discharge capacity, low charge voltage, and high discharge voltage.¹²

The coulombic efficiency, voltage efficiency, and energy efficiency are the three important indicators to evaluate the battery performance.⁶⁸ The coulombic efficiency is a performance indicator to evaluate the ratio of discharge capacity to charge capacity, and a high coulombic efficiency requires a membrane with good resistance to crossover of vanadium species. The voltage efficiency is an indicator for evaluating the overpotential losses during the charge–discharge processes, and a high voltage efficiency is attributed to the excellent ion conductivity of the membrane and the low internal resistance of the battery.^69,70 Energy efficiency is obtained from the product of the coulombic efficiency and voltage efficiency, which represents the overall performance of the battery system. These indicators can be calculated using the following equations:⁷¹

where t_d and t_c denote the discharge time and a charge time of the battery, respectively. I_d and I_c denote the current during charge and discharge of the battery, respectively. U_d and U_c denote the voltage during charge and discharge of the battery, respectively.

The limitation of polarization loss becomes more remarkable during the operation of VRFB under an increasing working current density, which is unfavorable for miniaturization and cost reduction of the battery system. A rate capability test can be performed to evaluate the charge–discharge performance of the battery at different current densities and the discharge capacity, coulombic efficiency, voltage efficiency and energy efficiency are observed during the testing process.¹² Extra attention should be given to the discharge capacity and efficiencies under a high current density, which are crucial in practical applications.¹⁹

In addition, the cycling stability of a battery is also a key indicator to evaluate the membrane performance. The capacity loss caused by self-discharge during long-term operation can be recovered by remixing the electrolyte.²⁸ However, the irreversible capacity loss caused by side reactions and liquid leakage will lead to the continuous deterioration of the battery performance.⁷² During the cycling of VRFB at a fixed current density, the ratio of the discharge capacity over different cycles can be measured and the capacity retention rate can be calculated.⁷³

3. Membrane classification

There are various types of VRFB membranes, which can be divided into two categories of charged membranes and non-charged membranes according to whether ion exchange groups are present in the membranes, as seen in Fig. 2. The charged ion exchange membrane is composed of polymer chains, which have attached functional ion groups. They are formed of fixed ions and dissociated ions (counter ions) with opposite charges.¹¹ Regarding the charge of the ions dissociated by the functional groups, ion exchange membranes can be divided into cation exchange membranes (CEM), anion exchange membranes (AEM), and amphoteric ion exchange membranes (AIEM). CEMs contain functional groups with a negative charge, such as –SO₃⁻, –COO⁻, and –PO₃⁻, which dissociate H⁺. AEMs contain functional groups with positive charge, such as –NH³⁺, –NRH²⁺, and –NR₂H⁺, which can transport OH⁻.⁷⁴ AIEMs contain two functional groups distributed in the same polymer matrix. In addition, non-charged membranes are made of membrane materials with lower polarity such as polysulfone, polyethersulfone, and polyvinylidene fluoride, which have better solvent resistance and dimensional stability compared to charged membranes.⁷⁵

The mass transfer of charged membranes usually relies on the dissolution/diffusion mechanism, which is related to the charged ion exchange groups in the membrane. CEMs and AEMs allow the ions that are the same as that in membranes to pass through them and repel dissimilar charge ions. Furthermore, the ions diffuse through the membrane by the interaction of ion exchange groups and pores, and their directional transport is the result of osmotic pressure and an external electric field.⁷⁶ Non-charged membranes are usually made with a porous structure to realize the capability of transporting charge carriers, and their selective permeability mainly depends on the screening mechanism. The pore size of non-charged membranes is a prerequisite for ion transfer in the membrane. The hydrated ions can migrate from one side of the membrane to the other along the channels in the membrane when their radius is smaller than that of the pores.^77,78

4. Crossover of vanadium ions

During the operation of VRFB, proton transport within the membrane is mainly achieved through two modes, media transport and the Grotthuss mechanism.⁷⁹ Besides proton transport, transmembrane transport of vanadium ions in VRFB occurs during the charge and discharge processes. As shown in Table 1, the unwanted transmembrane transport of vanadium ions can trigger a series of self-discharge reactions,^80,81 which have a negative influence on the capacity and lifetime of VRFB. The main driving force for the transmembrane transport of vanadium ions is a concentration gradient and electric field.

Table 1 Self-discharge reactions in a vanadium redox flow battery (VRFB)

In positive electrode	In negative electrode
2VO2^+ + V^2+ + 2H^+ → 3VO^2+ + H2O	V^2+ + VO^2+ + 2H^+ → 2V^3+ + H2O
VO2^+ + V^3+ → 2VO^2+	2V^2+ + VO^2+ + 4H^+ → 3V^3+ + 2H2O
V^2+ + VO^2+ + 2H^+ → 2V^3+ + H2O	VO2^+ + V^3+ → 2VO^2+

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Mohammadi et al. conducted early research on water migration in various commercial ion exchange membranes, and found that water migration occurred from the positive electrode to the negative electrode in the VRFB equipped with AEMs, whereas that equipped with CEMs exhibited water migration in the opposite direction.⁸² In addition, the diffusion coefficients of vanadium ions in Nafion 115 follow the order of V²⁺ > VO²⁺ > VO₂⁺ > V³⁺. The transport of vanadium ions and protons is often carried out in the form of bound water and its amount is dependent on the type of ions. As shown in Fig. 4a, each V²⁺ or V³⁺ is connected to 6 bound water molecules, while VO²⁺ is linked to 5 bound water molecules. VO₂⁺ is connected to 4 bound water molecules and protons are connected to 2.5 bound water molecules.^80,81 The migration of vanadium ions is driven by the concentration gradient of vanadium ions between the positive and negative electrodes.⁸³

Fig. 4 (a) Illustration of chemical reactions, species crossover and water transport in VRFB. (b) Comparison of the coulombic efficiency, voltage efficiency and energy efficiency of VRFB. (c) Simulation and experimental results of electrolyte volume changes in the catholyte and anolyte tanks after each cycle. Reproduced from ref. 80 with permission from Elsevier, Copyright 2023.

In the case of the self-discharge reaction, water transfer is caused by the migration of bound water with vanadium ions and protons, and the permeation (approximately 75%). In the process of charge–discharge, the migration of protons to form internal circuits and the migration of bound water also play a crucial role in the water transfer process. However, the migration of vanadium ions with bound water and their permeation are the main reasons for water transfer during long-term cycling. As shown in Fig. 4b and c, the electrolyte imbalance is enhanced with an increase in the operation time, leading to the severe degradation of the capacity and output performance for VRFB.⁸⁰

Skyllas-Kazacos et al. also highlighted that different charge states will also affect the direction of water migration.⁸² VRFB equipped with CEM exhibits water migration from the negative electrode to the positive electrode in a high charge state (SOC = 50%–100%), while the opposite direction is observed in the low charge state (SOC = 0%–50%). Moreover, the water migration is more severe in the low charge state compared to that in the high charge state.

The influence of an external electric field on the transmembrane migration of vanadium ions or water also needs to be considered during the operation of a liquid flow battery. Luo et al. found that the presence of an electric field accelerates the transmembrane migration of vanadium ions from the positive electrode to the negative electrode during the charge process and inhibits this migration in the reversed direction.⁷⁶ Furthermore, the effect of an electric field on the migration of vanadium ions will be reversed during the discharge process. The migration coefficient of vanadium ions under the action of an electric field follows the order of VO²⁺ > V²⁺ > V³⁺ > VO₂⁺, and the migration effect caused by an electric field is three orders of magnitude greater than that from a concentration gradient.⁷⁶

5. Nafion-based cation exchange membranes (CEMs)

CEMs are the most developed and widely used membranes in redox flow battery systems. Anionic functional groups, such as –SO₃⁻, –COO⁻, and –PO₃⁻, are immobilized on the main polymer chain, resulting in a relatively high migration rate for protons and other cations. This section focuses on the latest developments of typical Nafion-based membranes in the large-scale application of VRFB.

5.1 Pure perfluorosulfonic acid membranes

Nafion was first developed by DuPont. The chemical structure and its cluster-network model are shown in Fig. 5a.⁸⁴ Nafion membranes are thermally and chemically stable, and they have been widely used in the chlor-alkali production industry, energy storage and conversion systems.^85,86 Their excellent chemical stability in various environments, such as acidic, oxidation, and reduction, enables their application in VRFB systems with high concentration of sulfuric acid. The utilization of Nafion membranes is beneficial for the implementation of a high acid concentration in VRFB, thereby enhancing the solubility of the vanadium active materials and the conductivity of the electrolyte.⁷⁰ In addition, the integration of a polytetrafluoroethylene (PTFE) backbone with sulfonic acid functional groups in polymer membranes can endow them with hydrophobicity and hydrophilicity (as shown in Fig. 5b), ensuring the high chemical and mechanical strength of the membrane for application in VRFB.

Fig. 5 (a) Structure of Nafion and the cluster-network model. Hydrophilic clusters are connected by short narrow channels. Short curves represent Nafion side chains and dots represent sulfonic acid groups. Reproduced from ref. 84 with permission from Elsevier, Copyright 1983. (b) Proton conduction mechanism in hydrophilic region of hydrated proton exchange membrane. Reproduced from ref. 87 with permission from John Wiley and Sons, Copyright 2023.

However, Nafion membranes exhibit very low selectivity towards cations, leading to severe crossover of vanadium ions. Also, the resulting migration of bound water aggravates the imbalance in electrolyte volume on the positive and negative sides, reducing the lifespan of the battery.^76,83 In addition, the high cost of Nafion membranes also hinders the further application of redox flow batteries. Thus, extensive effort has been devoted to addressing the above-mentioned issues and novel strategies such as the addition of inorganic and organic components have been proposed.

