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Review Article

# Redox Flow Batteries for Energy Storage: A Technology ReviewPUBLIC ACCESS

[+] Author and Article Information
Ruijie Ye, Zhifeng Huang, Sangwon Kim

Transfercenter Sustainable Electrochemistry,
Saarland University,
Saarbrücken 66125, Germany;
Bio Sensor and Materials Group,
KIST Europe,
Campus E7 1,
Saarbrücken 66123, Germany

Dirk Henkensmeier

Fuel Cell Research Center,
Korea Institute of Science and Technology,
Seoul 02792, South Korea;
ET-GT,
University of Science and Technology,
Seoul 02792, South Korea;
Green School,
Korea University,
Seoul 136-713, South Korea

Sang Jun Yoon

Transfercenter Sustainable Electrochemistry,
Saarland University,
Saarbrücken 66125, Germany;
Bio Sensor and Materials Group,
KIST Europe,
Campus E7 1,
Saarbrücken 66123, Germany;
Center for Membranes,
Korea Research Institute of Chemical Technology,
Daejeon 34114, South Korea

Dong Kyu Kim

Transfercenter Sustainable Electrochemistry,
Saarland University,
Saarbrücken 66125, Germany;
Bio Sensor and Materials Group,
KIST Europe,
Campus E7 1,
Saarbrücken 66123, Germany;
Department of Mechanical and
Aerospace Engineering,
Seoul National University,
Seoul 08826, South Korea

Zhenjun Chang

Transfercenter Sustainable Electrochemistry,
Saarland University,
Saarbrücken 66125, Germany;
Bio Sensor and Materials Group,
KIST Europe,
Campus E7 1,
Saarbrücken 66123, Germany;
College of Materials Science and Engineering,
Jiangsu University of Science and Technology,
Zhenjiang 212003, China

Ruiyong Chen

Transfercenter Sustainable Electrochemistry,
Saarland University,
Saarbrücken 66125, Germany;
Bio Sensor and Materials Group,
KIST Europe,
Campus E7 1,
Saarbrücken 66123, Germany
e-mail: r.chen@kist-europe.de

1Corresponding author.

Manuscript received May 15, 2017; final manuscript received July 5, 2017; published online September 19, 2017. Assoc. Editor: Kevin Huang.

J. Electrochem. En. Conv. Stor. 15(1), 010801 (Sep 19, 2017) (21 pages) Paper No: JEECS-17-1050; doi: 10.1115/1.4037248 History: Received May 15, 2017; Revised July 05, 2017

## Abstract

The utilization of intermittent renewable energy sources needs low-cost, reliable energy storage systems in the future. Among various electrochemical energy storage systems, redox flow batteries (RFBs) are promising with merits of independent energy storage and power generation capability, localization flexibility, high efficiency, low scaling-up cost, and excellent long charge/discharge cycle life. RFBs typically use metal ions as reacting species. The most exploited types are all-vanadium RFBs (VRFBs). Here, we discuss the core components for the VRFBs, including the development and application of different types of membranes, electrode materials, and stack system. In addition, we introduce the recent progress in the discovery of novel electrolytes, such as redox-active organic compounds, polymers, and organic/inorganic suspensions. Versatile structures, tunable properties, and abundant resources of organic-based electrolytes make them suitable for cost-effective stationary applications. With the active species in solid form, suspension electrolytes are expected to provide enhanced volumetric energy densities.

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## Introduction

Electrochemical energy storage is currently attracting significant interest, considering the expanding market for portable electronics, electric vehicles, smart grid, and off-grid energy storage. Among various energy storage systems, redox flow batteries (RFBs) are promising for stationary large-scale applications in terms of cost, reliability, and safety [1]. RFBs consist of cathode and anode chambers, membranes, and flowable electrolytes. Energy is converted in electrochemical reactions and stored by keeping the active species of the electrolytes in external tanks. The energy density of RFBs is determined by the volume of the electrolytes, concentration of active species, the cell voltage, and the number of stacks. The power generation capability is related to the kinetic behavior of redox-active species and the size of the electrodes. Such characters make the RFBs flexible energy storage systems.

Various metal-based redox couples have been investigated over the past few decades [2]. Among them, all-vanadium RFBs (VFRBs) are the most investigated. Sulfuric acid is typically used as supporting electrolyte. The maximal energy density of VRFBs is only about 30 Wh L−1. This is limited by the solubility and stability of the four involved vanadium ions (VO2+, VO2+, V3+, and V2+) and can be slightly enhanced by using mixed sulfate–chloride electrolytes [3]. Membranes with low vanadium permeability and thus high Coulombic efficiency (CE) and high energy efficiency (EE) are required. Crossover of vanadium ions through the membranes will reduce the cell efficiency and require remixing of electrolytes. Low-cost and high-performance membranes are of great importance. Carbon-based electrodes are generally used to catalyze the VO2+/VO2+ and V3+/V2+ redox reactions. Rational processing and modification of the carbon electrode can increase the surface area and the number of active functional groups on the surface [4,5]. Catalyst particles on the carbon electrode surface can further promote the kinetics and enhance the voltage efficiency (VE) [6]. To enhance the energy storage capability of RFBs, it is necessary to find alternative energy-dense electrolytes (high concentration of active species and with high cell voltage [7]). Recently, efforts have been devoted to energy storage systems that use low-cost, environmentally safe organic and polymer electrolytes [8] and suspension electrolytes with solid active species [9]. Optimization of the VRFBs and the development of new electrolyte and cell systems for a practical application need to overcome many challenges, including the fundamental understanding of the electrolyte electrochemistry, development of improved electrode materials, and innovation in the critical aspects of cell and stack design.

In this review, we address the critical issues and challenges that are related to the performance of RFBs. In Sec. 2, we discuss the types and properties of membranes and their impact on the VE, CE, and EE for the VRFBs. In Sec. 3, we introduce the modification of carbon-based electrode materials, the application of catalysts, and their effect on cell performance. The main focus of Sec. 4 is on organic redox-active species, which possess great potential to meet the low-cost and high-power requirements. Section 5 discusses novel electrolytes using solid redox-active compounds in semi-solid RFB systems, which can allow for large improvements in the energy density. Section 6 presents system configurations for RFBs. In the end, we present concluding remarks and prospects for the future development of RFBs.

## Membranes

In flow batteries, membranes separate the two electrode chambers, preventing mixing of anolyte and catholyte. They are permeable for supporting ions, such as protons or sulfates. However, membranes should be impermeable for the reactive species. Membranes should be chemically stable against oxidation and acid-catalyzed reactions like hydrolysis, which excludes polymers with ester bonds, for example. In general, membranes can be classified as: porous membranes and dense ion-conducting membranes. The latter can be further separated into three categories: cation exchange membranes (CEMs), anion exchange membranes (AEMs), and membranes which conduct both cations and anions. Historical development of membranes for flow batteries has been reviewed elsewhere [10,11]. In this section, we focus on the very recent development, highlighting sulfuric-acid-doped polybenzimidazole (PBI) membranes, which seem to be promising for VRFBs [12,13]. Another example is the use of spongelike porous membranes, in which the pores are filled with highly conductive sulfuric acid and the thin polymeric pore walls can block transport of vanadium ions [1317].

###### Porous Membranes.

Earlier development of flow batteries was restricted by the availability of suitable membranes. For example, a diaphragm was prepared by drilling holes in a Plexiglass disk (60 holes of 0.6 mm diameter per 1 cm2) [18]. Such diaphragm cannot prevent mechanical mixing of two electrolytes, resulting in low CE. Nowadays, well-defined porous membranes are commercially available, such as microfiltration membranes (block particles above 100 nm), ultrafiltration membranes (block particles down to 2 nm), and nanofiltration membranes (a cut-off below 2 nm) [19]. Ultrafiltration membranes can be used for flow batteries with polymeric redox couples. For example, a commercial Celgard 2400 microporous membrane (25 μm thick, 39% porosity, and 28 nm average pore diameter) rejected 86.3% of a redox-active polymer when the molecular weight of the polymer was 318 kDa [20]. Schubert and coworkers developed an aqueous solution of polymers [21], allowing the use of a cellulose-based dialysis membrane.

For nanofiltration membranes, the size exclusion effect can even be used to separate protons and vanadium ions [22,23]. For this purpose, a poly(acrylonitrile) membrane was prepared by a phase inversion process [22]. Based on the scanning electron microscope (SEM) images, the preparation process probably involved the casting of the membrane on a substrate (e.g., glass plate), followed by immersion in a nonsolvent, e.g., water. Membranes prepared in this way usually show an anisotropic morphology, consisting of large pores and a dense, thin selective layer. Critical process parameters include the choice of solvents, processing temperature, and polymer concentration in the casted membrane. In order to improve the wetting behavior of the membrane surface, the nitrile groups of the porous membranes were partially hydrolyzed by immersion in 10 wt. % NaOH solution. A polyethersulfone-based nanofiltration membrane was further developed [23]. Different pore structures were obtained by adding a water soluble polymer to the casting solution. CE in the range of 90–95% has been achieved with an EE of 75% at 80 mA cm−2.

