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Manganese (Mn), possessing ample reserves on the earth, exhibits various oxidation states and garners significant attentions within the realm of battery technology. Mn-based flow batteries (MFBs) are recognized as viable contenders for energy storage owing to their environmentally sustainable nature, economic feasibility, and enhanced safety features. Nevertheless, the advancement of MFBs is hindered by contentious reaction mechanisms, suboptimal energy density, and inadequate cycling stability. This review offers a comprehensive analysis of various MFBs based on the specific redox couples utilized in the catholyte, including Mn3+/Mn2+, MnO2/Mn2+, and MnO4−/MnO42−. Moreover, recent advancements and concerns encountered by each type of MFBs are subsequently addressed and discussed in detail. Additionally, the current understanding of the mechanisms for different Mn-based pairs and their potentials for energy storage applications are introduced. Finally, challenges for the future development of MFBs, along with suggested improvement strategies are outlined. [ABSTRACT FROM AUTHOR]

Anthraquinone-based molecules are promising electroactive materials for use in aqueous organic flow batteries. At high concentrations in aqueous solutions, the well-known negolyte molecule anthraquinone disulfonic acid (AQDS) molecule has been observed to aggregate under near-neutral and acidic conditions. Aggregation has been hypothesized to be directly linked to observed concurrent capacity reduction. In this study, we investigated three different water-soluble anthraquinones in electrolytes of varying compositions and pH to gain further insight into the possible causes of capacity loss. We used low-field benchtop 1H-NMR and diffusion NMR measurements directly in non-deuterated aqueous flow-battery electrolytes to investigate molecular aggregation. Single-cell testing was performed under exhaustive electrolysis conditions to determine the number of electrons exchanged per molecule. We observed a decrease in the number of electrons exchanged per molecule in the presence of carbonate ions due to CO2 adduct formation. The aggregation constants were determined from both concentration-dependent chemical shifts and self-diffusion coefficients. We show that aggregation of the oxidized form of all three molecules studied here occurs under near-neutral and alkaline conditions and does not affect the number of accessible e−. [ABSTRACT FROM AUTHOR]

In this installment of our "Looking at Patent Law" series, we present a case study of the prosecution of US Patent No. 9,559,375: "Iron Flow Battery." This invention aligns with several ECS divisions, including Industrial Electrochemistry and Electrochemical Engineering (IE&EE), Battery (BATT), Energy Technology (ETD), and Electrodeposition (ELDP). The '375 patent issued on January 31, 2017 with inventors Robert F. Savinell and Jesse S. Wainright. The assignee of the patent was Case Western Reserve University (CWRU). The case study begins with a brief synopsis of the background of the invention followed by 1) summary of key drawings and the specification of the invention, 2) inventor assignment and power of attorney designations, 3) submission of the Invention Disclosure Statement (IDS) and associated Duty of Candor, 4) summary of the non-final office action (NF-OA) and rejection, and 5) applicant response and allowance of the patent application. The case study illustrates overcoming "lack of novelty" and "obviousness" rejections by combining the limitation of dependent claims with the independent claim. With this case study, we hope to de-mystify the patent prosecution process and better prepare electrochemical and solid-state scientists, engineers, and technologists to interact with their patent counsel regarding their inventions. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) can achieve long lifetimes and high performance when employing highly selective and conductive membranes. Neutral and alkaline RFBs suffer from higher resistances due to lower cation conductivity, compared to acidic RFBs utilizing proton transport. We report the use of a sulfonated Diels-Alder poly(phenylene) membrane that exhibits low and stable potassium area specific resistance and high efficiency RFB cycling relative to Nafion, as well as undetectable ferricyanide crossover. An alkaline (pH 12) organic anthraquinone derivative RFB using this membrane demonstrates over 10 days of cycling without capacity loss from crossover. A neutral chelated chromium complex RFB using this membrane demonstrates a peak discharge power of 1.23 W cm-2, and 80% energy efficiency (EE) cycling at an average discharge power density of 446.3 mW cm-2. Finally, the membrane exhibits similar favorable conductivity for many monovalent cations, opening the opportunity to improve the cycling and crossover performance of other acidic, neutral, and alkaline RFBs. [ABSTRACT FROM AUTHOR]

Improvements in redox flow battery (RFB) performance and durability can be achieved through the development of new active materials, electrolytes, and membranes. While a rich design space exists for emerging materials, complex tradeoffs challenge the articulation of unambiguous target criteria, as the relationships between component selection and cycling performance are multifaceted. Here, we derive zero-dimensional, analytical expressions for mass balances and cell voltages under galvanostatic cycling, enabling direct connections between material/electrolyte properties, cell operating conditions, and resulting performance metrics (e.g., energy efficiency, capacity fade). To demonstrate the utility of this modeling framework, we highlight several considerations for RFB design, including upper bound estimation, active species decay, and membrane/separator conductivityselectivity tradeoffs. We also discuss modalities for extending this framework to incorporate kinetic losses, distributed ohmic losses, and multiple spatial domains. Importantly, because the mass balances are solved analytically, hundreds of cycles can be simulated in seconds, potentially facilitating detailed parametric sweeps, system optimization, and parameter estimation from cycling experiments. More broadly, this approach provides a means for assessing the impact of cell components that simultaneously influence multiple performance-defining processes, aiding in the elucidation of key descriptors and the identification of favorable materials combinations for specific applications. [ABSTRACT FROM AUTHOR]

Increasing energy demand throughout the world produces great environmental issues, therefore, renewable and clean energy sources, such as tidal energy, wind energy, solar energy and geothermal energy, are desirable request. Meanwhile, developing a new critical energy storage technology to balance the instantaneous energy supply and demand of arranged electric energy is urgent. Rechargeable flow batteries are solutions for storing electricity in form of chemical energy, containing positive and negative electrodes reserved in two separate containers, which have the advantages of low self—discharge and independent scaling of power, therefore considered as promising energy storage technologies. Ionic liquids (ILs) have been widely studied and used in energy storage devices, such as lithium ion battery, for their unique prospective properties. Herein, the key role of ILs and their applications in supporting electrolytes, separators and additives in flow batteries are highlighted in this review. The approaches and challenges in developing ILs supported flow batteries are discussed, and a significative overview of the opportunities of ILs promote flow batteries are finally provided, which is expected to help achieving further improvements in flow batteries. [ABSTRACT FROM AUTHOR]