5.2 Nafion-based inorganic composite membranes

5.2.1 Modification with inorganic nanoparticles. As shown in Fig. 6a, utilizing the shielding effect of inorganic nanoparticles, which are embedded into a polymer matrix, can reduce the permeability of vanadium ions and improve ion selectivity of Nafion based membranes. It is necessary to consider two main factors for material selection, cost and stability. Typical selection of inorganic nanoparticles includes silicon dioxide (SiO₂), titanium dioxide (TiO₂) and tungsten trioxide (WO₃).^88–90

Fig. 6 (a) Schematic illustration of Nafion/inorganic hybrid membrane. (b) Preparation of Nafion/amino-SiO₂ hybrid membrane. (c) Effect of Nafion and Nafion/amino-SiO₂ membrane on intracellular VO²⁺ and VO₂⁺ concentration. (d) OCV of VRFB over time using Nafion and Nafion/amino-SiO₂ membrane in the charged state of 75%. Reproduced from ref. 92 with permission from Elsevier, Copyright 2015.

Xi et al. doped SiO₂ in Nafion and obtained a Nafion/SiO₂ hybrid membrane with ion exchange capacity and proton conductivity similar to Nafion117.⁹⁰ However, the incorporation of nanoparticles in the channels of Nafion membranes not only inhibits the permeation of vanadium ions, but also reduces the conductivity of protons, resulting in a significant voltage and energy loss in the battery.

Therefore, it is an advanced strategy to prepare composite membranes with functionalized nanoparticles, which are mixed with the polymer solution before the formation of the membrane. Nanoparticles with hydrophilic groups, such as hydroxyl, amino and sulfonic acid groups, can provide intense pathways for proton transport and promote the ionic conductivity.⁹¹ Lin et al. prepared Nafion/amino silica composite membranes via the sol–gel method, as shown in Fig. 6b.⁹² The results of the potentiometric titration and self-discharge tests showed that the exchange capacity of vanadium ions through the hybrid membrane is only 26.8% of that in the original Nafion membrane. Fig. 6c and d suggest that the self-discharge rate of the battery was also greatly reduced, and Teng et al. reported the similar results.^93,94

5.2.2 Modification with inorganic nanowires/nanotubes. The above-mentioned inorganic nanoparticles/Nafion composite membranes are effective to reduce vanadium ion permeation and improve ion selectivity. However, nanoparticles with a high specific surface area tend to agglomerate, leading to a decrease in the membrane stability and accelerating the penetration rate of vanadium ions.⁹⁵ Therefore, inorganic nanowires/nanotubes have been introduced to modify ion exchange membranes.

Aziz et al. prepared ZrO₂ nanotubes (ZrNT) with a diameter of less than 50 nm and incorporated them in Nafion to prepare a Nafion/ZrNT composite membrane via the solution casting method.⁹⁵ Two-dimensional inorganic nanofillers serve as barriers to the permeation of vanadium ion, delivering a significant increase in ion selectivity. Similarly, Ye et al. prepared a Nafion/TiO₂ nanotube composite membrane (Fig. 7a–c).⁷³ The super-hydrophilic TiO₂ nanotubes significantly inhibited the permeability of vanadium ions without a decrease in proton conductivity. The battery with the composite membrane maintained 55.7% discharge capacity after 1400 cycles (Fig. 7d), while the battery using commercial Nafion membrane could only deliver 20% discharge capacity after 410 cycles. Aziz et al. further mixed TiO₂ and ZrO₂ nanotubes to prepare a Nafion/TiZrO₄NT composite membrane (Fig. 7e). The Nafion/TiZrO₄NT composite membrane exhibited excellent proton conductivity (75.9 mS cm⁻¹) and impressive ion selectivity (3.61 × 10⁶ S min cm⁻³) at room temperature. This proves the superiority of inorganic co-doping in membrane materials for high-performance redox flow batteries.⁹⁶

Fig. 7 (a) Top, (b) bottom and (c) cross-sectional SEM images of TiO₂ nanotube arrays. (d) Coulombic efficiency, energy efficiency and discharge capacity retention of VRFB single cells with Nafion and Nafion/TiO₂ membrane for cycling at 120 mA cm⁻². Reproduced from ref. 73 with permission from John Wiley and Sons, Copyright 2020. (e) Schematic diagram of proton transfer and vanadium ion diffusion mechanism in the fabricated Nafion/TiZrO₄NT composite membrane. (f) VO²⁺ permeability and H⁺/VO²⁺ selectivity of the Nafion and composite membrane. Reproduced from ref. 96 with permission from the American Chemical Society, Copyright 2021.

5.2.3 Modification of inorganic nanosheets. Besides solution casting, spin coating is another common method to modify polymer membranes and the direct coating of nanoparticles on the surface of the membrane can form an even barrier layer, hindering the permeation of vanadium ions (Fig. 8a and b). Furthermore, the utilization of nanosheets with a large transverse size can significantly improve the permeation blocking efficiency (Fig. 8c). The typical nanosheet materials include graphene oxide (GO)⁵⁵ and graphite nitride carbon (g-C₃N₄).⁹⁷ Zhang et al.⁹⁸ successfully constructed a composite membrane with an enhanced interface, which consisted of cross-linked graphene oxide (CLGO), as shown in Fig. 8d. The cross-linked GO layer and angstrom-scale channels significantly reduced the vanadium ion permeability, and the composite membrane showed good mechanical and chemical stability, improving the electrochemical performance of the assembled battery.

Fig. 8 SEM images of (a) surface and (b) cross-section of graphene oxide/Nafion membrane. (c) Schematic diagram of permeation of vanadium ions in pure Nafion and graphene oxide/Nafion composite membrane. Reproduced from ref. 55 with permission from Elsevier, Copyright 2017. (d) Process for the preparation of amino-functionalized Nafion212 covered with graphene oxide nanosheets. Reproduced from ref. 98 with permission from Elsevier, Copyright 2019.

5.3 Nafion-based organic composite membranes

Based on the bonding between the organic components and Nafion polymer matrix, Nafion-based organic composite membranes can be classified as covalent-modified membranes and non-covalent-modified membranes. There are two typical configurations, the sandwich structure and blending organic components with Nafion. Between them, the sandwich structure is composed of a Nafion inner layer sandwiched by organic surface layers. The performance of this type of composite membrane is synergistically improved by the excellent chemical stability and high ion conductivity of the Nafion layer, as well as the high ion selectivity of the surface layer. Alternatively, blending composite membranes are fabricated via the direct mixing of organic materials with Nafion.

5.3.1 Covalently modified composite membranes. Covalently modified composite membranes form a functional layer through the chemical reaction between the modified components and the Nafion membrane, effectively reducing the permeation of vanadium ions and improving the ion selectivity and chemical stability of the membrane. Luo et al. introduced a polyethylene imine (PEI) layer on the surface of a Nafion membrane by interface polymerization.⁹⁹ Due to the presence of a large number of amino groups in PEI, the vanadium ion permeability of the composite membrane significantly decreased due to its positive charge repulsion effect. Similarly, Ma et al. grafted copolymerized N,N-dimethylaminoethyl methacrylate (DMAEMA) on a Nafion substrate through radiation induction and protonated it to reduce the crossover of vanadium ion (as shown in Fig. 9a).¹⁰⁰ Dai et al. used surface-initiated atom transfer radical polymerization (SI-ATRP) technology to graft zwitterionic sulfobetaine methacrylate (SBMA) on Nafion to form a composite membrane (N115-g-PSBMA).¹⁰¹ The amphoteric layer of the N115-g-PSBMA membrane had low vanadium permeability and the VRFB assembled with this membrane showed high coulombic efficiency and energy efficiency.

Fig. 9 (a) Preparation of radiation-induced grafted Nafion-g-PDMAEMA and protonated Nafion-g-PDMAEMA. Reproduced from ref. 100 with permission from Elsevier, Copyright 2013. (b) Schematic diagram of layer-by-layer self-assembly of Nafion-[CS-PWA]_n membrane. Reproduced from ref. 50 with permission from The Royal Society of Chemistry, Copyright 2014.

It is worth noting that the dense surface layer formed by covalent modification can significantly reduce the vanadium ion permeability, water absorption and water migration in the membrane, but it inevitably leads to a decrease in proton conductivity and an increase in cell resistance.

5.3.2 Non-covalent modified composite membranes. The methods for the preparation of non-covalent-modified membranes include blending, layer-by-layer self-assembly, and coating. Due to the absence of covalent bonds between the additives and the Nafion matrix, the water absorption and expansion behavior of the Nafion membrane will intensify the permeation of the vanadium ions in the composite membrane, leading to the degradation of battery performance. Therefore, it is crucial to obtain a uniform and stable composite membrane.

To suppress the water absorption and swelling of Nafion membranes, they can be blended with some hydrophobic components. There is a strong dipole–dipole interaction between the polar carbon fluorine bond of polyvinylidene fluoride (PVDF) and the carbonyl group of ester-containing polymers, which promotes the miscibility of PVDF with other polar polymers.^102,103 Mai et al. first blended PVDF with Nafion to prepare ion exchange membranes for VRFB. The addition of PVDF with high crystallinity and hydrophobicity effectively limited the swelling behavior of the Nafion membranes.²⁹ The VRFB assembled with PVDF/Nafion membrane exhibited a higher coulombic efficiency at different current densities and had a longer open circuit voltage retention time (80 h) compared to the battery with the original Nafion membrane. Similarly, Teng et al. blended hydrophobic polytetrafluoroethylene (PTFE) with Nafion to prepared a Nafion/PTFE composite membrane via the solution casting method,^104,105 which improved the crystallinity and thermal stability of the Nafion membrane and decreased the permeability of vanadium ions. However, although the above-mentioned polar polymers have high miscibility with Nafion, their hydrophobicity also reduces the ion exchange capacity and conductivity of the composite membrane.

