Electrolytes For Redox Flow Batteries Research Articles

Improved properties of positive electrolyte for a vanadium redox flow battery by adding taurine

Taurine was employed as an additive to improve the thermal stability and electrochemical performance of positive electrolyte for a vanadium redox flow battery. The addition of taurine could significantly improve the thermal stability of positive electrolyte, and 2 M V(V) electrolyte with 4 mol% taurine could keep it stable at 40 °C for 120 h, which was 54 h longer than the pristine one. Electrochemical measurements showed that the electrolyte with taurine exhibited superior electrochemical activity and reaction kinetics with a larger diffusion coefficient, exchange current density and reaction rate constant compared with the pristine one. Moreover, the cell using taurine as additive achieved higher average energy efficiency (81.75%) than the pristine cell (79.15%). The Raman and XPS spectroscopy illustrated that taurine could combine with VO2+ to form a small molecule complex and the –NH2 in taurine could be adsorbed on the surface of the electrode to provide more active sites for the electrode reaction, which led to the improvement of mass transfer and the charge transfer process for the V(IV)/V(V) redox reaction.

Effect of quinone additives on the performance of electrolytes for vanadium redox flow Batteries

Anthraquinone-2, 6-disulfonic acid (AQDS) was added to the electrolyte of a vanadium redox flow battery (VRFB) with the goal of enhancing its performance. This study utilized carbon paper electrodes and a Nafion 117 ion exchange membrane for the membrane-electrode assembly. The cycling performance was investigated over multiple charge and discharge cycles. The addition of AQDS was found to increase capacity efficiency by an average of 7.6% over the control VRFB containing no AQDS, while decreasing the overall cycle duration by ~18%. It is thus reported that the addition of AQDS to a VRFB electrolyte has the potential to inexpensively increase the activity and capacity.

The role of phosphate additive in stabilization of sulphuric-acid-based vanadium(V) electrolyte for all-vanadium redox-flow batteries

Catholyte in all-vanadium redox-flow battery (VRFB) which consists of vanadium salts dissolved in sulphuric acid is known to be stabilized by phosphoric acid to slow down the thermal aging at temperatures higher than 40 °C. To reveal the role of phosphoric acid, the thermally-induced aggregation is investigated using variable-temperature 51V, 31P, 17O, 1H nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering (DLS). The results indicate that the thermal stabilization of vanadium(V) electrolyte is attained by the involvement of monomeric and dimeric vanadium(V) species in the reaction with phosphoric acid which is concurrent to the formation of neutral hydroxo-aqua vanadium(V) precipitation precursor. The dimers are stabilized by counter ions due to association reaction or if such stabilization is not possible, precipitation of vanadium pentoxide is favored. The evolution of particles size distributions at 50 °C in electrolyte samples containing 1.6 M vanadium and 4.0 M total sulphate and the pathways of precipitate formation are discussed. The optimal total phosphate concentration is found to be of 0.15 M. However, the induction time is assumed to be dependent not only on the total phosphate concentrations, but also on the ratio of total vanadium(V) to sulphate concentrations.

MetILs3: A Strategy for High Density Energy Storage Using Redox‐Active Ionic Liquids

A systematic approach is presented for increasing the concentration of redox‐active species in electrolytes for nonaqueous redox flow batteries (RFBs). Starting with an ionic liquid consisting of a metal coordination cation (MetIL), ferrocene‐containing ligands and iodide anions are substituted incrementally into the structure. While chemical structures can be drawn for molecules with 10 m redox‐active electrons (RAE), practical limitations such as melting point and phase stability constrain the structures to 4.2 m RAE, a 2.3× improvement over the original MetIL. Dubbed “MetILs3,” these ionic liquids possess redox activity in the cation core, ligands, and anions. Throughout all compositions, infrared spectroscopy shows the ethanolamine‐based ligands primarily coordinate to the Fe2+ core via hydroxyl groups. Calorimetry conveys a profound change in thermophysical properties, not only in melting temperature but also in suppression of a cold crystallization only observed in the original MetIL. Square wave voltammetry reveals redox processes characteristic of each molecular location. Testing a laboratory‐scale RFB demonstrates Coulombic efficiencies >95% and increased voltage efficiencies due to more facile redox kinetics, effectively increasing capacity 4×. Application of this strategy to other chemistries, optimizing melting point and conductivity, can yield >10 m RAE, making nonaqueous RFB a viable technology for grid scale storage.

