Ionic Liquid and Ionanofluid-Based Redox Flow Batteries—A Mini Review

Abstract

Stationary energy storage methods such as flow batteries are one of the best options to integrate with smart power grids. Though electrochemical energy storage using flow battery technologies has been successfully demonstrated since the 1970s, the introduction of ionic liquids into the field of energy storage introduces new dimensions in this field. This reliable energy storage technology can provide significantly more flexibility when incorporated with the synergic effects of ionic liquids. This mini-review enumerates the present trends in redox flow battery designs and the use of ionic liquids as electrolytes, membranes, redox couples, etc. explored in these designs. This review specifically intends to provide an overview of the research prospects of ionic liquids for redox flow batteries (RFB).

1. Introduction

The globally growing demand for energy is now one of the major issues facing humanity. Renewable energy sources such as solar and wind are intermittent, which therefore creates the need for new and improved large-scale stationary energy storage systems to be integrated with smart grid facilities. The current state-of-the-art (SoA) technologies for energy storage are becoming inadequate due to the high demand, which is predicted to increase significantly with time. Energy storage systems such as solid state batteries, super capacitors, flow batteries, flywheels, compressed-air storage, pumped storage, thermal energy storage, etc. are being explored prudently day by day. One reason for this is the awareness created about the necessity of creating a fossil-fuel free society for a sustainable ecosystem. Clean energy batteries are definitely one of the answers to the demand for a safe and a low-cost substitute to carbon-based fuels. Therefore, the current rate of exploration of greener alternatives for energy generation and storage with the help of all these technologies is very high. The contributions to the energy industry by the 2019 chemistry noble laurates John Goodenough, Stanley M. Whittingham, and Akira Yoshino through the development of lithium-ion batteries has transfigured the way of life since 1990. This battery technology, which replaced nickel-cadmium batteries ahead of time, is based on lithium ions, which also possess limitations. The limited availability of lithium and its safety limitations prompts us to find better alternatives. Ever since their invention, new research has been triggered by the inevitable need for superior lightweight, longer-duration, and hardwearing batteries for energy storage.

The high maintenance cost, safety limitations, limited raw material availability, diminishing effectiveness, toxicity, and non-recyclable nature of conventional energy storage systems demands more efficient alternatives. Among rechargeable electrochemical energy storage systems, flow batteries hold a very significant place, offering an alternative for quick recharging. In the future, they might also overcome the market dominance of high-cost Li-ion batteries (LIB) used in portable devices and grids. Large-scale grid storage requires long life time–low cost batteries that have scalability, calendar life, and round-trip efficiency. Previously, in 2021, 0.25 GWh of RFBs could only be used for stationary energy storage compared to 8.8 GWh of LIB [1]. Presently, there are numerous business enterprises and venture capital firms investing in the promising flow battery technology.

Redox flow batteries (RFBs) with soluble organic and inorganic flow species, semi-solid lithium-based solid suspensions, etc. have been developed and some of them are being commercialized now. The first generation RFBs do not have a high enough volumetric energy density for large-scale stationary energy storage and transportation applications. However, new approaches to battery chemistries such as using ionic liquids and polyionic liquid membranes provide more fertile ground for scientific exploration to revolutionize the practice of energy storage. There are many reviews published on redox flow battery aspects, the performance of various RFB systems, and the current progress in research [1,2,3,4,5,6,7]. Here, we have specifically discussed the effective contributions of ionic liquids in improving the efficiency of present redox flow battery technology.

