Aqueous titanium redox flow batteries—State-of-the-art and future potential

Market-driven deployment of inexpensive (but intermittent) renewable energy sources, such as wind and solar, in the electric power grid necessitates grid-stabilization through energy storage systems Redox flow batteries (RFBs), with their rated power and energy decoupled (resulting in a sub-linear scaling of cost), are an inexpensive solution for the efficient electrochemical storage of large amounts of electrical energy. Titanium-based RFBs, first developed by NASA in the 1970s, are an interesting albeit less examined chemistry and are the focus of the present review. Ti, constituting 0.6% of the Earth’s crust and an ingredient in inexpensive white paints, is amongst the few elements (V and Mn being some others) which exhibit multiple soluble oxidation states in aqueous electrolytes. Further, the very high (approaching 10 M) solubility of Ti in low pH solutions suggests the possibility of developing exceptionally high energy density aqueous Redox Flow Batteries systems. With these advantages in mind, we present the state-of the-art in Ti-RFBs with a focus on Ti/Mn, Ti/Fe and Ti/Ce couples and systems that use Ti as an additive (such as Ti/V/Mn). The inherent advantages of inexpensive Ti actives and relatively high energy density is contrasted with potential side-reactions resulting in reduced energy efficiency. Technological pathways are presented with a view to overcoming critical bottlenecks and a vision is presented for the future development of Ti-RFBs.

1 Introduction

The rapid, market-driven deployment of economical but intermittent renewable energy sources such as solar and wind necessitates the integration of reliable energy storage solutions with the electric grid to ensure grid stability and reliability. Amongst various energy storage technologies redox flow batteries (RFBs) are an economical solution at scale due to their characteristic decoupling of energy and power that ensures sublinear scaling of cost (Chen et al., 2009; Zhao et al., 2015). A plethora of possible RFBs have been investigated and proposed in the literature, such as, Fe-X (X = Cr, Mn, Fe, Zn) (Fedkiw and Watts, 1984; Skyllas-Kazacos et al., 2011; Gong et al., 2016; Selverston et al., 2017; Archana et al., 2020; Zhen et al., 2020), V-X (X = Mn, Ce, Br, V) (Chen et al., 2009; Prifti et al., 2012; Cunha et al., 2015; Zhao et al., 2015; Sankarasubramanian et al., 2019; Reynard et al., 2020; Raja et al., 2021; Wang et al., 2021) and Zn-X (X = Ce, Br, Mn, V) (Chen et al., 2009; Leung et al., 2011; Dewage et al., 2015; Zhao et al., 2015; Jiang et al., 2018; Ulaganathan et al., 2019; Naresh et al., 2021) RFBs. Critically, the translation of these RFBs to the market hinges on numerous factors, namely - 1) cell potential, 2) energy density (a function of salt solubility in the electrolyte), 3) chemical and electrochemical stability of the cell components, and finally (and possibly most importantly) 4) availability of the redox active species at low marginal cost and at scale. The energy storage cost of RFBs hinges on the cost of the electrolyte actives and their degradation and loss during operation. The loss of electrolyte due to crossover results in poor coulombic efficiency at the system level and hinders economical operation. The crossover of electrolyte species is largely determined by the nature of the separator employed. Three classes of separators are typically encountered in RFBs i.e., cation exchange membrane (CEM), anion exchange membrane (AEM), and porous membrane (PM) (Varcoe et al., 2014; Barry et al., 2021). The two classes of ion exchange membranes operate on the principle of charge-based repulsion and hence exclusion of redox active species. Selectivity is conferred by the nature of the charged species attached to the separator backbone and density of these species. Porous separators, on the other hand, rely on size-based exclusion of redox active species. Here, ionic species and chemical species in solution (irrespective of charge) are prevented from crossing over on the basis of their size relative to the pores across the separator. CEMs (typically Nafion®) are relatively expensive and exhibit high ionic conductivity. Given that most RFBs utilize cationic redox species, the use of CEMs in these systems results in cation cross-over and hence a drastic reduction in capacity over time (Gubler, 2019). This makes CEMs suitable only for systems employing catholytes and anolytes consisting of different oxidation states of the same chemical species (e.g., V²⁺/V³⁺ and V⁴⁺/V⁵⁺ in all-V RFBs). AEMs, on the other hand, mitigate cation crossover but typically exhibit lower ionic conductivity and chemical stability compared to CEMs (Barry et al., 2021). PMs allow the cross-over of the ions that have smaller diameter than the pore size of the separator irrespective of the nature of the charge they carry and hence show poor ion selectivity (Lu et al., 2017). All separators may require mechanical rebalancing to adjust the osmotic pressure (Bhattarai et al., 2019) and chemical rebalancing to maintain electrolyte purity (Wu et al., 1983; Fedkiw and Watts, 1984).