As shown in Fig. 9b, another commonly used method for non-covalent modification is layer-by-layer self-assembly. Lu et al.⁵⁰ prepared Nafion-[CS-PWA]_n by the self-assembly of cationic chitosan (CS) and negatively charged phosphotungstic acid (PWA). The Nafion-[CS-PWA]_n membrane exhibited much lower vanadium ion permeability and the battery with this membrane showed a higher coulombic efficiency and energy efficiency, and slower self-discharge rate in comparison to that with Nafion 212. J. grosse Austing et al. also obtained an N117-(PEI/Nafion)_k composite membrane using the same method and this membrane was used in a vanadium/air redox flow battery.¹⁰⁶ However, the process of self-assembly is based on the electrostatic interaction between different layers with opposite charges. Therefore, delamination or detachment of the multi-layer composite membranes may occur during the operation of VRFB, leading to the degradation of battery performance.

In recent years, several organic materials for the modification of Nafion membranes have emerged, which are usually immiscible with the membrane substrate, thus exhibiting good dispersion and chemical stability. Yang et al. found that PWA is beneficial for proton transport, while nano Kevlar fibers (NKF) inhibited the permeation of vanadium ions.¹⁰⁷ Therefore, NKF were immobilized on PWA by hydrogen bonding, and then NKF@PWA was dispersed in a Nafion matrix to prepare a Nafion-(NKF@PWA) composite membrane via the solution casting method. Yang also coupled PWA with ammonified UiO-66 to prepare a Nafion-(UiO-66-NH₂@PWA) composite membrane.¹⁰⁸ In addition, Ye et al. utilized lignin, a natural polymer rich in hydroxyl groups with excellent dispersibility, to prepare a Nafion/lignin composite membrane through simple solution casting, and the assembled batteries could stably cycle 1000 times.¹⁰⁹ Lee et al. doped organic sulfonated copper phthalocyanine in Nafion to improve its resistance to vanadium ions.¹¹⁰

As mentioned above, in recent years, there have been many studies on the modification of Nafion membranes aimed at improving their performance. The research results are summarized in Table 2. It cannot be ignored that the high cost of Nafion greatly limits the large-scale application of flow batteries, and thus the development of CEM without Nafion has become a research hotspot in recent years.

Table 2 Preparation approaches, properties, and cell performance of modified membranes for VRFB

		Membrane properties							Mechanical properties	Cell performances
Membrane name	Preparation approach	Thickness (μm)	Proton conductivity (mS cm^−1)	Swelling ratio (%)	Water uptake (%)	Permeability of VO^2+ (×10^−7 cm^2 min^−1)	Ionic selectivity (×10^5 S min cm^−3)	IEC (mmol g^−1)	Tensile strength (MPa)	CE (%)	VE (%)	EE (%)	Current density (mA cm^−2)	Capacity retention (%)	Cycle numbers	OCV holding time (h)	Ref.
Nafion/TiO2	Sol–gel reaction	225	—	—	22.5	4.3	—	0.95	—	94.8	82.2	77.9	30	—	100	65	88
Nafion/SiO2	Sol–gel reaction	204	56.2	—	21.5	—	—	0.96	—	86.7	92.1	79.9	20	—	100	35	90
Nafion/TiO2 nanotube	Solution casting	36	—	—	—	—	—	—	26.54	98.3	83.0	84.4	120	55.7	1400	57.5	73
Nafion/TiZrO4	Solution casting	—	75.9	—	—	0.21	36.1	—	—	99.8	84.1	83.9	80	74.7	300	382	96
Nafion/GO	Spin coating	57.4	0.41	9.6	13.1	0.82	—		21	95.0	84.2	80.0	80	—	200	—	55
CLGO/Nafion212	Spin coating	—	—	11.5	17.1	3	—	0.89	18.5	96.9	86.0	83.3	100	55.0	180	—	98
Nafion-(NKFs@PWA)	Solution casting	—	61	—	—	2.46	2.48	0.952	35.2	87.0	92.0	80.2	40	82.55	100	76	107
Nafion-(UiO-66-NH2@PWA)	Solution casting	—	92	—	25.1	3.46	2.66	0.964	31.3	86.4	94.0	81.2	100	—	—	56	108
Nafion/lignin	Solution casting	∼39	15.7	—	4.5	0.85	—	—	—	97.4	84.9	82.7	120	52.8	1000	87	109
PTFE@PFSA/f-SiC	Solution casting	37	—	—	—	0.67	2.31	—	27.23	96.2	90.5	87.1	120	—	—	113.1	43
d-GO@PFSA-PTFE	Solution casting	—	14.3	—	—	2.78	0.73	—	24.4	97.0	89.6	86.9	120	—	400	—	112
SPEEK/G-AMH-3	Solution casting	—	19.7	12	30.2	9.6	0.25	1.65	34.3	∼91	—	∼88	40	84.7	100	—	115
SPEEK/(WO3)0.20	Solution casting	—	—	—	30.2	—	0.21	1.52	—	∼95	∼88	∼84	20	—	—	60	116
SPEEK/TiO2	Solution casting	82	18.3	—	—	0.18	1.02	—	—	99.3	85.4	84.8	120	86.8	200	87.5	117
SPEEK/s-TiO2	Solution casting	93	61	—	30.1	8.55	0.713	1.59	—	95.3	84.9	82.3	50	86.9	20	—	119
SPEEK/0.05 OH-BN	Phase inversion	135	26.4	17.5	36.8	—	0.53	—	—	94.2	87.2	82.12	40	—	—	14.21	120
SPEEK/FCB-3	Solution casting	62	—	9.84	18.52	—	—	1.24	—	99.5	83.9	83.5	120	44.64	1062	350	45
SPEEK/SGO-3	Solution casting	—	17.6	9.1	34.7	—	0.26	1.86	—	95.6	84.8	81.1	30	—	—	∼52	122
SPEEK/MWCNTs-OH	Solution casting	—	—	—	—	1.93	—	—	61.8	99		80	50	73	500	100	124
SPEEK/PVDF (S0.7/P0.3)	Solution casting	200	70.0	—	24.5	1.60	4.4	1.59	42.3	97.4	85.4	83.2	30	—	100	150	126
SPEEK/PTFE	Solution casting	—	—	—	—	—	—	—	39.47	93.4	89.6	83.7	80	—	700	∼28	128
SPI/TiO2	Solution casting	49	31.2	12.5	32.94	2.02	1.54	1.243	—	∼97	∼72	∼70	70	—	—	∼165	129
SPI/s-MoS2	Solution casting	66	27.5	16.18	23.2	1.23	2.24	1.698	—	∼100	∼72	∼72	70	—	500	193	132
bSPI/s-MWCNTs-1%	Solution casting	47	—	14.7	36.5	2.01	1.06	1.23	28.4	96	83	79.7	80	—	400	44	133
PVA/OMS-7	Solution casting	—	69.14	—	27.5	0.135	—	0.695	—	97.3	83.8	81.51	30	—	100	∼71	139
SPI5/PVA5	Solution casting	84	31.7	27.3	35.2	11.3	—	0.65	—	96.3	82.6	79.5	30		100	68	140
SPI-B-50	Solution casting	40	16.4	10.0	24.3	0.71	2.31	1.25	38.26	97.5	—	∼92	80	68.3	100	44	138
6F-s-bSPI	Solution casting	35	20.6	19.0	25.7	1.18	1.75	1.54	78.3	∼99	∼77	∼76	60	69.8	100	—	141
TM-OPBI-12	Solution casting	—	13.3	13.8	22.1	—	—	—	—	∼100	∼67	68.2	160	93.9	500	263	145
B-PBI	Solution casting	33	—	—	—	1.59	—	—	—	99.2	84.1	84.8	80	—	200	—	146
P-20-7	Electrostatic spinning	—	57.0	4.4	200.2	7.30	—	—	36.2	∼99	∼84	82.0	150	62.0	200	—	148
SPPEK-P90	Solution casting	53	—	15.6	23.2	—	—	1.51	71	98.2	88.8	87.2	40	—	100	290	151
SPES/S-ZrO2-10	Solution casting	55	—	9.2	36.2	∼0.3	—	1.31	—	98.9	87.8	86.8	100	—	70	63	154
HSPAEK	Solution casting	60	33.4	11.6	38.5	5.50	0.367	1.72	27.4	99.0	84.8	84.0	80	83.0	100	202	156
QPAPEKK-110	Solution casting	40	—	∼10	—	8.57	—	0.62	33	∼91	∼89	86.7	160	∼61	1000	—	159
PFDP-90	Solution casting	42	—	5.9	—	0.10	—	∼2.7	45.2	∼96	∼88	83.1	120	—	1000	∼30	160
PS-DTQG-5	Solution casting	12.5	60.6	18.0	32.5	0.32	0.36	2.14		98.1	78.0	76.4	60	—	80	120.8	161
PSf-MIm-1.2	Solution casting	—	22.6	19.8	67.5	18.12	—	1.53	10.8	97.0	84.9	82.4	120	—	4800	—	162
PBI-21%EPTMA	Solution casting	47	—	—	—	5.11	—	—	56	∼99	∼81	81.0	100	—	450	—	163
HPSf-Im-CD30%	Solution casting	61	100.0	7.5	25.5	0.92	10.82	0.76	—	∼97	∼82	80.0	120	—	50	73	168
BVSN-porous-3	Solution casting	—	77.0	13.2	30.2	0.78	9.9	2.31	—	97.3	85.0	82.7	80	—	100	300	169
PE/PP-g-PVBTMA (210%)	Phase inversion	21	39.4	38.0	93.6	2.3	—	3.6	22.6	∼93	—	∼82	80	—	100	50	170
ETFE-g-PSSA	Radiation grafting	38	24.0	—	14.7	0.39	—	0.88	—	∼95	∼78	∼72	40	—	40	300	177
C-SPEEK-50	Solution casting	—	—	—	—	—	—	1.34	—	98.0	87.0	85.3	80	—	180	—	178
SPI/0.5% PDAP	Solution casting	60	133.0	28.0	38.5	0.14	1.55	1.49	—	96.6	87.0	84.0	100	—	500	—	180
PFSPI-PAA-25	Solution casting	—	30.7	18.7	37.2	0.06	—	1.53	—	98.0	86.7	85.0	140	—	500	52.3	137
SPI/PEI-rGO-2	Solution casting	—	∼4.8	7.1	44.2	—	0.09	1.16	52.6	95.0	79.6	75.6	40	—	100	∼80	181
bSPI-8	Solution casting	60	4.4	23.2	40.2	6.27	0.7	1.40	72.94	97.0	82.5	80.0	50	—	800	180	182
TA15-SPBP	Solution casting	—	116.0	11.0	38.0	0.79	14.7	1.98	44.3	99.1	87.8	87.0	80	80.5	200	—	187
PBI-OSO3^−/OHN^+-0.47	Solution casting	—	—	—	—	0.02	58.9	0.47	—	99.2	86.6	85.9	100	69.0	100	—	184
PVDF/SiO2–SO3H-42	Phase inversion	—	14.0	0.5	52.1	1.12	1.25	—	—	90.3	83.7	75.6	60	—	30	∼32	195
M90	Phase inversion	130	—	—	—	—	—	—	—	90.3	86.8	78.4	80	—	300	—	196
TMA-5	Phase inversion	45	—	6.8	—	—	—	4.41	—	∼99	∼90	∼88.5	80	—	1500	100	201
PPY/PES	Phase inversion	—	—	—	—	0.07	—	—	—	87.3	91.9	80.2	80	—	100	—	202
PS-IPA	Phase inversion	—	—	—	—	—	—	—	—	98.5	91.8	90.4	80	—	500	—	205
PES/SPEEK-M3	Solvent-template	65	—	—	—	—	—	—	—	99.0	92.9	92.0	40	—	100	—	206
PES (10% P)	Phase inversion	—	—	—	—	0.14	—	—	—	94.5	86.4	81.7	80	—	70	—	207
P-5	Solution casting	30	—	—	—	8.24	—	—	—	95.2	92.2	87.8	80	—	200	—	208