Redox flow batteries are now among the most promising technologies for energy storage. An industrial race to commercialize this technology is evidently noticeable. The first known vanadium-based RFB technology, introduced in 1980 is currently the most researched one [8]. Vanadium-based and bromine-based chemistries are gaining noticeable commercial footing. Vanadium-based systems account for about 75 MWh of deployed systems. However, this type of flow battery possesses certain limitations such as short life time, shortage of vanadium-based salts, toxicity, poor power density, expensiveness of ion-selective membranes, etc. [9,10]. Competing with the fast-moving targets in Li-ion and other next-generation storage technologies was nearly out of the question for conventional vanadium-based RFBs. The UNSW all-vanadium redox flow battery (VRFB) has previously attracted substantial commercial interest despite its relatively low energy density (max. 40 W h L${}^{-1}$) [11] when compared to Li-ion batteries (∼700 W h L${}^{-1}$ ) [12]. Their exceptional cycle life, reasonable cost, and energy efficiency for storage capacities greater than 4 h is admirable [13,14]. A few major limitations of RFBs in general are the following:

• Increased system complexity;
• Low power and energy densities;
• High expensiveness;
• Complexity of handling corrosive redox active species and electrolytes.

Current research is focused on not only improving the energy density and current density, but also the inexpensiveness, stability, and membrane selectivity of redox flow batteries. Consequently, the VRFBs and zinc–bromine redox flow batteries (ZBFBs), which were once considered as the current SoA technology, are being replaced by a group of innovative RFB technologies preparing for market launch.

The current trends in RFB research evolution involve an overlap from other energy storage systems as well. In the recent years, many categories of RFB, such as membraneless RFBs, AORFBs, MAFBs, SSFB, SRFBs, SMFB etc., have been developed [1]. These technologies focus on reducing cost and improving energy density. A diagram showing the timeline of the evolution of numerous flow battery systems after the VRFB is shown in Figure 1 below.

There are numerous flow battery designs presently used. A detailed conceptual figure of different flow battery designs [15], such as (1) conventional redox flow batteries that use liquid phase redox electrolytes, (2) a hybrid redox flow battery with gas supply at one electrode such as the metal–air flow battery (MAFB), (3) membraneless flow batteries with no electrolyte separation, and (4) a redox flow battery with solid particle suspension as flowing media, is presented in Figure 2. The main attraction of these RFBs is that an apt combination of a perfect flow battery design and a suitable chemical redox system can give a specific charge density greater than solid state batteries. The ability to decouple power and energy is also a notable advantage in some of these systems [9]. However, in some membraneless redox flow batteries with solid active species such as Li, Cd, Zn, graphite, etc., the advantage of decoupling energy and power is unattainable [16,17]. Another difficulty in large-scale commercialization of this technology is the high expense of ion-selective membranes. A membraneless large-scale redox flow battery systems would be rather difficult to optimize since it requires the wise modulation of fluidodynamics of the electrolytes without causing mixing of the electrolytes [17].

Recently, a new group of materials with complex thermophysical and electrical properties gained the interest of scientists: suspensions of nanoparticles in ionic liquids—ionanofluids. One can find some review papers summarizing the physical properties of these materials [18,19,20,21,22] and their potential applications in renewable energy systems [23,24,25,26].

The goal of this review paper is to summarize the latest findings in the field of application of ionic liquids and ionanofluids in energy storage systems with redox flow batteries.

2. Working Mechanism of a Conventional Redox Flow Batteries

Before discussing the different roles of ionic liquids in flow batteries, we shall look at the components and mechanism of working of RFBs. As shown in Figure 3, conventional types of RFB systems have the following basic components: electrodes, bipolar plates to avoid direct contact between the electrolyte and current collectors, membranes, and two external tanks, a negative electrolyte tank with anolyte and a positive electrolyte tank with catholyte. The anodic and cathodic electroactive species are dissolved in electrolytes that are stored in these two external tanks. The electrical energy is stored directly in these flowing liquid electrolytes that undergo oxidation and reduction during the charging and discharging processes [10]. Organic redox molecules such as quinones, pyridines, phenothiazine, viologens etc. in acid and alkaline electrolytes are also very actively researched materials for this system [17,27,28,29,30].

