Solubility Challenges in Battery Electrolytes.

The Kansai Electric Power Company has been engaged in the ongoing development of the redox flow battery (RFB) since 1985 in collaboration with Sumitomo Electric Industries, Ltd. The fundamentals and performance of RFB have been verified through field test projects such as the prototype 450 kW 2 h system and the advanced design 168 kW 8 h system installed in Tatsumi substation. Now RFB has total energy efficiency of more than 70% including inverter loss, etc., and RFB has been applied to a number of practical uses. Several RFB systems have been installed in a semiconductor factory (3000 kW for UPS), a university (500 kW for load leveling), and other locations.

Redox flow batteries are growing in interest as potential systems for electrochemical energy storage at the grid-level, where high cycle number durability, long calendar life, high efficiency, low cost and fast response times are required.1,2 However, due to current issues of low energy density3–5 and high capital costs3,4 the industrial implementation has been slow. Electrolytes are a topic of active research aiming to increase the energy density while reducing the cost. Battery energy density can be increased by expanding the voltage window, or by minimising the mass and/or volume of the electroactive species per electron transferred6 which opens up many avenues through which novel electrolytes can be explored. Previous examples of electrolytes for flow batteries have included a variety of metal-based systems and a range of organic molecules.7,8 Of these, quinones are a common electroactive class of organic molecules that have been investigated and offer fast kinetics, high tunability and low cost.8 In the current work, benzoquinone derivatives were synthesised and investigated as potential anolytes for redox flow batteries. Benzoquinone has a higher aqueous solubility and a lower molecular weight than anthraquinone, but it is less stable electrochemically.8 The derivatised quinones explored in this work are anticipated to benefit, in comparison to previous anthraquinones, from a higher energy density through greater solubility and lower cost. Electrochemical screening was carried out using cyclic voltammetry as an initial test of redox properties and stability. Selected molecules that exhibited favourable behaviour were then run in a full cell on a lab-scale and through the aid of in situ NMR spectroscopy, the behaviour of the species under cycling conditions was investigated. Density functional theory modelling was used to complement the analysis. Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).Gyuk, I. et al. Grid Energy Storage. (2013).Potash, R. A., McKone, J. R., Conte, S. & Abruña, H. D. On the Benefits of a Symmetric Redox Flow Battery. J. Electrochem. Soc. 163, A338–A344 (2016).Wang, W. et al. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986 (2013).Alotto, P., Guarnieri, M. & Moro, F. Redox flow batteries for the storage of renewable energy: A review. Renew. Sustain. Energy Rev. 29, 325–335 (2014).Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652–657 (2008).Noack, J., Roznyatovskaya, N., Herr, T. & Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chemie - Int. Ed. 54, 9776–9809 (2015).Ding, Y., Zhang, C., Zhang, L., Zhou, Y. & Yu, G. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev. 47, 69–103 (2017).

Impact of carbonaceous particles concentration in a nanofluidic electrolyte for vanadium redox flow batteries