Recently, ultrafiltration membranes were also considered for the use in VRFBs. Wessling and coworkers [24] coated a porous polyacrylonitrile membrane with either a 0.75- or a 2.8-μm thick PIM-1 (a polymer of intrinsic microporosity) layer. PIM-1 was designed to have a high free volume with a rigid, kinked polymer backbone (Fig. 1) [25]. The nanosized space between the polymer chains is rather hydrophobic, limiting its water absorption and blocking large vanadium ions. Protons can enter the polymer and interact with the water molecules in the nanopores of PIM-1. The CE of these membranes is hardly affected by the current density, which is around 100% at open circuit voltage and a little lower at current densities up to 40 mA cm−2. The VE is similar to Nafion 112. In order to reduce the porosity and enhance the conductivity, some strategies could be helpful such as hydrolysis of the nitrile group [26] or copolymerization with less rigid monomers. Note that PIM-1 has poor acid resistance. The structure of the spiro compound is complicated, but it can be easily obtained by acid-catalyzed condensation of acetone and catechol. The back reaction of the monomer synthesis opens a pathway for degradation of the polymer backbone under acidic conditions [27].

In conclusion, porous membranes can effectively block permeation of the redox compounds through the size exclusion effect. Further optimization of these membranes could be obtained by modifying the pore structure, adding inorganic fillers, or blending with more hydrophilic materials (to enhance proton conductivity).

###### Cation Exchange Membranes.

CEMs consist of polymers substituted with acidic groups. For weak ion exchange materials, which are less relevant to flow batteries, these groups can be carboxylic acid groups. For strong ion-exchange materials, the functional groups are mainly sulfonic or phosphonic acids, which easily dissociate into a mobile cation and the respective immobilized counter ion. Due to different hydrophilicities of the backbone (hydrophobic) and the functional groups (hydrophilic), CEMs show phase separation into hydrophobic domains (which provide mechanical integrity) and connected hydrophilic domains (which provide conductivity). This phase separation is especially pronounced in perfluorinated materials like Nafion (Fig. 2) [28]. In a strong contrast to typical porous membranes discussed in Sec. 2.1, the shape and size of the hydrophilic domains for CEMs may vary, depending on the water contents, and also show dynamic reorientations due to mobility of the polymer chains. CEMs have several advantages such as high conductivity, commercial availability (mainly developed for chlor-alkali electrolysis, fuel cells, electrolyzers, and water treatment), high chemical stability (e.g., Nafion), and structural variety. The main drawback of CEMs is that they are susceptible to metal crossover, e.g., vanadium ions. During operation, especially at high current densities, this effect is less pronounced because protons have a higher diffusivity than metal ions. At open circuit voltage, cells with a CEM will rapidly self-discharge.

If CEMs are in the protonated form, they will easily show much higher conductivity than a membrane which selectively conducts anions like Cl or $SO42−$, considering protons have the highest molar conductivity among all known ions [29]. Protons are transported not only via the vehicle mechanism (a moving charge) but also via structural diffusion (Grotthus mechanism). In the latter case, charge is not transported by individual protons between the electrodes, but transported by a reorientation of bonds between hydronium ions (H3O+) and a neighboring water molecule. For some flow batteries, also the ability of CEMs to conduct Na+ can be useful.

The most prominent CEM membrane is Nafion, which is available in different thicknesses (25, 50, 125, and 175 μm for Nafion 211, 212, 115, and 117, respectively). The dependence of VE, CE, and EE on the thickness in flow batteries has been well investigated [30,31]. In general, thin membranes increase the VE (low resistance), and thick membranes increase the CE (reduced vanadium ion crossover). Between 120 and 240 mA cm−2, Nafion 115 showed the highest EE, balancing CE and VE [31]. Further optimization is possible by controlling the hygrothermal history of the membranes, which influences the degree of crystallinity and channel structure [32]. Therefore, when handling the membranes, care needs to be taken to follow an exact protocol. For example, the water content of a membrane during assembly can have an effect on the swelling direction (in-plane versus thickness direction) of the membrane in the assembled cell [33,34]. Also, the preparation conditions are expected to have a major effect on the membrane performance. Membranes casted from commercial dispersions are brittle, unless the solvents are evaporated at a minimum temperature of about 60 °C [35]. The brittleness can also be reduced by an annealing step, at which the polymer is heated above its glass transition temperature [36]. For the use for RFBs, the requirement of mechanical stability is not as critical as for fuel cells, considering that membranes (ideally) are always in contact with the liquid electrolyte. Future work therefore may focus on membranes casting under different conditions or solvents [37], in order to rebalance the membrane properties (conductivity, permeability, and mechanical stability).

Many hydrocarbon-based CEMs with a large structural variety, developed previously for fuel cells, can be considered for their use in flow batteries. Structurally, most hydrocarbon-based membranes are derived from condensation polymers like poly(ether ether sulfone) or poly(ether ether ketone). Design parameters are the number, position, and distribution of sulfonic acid groups. Sulfonation of polymers leads to a random distribution over the chains and connecting points are activated positions (e.g., ortho to ether groups). The use of sulfonated monomers allows controlling the degree of sulfonation and the position of the sulfonic acid groups. The distribution of the functional groups can be controlled by changing from random copolymers to block copolymers, which has a large influence on the microstructure and morphology and therefore on the water uptake and conductivity [38]. Another way to enhance the phase separation is to use spacer groups between the aromatic backbone and the sulfonic acid groups. Kim and coworkers [39] prepared a polymer in which some of the hexaflourinated bisphenol A units bear two bis(propylsulfonic acid)amine substituents. As expected, both conductivity and vanadium permeability increased with the degree of sulfonation. However, a membrane with a thickness close to Nafion 117 showed 50% lower permeability, but 10% higher conductivity than Nafion 117. Stability test in 1 M VO2+ suggested a good stability with 1.5% weight loss. In another work, Fujiwara and coworkers [40] compared the effect of three different ways to connect the acid groups: (1) directly attached to an aromatic ring, (2) with an allylic spacer, and (3) attached as styrenesulfonic acid. Most tested membranes showed an EE of up to 86%, higher than that of Nafion 212. Especially, the membranes based on styrenyl sulfonic acid showed slightly higher proton conductivities and performed well in the VRFB, indicating that the longest side group has the most beneficial impact on the properties. Kwon and coworkers [41] analyzed the effect of the membrane thickness (30–150 μm, sulfonated poly(ether ether ketone) (sPEEK) with a degree of sulfonation of 55%) on the VRFB performance. As expected, the VE decreases and the CE increases with the thickness. However, all the tested membranes showed higher EE than Nafion 212, due to the higher CE (around 95%) of sPEEK membranes than that of Nafion 212 (below 80%).

Similar to Nafion, the properties of hydrocarbon-based membranes can be strongly influenced by the casting solvents. By varying the degree of sulfonation and casting sPEEK membranes from N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), and dimethylsulfoxide (DMSO) solutions, Xi et al. [42] found that the membrane with a degree of sulfonation of 67%, casted from DMF, provided the best efficiencies and a better cycle-life performance at 80 mA cm−2.

It was shown that sulfonated hydrocarbon-based membranes usually lead to a higher CE than Nafion. As discussed, ultrafiltration membranes can have high proton/vanadium selectivity, based on the molecular sieving effect. For comparison, if we consider CEMs as ultrafiltration membranes with the pores as hydrophilic channels, Kreuer's comparison of the morphologies between Nafion and sPEEK membranes (Nafion has wider channels than sPEEK) may provide explanation for the enhanced CE [43]. It is plausible that such a sieving effect should be further enhanced by the more rigid structure of hydrocarbon-based membranes (lower chain mobility). This finding is suggestive to optimize Nafion membranes. Wang and coworkers [44] analyzed Nafion membranes with an equivalent weight of 1000, 1200, and 1500 g mol−1 H+ by small-angle X-ray scattering (SAXS) and calculated that the average pore size decreases from ca. 3.3 nm to 3.15 and 3.05 nm, respectively. In the VRFB, a 47-μm thick EW 1500 membrane showed the highest capacity retention, but a lower VE. When the membrane was produced with just 31 μm thickness, it showed an area resistance comparable to Nafion 115 (EW 1100), but a 40% lower VO2+ ion flux.