The RFB performance is evaluated in terms of the voltage efficiency, coulombic efficiency, and energy efficiency. The energy-bearing complexes are stored in external tanks and not inside the electrode compartment. Therefore, the tailoring and optimization of the system is independent and more effective [2]. It also has a quick response time. The total energy output is influenced by the volume of the electrolyte tanks. High-solubility of anolytes and catholytes will give high volumetric energy densities. The theoretical energy density of RFB is proportional to three important factors: (a) the number of transferred electrons (n), (b) the concentrations of the electrolytes, and (c) the potential difference of the two half cells ($\mathsf{\Delta }$ E). The electrode reactions possess the least mechanical stresses from the inclusion or exclusion of the electroactive species. The active area inside the stack determines the total power output. Thus, the areal power density is an important factor linked to the capital cost.

The thermal and electrochemical stability of the electrolyte mixture used in RBF systems is very important. When the low operation voltage of conventional aqueous-based RFBs resulting from the insufficient electrochemical window of water limits its application, new kinds of ILs mixed with organic solvents offer greater opportunities. In the present day, curiosities in ionic-liquid applied RBF technologies have surfaced. To mention one among the many efforts targeted at support increasing demands for stationary energy storage, a research team at Sandia National Laboratories, USA have been developing flow cell designs using a series of ionic-liquid electrolytes with compatible non-aqueous membranes for improved energy density RFBs [32]. Nevertheless, the chemistries of these IL electrolytes need more research in order to improve the performance of RFBs based on them. Most of the ILs are inflammable and basically non-volatile in nature. RFB electrolytes based on an organic solvent–IL mixture have been observed to support high cell voltage, and certain H₂O-IL electrolyte have been reported to exhibit stability against water hydrolysis [33,34,35]. Both these factors are significant. Besides, several other challenges such as low solubility and sluggish mass transport of electroactive species in organic solvent/ionic liquid mixture, high viscosity, and low conductivity of most of the ILs, evaporation loss and inflammable nature of organic solvents etc. needs to be addressed in this scenario.

The use of mixed halide [36] as well as HCl-H₂SO₄-supporting electrolytes [11] to increase the energy density of the vanadium-based RBFs for purpose of large-scale energy storage has previously been reported. However, the presence of compounds such as HCl or HBr in the electrolyte offers potential safety risks, causing unwanted product formation during overcharging or overheating [13]. This might restrict the operating efficiency of the batteries, unlike in RFB systems that use acidic and aqueous electrolytes. This will allow the use of less robust materials to design the pump, tank, and channels.

For a highly efficient charge–discharge process, the electrochemical activity at the electrode surface needs to be significantly improved. Ionic liquids are successfully used even for the surface treatment of electrodes because they are rich in nitrogen content. Yoon et al. has reported the preparation of nitrogen-doped graphite felt electrodes for RFBs using 1-ethyl-3-methylimidazolium dicyanamide (EMIM dca) [37]. Since EMIM dca has high nitrogen content, it was used as an N doping precursor that was coated over the graphite felt electrodes followed by thermal treatment. This subsequently improved the electrocatalytic activity of vanadium redox reactions.

3. Ionic Liquid Electrolytes

Ionic liquids can definitely improve the energy density of RFBs by providing a wide electrochemical window, thereby increasing $\mathsf{\Delta }$ E [16]. They can also offer higher cell voltage and stability against hydrolysis. A few reports on the advantages of using ILs as electrolytes are discussed here. Periyapperuma et al. first demonstrated the use of an ionic liquid electrolyte 1-ethyl-3-methylimidazolium dicyanamide, [Emim][dca], under a realistic flow environment for applications in redox-flow batteries [38]. This was based on the Zn${}^{2+}$/Zn${}^0$ redox couple in which an electrolyte mixture containing Zn(dca)${}_2$ and 3 wt% H₂O in [Emim][dca] was used. They studied the influence of Zn${}^{2+}$ concentration and the flow rate on the Zn${}^{2+}$/Zn${}^0$ electrochemical performance. The electrolyte mixture with a higher Zn concentration of about 18 mol% showed higher discharge current density of ∼100 mA/cm${}^2$. At this concentration, the Zn electrodeposition potentials was more positive (−1.33 V vs. Ag/AgOTf) and the cycling efficiency was higher (45 ± 3%) compared to the values observed at a lower Zn concentration of 9 mol% (−1.44 V vs. Ag/AgOTf and 33 ± 3% cycling efficiency).

