A Novel Vanadium–Titanium Redox Flow Battery with Enhanced Electrochemical Performance & Greener Alternative

ChivukulaKalyan1

SundarKrishna1

YansongZhao1✉Emailyansong.zhao@hvl.noEmailyansong.zhao2004@gmail.com

1Department of Safety, Chemistry and Biomedical Laboratory SciencesWestern Norway University of Applied Sciences (HVL)5063BergenNorway

Chivukula Kalyan Sundar Krishna, Yansong Zhao*

Department of Safety, Chemistry and Biomedical Laboratory Sciences, Western Norway University of Applied Sciences (HVL), 5063 Bergen, Norway.

*Corresponding author: Yansong Zhao (yansong.zhao@hvl.no or yansong.zhao2004@gmail.com).

Abstract

In the pursuit of efficient and cost-effective grid-scale energy storage solutions, redox flow batteries (RFBs) have emerged as champions by offering a promising solution owing to their design scalability. However, conventional vanadium RFBs are limited by high material costs. Here, we present a novel vanadium–titanium redox flow battery (VTRFB) that combines the redox potential of vanadium (V⁵⁺/V⁴⁺) with the low cost and abundance of titanium (Ti³⁺/Ti⁴⁺). The system demonstrates a cell voltage of 1.3 V, high ionic conductivities (0.319 and 0.353 Sꞏcm^− 1), and low dynamic viscosities (2.7 and 1.3 mPa·s) for catholyte and anolyte, respectively, at 298 K. Charge–discharge testing over 500 hours yielded ~ 100% CE, ~ 80% VE, and ~ 80% EE. Further, an ionic-liquid-based catholyte was employed to enhance sustainability, with performance evaluated and compared using Nafion and SPEEK membranes. This VTRFB design integrates low-cost materials and high electrochemical performance, representing a significant step toward next-generation sustainable RFBs.

The ever – growing demand for the sporadic renewable energy necessities the urgent need for the incorporation of cost effective, and efficient energy storage devices in the grid – scale[1–3]. However, the current landscape is dominated by the commercialized all vanadium redox flow batteries which exhibits a low open-circuit potential (OCP), and mid-voltage during the discharge with an efficiency of approximately 80%, at an expensive material procurement cost due to the extremely volatile prices of vanadium resources[4, 5]. On the other hand, a plethora of studies have been performed in the domain of redox flow batteries in a dire need to replace the vanadium – based catholyte or anolyte solutions with a cheaper, yet an effective alternative. In this regard, the low cost catholyte materials such as Fe, Ce, Cr, Mn etc., have been utilized in conjunction with various component combinations of Zn, Pb, Br, I etc., in an effort to enhance the OCP of the redox flow battery, and energy density while retaining the capacity, efficiencies, and cyclic stability of the flow battery[6–20]. Apart from these anolyte combinations, when compared with to vanadium, a stable, low cost and abundant mineral-Titanium has been utilized only by a handful of researchers despite of its advantages. In this regard, a low cost, and novel iron-titanium based redox flow battery was introduced for the first time utilizing titanium as the anolyte solution, and the resulting system demonstrated an extremely low OCP of 0.67 V. However, the 1st generation Fe-Ti RFB suffered from a consequence of titanium dioxide particle formation, and thereby reducing the overall efficiency of the flow battery, which was later improved in the 2nd generation Fe-Ti flow batteries by introducing catalysts and altering the initial salts and their concentrations, of titanium and iron. In spite of enhancing the efficiencies of the Fe-Ti RFBs, their imminent drawback of extremely low OCP persists[21]. To tackle this, the titanium based anolyte solution was used in combination with manganese and cerium which exhibits an extremely high standard electrode potentials of 1.5 V, and 1.7 V respectively vs SHE. In these lines, Dong et al. [22], introduced a novel manganese – titanium mixed electrolyte redox flow battery which exhibited an OCP of 1.55 V, and an energy density of 23.5 Wh.L^− 1 while maintaining the coulombic, voltage, and energy efficiencies (CE,VE and EE) at 99.8%, 86.9%, and 86.7% respectively for more than 40 cycles of operation. In a similar vein, to reduce the formation of MnO₂ during the discharging of manganese based redox flow batteries, M. Nan et al.[9], implemented a charge-induced slurry (CMRFB), which composed of utilizing MnO₂ nanoparticles as redox active materials. The flow battery utilized titanium as anolyte solution in combination with manganese based catholyte solution, and exhibited a coulombic, and energy efficiency of 99%, and 85% at a discharge current of 40 mA.cm^− 2 for more than 1000 cycles, and thereby effectively minimizing the imminent problem occurred in the manganese-titanium based redox flow batteries. These results suggests that titanium can indeed be utilized as anolyte, and it exhibits an excellent cyclic charge-discharge performance. Further, as per the review article published recently by Ahmed et al.[5], on the state of the art research on titanium – based aqueous electrolytes, the availability of the titanium resources is 50 times more as compared to the vanadium resources which are being extracted at a rate of 100x, however, it suffers from an imminent drawback of low standard electrode potential of 0.1 V vs SHE, during the conversion of Ti⁴⁺ to Ti³⁺ in an acidic medium. In spite of its low electrode potential, due to the abundance of the material availability and its extreme stability as illustrated by the other researchers, it can be utilized as a potential candidate to act as an anolyte material by replacing the vanadium (V³⁺ to V²⁺) redox couple, which has a similar reduction potential. So, to complement the low electrode potential of titanium, this study utilizes the well-known, and commercialized redox chemistry of vanadium based catholyte solution which possess a high standard electrode potential of 1 V vs SHE, and effectively reducing the cost of the vanadium – titanium redox flow battery (VTRFB) as compared to the commercialized VRFBs due to a decrement in the initial chemical procurement costs.