The mass transfer performance is unclear to limit the development of organic flow battery, which is regarded as the emerging electrochemical energy storage technology. The mass transfer behaviors in the electrode are influenced by the flow fields, which is indispensable. A three-dimensional and steady numerical model of the organic flow battery is established and the results are verified by the experiments data. The battery performance and mass transfer behaviors are analyzed under different flow field for the charge/discharge processes based on this model. Compared with other flow fields, the interdigital flow field can achieve the best charge-discharge performance, which is mainly due to the improvement distribution uniformity of active species. The average concentration of the interdigital flow field is 45.1% higher that of the conventional flow field. The effects of the flow rates and initial concentration of electrolyte on the battery performance are investigated, and the results indicate that appropriate inlet flow rate can lead to the highest net discharge powers and power-based efficiency of the flow battery. Although the increasing of the initial concentration can improve the battery performance, the improvement is too slight when the concentration is high enough, which can lead to the waste of active species. [ABSTRACT FROM AUTHOR]

The Vanadium Flow Battery (VFB) was taken from the initial concept stage at UNSW in 1983 through the development and demonstration of several 1–5 kW prototypes in stationary and electric vehicle applications in the 1990s with on-going research activities continuing to the present day. As part of this 38 year R&D program, a wide range of research projects was undertaken in the areas of electrodes and membranes, electrolyte process development and characterisation, electrolyte additives and precipitation inhibitors, conducting plastic bipolar plate formulation and manufacturing trials, mathematical modelling of membrane transfer, shunt currents and pumping energy losses in bipolar stacks, thermal modelling of redox cells under a range of operating conditions, impedance spectroscopy and control system development, sensors and state-of-charge monitoring, chemical and electrochemical rebalancing, gelled electrolyte, vanadium bromide cell and V/O redox fuel cell. Several field trials of the VFB were conducted by UNSW in the mid-1990s with early licensing leading to large-scale demonstrations and significant commercialisation activities in a wide range of applications around the world. This paper presents an historical overview of the research, development and early field trials of the All-Vanadium Redox Flow Battery at UNSW between 1983 and 2021. [ABSTRACT FROM AUTHOR]

Aqueous Zn-iodine redox flow batteries have aroused great interest for the features of high capacity, excellent stability, low cost, and high safety, yet the dissatisfying energy efficiency still limits their future advancement. In this work, three-dimensional semiconductor BiVO4 nanoparticles decorated hierarchical TiO2/SnO2 arrays (BiVO4@TiO2/SnO2) were applied as photocathode in Zn-iodine redox flow batteries (ZIRFBs) for the realization of efficient photo-assisted charge/discharge process. The photogenerated carriers at the solid/liquid interfaces boosted the oxidation process of Iâˆ', and thus contributed to a significant elevation in energy efficiency of 14.9% (@0.5 mA cmâˆ'2). A volumetric discharge capacity was extended by 79.6% under light illumination, owing to a reduced polarization. The photocathode also exhibited an excellent durability, leading to a stable operation for over 80 h with a maintained high energy efficiency of ∼90% @0.2 mA cmâˆ'2. The research offers a feasible approach for the realization of high-energy-efficiency aqueous Zn-iodine batteries towards high-efficiency energy conversion and utilization. [ABSTRACT FROM AUTHOR]

Batteries which use dissolved redox-active species, such as redox flow batteries (RFBs), are often considered to be constrained in their operation and energy density by the solubility limit of the redox species. Here, we show that soluble redox active electrolytes can be reversibly cycled deeply into the precipitation regime, permitting higher effective concentrations, energy densities, and lower costs. Using aqueous sodium polysulfide negative electrolytes cycled in the nominal Na2S2 to Na2S4 capacity range as an example, we show that the effective solubility can be increased from 5 M in the fully-dissolved state to as much as 10 M using the precipitation strategy. Stable cycling was observed at 8 M concentration over more than 1600h at room temperature. We also analyze the range of polysulfide electrochemical stability, and characterize the precipitate composition. This enhanced effective concentration approach may be generalized to other redox chemistries that utilize solubilized reactants, and may be especially useful for long-duration storage applications where slow charge-discharge rates allow equilibration of precipitated species with the redox-active solution. [ABSTRACT FROM AUTHOR]

Zinc-based redox flow batteries (ZRFBs) have been considered as ones of the most promising large-scale energy storage technologies owing to their low cost, high safety, and environmental friendliness. However, their commercial application is still hindered by a few key problems. First, the hydrogen evolution and zinc dendrite formation cause poor cycling life, of which needs to ameliorated or overcome by finding suitable anolytes. Second, the stability and energy density of catholytes are unsatisfactory due to oxidation, corrosion, and low electrolyte concentration. Meanwhile, highly catalytic electrode materials remain to be explored and the ion selectivity and cost efficiency of membrane materials demands further improvement. In this review, we summarize different types of ZRFBs according to their electrolyte environments including ZRFBs using neutral, acidic, and alkaline electrolytes, then highlight the advances of key materials including electrode and membrane materials for ZRFBs, and finally discuss the challenges and perspectives for the future development of high-performance ZRFBs. [ABSTRACT FROM AUTHOR]

It is a privilege for me to write this brief perspective on the transition of John B. Goodenough from Oxford to the University of Texas at Austin (UT Austin). While most people will tend to retire, Goodenough transitioned to UT Austin at the age of 64 in 1986 and has been making phenomenal contributions during the past 35 years, with a genuine passion for science and profound impact on the society at large. To highlight a few, the contributions at UT Austin include the following: chemistry and physics of hightemperature copper oxide superconductors; battery electrodes and electrolytes, including polyanion family of oxide cathodes, niobium titanium oxide anode, and Prussian Blue cathode; alkali-metal plating/stripping; solid-state batteries; flow batteries; oxygen reduction/evolution reaction catalysts; and solid oxide fuel cell electrodes and electrolytes. Finally, what distinguishes Goodenough more than anything else is his unique personal attributes. [ABSTRACT FROM AUTHOR]