Research is also being carried out using room temperature ionic liquids (RTILs) as solvents for RFBs containing metal complexes. Bahadoori et al. reported single-component non-aqueous redox flow batteries (NARFBs) using vanadium (III) acetylacetonate (V(acac)${}_3$) in two ILs, namely, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, [C4mpyr][NTF${}_2$] and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C4mim][NTF${}_2$] [39]. A cell potential of 2.2 V was observed in both ILs. The cyclic voltammetry showed a quasi-reversible nature for the V${}^{+2}$/V${}^{+3}$ and V${}^{+3}$/V${}^{+4}$ redox couples. They also observed that the V(acac)₃/[C₄mpyr][NTF₂] cell exhibited a coulombic efficiency of 88–92%.

Ejigu et al. studied the electrochemical properties of the metal acetylacetonate (acac) complexes Mn(acac)₃, Cr(acac)₃, and V(acac)₃ in imidazolium–based RTILs [33]. V²⁺/V³⁺, V³⁺/V⁴⁺, and V⁴⁺/V⁵⁺ redox couples were quasi-reversible in 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl) imide, [C₂C₁Im][N(Tf₂)] with glassy carbon (GC) electrodes, whereas, voltametry measurements with Mn(acac)₃ and Cr(acac)₃ exhibited an irreversible nature of [C₂C₁Im][N(Tf₂)]. However, when using Au electrodes, the rate of the Mn²⁺/Mn³⁺ reaction improved. They have reported to achieve a coulombic efficiency of 72% in the V(acac)₃/[C₂C₁Im][N(Tf₂)]/GC cell. In another attempt, Miller et al. reported the synthesis of iron ionic liquid electrolytes containing up to 6.3 M iron from iron chloride, choline chloride, and ethylene glycol for use in non-aqueous RFBs [40]. The iron electroplating efficiency and reaction kinetics were highly influenced by the molar ratio of iron chloride, choline chloride, and ethylene glycol, with the highest observed at a ratio of 1:1:4.

Takechi et al. showed that “solvate ionic liquid” can be used as catholyte for rechargeable flow batteries [41]. For this, a redox-active supercooled liquid was prepared from a mixture of a stabilized organic radical and a Li salt. This mixture was stabilized below its melting points. They reported that this liquid exhibited a high energy density of 200 W h L${}^{-1}$. Furthermore, the addition of water enhanced the electrochemical advantage and maintained its supercooled nature.

4. Ionic Liquid Redox Couples

New strategies such as replacing vanadium compounds with less expensive and eco-friendly organic compounds with redox properties are also being explored [38]. Drawing inspiration from nature itself, Modrzynski and Burger proposed an RFB based on imidazolium IL and iron-sulfur clusters [Fe₄S₄(SR)₄]${}^{2-}$, the structure and synthesis of which is shown in Figure 4 [16]. They used the positive electrolyte as a bromide/bromine redox-couple in an IL solution in the second half cell. This system demonstrated a high coulombic efficiency (>95%) and energy efficiency (∼69%), along with a high theoretical energy density (88 W h L${}^{-1}$). This energy density was significantly higher than that of the aqueous all–vanadium RFBs. Another report on an ionic liquid with the redox active 2, 2, 6, 6–tetramethyl 1–piperidinyloxy moiety was recently reported by Zhang et al. [42]. This redox active IL can perform multiple functions as a redox mediator, oxygen shuttle, lithium anode protector, and an electrolyte solvent.