A very effective way to improve CEMs is to blend them with small amounts of a basic polymer. As an example, Kerres and coworkers [45] blended a partially fluorinated, sulfonated polymer (based on bisphenol A and biphenyl) with a partially fluorinated PBI. The strong ionic interaction between the acidic and basic groups leads to ionic crosslinking, which reduces water swelling and therefore the average pore size. Accordingly, such membrane showed a higher CE (close to 100%) than Nafion 117 and a good stability over 200 cycles.

Composite membranes with added components, which are not miscible with polymers, have been developed for use in fuel cells [46]. Such strategies have been applied to RFBs. For instance, 2.5–10 wt. % of Al2O3, SiO2, or TiO2 nanoparticles were mixed into a sPEEK membrane [47]. Since the particles do not provide acidic groups, the ion-exchange capacity (IEC) decreases with increasing the amount of nanoparticles. This leads to decrease of water uptake, swelling ratio, and proton conductivity. At a load of 5 wt. %, the addition of nanoparticles significantly decreased the VO2+ permeability of the sPEEK membrane. When the thickness was adjusted to 60–70 μm, the composite membranes showed not only higher CE but also higher VE than Nafion 117. The stability in 0.1 M VO2+/3 M sulfuric acid was good (no visible degradation over 21 days). The concentration of V5+ was very low in this test (others use 1.7 M [48]), but even 0.1 M is enough to strongly degrade sulfonated Radel [49]. The discharge capacity retention of the membrane with 5 wt. % silica was 53.3%, which was more than twice the value observed for Nafion 117. In another work, Nafion was filled with silica nanoparticles with a sulfonated surface [50]. The additional sulfonic acid groups increased the IEC from 0.99 to 1.24 mmol g−1. Both Nafion 117 and a composite membrane (similar thickness) showed the same VE of 87%, while the composite membrane showed a CE of 94%, compared to 82% for Nafion 117. Xi and coworkers [51] mixed graphene and graphene oxide into Nafion. Graphene is impermeable for most molecules, but permeable for protons, which could boost the membrane selectivity [52]. In comparison to recast Nafion, the composite membranes improved self-discharge and EE at 80 mA cm−2 (from 80% to 85%).

All the membranes discussed earlier were prepared by solution casting, which implies that all the components are freely miscible and soluble in the casting solvent. Radiation grafting is a way to circumvent this limitation. For example, partially fluorinated ethylene tetrafluoroethylene (ETFE) membranes are commercially available at a relatively low price. They are nearly insoluble in common solvents. By electron-beam irradiation, which is a well-established industrial scale process, radicals are formed throughout the whole membrane. When such an activated membrane is immersed in a monomer-containing solution, the monomers start to diffuse into the membrane and then to polymerize. Usually, the crystalline areas of the ETFE substrate absorb much less (or no) monomers, leading to a unique morphology of crystalline, unreacted ETFE domains (enhancing mechanical stability) and amorphous, grafted domains [55]. By choosing the right combination of monomers, copolymers can be also grafted. Gubler and coworkers [56] grafted ETFE films with a styrene/acrylonitrile copolymer. The aromatic groups were sulfonated, and some of the nitrile groups were transferred into amidoxime groups (Fig. 3), which are known to complex vanadium ions. However, the complexes are not stable under acidic conditions [57]. Therefore, the observed good membrane properties (similar conductivity to Nafion 117, but five times lower VO2+ uptake, resulting in higher VE and CE than Nafion 212 and 117) are probably a result of electrostatic repulsion of vanadium ions by the protonated amidoximes.

###### Anion Exchange Membranes.

For AEMs, the positively charged functional groups electrostatically repel cations. This will inhibit the crossover of vanadium ions. But in spite of the Donnan exclusion effect, AEMs were found to absorb some additional sulfuric acid when immersed in the electrolytes, which leads to higher conductivity than expected for a pure membrane [58]. Ramani and coworkers [59] tested the suitability of AEMs for the V/Ce RFBs. The results showed that the employed trimethylammonium functionalized poly(etherketone)-based AEM is chemically and mechanically stable and can prevent mixing of the active species. After 20 cycles, the capacity retention of a system with Nafion 212 was 55%, while the capacity loss was <5% for the AEM-based system.

The main challenge on AEMs is the low alkaline stability of the cationic groups, which are typically quaternary ammonium groups. Marino and Kreuer [60] reported that a spiro-ammonium model compound has a 1000 times longer half-life in 6 M NaOH at 160 °C than the 1,2,3-trimethylimidazolium ion. Another functional group, which is considered unstable, is the 1-methyl pyridinium group [61]. However, under acidic conditions, it becomes attractive again to investigate AEMs with these functional groups. Zhang et al. [62] modified chloromethylated Radel with pyridine. Zhang and coworkers [63] prepared crosslinked membranes by adding bipyridine to a casting solution containing chloromethylated Udel. Zhao and coworkers [64] followed a very similar protocol, but crosslinked bromomethylated PPO (poly(2,6-dimethyl-1,4-phenylene oxide) with 1,2-bis(4-pyridyl)ethane, which is a more flexible linker than bipyridine. All three groups reported excellent chemical stability, high CE, and good VE. Lu et al. [65] attached methylated imidazolium groups to a bromomethylated poly(ether ether sulfone) backbone. Membranes with an IEC of 1.5 and 2.0 showed a very low permeability. A membrane with a high IEC of 3.5 showed a permeability close to Nafion 117.

Polymers without heteroatoms in the main chain have also been studied. Fujimoto and corkers [69] evaluated a Diels–Alder poly(phenylene)-based AEM in VRFBs (Fig. 4). It is known that CEMs based on a similar structure degraded in the VRFBs, despite the fully aromatic structure. The similar materials, functionalized with quaternary ammonium groups, showed an improved stability in VO2+ solution. AEMs with an intermediate IEC value (0.8 mmol g−1) showed the lowest vanadium permeability (1.4 × 10−6 cm2 min−1 versus 2.1 × 10−6 cm2 min−1 for Nafion 212) and a low area resistance of 0.21 Ω cm2 (compared to 0.32 Ω cm2 for Nafion 212). In general, the IEC value was shown to be proportional to degradation rate and VE and negatively correlated to the CE. A direct comparison between a CEM and an AEM, both based on Diels–Alder polymers, showed unexpected findings [70]. Some sulfonated polymers showed a similar CE as the corresponding AEM, similar to that with Nafion 117. It is considered that the transport processes are strongly affected by water uptake. The bulky, rigid structure of the Diels–Alder polymer chains may lead to a high free volume, limiting the effect of the ionic groups. Finally, a Diels–Alder polymer, in which the ammonium groups are tethered by C6-spacers, was tested for application in a nonaqueous ionic liquid-based flow battery [71]. Membranes with an IEC of 1.5 were too brittle to be useful, while membranes with an ICE of 2.5 showed excessive movement of solvent across the membrane. Therefore, an IEC of 2.0 was mostly chosen. No chemical degradation was found. An increase in membrane resistance was observed, probably due to mechanical and thermal aging.

As will be discussed later for acid-doped membranes, membranes with closed pores (spongelike structures) can be beneficial for use in the VRFB, if the pores are filled with sulfuric acid. Then, the pores allow for an overall low resistance, and the polymeric pore walls block transport of vanadium ions. Zhang and coworkers [14] prepared a porous membrane from a chloromethylated polysulfone (CMPSF) solution by water-vapor-induced phase separation, followed by immersion of the membranes in an imidazole containing solution for 1–4 days. Due to the heterogeneous character of this reaction, reaction with imidazole should proceed from outside to inside. The VE improves for membranes treated for 1, 2, or 3 days, after which the VE reaches a constant value. The membrane immersed in the imidazole solution for 3 days showed a CE of 99% and an EE of 86% at 80 mA cm−2. Furthermore, when the membrane was tested over 6000 cycles at 120 mA cm−2, no efficiency loss was observed (Fig. 5). Very similar results were obtained, when the imidazole was substituted by 1,4-butanediamine [15].

###### Acid-Doped Membranes.

Although pure PBI is an insulating material, the basic nitrogen groups of PBI strongly interact with strong acids (such as phosphoric acid or sulfuric acid), rendering the acid-doped materials conductive (Fig. 6) [73,74]. It is expected that protons and anions (the conjugated base of the doping acid) contribute to the conductivity [75]. Through interaction with the first strongly bound acid molecules, additional acid molecules can be absorbed by hydrogen bond interactions. Zhao and coworkers [76] argued that PBI used in the VRFBs can be considered as a porous membrane with pore diameters of 0.5–2 nm, which is much smaller than the size of the ionic clusters in Nafion (connected by small channels of 1 nm diameter, see Fig. 2). Note that PBI will be fully protonated when immersed in electrolyte. Thus, the polymer backbone has a high positive charge density, which can hinder transport of vanadium ions through electrostatic repulsive forces. In addition, acid absorption will increase the pore size, as water uptake does for Nafion membranes [44]. PBI membranes showed excellent properties in the VRFB. After immersion for 3 months in 1 M V5+ solution, a PBI membrane showed a weight loss of 2.9%, similar to Nafion (2.1%). The conductivity of Nafion decreased 5.6%, while PBI decreased only 4.4% in contrast. No permeation of vanadium ions was measurable in ex situ tests. In the VRFBs, the CE of a 30-μm thick PBI membrane was close to 100% at very low current densities. The bottleneck is the low conductivity of sulfuric-acid-doped PBI (15.8 mS cm−1 versus 50.7 mS cm−1 for Nafion 211). As a consequence, the EE of PBI exceeds that of Nafion 211 only at current densities below 40 mA cm−2.