All-V RFBs are the farthest along the commercialization route and have been reported to operate at typical power densities of 100 mW cm⁻² with cycle life of 10–15 years with 1000 cycle per year (Holland-Cunz et al., 2018). A recent study has reported small, lab-scale (4cm² electrode area, 20 ml catholyte and anolyte) all-V RFB operating for ∼20,000 cycles at 600 mA cm⁻² (>8 months) (Jiang et al., 2020). Despite concerns stemming from component degradation while operating at such current densities, the demonstration of similar cycle life (even at lower current density) at a practical, pilot scale would be a major advancement in commercializing all-V RFBs. Although the V-X family of RFBs are quite successful in terms of providing high power densities with stability, the availability of V in the earth’s crust and its susceptibility to degradation during cycling is a limiting factor for successful industrialization. On the other hand, the Ti-X (X = Fe, Mn, Ce) family of RFBs offer several advantages over the V-X systems as -

1) Ti is ca 50x as abundant as V in the Earth’s crust and is produced at ca 100x the rate of V (Figures 1A,B).

2) The market price of Ti is 1/10^th that of V in the US (Figures 1A,B).

3) The half-cell potential of Ti⁴⁺/Ti³⁺ redox couple is 0.1 V (vs SHE) as compared to -0.26 V (vs SHE) for V³⁺/V²⁺ which makes Ti⁴⁺/Ti³⁺ redox couple less prone to hydrogen evolution side reactions (Figure 1C).

4) The maximum possible storage capacity of Ti-Ce RFBs would be 9.9 TWh as compared to 6.95 TWh for all-V RFBs considering all exploitable worldwide reserves of V, Ti, and Ce.

5) The Ti-X (X = Fe, Mn, Ce) RFBs also meets the DOE cost target of <100 $/kWh (Dong et al., 2015; Kaku et al., 2016; Funding opportunity announcement advanced research projects agency, 2016; Kaku et al., 2019; Sankarasubramanian et al., 2021).

FIGURE 1

FIGURE 1. (A) Cost per metric ton, worldwide production, and proven reserves of Ti, and V; (B) Abundance of Ti in the Earth crust (C) Half-cell potentials (vs SHE); for some redox couple for possible use in RFBs; (D) Schematic diagram of Ti- X (X = Fe, Mn, Ce) RFBs.

Given these advantages, in this work we critically review the developmental state of Ti-X RFBs and chart a course for their future development.

2 The Ti-X (X = Fe, Mn, Ce) family of RFBs

A schematic representation of Ti-X RFBs is shown in Figure 1D. The anolyte is the Ti salt dissolved in an acid and the catholytes are either Fe or Mn or Ce dissolved in their appropriate acidic counterparts. As discussed in literature, the oxidation states of Ti vary from +2 to +4 and the Ti ions exist stable in the salts as Ti³⁺ (+3 oxidation state), and TiO²⁺ (+4 oxidation state) but not as Ti²⁺. Their stability is confined to a very narrow region i.e., ∼1 pH and lesser, as seen in the Pourbaix diagram (Pourbaix, 1966). TiOSO₄ (titanium oxysulfate) and TiCl₃ (titanium chloride) with Ti in +4 and +3 oxidation state are the most widely used salts for Ti-X RFBs with supporting electrolytes including H₂SO₄, HCl, HNO₃ and H₃PO₄. Thus, the solvation and coordination of Ti species in the strong acidic electrolytes influences the reversibility and stability of the Ti⁴⁺/Ti³⁺redox couple and impacts the energy density of the Ti-X RFBs. The following section summarizes Ti solution chemistry in the context of RFBs.