Kinetic studies at carbon felt electrodes for vanadium redox-flow batteries under controlled transfer current density conditions

In this work we present a combination of mathematical modeling and experimental kinetic characterization of carbon felt electrodes in positive electrolyte (PE) and negative electrolyte (NE) of vanadium redox-flow batteries. The mathematical model is applied to check for homogeneous transfer current density within a radially connected carbon electrode intensively flown through by the electrolyte. Transfer current homogeneity depends mainly on electrolyte conductivity, electrode thickness and charge transfer coefficient. The transfer current inhomogeneity of the investigated electrode samples is below 5%, when single layers of carbon electrodes (415μm thickness at 89% porosity and 210μm thickness at 75% porosity) are applied and high electrolyte conductivities (>800mS∙cm−1) are maintained by low vanadium concentrations (<15mM). Experimental results show charge transfer coefficients for high overpotentials of 0.26±0.03 for the reduction reaction in NE and 0.37±0.04 for the oxidation reaction in NE. The charge transfer coefficients of the PE are 0.13±0.02 for reduction and 0.30±0.04 for the oxidation reaction. Application of the Butler-Volmer equation to describe the polarization behavior shows adequate agreement with the experimental results at states of charge between 25% and 75%. The rate constants for the PE reaction are nearly two times higher than for the NE reaction.

A high-temperature tolerance solution for positive electrolyte of vanadium redox flow batteries

Vanadium redox flow battery (VRFB) is attractive for energy storage applications, but there still remains a problem of preventing V-precipitation reaction (i.e., V2O5) to provide the thermal stability of electrolyte employed in VRFB. The V2O5 precipitation is accelerated in the positive electrolyte under high temperature, resulting in decreasing the charge capacity and energy efficiency of VRFB. So far, previous supporting materials, which are used to solve such precipitation problem, provide a disadvantage of decreasing voltage efficiency of VRFBs by increasing electrolyte resistance. This paper describes an advanced vanadium-positive electrolyte with high-temperature tolerance for high-efficiency VRFBs, which uses a sodium formate as a supporting material. The sodium formate plays a role as an agent capable of preventing precipitation reaction in the positive electrolyte and it also provides an effect of decreasing a viscosity of the positive electrolyte. The effectiveness of the proposed electrolyte solution is demonstrated through the following experiments: UV–vis spectrometry, viscosity measurement, cyclic voltammetry (CV), VRFB operation and scanning electron microscopy (SEM) analysis. Then, for the performance comparison of high temperature stability, all experiments are carried out at 60°C. Experimental results show that using the sodium formate leads to near 4.5 times increase of diffusion coefficient as compared to conventional electrolyte, and also provides 20.2% higher charge capacity (50th cycle) and 2.27% higher average energy efficiency (50cycles) at the current density of 80mAcm−2. In addition, it appears that the precipitation of vanadium species is not observed in the electrolyte during VRFB operation. Therefore, this paper provides that new direction about effect of the additive in the positive electrolyte and the sodium formate can be considered as a promising additive for high-performance electrolyte of VRFBs.

Reduction potential tuning of first row transition metal MIII/MII (M = Cr, Mn, Fe, Co, Ni) hexadentate complexes for viable aqueous redox flow battery catholytes: A DFT study

We systematically investigated the tuning of the reduction potentials (E0) for the first row transition metal (M=Cr, Mn, Fe, Co, Ni) complexes with the functionalized 1,4,7-Triazacyclononane-N,N',N”-triacetate (TCTA) ligands by means of DFT calculations. To predict reliable E0, the modified UB3LYP functional and equilibrium concept between high and low spin states were utilized. The functional groups NH2, CN, −F, and NO2 were attached to the carbon atoms carefully selected by considering the LUMO and steric hindrance. Based on firmed accuracy of DFT calculations, finally we obtained the calculated E0 for a series of complexes. It was found that electron donating group such as NH2 will cause a negative shift of E0 while electron withdrawing groups have the opposite effect. The overall trend of the calculated E0s according to ligand modifications were predicted to have the order as −NH2< Pristine <−F≈−CN <−NO2. In addition, optimized geometries, LUMO, vertical electron attachment and energy components constituting E0 were discussed in detail to assist the further understanding for E0s. Consequently, we suggested that 16 complexes can play a role as an electrolyte in aqueous redox flow battery. They can be classified into 5 groups having similar E0 ranges: Group I (- 0.6V∼− 0.7V), Group II (around 0.0V), Group III (around 0.3V), Group IV (0.6∼0.8V) and Group V (1.1∼1.2V). Especially, it is expected that [MnLF], [MnLCN] and [NiLNH2] can be used as promising catholyte candidates possibly possessing high E0s which almost reach to the reduction potential limitation 1.25V in aqueous redox flow battery. Our systematic approach to tune E0 can be applied to the design of other complexes via rational ligand modification.