5. Metal Ionic Liquids (MetILs) as Solvents and Electrolytes

Another approach is using metal ionic liquids (MetILs) containing a low cost first row transition metal cation and weakly pairing anion(s) in RFBs. With MetILs, greater active metal concentration can be attained. The research group at the Sandia National Laboratories have developed a new family of such redox-active ionic liquids for flow batteries. Small et al. proposed a similar method for maximizing the energy density in IL-based non-aqueous RFB electrolytes [43]. They showed that substituting ferrocene-containing ligands and iodide anions into an IL with a metal coordinated cation to form an Fe(EA)${}_{6-x}$(FcEA)${}_x$(OTf)${}_{2-y}$I${}_y$-type metal ionic liquid (MetIL) enhanced the capacity by about four times. The metal coordination cation consisted of a transition metal ion such as iron, copper, or manganese. The redox-active ligand was a transition metal such as tris(2,2${}^{\prime }$–bipyridine)nickel(II) or tris(2,2${}^{\prime }$-bipyridine)iron(II). “MetILs” exhibited redox activity in the cation core, anions, and the ligands. Similarly, Schaltin et al. reported the first use of an all-copper liquid metal salt, [Cu(MeCN)${}_4$][Tf₂N], in an RFB [37]. This Cu-based MetIL had high Cu concentration and could perform the roles of both electrolyte and solvent. The redox couple formed was Cu⁺/Cu and Cu²⁺/Cu⁺, separated by 0.9 V. Therefore, it could act as a good electroactive species in RFBs. The charge density attained was 300 kC L${}^{-1}$. They also reported an energy density up to 75 W h L${}^{-1}$ along with 85% coulombic efficiency.

6. Membranes

The performance of ion-exchange membranes (IEM) is crucial to determining the efficiency of any VRFB. Features such as negligible vanadium ion permeability, excellent proton conductivity, and low water permeability are expected characteristics of IEM. Recent reports show that incorporating ionic liquids to manufacture ion-exchange membranes for VRFBs helps to attain a high degree of proton selectivity. Proton selectivity is an essential requirement for membranes used in VRFBs. Song et al. showed that 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF₄) when used as a composite with poly (oxyphenylene benzimidazole) (OPBI) in membranes assisted proton transfer more effectively [44]. It also helped to reduce vanadium ion crossover through the membrane. Ionic liquids can be used to overcome many limitations of traditional membrane modification methods. Both the requirements of IEM—the increase in proton conductivity and decrease in vanadium ion permeability—can be achieved concurrently with the help of ILs. Besides, the battery efficiency after many cycles test was also not affected much. Since most of the IL anions can form hydrogen bonds with the polymer substrates used in the membrane, they can function as hopping sites for proton transport. The OPBI/BF₄ composite membrane reported by Song et al. had a proton selectivity of 1.41 × 10⁶ S min cm${}^{-3}$, higher than the unmodified OPBI membrane (6.06 × 10⁵ S min cm${}^{-3}$) and Nafion 115 membrane (1.61 × 10⁴ S min cm${}^{-3}$).

Nafion is generally preferred as a membrane material for batteries. In VRFBs as well, Nafion has been used after physical and chemical modification for property optimization. However, efforts are also been undertaken to replace it with less expensive materials. VRFB membranes must have these ideal characteristics: (1) to limit self-discharge, the permeation rates of vanadium ions and H₂O should be very low; (2) they should contain excellent proton conductivity; (3) ion conductivity must be high for the transport of the charge-carrying ions; (4) the area resistance must be low; (5) the chemical and operational stability must be very high; (6) they must be economical [45]. A report by Zhou et al. demonstrated the preparation of a porous asymmetric sulfonated poly(ether sulfone) (SPES) membrane via ionic-liquid-induced phase separation [46]. The ionic liquid affected the porous structure of the membrane. It has been stated that the vanadium ion permeability of the porous SPES membranes was lower than that of the commercially available Nafion 212 membrane. A VO²⁺ permeability of 1.41 × 10${}^{-8}$ cm² min${}^{-1}$ and an energy efficiency of 84.13% at 100 mAcm${}^{-2}$ were attained along with good cycling stability. Most of the commercially available machinery for RFBs is designed to be applied in aqueous solvents. However, it was observed that nonaqueous solvents gave higher operating voltages compared to their aqueous counterparts. In this context, Hudak et al. previously studied the conductivities of commercial anion exchange membranes such as Neosepta AFX, Neosepta AHA, Fumasep FAP-450 and Fumasep FAP-PK in non-aqueous solvents, including a few ionic liquids [47]. They soaked these anion exchange membranes in imidazolium-based ionic liquids and observed that their conductivities were higher than those of the microporous Celgard 2400 and Celgard 2500 separators.