2.1 Ti⁴⁺complexes in acids

Extensive literature report (Lingane and Kennedy, 1956; Miyanaga et al., 1990; Kavan et al., 1993; Cservenyák et al., 1996; Sole, 1999; Bahdad, 2020; Tsurumura et al., 2020; Choe et al., 2021) the solvation behavior of Ti⁴⁺/Ti³⁺ redox couple with different ligands in various acids solutions like H₂SO₄, HCl, HNO₃ and H₃PO₄. The redox stability of Ti⁴⁺/Ti³⁺ is influenced by the formation of different reaction/intermediate complexes that appear in various acids as discussed below. Critically, we are considering only strong mineral acids in our discussion given that the Ti⁴⁺/Ti³⁺ redox couple is stable only in low pH (< ca pH 1.5) conditions.

2.1.1 The H₂SO₄ system

In case of H₂SO₄ solution comprising dissolved TiOSO₄ salt, hereafter called the Ti-O-SO₄ system, the half-cell reaction is represented by the following equation,

${Ti}^{3+}+H_{2}O\rightleftharpoons Ti{O}^{2+}+2H^{+}+e^{-}(1)$

The Ti³⁺ and Ti⁴⁺ (i.e., as TiO²⁺) species of the redox couple co-exist in the concentrated Ti-SO₄ system. Ti⁴⁺ is the most stable oxidation state of Ti. The high charge density (ratio of charge to ionic radius) of Ti⁴⁺ prevents it from forming simply hydrated [Ti(H₂O)₆]⁴⁺ (Miyanaga et al., 1990). Ti⁴⁺ appears as [Ti(OH)₂(H₂O)₄]²⁺ in 1 M H₂SO₄ aqueous solutions (Bahdad, 2020) and in solutions where 0.04 < pH < 1 (Beukenkamp and Herrington, 1960; Kotsyubynsky et al., 2017), represented in short as TiO²⁺. These TiO²⁺ complexes tend to form oligomers (Figure 2A) when the oxo-oxygen of the titanyl ion is readily protonatable through hydrolysis reactions (Shepherd, 2013). The formation of oligomers is predominant when the concentration of TiO²⁺ is between 0.1–0.5 M, the H⁺ concentration between 1.0–2.5 M, and the temperature between 236–323 K (Comba and Merbach, 1987). In the presence of H⁺, SO₄^2- and HSO₄–ions (H₂SO₄ dissociation products in an aqueous solution (Choe et al., 2021)), Ti⁴⁺ forms complexes containing SO₄^2- or HSO₄^– ligands exhibiting the possible structures shown in Figure 2B. The competing coordination of Ti⁴⁺ to SO₄^2- or HSO₄^- depends on the strength of SO₄^2- concentration in the electrolyte [for e.g., 3 M H₂SO₄ concentration results in the dominance of Ti⁴⁺ to SO₄^2- coordination (Bahdad, 2020)]. The coordination of HSO₄^- with Ti⁴⁺ proceeds through a deprotonation pathway wherein H⁺ is accepted by a proton acceptor such as H₂O to form H₃O⁺ or Ti=O⁺ to form Ti-OH SO₄^2- and a Ti⁴⁺-SO₄^2- complex results. At higher SO₄^2- concentrations (and higher pH values), Ti⁴⁺ is predicted to exist as either mononuclear complexes (chelating complexes) or multinuclear complexes (bridging bidentate complexes) (Kotsyubynsky et al., 2017). Mononuclear complexes are formed by the coordination of Ti⁴⁺ ion with SO₄^2- leading to formation of [Ti(OH)₂SO₄(H₂O)₃]⁰ _, [Ti(OH)₂(SO₄)₂(H₂O)₂]²⁻ and [Ti=O(OH)₂(H₂O)₃]⁰. The multinuclear complexes are formed either due to polymerized Ti⁴⁺ complexes formed via oxygen atoms leading to -Ti-O-Ti-O- zigzag structures (Tsurumura et al., 2018) or via formation of [Ti₂O₂(H₂O)₅(OH)₂SO₄] (Choe et al., 2021). These Ti⁴⁺ complexes are either electrically neutral or anionic in the Ti-O-SO₄ system under high pH conditions. Ti⁴⁺ tends to form multinuclear complexes (nanoscale aggregates) in solutions of high SO₄^2- and Ti⁴⁺ concentrations which eventually results in the nucleation and precipitation to TiO₂. Thus, high pH conditions (typically with high SO₄^2- concentrations and low H⁺ concentration) are to be avoided when designing electrolytes for Ti RFBs.