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6. Other cation exchange membranes

6.1 Polytetrafluoroethylene (PTFE)-based CEM

The perfluorocarbon structure of PTFE ensures the high chemical and mechanical strength of membranes, but PTFE membranes have low proton conductivity due to the lack of ion exchange groups. Therefore, previous studies aimed to introduce components with high proton conductivity in the PTFE matrix to improve its proton conductivity.

Teng et al. used a solution impregnation method, in which a PTFE membrane was first impregnated with Nafion to prepare a PTFE/Nafion (P/N) composite membrane, and then the P/N membrane was further impregnated in tetraethoxysilane (TEOS) solution to fabricate the PTFE/Nafion/SiO₂ (P/N/S) composite membrane.^56,111 Ye et al. proposed a solution casting method by first dispersing functionalized silicon carbide nanowires (f-SiC) or acid-etched graphene oxide (d-GO) nanosheets in a perfluorosulfonic acid (PFSA) matrix, and then coating them on both sides of an ultra-thin PTFE layer to prepare a composite membrane.^43,112 As shown in Fig. 10a and b, the uniformly dispersed d-GO nanosheets or f-SiC nanowires provide more pathways for proton transport, and thus enhance the proton conductivity of the membrane (14.3 mS cm⁻¹ for d-GO@PFSA-PTFE composite membrane). The PTFE layer as the core layer can effectively suppress the expansion rate and improve the stability of the membrane. As shown in Fig. 10c and d, the battery assembled with the PTFE@PFSA/f-SiC membrane had a lower self-discharge rate (with an open circuit voltage holding time of 113.1 h), endowing the battery with a higher efficiency and cycle stability in 1000 cycles. In addition, Teng et al. used a layer-by-layer self-assembly method to prepared ultra-thin (∼30 μm) polycation poly(diallyl dimethylammonium chloride) (PDDA)/polyanion poly(sodium styrene sulfonate) (PSS) (named as P/S) films on PTFE.¹¹³ The membrane showed good ion selectivity and the VRFB with the P/S-(PDDA/PSS)₆ membrane exhibited improved cycling stability.

Fig. 10 Schematic illustration of the permeation of vanadium ions and proton transportation. (a) VRFB assembled with d-GO@PFSA-PTFE. Reproduced from ref. 112 with permission from Elsevier, Copyright 2022. (b) VRFB assembled with PTFE@PFSA/f-SiC. (c) Coulombic efficiency, voltage efficiency and energy efficiency and (d) open circuit voltage attenuation curves of VRFB assembled with Nafion 212 and PTFE@PFSA/f-SiC membranes. Reproduced from ref. 43 with permission from Elsevier, Copyright 2021.

6.2 Sulfonated polyetheretherketone (SPEEK)-based CEM

SPEEK is a polymer obtained by sulfonating polyetheretherketone (PEEK), and thus SPEEK membranes usually exhibit sufficient proton conductivity. In addition, they are easy to prepare and cost-effective, and present lower permeability to vanadium ions. However, SPEEK exhibits high swelling, poor mechanical stability and high vanadium ion crossover when the polymer matrix is highly sulfonated.^114,115

Previous reports have shown that the addition of inorganic nanoparticles, such as WO₃,¹¹⁶ and TiO₂,^117,118 is a feasible way to limit the swelling of SPEEK and reduce the permeability of vanadium ions due to the formation of hydrogen bonds. Wang et al. demonstrated that doping of hydrophilic TiO₂ nanoparticles in a SPEEK membrane could improve its proton conductivity (14.3 mS cm⁻¹).¹¹⁹ Myures et al. also prepared a hydroxy boron nitride (OH-BN)-reinforced SPEEK composite membrane using a simple phase conversion technique.¹²⁰

Carbon materials are also good candidates for the modification of SPEEK by doping. Luo et al. used functionalized carbon black (FCB) particles as fillers to obtain advanced SPEEK/FCB composite membranes.⁴⁵ As shown in Fig. 11a–c, the SPEEK/FCB-3 composite membrane exhibited excellent ionic selectivity (∼1.2 × 10⁶ S min cm⁻³), low vanadium permeability and high proton conductivity compared to the original SPEEK and commercial Nafion 212 membrane. Moreover, Dai et al. prepared SPEEK/GO composite membranes using the conventional solution casting method.¹²¹ As shown in Fig. 11d, the randomly embedded GO nanosheets in the SPEEK/GO membrane could act as an effective barrier to block the migration of vanadium ions, and significantly reduce their permeability. Zheng et al. used the same method to prepare a SPEEK/SGO composite membrane with sulfonated graphene oxide (SGO) nanosheets as a filler.¹²² The layered structure of SGO and Donnan repulsion effect successfully suppressed the permeability of vanadium ions and exhibited high proton conductivity. Jia et al. and Ding et al. contributed to the uniform distribution of functionalized carbon nanotubes in the SPEEK matrix.^123,124 However, despite the significant advances, the durability of these composite membranes with carbon materials has not been fully examined in VRFB systems.

Fig. 11 (a) Schematic of SPEEK/FCB composite membrane in a VRFB cell. Measurement of (b) concentration as a function of time and (c) permeability and ion selectivity of vanadium ions with different membranes. Reproduced from ref. 45 with permission from Elsevier, Copyright 2021. (d) Mechanism of vanadium ion migration in S/GO composite membrane. Reproduced from ref. 121 with permission from The Royal Society of Chemistry, Copyright 2014. (e) Cycling performance of VRFB assembled using various membranes. Reproduced from ref. 125 with permission from the American Chemical Society, Copyright 2016.

To improve the chemical stability and tensile strength of SPEEK, a polymer coating layer has been commonly used to reinforce the membrane. Zhao et al. used PVDF with high mechanical properties and good chemical stability to modify the surface layer of SPEEK.¹²⁶ The ion selectivity of the SPEEK/PVDF composite membrane was significantly improved and the VRFB with this membrane showed an enhanced performance. Xi et al. obtained a SPEEK/PDA composite membrane via the self-polymerization of a polydopamine (PDA) layer on the surface of SPEEK.¹²⁷ The PDA film acted as a barrier layer to protect the SPEEK membrane from corrosion by the high-oxidizing vanadium electrolyte. Recent research also utilized the reinforcing effect of PTFE to inhibit the water absorption and swelling ratio of SPEEK. In addition, the chemical and mechanical stability of PTFE can greatly improve the performance of the membrane. Wei et al. prepared PTFE-reinforced SPEEK composite membranes (SPEEK/PTFE)¹²⁸ and Yu et al. also demonstrated a PTFE/SPEEK/PTFE sandwich structure.¹²⁵ The VRFB based on this membrane exhibited an ultra-long lifespan (620 cycles), as shown in Fig. 11e.