Electrochemical systems, such as flow batteries, have the potential to enable cost-effective and environmentally friendly large-scale energy storage [1]. Membraneless laminar flow batteries leverage the laminar flow of co-flowing fluids to prevent reactant crossover [2,3,4]. These latter batteries are a promising class of electrochemical large-scale energy storage systems, as they do not require the use of ion exchange membranes, typically the single most expensive component of the flow battery stack [5]. Further, the power density of flow batteries can be limited by the presence of a membrane. For example, in hydrogen-bromine flow batteries, operating at hydrobromic acid concentration of order 1 M (desirable for high liquid-phase conductivity) can cause the membrane to dehumidify and increase membrane resistivity [6]. Laminar flow batteries can eliminate this issue, but still significant challenges remain in their practical implementation. One major challenge is the demonstration of high efficiency closed-loop cycling (charging and discharging) of laminar flow batteries. This cycling relies on maintaining pure (unmixed) anolyte and catholyte fluid streams to prevent crossover reactions in subsequent cycles. However, the mixing layer necessarily developed between co-flowing fluids in laminar flow batteries prevents the extraction of pure fluid streams downstream of the battery [2,3,7]. To our knowledge, the highest reported number of closed-loop cycles attained in a laminar flow battery is a single cycle at 20% energy efficiency, and with a maximum power of about 0.3 W/cm2[8].We here describe our work in the design and development of a unique prototype laminar flow battery. Unlike previous laminar flow batteries, our device is designed for closed-loop cyclability using innovative means of controlling stream mixing within porous media. This is achieved through two novel mechanisms: i) the use of a porous "dispersion blocker" layer to prevent rapid mixing within the porous structure via transverse mechanical dispersion, and ii) a two-dimensional flow field, including a flow component in the direction of the electric field (in addition to the typical flow which is perpendicular to the electric field), to inhibit oxidant crossover. Through the use of hydrogen-bromine chemistry and flow-through porous electrodes, we demonstrate that our battery can achieve an exceptionally high maximum power density of up to 0.66 W/cm2 in addition to, for the first time in a laminar flow battery, multiple closed loop cycles. References Skyllas-Kazacos, M., et al. "Progress in flow battery research and development." Journal of The Electrochemical Society 158.8 (2011): R55-R79.Ferrigno, Rosaria, et al. "Membraneless vanadium redox fuel cell using laminar flow." Journal of the American Chemical Society 124.44 (2002): 12930-12931.Kjeang, Erik, Ned Djilali, and David Sinton. "Microfluidic fuel cells: A review."Journal of Power Sources 186.2 (2009): 353-369.Salloum, Kamil S., and Jonathan D. Posner. "Counter flow membraneless microfluidic fuel cell." Journal of Power Sources 195.19 (2010): 6941-6944.Li, L, Kim, S., Xai, W., Wang, W., and Yang, Z., “Advanced Redox Flow Batteries for Stationary Electrical Energy Storage”, U.S. Department of Energy, (2012).Kreutzer, Haley, Venkata Yarlagadda, and Trung Van Nguyen. "Performance Evaluation of a Regenerative Hydrogen-Bromine Fuel Cell." Journal of The Electrochemical Society 159.7 (2012): F331-F337.Braff, William A., Martin Z. Bazant, and Cullen R. Buie. "Membrane-less hydrogen bromine flow battery." Nature communications 4 (2013). Lee, Jin Wook, Marc-Antoni Goulet, and Erik Kjeang. "Microfluidic redox battery." Lab Chip (2013). Figure 1: Schematic of the cyclable laminar flow battery using hydrogen-bromine chemistry. Undesirable bromine (oxidant) flux into the electrolyte channel is prevented through use of a dispersion blocker layer and 2D flow (blue arrows). Insets show numerical results of co-flowing fluids within porous media at high Peclet number, a) without a dispersion blocker layer, and b) with a dispersion blocker. The dispersion blocker can strongly inhibit mixing of co-flowing streams within porous structures of a flow battery.

Hydrated eutectic electrolyte as catholyte enables high-performance redox flow batteries

Flow battery systems were developed in the late 1970s, and have received renewed interest for grid-scale energy storage [1]. No flow batteries, though, have achieved mature commercial status, and this is partly because of poor stability and lifetime. Many flow battery systems have complications that results from electrolyte imbalances that result from undesired side reactions. Commercial lead-acid batteries have well-developed recombination systems that help extend battery lifetime and reduce maintenance [2]. This has not been the case for flow batteries though. For flow (and hybrid flow) batteries, there have been many proposed systems to deal with the electrolyte imbalances, but they are typically complicated open systems that require the continuous supply of rebalancing reactants such as hydrogen or chlorine, and which allow gas venting or the formation of other waste products. Typically, the reaction is carried out in a fuel cell or trickle-bed reactor, and it requires careful monitoring of state-of-charge, as well as a control system. Here, we show the behavior of all-iron flow battery systems [3] designed to be sealed and valve-regulated just like modern lead acid batteries. All-iron flow batteries charge and discharge according to Equation 1, and the undesired side reaction at the negative electrode is given by Equation 2. 3Fe2+ + 4e ↔ Fe0 + 2Fe3+ (1) 2H+ + 2e → H2 (2) The recombination reaction in the all-iron battery, which is the same reaction originally used by NASA’s rebalancing fuel cell, is given by Equation 3. H2 + 2Fe3+ → 2Fe2+ + 2H+ (3) This reaction is spontaneous (E0 = 0.77 V), and occurs readily at room temperature over a catalyst. In the all-iron flow battery system, recombination can also function as a pH control system, mitigating problems associated with the formation of iron hydroxide precipitates such as those given by Equations 4 and 5. Fe2+ + 2OH − → Fe(OH)2 , (4) Fe3+ + 3OH − → Fe(OH)3 . (5) In this study, we show that recombination in a sealed system can be used to keep the electrolyte balanced in terms of both iron and hydrogen species, without the need for an external reactants or control systems (see Figure 1). The pressure in the electrolyte reservoir and the pH of the electrolytes rise during charge, and fall during discharge as the recombination occurs. These results represent a major step forward toward sealed recombinant flow batteries and shed new light on flow battery electrolyte dynamics. Figure 1: System dynamics in a recombinant all-iron hybrid flow battery during charge/discharge cycling. System pressure – blue curve, Negative electrolyte pH – green curve, Positive electrolyte pH – red curve. Figure 1