Apart from the above discussed functions of ILs in membrane-based RFBs, it is also important to mention their applicability in membraneless RFB systems. As previously mentioned, membrane-free battery systems, if developed, would be remarkable. They have several advantages including cost reduction. Such a proof of concept was previously reported by Navalpotro et al. using a biphasic system of an acidic solution and an ionic liquid [17]. They have used an acidic solution of hydroquinone and a hydrophobic IL, namely, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) with dissolved parabenzoquinone. This mixture formed a liquid–liquid biphasic system that behaved as a reversible battery. It demonstrated an open circuit voltage of 1.4 V with a high theoretical energy density of 22.5 W h L${}^{-1}$ without any membrane or separators. Another similar report of a membraneless RFB system was given by Bamgbopa et al. [48]. They used an all-Fe redox system in which the aqueous anolyte was composed of iron(II) sulfate species and the catholyte was composed of iron(III) acetylacetonate dissolved in a water–ethyl acetate immiscible medium, supported by 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). Even after 25 cycles in a flow cell, they could sustain a columbic efficiency of around 80%.

7. Poly(ionic liquids)

Redox active polymers (RAPs) have been explored since 1950s as energy storage materials. Poly(ionic liquids) or PILs, very promising materials prepared by incorporating ionic liquid functionalities over polymer backbones, could be used as RAPs. These materials possess improved mechanical strength, electrical conductance, thermal stability, and ease of maintenance over ionic liquids. PILs with redox functionalities incorporate electro active groups, which enables oxidation–reduction processes. Such materials are being widely explored for electrochemical energy storage technologies. Furthermore, nanoporous PILs possess special advantages such as tunable porosity and active surface sites, which enable them to be used as membranes in RFBs. In these advanced PIL membranes, the advantages of anion-exchange membranes and porous membranes can be combined together. Zhao et al. has previously reported the design of a charged sponge-like membrane that incorporated positively charged imidazole groups on the pore walls of sponge-like poly(ether sulfone) (PES) porous membranes [49]. The energy efficiency and coulombic efficiency of a single cell assembly prepared using this porous membrane was about 86% and 99%. This was higher than Nafion 115 (82.3% and 93.5%). The ion conductivity of these membranes is usually explained using two mechanisms. One mechanism is selective “ion diffusion” through pore size exclusion. The other mechanism the “Grotthuss mechanism” from anion-exchanged groups. The common types of cations and anions used in PILs are shown in Figure 5 below.

The cationic part is grafted on to the polymer backbone and is not free to hop between these chains. However, the counteranions held to these cations by electrostatic forces provide hydrogen bonding sites for solvents such as water molecules. Developing innovative PILs such as these will be helpful in improving the performance of next-generation RFBs that might, in the future, match lithium-ion technologies. More elaborated reviews on polyionic liquid composites can be obtained from Refs. [50,51,52]. In addition to the conventional type of PILs shown in Figure 5, the insertion of redox-active counter-anions such as anthraquinone (AQ) and nitroxide 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-based derivatives into a poly(diallyldimethylammonium) (PDADMA)-type polyionic liquid has been reported by Hernández et al. [53]. Apart from the usual electroactive materials in which the redox active groups are covalently bonded to the polymer chains, here they used an electrostatic strategy, which enables ion exchange reactions with anionic redox-active molecules. Using N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI), the electrochemical properties of the copolymers of polyionic liquids were also studied by this group. The current density rises with the increase in the amount of redox-active counteranions in the polyionic liquid. Such materials are promising candidates that can be implemented in organic redox flow batteries.