In summary, most of the results based on SPEEK are listed in Table 2. However, due to its poor cycling stability and limited energy efficiency, especially at high operating current densities, the wider application of SPEEK based membranes is still hindered.

6.3 Sulfonated polyimide (SPI)-based CEM

SPI, as a low-cost fluorine-free aromatic polymer, has been utilized in VRFB because of its good mechanical and chemical properties.

The introduction of inorganic additives can improve the performance of SPI membranes and the main additives include TiO₂,¹²⁹ ZrO₂,¹³⁰ AlOOH,¹³¹ s-MoS₂ ¹³² and s-MWCNTs.¹³³ Among them, Li et al. prepared a composite membrane of SPI and sulfonated molybdenum disulfide (s-MoS₂) using a blending method.¹³² Due to the enhanced hydrogen bonding between SPI and s-MoS₂, as well as the barrier effect of s-MoS₂ sheets, which were uniformly distributed along the cross-section of the membrane (Fig. 12a and b), the SPI/s-MoS₂ membrane was found to enable high proton conductivity (27.5 mS cm⁻¹), lower vanadium ion permeability (1.21 × 10⁻⁷ cm² min⁻¹) and good stability (500 cycles). Also, the self-discharge time of the cell was longer than that with a Nafion membrane (Fig. 12c).

Fig. 12 (a) Proton transport mechanism and (b) surface morphology of SPI/s-MoS₂ composite membrane. (c) Open circuit voltage of VRFB with SPI/s-MoS₂, SPI/MoS₂ and Nafion 117 membranes. Reproduced from ref. 132 with permission from Elsevier, Copyright 2015. (d) Synthetic pathway of SPI-B membranes. (e) Performances of VRFB with SPI-B-50 and NR212 membranes during 100 cycles. (f) Performance of VRFB with SPI-B-50 membrane during 500 cycles. Reproduced from ref. 138 with permission from Elsevier, Copyright 2023.

In addition, the modification of SPI with polymers has been achieved by the immersion and self-assembly method. CS/SPI composite membranes have been proved to be effective in blocking vanadium ion permeation.^134–136 The cross-linking structure of CS and the blockage of micropores by CS in CS/SPI composite membranes significantly inhibit the crossing of vanadium ions. Li et al. synthesized a sulfonated polyimide polymer (PFSPI) with polyurea structure, and further modified PFSPI with polyacrylic acid (PAA) to prepare a novel PFSPI-PAA-X composite membrane.¹³⁷ This membrane exhibited low area resistance and low vanadium permeability. The assembled VRFB showed a lower self-discharge rate (with an open circuit voltage holding time of 52.3 h), demonstrating excellent durability in 500 cycles.

To enhance the proton conductivity of SPI, hydrophilic polyvinyl alcohol (PVA) can be blended with SPI to form a microstructure with highly dispersed phase separation, which keeps the hydrophobic main chain away from water molecules, thereby improving the chemical stability and proton conductivity of SPI.^139,140 It is worth noting that using the branching structure of SPI to prepare composite membranes can also improve the limitation of poor microphase separation in pure SPI. Li et al. developed a branched SPI-B membrane¹³⁸ and 6F-s-bSPI membrane¹⁴¹via the high-temperature condensation method. As shown in Fig. 12d, the branched sulfonated polyimide (SPI-B) membrane, consisting of 2-methyl-1,4-bis(4-amino-2-trifluoromethyl) benzene (FAPOB) as a functional diamine monomer, exhibited an excellent vanadium ion blocking performance and proton conductivity. The SPI-B membrane with a sulfonation degree of 50% (SPI-B-50) exhibited a higher coulombic efficiency, voltage efficiency and energy efficiency, and longer lifespan in VRFB compared to that with Nafion (Fig. 12e and f).

In summary, although modified SPI membranes result in an improved battery performance compared to the original SPI membrane, as shown in Table 2, their proton conductivity remains at a relatively low level throughout the ion exchange membrane, thus further efforts should be devoted to further optimizing the structural regulation/recombination.

6.4 Polybenzimidazole (PBI)-based CEM

PBI has a highly stable polymer framework and low cost, making it a promising membrane material for application in acidic and oxidizing conditions. The results show that the pore size of PBI films is about 0.5–2.0 nm, which is helpful to reduce the crossover of vanadium ions through the membrane.^142,143 However, the proton conductivity of pure PBI membranes is very low (∼10⁻¹² S cm⁻¹),¹⁴⁴ and acid-doping and protonating N atoms in PBI are needed to improve the proton mobility.

The dense skeletal structure of PBI and the strong hydrogen bonding between the main chains are insensitive to the general method of acid doping such as soaking pretreatment. Therefore, adding side chains and copolymerization modification are usually adopted to endow PBI with a larger free volume for higher degree of acid doping. For example, Ding et al. synthesized a PBI membrane with thiophene ring side group (TM-OPBI)¹⁴⁵ and the presence of thiophene side groups enlarged the free volume of the membranes. The alkaline heterocycle could improve the proton conductivity of the membrane, and the positively charged thiophene side could enhance the vanadium resistance performance through the Donnan effect. In addition, the VRFB assembled by the TM-OPBI membrane showed a capacity retention rate of 93.85% after 500 cycles at the current density of 160 mA cm⁻². In addition, Chen et al. introduced pyridine side chains and constructed a dual-channel PBI composite membrane with enhanced proton conductivity and vanadium ion selectivity.¹⁴⁶

In addition, the thickness of PBI membranes has a great influence on their degree of acid doping degree and performance.¹⁴⁷ For instance, the voltage efficiency of the battery will increase with a thinner membrane thickness due to the lower ohmic resistance, and a battery with a thick PBI membrane will require a higher charge potential. This extra charge potential promotes the occurrence of side reactions and the permeation of vanadium ions through the membrane.¹⁴⁷ Although thinner PBI membranes can result in improved battery properties by acid doping, their mechanical stability and durability strength will be insufficient. Wan et al. prepared a double-layer PBI composite membrane (P-20-7),¹⁴⁸ which consisted of a dense and thin PBI underlayer (7 μm) as well as a porous and thick PBI surface layer, as shown in Fig. 13a and b. The dense layer served as a selective layer to ensure high ion selectivity, while the porous layer served as a support layer to guarantee mechanical stability. The energy efficiency of the cell with the P-20-7 composite membrane was nearly 20% higher than that with the traditional PBI film (Fig. 13c and d) and the cell also showed stable operation for 200 cycles with a capacity retention rate of 99.81%.

Fig. 13 (a) Illustrative scheme of PBI composite membrane supported by electrospun nanofibers. (b) SEM images of the cross-section morphology of P-20-7. (c) Energy efficiency of flow batteries assembled with PBI-based membranes and Nafion 211. (d) Specific discharge capacity of flow batteries assembled with P-20-7 membrane and Nafion 211 at a current density of 80 mA cm⁻². Reproduced from ref. 148 with permission from Elsevier, Copyright 2021.

Besides the above-mentioned materials, polymers including sulfonated poly(phthalazinone ether ketone) (SPPEK),^149–151 sulfonated poly(ether sulfone) (SPES)^152–155 and sulfonated poly(fluorenyl ether ketone sulfone) (SPAEK)¹⁵⁶ have been explored to prepare CEMs for VRFB. However, there is no modified CEM that can completely replace or compete with commercial Nafion membranes. In the future, it is essential to develop more suitable membranes and simplify the production methods for achieving enhanced battery performances and low cost.

7. Anion exchange membranes (AEMs)

AEMs are another type of polymer electrolyte membrane, in which cationic groups are immobilized on the polymer backbone, and thus they fundamentally prevent the crossover of vanadium species through the Donnan effect, exhibiting excellent ion selectivity. However, the stability of the functional cationic groups in AEMs is lower compared to that in CEMs, and then the degradation of the membrane leads to a shortened lifespan of VRFB. Furthermore, batteries with AEMs may have a low voltage efficiency due to the unfavorable effect on proton conduction in membrane. There has been a significant amount of research on improving the performance of AEMs, and some representative research results will be discussed in detail next.

7.1 Modification with inorganic additives

The early attempt of AEM modification involved adding inorganic additives of different densities to pure AEM. Li et al. prepared a composite AEM based on quaternary ammonium poly(aryl ether sulfone)/nano-zirconia (QPAES/ZrO₂) via the solution casting method, and applied it in a fuel flow cell.¹⁵⁷ The introduction of nano-ZrO₂ inhibited the crystallization of the QPAES matrix, significantly improving the IEC of the composite membrane. Moreover, Leung et al. modified a commercial AEM (Fumatech GmbH) via the sol gel method and prepared an SiO₂-doped composite AEM.¹⁵⁸ The uniformly dispersed SiO₂ nanoparticles formed a physical barrier, which could effectively block the crossing of vanadium ions. The VRFB assembled with the composite membrane exhibited a coulombic efficiency of 92% and energy efficiency of 73% at a current density of 40 mA cm⁻². It is worth noting that further study is needed to enhance the long-term chemical stability and aging performance of AEMs modified by inorganic additives.