Toward the realization of a low-carbon society, renewable energies such as wind and solar power have been promoted rapidly around the world. Since such renewable energies have a fluctuating power output due to the variability of weather, the increasing amounts of such energy sources will bring stability problems to conventional power system. Electrical energy storage systems are expected to solve these problems, and large-scale batteries attach more attention than ever. Redox flow (RF) batteries have several advantages comparing to solid batteries, such as easy to capacity scale up, long cycle life, real time state of charge (SOC) monitor, which make them the most suitable for power network stabilization. RF batteries have been enthusiastically developed worldwide, since its principle was publicized in the 1970s by NASA[1], all vanadium system as a representative of them has been put in practical applications[2]. To meet the growing demand for electrolytes, it is generally desired that active metals should be lower cost, higher electrode potential and stable supply than vanadium. Mn(II)/Mn(III) redox couple as a positive active material meets the requirements mentioned above. However, Mn(III) ion is chemically unstable and tends to disproportionate to Mn(II) ion and MnO2 oxide, which make it difficult to be used in flow batteries. We found that the precipitation of MnO2 can be effectively suppressed by containing Mn ions as well as Ti(IV) ions in positive electrolyte. We gained a very good cycle charge-discharge performance in a cell test, even at SOC in a range of more than 100%, in this case SOC is calculated on Mn(II)/Mn(III) one-electron reaction. It considered that Ti(IV) ions exiting in positive electrolyte suppress the Mn(III) disproportion reaction, as well as particle growth of MnO2. The theoretical energy density of positive Mn and negative Ti electrolytes RF battery is around 26kWh/m3, which is comparable to that in all vanadium RF battery.

The study of electroactive species with multiple redox events is of interest due to their applications as electrolytes in Redox Flow Batteries (RFBs). This multi-electron behavior can be observed in organic molecules and coordination compounds containing ligands with heteroatoms such as O and N in π-extended systems. In this context, a study of 3,5-dinitrobenzoic acid, its respective cobalt complexes (1), and azole-pyridine co-ligands (2) was conducted.Electrochemical characterization of the cobalt complexes was performed using cyclic voltammetry. Measurements were conducted in a three-electrode cell, using glassy carbon as the working electrode, Ag/AgCl as the reference electrode, and a Pt wire as the counter electrode. 1 mM solutions of the benzoate ligand, 0.6 mM of complex 1, and 0.7 mM of complex 2 were prepared in acetonitrile with 0.1 M TBAPF6 as the supporting electrolyte.The 3,5-dinitrobenzoic acid ligand was studied in a potential window of -1.7 to 0 V. Two redox events were observed at potentials of -1.21 V and -0.79 V. Both events were quasi-reversible, with ΔE values of 88 mV and 53 mV, respectively. For complex 1, an exploratory window from -1.7 to 0 V was studied, and two quasi-reversible redox events were observed that appeared to coincide with those of the benzoate ligand. These events occurred at -1.24 V and -0.93 V. It is proposed that all these events are due to redox processes of the nitro groups present in the ligand, suggesting that cobalt may not be exhibiting redox activity.In contrast, complex 2 was studied in the potential window of 1 to 2.2 V, showing two irreversible oxidation events at around 1.73 V and 1.96 V. A decrease in peak current was observed over successive cycles, indicating a potential chemical process involving the oxidized species.To evaluate the stability of the species of interest, cyclic voltammetry over 20 cycles was performed It was observed that the 3,5-dinitrobenzoic acid ligand and its complex 1 presented more stable profiles, without significant changes in the peak currents or potentials. However, complex 2 exhibited instability because as the cycles pass, the current potentials decrease and the oxidation peaks disappear. Additionally, for the benzoate ligand and complex 1, cyclic voltammetries were carried out at different scanning speeds in order to obtain the diffusion coefficients of each species.In this order of ideas, both the 3,5-dinitrobenzoic acid ligand and complex 1 are the most suitable molecules to study their application in redox flow batteries. The benzoate ligand has the advantage of having a lower molecular weight, an important factor in energy density, but it has lower stability, while complex 1 has a higher molecular weight but provides greater stability. On the other hand, complex 2 does not present reversible redox events, which is why it is not applicable as an electrolyte in BFR. Figure 1