This study utilizes a novel combination of vanadium, and titanium based catholyte and anolyte solutions respectively in their + 5 and + 3 oxidation states which depict a completely charged battery, by utilizing vanadium pentoxide and titanium chloride in acidic medium using H₂SO₄ and HCl solutions respectively, to effectively stabilize the V⁵⁺ to V⁴⁺, and Ti³⁺ to Ti⁴⁺ ions, and to enhance the reversibility of the redox couples. The synthesized electrolytes were utilized in the VTRFB as catholyte and anolyte solutions, to evaluate their impact on the electrochemical performance of the battery. Further, the acid utilized in the catholyte has been replaced with an ionic-liquid called 1-butyl-3-methylimidazolium chloride (BmimCl) along with VCl₃, to promote green chemistry. In spite of the utilizing of the chloride-based systems, the evolution of its gas is restricted by limiting the charging potential to less than 1.3 V. Furthermore, to reduce to the cost of operation, the Nafion film has been replaced with SPEEK to evaluate the performance of the battery.

A couple of electrochemical studies such as the cyclic voltammetry (CV) tests to evaluate the stability window over numerous cycles, electrochemical impedance spectroscopy (EIS) studies to assess the resistance offered by the electrolyte solutions, and cyclic charge – discharge tests were conducted along with the physical property evaluations over a range of temperatures. These studies provided an in-depth understanding of the working of novel vanadium – titanium redox flow battery, and its effective implementation in the grid – scale by replacing the vanadium based anolyte solutions and thereby reducing the material procurement costs, and extending the overall life cycle while maintaining the stability of the flow batteries at high charging and discharging currents.

Physical and Electrochemical Characterization:

Prior to evaluating the electrochemical performance of the VTRFB, the physical properties such as the dynamic viscosity, and the density of the anolyte and catholyte solutions were measured over a temperature range of 25 ˚C to 60 ˚C with a step of 5 ˚C by utilizing a rolling ball viscometer. A capillary of 1.59 mm diameter was filled with the electrolyte solution, and inclined at an angle of 45˚ while a gold ball (due to its non-corrosive nature) immersed in the solution, which has a density of 7.848 g.cm^− 3, measures the viscosity of the solution while the capillary rotates from − 45˚ to + 45˚. The obtained dynamic viscosity, and the density of the anolyte and catholyte solutions are illustrated in the Figs. 1(a), and 1(b), respectively. It is evident from the obtained data that the solutions possess a low viscosity viz. 2.7 mPaꞏs, and 1.3 mPaꞏs for the catholyte and anolyte respectively at room temperature with densities of 1.265 gꞏcm^− 3, and 1.140 gꞏcm^− 3.

Fig. 1

Dynamic viscosity (a), and density (b) variation with temperature from 298 K to 333 K for the anolyte and catholyte solutions.

Subsequent to the physical property evaluation, the electrochemical studies such as CV, and EIS studies were performed to evaluate the overall stable operational window, and the resistances of the anolyte and catholyte solutions, prior to evaluating their performance as catholyte and anolyte in the redox flow battery. The CV tests were conducted for the anolyte, and catholyte solutions using a 3-electrode setup, and the obtained graphs are displayed in the Fig. 2.