7.2 Design of molecular structure

Most AEMs contain quaternary ammonium or other amine functional groups, which can be replaced by other groups to improve the membrane performance through molecular structure design. Zhang et al. prepared quaternized poly(aryl ether ketone ketone) (PAPEKK) membranes containing phenyl and adamantane groups, and the membranes were further soaked in phosphoric acid at different temperatures (40 °C, 90 °C and 110 °C) to induce selective swelling in the membranes with ion channels (QPAPEKK).¹⁵⁹ The area resistance of the QPAPEKK-110 membrane was only 0.24 Ω cm², while the VRFB assembled with QPAPEKK-90 showed an impressive energy efficiency of 91.4% and 80.0% at current densities of 80 mA cm⁻² and 220 mA cm⁻², respectively.

Ban et al. designed and synthesized a novel fluorinated poly(aryl piperidinium) (PFDP) anion exchange membrane.¹⁶⁰ As shown in Fig. 14a, the ether-free polymer skeleton and fluorinated structure of the PEDP membrane could synergistically improve the stability of the membrane. Meanwhile, the microphase separation channels were rich in quaternary ammonium groups, allowing the formation of continuous interconnected water clusters in the membrane, which achieved effective ion conduction, while effectively repelling vanadium ions (Fig. 14b). As shown in Fig. 14c–e, the prepared PFDP membrane exhibited low vanadium permeability (<1.4 × 10⁻⁸ cm² min⁻¹) and swelling ratio (<8%), high tensile strength (60 MPa) and chemical stability (>28 days in the antioxidant test). Due to these advantages, the cell assembled with PFDP-90 exhibited excellent energy efficiency (84.3%) at 120 mA cm⁻² (Fig. 14f) and stable cycling for over 1000 cycles.

Fig. 14 (a) Comparison of the chemical structure of fluorinated and nonfluorinated poly(aryl piperidine) membranes. (b) Schematic diagram of interconnected hydrophilic channels with microphase separation. (c) Electrolyte uptake and linear swelling ratio of PFDP and PIDP membranes in 3 M H₂SO₄ solution at 30 °C. (d) Changes in VO²⁺ concentration on the permeation side of magnesium sulfate solution with time. (e) Comparison of area resistance and vanadium permeability of AEMs with different molecular designs and Nafion membranes. (f) Comparison of EE at different current densities between PFDP-90 and the reported unmodified polymer ion exchange membranes applied to VFBs. Reproduced from ref. 159 with permission from The Royal Society of Chemistry, Copyright 2023.

7.3 Design of microstructure

Besides the design of molecular structure, the microstructure of AEMs can be tailored by introducing other side groups into the main chain of the polymer. Shukla et al. connected quaternized GO (QG), which is grafted with a long alkyl chain (C16), with the polyethersulfone (PS) polymer backbone by 1,4-diazabicyclic[2,2,2]octane (DABCO) coupling, and obtained the PS-DTQG membrane with high performance.¹⁶¹ The large number of amine groups in the PS-DTQG membrane improved the ionic conductivity, while its comb-shaped structure resulted in good hydrophilic/hydrophobic phase separation, which is beneficial for proton transport (60.6 mS cm⁻¹). Xing et al. transplanted imidazole groups in the main chain of polysulfone polymer (PSF) through an effective CuAAC click reaction, and the resulting membrane showed a very low area resistance.¹⁶² The VRFB assembled with this membrane has a coulombic efficiency of 97.0% and an energy efficiency of 82.4%, and a long-term durability (4800 cycles) at a current density of 120 mA cm⁻².

Similarly, the composite AEMs prepared with PBI^163–165 and poly(arylene ether sulfone) (PAES)¹⁶⁶ as the main chain of the polymer also showed great application potential. For example, Ren et al. synthesized a series of quaternary ammonium-grafted PBI membranes (PBI-EPTMA) using 2,3-epoxypropyl trimethyl ammonium chloride (EPTMA-Cl) as the grafting agent.¹⁶³ Grafting EPTMA group increased the hydrophilicity and free volume of the membrane, which led to an increase in the amount of sulfuric acid doping and decrease in the area resistance. The VRFB assembled with PBI-21% EPTMA exhibited a higher energy efficiency (87.6%–75.3%) at current densities varying from 50–150 mA cm⁻² compared to that with Nafion 115 (81.7%–61.2%), and good cycling stability for 450 cycles.

7.4 Control of preparation process

The microstructure of AEMs is also strongly related to their preparation method, and thus effective regulation of their preparation process is crucial for improving their performance.¹⁶⁷ For example, Ma et al. dispersed β-cyclodextrin (β-CD) into a hydroxyl modified polysulfone precursor (HPSf-Im) using the template method, and then removed the template to obtain AEMs with a fine pore structure.¹⁶⁸ The hydrogen bonding between β-CD and the polymer matrix was beneficial to reduce the agglomeration of β-CD, and then promote the formation of a pore structure in the membrane. Owing to the synergistic effect of nanopores and imidazole cation, the prepared AEM exhibited a low IEC, high conductivity, and low vanadium ion permeability. The VRFB using this membrane showed a coulombic efficiency of 99% and energy efficiency of 80% at a current density of 120 mA cm⁻².

Moreover, Ma et al. prepared a nano-porous AEM via the hydrolysis crosslinking method.¹⁶⁹ The pores in the membrane provided an effective pathway for proton transport due to the Donnan repulsion effect, while quaternary ammonium functionalized crosslinking further inhibited membrane expansion and vanadium ion penetration. Therefore, the prepared membrane combined the advantages of ionic and porous membranes. The VRFB assembled with this membrane showed stable cycling for 100 times at a current density of 80 mA cm⁻², with a coulombic efficiency of 97.3% and energy efficiency of 82.7%.

Abdiani et al. prepared a PE/PP-g-PVBC membrane via the radiation graft copolymerization of polyvinyl chloride benzyl (PVBC) and polyethylene/polypropylene (PE/PP) microfibers, and further converted it into a dense membrane with quaternary amine groups.¹⁷⁰ This membrane exhibited high ionic conductivity, low vanadium permeability and good chemical and mechanical stability. The VRFB assembled with the PE/PP-g-PVBC membrane exhibited a much lower vanadium ion permeability and higher energy efficiency than that with a Nafion membrane at a current density of 80 mA cm⁻².

Although numerous studies have made efforts to improve the ion conductivity and stability of AEMs, the adverse effects of proton conduction lead to a low voltage efficiency in VRFB. In addition, further molecular structure and microstructure design are needed, and preparation methods need to be further optimized to improve their comprehensive performance.

8. Amphoteric ion exchange membranes (AIEMs)

CEMs have excellent chemical stability and superior ionic conductivity, but typically have high vanadium ion permeability. On the contrary, AEMs can significantly inhibit the crossing of vanadium ions (Donnan effect), but their low ion conductivity leads to greater capacity loss and low voltage efficiency. To obtain both high ionic conductivity and low vanadium ion crossing in the membrane, Sollner first proposed a membrane containing both cationic and anionic groups in 1932, and named it an amphoteric ion exchange membrane (AIEM).¹⁷¹

The selectivity of vanadium ions can be significantly improved by introducing positively charged groups or doping inorganic materials in CEMs such as Nafion membranes, which are attractive routes to prepare AIEMs. However, the application of Nafion-based membranes is always limited by their high cost. The preparation of Nafion-based membranes was discussed in previous sections. Here, the development of partially fluorinated or non-fluorinated membranes as AIEM will be review and the typical polymer materials are PVDF,^172–177 SPEEK,^178,179 SPI^180–182 and other materials.^69,183–190

8.1 Poly(vinylidene fluoride) (PVDF)-based AIEM

The introduction of anionic functional groups in PVDF by the irradiation grafting process has been widely reported. As shown in Fig. 15a and b, Qiu et al. developed AIEM via one-step grafting (AIEM-I) and two-step modification (AIEM-II).^174,177 AIEM-II avoided the degradation of the quaternary ammonium groups caused by chlorosulfonic acid, showing high IEC, conductivity, and low vanadium ion permeability. Specifically, styrene (St) was first grafted to a poly(ethylene-co-tetrafluoroethylene) (ETFE) membrane, followed by sulfonation to obtain ETFE-g-PSSA, and then grafted with dimethylaminoethyl methacrylate (DMAEMA) and protonated to obtain AIEM-II. As shown in Fig. 15c, the open circuit voltage of VRFB assembled with AIEM-II could be maintained above 1.3 V after 300 cycles, and its coulombic efficiency and energy efficiency were higher than that with the Nafion membrane. As shown in Fig. 15d, Ma et al. also prepared an AIEM based on the radiation grafting of PVDF powder and solution phase inversion method.¹⁷⁶ The grafting ratio for powder irradiation grafting was higher than that for direct grafting on the membrane, and therefore the IEC of the AIEM is increased and its vanadium ion permeation was remarkably suppressed. However, the mechanical stability of AIEM will decrease if the grafting ratio is too high. In addition, the radiation source will be destructive to the polymer structure, which is unfavorable to the mechanical and chemical stability of AIEMs.

Fig. 15 (a) Preparation of AIEM-I via one-step modification. Reproduced from ref. 174 with permission from Elsevier, Copyright 2009. (b) Preparation of AIEM-II by two-step modification. (c) Open circuit voltage of VRFB with AIEM-II and Nafion 117 membranes. Reproduced from ref. 177 with permission from Elsevier, Copyright 2009. (d) Schematic diagram of AIEM prepared by radiation grafting method and solution phase inversion method. Reproduced from ref. 176 with permission from Elsevier, Copyright 2012.

