An energy-dense polysulfide/ferricyanide redox flow battery enabled by cation engineering

Harnessing Interfacial Electron Transfer in Redox Flow Batteries

Redox Flow Batteries (RFBs) are inherently well suited for large-scale electrical-energy-storage (EES) applications [1]. RFBs are entering a period of renaissance, buoyed by both the increasing need for affordable long-duration EES solutions, as well as recent substantial advancements in cell performance that leverage state-of-the-art (SOA) flow-cell technologies, such as those originally developed for polymer-electrolyte fuel cells (PEFCs) [2, 3]. A good example of this approach has been the recent dramatic improvements in RFB power density, illustrated in Fig. 1. There are multiple opportunities for advanced RFB materials, especially cell-stack components and RFB active materials, since the remaining components of a RFB system are typically comprised of commercial off-the-shelf parts [2]. This talk will focus on the key requirements for advanced materials for SOA RFB cells, since high power density cells enable inherently lower cell-stack cost and size.First-generation RFB chemistries have been based on single-element active materials dissolved in aqueous electrolytes. Next-generation RFB chemistries are likely to be engineered molecules or complexes. Both aqueous and non-aqueous options are being pursued because non-aqueous electrolytes enable a broader window of electrochemical stability, which is obviously advantageous from both an energy-density and cell-voltage perspective. However, non-aqueous electrolytes also have significant disadvantages, such as higher solvent costs, higher viscosities, and lower ionic conductivities. Detailed techno-economic analysis have recently made a quantitative assessment of these trade-offs [5, 6], and a brief summary of the key requirements for RFB active materials will be briefly summarized. In addition, the key requirement for some less conventional RFB systems will be discussed (e.g., mediated RFB systems with solid-phase storage [7]).Most RFBs today utilize ion-exchange membranes (IEMs), similar to those used in PEFCs. IEMs provide high ionic conductivities, good selectivity for the transport of the desired charge carrier relative to the active materials, and good mechanical and chemical stability. However, IEMs are inherently expensive materials, especially fully-fluorinated IEMs, which are typically used in RFB cells since hydrocarbon-based IEMs are generally not sufficiently stable when exposed to the highly oxidative conditions present in the positive reactant solution (e.g., see [8]). SOA RFB cells employ relatively thin IEMs to reduce material cost and to enable higher ionic conductivities; however, ion selectivity is also key requirement, which is highly dependent on the type of RFB chemistry and what happens to active molecules at the counter electrode [9]. A fundamental understanding of the different causes of crossover in RFB cells (i.e., diffusion, migration, and electro-osmosis) under a various operating conditions [10], as well as how these are related to the physical properties of the separator and the active materials, is also required to intelligently develop alternative RFB separators. Transport-property requirements for RFB separators have been derived for both aqueous and non-aqueous RFB chemistries [9].Many first-generation RFB chemistries are able to utilize simple carbon electrodes because simple redox reactions are facile and involve outer-sphere electrocatalysis. However, the fundamental reaction kinetics of even the relatively mature all-vanadium RFB is not well understood [11]. A major contributing factor to the complexity of RFB reactions on carbon electrodes is the fact that carbon is an extremely complex material. There are many types of carbons, and pretreatments of even the same carbon material can yield very different surface species, which can dramatically impact catalytic activity. Additionally, carbon itself is electrochemically active in the potential window of interest for most RFBs. Therefore, a better understanding of fundamental carbon properties, and stability in an electrochemical environment, is required for RFB cells that rely on carbon as a catalyst. A major conclusion of a recent review article on carbon materials in RFBs was that additional studies on degradation mechanisms are needed [12]. Catalyst materials for redox reactions, beyond carbon, also deserve more attention. Acknowledgements Thanks to the organizers of this Symposia for the invitation to present. The author is also grateful to many collaborators, especially at Vionx Energy and UTRC. References M. Perry, et.al., IEEE Proceedings, 102, 976 (2014). M. Perry & A. Weber, JECS, 163, A5064 (2016). M. Perry, et.al., ECS Transactions, 53, 7 (2013). R. Darling & M. Perry, JECS, 161, A1381 (2014). R. Darling, et.al., Energy & Environmental Science, 7, 3459 (2014). R. Dmello, et.al., J. Power Sources, 330, 261 (2016). C. Jia, et.al., Science Advances, 1, 10 (2015). S. Kim, et.al., J. Appl. Electrochem. 41, 1201 (2011). R. Darling, et.al., JECS, 163, A5029 (2016). M. Perry, et.al., JECS, 163 , A5014 (2016). N. Pour, M. Perry, Y. Shao-Horn, et.al., J. Physical Chemistry C, V119, 5311 (2015). M. Chakrabarti, et.al., J. of Power Sources, 253, 150 (2014). Figure 1

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.