Renewable energy technologies, such as wind or solar energy, depend upon intermittent sources, which make long-term storage an important issue for a large-scale application. A way to efficiently store this energy is to use redox flow batteries, where electricity is stored in a liquid electrolyte circulating through an electrochemical cell. The main advantage of this system is the decoupling of power and energy, allowing to increase the storage more efficiently than other technologies, which is advantageous for large-scale stationary systems.1 Most commercial redox flow batteries currently use vanadium as the active material. Although it is a stable metal, its price is high and volatile, and vanadium extraction is responsible for more than 80% of the environmental cost of a vanadium-based redox flow battery. 2 A cheaper, more environment friendly and safer alternative to vanadium is to use organic molecules as active material in an aqueous solvent.3 Organic molecules also offer opportunities to easily tune properties by modifying their structure. This tunability of organic molecules is a great advantage to maximize desired properties: high solubility in water, low viscosity and optimal redox potential (high for positive electrolyte and low for the negative one). The possibilities of modification are however almost infinite and it would be unrealistic to evaluate experimentally all the possible derivatives of even only one family of redox centers. Therefore, computational chemistry, which permits to study molecules properties and draw tendencies within a shorter time frame, is an especially useful tool to assist the design of better performing molecules.4 In this work, we use DFT to study the effect of some structural modifications on the properties of viologen core molecules, with the aim of using them in aqueous organic redox flow batteries (AORFB). In the first part of this presentation, we show the study of PEG chain conformation of PEGylated viologen derivatives in order to explain experimental trends in solubility for different PEG chain lengths. We show that the experimental measure of solubility correlates with the folding of the PEG chain, which is more favored for longer chains. We also see a correlation between asymmetry, dipolar moment and solubility for these molecules, meaning that calculation of the dipole can give an approximate idea of solubility. However, no significant change in redox potential is measured with this type of structural modification.Therefore, in the second part of this study, we present a study of the effect of adding small functional groups directly on the viologen bipyridine core to tune their redox potential. We developed a computational method to calculate the theoretical redox potentials of these derivatives, and we correlate the values to experimental results, and obtained a R factor correlation of 0,9, supporting the validity of the method, which could be used to accurately predict redox potential of other viologen derivatives.These results show the insight computational chemistry can provide for interpretation of experimental data, and the potential of this method to predict and design new optimized molecules for aqueous organic redox flow batteries.References(1) Bai, H. Y.; Song, Z. Y. Lithium-ion battery, sodium-ion battery, or redox-flow battery: A comprehensive comparison in renewable energy systems. J Power Sources 2023, 580.(2) Weber, S.; Peters, J. F.; Baumann, M.; Weil, M. Life Cycle Assessment of a Vanadium Redox Flow Battery. Environ Sci Technol 2018, 52 (18), 10864-10873.(3) DeBruler, C.; Hu, B.; Moss, J.; Liu, X. A.; Luo, J. A.; Sun, Y. J.; Liu, T. L. Designer Two-Electron Storage Viologen Anolyte Materials for Neutral Aqueous Organic Redox Flow Batteries. Chem-Us 2017, 3 (6), 961-978.(4) Asenjo-Pascual, J.; Salmeron-Sanchez, I.; Mauleón, P.; Agirre, M.; Lopes, A. C.; Zugazua, O.; Sánchez-Díez, E.; Avilés-Moreno, J. R.; Ocón, P. DFT calculation, a practical tool to predict the electrochemical behaviour of organic electrolytes in aqueous redox flow batteries. J Power Sources 2023, 564. Figure 1