Dopamine (DA) exhibits quinone/hydroquinone redox activity at ~0.77 V vs SHE. As the potential is sufficiently high, this can be a replacement to the VO2+/VO2 + redox couple (catholyte) of the vanadium redox flow battery. But DA undergoes polymerization into polydopamine (PDA) while cycling in the flow battery. Therefore, a hybrid flow battery was constructed with PDA coated thermally activated graphite felt positive electrode and V3+/V2+ in 3 M H2SO4 anolyte. The vanadium-PDA flow battery exhibits a capacity of ~275 mAh gPDA -1 in the first cycle. When the battery was subjected to continuous galvanostatic charge-discharge up to 300 cycles, a capacity retention of ~86% was observed with coulombic efficiency close to > 99%. Besides, energy efficiency of ~63% at a current density of 5 A g-1 was observed. The peak power density of the battery was found to be ~8.83 W g-1 at a current density of ~14.31 A g-1. [ABSTRACT FROM AUTHOR]

High voltage aqueous Li-ion batteries have the potential for sustainable large-scale energy storage due to their intrinsic advantages of safety, low cost, as well as environmental friendliness. "Water-in-salt" electrolytes have significantly enhanced the energy density of aqueous Li-ion batteries by extending the aqueous electrolyte stability window to 3.0 V. However, the cathodic limit and salt concentration need to be reduced, enabling to operate low cost LiMn2O4/Li4Ti5O12 cells. In addition, gravimetric energy density of LiMn2O4/Li4Ti5O12, self-discharge rate, and operation temperature range remain rarely studied. This perspective aims to briefly summarize the opportunity and challenges of aqueous batteries for practical applications. [ABSTRACT FROM AUTHOR]

This perspective provides a brief overview of the recent work on electrolytes with hydrogen (H)-bonding network, specifically the deep eutectic solvents (DESs), and outlines important factors to consider when adapting these electrolytes in redox flow batteries (RFBs). The redox behavior, solubility, and stability of several redox molecules of relevance to RFBs in DESs are presented, including some of our work within the Breakthrough Electrolytes for Energy Storage (BEES)--an Energy Frontier Research Center of the United States Department of Energy. Particularly, the challenges and opportunities for further development of DESs for energy storage are discussed. [ABSTRACT FROM AUTHOR]

Metal electrocatalysts have been reported to improve the electron transfer kinetics of aqueous redox flow battery electrolytes on various types of carbon electrodes. In this work, we electrodeposited bismuth metal onto a carbon paper anode of a redox flow battery containing our previously reported polyaminocarboxylate-chelated chromium electrolyte. Depositing 0.58 mg cm-2 of bismuth metal enabled an electrochemically reversible electron transfer for the Cr(II)/Cr(III) couple, resulting in a 3.9% voltage efficiency increase over ten cycles at 100 mA cm-2 across an 80% state of charge window, while maintaining >99% current efficiency. The bismuth electrocatalyst provided other improvements, such as a 13% increase in average discharge power density when cycling at 80% energy efficiency, along with a 60% decrease in charge transfer resistance and 12% decrease in the full cell area specific resistance. A chelated bismuth complex was also utilized as a bismuth source for electrodeposition, providing an example of how polyaminocarboxylate ligands can be further implemented into efforts towards improving the energy efficiency of aqueous redox flow batteries. [ABSTRACT FROM AUTHOR]

This study reports a sodium polysulfide catholyte for nonaqueous redox flow batteries (RFBs). We demonstrate reversible capacities up to 200 mAh/gS with negligible fade over 250 cycles at room temperature for sodium polysulfide|biphenyl full cells containing Na+ß"-Al2O3 solid electrolyte (BASE) membranes. Interestingly, formation of insoluble S and Na2S4 phases did not inhibit the catholyte's cycle life which is likely due to the low concentrations used in the lab-scale prototypes. 3-electrode galvanostatic AC impedance measurements demonstrate that voltage losses were dominated by charge transfer at the cathode, and relevant kinetic parameters (i.e., transfer coefficients and exchange current density) were calculated through a Tafel analysis. To the best of our knowledge, this is the first report applying such an impedance approach to nonaqueous RFBs. Overall, the use of low-cost active materials makes sodium polysulfide|biphenyl RFBs promising for long duration energy storage applications. If strategies are developed to increase the solubility of S and/or low order polysulfides (Na2Sx, x ≤ 4), specific energies up to 100 Wh kg-1 (including combined mass of the anolyte and catholyte) can be achieved. [ABSTRACT FROM AUTHOR]

Comprehensively understanding the behavior of redox-active compounds in organic flow cells is essential to developing low-cost and long service life flow batteries. We develop and validate a zero-dimensional model of the electrochemical performance of an organic flow cell. The model simulates voltage losses from Faradaic charge transfer, Ohmic resistance, and mass transfer, along with the influence of spatial variations in the electrolyte's state-of-charge between the cell and electrolyte reservoir, on the cell's cycling behavior. The model's predictions agree with constant current and constant voltage cycling data for a symmetric ferro-/ferricyanide cell across a wide range of current densities and electrolyte flow rates. We determine the model's voltage loss parameters from electrochemical impedance spectroscopy and voltammetry measurements acquired prior to cycling, rather than fitted a posteriori. In operando measurements of the electrolyte's state-of-charge demonstrate that the finite time for electrolyte flow between its reservoir and the electrochemical cell may significantly affect voltage-current behavior. By modelling active reactant decay, we demonstrate how capacity fade measured in a cell depends on the cycling protocol and reactant decay mechanism. This work shows that zero-dimensional electrochemical modeling helps in elucidating capacity fade mechanisms and optimizing the performance of chemistries under consideration for practical organic flow batteries. [ABSTRACT FROM AUTHOR]

Redox flow batteries have recently received considerable attention as possible large-scale energy storage devices, but their low energy density has inhibited widespread application. In this work, a novel strategy of decoupling conductivity and solubility of electrolytes using microemulsion is put forward to enhance ionic conduction of non-aqueous electrolytes, increase the selectivity of active species, improve the battery voltage, and eventually achieve the possibility of high energy density. We report a study of the electrochemistry of ferrocene in single phase Tween® 20/1-butanol/H2O/toluene microemulsion system at 20 °C. At low and intermediate surfactant to water weight ratios (<0.5/0.5), the voltammogram exhibits reversible electrochemical behavior, while at high surfactant levels the curves show lower levels of reversibility. The latter voltammograms have a form typically associated with high resistance in solution, consistent with a gradual transition in microstructure as surfactant levels increase. This change in structure is supported by correlations with conductivity results based on the literature. The voltammograms show little evidence of anomalies in double layer capacitance or electrode "blocking" by droplets, suggesting that the electron transfer is indeed occurring in a facile manner between the electrode and the ferrocene inside the oil phase. [ABSTRACT FROM AUTHOR]