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

The growing need to transition from fossil fuels to renewable energy sources will require innovative development of grid-level electrochemical energy storage.1 Suitable grid-level storage must provide a high cycle number durability, long calendar life, high efficiency, low cost and fast response time.2–4 Redox flow batteries (RFBs) are one potential solution boasting a decoupled power and capacity scaling.1,4,5 However, the low energy density1,6,7 and high capital costs6,7 of current systems preclude wide-scale deployment of this technology. Increasing energy density can be achieved in various ways: expanding the voltage window or by minimising the mass and/or volume per electron transferred.8 These offer potential strategies for electrolyte exploration but the challenge is to minimise the cost while providing as much electrical energy storage as possible.Current electrolyte systems for RFBs rely on a variety of metal-based systems (vanadium,9 iron,10 chromium10)1,11 and a range of organic molecules (nitroxide radicals,12 phenazines,12–14 viologens,15 and quinones16–18).4,5 Quinones offer fast kinetics, high tunability and low cost.5 Of these, higher order quinones offer increased chemical and electrochemical stability.5 In this work, an exploration of nitrogen-rich fused heteroaromatic quinones was carried out to investigate new avenues for electrolyte development. The electrolytes were screened using electrochemical techniques and the most promising candidate was tested in a lab-scale flow battery as an anolyte under aqueous conditions. Sitting at -0.7 V(SHE), a capacity fade rate of 0.004%.cycle-1 was found in symmetric cycling. In situ UV-Vis, NMR19,20 and EPR19 spectroscopy were used to investigate the electrochemical stability and nature of the charged species involved during operation, complemented by density functional theory modelling. These studies indicate that fused systems of this type may be promising candidates for aqueous RFBs. 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).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).Cao, J., Tian, J., Xu, J. & Wang, Y. Organic Flow Batteries: Recent Progress and Perspectives. Energy and Fuels 34, 13384–13411 (2020).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 (2018).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).Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652–657 (2008).Ulaganathan, M. et al. Recent Advancements in All-Vanadium Redox Flow Batteries. Adv. Mater. Interfaces 3, (2016).Sun, C. & Zhang, H. A review of the development of the first‐generation redox flow battery : iron chromium system. ChemSusChem (2021). doi:10.1002/CSSC.202101798Noack, J., Roznyatovskaya, N., Herr, T. & Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chemie - Int. Ed. 54, 9776–9809 (2015).Winsberg, J. et al. Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High-Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery. ACS Energy Lett. 2, 411–416 (2017).Kwon, G. et al. Multi-redox Molecule for High-Energy Redox Flow Batteries. Joule 2, 1771–1782 (2018).Romadina, E. I., Komarov, D. S., Stevenson, K. J. & Troshin, P. A. New phenazine based anolyte material for high voltage organic redox flow batteries. Chem. Commun. 57, 2986–2989 (2021).Hu, S. et al. Phenylene-Bridged Bispyridinium with High Capacity and Stability for Aqueous Flow Batteries. Adv. Mater. 33, 2005839 (2021).Suo, L. et al. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chemie - Int. Ed. 55, 7136–7141 (2016).Shimizu, A. et al. Liquid Quinones for Solvent-Free Redox Flow Batteries. Adv. Mater. 29, 1606592 (2017).Yang, Z. et al. Alkaline Benzoquinone Aqueous Flow Battery for Large-Scale Storage of Electrical Energy. Adv. Energy Mater. 8, 1702056 (2018).Zhao, E. W. et al. Coupled in Situ NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries. J. Am. Chem. Soc. 143, 1885–1895 (2021).Zhao, E. W. et al. In situ NMR metrology reveals reaction mechanisms in redox flow batteries. Nature 579, 224–228 (2020). Figure 1