By changing the backbone structure of PBI (BIpPBI, see Fig. 6), Hong and coworkers [77] obtained a membrane that can absorb about 25% more acid than standard meta-PBI, when immersed in the VRFB electrolyte. It has been reported that the conductivity increases exponentially with the phosphoric acid uptake [78]. The area resistance of BIpPBI was about half of that of PBI. An ex situ test showed a slightly increased VO2+ permeability (but far below that for Nafion 115), as expected for a membrane with higher acid uptake. After 200 cycles at 50 mA cm−2, both mPBI and BIpPBI showed a capacity retention of >80%, while Nafion 115 already lost >50% of the capacity after 100 cycles.

As discussed, the positively charged PBI backbones hinder vanadium ions in the electrolyte solution from entering the polymer. Li and coworkers [13] hypothesized that establishing several liquid/polymer interfaces, as in a membrane with a closed pores structure, would block vanadium ions even more efficiently. A cell with a 68-μm thick porous O-PBI membrane can withstand over 13,000 cycles between 80 and 120 mA cm−2, with good efficiencies (CE close to 100%, VE and EE around 80% after 13,500 cycles, see Fig. 7). Furthermore, the material was used in a 1 kW stack, which showed stable efficiencies over 100 cycles at 120 mA cm−2. Such porous PBI membranes are promising for VRFBs in terms of performance and cost [13].

Moon and coworkers [16] reported the use of porous PBI copolymer membranes, which contain 50% para-phenyl and 50% C7 alkyl chain links. The pores were rather large. Because of the additional interfacial resistance and poor transport of vanadium ions through PBI membranes, the CE of a porous PBI membrane was higher than that of a dense PBI membrane. An improved VE was obtained. Wessling and coworkers [17] compared abPBI-based membranes. Again, a porous membrane filled with sulfuric acid showed a higher conductivity than the dense membrane.

The chemical structure of PBI can be varied. Not only can the backbone structure be freely modified (Fig. 6) but it is also possible to crosslink PBI or to sulfonate activated phenyl rings. Zhang and coworkers [79] prepared a series of crosslinked PBI membranes, in which the ratio of an aniline moiety and a sulfonated phenylether was varied. These membranes showed stable CE close to 100% and EE above 80% for over 300 cycles at 60 mA cm−2. Note that the crosslinker is bisphenol A based, a structure which is known to be acid labile and therefore will degrade over time [80,81].

Since the membrane resistance decreases with the acid uptake, efforts were made to increase the acid uptake. It is known that PBI membranes absorb more phosphoric acid molecules per repeat unit (the acid doping level (ADL)) than sulfuric acid molecules. He and coworkers [82] preswelled membranes in phosphoric acid, followed by immersion in a sulfuric acid solution. Apparently, the ADL increased, leading to reduced resistance. In order to further minimize the membrane resistance, a porous O-PBI membrane was fabricated recently with a 2–5 μm thin defect-free dense blocking layer [12]. When the porogen content (dibutylphthalate) reached 250%, the highly porous membranes showed a similar resistance to Nafion 211, but no measurable VO2+ permeability. The EE reached 92% and 82% at 20 and 100 mA cm−2, respectively.

## Electrodes

Electrodes used in an RFB are required to have a high surface area, suitable porosity, good wettability for electrolytes, low electronic resistance, high activity toward the redox reactions (while suppressing water electrolysis), and good (electro)chemical stability. Due to the commonly corrosive environment in a RFB, there are limited choices of materials to fabricate electrodes. Carbon-based materials have been used as electrodes in various RFBs such as VRFBs, polysulfide/Br, Zn/Br, and V/Ce flow batteries [83101]. Because of inadequate electrochemical activity, catalysts have been employed for facilitating the redox reactions [102,103]. So far, electrodes for VRFBs have been mostly investigated, as will be discussed in this section.

Since the VRFB was first proposed by Skyllas-Kazacos et al. [104], various approaches have been reported to modify the carbon-based electrode materials, including thermal treatment, chemical treatment, electrochemical oxidation, and metal doping. Sun and Skyllas-Kazacos [97] adopted heat treatment of graphite felts (GFs) (at 400 °C for 30 h) to improve its EE for the VRFBs. The improved performance was attributed to the enhanced surface hydrophilicity and the formation of the oxygen functional groups (C–O–H and C=O groups) on the graphite felt surface, which act as active sites to catalyze the electrochemical reactions, as illustrated in Fig. 8 [101]. Another method to modify the surface of graphite felts is chemical treatment with nitric acid or sulfuric acid [96]. A significant enhancement in VRFB performance was achieved (EE of about 88% at 25 mA cm−2) after the graphite felt was treated with concentrated sulfuric acid solution for 5 h. A combination of thermal and chemical treatments has also been employed [105]. The graphite felt was first treated in 98 wt. % sulfuric acid solution for 5 h and then kept at 450 °C for 2 h. The synergetic effect of thermal and chemical treatment increased the number of the active sites on the graphite electrode surface and enlarged the specific surface area of the graphite felt electrode (from 0.31 to 0.45 m2 g−1). Li et al. [106] studied chemical treatment of graphite felts using mixed acids (nitric acid/phosphoric acid = 3:1, at 80 °C for 8 h). The improved electrochemical properties (EE of the VRFB reached about 78% at 20 mA cm−2) of the treated graphite felt electrode was ascribed to the increase of the interaction between vanadium ions and OH groups that were formed by mixed acid treatment.

Electrochemical oxidation is another method to modify the graphite felts [107]. After electrochemical oxidation treatment, the specific surface area of the graphite felt electrode increased from 0.33 to 0.49 m2 g−1. The improvement of electrochemical activity for the electrode was ascribed to the increase of the number of COOH group and the special surface area. Tan et al. [108] investigated the activation mechanism of electrochemically treated graphite felts by electrochemical impedance spectroscopy. The decrease in the value of impedance can be explained by the increase of COOH group on the graphite felt surface. Zhang et al. [109] studied the electrochemical activation of graphite felts at different oxidation degrees. The graphite felt was modified through electrochemical oxidation at 100 mA cm−2 in 1 M sulfuric acid solution. Active sites on graphite felt surface for VO2+/VO2+ redox reactions were created. As a result, the VE and EE for VRFB with oxidized graphite felts were 4–5% higher than that with pristine graphite felts.

Sun and Skyllas-Kazacos [98] deposited metal on the surface of graphite fiber electrodes by impregnation or ion-exchange with solutions containing Pt4+, Pd2+, Au4+, In3+, Mn2+, Te4+, or Ir3+. Among them, the electrode modified by Ir3+ showed the best electrochemical properties for the vanadium redox species. Wang and Wang [110] reported an Ir-modified carbon felt as positive electrode of VRFBs. After Ir-modification, the conductivity of electrode materials increased and the cell internal resistance decreased. Jeong et al. [111] proposed Pt-based catalyst synthesized by polyol process. To promote the VO2+/VO2+ redox reaction, Pt/C catalyst was synthesized and coated on the surface of graphite felt using air spray for VRFB tests.

Apart from using noble metals, González et al. [112] proposed graphite felt modified with nanodispersed Bi as positive electrode in VRFBs. The graphite felt was modified by immersion in a Bi2O3 solution in 0.01 M HCl under vacuum for 3 h and then treated in air at 450 °C for 3 h. Eventually, Bi-modified graphite felt contains 1.09 at % Bi on the surface of the fibers with a high concentration of defects or holes. The well-dispersed Bi nanoparticles on the carbon surface served as stable active sites. Furthermore, they reported an additional achievement by incorporating Bi nanoparticles into the graphite felt electrode in the negative half-cell of VRFBs [113]. Unlike previous study, Bi was electrodeposited onto thermally oxidized graphite (at 450 °C in air for 3 h) by immersing the graphite felt in 5 mM Bi(NO3)3/1 M HNO3 solution and applying −0.2 V for 1 min. Then, the graphite felt was treated in air at 450 °C for 30 min to stabilize the electrodeposited Bi. The reversibility of the V3+/V2+ redox reactions and the long-term cyclability of the electrode were improved by preventing H2 evolution through incorporating Bi nanoparticles into the graphite felt.