The EIA projects that 60% of cumulative capacity additions in the U.S. by 2050 will be renewable electric generating technologies.1 The use of intermittent renewable energy in the U.S. electricity grid requires energy storage. NREL predicts that for a scenario in which 80% of electricity in the U.S. comes from renewable energy by 2050, 120 GW of energy storage would be needed,2 yet as of 2020, the U.S. has only 24 GW of storage capacity.3 Redox flow batteries (RFBs) are a useful technology for ensuring the smooth integration of renewable energy into the U.S. electricity grid because of their long lifecycles and discharge times. RFBs are currently too expensive for market deployment, however, with the all vanadium RFB (VRFB) costing double the DOE target.4,5 One way to improve the cost effectiveness of RFBs is to explore chemistries that increase the voltage window. The replacement of the VO2+/ VO2 + chemistry at the positive electrode of a VRFB with the Ce3+/Ce4+ chemistry would result in an increased theoretical voltage, but it is unclear how the kinetic, ohmic, and mass transport overvoltages would change. Additionally, studies of the environmental burdens of life cycle phases of Ce RFBs are limited. To advance the most cost effective and least environmentally harmful RFB, in this study, we develop a combined Technoeconomic Assessment-Life Cycle Assessment (TEA-LCA) model that is informed by our performance measurements to compare the levelized cost of electricity (LCOE) and levelized greenhouse gas (LGHG) emissions of VRFBs and Ce-V RFBs.The TEA-LCA model allows the user to select from a list of positive and negative electrode redox chemistries, solvents, electrode materials, and electricity grid generation profiles to calculate the LCOE and LGHG emissions of the battery for the delivery of 1 kWh of energy. A solver function optimizes the current density that minimizes either LCOE or LGHGs. The TEA-LCA model uses a bottom-up approach, in which energy- and power-dependent capital costs and environmental burdens are calculated by converting the amount of material to a kWh basis. Cost estimates are sourced from vendors and GHG emissions are pulled from the GREET database and literature. The amount of active species required to deliver 1 kWh of electricity at a specified discharge time is calculated using the redox couple properties, including redox potential, exchange current density, and limiting currents. These performance parameters are based on measurements collected in lab. The use phase burdens are calculated using the roundtrip efficiency of the battery and the price and GHGs associated with the electricity grid generation mix. End-of-life costs consist of the economic and environmental burdens of recycling and disposing of the battery material and are collected from vendors and GREET.The TEA-LCA model answers important questions related to the optimal operating conditions of an RFB. In addition to comparing the economic and environmental performances of the VRFB and Ce-V RFB, it demonstrates how different electricity grid mixes influence total cost and emissions and highlights the difference in optimal operating current density if cost or GHG emissions are prioritized, e.g., lower current density results in fewer emissions but higher cost in carbon-intensive electricity grid profiles. Preliminary results using literature values show that the Ce-V RFB has an LCOE that is 45% lower than the VRFB LCOE, with capital costs dominating. We will present the finalized LCOE for the VRFB and Ce-V RFB, as well as LGHGs, as a function of discharge time and electricity grid mix. Sensitivity analyses of the input parameters found that for both RFBs, the discount rate, discharge faradaic efficiency, and lifetime of battery had the most influence on LCOE, with a 20% decrease in discharge faradaic efficiency resulting in a 16% increase in LCOE for the VRFB. The results of this TEA-LCA model show that the use of cerium is a viable option for reducing the cost of RFBs to advance their use in renewable energy storage grid applications. Additionally, this model is generalizable to other batteries and electrochemical systems, such as CO2 conversion. Thus, the development of this TEA-LCA model represents not only an advancement to the field of redox flow batteries but also the wider field of electrochemistry. U.S. EIA. Annual Energy Outlook. (2021).Mai, T. et al. Renewable Electricity Futures Study. NREL. (2012).CSS University of Michigan. U. S. Grid Energy Storage Factsheet. (2021).Mongird, K. et al. 2020 Grid Energy Storage Technology Cost and Performance Assessment. (2020).Weber, A. Z. et al. J. Appl. Electrochem. 41, 1137–1164 (2011).Smith, G. F. & Getz, C. A. Ind. Eng. Chem. Res. 10, 191–195 (1938).