FIGURE 2

FIGURE 2. Schematic representation of various Ti⁴⁺ complexes under weakly and strongly acidic conditions.

Upon electrochemical reduction of Ti⁴⁺ to Ti³⁺, there occurs substantial change in the structures of Ti-ion complexes and nanoscale Ti⁴⁺ aggregates are gradually disrupted to yield mononuclear Ti³⁺ complexes (Tsurumura et al., 2020). The Ti³⁺ ion is stable at very low pH (< ca 1.5) as seen through Pourbaix diagram. In dilute (higher pH) aqueous solutions, Ti³⁺ usually exists in the form of [Ti(H₂O)₆]³⁺. Literature also report using EXAFS (Extended X-ray Absorption Fine Structure) analysis, that Ti-Ti bond does not exist in Ti³⁺ solution, and the possibility for Ti-O bond exists in Ti³⁺ solution (Miyanaga et al., 1990) with Ti³⁺ ions existing in various other forms in aqueous solutions as Ti(OH)²⁺, TiO⁺, Ti(OH)₂⁺, and other complexes (Sole, 1999). However, with H₂SO₄ solution, Ti³⁺ react with SO₄^2- ion to form Ti³⁺-SO₄^2- complexes (Cservenyák et al., 1996).

2.1.2 The HCl system

In case of HCl solutions containing TiCl₃ salt, both the H^+, and Cl⁻ ions play a significant role in the reversibility of Ti⁴⁺/Ti³⁺ redox couple. Ti³⁺ exist as a TiOH³⁺ complex in acidified solutions (pH 2–2.5) and, due to hydrolysis of Ti³⁺, Ti(OH)₂²⁺ is formed with proton liberation as shown below (Kavan et al., 1993; Lokhande et al., 2004),

${TiOH}^{3+}+H_{2}O=Ti{\left(OH\right)}_2^{2+}+H^{+}\left(\mathit{\text{Fast}}\mathit{\text{Reaction}}\right)(2)$

$Ti{\left(OH\right)}_2^{2+}\to Ti\left(IV\right)\mathit{\text{oxo}}\mathit{\text{s}}\mathit{\text{p}}\mathit{\text{e}}\mathit{\text{c}}\mathit{\text{i}}\mathit{\text{e}}\mathit{\text{s}}+e^{-}\to Ti{O}_2\left(\mathit{\text{Undesired}}\mathit{\text{side}}\mathit{\text{reaction}}\right)(3)$

Unfortunately, Ti(OH)₂²⁺ leads to the formation of TiO₂ by precipitation as shown above, through an intermediate Ti(iv)oxo species which limits the reversibility of the redox couple. The Ti(iv)oxo species consist of partly dehydrated polymeric Ti(IV) hydroxide (Kavan et al., 1993), which get converted to TiO₂ (Lokhande et al., 2005). The Ti⁴⁺ ions in HCl exist in the form of TiOCl⁺, an oxy-chloro ion which reduces to a Ti³⁺ chloro complex, TiCl₄^- as shown below,

${TiOCl}^{+}+e^{-}+2H^{+}+3C{l}^{-}={TiCl}^{4-}+H_{2}O(4)$

The Ti³⁺/Ti⁴⁺ redox couple was found to be reversible only in >1 M HCl solution. The irreversibility observed in <1 M HCl solutions indicates the necessity of Cl⁻ ion for the reversibility of Ti⁴⁺ and Ti³⁺ as shown in Eq. 4 (Lingane and Kennedy, 1956). In the presence of HCl and H₂O, Ti⁴⁺ ions form unstable [Ti(OH)₂(H₂O)₄]Cl₂ which eventually results in the formation of TiO₂. In the context of RFBs requiring high reversibility of the Ti⁴⁺/Ti³⁺ redox couple, addition of (unfortunately unstable) organic compounds with oxygen-containing functional groups, such as acetylacetone, can partially suppress the hydrolysis reaction owing to the affinity between TiO²⁺ and oxygen-containing functional groups (Wang et al., 1984). HCl concentrations up to 6 M have been found to mitigate the precipitation of TiO₂ (Qiao et al., 2022). However, it enhances H⁺ concentration in the electrolyte and accelerates another undesired side reaction, namely the hydrogen evolution reaction (HER), thereby decreasing the RFB efficiency. The choice of the catholyte to be paired with the Ti anolyte can also preclude the use of HCl supporting electrolytes due to the occurrence of the chlorine evolution reaction (+1.36V vs. SHE).