Metal oxides with relatively low price and comparable catalytic properties with noble metals have been investigated for VRFBs [101]. Kim et al. [114] investigated catalytic effects of Mn3O4 nanoparticles formed on carbon felts using a hydrothermal treatment. A carbon felt was soaked in 1 M manganese acetate solution and heated to 200 °C for 12 h for hydrothermal reactions. Afterward, the modified carbon felt was treated at 500 °C for 5 h under an Ar atmosphere to obtain the final product. The well-dispersed Mn3O4 nanoparticles can catalyze the VO2+/VO2+ redox reaction. The VRFBs using Mn3O4-modified carbon felts exhibited improved CE, VE, and EE compared to the cell using pristine carbon felts. Li et al. [115] proposed Nb2O5 nanorods as efficient catalysts toward both VO2+/VO2+ and V3+/V2+ redox couples. They investigated and optimized the amount, size, and distribution of Nb2O5 nanorods on graphite felt surface and added salt containing W in the precursor solutions during synthesis for more uniform distribution and minimum agglomeration. VRFBs using Nb2O5 nanorods on graphite felt exhibited about 10.7% higher EE at 150 mA cm−2, compared to the cell without catalysts. Tseng et al. [116] investigated the effect of adding TiO2 particles to the negative electrode in VRFBs. Hydrophilic TiO2/C composite electrodes were prepared by a sol-gel method. The specific capacitance of electrode and CE of VRFBs were improved by adding an adequate amount of TiO2 particles to the carbon electrode. Wu et al. [117] electrodeposited PbO2 onto graphite felts to improve the activity and kinetic properties. Zhou et al. [118] decorated graphite felts with CeO2 as a high-performance electrode for VRFBs. Compared to the pristine material, CeO2 nanoparticle decorated graphite felts, prepared by a facile precipitation method, showed higher activity and reversibility toward the VO2+/VO2+ redox reaction. The highest EE was observed at a CeO2 content of 0.2 wt. %.

Carbon nanotubes (CNTs) have been widely used as catalyst support because of their high-specific surface area, chemical stability, and excellent electrical conductivity [119122]. Zhu et al. [123] first proposed graphite–carbon nanotube composite electrodes for VRFBs. Yan and coworkers [124] also employed CNTs as catalysts for VRFBs. Multiwalled carbon nanotubes (MWCNTs) and hydroxyl and carboxyl functional MWCNTs were coated on the surface of the carbon electrodes. Modified carbon electrodes showed enhanced electrochemical activities and kinetic behavior in the order: carboxyl MWCNTs > hydroxyl MWCNTs > pristine MWCNTs, suggesting that the oxygen functional groups could significantly facilitate the VO2+/VO2+ redox reaction.

Nitrogen-doped carbon electrode materials have been explored with improved electroactivity [125131]. Also, N-doped polyacrylonitrile-based graphite felt by a hydrothermal ammoniated treatment was used for VRFBs [132]. Wang et al. [133] employed N-doped carbon nanotubes on graphite felt (N-CNT/GF) by a chemical vapor deposition (CVD) process. The cell using N-CNT/GF electrodes showed increased EE in VRFBs, compared to that with pristine graphite felts. The significantly improved performance was attributed to the unique porous structure and increased surface area. Jin et al. [134] developed N-doped graphene sheet (NGS). NGS was synthesized by annealing graphite oxide with urea at 700–1050 °C and used as positive electrodes in VRFBs. The catalytic activity was related to the types of nitrogen species (pyridinic, pyrrolic, quaternary, and oxidic) in the graphene sheets, not to the N-doping level. Among them, the quaternary nitrogen was verified as catalytic active center for the VO2+/VO2+ reaction.

Bio-derived carbon materials were also investigated as electrodes for the VRFB. Park et al. [135] employed carbon-based catalysts by corn protein self-assembly. Carbon black nanoparticles were coated with N-doped graphite layers with oxygen-rich functionalities. This treatment increased catalytic activity toward V3+/V2+ and VO2+/VO2+ redox reactions. As a result, a significant improvement in the EE of VRFB was observed. The abundant oxygen active sites and nitrogen defects in the corn protein-derived N-doped carbon black enhanced the electron transfer rate and vanadium ion transfer kinetics. Ulaganathan et al. [136] investigated the multicouple reactions in VRFBs by using coconut shell-derived mesoporous carbon with high surface area as electrodes.

Recently, Park et al. [137] proposed a new fabrication method for highly porous graphite felt electrodes. They conducted etching treatment on graphite felts by repeating the NiO/Ni redox reactions to produce a high surface area. The etched graphite felts have stepped-edges incorporating oxygen defects, which showed increased catalytic activity and wettability, leading to enhanced electrochemical performance. González et al. [138] employed graphene-modified graphite felts as electrodes for VRFBs by electrophoretic deposition. Blasi et al. [139,140] employed an electrospinning method to fabricate carbon nanofibers with oxide materials. Apart from these studies focusing on electrode materials, Bhattarai et al. [141] found that porous electrodes with flow channels improved the overall EE by reducing pumping power and enhancing flow distribution of electrolyte.

## Organic Redox-Active Species

Compared to conventional metal ion-based RFBs, organic materials are relatively inexpensive and structurally diverse. Recent researches on RFBs using organic redox-active materials have been comprehensively reviewed elsewhere [142]. In this section, we introduce some representative organic redox species developed for RFBs.

###### Benzoquinone.

Benzoquinones (BQs) are known as reversible redox compounds. BQs undergo a reversible two-electron process with protons in aqueous media to form hydroquinone (BQH2) [143] Display Formula

(1)

The reduction of BQ in aprotic media proceeds in two steps (first to anion radical and then to dianion) [144,145]: Display Formula

(2)

A BQ-based compound, chloranil (tetrachloro-p-benzoquinone, BQCl4), exhibiting good electrochemical reversibility with a high redox potential of E0 = 0.71 V (potentials are referred to the standard hydrogen electrode (SHE) in this section, if not specified) in strongly acidic solutions, was used in a single flow acidic Cd/BQCl4 RFB [146]:

Positive electrode Display Formula

(3)

Negative electrode Display Formula

(4)

A smooth copper sheet was employed as a substrate, on which the Cd2+/Cd redox reaction took place. A mixture of H2SO4–(NH4)2SO4–CdSO4 was continuously circulated to pass through the cell. No ionic membrane was needed in this battery. An average CE of 99% and EE of 82% were achieved at 10 mA cm−2 over 100 cycles. However, the energy density of the battery is limited by the size of the electrode and the amount of chloranil on it.

Tiron (4,5-dihydroxy-1,3-benzenedisulfate, BQDS) is a weakly acidic aromatic organic compound. Two sulfonic groups on the benzene ring can improve the solubility of BQDS in water. In a hybrid BQDS/Pb RFB [147], 0.25 M BQDS in 3 M H2SO4 medium was employed as catholyte and 3 M H2SO4 solution as anolyte. A graphite felt positive electrode and a Pb negative electrode were used, separated by a Nafion 115 membrane. An irreversible process occurred for the first cycle (a possible hydroxylation reaction, resulting in new substances containing p-benzoquinone structure [148,149]) with a low CE of only 38% [150]: Display Formula

(5)

From the second cycle, the CE was over 90% and maintained around 93% over the next 10 cycles. Although the hydroxylation of BQDS leads to a decrease in the cell voltage, BQDS is promising as active species for catholytes in organic-based aqueous RFBs due to its high solubility (about 1.7 M at 25 °C in water) and high redox potential (0.85 V) [151]. To inhibit the hydroxylation reaction, Hoober-Burkhardt et al. [149] have introduced 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid and achieved a high CE of 100% over 25 cycles and good capacity retention (0.05% decay per cycle).

###### Anthraquinone.

The redox mechanism of anthraquinones (AQs), taking 9,10-AQ as an example, is similar to that of BQs. In solid state or aprotic media, the reduction of AQ involves two steps [152,153]: Display Formula

(6)

In aqueous solutions [154], especially under acidic conditions [151], AQs tend to undergo a rapid reversible two-electron reduction.

Aspuru-Guzik and coworkers [155] systematically investigated the structure–property relationships of quinones using computational approaches. Taking redox potential and solubility into account, the 9,10-AQs compounds were considered to be suitable for the negative side of aqueous RFBs, whereas the 1,2-BQs, 2,3-naphthoquinones, and 2,3-AQs could be appropriate for the positive side. The redox potentials of quinones can be tuned by utilizing electron-donating groups (such as –OH and –NH2, which decrease the electron affinity and redox potential) or electron-withdrawing groups (such as –SO3H and –NO2, which increase the redox potential). Substitutions of hydrophilic groups (e.g., –OH, –NH2, –COOH, –SO3H, and –PO3H2), especially on the positions far from the ketone groups, can increase the solubility.