Fig. 2

The variation of anodic peak current with scan rates for ‘V’ catholyte solution (a), CV study of ‘V’ catholyte solution at scan rate of 0.01 mVꞏsec^− 1 (b), CV of ‘V’ catholyte at various scan rates ranging from 0.01 mVꞏsec^− 1 to 0.05 mVꞏsec^− 1 (c), The variation of anodic, and cathodic peak currents with scan rates for ‘Ti’ anolyte solution (d), Cyclic stability study of ‘Ti’ anolyte solution at a scan rate of 0.05 mVꞏsec^− 1 for 50 cycles (e), and CV of ‘Ti’ anolyte solution at various scan rates ranging from 0.01 mVꞏsec^− 1 to 0.1 mVꞏsec^− 1 (f).

Utilizing the obtained CV graphs of ‘Ti’, and ‘V’ based catholyte and anolyte solutions, the peak currents have been plotted with the square root of scan rates, and a linear relation has been obtained as illustrated in the Figs. 2(a), and 2(d), signifying a diffusion-controlled mechanism, allowing the implementation of Randles-Sevcik equation to obtain the diffusion coefficients (D).

$\:{I}_{p}=\left(2.69\:\times\:\:{10}^{5}\right)\:{n}^{\frac{3}{2}}A\:{D}^{\frac{1}{2}}\:C{v}^{\frac{1}{2}},$

Where, ‘I_p’ denotes the peak current, ‘n’ is the electron transfer, ‘A’ is the active surface area of the electrode immersed in the electrolyte solution, while ‘D’ denotes the diffusion coefficient, ‘C’ represents the concentration of the electrolyte solution, and ‘v’ is the scan rate. Utilizing the above-mentioned expression, the values of ‘D’ for the ‘Ti’ and ‘V’ based solutions are evaluated to be 5.78 x 10^− 13 cm²ꞏsec^− 1, and 2.82 x 10^− 9 cm²ꞏsec^− 1 signifying a faster mobility of vanadium ions, and lower mass transport resistance as compared to the anolyte. Further, utilizing Laviron’s expression for the quasi-reversible systems, the value of apparent electron transfer coefficient ‘α’ and the rate of electron transfer ‘k_s’ can be calculated by utilizing the ln(v) vs peak potential values.

$\:E=k+\frac{RT\text{ln}v}{{\upalpha\:}\:n\:F}\:,$

Where, the slope from the peak potentials versus the ln(v) provides us with the values of ‘α’, which can be further utilized to evaluate the rate of electron transfer ‘k_s’ as,

$\:\text{log}\left({k}_{s}\right)={\upalpha\:}\text{log}\left(1-\alpha\:\right)+\left(1-{\upalpha\:}\right)\text{log}\left(\alpha\:\right)-\text{log}\left(\frac{RT}{nFv}\right)-(1-{\upalpha\:}){\upalpha\:}\frac{nF{\Delta\:}E}{2.303\text{R}\text{T}}\:,$

Where, ‘

$\:{\Delta\:}E$

’ denotes the peak separation, ‘R’, and ‘F’ represents the gas constant (8.314 Jꞏmol^− 1ꞏK^− 1) and Faraday constant (96485 Cꞏmol^− 1), and ‘T’ is the temperature denoted in ‘K’. The values of ‘α’ for the ‘Ti’ and ‘V’ are evaluated to be 0.191, and 0.2045 respectively, signifying the need for higher overpotentials for the oxidation reactions, which will be implemented during the charge-discharge studies of the battery. Moreover, utilizing the obtained values of ‘α’, the rate transfers are evaluated to be 0.0615 sec^− 1, and 0.0264 sec^− 1 respectively for ‘Ti’, and ‘V’ based solutions signifying the quasi-reversible kinetics with a slower electron transfer rate at the electrode-electrolyte interface, which will be further studied utilizing electrochemical impedance spectroscopy technique.

On evaluating the kinetics, and stability of the catholyte and anolyte solutions using the data obtained CV, EIS study was utilized to evaluate the resistances offered by the corresponding solutions, and the obtained graphs are illustrated in the Figs. 3(a), and 3(b).

Fig. 3

EIS graph obtained for ‘Ti’ anolyte (a), and ‘V’ catholyte (b) solutions measured w.r.t to the OCP at a frequency range varying from 0.01 Hz to 100 kHz, and an amplitude of 5 mV.