This is a critical review of the advances in the molecular design of organic electroactive molecules, which are the key components for redox flow batteries (RFBs). As a large-scale energy storage system with great potential, the redox flow battery has been attracting increasing attention in the last few decades. The redox molecules, which bridge the interconversion between chemical energy and electric energy for RFBs, have generated wide interest in many fields such as energy storage, functional materials, and synthetic chemistry. The most widely used electroactive molecules are inorganic metal ions, most of which are scarce and expensive, hindering the broad deployment of RFBs. Thus, there is an urgent motivation to exploit novel cost-effective electroactive molecules for the commercialization of RFBs. RFBs based on organic electroactive molecules such as quinones and nitroxide radical derivatives have been studied and have been a hot topic of research due to their inherent merits in the last decade. However, few comprehensive summaries regarding the molecular design of organic electroactive molecules have been published. Herein, the latest progress and challenges of organic electroactive molecules in both non-aqueous and aqueous RFBs are reviewed, and future perspectives are put forward for further developments of RFBs as well as other electrochemical energy storage systems.

Carbonyls, and quinones among them in particular, occupy a place of choice among organic reversible redox systems for electrocatalysis [1] and energy storage [2]. Their electrochemical behavior is marked by coupled proton and electron transfer (CPET) [3-5]. We have studied this subject in the context of flow batteries [6] and hydrogen carriers [7]. On the other hand, metal ion – catecholate (o-dioxolene) binding [8] attracts attention in different and rather independent domains, including active materials for metal-ion and redox flow batteries [2]. Redox transformations of o-dioxolene-metal complexes are therefore coupled to competitive binding with metal ion and proton at the electrochemical timescale, raising the complexity in the system above that of CPET. These effects have not received proper attention e.g. in aqueous metal-ion battery research.The subject of this communication is the electrochemistry of Al3+ - catechol system in aqueous solution. We discuss the conditions and measurable electrochemical signatures of complex formation, as well as the role of buffer. We explain why our observations are contrasting those on catechol containing polymers proposed for universal metal-ion batteries [9, 10] by considering which reactions attain equilibrium and which are obeying kinetics only.[1] K. Tammeveski et al. J. Electroanal. Chem. 2001, 515 , 101; DOI: 10.1016/S0022-0728(01)00633-7[2] B. Häupler et al. Adv. Energy Mater. 2015, 5 , 1402034; DOI: 10.1002/aenm.201402034[3] M. Quan et al. J. Am. Chem. Soc. 2007, 129 , 12847; DOI: 10.1021/ja0743083[4] J. Wang et al. J. Electroanal. Chem. 2007, 601 , 107; DOI: 10.1016/j.jelechem.2006.10.036[5] Q. Lin et al. J. Phys. Chem. C 2015, 119 , 1489; DOI: 10.1021/jp511414b[6] H. Ghorbani Shiraz et al. J. Energy Chem. 2022, 73 , 292; DOI: 10.1016/j.jechem.2022.06.015[7] M. Vagin et al. Adv. Funct. Mat. 2020, 30 , 2007009; DOI: 10.1002/adfm.202007009[8] R. Maskey et al. Encycl. Inorg. Bioinorg. Chem.; DOI: 10.1002/9781119951438.eibc2810[9] N. Patil et al. ACS Appl. Energy Mater. 2019, 2, 3035; DOI: 10.1021/acsaem.9b00443[10] K. Pirnat et al. Macromolecules 2019, 52, 8155; DOI: 10.1021/acs.macromol.9b01405