Wang et al. [156] reported a nonaqueous hybrid Li/AQ RFB, using graphite felt as positive electrode. In a static cell, an overall EE of 82% and a specific discharge energy density of 25 Wh L−1 were achieved. A metal-free organic/inorganic hybrid aqueous RFB was demonstrated by Aziz and coworkers [157], using 1 M 9,10-anthraquinone-2,7-disulfonic acid (AQ27DS) in 1 M H2SO4 (anolyte), 0.5 M Br2 in 3 M HBr (catholyte), and a Nafion 212 membrane:

Positive side Display Formula

(7)

Negative side Display Formula

(8)

Although this flow battery yielded a peak power density of 0.6 W cm−2 at 1.3 A cm−2 and a high CE of around 95%, crossover of bromine occurred. Bromine can react with the hydroquinone and oxidize it back to quinone, resulting in capacity loss. Yang et al. [151] introduced an aqueous all-organic RFB, using 0.2 M BQDS in 1 M H2SO4 as catholyte and 0.2 M 9,10-anthraquinone-2-sulfonic acid (AQS) as anolyte, equipped with carbon paper electrodes and a Nafion 117 membrane:

Positive side Display Formula

(9)

Negative side Display Formula

(10)

Argon gas was purged to the solutions to avoid reoxidation of the reduced form to quinone. A power density of only 0.025 W cm−2 was achieved. Later, the researchers demonstrated another aqueous organic RFB with BQDS (catholyte) and AQ26DS (anolyte), which showed an EE of 70% at 100 mA cm−2 and a high CE (about 100% over 100 cycles) [154]:

Positive side: the same as Eq. (9).

Negative side Display Formula

(11)

AQDS exhibited fast reaction kinetics (a rapid two-electron redox reaction), high stability, low membrane crossover, and high aqueous solubility (1 M at pH 0).

Li and coworkers [158] reported 3,4-dihydroxy-9,10-anthraquinone-2-sulfonic acid (ARS) with a redox potential of 0.082 V. For the RFB test, 0.05 M ARS in 1 M H2SO4 and 0.05 M BQDS in 1 M H2SO4 were used as anolyte and catholyte, respectively:

Positive side: the same as Eq. (9), E0 = 0.908 V (note that this is different from a previously reported value of 0.85 V [151,154])

Negative side Display Formula

(12)

The electrolytes were sealed to prevent oxidation and pumped into a cell with carbon paper electrodes and a Nafion 212 membrane. A CE of 99% was achieved. A maximal power density of 10.6 mW cm−2 at 80% state-of-charge has been obtained.

All the previously mentioned AQs were tested under acidic conditions. An alkaline quinone RFB was first introduced by Aziz and coworkers [159]. A commercially available 2,6-dihydroxy-9,10-anthraquinone (2,6-DHAQ), which has a solubility of >0.6 M in 1 M KOH, was used as negative material. Ferrocyanide was used as positive material:

Positive side Display Formula

(13)

Negative side Display Formula

(14)

A 0.5 M 2,6-DHAQ dipotassium salt and 0.4 M K4Fe(CN)6, both dissolved in 1 M KOH, were pumped through a flow cell with carbon paper electrodes and a Nafion 212 membrane. A CE exceeding 99% with a stable EE of 84% was achieved at 0.1 A cm−2 over 100 cycles. A 2,6-DHAQ exhibited good stability when heated in 5 M KOH at 100 °C for 30 days. Moreover, all the electroactive species remain negatively charged in all the states of charge/discharge, leading to a dramatic decrease in crossover through the CEM membrane.

###### Viologen.

Viologens (1,1′-disubstituted 4,4′-bipyridinium ions) have been investigated as redox indicators in biological studies [160]. The electrochemical behavior of viologens was further studied by Bird and Kuhn [161]. The viologens exist in three oxidation states: Display Formula

(15)

The first reduction step is highly reversible. The second reduction is less reversible, because the fully reduced state is an insoluble uncharged species. Methyl viologen (1,1′-dimethyl-4,4′-bipyridinium, MV) is chemically stable, but will dealkylate in alkaline solutions [162]: Display Formula

(16)

The potential of the first reduction for MV dichloride (MVCl2) is −0.446 V and the second one is −0.88 V [163]. The solubility of MV in water can reach 2.6 M [164]. Aqueous RFBs using viologens were demonstrated by Liu et al. [165] and Schubert and coworkers [21,166]. Viologens were combined with TEMPO-based species in these RFBs, which will be discussed later.

###### TEMPO.

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is a heterocyclic nitroxide radical compound. TEMPO and its derivatives were investigated and used as organic electrode materials [167]. The nitroxide radical has two redox couples: Display Formula

(17)
The nitroxide radical can be oxidized to oxoammonium cation (reversible process) and reduced to aminoxyl anion (less reversible process). The high stability of the nitroxide radical can be ascribed to (1) the delocalization of the unpaired electron by conjugation over the N–O bond and (2) steric restriction from the four methyl groups on adjacent carbons.

Wei et al. [168] reported a nonaqueous RFB using Li metal anode and 2 M TEMPO in 2.3 M LiPF6/EC-PC-EMC (EC: ethylene carbonate, PC: propylene carbonate, and EMC: ethyl methyl carbonate) electrolyte in a static cell:

Positive electrode reaction Display Formula

(18)

Negative electrode reaction Display Formula

(19)

This flow battery with a high concentration and a high voltage delivered an energy density of 126 Wh L−1. Another flow cell with a lithium–graphite hybrid anode, using a diluted catholyte of 0.1 M TEMPO in 1.0 M LiPF6, was able to operate at 10 mA cm−2 with an EE above 75% and a CE above 99%. An enhanced EE (86%) was achieved when operating at 5 mA cm−2. Over 100 cycles, the average capacity retention was 99.8% per cycle.

A 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a methoxy-substituted TEMPO (MeO-TEMPO) was investigated by Takechi et al. [169]. MeO-TEMPO was mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a molar ratio of 1:1 to form a self-melting liquid, which could be recognized as a super-cooled liquid. This liquid, with a concentration of over 2 M consisting of 17 wt. % of water, was used as catholyte. Li-metal and 1 M LiTFSI in propylene carbonate were used as the anode and electrolyte, respectively. A piece of Li+-conducting glass ceramics was employed as separator. MeO-TEMPO exhibited a higher redox potential than TEMPO. The charge and discharge capacities at 0.1 mA cm−2 were 93% and 92% of the theoretical capacity, respectively. The CE was 99% and the capacity retention was 84% after 20 cycles. The energy density and specific energy were 200 Wh L−1 and 155 Wh kg−1, respectively.

By introducing the hydrophilic –OH group at the para-position for the nitroxide radical compound, Liu et al. [165] obtained a water solubility up to ca. 2.1 M and demonstrated an all-organic aqueous RFB using 4-hydroxy-TEMPO as catholyte and MV as anolyte (0.5 M for both redox-active materials in 1.5 M NaCl):

Positive side Display Formula

(20)

Negative side Display Formula

(21)

At 60 mA cm−2, the cell delivered 71.5% theoretical capacity and the CE remained above 99% over cycling. VE and EE were 62.1% and 62.5%, respectively. After 100 cycles, the capacity retention was 89%.

Schubert and coworkers [166] developed a so-called TEMPTMA by choosing the ionic trimethylammonium group (–TMA) as a substituent and accordingly obtained a water solubility of up to 3.2 M in 0.3 M NaCl. The TEMPTMA has a redox potential of 0.79 V versus Ag/AgCl, surpassing the 4-hydroxy-TEMPO by 0.15 V. Combining TEMPTMA with MV leads to a theoretical cell voltage of 1.42 V. Owing to the high mobility of Cl counter ions in both electrolytes, no additional supporting electrolyte was necessary. The flow cell with 2 M TEMPTMA and MV was assembled with two graphite electrodes and an AEM. At 75 mA cm−2, the EE was over 70% and capacity stayed unchanged over 100 cycles.

Recently, a so-called VIOTEMP was created by combining viologen unit and TEMPO radical in one single molecule [170], which can serve as both anode and cathode. The idea is to mimic vanadium RFB, in which the same electrolytes are employed on both sides. In the anolyte, during the first charge, the TEMPO radical was first reduced to its hydroxylamine form, followed by the reduction of the viologen unit. In the catholyte, two equivalents of TEMPO were oxidized to its oxoammonium form. Half of the catholyte was used as sacrificial agent. In addition, an RFB utilizing VIOTEMP also suffered from fast capacity loss (nearly 50% after 30 cycles).