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 predicted to be a major contributing technology in the need for four-to-ten-hour energy storage. Other energy storage technologies, such as lithium-ion batteries, provide well-documented performance indicators such as efficiency and especially energy storage capacity utilization. While redox flow batteries are compared similarly based on cost and round-trip efficiency, they often neglect a comparison of their operational energy storage capacity utilization. The breakdown of operational losses that represent the current state of operation for RFBs are pivotal to further compare the development of these technologies to the standard of other electrochemical storage devices. Quantifying the current state-of-the-art in relation to the theoretical potential of RFBs allows us to assess the extent of development required to enhance their effectiveness in contributing to future energy storage goals.In this work, we set out to depict the operational energy storage density (W-h L-1) of various redox flow battery technologies based on consideration from experimental parameters and results from literature as key inputs. After reviewing five RFBs including the all-vanadium RFB, Hydrogen-Bromine RFB, All-Iron RFB, Zinc-Bromine RFB, and the Iron-Chromium RFB, results indicate that the VRFB is the most developed technology attaining around 60% of its theoretical energy storage density. While the Zinc-Bromine RFB has approximately the largest theoretical energy storage density (due to its high electroactive species concentration and standard potential difference), it is currently only able to operate at about a lowly 15% of it, dependent on its operational current density. We also assess the volumetric footprint for each RFB electrolyte system by considering a 6 MWh (6 MW, 1 hr) system. This assessment considers how large the electrolyte component of the RFB would be for operation. The Hydrogen-Bromine RFB would require electrolyte tanks to be more than 1600 m3 based on operational parameters, but if it were able to operate at 100% of its capability, the electrolyte tanks would shrink to be about 400 m3.

With the exponential growth of electricity generation using intermittent renewable power sources, there is a need for affordable energy storage. For grid level storage in past years, redox flow batteries (RFB) have drawn considerable attention. Most successful RFBs till now are all-vanadium RFB and all-iron RFB. But these RFBs have a few drawbacks associated with them like; First RFB developed by NASA[1], Iron-chromium RFB, had problems of ions crossing over; all-iron RFB [2] have a problem of hydrogen evolution reaction (HER) at the negative electrode which causes capacity decay; and the commercialized all-vanadium RFB's [3] is expensive and has limited vanadium reserves.In an effort to address these problems, we present here the initial development of an all-tungsten RFB. Its key advantage is no capacity fading with cycles due to ions crossing over. The tungsten salt used was phosphotungstic acid (PTA). As shown in cyclic voltammograms (CV) of PTA in Figure 1(a), it is clear that PTA ions exist in four different oxidation states (PTA3-, PTA4-, PTA5- and PTA6-). For all-PTA redox flow battery experiments, we used the same PTA5- active species for both negative as well as positive electrolyte. Prior to the all-PTA RFB tests, PTA3- was reduced to PTA5-. Since PTA3- is the only salt available, it was reduced to PTA5- by using it as a negative electrolyte for the iron-tungsten RFB using a procedure developed earlier [4].As shown in reaction 1, during charging on the negative side, PTA5- gains 2 electrons and reduces to PTA6- oxidation state. Simultaneously, on the positive side PTA5- loses 2 electrons and gets oxidized to PTA3-. Moreover, the maximum potential difference that can be achieved by using the highest reduced form of phosphotungstic acid as a single cell is +0.611 V vs. RHE (Eq. 4). Although we are getting half the potential when compared to an all-vanadium or an all-iron battery. But due to its extremely fast kinetics and large discharge currents makes all-PTA RFB a promising alternative. For the large capacity of the battery, one of the deciding factors is the solubility of its active species salts and the maximum solubility of PTA3- is 0.7 M.Flow cell experiments were conducted using an in-house constructed standard fuel cell setup [4]. Meanwhile, CV experiments were conducted in 3 electrode setup using glassy carbon as working electrode, platinum mesh as counter