The transport delay associated with fluid movement within the pipe inevitably exists in the piping system of a flow battery and this can impose a notable impact on battery design and operational performance. In this paper, comprehensive numerical all-vanadium flow battery models with and without transport delay are proposed on the basis of mass balance. Based on such models, the cell performance and prediction precision are evaluated for different cell designs and various operational conditions. The simulation results demonstrate that it is preferable to adopt a longer and thicker piping, a relatively high flow rate, a small electrolyte volume, a high electrolyte concentration, and an appropriate current density for flow battery design and operation. Furthermore, transport delay can be minimized for energy efficiency investigation and incorporated for electrolyte utilization analysis. Such in-depth studies can significantly help to optimize the cell design and operation management and effectively guide the establishment of the mathematical models for flow battery as well. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) possess multiple advantages as a flexible energy storage solution. However, RFB researchers are still facing many challenges in finding an appropriate electrolyte. Microemulsions have recently been proposed as a promising alternative RFB electrolyte because of their ability to accommodate organic redox species with fast electron transfer rates that are not soluble in aqueous phase, while still offering the high conductivity of an aqueous salt electrolyte. In this work, we focused on understanding the transport of ferrocene (Fc) in a toluene/Tween 20/1-butanol/water model microemulsion and studied the compositional influence on Fc diffusion. The results show that Fc redistributes among the oil, surfactant, and water microenvironments, and the corresponding diffusion and partition coefficients are quantified. Thus, a tortuous path diffusion model is proposed to describe the mass transport of Fc to an electrode surface. Diffusion coefficients are also obtained by pulsedfield gradient nuclear magnetic resonance (PFG NMR), while the values for Fc diffusion are substantially higher than those from electrochemical measurements, suggesting that they measure samples in different ways. The current contributions from each microenvironment indicate that the Fc permeability is much higher in the oil, even though the electron transfer reaction is likely occurring in the surfactant. [ABSTRACT FROM AUTHOR]

Deep eutectic solvents (DESs) are promising electrolytes for electrochemical redox reactions, which can be used in redox flow batteries (RFBs). However, in some systems like the Fe2+/3+ redox reaction in ethaline, traditional Tafel-based kinetic analysis generates unreasonable kinetic parameters (i.e., large anodic/cathodic charge transfer coefficients (aa, ac) along with low exchange current densities (i0)). This hinders a comprehensive kinetic and kinetic mechanism study. Here, we perform a detailed investigation of the Tafel analysis using a series of synthetic rotating disk electrode (RDE) data. We find the Tafel analysis only works well when i0 < 0.57 ilim (limiting current density) in our scenario and leads to abnormal kinetic values once i0 exceeds this limiting value. Thus, we propose an extended Butler-Volmer (ex-BV) analysis based on modern non-linear fitting techniques to obtain the actual kinetic parameters for such systems. The results show that this method fits the RDE data closely and generates reliable aa, ac and i0 values, demonstrating that it is a good replacement for traditional Tafel analysis for kinetic studies in highviscosity electrolytes such as DES systems. [ABSTRACT FROM AUTHOR]

Redox flow batteries are a nascent, yet promising, energy storage technology for which widespread deployment is hampered by technical and economic challenges. A performance-determining component in the reactor, present-day electrodes are often borrowed from adjacent electrochemical technologies rather than specifically designed for use in flow batteries. A lack of structural diversity in commercial offerings, coupled with the time constraints of wet-lab experiments, render broad electrode screening infeasible without a modeling complement. Herein, an experimentally validated model of a vanadium redox flow cell is used to generate polarization data for electrodes with different macrohomogeneous properties (thickness, porosity, volumetric surface area, and kinetic rate constant). Using these data sets, we then build and train a neural network with minimal average root-mean squared testing error (17.9 ± 1.8 mA cm-2) to compute individual parameter sweeps along the cell polarization curve. Finally, we employ a genetic algorithm with the neural network to identify electrode property values for improving cell power density. While the developed framework does not supplant experimentation, it is generalizable to different redox chemistries and may inform future electrode design strategies. [ABSTRACT FROM AUTHOR]

Engineering redox-active compounds to support stable multi-electron transfer is an emerging strategy for enhancing the energy density and reducing the cost of redox flow batteries (RFBs). However, when sequential electron transfers occur at disparate redox potentials, increases in electrolyte capacity are accompanied by decreases in voltaic efficiency, restricting the viable design space. To understand these performance tradeoffs for two-electron compounds specifically, we apply theoretical models to investigate the influence of the electron transfer mechanism and redox-active species properties on galvanostatic processes. First, we model chronopotentiometry at a planar electrode to understand how the electrochemical response and associated concentration distributions depend on thermodynamic and mass transport factors. Second, using a zero-dimensional galvanostatic charge/discharge model, we assess the effects of these key descriptors on performance (i.e., electrode polarization and voltaic efficiency) for a single half-cell. Finally, we extend the galvanostatic model to include two-electron compounds in both half-cells, demonstrating compounding voltage losses for a full cell. These results fundamentally show why multi-electron compounds with disparate redox potentials are less attractive than those with concerted electron transfer. As such, we suggest new directions for molecular and systems engineering to improve the prospects of these materials for RFBs. [ABSTRACT FROM AUTHOR]

Zinc-iodide flow battery (ZIFB) is one of the best potential candidates for future grid-scale energy storage, due to its eye-catching features of benign, high energy density and non-corrosive nature. However major investigations have not done yet on the negative electrode of this battery where the Zn deposition/dissolution mechanism takes place, which may have an impact on the battery performance. Herein, we have reported a comparative study of different carbon-based anodes which are conventional graphite felt, carbon paper and graphite foil. Single-cell charge/discharge performances among these three different anodes depicts that the cell with planar, hydrophilic graphite foil anode is showing the best energy efficiency and the lowest cell resistance among the carbonaceous electrodes. Zinc dissolution process during discharge process seems to be the bottleneck for having a stable cell, which was corroborated by the use of a Zn foil anode that shows excellent efficiencies along the successive cycles. [ABSTRACT FROM AUTHOR]