Some other organic redox-active species are shown in Fig. 9. The redox mechanisms of these materials are most related to formation of radicals. With respect to their low water solubility, these species are often investigated in nonaqueous RFBs or in form of suspensions. Figure 10 summarizes the solubility of some organic redox-active species discussed earlier and their redox potentials.

###### Redox Polymers.

As discussed, redox-active polymers allow the use of low-cost porous membranes. The crossover of active species can be avoided by the size exclusion effect. Schubert and coworkers [21] demonstrated an all-organic aqueous RFB based on TEMPO and MV. Two polymers (Fig. 11), P1 containing the TEMPO unit and P2 containing the MV unit, were synthesized and investigated as redox-active materials. The redox potential of P1 and P2 were determined as ∼0.7 V and −0.425 V versus Ag/AgCl, respectively, suggesting a cell voltage of ca. 1.12 V. Using a cellulose-based dialysis membrane, a CE of ca. 98% and an EE of 75% were achieved at 40 mA cm−2.

Schubert and coworkers [171] further investigated poly(1-methyl-1′-(4-vinylbenzyl)-[4,4′-bipyridine]-1,1′-diium dichloride) and poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl-co-2-(methacryloyloxy)-N,N,N-trimethylethane ammonium chloride) for RFBs. The rheological, thermal, and electrochemical properties were studied. Winsberg et al. [172] reported a hybrid RFB using TEMPO-methacrylate/styrene block copolymers (PTMA-b-PS) as cathode active material and Zn2+/Zn couple for anode reaction (Fig. 12). The copolymer was composited with a polar PTMA and an unpolar PS to enable the formation of core–corona micelles. Such hybrid RFBs showed an excellent cycling stability over 1000 cycles with 95% capacity retention and a stable voltage.

Boron dipyrromethenes (BODIPYs) have chemically reversible oxidation and reduction reactions, allowing their utilization as bipolar redox-active material for organic flow batteries. Winsberg et al. [173] prepared two different BODIPY copolymers (P3 and P4) (Fig. 13). They found that the redox polymers have good electrochemical stability. A new battery was prepared with P3 as anolyte and P4 as catholyte, which showed a mean discharging voltage of 1.28 V and stable cycling over 90 cycles.

## Suspension Electrolytes

Different from the conventional solid-state lithium-ion batteries, RFBs store energy in redox-active electrolytes, contained in external reservoirs. The energy density of RFBs is limited by the concentration of the active species (typically below 2 M) and the narrow electrochemical potential window of aqueous electrolytes (∼1.23 V). The development of semi-solid flow batteries, with a high concentration of active species (in solid form) up to 10–40 M, can lead to an enhanced energy storage capability [9]. As illustrated in Fig. 14, in the semi-solid flow batteries, the solid active compounds and the conductive additives are well mixed in the electrolytes [174178]. With sufficient conductive additives, an electron transfer percolation network can form, which ensures that the active materials can effectively accept or dispatch charges. In this section, we discuss several reported semi-solid suspensions, including inorganic intercalation-based materials, redox-active organic compounds, and polysulfides.

###### Intercalation-Based Materials.

In order to improve the energy density of flow battery systems, a Li+-based semi-solid RFB was proposed by Chiang and coworkers [9]. With appropriate content of active materials (from 0 to 40 vol %), the semi-solid system using lithium intercalation compounds (LiCoO2, LiFePO4, and Li4Ti5O12) delivered a high-energy density of about 300–500 Wh L−1. When the operating voltage was below 1 V versus Li/Li+, an insulating solid electrolyte interphase (SEI) film can form at the graphite anode due to the decomposition of alkyl carbonate solvents [9], which hindered the electron transfer between the fluid electrode and the current collector. Using anode materials with a suitable lithiation potential (1.55 V versus Li+/Li for Li4Ti5O12, for instance) can minimize the formation of SEI [179].

Biendico et al. [180] studied the role of carbon black on the electrochemical and rheological properties based on LiNi1/3Co1/3Mn1/3O2 suspensions. With an increase in the volume percentage of Ketjenblack (KB), the electrolyte resistance and charge transfer resistance decreased. Meanwhile, the viscosity of the suspensions increased. During static and dynamic measurements, large capacities can be delivered as the volume percentage of KB increased. Moreover, Lewis and coworkers [181] introduced a nonionic dispersant of polyvinylpyrrolidone (PVP) into the LiFePO4/KB suspensions. Such electrode suspensions allowed for high content of active materials, without deteriorating the flow behavior and electronic conductivity. Recently, carbon-free suspensions with a low viscosity have been reported, functioning through the collision of active particles with the current collector [182,183]. However, only a low concentration and low operating currents (from 0.001 to 0.02 mA) were applied. The utilization of the active particles was too low from their preliminary work.

An aqueous RFB based on LiFePO4–LiTi2(PO4)3 was first reported by Chiang and coworkers [184]. Aqueous suspensions with high ionic conductivity are of interest for safe and cost-effective applications [7]. In static cell configuration, the suspension containing KB in 1 M LiNO3 aqueous electrolyte showed a capacity retention of about 85% after 100 cycles at 10 mA cm−2. Side chemical reactions, such as (1) oxidation reactions due to the presence of dissolved oxygen in the aqueous electrolytes, (2) hydrolysis reaction, and (3) precipitation reaction, caused capacity fade over time.

Na+-based batteries have attracted increasingly attention due to abundance in resources. Recently, Ventosa et al. [185] applied Na+ intercalation materials as suspensions for nonaqueous RFBs, using a P2-type NaxNi0.22Co0.11Mn0.66O4 suspension cathode and a NASICON-type NaTi2(PO4)3 suspension anode. NaTi2(PO4)3 has a flat potential plateau at about 2.1 V versus Na+/Na and is suitable as SEI-free anode [186]. The full-cell delivered a discharge capacity of 80 mAh g−1 (based on cathode material) at 0.17 mA cm−2 with an average discharge voltage of about 0.8 V. Large overpotential was observed from the positive electrode.

###### Organic Materials.

Metallocenes have demonstrated good electrochemical performance in nonaqueous electrolytes with high-energy density and CE [187,188]. Ferrocene (Fc) dendrites were first introduced as a redox mediator in RFBs by Wang and coworkers [189]. Recently, a novel membrane-free semi-solid RFB with a high cell voltage (∼3.0 V) using a ferrocene-based catholyte and a passivated lithium anode has been investigated by Yu and coworkers [190]. Owing to the fast mass diffusion in the liquid phase and fast redox reaction kinetics of ferrocene/ferrocenium, the full-cell showed small polarization and high capacity retention (94% of the theoretical capacity of ferrocene at 60 °C). The power density and energy density based on the volume of catholyte reached 1400 W L−1 and 40 Wh L−1, respectively. Excellent cycling stability (a capacity retention of 80%) was observed over 500 cycles.

Tomai et al. [191] reported a promising approach to impregnate nanoporous carbon beads with redox-active tetrachlorohydroquinone and 1, 5-dichloroanthraquinone to form a flowable, low viscosity slurry electrolyte. Through shuttling protons between the slurry electrolytes, the energy density of this flow cell was 19.4 Wh kg−1 at 0.26 A g−1, based on the total weight of solid compounds.

###### Li–S Flow Battery.

A concept of semi-solid Li/polysulfide batteries was introduced by Cui and coworkers [192]. Carbon fiber paper was used as current collector for the polysulfide catholyte and assembled against a passivated Li film. The catholyte was cycled only between sulfur and Li2S4, to avoid the formation of insoluble Li2S2 and Li2S. When the concentration of Li2S8 reached up to 5 M, an energy density of 108 Wh L−1 was achieved, which is about three times that of the VRFBs. The addition of LiNO3 in the ether solvent contributed to the formation of an SEI film, which prevented parasitic reactions between the Li anode and the polysulfide.

Lu and coworkers [193] investigated a concentrated sulfur-impregnated carbon suspension in order to increase the volumetric capacity of a Li/polysulfide RFB. The catholyte with 20 vol % sulfur and 26 vol % KB showed a higher volumetric capacity of up to 155 Ah L−1 at 4 mA cm−2. Furthermore, Chen and Lu [194] demonstrated a catholyte suspension with mixed liquid lithium iodide and solid sulfur/carbon active components. As observed from the cyclic voltammetry analysis for the individual S/S2− redox couple (Fig. 15(a)), I3/I redox couple (Fig. 15(b)), and their mixture (Fig. 15(c)), there were no electrochemical interferences, confirming that LiI is compatible with sulfur. A high volumetric capacity of 550 $Ah Lcatholyte−1$ has been achieved using such multiple redox reactions of LiI and sulfur. For further application, it is necessary to replace the lithium metal anode with semi-solid negative flow electrode, such as silicon anolyte [195]. Table 1 summarizes the test conditions and performance for several selected compounds.