electrode and mercury/mercurous sulphate (Hg/Hg2SO4) as reference electrode. Hg/Hg2SO4 was calibrated in 0.5 M H2SO4 using an in house RHE. Also, all the potentials mentioned here are with respect to RHE. A CV (see Figure 1(a)) for phosphotungstic acid was obtained which showed 3 distinct peaks corresponding to the 3 reactions. Reactions 2 and 3 involve transfer of 1 electron each, while reaction 1 requires 2 electrons as proven from the current peaks of the CVs as well as limiting currents obtained from their RDEs [5]. The involvement of parasitic HER is clear from the 3rd reduction peak. Kinetic parameters such as rate constant (k0), Tafel slope, diffusion coefficients (Do) and transfer coefficient (α) were determined using RDE studies. From RDE analysis at different RPM, rate constant for PTA3- was found to be 4.49 x 10-3 cm s-1.Meanwhile, for the flow cell experiments we have taken electrolyte solution concentrations similar to that for the CVs. The flow rate (120 mL min-1) and volume (40 mL) for both the electrolyte were kept same. As shown in Figure 1(b), the all-PTA battery was charged and discharged using chronopotentiometry at a current density of 50 mA cm-2. Furthermore, to obtain the i-V performance curve the cell was discharged from 2 mA to 1000 mA for 10s. The potential at the end of 10th second was reported to eliminate the pseudo steady state [6]. i-V and power density performance curve was plotted from these current and potential values (see Figure 1(c)).From just 40mM concentration of PTA5- the maximum volumetric capacity we got is around 200 mAh L-1, the maximum power density obtained was 10 mW cm-2 and the maximum current that can be drawn was 85 mA cm-2. The above results strongly suggest that the all-PTA RFB could indeed be a promising alternative. Figure 1

Redox flow batteries (RFBs) are one potential solution to grid-level electrical energy storage (EES) benefiting from a decoupled power and capacity scaling.1–3 High durability, long-calendar life, high efficiency EES with a low cost and fast response time is needed1,4 for the transition from fossil fuels to renewable sources.3 However, the low energy density3,5,6 and high capital costs5,6 of current systems preclude wide-scale deployment of this technology.In recent years, several new RFB chemistries have been explored to address these concerns.1,2,7 However, a high solubility for a high volumetric energy density remains a troublesome target.1 It is, therefore, no surprise that one growing trend in this regard is the design of redox active liquids (RALs).8–13 RALs provide a means of dramatically increasing the volumetric energy density of RFBs through either miscibility with typical supporting electrolytes, or by acting as both solvent and electrolyte themselves.9,12 In this work, we investigate a series of RALs that offer a similar theoretical energy density to conventional intercalation materials. A combination of computational and experimental techniques was employed herein for both molecular design and explanation of the physio-chemical phenomena. The candidate compounds were initially screened via electrochemical techniques to identify their electrochemical reversibility and stability. Exploration of the bulk properties was then carried out before system-level characterisation was undertaken. In tandem, the electrochemical and chemical stability of the samples was also investigated through the typical routes (NMR, EPR, UV-Vis). These systems show much promise for organic, tuneable electrical energy storage. Cao, J., Tian, J., Xu, J. & Wang, Y. Organic Flow Batteries: Recent Progress and Perspectives. Energy and Fuels 34, 13384–13411 (2020).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 (2018).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).Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).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).Li, Z., Jiang, T., Ali, M., Wu, C. & Chen, W. Recent Progress in Organic Species for Redox Flow Batteries. Energy Storage Mater. 50, 105–138 (2022).Shimizu, A. et al. Liquid Quinones for Solvent-Free Redox Flow Batteries. Adv. Mater. 29, 1606592 (2017).Robertson, L., Udin, M. A., Shlrob, I. A., Moore, J. S. & Zhang, L. Liquid Redoxmers for Nonaqueous Redox Flow Batteries. ChemSusChem e202300043 (2023) doi:10.1002/cssc.202300043.Chen, N., Chen, D., Wu, J., Lai, Y. & Chen, D. Polyethylene glycol modified tetrathiafulvalene for high energy density non-aqueous catholyte of hybrid redox flow batteries. Chem. Eng. J. 462, 141996 (2023).Smith, L. O. & Crittenden, D. L. Acid‐Base Chemistry Provides a Simple and Cost‐Effective Route to New Redox‐Active Ionic Liquids. Chem. – An Asian J. 18, e202201296 (2023).Zhao, Y. et al. TEMPO allegro: liquid catholyte redoxmers for nonaqueous redox flow batteries. J. Mater. Chem. A 9, 16769–16775 (2021).Huang, J. et al. Liquid Catholyte Molecules for Nonaqueous Redox Flow Batteries. Adv. Energy Mater. 5, 1401782 (2015).