Developing Electrolyte for a Soluble Lead Redox Flow Battery by Reprocessing Spent Lead Acid Battery Electrodes

The archival value of this paper is the investigation of novel methods to recover lead (II) ions from spent lead acid battery electrodes to be used directly as electrolyte for a soluble lead flow battery. The methods involved heating electrodes of spent lead acid batteries in methanesulfonic acid and hydrogen peroxide to dissolve solid lead and lead dioxide out of the electrode material. The processes yielded lead methanesulfonate, which is an electrolyte for the soluble lead acid battery. The lead (II) ions in the electrolyte were identified using Inductively Coupled Plasma Mass Spectroscopy and their electrochemistry confirmed using cyclic voltammetry. The concentration of lead (II) ions was determined and it was found that using the higher concentration of hydrogen peroxide yielded the highest concentration of lead (II) ions. The method was therefore found to be sufficient to make electrolyte for a soluble lead cell.

High Capacity Electrolytes for Non-Aqueous Redox Flow Batteries – a Positive Focus

The development of low-cost, large-scale energy storage technologies is required for increased reliance on non-polluting, intermittent renewable energy sources connected to our electrical grid. Our research focuses on the development of organic materials for charge storage in non-aqueous redox flow batteries (RFBs). At present, most commercial RFBs contain aqueous, vanadium-based electrolytes, which have been demonstrated on scales as large as 1.5 MW. With non-aqueous electrolytes, a wider electrochemical window allows for 2-3 times higher operating voltages, while avoiding the use of corrosive electrolytes. Most of the organic species reported as electron donors are only stable in the neutral and singly oxidized states; removal of a second electron results in an unstable dication. Here we sought to increase the stability of dication species without significantly raising molecular weight, thus preserving atom economy. Considering phenothiazine donors, we found that the addition of certain substituents led to improved stability of the dication form; specifically, electron-donating methoxy groups (Figure 1) are more effective in stabilization than non-conjugated substituents. This presentation will include the synthesis of these materials, as well as electrochemical and spectroelectrochemical analysis. Furthermore, the results of density functional theory calculations will be presented to evaluate the design of a stable two-electron donor. Figure 1. Structural representation of 3,7-dimethoxyphenothiazine derivatives (a) and cyclic voltammograms of N-ethyl-3,7-dimethoxyphenothiazine (DMeOEPT) showing reversible first and second oxidations (b). Figure 1

Thermodynamics of Polymer Electrolyte Membranes in Contact with Concentrated Electrolyte Solutions

In this study, we build off our recent development of a database of data describing the composition of polymer electrolytes in contact with various acids, bases and salts. This situation is common in practical electrochemical systems such as redox flow batteries and electrochemical reactors. For example, the vanadium redox flow battery (VRFB) electrolyte solutions consist of several molar solutions of aqueous sulfuric acid (with other acids used in certain cases) and up to 2M vanadium salts. As we have shown, these concentrations cause the general phenomenon of breakdown of the Donnan exclusion effect in a variety of polymer electrolyte membranes, both cation and anion exchanging membrane types.1,2,3 A description of these processes would be useful in helping to separate the effects of various modifications of the membrane composition and structure on its behavior. We develop the groundwork for a thermodynamic description of uptake of ionizable species by polymer electrolyte membranes in contact with concentrated solution electrolyte solutions using the VRFB membrane-electrolyte combination as our test case. We note that all such descriptions of the behavior of polymer systems entails the assumption of a ‘local’ equilibrium, relevant for short times. Here the adjective short refers to times long enough for uptake processes to occur but short enough that the polymer itself does not have time to change from its initial state, set by its thermal history. This allows us to assess membranes that might be thermally ‘re-arranged’ as separate entities, thus isolating any effects of the treatment. We first develop a description of the local equilibrium of the acid solution with the membrane. As Tang et al showed,1 this has several interesting features that are generic to acid uptake into Nafion and related polymer electrolytes. A convenience of this approach is the broad availability of descriptions of the acid solution properties. Once a description of the acid-membrane equilibrium is available, we proceed to consider the situation with added vanadium. For simplicity, we will begin by developing the case for VOSO4 solutions in acid. In this case, we need to reassess the solution thermodynamics, adding some measurements specific to a given ternary composition. We will describe these measurements in the oral presentation. Acknowledgements We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk) and by the Office of Naval Research. We also thank 3M for providing membranes.