8. Ionic Liquid-Based Nanofluids

Solid suspension flow batteries that incorporate the advantages of both flow batteries as well as solid-electrode batteries are highly attractive. These systems are capable of large-scale energy storage along with added benefits such as long life span, high energy density, and low capital cost. Rechargeable nanofluid-based flow battery technology is a transformational advancement of conventional redox flow battery technologies in which energy is stored and released through a reversible redox reaction in nanoparticle suspensions. There have been limited reports on studying the feasibility of ionic liquid-based nanofluids (ionanofluids) in FB technology. Issues such as high price and oddities in the chemical nature of ionic liquids add to the unpredictable future of ionanofluids in flow batteries. However, previous research indicates the possibility of a gain in energy density when using nanofluids. In 2015, Jia et al. reported that by using LiFePO₄ and TiO₂ as the cathodic and anodic Li storage materials, the tank energy density of the redox flow lithium battery can be increased ∼500 W h L${}^{-1}$, i.e., about 10 times higher than that of a vanadium redox flow battery [54]. New research indicates that use of stable dispersions of solid electroactive nanoparticles in the liquid electrolyte allows for the increase of the energy density in flow batteries by up to 30 times [55,56].

It is well known that the allotropic variety of carbon-graphene has received great interest in the energy storage field [57,58,59]. Mubeen et al. previously reported a non-Li-based flow battery type that uses solid suspensions [60]. Rather than redox-active molecules, they used a hydrophilic carbon particle suspension coated with earth-abundant redox-active metals that acted as electrodes. During the charging process, the earth-abundant redox-active metals are deposited over the carbon particle suspension. During the discharge process, these deposited redox-active metals are stripped off. The direct plating of metals on the redox-inert hydrophobic carbon current collectors was avoided due to its hydrophobic surface. They could also successfully apply this “contact charge transfer mechanism” along with Zn–Cu, Zn–MnO, Zn–Br, and Zn–S battery chemistries.

Lobato et al. explored the behavior of graphene nanoparticles dispersed in vanadium electrolytes in redox flow batteries, and it turned out that it improves the performance of energy storage as well as durability [61]. The use of graphene nanoparticles in the electrolyte contributed to a greater than 14% increment in charge capacity. In another work by Aberoumand et al., reduced graphene oxide (rGO) nanofluidic vanadium electrolyte was used. They observed that 0.4 wt.% nanofluid electrolyte had a positive effect on the electrochemical activity of the electrode. It contributed to the enhancement in performance of both the electrode and the electrolyte. Research on a different form of magnetic-modified multiwalled carbon nanotube (MMWCNTs) was carried out, where it was observed that adding MMWCNTs to the positive electrolyte at the mass concentration of 0.3 g L${}^{-1}$ had effect on enhanced performance of the polysulfide-iodide redox flow; battery-power density is enlarged by 45% and an energy efficiency of 79.91% was reached [62]. A simplified drawing showing the preparation of the magnetic nanofluidic electrolyte using the two-step method, as well as the schematic redox flow battery test system are depicted in Figure 6.

While choosing the type of nanofluid in redox flow batteries, the nanoparticle content should also be taken into account. Carbon-based vanadium nanofluids with different concentrations (0, 0.02, 0.04, and 0.08 wt.%) were used as electrolytes in redox flow batteries by Lobato et al. [63]. The outcome indicated that not only did the greatest nanocarbon particle content have the lowest ohmic resistance, but also that all used nanofluids had stable behavior. Furthermore, it has been found that the addition of nanocarbon particles into the commercial vanadium electrolyte increased the active surface area where the redox reactions occurs. This thus increased the redox flow battery performance.