Redox flow batteries (RFBs) are a promising energy storage system for grid-level storage, where low-cost and scalability are essential1. To date, many different organic molecules including quinones2–4, viologens5,6, phenazines7,8, and alloxazines7,9 have been investigated as potentially-cheap RFB active molecules. Although a few molecules have shown a good performance in alkaline solution (pH 14)7,8, most organic molecules considered for RFBs generally experience degradation, reducing cell lifetime1. In 2016 Orita et al.10 reported a RFB comprising flavin mononucleotide (FMN3−) at pH 14 as the anolyte against a potassium hexacyanoferrate K4[FeII(CN)6] catholyte. The cell showed a remarkable capacity retention of 99% over the course of 100 cycles. Despite the encouraging capacity retention, an additional FMN reduction plateau appeared during charge, which was assigned to a dimerization process. 10 The process was not seen on discharge leading considerable cell hysteresis. Consequently, the resulting capacity retention and energy efficiency were not good enough for grid-scale storage systems, where even longer long-life times with minimal degradation and high coulombic efficiency are required. More recently, Nambafu et al. attached a 2,2,6,6- tetramethylpiperidinyl-N-oxyl (TEMPO) radical to FMN to form a bifunctional redox active material, which showed improved stability at neutral pH.11 However, significant capacity loss was seen within 100 cycles which was largely ascribed to degradation of the TEMPO functionality.FMN is a commercially available, non-toxic biomolecule, readily derived from vitamin B2, motivating its further study in an RFB. Flavins generally act as a cofactor in many enzymes that catalyze a wide variety of biological reaction and contain one of the most versatile in vivo redox centers 10. In nature, flavins are often found dissolved in water, fat, or blood, such as in biological systems 10. The molecules are also used in the food industry as an orange-red food color additive, utilized in Europe as E101a 12; the sodium salt is commonly known as E106 and is found in foods for babies and young children as well as jelly, milk products, and sweet products 12.Here we demonstrate a powerful strategy to study the degradation of FMN3− by coupling in-situ NMR and EPR techniques 4. We explain how degradation, which we show involves the hydrolysis of FMN3− rather than a dimerization process, leads to the additional charging plateau. We investigate the electrochemical behavior of hydrolyzed FMN3− with in-situ NMR and demonstrate that FMN3− acts as a redox mediator, helping to reduce the hydrolyzed product, explaining the lack of an additional plateau on discharge yet good cycling behavior, despite degradation and poor reversibility of hydrolyzed FMN3− redox reactions. Lastly, we provide a strategy to avoid the hydrolysis by lowering the pH. The battery performance is improved significantly and the FMN solubility is increased dramatically, by addition of a simple salt. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M. D. & Schubert, U. S. Redox-Flow Batteries: From Metals to Organic Redox-Active Materials. Angew. Chemie - Int. Ed. 56, 686–711 (2017).Lin, K. et al. Alkaline Quinone Flow Battery. Science (80-. ). 349, 1529–1532 (2015).Zhao, E. W. et al. In situ NMR Metrology Reveals Reaction Mechanisms In Redox Flow Batteries. Nature 579, 224–228 (2020).Zhao, E. W. et al. Coupled in Situ NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries. J. Am. Chem. Soc. 143, 1885–1895 (2021).Hu, B., DeBruler, C., Rhodes, Z. & Liu, T. L. Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) Toward Sustainable And Safe Energy Storage. J. Am. Chem. Soc. 139, 1207–1214 (2017).Jin, S. et al. Near Neutral pH Redox Flow Battery with Low Permeability and Long-Lifetime Phosphonated Viologen Active Species. Adv. Energy Mater. 10, 1–10 (2020).Lin, K. et al. A redox-flow battery with an alloxazine-based organic electrolyte. Nat. Energy 1, 1–8 (2016).Hollas, A. et al. A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat. Energy 3, 508–514 (2018).Wei, X. et al. Materials and Systems for Organic Redox Flow Batteries: Status and Challenges. ACS Energy Lett. 2, 2187–2204 (2017).Orita, A., Verde, M. G., Sakai, M. & Meng, Y. S. A Biomimetic Redox Flow Battery Based On Flavin Mononucleotide. Nat. Commun. 7, 1–8 (2016).Nambafu, G. S. et al. An organic bifunctional redox active material for symmetric aqueous redox flow battery. Nano Energy 89, 106422 (2021).Turck, D. et al. Dietary Reference Values for riboflavin. EFSA J. 15, (2017).

The progressive integration of renewable energy sources such as wind turbines and photovoltaic pannels in the current electrical network, and the rise of the electrical mobility, provoke a change of paradigm in the sector of energy management. The increasing variability of the electricity production and consumption profiles, together with the requirement for a reliable supply of energy, necessitates the implementation of energy storage means of different scales and for a wide range of applications. Redox flow batteries (RFBs) are well adapted for buffering the fluctuations of solar or wind energy production. They present a fast response time, can withstand a large number of charge-discharge cycles and their ouput power is independant of their energy capacity. The main limitation of RFBs resides in their low energy density. The objective of the present work was to test a mean to overcome this low energy density. A new concept was developed, which rests on the addition of a second pathway to discharge the RFB. This pathway allows to generate hydrogen and oxygen, without affecting the functioning of the RFB. This concept was called dual-circuit RFB and was patented. RFBs are based on two liquids electrolytes, each stored in a reservoir, and flowing through an electrochemical cell for their electrochemical conversion. Each electrolyte contains one redox couple and Ce(IV)/Ce(III) and V(III)/V(II) were selected as positive and negative redox couples in the RFB developed here. Charge-discharge curves of a V–Ce RFB were measured for characterisation purposes and for the preparation of the charged electrolytes. The latter ones can be discharged electrochemically in the RFB to generate electricity (electrochemical discharge mode), or they can be directed in a secondary circuit where they are discharged for the production of hydrogen or oxygen (chemical discharge mode). The chemical discharge of the negative electrolyte consists of the reaction of V(II) with protons to produce hydrogen and V(III). Mo2C was selected as heterogenous catalyst for the characterisation of the reaction. A conversion close to 100% suggested no loss of current. A kinetic analysis provided some insights into the catalytic mechanism of this reaction. The chemical discharge of the positive electrolyte aimed at the conversion of Ce(IV) to Ce(III) by the oxidation of water to oxygen and also necessitates a catalyst. Iridium dioxide (IrO2) and ruthenium dioxide (RuO2) were evaluated in terms of conversion and kinetics. The composition of the positive electrolyte was shown to be of importance for this reaction. To show the feasibility of this concept, a larger-scale demonstrator system was designed based on a 10 kW (40kWh) commercially available vanadium RFB. A characterisation of this RFB was first performed. The design a suitable Mo2C catalyst and its corresponding catalytic bed is also discussed. As a conclusion, the concept developed and experimentally tested in the present work leads to a RFB system which is characterised by two discharge modes, increasing its energy storage capacity and energy density. The dual-circuit RFB also represents a crossing between electrical grid and hydrogen mobility as the hydrogen produced by surplus electricity could be delivered to fuel cell cars. The application of this system in a local distribution electrical network, close to a wind or solar source seems a promising approach.