Predictive approach to better batteries

Redox-flow batteries (RFBs) aid solar and wind energy facilities by storing energy when it’s generated and then giving it back to the electrical grid when the sun isn’t shining or the wind isn’t blowing. Scientists and engineers would like to improve the size, cost, and longevity of RFB systems, and a new predictive chemistry-based strategy could help. The technique identifies nonaqueous electrolytes for RFBs that will enable the batteries to have high energy density and high stability. It was developed by Melanie S. Sanford of the University of Michigan, Shelley Minteer and Matthew Sigman of the University of Utah, and coworkers (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b00147). Sanford discussed the approach at a Presidential Events session at the ACS national meeting in San Francisco. To charge an RFB, generated energy drives electrons in a circuit toward an electrode where a redox-active electrolyte, the anolyte, is reduced. Another redox electrolyte,

Vanadium proton exchange membrane water electrolyser

Abstract In order to reverse the reactions of vanadium oxygen fuel cells and to regenerate vanadium redox flow battery electrolytes that have been oxidised by atmospheric oxygen, a vanadium proton exchange membrane water electrolyser was set up and investigated. Using an existing cell with a commercial and iridium-based catalyst coated membrane, it was possible to fully reduce V3.5+ and V3+ solutions to V2+ with the formation of oxygen and with coulomb efficiencies of over 96%. The cell achieved a maximum current density of 75 mA/cm2 during this process and was limited by the proximity of the V(III) reduction to the hydrogen evolution reaction. Due to the specific reaction mechanisms of V(IV) and V(III) ions, V(III) solutions were reduced with an energy efficiency of 61%, making this process nearly twice as energy efficient as the reduction of V(IV) to V(III). Polarisation curves and electrochemical impedance spectroscopy were used to further investigate the losses of half-cell reactions and to find ways of further increasing efficiency and performance levels.

Electroactive nanofluids with high solid loading and low viscosity for rechargeable redox flow batteries

A typical dispersion of iron oxide nanoparticles in alkali electrolyte is limited to 20 wt% solids, above which it has a paste-like consistency (>370 cP), incompatible with flow applications. The formulation of stable electrochemically active colloids of nanoscale iron oxide (30–50 nm) with up to 70 wt% solids, low viscosity ( 2 weeks at rest), and 55% enhancement in thermal conductivity is reported herein. A thin surface coating allows for good particle dispersion, which is shown to be crucial for these enhanced features. These suspensions of electrochemically active nanoparticles (nanoelectrofuels) can undergo electrochemical charge and discharge in fluidized format through particle–electrode impact events with potential for application in redox flow batteries. The surface coating is found to partially suppress electrochemical access to the nanoparticle in a fluidized format but has no detrimental effects on discharge capacity in the solid state. This approach is also shown to suppress a parasitic nanoparticle agglomeration process, which is otherwise dominant during electrochemical cycling of pristine iron oxide. A dissolution re-precipitation mechanism is proposed to offer insight into this auxiliary benefit. This study provides the first insight into the feasibility of adopting electrochemically active nanofluids as high energy density redox flow battery electrolytes.

Bulk Aging of Graphite-Polypropylene Current Collectors Induced by Electrochemical Cycling in the Positive Electrolyte of Vanadium Redox Flow Batteries

Carbon-based bipolar plates (BPP) are essential components of the vanadium redox flow battery (VRFB) stack guaranteeing electrical conductivity and physical separation of adjacent cells. They are usually made of composite materials with complex structures containing conductive carbon components and polymer binder materials. In order to investigate their aging by simulation of repetitive charging/discharging conditions in areas with sluggish electrolyte flow, commercially available graphite-polypropylene BPPs were subjected to galvanodynamic treatment under diffusion limitation conditions in a three electrode electrochemical cell with positive electrolyte (VO2+/VO2+) using different states of charge (SOC). The current was swept between 100 and −100 mA/cm2. After aging, cyclic voltammetry (CV) in diluted H2SO4 was used to analyze the change in double layer capacitance and redox activity of the BPPs. Morphological investigations of the surface texture and bulk material were performed by means of scanning electron and confocal microscopies as well as by X-ray computed microtomography. After 3000 sweeping cycles in the positive electrolyte at high SOC the BPP showed an increase in double layer capacitance as well as higher roughness and increase in open pore volume in depths up to 100 μm. The aging effects are ascribed to oxidation and corrosion of the surface.