2.1.3 The HNO₃ system

In case of HNO₃ solutions containing TiOSO₄, the salt dissolves as small clusters as observed through Small-angle X-ray scattering (SAXS) experiments (Molina et al., 2017). The analysis of TiOSO₄ dissolved in 1 M HNO₃ solution, suggests that the clusters contain a dense 1.2 nm diameter core (dominated by Ti–oxo) with a dynamic shell of water, sulfate, and nitrate which also results in precipitation for any dilution below 0.25 M TiOSO₄ (Molina et al., 2017). No complexation of Ti⁴⁺ was observed in dilute HNO₃ solutions (0.73–2.2 mM.L⁻¹) with 0.05 mM.L⁻¹ ortho-titanic acid (TiH₄O₄) due to their weak tendency to form nitrato complexes with most metal ions (Morris et al., 1978). This is markedly different from the formation of divalent mononuclear species like [Ti(OH)₂]²⁺ in H₂SO₄ solutions (Mangold et al., 2021). But, however HNO₃ is not actively used as supporting electrolyte due to the reduction of NO³⁻ leading to degraded performance during cycling in RFBs (Xie et al., 2011a).

2.1.4 The H₃PO₄ system

Studies with H₃PO₄ solutions containing Ti salts are very scarce in the literature (Lingane and Kennedy, 1956; Oldenburg et al., 2018; Mangold et al., 2021) as the solubility and stability of Ti⁴⁺ ions in these systems is a practical limitation (Lingane and Kennedy, 1956). The reversibility of Ti⁴⁺/Ti³⁺ redox couple and their stability was studied under different concentrations (1–10 M) of H₃PO₄ with 10 mM and lower concentrations of Ti⁴⁺ [as the Ti(OH)₄ salt]. It was found that a 10 mM solution of Ti⁴⁺ ions in 1M H₃PO₄ was unstable and precipitated after 24 h but stabilized in 4M H₃PO₄ without any phase separation. But upon increasing the Ti⁴⁺ ion concentrations to >10 mM, the electrolyte was again unstable in 4M H₃PO₄ leading to precipitation. Interestingly, the reversibility of Ti⁴⁺/Ti³⁺ redox couple is more pronounced at 1M H₃PO₄ as compared to 4M H₃PO₄ (Shepherd, 2013). In this electrolyte system, Ti⁴⁺ is present as mononuclear (µ = 1) or polynuclear (µ > 1) free cation(s), [(TiO)_µ] ^2µ+ in diluted H₃PO₄ solutions (<0.1 mol.L⁻¹). Ti⁴⁺ ions progressively form [(TiO)_µ(H_3-mPO₄)_δ]^2µ−δm complexes as the concentration of H₃PO₄ is increased to >1 mol.L⁻¹ and [(TiO)_µ(H_3-mPO₄)_δ(H_3-nPO₄)_β]^{2µ−δm-βn} complexes at >6 mol.L⁻¹ H₃PO₄ (Mangold et al., 2021). The poor solubility and reversibility of Ti solutions in H₃PO₄ precludes their use in RFBs.

2.2 Performance of Ti RFBs

Given the discussion above, reports on the Ti-X family of RFBs consist predominantly of systems using H₂SO₄ as the supporting electrolyte due to the stability and reversibility of the Ti⁴⁺/Ti³⁺ redox couple at relatively high Ti concentrations (∼0.5–1.5 M). This configuration also avoids any side reactions (H₂-, Cl₂-, and NO₂- evolution) thereby increasing the overall energy efficiency of the RFBs.