Slurry electrodes have been proposed as a means to enhance the scalability of hybrid redox flow battery (RFB) chemistries for better usability in utility scale energy storage applications1–3. In conventional hybrid RFB’s, scalability is limited due the spatial constraints of the flow cell and the metal deposited by the negative half-reaction on charge1. By using a slurry electrode, the solid metal can be deposited onto electrically conductive particles dispersed in the electrolyte instead of on the stationary electrode within the flow cell. In this way, hybrid RFB chemistries can achieve the same scalability as more commonly studied true RFB chemistries, such as all-vanadium. Due to the high abundance, low cost, and low toxicity of iron electrolytes, the all-iron RFB chemistry is of particular interest for use with a slurry electrode2,4. The usefulness of the slurry electrode depends on the current distribution of the plating reaction. To successfully decouple the storage and power capacities of the RFB and thus enhance its scalability5, the faradaic current of the plating reaction must occur predominantly on the mobile slurry particles, as opposed to on the stationary current collector1. This current distribution is dependent on a variety of factors, such as the applied overpotential, the electrical conductivity of the slurry, the ionic conductivity of the electrolyte, the kinetics of the reaction, and the rate of ionic mass transport to reaction sites. Ionic mass transport in electrolytes containing slurry electrodes may differ from ionic transport in neat electrolyte in interesting and important ways. Due to the volume fraction of the electrolyte occupied by solid particles, the effective concentration of the ionic species may be lower than in neat electrolyte. Further, the solid particle volume fraction hinders ionic diffusion by introducing diffusion path tortuosity. This effect is more severe in higher slurry particle loadings.In this work, the effect of varying dispersed solid particle loading on ionic diffusivity is investigated via voltammetry using a rotating disk electrode. The diffusivities of ionic iron species are measured as a function of the volume fraction of solids dispersed in the electrolyte. Comparisons with the Bruggeman correlation6,7 are made and amendments to the Levich equation are considered.(1) Petek, T. J.; Hoyt, N. C.; Savinell, R. F.; Wainright, J. S. Slurry Electrodes for Iron Plating in an All-Iron Flow Battery. J. Power Sources 2015, 294, 620–626. https://doi.org/10.1016/j.jpowsour.2015.06.050.(2) Petek, T. J. Enhancing the Capacity of All-Iron Flow Batteries: Understanding Crossover and Slurry Electrodes. Ph.D. Thesis 2015, No. May.(3) Narayanan, T. M.; Zhu, Y. G.; Gençer, E.; McKinley, G.; Shao-Horn, Y. Low-Cost Manganese Dioxide Semi-Solid Electrode for Flow Batteries. Joule 2021, 5 (11), 2934–2954. https://doi.org/10.1016/j.joule.2021.07.010.(4) Dinesh, A.; Olivera, S.; Venkatesh, K.; Santosh, M. S.; Priya, M. G.; Inamuddin; Asiri, A. M.; Muralidhara, H. B. Iron-Based Flow Batteries to Store Renewable Energies. Environ. Chem. Lett. 2018, 16 (3), 683–694. https://doi.org/10.1007/s10311-018-0709-8.(5) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Jeffrey, T.; Liu, Q. Redox Flow Batteries , a Review Environmental Energy Technologies Division , Lawrence Berkeley National Laboratory , Department of Mechanical , Aerospace and Biomedical Engineering , University of Tennessee , Department of Chemical Engineering , McGill Un. 1–72.(6) Tjaden, B.; Cooper, S. J.; Brett, D. J.; Kramer, D.; Shearing, P. R. On the Origin and Application of the Bruggeman Correlation for Analysing Transport Phenomena in Electrochemical Systems. Curr. Opin. Chem. Eng. 2016, 12, 44–51. https://doi.org/10.1016/j.coche.2016.02.006.(7) Chung, D. W.; Ebner, M.; Ely, D. R.; Wood, V.; Edwin García, R. Validity of the Bruggeman Relation for Porous Electrodes. Model. Simul. Mater. Sci. Eng. 2013, 21 (7). https://doi.org/10.1088/0965-0393/21/7/074009.