The solution resistances offered by the anolyte and catholyte solutions were calculated to be 4.25 Ω, and 4.70 Ω, and the charge transfer resistances are 1.75 Ω, and 13.46 Ω respectively implying a faster electron transfer for the anolyte solution rather than the catholyte which has been corroborated by the obtained values of apparent electron transfer rates utilizing the CV graphs. It is quite evident from the obtained values that the resistance offered by the solutions were extremely low, and can aid in the transport of the ions by exhibiting an enhanced ionic conductivity in the respective solutions, owing to their low dynamic viscosity values as measured earlier. Utilizing the obtained values for the resistances of the catholyte and anolyte solutions, the approximate ionic conductivities were evaluated to be 0.319 Sꞏcm^− 1, and 0.353 Sꞏcm^− 1 respectively. Upon evaluating the following parameters, the charge – discharge tests were conducted for the VTRFB to evaluate their influence on the electrochemical characteristics, which will be discussed in the upcoming section.

VTRFB Charge – Discharge Evaluation:

To evaluate the impact of incorporating titanium, and vanadium as the anolyte and catholyte solutions in a VTRFB, a series of cyclic charge-discharge tests were conducted for greater than 500 hours, which corresponds to more than 150 cycles of operation. The working of the VTRFB has been depicted in the Figs. 4, and 5. The synthesized anolyte and catholyte solutions, were in a state of completely charged i.e., the first cycle involved complete discharge of the battery, to ensure no residual charge during the subsequent operation of the battery. A charging current of 100 mAꞏcm^− 2 has been provided with a cut-off voltage of 1.3 V and various discharging current ranging from of 10–100 mAꞏcm^− 2 were provided, till the potential reaches 0.5 V, as a lower cut-off.

Fig. 4

The variation of voltage and current with testing time for the VTRFB(a), and its efficiency variation over various cycles (b). The efficiencies measured with respect to the normalized cycle number at various discharge current densities while the charging voltage is fixed at 1.3 V (c).

Fig. 5

The efficiencies measured with respect to the normalized cycle number at various discharge current densities while the charging voltage is fixed at 1.1 V (a). The voltage versus capacity variation for the 2nd, and 115th cycle while being charged at constant current, and constant voltage (b), and the nominal discharge potentials during the discharge of the VTRFB at various discharging current densities ranging from 10–100 mAꞏcm^− 2 (c).

On the basis of the obtained data, the following coulombic, voltage, and the energy efficiencies (CE,VE, and EE) have been evaluated utilizing the following expressions,

$\:CE=\:\frac{{I}_{D}{t}_{D}}{{I}_{C}{t}_{C}}\:\times\:100\%,$

$\:VE=\:\frac{{V}_{D}}{{V}_{C}}\:\times\:100\%,$

$\:EE=CE\:\times\:VE,$

wherein,

$\:{I}_{D},{t}_{D}$

are the discharge currents, and time of discharge, and

$\:{I}_{C},{t}_{C}$

are the charging currents and time taken for the VTRFB to completely charge. The terms

$\:{V}_{D}$

and

$\:{V}_{C}$

represent the discharging, and the charging potentials. The VTRFB exhibited a CE of greater than 90% almost the entire discharging process. As showcased in the Fig. 4(b), the significant drops at various cycles are the cycles of transformation of the discharging currents i.e., at those corresponding cycles, the discharge current has been increased by 10 mAꞏcm^− 2. The stabilized efficiencies w.r.t normalized cycle numbers are depicted in the Figs. 4(c), and 5(a). The charging voltage has been restricted to 1.1 V rather than 1.3 V in an attempt to increase the VE%, and thereby the EE% of the VTRFB, and the obtained data proves the same. On limiting the charging voltage to 1.1 V rather than 1.3 V, the EE% of the VTRFB can be increased to almost 90% at a discharging current of 10 mAꞏcm^− 2 rather than 69% at 1.3 V. However, this comes with a drawback of lower charging currents due to the reduced charging potential. The average charging currents for the 1.1 V, and 1.3 V are 48 mAꞏcm^− 2, and 95 mAꞏcm^− 2 respectively, at a discharging current of 10 mAꞏcm^− 2. Furthermore, the variation of nominal discharge voltages as a function of various discharge currents has been obtained, and depicted in the Fig. 5(c), signifying a negligible ohmic drop at lower currents, and progressively increasing as the discharging current raises. Moreover, to evaluate the energy density of the VTRFB, it has been charged to a capacity of 300 mAh viz. when the charging current drops below 5 mA, and the obtained discharging capacity is approximately 246 mAh which corresponds to a CE, VE, and EE of 83%, 67.44%, and 55.98% respectively, with a nominal discharge potential of 0.888 V, and an energy density of 21.84 WhꞏL_{w.r.t catholyte or anolyte}.