Ti - Fe RFB: Fe based RFBs (coupled with Ti or Cr) have been widely investigated by NASA in the early 1970s due to the low cost and abundant supply of Fe. These RFBs can achieve a theoretical energy density of nine Wh. L⁻¹ (at 0.67V open circuit potential (OCP)). Cr-Fe RFBs was initially assessed for space applications and scale-up studies were conducted, but the system was not commercially developed due to several drawbacks like, low energy density of the mixed electrolyte (containing both Fe and Cr in the anolyte and catholyte), membrane fouling, the slow Cr redox kinetics requiring expensive noble metal catalysts and parasitic HER on the Cr side (Skyllas-Kazacos et al., 2011). The Ti-Fe RFBs was studied by Thaller in aqueous HCl solution (Thaller and inventor, 1976). In this system, during the charge cycle, Ti⁴⁺ (i.e., TiO²⁺) reduced to Ti³⁺ on the negative side and Fe²⁺ is oxidized to Fe³⁺ in positive side. The half-cell charge reactions of the Ti-Fe RFB are,

$\mathit{\text{Negative}}\mathit{\text{Electrode}}:{TiO}^{2+}+2H^{+}+e^{-}\rightleftharpoons {Ti}^{3+}+H_{2}O\left(+0.1Vvs\mathit{\text{SHE}}\right)(5)$

$\mathit{\text{Positive}}\mathit{\text{Electrode}}:{Fe}^{2+}\rightleftharpoons {Fe}^{3+}+e^{-}\left(-0.771Vvs\mathit{\text{SHE}}\right)(6)$

Initially, TiCl₃ and FeCl₃ were the salts used at the anolyte and catholyte respectively in the HCl supported electrolytes. This system suffered from the formation of TiO₂ particles as an undesired side reaction, decreasing the overall coulombic efficiency. The maximum current density of these initial Ti-Fe RFBs with HCl supporting electrolyte was 8.16–14 mAcm⁻² with a nominal cell voltage around 0.67V (Savinell et al., 1979). Recent improvements in Ti-Fe RFBs have consisted of using H₂SO₄ as supporting electrolyte instead of HCl and using TiOSO₄ and FeSO₄ salts in the anolyte and catholyte respectively. In the presence of H₂SO₄, the interaction between H₂O and Ti⁴⁺ ions are diminished, thereby inhibiting the formation of Ti(OH)₂²⁺ and improving the stability of the electrolyte. Such as second generation Ti-Fe RFB with bismuth (Bi) catalyst at the positive electrode and a carbon felt at the negative electrode exhibited a diffusion coefficient of 19.18×10^–8cm² s⁻¹ for Fe³⁺/Fe²⁺ and 0.36×10^–8cm² s⁻¹ for Ti⁴⁺/Ti³⁺ (Qiao et al., 2022) with a rate constant of 3.828×10^–4cm s⁻¹ for Fe³⁺/Fe²⁺ and 0.203×10^–4cms⁻¹ for Ti⁴⁺/Ti³⁺ respectively (Qiao et al., 2022). It suggests that both the diffusion coefficient and rate constant for Fe³⁺/Fe²⁺ is higher than Ti⁴⁺/Ti³⁺ with the reactions of Ti redox couple being rate limited. The Ti-Fe RFBs in 3M H₂SO₄ were cycled at current densities as high as 120 mAcm⁻² with the highest energy efficiency of 85.6% (at 40 mAcm⁻²). This system showed 80% discharge capacity after 1000 cycles (30 min per cycle) with a low-capacity decay of 0.193 Ah. cycle⁻¹ (Qiao et al., 2022). CEMs like Nafion® 212, sulfonated poly (ether ketone) (SPEEK) have been used in Ti-Fe RFB. Non-fluorinated SPEEK is predominantly used as it reduces the cost for energy production from $165.79/kWh (Nafion® 212) to $88.22/kWh (SPEEK) (Qiao et al., 2022).

Ti-Mn RFB: Ti-Mn RFBs was first developed by Dong et al, (2012) where a relatively high OCP of 1.41 V was obtained (as compared to 0.67 V for Ti-Fe RFBs) resulting in superior power density (Dong et al., 2015; Kaku et al., 2016). These RFBs can achieve a theoretical energy density of 18.9 Wh. L⁻¹. The half-cell charge redox reactions of Ti-Mn RFB are represented by the following equations.