Bioinspired, high-stability, nonaqueous redox flow battery electrolytes

A promising nonaqueous redox flow battery electrolyte has been developed by leveraging natural selection to elucidate stable, redox-active molecules.

Systematic selection of solvent mixtures for non-aqueous redox flow batteries – vanadium acetylacetonate as a model system

Employing solvent mixtures in the electrolyte of non-aqueous redox flow batteries can increase energy density and efficiency. In this paper, active species solubility, electrolyte conductivity, and redox reaction rates were examined systematically among a number of binary and ternary mixtures, consisting of acetonitrile and 5 polar aprotic co-solvents to identify mixtures with enhanced active species solubility and redox reaction rates. Although we used vanadium acetylacetonate as a model, the methodologies presented here are applicable when evaluating solvent mixtures for other active species. Our approach is distinctive in that it elucidated the trade-offs in desirable properties that are necessary when solvent mixtures are used for non-aqueous redox flow batteries. We found that in a vanadium acetylacetonate–based non-aqueous redox flow battery, the use of an 84/16vol% acetonitrile/1,3-dioxolane binary solvent mixture resulted in an increase in the positive and negative sides reaction rates compared to that observed with pure acetonitrile. This binary mixture resulted in an improvement in reaction rate with no decrease in energy density and is a promising solvent system for other active species used for non-aqueous flow batteries.

Cost-driven materials selection criteria for redox flow battery electrolytes

Abstract Redox flow batteries show promise for grid-scale energy storage applications but are presently too expensive for widespread adoption. Electrolyte material costs constitute a sizeable fraction of the redox flow battery price. As such, this work develops a techno-economic model for redox flow batteries that accounts for redox-active material, salt, and solvent contributions to the electrolyte cost. Benchmark values for electrolyte constituent costs guide identification of design constraints. Nonaqueous battery design is sensitive to all electrolyte component costs, cell voltage, and area-specific resistance. Design challenges for nonaqueous batteries include minimizing salt content and dropping redox-active species concentration requirements. Aqueous battery design is sensitive to only redox-active material cost and cell voltage, due to low area-specific resistance and supporting electrolyte costs. Increasing cell voltage and decreasing redox-active material cost present major materials selection challenges for aqueous batteries. This work minimizes cost-constraining variables by mapping the battery design space with the techno-economic model, through which we highlight pathways towards low price and moderate concentration. Furthermore, the techno-economic model calculates quantitative iterations of battery designs to achieve the Department of Energy battery price target of $100 per kWh and highlights cost cutting strategies to drive battery prices down further.

Understanding Speciation in Ionic-Liquid Electrolytes for Non-Aqueous Redox Flow Batteries