Considering the high-energy consumption for pumping suspensions with a typically high viscosity, Carter et al. [196] modeled the hydrodynamic (residence time of a fluid volume within a cell) and electrochemical efficiency properties (time required to charge/discharge a single volume of suspensions within a cell) for the semi-solid RFBs. High-flow rates increase the pumping-energy dissipation, whereas low-flow rates will give rise to shunt currents. In addition, at low-flow rates, the presence of gradients in the state-of-charge, current distribution, and overpotential along the length of the flow channels needs to be considered. Rational operational mode of semi-solid flow cells is thus important. In addition, a suitable battery module design may improve the rheological properties of the suspension and reduce the energy consumption from the pump.

## Systems

###### Historical Development.

The earlier Fe/Cr systems (1 kW prototype) were developed in 1980 by NASA Glenn Research Center (Cleveland, OH) [197]. Since then, several commercial attempts were followed [198200]. Although many pilot-scale systems were demonstrated, they were not sufficiently satisfactory for commercial systems [201]. Commercial attempts using the polysulfide-Br flow batteries were conducted by Regenesys [202]. Moreover, Exxon, Johnson Controls (Milwaukee, WI), and Meidensha developed large-scale Zn/Br flow batteries. NASA Glenn Research Center and the U.S. Electric Power Research Institute (Palo Alto, CA) installed a large-scale electric storage system using a Fe/Cr RFB [203]. Japanese industries featured prominently for the early development of RFBs. The Japanese Ministry of International Trade and Industry initiated a “Moonlight Project.” Kansai Electric Power Co., Inc. (KEPCO, Amagasaki, Japan) tested 60 kW Fe/Cr redox battery prototypes [203]. Meidensha Electric Company developed a Zn/Br battery, and Furukawa Electric Company developed a Zn/Cl battery [203]. Although previous studies can demonstrate the technical possibilities, commercialization of large-scale system was not successful, due to inherent cross-contamination. Low energy density and high primary capital cost also restricted the commercialization [103].

In contrast, VRFB systems were successfully commercialized since 1990s. The first system appeared in 1993. Thai Gypsum Products, Ltd., installed a vanadium battery system for Solar House in Thailand in cooperation with UNSW Center for Photovoltaic Devices and Systems [204]. This integrated system included solar cells of 2.2 kW and the VRFB system with 12 kWh. They developed a 47 V, 36 cell stack for long-term field trials. In 1997, Mitsubishi Chemicals (Kashima-Kita Electric Power, Tokyo, Japan) installed a 200 kW/800 kWh grid-connected vanadium battery for load-leveling. This system could run over 150 charge–discharge cycles and showed a high EE of 80% at 80–100 mA cm−2 [205]. Since the installation of a VRFB system in 1996 at Tatsumi Sub-Station for peak shaving, Sumitomo Electric Industries (SEI) have developed VRFB systems for a wide range of applications [206]. They demonstrated that overall EE of the previously mentioned systems could be maintained about 80% over 270 thousand cycles [207]. To store wind energy, in 2005 SEI installed 4 MW/6 MWh VRFB plant at Subaru Wind Farm in Japan [208]. The power of each stack is 45 kW, which consisted of 108 cells. Recently, MW-class energy storage system has emerged to meet the market needs. Although SEI demonstrated already several MWh scale VRFB systems, they seem to be impractical due to the expensive cost. For large-scale systems, the modularization and optimization are important. It is necessary to understand the system configuration from the engineering perspective.

###### System Configuration.

A VRFB system is mainly categorized by the cell stack, electrolyte storage, and the balance of plant (BOP). Electrolytes are circulated between the tanks and the stack during operation. VRFB system needs a power conditioning system and a system controller. Through the proper design of direct current (DC)/alternating current (AC) converter, the system can obtain high round-trip efficiency. In the case of system controller, there is a battery management system (BMS) to operate the system safely and to maximize the stack performance and system lifetime by reducing undesired side reactions and local depletion of the redox reactants. A sufficient quantity of electrolyte should be circulated between storage tank and stack for preventing side reactions. The upper and lower limits of the state-of-charge are approximately 80–85% and 15–20%, respectively, to avoid undesired side reactions. Furthermore, a VRFB system typically uses centrifugal pumps to prevent leaking or corrosion [209].

VRFB systems have several advantages from the engineering aspects. First, such systems have dominant feature for safety and thermal management. Flowing electrolytes can remove heat generated in the cell stack. Second, the unique system configuration can simplify the manufacturing process. Through the modularization of stacks, electrolyte containers, plumbing, and electrical systems, VRFB can reduce expensive and complicated production steps. Finally, the rebalancing process in the VRFB system is simple. For VRFB systems, it is necessary to manage the active species crossover and water transport through the membrane.

Each single cell of a VRFB stack includes several components, such as the ion-exchange membrane, electrodes, and bipolar plates. In addition, nonconductive plastic frames and sealing materials are also required. A complex manifold design is often necessary to reduce undesirable shunt currents in the ionically conductive solutions. For the development of stacks, the design of channel is important for supplying reactants uniformly. There are two methods for feeding the electrolyte along the stack: flow-by and flow-through (Fig. 16) [209,210]. In the flow-through geometry, there is no channel to flow the electrolyte in the bipolar plate. Forced-convection occurs in the porous electrode. Thus, reactants and products are easily transferred. On the other hand, in order to reduce pressure drop, relatively thick electrodes are required, which leads to large ohmic loss. The overall system efficiency is low for such configuration. In contrast, the pressure drop can be reduced when flow-by channels are placed adjacent to the porous electrodes. However, there is no forced-convection flow in the electrode. To tackle the disadvantages of flow-by channels, several types of channels have been introduced recently. A serpentine channel can reduce the thickness of the electrode without severe pressure drop and cause forced-convection [211]. Interdigitated channels can be one of the solutions for optimal channel design [212]. This configuration allows electrolyte to flow in the electrode, perpendicular to the channels, with forced-convection. The pressure drop at interdigitated channels is higher than that for flow-by type channels, but lower than that for flow-through ones.

###### Engineering Factors.

VRFB system can be optimized by focusing on the engineering factors: system scale-up, structural and operational parameters, and supervisor systems. Efforts were made to understand the accurate internal behavior by developing multiscale, multidimensional, multiphysical, steady-state and dynamic models. Some models were used to show the transient behavior of VRFB system [213]. Some sophisticated models were also developed to examine the changes of flow field for different channel designs, aiming at supplying the reactant uniformly over the electrode areas with a low-pressure drop and reducing shunt currents by advanced bipolar plates [214]. Further studies for channel design are needed to reduce pumping power. Furthermore, computational studies are required to solve the problems related to cell elements, nonlinear material behaviors, and optimal geometries. Modeling can also help to optimize the operation parameters of the system [215].

The flow rate is also a significant factor affecting the performance of the VRFB system. By increasing the flow rate stepwise during operation, the EE of stack can be increased and the power consumption of pump can be decreased [216]. Capacity loss can be decreased by controlling operating parameters. Capacity loss may be recovered by periodic mixing of the electrolyte. When the battery is off, stopping the pump is a simple solution for reducing the capacity loss. The species crossover can be decreased by controlling the hydraulic pressure [216]. Increasing the current density may be a solution to reduce the crossover, because it can shorten the charge and discharge time. Increasing current and power density may reduce stack size and overall system costs [217,218]. To accelerate commercialization of VRFB systems, more engineering efforts are necessary.

## Conclusions

RFBs are promising and competitive systems for stationary energy storage applications for the future. To promote the market penetration of VRFBs, the performance of membranes and electrodes needs to be further improved. Rational cell/stack design and system management can lead to a reduction in the construction and operation costs. RFBs with high energy and power densities can be obtained by developing new active redox couples with high concentration, high cell voltage, and fast reaction kinetics. Recent development in active organic molecules opens new opportunities for sustainable energy storage strategy. Fundamental understanding for the physiochemical and electrochemical properties of the electrolytes is needed. In addition, finding compatible membranes, electrodes, and supporting salts is critical for successful applications. The use of energy-dense solid suspension electrolytes for RFBs may largely enhance the energy density of the system and make it possible for their use in mobile applications.

## Acknowledgements

We gratefully acknowledge the support from the basic research funding of KIST Europe. D.H. thanks the support from the German-Korean joint SME R&D projects program of ZIM-AIF and MOTIE/KIAT. Z.H. gratefully acknowledges the scholarship from China Scholarship Council (CSC). D.K. thanks the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A6A3A03007749). R.C. thanks Professor R. Hempelmann for his continuing support.

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