The concept of redox flow batteries (RFB) was originally patented in 1949 by Dr. Kangro with inorganic active materials like iron- or chromium-ions in mind.[1] Intensive research on inorganic active materials led to the proposal of vanadium based RFBs in 1985 by the Skyllas-Kazacos group, which is the most intensively studied RFB type currently.[2],[3] Organic active materials were introduced in 2009 for RFBs and offer a potential alternative to vanadium as active species, due to potentially lower costs and the replacement of rare metal ions.[4],[5] The class of verdazyl radicals has gathered interest as organic active material for the battery application since the publication of symmetrical coin cells.[6] Verdazyl radicals can be transformed into an oxidized or reduced species by a one electron reaction in organic electrolytes, which motivates the application in symmetrical RFBs.[7] Still, degradation of the verdazyl species during operation of the symmetrical battery in organic electrolytes as well as the theoretical voltage of ≈ 1 V show the need for further optimization of the material class and electrolytes.[6] While previous electrochemical investigations of verdazyl species were limited to organic electrolytes, we used a different approach and investigated the influence of acidic electrolytes on the redox chemistry of this material class.[8] Strongly acidic electrolytes lead to a disproportionation of the radical form. The resulting species differ by two electrons and two protons and show a reversible redox reaction in cyclovoltammetry measurements. This different reaction is of interest for the RFB application due to the fast rate constant as well as the utilization of two electrons.[8] Herein, we take the next step for the possible application of the redox chemistry of verdazyl species in acidic electrolytes into aqueous RFBs by analyzing the influence of the separating membranes. We use 1,3,5-triphenylverdazyl species as model compounds in 1 M sulfuric acid. To investigate the influence of the separating membranes in aqueous RFBs, we use Nafion 211 as commercial standard and compare it to self-made mPBI and O-PBI-based polybenzimidazole (PBI) membranes with a target thickness of ≈ 25 µm. When Nafion membranes are immersed in verdazyl cation solutions, they absorb ≈ 0.5 verdazyl cations per sulfonic acid group from the solution, whereas no absorption was observed for PBI membranes. UV/vis also did not show any changes in the UV/vis spectra of the solution, indicating that the verdazyl cations did not degrade. Beside the chemical stability, the permeability through the membranes is characterized, which is detrimental for the capacity retention in an RFB. To fully characterize the membranes with verdazyl species, the conductivity of the membranes is measured and the influence of the verdazyl species onto the conductivity is investigated. As a last step, symmetrical RFBs of verdazyl species in acidic electrolytes are built and the capacity retention is compared.With this study, we show the detrimental effect of using Nafion in combination with verdazyl species. Nafion 211 shows a high affinity to verdazyl cations and absorbs them from the electrolyte, leading to fast capacity fading of the RFB. PBI membranes show chemical stability as well as low absorption and permeability.Our study presents the one of the next steps towards the implementation of verdazyl species into water-based RFBs by investigating the suitability of PBI membranes for this application. PBI membranes offer overall good stability and beneficial properties for this application, while state-of-the-art Nafion is not applicable.