Ionic-liquid Incorporation and Green Chemistry Advancement:

Subsequent to the evaluation of the acid-based system, the vanadium catholyte has been replaced with a novel ionic-liquid based catholyte synthesized in-house by utilizing BmimCl and VCl₃ to promote green chemistry. Solutions of 1 M concentration have been utilized for catholyte, and anolyte to evaluate the comparative performance with Nafion, and SPEEK membranes. The obtained data is illustrated in the Fig. 6. As compared to the acid-based system, the ionic-liquid based one showcased a lower charging speed due to a lower H⁺ ion concentration.

Fig. 6

The variation of voltage and current w.r.t to the test time for the ionic-liquid incorporated VTRFB(a), and its efficiency variation over various cycles (b). The nominal discharge potentials during the discharge of the battery at various discharging current densities ranging from 10–50 mAꞏcm^− 2 (c), and the variation of voltage with capacities for various cycles (d), and the comparison of the same with respect to non-liquid based system (e).

Apart from the lower charging speed, the ionic-liquid based system showcased a similar performance as illustrated in the Fig. 6(b) i.e., CE greater than 90%, and the VE, and EE greater than 50%, and 50% respectively. Moreover, the ionic-liquid based system showcased an appreciable performance even at higher charging capacities when compared to the acid-based one as depicted in the Fig. 6(e). The electrolyte is rather stable as showcased in the Figs. 6(a), 6(b), and 6(d), and promotes green chemistry rather than utilizing acid, and the detailed study on the performance metrices of BmimVCl₄ have been studied in detail in our earlier works[23]. Furthermore, it is noteworthy to mention that during the discharging of the VTRFB, the nominal potential is greater than 0.9 V at a discharging 10 mAꞏcm^− 2 as showcased in Figs. 5(c), and 6(c), while the theoretically obtainable maximum potential is 1.1 V. The flow battery showcases an exceptionally stable discharge characteristics throughout the discharging process, yielding a maximum power density of 55 mWꞏcm^− 2, and an energy density of greater than 21.84 WhꞏL^− 1_{w.r.t anolyte or catholyte}. To further reduce the cost, while implementing a greener alternative viz. ionic-liquid based electrolyte, the Nafion film has been replaced with SPEEK. The relatively low-cost, non-fluorine proton ion exchange membrane, SPEEK was utilized in the ionic-liquid based VTRFB, and the obtained results are showcased in the Fig. 7. The VTRFB retained a similar performance with an average discharge current of 40 mAꞏcm^− 2, and 50 mAꞏcm^− 2, and delivered a CE, EE, and VE of greater than 95%, 50%, and 50% respectively, similar to the VTRFB utilizing Nafion, as displayed in the Fig. 7(b). The nominal discharge voltage has been showcased in Fig. 7(c), and it is constant at 0.7 V, similar to the Nafion based system. The first few cycles slightly distorted due to the external disturbances to the battery and residual charges, however, from the cycle ‘6’, the battery has demonstrated a stable performance, while the 2nd cycle delivered an extremely high efficiency viz. closer to 270% as an anomaly, depicting the removal of any residual charges as a result of utilizing a completely charged setup. As the battery has not been charged, and discharged to a larger extent than intended due to the completely charged setup, the efficiency rose to a large extent.

Fig. 7

The variation of voltage and current with the testing time for the ionic-liquid incorporated VTRFB with SPEEK membrane (a), and its efficiency variation over various cycles (b). The nominal discharge potentials of the battery at a current density of 40 mAꞏcm^− 2 (c).

The future works of the research aligns in the direction of improving the performance of the novel VTRFB by incorporating ionic-liquids in the anolyte and catholyte while increasing the concentrations of the solutions, and extrapolating the lab-scaled version to a large-scale energy storage system, while promoting green chemistry at lower material procurement costs as compared to the commercialised flow batteries while achieving high efficiencies, and operational stability at elevated charge, and discharge current densities.