Conventional aqueous redox flow batteries (RFB) are limited by low energy and power densities; thus, recent focus has been on development of non-aqueous electrolytes. Although organic solvents allow for increased voltage windows, they possess a several drawbacks, including sensitivity to moisture, low solubility of active species, and high flammabilities and toxicities. Ionic liquids (IL) offer a “greener” alternative to volatile organic electrolytes as they are non-toxic and nonflammable [1, 2]. Iron chloride based IL electrolytes containing up to 6.3 M iron have been synthesized. It was found that electrolyte conductivity and viscosity, as well as the nature of the electroplated iron, are greatly influenced by electrolyte composition. An electrolyte containing iron chloride, choline chloride, and ethylene glycol in a 1:1:4 molar ratio was attractive due to its high plating efficiencies, fast kinetics, and acceptable conductivity [3]. Electrochemical reactivity of the solute ions as well as the physical properties of the electrolyte are controlled by speciation of the metals in solution. To understand what ions are responsible for the observed properties in these iron IL electrolytes, chemical speciation was investigated using x-ray absorption spectroscopy (XAS) as well as x-ray photoelectron spectroscopy (XPS). The two techniques indicate the presence of organometallic compounds which may hinder the electrokinetics as well as the fluid properties. For example, the conductivity is a function of the mole fraction of iron as well as the iron to chloride ratio (Fig 1 below). Complexes such as [FeCl4]-have been shown to exhibit enhanced fluid properties [4], however, when the iron:chloride ratio is less than 1:4, di-iron species have been known to form [5]. The properties of these iron IL electrolytes as a function of composition coupled with results from XAS and XPS to explain observed trends will be presented. [1] M. J. Earle and R. K. Seddon, Pure Appl. Chem., vol. 72, p. 1391, 2000. [2] K. N. Marsh, A. Deev, A. C.-T. Wu, T. E., and A. Klamt, Korean J Chem. Eng. , vol. 19, 2002. [3] J. S. W. M. A. Miller, and R. F. Savinell, "Iron Ionic Liquid Electrolytes for Redox Flow Battery Applications," Journal of The Electrochemical Society, vol. 163, pp. A578-A579, 2016. [4] I. M. H. C. A. Angell, P. A. Cheeseman, "Physico-chemical and computer simulation studies of the role of cation coordination numbers on melt physical properties," Electrochemical Society Proceedings, p. 138, 1976. [5] M. S. Sitze, E. R. Schreiter, E. V. Patterson, and R. G. Freeman, "Ionic Liquids Based on FeCl3 and FeCl2. Raman Scattering and ab Initio Calculations," vol. - 40, pp. - 2304, 2001. Figure 1

Effects of Additives on the Stability of Positive Electrolytes for All-Vanadium Redox Flow Battery

The all- Vanadium redox flow battery (VRFB) is a promising technology for large-scale electrical energy storage systems[1, 2]. VRFB is an electrochemical system using reactions of two redox couples of V2+/V3+ in a negative half-cell and VO2+/VO2 + in a positive half-cell to perform a reversible conversion between electrical energy and chemical energy. However, poor stability of the electrolytes, especially the positive electrolyte, greatly limits the concentration of redox couples in the electrolyte solution, which significantly affects the performance of VRFBs[3, 4]. In order to achieve high electrolyte performance, a sodium pyrophosphate dibasic (SPD) introduced as an inorganic additive in positive electrolyte VO2 + for VRFBs. And its effects on long-term stability and electrochemical performances were investigated, through UV-vis spectrophotometry, cyclic voltammetry, and through the performance of a charge/discharge test. [1] A. Parasuraman, T.M. Lim, C. Menictas, M. Skyllas-Kazacos, Review of material research and development for vanadium redox flow battery applications, Electrochimica Acta, 101 (2013) 27-40. [2] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, Redox flow batteries: a review, Journal of Applied Electrochemistry, 41 (2011) 1137-1164. [3] J. Zhang, L. Li, Z. Nie, B. Chen, M. Vijayakumar, S. Kim, W. Wang, B. Schwenzer, J. Liu, Z. Yang, Effects of additives on the stability of electrolytes for all-vanadium redox flow batteries, Journal of Applied Electrochemistry, 41 (2011) 1215-1221. [4] S.-K. Park, J. Shim, J.H. Yang, C.-S. Jin, B.S. Lee, Y.-S. Lee, K.-H. Shin, J.-D. Jeon, Effect of inorganic additive sodium pyrophosphate tetrabasic on positive electrolytes for a vanadium redox flow battery, Electrochimica Acta, 121 (2014) 321-327.

Effect of Ti(IV) Ion on Mn(III) Stability in Ti/Mn Electrolyte for Redox Flow Battery

The mixed electrolyte of titanium and manganese is one of the promising candidates as low-cost electrolytes for redox flow batteries. In this study, the effect of Ti4+ ion in the positive electrolyte was examined in detail. With an increase of Ti4+ ion concentration, the formation reaction of MnO2 particle was slowed down significantly, which means the stability of Mn3+ ion is enhanced. Moreover, the particle size of MnO2 precipitate became smaller in electrolytes containing certain amount of Ti4+ ion. These MnO2 particles were small enough to be flowed in the electrolyte and to be reduced to Mn2+ ion again. Ti4+ ion also affected the crystal structures of MnO2, limiting to gamma-MnO2 which is assumed to be reactive. In cyclic voltammetry study, the higher Ti4+ ion concentration was, the better reversibility of Mn redox reaction was observed. The ion interaction between Ti4+ and Mn3+ was confirmed by UV-visible spectroscopy.