Redox flow batteries (RFBs) are a promising technology for large-scale energy storage. Rapid research developments in RFB chemistries, materials and devices have laid critical foundations for cost-effective and long-lasting RFB systems. However, the lack of consistency in testing methods and assessment metrics makes it challenging to compare reported RFBs and evaluate their potential for practical applications. Here we discuss RFB assessment methods and performance metrics in direct relation to their working principles and degradation mechanisms. We first introduce basic cell attributes and performance metrics and describe common misconceptions in testing and performance comparison. We discuss major RFB decay mechanisms and highlight bottlenecks in organic, inorganic and solid-hybrid RFBs. Testing protocols, reporting practices and comparison criteria are proposed under a general framework of symmetric and asymmetric full RFBs. These recommendations can be broadly applied to a wide range of flow battery chemistries to facilitate future benchmarking and RFB development. Performance assessments of redox flow batteries (RFBs) can be challenging due to inconsistency in testing methods and conditions. Here the authors summarize major performance metrics of RFBs, analyse their degradation mechanisms and propose testing protocols for benchmarking.

The continued integration of renewable energy sources into the power grid, as well as the development of decentralised energy systems, requires energy storage in order to capture and store the energy produced intermittently from renewable sources. One energy storage option is the Redox Flow Battery (RFB) which has several advantages over other secondary batteries. As the active species of an RFB remain in solution during charge and discharge, degradation of the electrodes is minimised. Further, due to external storage of the electrolytes, the energy and power of an RFB can be adjusted independently by varying the volume of stored electrolyte or size of the cell stack. The most mature RFB is the all-vanadium RFB but this also has drawbacks, such as slow electron transfer kinetics of the vanadium ions1 and a requirement for the use of acidic electrolytes. Polyoxometalates (POMs) are anionic metal-oxygen clusters of early transition metals, commonly vanadium, molybdenum and tungsten, which can undergo redox reactions in which multiple electrons are transferred. The electrochemical properties of POMs prompted their investigation by our group and the development of a POM RFB which was based on the reactions of [PV14O42]9- at the cathode and [SiW12O40]4- at the anode.2 This work demonstrated that an RFB cell with multi-electron redox reactions occurring in each half-cell could be created by using POMs and that the charge transfer reactions of POMs are relatively fast.2 The [PV14O42]9-/[SiW12O40]4- RFB was classified as an asymmetric RFB due to the use of different POM active species in each half-cell. Alternatively, symmetric RFBs can be designed which utilise the same active material in both half-cells. Our group has also investigated the approach of using bifunctional molecules, which can undergo redox reactions at positive and negative potentials, as the active species in symmetric RFBs. This includes the use of fullerene species functionalised with ferrocene units.3 POMs can also be bifunctional species when composed of two different redox-active metals. In this poster, we outline the results of work in our group on the use of bifunctional species in symmetric RFBs, including the study of a POM for a symmetric RFB which has shown good stability in charge-discharge testing so far in conditions that are only mildly acidic. References J. Friedl and U. Stimming, Electrochimica Acta, 2017, 227, 235–245.J. Friedl, M. V. Holland-Cunz, F. Cording, F. L. Pfanschilling, C. Wills, W. McFarlane, B. Schricker, R. Fleck, H. Wolfschmidt and U. Stimming, Energy Environ. Sci., 2018, 11, 3010–3018.J. Friedl, M. A. Lebedeva, K. Porfyrakis, U. Stimming and T. W. Chamberlain, J. Am. Chem. Soc., 2018, 140, 401–405.

Chapter 9 - Oxides free materials as anodes for zinc-bromine batteries