$\mathit{\text{Negative}}\mathit{\text{Electrode}}:{TiO}^{2+}+2H^{+}+e^{-}\rightleftharpoons {Ti}^{3+}+H_{2}O\left(+0.1Vvs\mathit{\text{SHE}}\right)(7)$

$\mathit{\text{Positive}}\mathit{\text{Electrode}}:{Mn}^{2+}\rightleftharpoons {Mn}^{3+}+e^{-}\left(-1.51Vvs\mathit{\text{SHE}}\right)(8)$

Unfortunately, Mn³⁺ is highly unstable and inclined to form manganese dioxide (MnO₂) via the following reaction-

${2Mn}^{3^{+}}+2H_{2}O\to M{n^2}^{+}+Mn{O}_2+4H^{+}\left(\mathit{\text{Disproportionation}}\mathit{\text{Reaction}}\right)(9)$

The precipitated MnO₂ particles start to aggregate and hinder the flow of electrolyte by blocking the pores of the membrane thereby reducing the columbic efficiency. So, it is required to reduce the formation of MnO₂ particles as well as to ensure that the MnO₂ particle are small enough to avoid aggregation and prevent membrane fouling (Kaku et al., 2016). Several approaches have been proposed for stabilization of Mn³⁺ such as by increasing the acidity, by increasing the Mn²⁺ concentration, or via complex formation (Davies, 1969). However, increasing Mn²⁺ concentration necessitates limiting the cycling of the cell to only 50% state of charge (SOC) to prevent Mn³⁺ disproportionation, thereby negating any advantages due to increased reactant concentration. On the other hand, the formation of Mn complexes (i.e., MnOOH) results in loss of electro-activity (Dong et al., 2015; Bahdad et al., 2021).

$Mn{O}_2+H^{+}+e^{-}\to \mathit{\text{MnOOH}}(10)$

The disproportionation reaction and the morphology of MnO₂ were significantly influenced by addition of H₂SO₄ solution containing TiO²⁺ ions (Kaku et al., 2016). TiOSO₄ solutions of varying molarities was added to 1M MnSO₄ and the characteristic of the composite electrolyte was studied in the context of suppressing the disproportionation of Mn³⁺ ions. The MnO₂ aggregates were found to be > 1000 nm without adding TiOSO₄ or with the addition of 0.25M TiOSO₄ in MnSO₄. The particles size reduced to less than 100 nm with addition of 0.5M–1M of TiOSO₄ in MnSO₄ (Kaku et al., 2016). The optimal composition of 1.5 M TiOSO₄ in the 1 M MnSO₄ electrolyte improved the performance of the Ti-Mn RFB in terms of energy density to achieve ∼11.75 Wh. L⁻¹ (accounting for the electrolyte in both tanks) with coulombic efficiency of 99.8% and energy efficiency of 88.7%, both of which were stable over 40 cycles (Dong et al., 2015; Dong et al., 2017; Kaku et al., 2019). Unfortunately, the addition of TiOSO₄ with MnSO₄ also reduces the cell voltage by more than 100 mV and increases the cost of energy components (Kaku et al., 2016). An alternate approach using V⁵⁺ ions to stabilize the Mn electrolyte has also been proposed (Reynard et al., 2020). These V/Ti/Mn RFB systems will exhibit higher voltages compared to the Ti/Mn system given the lower standard electrode potential of the V³⁺/V²⁺ couple (-0.26V vs SHE). But this system is economically unattractive given the increased cost associated with the use of vanadium and thus we do not believe this is a viable future direction. Various thicknesses of Nafion® i.e., Nafion® 115, Nafion® 212, Nafion® 211 have been evaluated to investigate their impact on the performance of Ti-Mn RFBs. The energy efficiency was found to be function of separator thickness with the energy efficiency being 84%, 83%, and 81% for Nafion® 211 (25 μm), Nafion® 212 (51 μm), and Nafion® 115 (127 μm) respectively (Kaku et al., 2017).

Ti-Ce RFB: An alternative Ti based RFB which provides a higher OCP compared to, Ti-Fe and Ti-Mn RFBs, are Ti-Ce RFB. The OCP of Ti-Ce RFB is 1.61V, which results in higher operating power density at the same operating current density and higher energy density for the same electrolyte concentration compared to Ti-Fe and Ti-Mn systems (Sankarasubramanian et al., 2021). These RFBs can achieve a theoretical energy density of 19.4 Wh/L. The half-cell redox reactions of Ti-Ce RFB are represented by the following equations.