In can be observed that recent years, the science world has put emphasis on the subject of energy storage, including attempts to extract it from waste through recycling, or to improve particular elements in technological process. Research and reflection connected to electrochemical energy storage as also been emphasized. Therefore, intensive development of the electronics industry to solve the problem by improving batteries has become an important issue. The batteries produced so far (for example nickel-metal hydride, nickel-cadmium, or lithium-ion batteries) have their limitations due to the presence of salts, most often in flammable organic solvents in electrolytes, which compromise occupational safety and which also did not accumulate significant amounts of energy.

Ionic liquids, due to their outstanding advantages [64,65], have great potential due to the fact that they are non-volatile and non-flammable, which translates into increased safety of battery operation [66]. Research points out that ionic liquid-based nanofluids are potential candidates for implementation in RFBs and would be beneficial due to high thermal and electrochemical stability, relatively low viscosity, and relevant charge–discharge efficiency [67]. The implementation of ionic liquids is expanding, i.e., it can be carried out at the battery element, such as in membrane. Deb et al., in [68], synthesized a polymer membrane based on hexafluoropropylene copolymer (P(VDF-HFP)). The preparation included ensnaring a different extent of pyrrolidinium ionic liquid-based nanofluid. Such a procedure contributed to an amelioration in the electroactive phase nucleation of P(VDF-HFP) and, more importantly—simplified ion-conducting channels within the membranes.

The present researches focus on developing high-solubility redox couples in aqueous systems using less expensive materials. Aqueous systems possess high power density, better safety, and environmental compatibility [66]. The future prospects of aqueous ionanofluids with electroactive nanoparticles cannot be ruled out, since the presence of ionic liquids offers added advantages. We have previously reported the synthesis and electrochemical characterization of iron oxide-based ionanofluid from ferrous sulphate [67]. This ionanofluid was prepared using 1-butyl-4-methylpyridinium chloride and showed high dielectric constant, high aqueous stability, and low viscosity. It exhibited pseudo capacitance redox curves and charge–discharge efficiency greater than 94% even after 100 cycles. The challenges in using ionanofluids in FB technology are numerous. The appropriate data, including simulations and theoretical models (with particular emphasis on the dependence of the viscosity, which can inhibit the mobility of the ions, and ionic conductivity values) is crucial for their potential implementation in power devices. The attention should be also focused on the concentration of dispersed nanoparticles in ionic liquids, due to the fact that high content of nanoparticles may deteriorate electrical conductivity. This phenomenon was noted in the research performed by Cherecheş et al. [69], where 1-ethyl-3-methylimidazolium methanesulfonate ([C₂mim][CH₃SO₃]) ionic liquid and its ionanofluids with Al₂O₃ nanoparticles in different mass concentrations (0.05–10 wt.%) were experimentally investigated, with the selection of suitable ionic liquids that are water stable and less expensive. Besides, maintaining high power output, low cell resistance, low viscosity, high physicochemical and thermal stability, etc. are the other concerns. More research is needed in this direction to explore the possibilities. Most importantly, the developed composition on ionanofluids that are to be used in RFBs should not exhibit any environmental toxicity and should be reusable.

9. Conclusions

As a summary, the growing urge for global decarbonization to mitigate climate changes demands an assortment of battery technologies. The solutions for numerous challenges involved in this are continuously being resolved by research teams around the world. Redox flow batteries hold several advantages when compared to conventional rechargeable batteries such as Li-ion and lead-acid batteries. The cost–effective approach put forward by these flow technologies for large-scale energy storage is highly promising especially with the advent of ionic liquids. The emerging roles of ionic liquids in improving the efficiency of present SoA RFBs are very diverse. In this review, we have discussed the different roles of ionic liquids as solvents, electrolytes, redox couples, membranes, electrode modification materials, etc. in RFBs. An effort has been made to enumerate the challenges and future prospects of ionic liquids and ionic liquid-based materials for applications in redox flow batteries. As a conclusion, ILs has already been identified as a potential material for battery applications. Its significant role as a substitute to reduce metal toxicity and increasing recyclability is also appreciable. There are immense possibilities for these materials to be used in next generation RFBs for enhancing stationary energy storage capacity, and using ionanofluids brings even more opportunities.