Ferrocyanide (FCY) based alkaline redox-flow batteries are considered promising alternatives to traditional redox-flow batteries that are based on high cost and strategic metals and corrosive acidic media. FCY has been used as active species in the positive side of a variety of redox-flow batteries, e.g. anthraquinone/FCY[1] and phenazines/FCY[2]. However, in all these cases, the energy density is limited by the low solubility of FCY in alkaline media (0.4 M equivalent to 10 AhL-1). In contrast, electroactive solid species that are frequently used in non-flow batteries are able to store more charge per unit volume than soluble electroactive species, e.g. Ni(OH)2 has a volumetric capacity of 1180 Ah·L-1 that is two orders of magnitude higher than that of a FCY solution. However, solid materials cannot be used in flow batteries as there is no electrical contact between the electrode and the solid located in the external tank. In this talk, we will show that soluble electroactive species can be also used as molecular wiring to transport charges to high-energy solid materials confined in the external reservoir.[3] We demonstrate that by adding solid material in the reservoir, volumetric capacities of >25 AhL-1 are achieved (40% utilization rate of Ni(OH)2). We apply the concept in two types of FCY flow batteries: quinone/FCY and phenazines/FCY. For the latter, energy densities of >15 WhL-1 are demonstrated representing the highest value for an alkaline flow battery. This work open up the field for a new generation of batteries, which benefits from the advatages of flow and non-flow configurations. References Lin, K. et al. Alkaline quinone flow battery. Nat. Energy 349, 1–4 (2015).Hollas, A. et al. A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat. Energy 3, 508–514 (2018). Ventosa, E.; Paez, T.; Palma, J, Redox flow battery for energy storage, European Patent Application, EP18382971 Figure 1. a Scheme of a redox-flow battery based on the use of ferrocyanide-Ni(OH)2 in the positive container. b Reversible stored charge during the first 20 cycles of a 2,6-dihydroxi- anthraquinone // ferrocyanide flow battery in the absence (black circles) and in the presence (red circles) of solid Ni(OH)2 in the external positive container. Figure 1

Redox flow batteries (RFBs) are secondary battery systems suitable for large-scale, stationary energy storage applications, and are capable of storing large quantities of energy (MWh) and power (MW).1 One principle advantage of flow batteries is the ability to decouple energy and power density, and scale both independently. The all-vanadium RFB represents the current state-of-the-art in flow battery technology, but uses expensive ion exchange membranes including Nafion®, and the relatively high cost of vanadium leads to expensive electrolytes.2 The development of manganese-based anolytes as a suitable alternative to vanadium anolytes for redox flow batteries is attractive for various reasons, including a higher reversible potential for Mn2+/Mn3+ than VO2+/VO2 +, higher natural abundance, and lower cost than vanadium. Flow battery anolytes based on the Mn2+/Mn3+ redox couple have been reported in the literature, and the high standard electrode potential of Mn2+/Mn3+ (1.51 V) has been utilized in manganese anolyte (Mn2+/Mn3+)/vanadium catholyte (V2+/V3+) redox flow batteries, featuring a theoretical open circuit voltage of 1.77 V.2,3 The usage of manganese anolytes can lead to a higher cell voltage, yet the disproportionation reaction of Mn3+ is a technical hurdle that needs to be resolved in order for manganese-based anolytes to find widespread utility in redox flow batteries.2,4 This presentation will disclose investigations on the development of manganese-anolyte based redox flow batteries, and will show results from two different systems, including Ti/Mn, and V/Mn. Half–cell and full cell performance metrics, including cycle life testing, will be presented for each system. References1) Wang, W. et al. J. Power Sources. 2012, 216, 99. 2) Xue, F.-Q. et al. Electrochimica Acta. 2008, 53, 6636. 3) Hong, T.; Xue, F. “Investigation on manganese (Mn2+/Mn3+)-vanadium (V2+/V3+) redox flow battery.” 2009 Asia-Pacific Power and Energy Engineering Conference (APPEEC). 4) Swartz, C. R.; Lipka, S. M.; Rogers, F. III; Chen, R.; Kodenkandath, T. “Aqueous Manganese-Based Electrolytes for Redox Flow Batteries.” ECS abstract MA2014-02, 616.