For the application in Fe/Fe-Redox-Flow-Batteries some important factors concerning the composition of the electrolyte and the influence of temperature on the properties of half-cell reactions were investigated. In contrast to previous investigations, the measurements were performed more realistically on deposited iron and by means of linear sweep voltammetry. Since the distinction between cathodic iron deposition and hydrogen generation is not possible by convention, with quantitative stripping analysis on a rotating disk electrode, partly a method was used to distinguish between these two reactions. The investigations were carried out at temperatures up to 80 °C, with addition of 10 mM of chlorides of Bi, Cu, In, Pb, Sn, Tl, Cd, Sb and Hg and different supporting salts of NH4+, Li+, K+. Na+, Cs+, Mg2+ and Al3+. [ABSTRACT FROM AUTHOR]

Aiming at the shortcoming of low specific surface area of the most commonly used carbon felt (CF) electrodes in vanadium flow battery (VFB), there are mainly two approaches to enhancing its specific surface area: anchoring effect and digging effect. Based on the digging effect, the CO2-etched CF has been fabricated by a simple heat treatment under the CO2 atmosphere. As expected, lots of etch pits are formed and evenly distribute on the fiber surface, indicating a greatly enhanced surface area. Meanwhile, the content of O in the CO2-etched CF increases obviously, meaning the formation of more oxygen-containing groups, which are conducive to the improvement of hydrophilicity and electrochemical surface area. Furthermore, the oxygen-containing groups newly generated during the CO2 etching process not only improve the hydrophilicity, but also enhance the electrocatalytic activity toward to the V3+/V2+ redox couples. Consequently, VFB with CO2-etched CF as its electrodes presents significantly improved energy conversion efficiencies, discharge capacity and cycling stability. This work supplies a convenient and efficient way to improve the VFB performance by enhancing the specific surface area of CF electrodes through digging effect. [ABSTRACT FROM AUTHOR]

Using our standard methodology, we examined the thermal stability of vanadium flow battery positive electrolytes over a range of temperature from 30 to 70 °C with stable lifetimes from 11 min to 87 days. At higher temperatures (45 °C–70 °C) measurements showed excellent reproducibility but at lower temperatures (30 °C–45 °C) showed some scatter. Measurements at higher temperatures are in good agreement with our (single-slope) model which is based on earlier data but there is some divergence from the model at lower temperatures. Arrhenius plots of the data show two linear regimes: one in the range 45 °C–70 °C and another in the range 30 °C–45 °C, the latter having a higher Arrhenius slope. Based on linear least-squares best fits in these two regimes, we have formulated an improved stability model (two-slope model). We use our models to derive expressions for accelerated testing of thermal stability using increased temperature, increased vanadium concentration and decreased sulfate concentration and estimate values for the acceleration factors over a range of test and use temperatures and concentrations. We analyse the effect of changing concentration to counteract the decrease in electrolyte stability at higher temperatures and derive expressions to calculate the necessary concentrations. [ABSTRACT FROM AUTHOR]

Instead of an organic medium, a simple change in pH could lead to a high energy density redox flow battery (RFB). Besides, ion crossover and membrane optimization are problems that limit its commercialization. In this investigation, a zeolite-coated ceramic single membrane is adopted in an acid-base pH electrolyte combination for the vanadium (V4+/V3+)/sulfur (S4 2-/2S2 2-) (V/S) redox couple as a model system. First, the potential widening with a change in pH is explained by difference in OCP (open circuit potential) between the acid-acid and acid-base electrolyte combination that differs by 0.8 V. A 300 mV decrease in the V4+/V5+ redox peak potential and the 10 mV increase in the negative direction in the S4 2-/2S2 2- redox peak potential between acid-acid and acid-base electrolyte combination show the pH effect predominant in anodic half-cell than the cathodic half-cell. UV-visible analysis for the migration of vanadium and sulfur ions demonstrates no migration of vanadium and sulfur ions to each other halfcell via zeolite coated ceramic membrane. The current efficiency of 94%, voltage and energy efficiencies of 45%-50% are achieved under the given current density of 5 mA cm-2. In addition, the acid-base combination of V/S RFB system shows an energy density of 233.2 Wh l-1. [ABSTRACT FROM AUTHOR]

The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame-thylimidazoline-1-oxyl-3-oxide (PTIO), was investigated for the first time in a deep eutectic solvent (DES)-like system consisting of a 1:4 molar ratio of choline chloride and ethylene glycol (Ch1EG4) as a redox flow battery electrolyte. PTIO is a single molecule with three oxidation states, and can provide both positive and negative redox couples for a flow battery. A flow battery using the PTIO/Ch1EG4 electrolyte demonstrated nearly 50% round trip efficiency with an approximately 1 V open circuit potential. Inefficiencies were primarily due to membrane resistance which can be significantly lowered with increased temperature. While PTIO appears stable over short periods (hours), the oxidized form is not stable in the DES-like electrolyte over longer times. Molecular modeling was performed to investigate the relative stability of PTIO in DES as compared to the previously studied 4-hydroxy-TEMPO (4HT). It was found that the oxoammonium cation 4HT+ exhibits a noticeably larger nucleophilic reactive cloud as compared to PTIO+, indicating a higher reactivity. This method to predict stability of the oxoammonium cation shows promise to inform the design and synthesis of promising redox systems based on nitroxide radicals in DES electrolytes to identify new chemistries for large scale energy storage. [ABSTRACT FROM AUTHOR]