8.2 Sulfonated poly(ether ether ketone) (SPEEK) based AIEM

Chen and Li dissolved sulfonated poly(etheretherketone) (C-SPEEK) in dimethylacetamide, and then poured the solution on a glass plate and dried it at 70 °C for 12 h to obtain a film. The film was peeled and soaked in 0.5 M sulfuric acid for 24 h to fabricate a SPEEK membrane with suspended carboxylic acid groups.¹⁷⁸ The introduction of carboxylic acid groups could effectively improve the IEC of the C-SPEEK membrane. Compared with Nafion 115, the VRFB assembled with the C-SPEEK-50 membrane showed a higher energy efficiency (85% vs. 82%) and coulombic efficiency (97.3% vs. 94.6%). Also, it still had good stability after more than 180 cycles. Jung et al. also introduced a small amount of urethane acrylate non-ionomer (UAN) in a blend of SPEEK and PVDF and prepared a SPEEK/PVDF/UAN membrane via the solution casting method.¹⁷⁹ The blended membrane combined the advantages of high proton conductivity of SPEEK and low vanadium permeability of PVDF. Therefore, the VRFB assembled with the three-phase blended membrane showed a significant improvement in discharge capacity and energy efficiency compared to that with the SPEEK/PVDF membrane or Nafion 212 membrane.

8.3 Sulfonated polyimide (SPI)-based AIEMs

Zhang et al. prepared a blended membrane of SPI and poly[bis(4,4′diaminobenzidine-2,2′-disulfonic acid) phosphazene] (PDAP) via the solution casting method.¹⁸⁰ PDAP contains amphoteric functional groups, which provide more groups (–SO₃H) for proton transport, and the protonated N atoms can block the permeation of vanadium ions by the Donnan repulsion effect. Therefore, the SPI/PDAP membranes exhibited high proton conductivity (up to 1.33 × 10⁻¹ S cm⁻¹ at room temperature) and low vanadium permeability (1.37 × 10⁻⁸ cm² s⁻¹). As shown in Fig. 16a–c, the coulombic efficiency and energy efficiency of the VRFB assembled using the SPI/PDAP membrane were higher than that with Nafion 212. Moreover, the battery could be continuously and stably cycled for 500 times at the current density of 100 mA cm⁻². Li et al. developed a new covalently cross-linked polyfluorinated sulfonated polyimide membrane (PFSPI-PAA-X) using hydrophilic PAA as a crosslinking agent.¹³⁷ As shown in Fig. 16d, the polymer structure containing C–F bond exhibited excellent chemical stability in the VRFB, and the hydrogen bond network formed by F element and H₂O could improve the proton conductivity of the membrane. In addition, the cross-linking of PAA and PFSPI promoted the proton transport and effectively limited the penetration of vanadium ions due to the hydrophilic/hydrophobic phase separation structure. As shown in Fig. 16e, the membrane exhibited very low vanadium permeability, which is two orders of magnitude lower than that of the Nafion 212 membrane. The VRFB assembled with the PFSPI-PAA-25 composite membrane maintained a high efficiency, peak power density, and excellent durability during 500 cycles (Fig. 16f–h). Similar studies on SPI-based AIEMs include a new acid–base hybrid membrane (SPI/PEI-rGO) synthesized by Cao et al.¹⁸¹ and branched sulfonated polyimide (bSPI) membrane reported by Zhang et al.¹⁸²

Fig. 16 (a) Coulombic efficiency and (b) energy efficiency of VRFB with different SPI/PDAP and Nafion 212 membranes. (c) Cycling performances of VRFB with SPI/0.5% PDAP membrane at a current density of 100 mA cm⁻². Reproduced from ref. 180 with permission from Elsevier, Copyright 2021. (d) ¹H NMR spectrum of PFSPI-PAA-25 membrane. (e) Concentration of VO²⁺ across PFSPI, PFSPI-PAA-X and Nafion 212 membranes versus time. (f) Coulombic efficiency and energy efficiency of VRFB with different membranes. (g) Polarization curves of VRFB. (h) Efficiencies of VRFB assembled with PFSPI-PAA-25 membrane. Reproduced from ref. 137 with permission from The Royal Society of Chemistry, Copyright 2021.

8.4 Other AIEMs

Xu et al. prepared sulfonated polybiphenyl tethered with pendant tertiary amine groups (TA-SPBP) via the solution casting method.¹⁸⁷ The high stability of the ether-free skeleton and acid–base interaction (sulfonic acid group and tertiary amine group) greatly improved the structural stability of the membrane, and the Donnan repulsion effect significantly reduced the permeability of vanadium ions. Compared with the Nafion 212 membrane, the TA15-SPBP membrane showed higher proton conductivity and excellent ion selectivity. The VRFB assembled with the TA15-SPBP membrane showed stable cycling of 1188 times at a current density of 80 mA cm⁻², whereas the capacity retention rate of the battery with Nafion 212 was only 11.3% after 200 cycles. Similarly, Pang et al. proposed the strategy of synthesizing the side chain of hydroxy bromopropane anion precursor followed by quaternization, forming a polybenzimidazole ionic conductive film with sulfate/quaternary ammonium zwitterion pair (PBI-OSO₃⁻/OHN⁺).¹⁸⁴ The assembled VRFB exhibited an excellent performance with an coulombic efficiency of 99.2%, energy efficiency of 85.9% and capacity decay rate of 0.31% per cycle. Liao et al. prepared a novel fluoro-methyl sulfonated poly(aryl ether ketone) AIEM (60SPAEK-6F-co-10% BI).⁶⁹ The positively charged benzimidazole group effectively inhibited the crossing of vanadium ions, and the antioxidant C–F bond enhanced the chemical and mechanical stability of the membrane. The VRFB assembled with this AIEM exhibited a higher efficiency than that with Nafion 117 and maintained stable operation for 220 cycles.

Although many AIEMs have been modified by partially fluorinated/non-fluorinated polymers, finding a good balance between high ionic conductivity and dimensional stability is still an important direction for future work. In addition, the durability of AIEMs is also a key factor to be considered because the electrolyte in VRFB has strong acidity and oxidizability. Therefore, the strategies of developing polymer skeletons and new functional groups, as well as optimization of the microphase structure, are promising pathways to obtain high performance AIEMs with strong stability and low cost.

9. Porous membrane

To overcome the disadvantages of high cost and complicated preparation process of the above-mentioned polymer electrolyte membranes, nanofiltration (NF) membranes with a porous structure, which is known as reverse osmosis (RO), were proposed in 1984.¹⁹¹ The principle of nanofiltration membranes is to prevent the passage of vanadium ions (hydrated) with a larger radius but allow protons with a smaller radius to pass through pore sieving.⁷⁷ In 2011, Zhang et al. first proposed and designed a polyacrylonitrile (PAN) nanofiltration membrane with an adjustable distribution of pore size for VRFB and then porous membranes for VRFB have attracted considerable attention.¹⁹²

9.1 Design criteria for porous membranes

The migration mechanism of vanadium ions can be divided into three types, namely diffusion, convection, and migration. Zhou et al. investigated the transport phenomenon of vanadium ions in porous membranes, and found that the main mechanism for vanadium ion crossover is convection.¹⁹³ Reducing the pore size (<15 nm) of membranes can effectively reduce the driving force of convection, but the driving force of migration and diffusion can only be effectively reduced when the pore size is down to the size of vanadium ion (<2 nm). Therefore, reducing the pore size of porous membranes is a feasible method to reduce their permeability. However, the voltage efficiency of the battery will also decrease with a smaller pore size due to the increased ohmic resistance of the film. Thus, to balance the vanadium resistance and ohmic resistance of the membrane, the appropriate pore size of the membrane should be regulated. In addition, the influence of membrane thickness, electrolyte flow rate and other factors should also be considered.

9.2 Methods for the preparation of porous membranes

The methods for preparing porous membranes can be classified into two categories, pore-forming agent and phase inversion. The drying process of the casting solution or the separation process of the pore-forming agents from the matrix can form pores naturally to obtain porous membranes. In the phase inversion method, the solvent and non-solvent diffuse and exchange with each other in a coagulation bath containing polymer, and spontaneously form liquid–liquid phase separation. The polymer-rich phase coagulates to fabricate the membrane body, while the polymer-poor phase forms pores.

The polymer materials for porous membranes include PAN, PVDF, PBI and polyether sulfone (PES) because their ion selectivity is inferior to that of ion exchange membranes, which usually have high vanadium ion permeability. To achieve effective blocking of vanadium species without the loss of too much proton conductivity, previous reports usually adopted organic/inorganic nano-materials filling, grafting and copolymerization, introduction of functional layer, and polymer blending to modify porous membranes.

9.2.1 Organic/inorganic nano-materials filling. A porous polymer filled with organic/inorganic nanomaterials can reduce the pore size and improve the ion selectivity. Zhang et al. filled a PAN matrix with SiO₂ nanoparticles by the in situ sol–gel process of TEOS to reduce the pore size of the film.¹⁹⁴ This method can effectively improve the ionic selectivity of the membrane and maintain good ionic conductivity. Ling et al. also used the sol–gel method to introduce sulfonated silica in the porous PVDF matrix.¹⁹⁵ Pore-filled sulfonated silica can provide a proton transfer carrier and inhibit the permeation of vanadium ions in the membrane.

9.2.2 Grafting and copolymerization. Li et al. prepared a hydrophilic porous PSF membrane by grafting sodium p-styrene sulfonate (NaSS) on a PSF phase inversion membrane, which was carried out via ultraviolet-induced polymerization of vinyl monomer.¹⁹⁶ The grafting degree of the PSF films could be adjusted by controlling the ultraviolet wavelength, photo-initiator concentration and irradiation time. As shown in Fig. 17a, Cao et al. also introduced a hydrophilic polyvinylpyrrolidone (PVP) layer on the surface of a PVDF porous membrane through graft polymerization and crosslinking reaction, which improved the overall hydrophilicity of the PVDF membrane and adjusted the pore size distribution.⁶² The VRFB assembled with the modified membranes could deliver a coulombic efficiency of 94.4% and energy efficiency of 83.3% at a current density of 80 mA cm⁻², which are comparable to that with commercial Nafion 115.