Conclusion:

In this study, we have successfully developed and demonstrated a novel vanadium–titanium redox flow battery (VTRFB) which addresses two key challenges that are primarily faced by conventional all-vanadium redox flow batteries: the expensive material procurement costs due to the volatile cost of vanadium, and the moderate efficiencies along with their nominal operational potential. By replacing the traditional vanadium-based anolyte with a titanium-based one (Ti³⁺/Ti⁴⁺), and retaining the vanadium catholyte (V⁴⁺/V⁵⁺), we have achieved a cost-effective and highly stable flow battery configuration that combines the strengths of both redox systems, and delivers an excellent efficiency at a stable nominal voltage.

This work addresses the material, cost, and performance limitations of conventional vanadium redox flow batteries through three key innovations. First, the use of an abundant and inexpensive titanium-based anolyte to reduce the overall system cost by reducing the chemical procurement costs. Second, the incorporation of a ionic-liquid to the electrolyte enables a wider electrochemical window and supports greener electrolyte chemistry. Third, the replacement of Nafion with a cost-effective SPEEK membrane further reduces material expenses while maintaining compatibility with the ionic-liquid system. Collectively, these advances deliver high electrochemical performance and long-term cycling stability, marking a significant step toward scalable, sustainable, and economically viable energy storage solutions for grid-level applications.

Experimental Section

Materials and Chemicals:

Titanium chloride solution (TiCl₃), vanadium pentoxide (V₂O₅), sulfuric acid (H₂SO₄), and hydrogen peroxide (H₂O₂) were utilized in the preparation of the anolyte, and catholyte solutions respectively without further purification. Further, 1-butyl-3-methylimidazolium chloride (BmimCl), and vanadium chloride (VCl₃) were utilized to synthesize ionic-liquid based catholyte solution. Moreover, commercially available Nafion, and SPEEK membranes were utilized along with graphite current collector plates, and carbon felt electrodes.

Electrolyte preparation:

The catholyte was prepared by dissolving 1M (molꞏL^− 1) V₂O₅, in an aqueous solution of 3M H₂SO₄, 1M H₂O₂ and deionized water at room temperature while stirring for approximately 4 hours, whereas, the homogenous solution of TiCl₃ was utilized as obtained without any further purifications or dilutions. The preparation process of ionic-liquid based catholyte has been detailed in our earlier studies[23].

Electrochemical Characterization:

Cyclic voltammetry (CV) measurements were conducted using a conventional three-electrode configuration connected to a potentiostat (Palmsens MultiTrace 4). A carbon felt electrode (1 cm²) was employed as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl (3 M KCl) electrode served as the reference electrode. All measurements were carried out in 1 M solutions of the anolyte and catholyte, prepared using analytical grade chemicals and deionized water. Prior to each experiment, the carbon felt working electrode was washed thoroughly with ethanol and deionized water, and dried under ambient conditions. Cyclic voltammograms were recorded over an appropriate potential window (from − 0.4 V to 0.2 V vs. Ag/AgCl for anolyte, and − 0.2 V to 1.2 V vs Ag/AgCl for catholyte) at varying scan rates (10–100 mV/s), depending on the system. All measurements were performed at room temperature under ambient atmosphere.

Furthermore, the same setup has been used while performing the electrochemical impedance spectroscopy (EIS) studies. Palmsens MultiTrace 4 system has been utilized in this measurement, and the test was conducted over a frequency range of 0.01 Hz to 100 kHz with an AC potential perturbation of 5 mV amplitude, measured against the OCP for both the anolyte and catholyte solutions separately.

Redox Flow Battery Testing:

A symmetric redox flow battery with different catholyte and anolyte solutions, has been assembled which comprised of two pieces of graphite plates, and carbon foam as the electrode with an area of 1 cm² (1cm x 1 cm), and Nafion, and SPEEK membranes were utilized for the proton transfer. The flow rate of the catholyte, and the anolyte solutions has been fixed at 50% of the overall RPM of the pump, which corresponds to a mass flow rate of 0.54 gꞏsec^− 1. Furthermore, the volumes of the catholyte and anolyte solutions utilized were 10 mL respectively. The charge and discharge studies of the novel VTRFB, were evaluated by utilizing LANHE – CT3002A (LANHE, Wuhan, China) at an average charge and discharge currents of 100 mA, and 50 mA respectively. Unless otherwise specified, the cut – off voltage was specified at 0.5 V for the discharge, and 1.3 V during the charging process at a charge capacity of 100 mAh, throughout this work.

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