A detailed study characterizing the degradation of non-aqueous vanadium flow batteries under moisture-free conditions is presented. Despite assembly and operation in an inert glovebox, rapid cell failure is still observed. The capacity loss is attributed to a combination of electrolyte degradation and increased cell impedance. Cyclic voltammetry verifies the formation of VO(acac)2 within the positive electrolyte, while Raman spectroscopy reveals the formation of additional degradation products in the negative electrolyte. Scanning electron micrographs and X-ray photoelectron spectroscopy reveal the growth of a surface film on cycled electrodes harvested post-mortem, particularly in the negative half-cell. The appearance of the surface film is correlated with electrochemical impedance measurements, which indicate a dramatic increase in charge-transfer resistance post-cycling. The individual electrode contributions to the total cell impedance are resolved post-mortem. It is determined that even when great care is taken to exclude water from the system, the stability of the non-aqueous vanadium flow battery is limited by degradation processes occurring at the negative electrode. Furthermore, the lifetime is not noticeably enhanced, compared to earlier studies where moisture was present. [ABSTRACT FROM AUTHOR]

We present a method for comparing porous electrodes for use in redox flow batteries. By comparing physical and electrochemical properties of six carbon cloth electrodes (CCEs) including hydraulic permeability, Ohmic resistance, and mass transport overpotential, we show how optimal engineering conditions such as compression and electrolyte flow rates can be derived. These results are combined and applied to a quinone-based redox flow battery, leading to a peak power density around 0.4 W cm−2 for several different CCEs. This method can be applied broadly to other electrode architectures and flow battery chemistries, and provides a comprehensive framework for their comparison. [ABSTRACT FROM AUTHOR]

The need for viable energy storage technologies is becoming more apparent as the amount of renewable energy being wasted increases. Here, we have provided an in-depth quantification of the theoretical energy storage density possible from redox flow battery chemistries which is essential to understanding the energy storage capacity of a battery system. This improved energy storage density model captures a wide range of conditions and reaction types based on fundamental electrolyte chemistry principles and thermodynamics. The model proposed here Requires standard Gibbs energy, activity coefficients, and state of charge limits. We also demonstrate that energy efficiency values can be incorporated to account for non-thermodynamic contributions. Allvanadium and iron-chromium redox flow battery chemistries were modeled using literature data to confirm the accuracy of the proposed approach. Excellent agreements were obtained between our modeling results and experimental energy storage values obtained from literature. Using this approach, we can precisely quantify the storage capacity possible with new chemistries and assess the tradeoffs we make by enhancing one property at the expense of another. [ABSTRACT FROM AUTHOR]

Redox flow batteries based on aqueous electrolytes with organic active material (ORFB) have great potential for the development of environmentally safe and ecologically sustainable energy storage systems. To be competitive with the state-of-the-art vanadium redox flow battery, organic electrolytes must meet a whole range of requirements. We investigated different anthraquinone-based electrolytes, i.e. anthraquinone-2,6-disulfonic acid, anthraquinone-2,7-disulfonic acid (2,7-AQDS), anthraquinone-2-sulfonic acid, and 1,2-dihydroxybenzene-3,5-disulfonic acid (BQDS) with respect to their solubility in sulfuric acid, their electrical conductivity, and their viscosity. For this purpose, the influence of the concentration of sulfuric acid and the active species on the electrolyte properties was determined. Using NMR spectroscopy we analysed the thermal and electrochemical stability of 2,7-AQDS and BQDS electrolytes. The electrochemical stability was also monitored by cyclic voltammetry. Both methods have also indicated the absence of crossover phenomena. Furthermore, the influence of the electrolyte properties on the performance of the ORFB was investigated. Comparison with the vanadium electrolyte allowed us to estimate these kinds of requirements in order to develop a comparable all-organic flow battery. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are considered an outstanding candidate for the integration of renewable energy sources into the existing power grids. A key property of RFBs is the open circuit voltage (OCV) corresponding to the currentless equilibrium state. In the literature, the Nernst equation describing this property is often simplified by neglecting the activity coefficients. In this work, using a thermodynamically rigorous approach, we show that activity coefficients have a significant influence on the OCV of the Iron-Cadmium and All-Vanadium RFBs. Moreover, this influence varies with the state of charge. Therefore, activity coefficients should not be neglected in the Nernst equation. We show that when doing so, the resulting offset in OCV is actually comparable to typical voltage losses occurring during operation. Hence, fitting kinetic parameters to measurement data of voltage losses can lead to ambiguous results if only the idealized OCV, obtained by neglecting the activity coefficients, is used in that evaluation. Therefore, the implementation of a thermodynamically rigorous model has the potential to significantly improve state-of-the-art models for RFBs. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are attractive energy storage solutions for the grid, the simulation of which enables system and material optimization. In this article, we introduce tailored numerical schemes to model coupling between the transport of dissolved species, electrons, and fluid with redox reaction kinetics within RFBs in a robust way. The macro-scale transport of species (including advection, migration, and hydrodynamic dispersion) is coupled with the volume-averaged pore-scale processes of reaction kinetics, and mass transport, while the Poisson equation is used to model Donnan exclusion across ion exchange membranes that limit capacity fade due to the crossover of redox-active species. The governing equations are discretized using the finite volume method with a Newton-Raphson iteration scheme to resolve non-linearity. We introduce several numerical schemes to increase solver robustness, including reaction rate damping and logarithmic transform of concentration fields. Based on fixedpoint iteration convergence criteria we also show that the mechanistic Marcus-Hush-Chidsey (MHC) redox kinetics model can tolerate larger time steps than the empirical Butler-Volmer (BV) kinetics model. The numerical schemes presented in this article can also find application in other electrochemical systems, including desalination devices, fuel cells, and electrodialysis. [ABSTRACT FROM AUTHOR]

The bromine-bromide redox couple is a promising solution for aqueous cathodes of several flow batteries. In this work, we present an experimental characterization of the system Br2-HBr-H2O at chemical equilibrium, featuring isothermal measurements of Open Circuit Voltage (OCV), density and conductivity in solutions with concentrations up to 2 M Br2 and 4 M HBr. An equilibrium model considering polybromides formation that fits experimental data with an accuracy of 1.8 mV is presented. Information on activity coefficients and equilibrium constants of polybromides can be extrapolated from the model. Furthermore, alternative approaches are considered to effectively fit the measured voltage using algebraic equations, that provide a convenient tool for flow battery modeling at system-level. [ABSTRACT FROM AUTHOR]