Fig. 17 (a) Hydrophilic ion transport networks and chemical reaction between PVP and PVDF by K₂S₂O₈ radical initiation. Reproduced from ref. 62 with permission from Elsevier, Copyright 2015. (b) Design principle of zeolite flake porous membrane for VRFB. Reproduced from ref. 197 with permission from Wiley, Copyright 2016. (c) Designed membranes with symmetric spongy structures in VRFB. Reproduced from ref. 198 with permission from The Royal Society of Chemistry, Copyright 2013. (d) Solvent-induced reorganization of polymer chains in porous PES/SPEEK blended membrane. Reproduced from ref. 77 with permission from Wiley, Copyright 2016.

9.2.3 Introducing functional layers/charged groups. Introducing functional layers on the membrane surface is also an effective strategy. Yuan et al. used porous PES prepared by the phase inversion method as the carrier, and sprayed zeolite on the top layer to fabricate a composite membrane.¹⁹⁷ As shown in Fig. 17b, the pore size of the zeolite was about 0.5 nm, which is between hydrated vanadium ions (>0.6 nm) and protons (<0.24 nm), and thus the functional top layer perfectly separated vanadium ions from protons. At a current density of 200 mA cm⁻², the assembled VRFB exhibited a coulombic efficiency of over 99% and energy efficiency, which is the highest value reported thus far. Chen et al. transferred multilayer graphene directly to the surface of a PES porous membrane, which significantly reduced the vanadium crossover in the membrane and increased the proton selectivity.¹⁹⁹ Teng et al. also introduced a polypyrrole (PPY) top layer on a porous PES substrate via interfacial polymerization, and the composite membrane had lower vanadium ion permeability and area resistance than that with commercial Nafion 115.²⁰⁰

To reduce the permeability of vanadium ions, the introduction of charged groups in the polymer membrane substrate is another potential strategy. As shown in Fig. 17c, Zhang et al. grafted pyridine groups on chloromethylated polysulfone (CPSF), and formed a positively charged sponge-like structure, which could repel the migration of vanadium ions.¹⁹⁸ In addition, weak bases can further promote protons to pass through the pore wall by acid–base interaction. The cross-linking of CPSF with hydrophilic trimethylamine can form a microphase separation structure, which is beneficial for proton conduction. The composite membrane also reduced the cross-crossing of vanadium ions by the Donnan effect and pore size repulsion and the rigid cross-linking network ensured the chemical stability of the membrane.²⁰¹ Similarly, the introduction of positively charged imidazole groups on the pore wall of spongy PES or in situ polymerization of PPY to obtain a PPY/PES composite film²⁰² could improve the ion selectivity of membranes.

9.2.4 Inducing the formation of phase separation structures. The process for the formation of a phase separation structure in polymer membranes is difficult to control, and in this case, polymer–solvent interactions provide a method for inducing polymer chain recombination,^203,204 particularly for the formation of unique phase separation structures in hydrophobic/hydrophilic polymer blends. Lu et al. introduced isopropanol (IPA) in a PES/SPEEK blend porous membrane to induce the recombination of sulfonated groups, which formed ion transport channels by the interaction between IPA and functional groups in SPEEK.⁷⁷ The highly stable hydrophobic porous PES matrix and interconnected pores with hydrophobicity formed a fine phase-separated structure (Fig. 17d), and the assembled VRFB showed a coulombic efficiency of over 99% and energy efficiency of over 91% at a current density of 80 mA cm⁻². The control of the pore size and phase separation structure in PES/SPEEK porous composite membranes via the selection of different solvents has also been reported in the literatures.^205–207 In addition, Shi et al. used hydrophilic polyethylene glycol (PEG 400) to induce the aggregation of SPEEK, resulting in phase separation from the hydrophobic PVDF polymer during the formation of the PVDF/SPEEK porous membrane.²⁰⁸

Porous membranes are generally fabricated using polymers with high chemical stability and improved ion selectivity by reducing the transmission channels or introducing charged groups/phase separation structures. Therefore, VRFBs with porous membranes present high coulombic efficiency at low self-discharge rates, but the lower proton conductivity of membranes will cause greater membrane resistance. Therefore, it is necessary to improve the performance of porous membranes to achieve high-performance VRFB.

10. Conclusion and outlook

Flow batteries are considered as the most promising candidates among the numerous large-scale energy storage technologies because of their high reliability, fast response time, long life-time and decoupling of capacity and power. The membrane is one of the key components in flow batteries and its function is to conduct charge carriers and to separate the catholyte from the anolyte in space. Currently, the widely used membrane in flow batteries is perfluorosulfonic acid proton exchange membranes (Nafion) with excellent chemical stability and ionic conductivity. However, their high cost, poor ionic selectivity and low mechanical stability limit their application in VRFB. Tremendous effort has been devoted to the development of polymer electrolyte membranes with enhanced ionic conductivity, long-term durability, and low cost. This review comprehensively summarized the recent research achievements in new membranes in VRFB systems, including CEM, AEM and AIEM, which are charged membranes, and porous membranes.

(1) CEMs are well-developed and widely used membranes in redox flow batteries, and their main challenge is the capacity degradation of the battery systems caused by high vanadium ion permeability. Therefore, previous reports developed multi-scale inorganic material composite membranes and covalent/non-covalent modified organic composite membranes. To reduce the cost of the membrane, new types of CEMs including PTFE, SPEEK, SPI, and PBI have been studied as substitutes for Nafion membranes. These membranes without Nafion exhibit improved ionic selectivity and lower cost, but their poor conductivity and long-term stability need to be improved.

(2) AEMs with cationic groups significantly reduce the permeability of vanadium ions through the membrane but limit the proton conductivity simultaneously, resulting in a lower voltage efficiency. Researchers have regulated the microstructure of membranes by adding inorganic additives, acid treatment and organic modification methods. However, the low redox stability of AEMs still hinders their wide application in VRFB.

(3) To obtain both high ionic conductivity and low vanadium ion crossover, the development of AIEMs has become a major research direction in recent years. However, the methods for the preparation of AIEMs are usually complicated and the distribution of two ion exchange groups in the membrane matrix has limited accuracy, which also hinder their potential application.

(4) In recent years, non-ionic porous membranes based on pore size screening have attracted intense attention in the field of VRFB. To achieve effective blocking effect of vanadium ions without compromising the proton conductivity, it is essential to maintain a hydrophilic pore structure with adjustable pore size. Preparation methods such as filling with organic/inorganic nanomaterials, grafting and copolymerization modification, introducing functional surface layers, and polymer blending have been explored to modify porous membranes. It is worth emphasizing that high-stability and low-cost porous membranes have not been fully achieved for VRFB.

The high cost of membranes has become a bottleneck in the future application of VRFB. Specifically, Nafion membranes are expensive ($600–800 m⁻²), whereas several modified non-fluorinated membranes (such as PSF and SPEEK) effectively reduce the membrane cost ($40 m⁻²). However, due to the presence of charged functional groups, their chemical stability is generally poor. In contrast, porous membranes greatly reduce the cost of VRFB. Thus, a further cost reduction is the direction that the entire industry needs to focus on.

From the perspective of the structure–activity relationship of modified Nafion membranes, the functionalization of Nafion membranes has become an important strategy. However, Nafion membranes with high vanadium ion permeability are generally used as cation exchange membranes. Alternatively, by doping fillers, their ion transport channels are reduced, and positively charged groups are introduced to build a vanadium barrier, which effectively hinders the cross penetration of vanadium ions. However, the reduction in the number of sulfonic acid groups during the modification process still limits the energy efficiency of VRFB. Therefore, it is still a huge challenge to coordinate the balance between vanadium ion permeability and proton conductivity and reasonably design and prepare advanced modified Nafion membranes with excellent electrochemical properties.

From the perspective of materials science, the performance of membranes is closely related to their microstructure. In many novel ion exchange membranes, the hydrophilic groups responsible for conducting protons and the hydrophobic backbones are arranged arbitrarily, which makes the proton transfer channel unable to effectively transport protons, and even directly leads to dead ends inside the membrane, completely blocking the proton transport channels. In addition to optimizing the composition and polymerization degree of membranes, researchers should also reasonably design their proton channels, constructing more microstructures that are conducive to proton transport but effectively block vanadium ion transport, thus breaking through the constraints of membrane materials, providing support for the wide application of high-performance membranes.

The method for the preparation of membranes also has a significant impact on production costs and future sustainable development. As seen in Table 2, most membranes were prepared via the solution casting method, which often requires N,N-dimethylformamide and N,N-dimethylacetamide as solvents, leading to potential hazards. In addition, the membranes formed by the polymer blending casting method are mostly composed of petroleum derivatives. Therefore, the development of biopolymers and environmentally friendly solvents is a future trend.

In summary, balancing the selection of material composition, preparation methods, and macro and microstructures is expected to further promote the development of VRFB membranes towards high chemical stability, high ion conductivity, low vanadium permeability, and low cost. Simultaneously, it is also of great significance for accelerating the commercial application of VRFB and the utilization of clean and renewable energy.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by National Natural Science Foundation of China (U21B2057) and Chalmers Areas of Advance Materials Science and Energy and Batteries Sweden (BASE).

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