$\mathit{\text{Negative}}\mathit{\text{Electrode}}:{TiO}^{2+}+2H^{+}+e^{-}\rightleftharpoons {Ti}^{3+}+H_{2}O\left(+0.1Vvs\mathit{\text{SHE}}\right)(11)$

$\mathit{\text{Positive}}\mathit{\text{Electrode}}:{Ce}^{3+}\rightleftharpoons {Ce}^{4+}+e^{-}\left(-1.72VvsSHE\right)(12)$

The Ce Pourbaix diagram shows that cerium ions are soluble in strong acids but forms stable, insoluble hydroxyl complexes above pH ∼7. Given the exceptionally high standard reduction potential for Ce (the highest amongst all the catholyte candidates considered here), the stability of the supporting electrolyte is a particular concern - HCl and HNO₃ cannot be used due to their side reactions that produce Cl₂ and NO₂ respectively (Sankarasubramanian et al., 2021). The Ce⁴⁺/Ce³⁺ redox couple exhibits unusual solubility behavior as a function of the supporting electrolyte (acid) concentration. The solubility of both species in this redox couple decreases in inverse proportion to H₂SO₄ concentration and the highest concentration achieved was 0.5M Ce in 1M H₂SO₄ (Xie et al., 2011b). But interestingly, in CH₃SO₃H, the solubility of Ce³⁺ decreases and the solubility of the Ce⁴⁺ increases with increasing acid concentration and this results in a solubility maximum of 0.9M Ce in 4M CH₃SO₃H (Kreh et al., 1989; Shi et al., 1989; Sankarasubramanian et al., 2021). Thus, the energy density of this system is circumscribed by the solubility of the Ce catholyte as TiOSO₄ is highly soluble in both H₂SO₄ and CH₃SO₃H (Sankarasubramanian et al., 2021). On the other hand, the Ti electrode is the rate-limit electrode (relevant for achieving higher operating current densities) as the rate constants of the Ce⁴⁺/Ce³⁺ couple is 3x that of the Ti⁴⁺/Ti³⁺ redox couple (Klingler and Kochi, 1981; Sankarasubramanian et al., 2021). Cyclic voltammetry shows the anodic to cathodic peak separation for Ti³⁺/Ti⁴⁺ to be 1V and 0.67V for the Ce⁴⁺/Ce³⁺ couple (Sankarasubramanian et al., 2021), indicating these reactions are not electrochemically reversible and suggesting high charge and discharge overpotentials (Bard and Faulkner, 2000). Nevertheless, in both H₂SO₄ and CH₃SO₃H supporting electrolytes, the Ti-Ce RFB exhibited nearly 100% coulombic efficiency with over 70% energy efficiency (charging and discharging at 100 mA/cm²) during 1300 and 700 h of diurnal cycling, respectively (Sankarasubramanian et al., 2021). These cells employed highly permselective quaternary cardo-poly ether ketone (QPEK-C) AEM separators (Yun et al., 2014; Yun et al., 2015; Yun et al., 2016) which demonstrated negligible crossover (<0.4%) over 1000 h of operation with 24 h cycle as compared to commercial CEM which suffers from drastic cation crossover and loss in capacity early in the cycling of the RFB (Sankarasubramanian et al., 2021).

3 Future Directions and Prospects

The Ti-X family of RFBs represent an interesting new direction in the development of aqueous RFB systems given their high theoretical energy density and economic competitiveness enabled by the high solubility and low cost of Ti. We anticipate the following future directions –

1) Unlocking the high energy density of the Ti electrolyte by pairing it with a stable and high solubility counter electrolyte. The long-term stability of the low pH Ti electrolytes needs to be demonstrated.

2) Catalyzing Ti⁴⁺/Ti³⁺ redox kinetics to overcome its nature as the rate-limiting electrode. The catalysts should be low cost to preserve the cost advantage enjoyed by the Ti electrolyte.

3) Increasing the thermal stability of the Ti electrolyte to prevent TiO₂ formation by hydrolysis.

4) Using AEM and pore tailored PM instead of CEM to reduce crossover of the predominantly cationic redox active species, thereby enabling electrode decoupled RFBs. (Wang et al., 2018).

The continued development of these systems is anticipated to result in a commercially viable, high-energy density aqueous RFB that can economically be integrated into the electric grid.

Author contributions

SIUA and MS contributed equally to this manuscript. SIUA and MS prepared the first draft of the manuscript. All authors reviewed and revised the manuscript. SS conceived and supervised the project.

Funding

The authors acknowledge funding from the US Department of Energy through grant # DE-FE0032011. SS gratefully acknowledges funding through a start-up grant from the University of Texas at San Antonio.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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