Non-aqueous organic redox flow batteries (NORFBs) have emerged as a promising technology for renewable energy storage and conversion. High capacity and power density can be achieved by virtue of high solubility and high operating voltage of the organic anolytes and catholytes in organic media. However, the lack of anolyte materials with high redox potentials and their poor electrochemical stability retard the wider application of NORFBs. Here, we investigated an evolutionary design of a set of bipyridines and their analogues as anolytes and examined their performance in full flow batteries. Using combined techniques of repeated voltammetry, lower scan rate cyclic voltammetry, proton nuclear magnetic resonance, and density functional theory calculations, we could rapidly evaluate the redox potential, stability, and reversibility of these redox candidates. The promising candidates, 4-pyridylpyridinium bis(trifluoromethanesulfonyl)imide (monoMebiPy) and 4,4'-bipyridine (4,4'-biPy), were subjected to battery cycling. Extended studies of the post-cycling electrolytes were conducted to analyze the pathway of capacity fading and revealed a reduction-promoted methyl group shift mechanism for monoMebiPy. A family of easily accessible anolyte molecules with high redox stability and redox potentials was discovered that can be applied in NORFBs. [ABSTRACT FROM AUTHOR]

Designing competitive and reliable Redox Flow Batteries (RFBs) requires the development of specific analytical tools at lab-scale. Specifically, the influence of the cycling conditions on the system performance must be well identified. An instrumented test bench dedicated to the tuning of the operating parameters and the monitoring of the battery internal behavior was developed. The use of a segmented cell with a 1D-like design allowed to focus on the local current distribution along the electrolyte flow. The study started by examining the performances of a RFB coupling the ferri/ferrocyanide couple (Fe(CN)6 3-/Fe(CN)6 4-) to the anthraquinone Alizarin Red S, over 300 cycles. Once cycling in standard conditions was reviewed, a parameter study was performed to scan a varied range of operating points, by changing the current density, the flow rate and the temperature successively. Thanks to the spatially-resolved analysis of the cell, the global performance could be related to local processes. This investigation gave unprecedented insight of the internal operation of a RFB, with peculiar features appearing at the start of the charge and the discharge. It was shown that a same global response of the cell could actually hide very different internal operations. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are energy storage devices designed for grid-scale application. For next generation RFBs it is desirable to develop low cost materials with low ohmic resistance and high transport selectivity. We present a composite membrane for the vanadium redox flow battery (VRFB) consisting of a composite of a porous polypropylene separator laminated with a thin film of polybenzimidazole (PBI). PBI layers are prepared by solution casting to obtain thicknesses in the range of 0.2 to 10 µm. The ohmic resistance of vanadium electrolyte imbibed PBI is ~50 mOhm·cm2 per micrometer of film thickness at room temperature. In cell tests, composite membranes show higher coulombic efficiency compared to Nafion® 212. Composite membranes with a PBI layer thickness of 1 µm and below outperform Nafion® 212 in terms of energy efficiency and discharge capacity up to a current density of 250 mA cm-2. With thicker PBI films the ohmic cell resistance is excessively high. Over 100 charge-discharge cycles a higher rate of capacity fading is observed for a composite membrane with 0.7 µm PBI compared to Nafion® 212, which is a result of a more pronounced net electrolyte flux from the negative to the positive electrolyte. [ABSTRACT FROM AUTHOR]

Charge storage in concentrated electrolytes gives redox flow batteries (RFBs) a unique edge in grid-scale energy storage. It also creates a unique challenge in the design of organic molecules as alternatives to expensive vanadium salts. Molecules often come in the form of organic salts, and dissolve by ionic bond breakingand ion solvation, which can hardly be interpreted by "like dissolves like." Here we use anthraquinone sulfonate salts as a model system to investigate factors underlying their aqueous solubility. We synthesize fifteen salts, measure their solubilities with UV-vis spectroscopy, and find that the solubility of the same quinone could change by three orders of magnitude with a change in its counter-cation. The same impact can come from the position and number of the sulfonate groups. We explain the results with the differences in the ion solvation energy and the lattice energy, the latter of which is corroborated by single-crystal X-ray diffraction. We also derive a simple method to semi-quantitatively predict the solubility of a quinone sulfonate salt based on that of a common sulfate salt. The work provides a physical-chemical base for understanding the solubilities of organic salts for the design of high capacity electrolytes for aqueous flow batteries. [ABSTRACT FROM AUTHOR]

Several metal-free, nonaqueous, disproportionation redox-flow-battery chemistries based on electrochemically active organic molecules are presented. The electrochemistry of 9,10-diphenylanthracene (DPA), a polycyclic aromatic compound, involves two reversible redox couples separated by more than 3 V, which are associated with electrochemical disproportionation of the neutral molecule. Nonaqueous solvents are investigated with the dual aims of realizing this high voltage in a battery cell and maximizing active-species solubility. Functionalized DPA analogues are synthesized and shown to exhibit electrochemical responses similar to pristine DPA; appending diethyleneglycoxy esters on each phenyl group to form DdPA (9,10-Bis(4-(2-(2-methoxyethoxy)ethoxy) carbonyl-phenyl)anthracene) improves solubility over DPA by a factor of 20 in acetonitrile and 5 in dimethoxyethane. The 0.21 M maximum concentration of DdPA in dimethoxyethane suggests an energy density of 8 Wh l-1, which begins to approach the energy density of state-of-the-art aqueous RFBs. Charge/discharge of a stagnant one-dimensional cell delivers the highest cell voltages from an organic single-active-species RFB chemistry yet reported. Energy and power efficiencies for DPA in dimethoxyethane and DdPA in acetonitrile are similar to nonaqueous vanadium acetylacetonate in cells of similar construction. [ABSTRACT FROM AUTHOR]

A new redox flow battery system based on iron sulfate and anthraquinone disulfonic acid (AQDS) is shown here to have excellent electrical performance, capacity retention, and chemical durability. While these redox couples, iron(II)/iron(III) and AQDS are well known individually, their combination in a redox flow battery is shown here for the first time to provide unique benefits for large-scale energy storage. Based on iron sulfate, a waste product of the steel industry, the active materials cost for this battery is anticipated to be $66/kWh. Cycling studies of over 500 cycles in the symmetric cell configuration show a negligibly low capacity fade rate of 7.6 × 10−5 % per cycle. This symmetric cell also shows a notably high average coulombic efficiency of 99.63%. Using a graphite felt electrode modified with multi-walled carbon nanotubes (MWCNTs), we could achieve a peak power density of 194 mW cm−2 . The major voltage losses are ascribed to the ohmic resistance of the electrode and electrolyte. Despite the lower cell voltage of the system relative to the vanadium flow battery, the iron–AQDS flow battery system presents a good prospect for simultaneously meeting the demanding requirements of cost, durability and scalability for large-scale sustainable energy storage. [ABSTRACT FROM AUTHOR]

The membrane, as one of the key components in vanadium flow battery, plays an important role in battery performances and cost. In this paper, a series of new blend membranes based on polybenzimidazole and polyvinylpyrrolidone are proposed to combine the beneficial features of the two components, such as great chemical stability, low vanadium ion permeability and low area resistance. To begin with, blend membranes with different ratios of the two compositions are fabricated, followed by a microstructure characterization, demonstrating that the polybenzimidazole and polyvinylpyrrolidone can be compatible with each other and of good film-forming property. After that, the membrane performance investigation shows that the low area resistance, superior vanadium resistance and strong chemical stability can be simultaneously achieved. Furthermore, the cell testing indicates that the energy efficiencies are respectively 79% and 67% at the current densities of 100 mA cm−2 and 200 mA cm−2, and the cycling life could be over 120 times for the cell with the blend membrane of 30% polyvinylpyrrolidone and 70% polybenzimidazole. Accordingly, this work not only provides a kind of advanced membrane for the vanadium flow battery, but also offers an effective method to design, optimize and fabricate the membranes incorporating the features according to needs. [ABSTRACT FROM AUTHOR]

The penetration of renewable sources (solar and wind power) into the power system network has been increasing in the recent years. As a result of this, there have been serious concerns over reliable and satisfactory operation of the power systems. One of the solutions being proposed to improve the reliability and performance of these systems is to integrate energy storage devices into the power system network. Zinc-bromine batteries systems among other energy storage technologies has appeared as one of the best options. This paper presents the performance of three different electrodes feedermaterials (carbon, nickel and a titanium) coupled and investigated within a fabricated ZnBr2 cell system via numerical modelling, DDPM+DEM model in ANSYS Fluent to simulate an incorporated anode zinc-electrode and COMSOL Multiphysics for the electrochemical behavior of the cell. After introducing briefly other alternatives to store energy, ZnBr2 cell systems, and its mode of operation were then discussed, before focusing on the numerical modelling and simulation and the laboratory experiments. Several extensive electrochemical experiments were implemented on the cell to achieve fast deposition of zinc onto the electrode surface during charge and fast dissolution during discharge for high performance. The mechanical action of the fluidised design of electrode is intended to improve deposit morphology, obviate the risk of dendrite growth and provide high transport rates of reactant to and from the active electrode surface. In conclusion, this paper has analyzed electrochemical techniques like chronopotentiometry, cyclic voltammetry (CV), and electrochemical impedance spectroscopy that were used to understand the behavior of the zinc bromide cells at a particular flow rate of 166.7cm³ min-1 required to give good fluidization of the anode. [ABSTRACT FROM AUTHOR]

The fibrous electrodes used in redox flow batteries are a key component of the batteries and have a determining effect on their performance. In this work, a two-dimensional numerical model of redox flow batteries was developed and used to optimize the architecture of the electrodes employed in vanadium redox flow batteries with interdigitated flow fields. The developed model was validated and subsequently used to determine the optimized electrode architecture. During the optimization process, we considered the fiber diameter, porosity, and thickness of the fibrous electrode as well as the geometrical properties of the channel. Numerical simulations revealed that the cell performance can be improved significantly by employing electrodes consisting of finer fibers. We also show that multiple-parameter optimization that considers the electrode properties and channel geometry is essential for improving the design of redox flow batteries. [ABSTRACT FROM AUTHOR]

Although p-benzoquinone (BQ) is electrochemically reversible in non-aqueous media, the physicochemical properties of BQ are insufficient for its practical use as a negative redox couple for flow batteries. The voltage and solubility drawbacks of BQ can be resolved by using an aliphatic chain, which introduces electron-donating and melting-point-decreasing functional groups. The former negatively shifted the redox potential of BQ and the latter increased the solubility of BQ 1.5-fold. Furthermore, the reversibility of the redox couple is retained even after tert-butyl substitution with increased energy density. Finally, the designed tert-butyl-p-benzoquinone molecule exhibits a stable voltage profile during the flow battery cycling. [ABSTRACT FROM AUTHOR]

Redox targeting reactions between lithium-ion battery materials and redox shuttles have been proposed to design high energy density redox flow batteries. Designing these batteries would require a deeper understanding of the kinetics of redox targeting reactions and the phase transformation of the materials involved. In this study, the oxidation and reduction of lithium iron phosphate, LiFePO4, via chemical and electrochemical routes will be compared. Ultraviolet-visible spectroscopy was used as a technique to characterize the extent of chemical lithiation/delithiation during chemical redox of LiFePO4, while the electrochemical redox was characterized using battery coin cells. The kinetic parameters extracted using the Johnson-Mehl-Avrami-Erofeyev-Kolomogorov model suggested that chemical redox was slower than electrochemical redox within the experimental regimes. Calculated apparent activation energies suggested the limitations in the chemical redox rate were due to different processes than the electrochemical redox. In addition, asymmetry observed for oxidation and reduction of LiFePO4 materials will be discussed. As pairs of solid battery electroactive particles and soluble redox shuttles are designed, tools and analysis such as those in this study will be needed for interrogating and comparing electrochemical and chemical oxidation and reduction of the solid particles to understand and design these systems. [ABSTRACT FROM AUTHOR]

Transference numbers for the four vanadium cations present in flow batteries were measured and compared to values derived from permeability and conductivity measurements. The agreement between measured and predicted transference numbers is good. The transference numbers range from ~0.02 for V(II) and V(III) to ~0.002 for V(IV) and V(V) at technically relevant vanadium and acid concentrations. The level of agreement between measured and predicted transference numbers suggests that the dilute-solution framework provides a reasonably accurate representation of transport in ion-exchange membranes in vanadium flow batteries. [ABSTRACT FROM AUTHOR]