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Here, we present a new aqueous biphasic system (ABS) with near neutral pH and containing highly soluble organic/organometallic active species that avoids crossover due to the thermodynamic separation of active species. This ABS become the best example of “Static” Membrane-Free Battery with the highest maximum theoretical energy density (21.7 Wh L􀀀 1), enhanced coulombic efficiency (96%) and stable performance over 250 cycles. Moreover, this work assesses the process of the self-discharge stablishing for the first time a protocol to quantify this phenomenon that results in low coulombic efficiencies in membrane-free batteries. Remarkably, a radically new membrane-free flow-reactor was specifically designed to maintain a stable liquid-liquid interphase that allows the battery to operate under flowing conditions. This resulted in flow battery with a two-fold increase of power density, high coulombic efficiencies and excellent capacity retention over 100 cycles. This work demonstrates for the first time the feasibility of this biphasic electrolyte concept in a truly Membrane-free “Flow” Battery

The accumulation of energy produced by renewable sources to be used when demand is high is a key aspect of a truly decarbonized grid. Among all energy accumulation technologies, Redox Flow Batteries (RFBs) are specially promising due to its safety, flexibility of design and the ability to decouple capacity and power of the system. In a standard configuration, both redox electrolytes that are stored in external tanks are pumped in the electrochemical reactor separated only by an ion-selective membrane. This membrane is a critical part of the battery as it allows the selective pass of certain ions that compensate the charge imbalances while maintaining separated the active species in their respective compartments. Membranes used in RFB are expensive (up to 25% of the total cost in vanadium RFB), contribute greatly to the ohmic resistance of the battery, and the undesired crossover through it is never fully avoided which produces a performance decay of the battery over time. These issues are addressed by membrane-free RFB designs developed in our group. The idea behind MFreeB ERC project is to take advantage of the immiscibility of certain liquids to naturally separate redox active species in a membrane-free battery design [1]. Aqueous Biphasic Systems (ABSs) are a special example of all-aqueous natural occurring immiscible phases made by mixing water with two compounds that can be either polymers, inorganic salts or both. The two redox active species added to the biphasic system can be rationally selected and optimized so that they partition specifically to one of the two phases used as anolyte and catholyte [2]. These types of batteries have been proposed by several groups, including us, mostly as “static batteries” because filter-press reactors, typically used in conventional RFB, cannot be easily adapted to this membrane-free concept. In order to develop truly flowing systems, a totally different electrochemical reactor should be designed. Here, we report on the development of an electrochemical reactor that allows the formation of the two immiscible phases making possible the testing of immiscible batteries under flowing conditions for the first time. A counterflow flow-through configuration was used since it reduces flow instabilities while at the same time it allows high conversion rates in the carbon felt electrodes avoiding mass transport issues [3]. Several tests were performed in the battery; e.g. polarization curves, charge and discharge cycling and efficiency over time were measured. Moreover, the self-discharge phenomena, occurring near the interface when charged species enter in contact and provoking a loss of efficiency, was evaluated as a function of operating conditions. Special attention was given to the study of the different processes occurring in the region near the interface in order to mitigate losses and further optimize the design. Acknowledgments This work has been partially funded by project MFreeB which have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 726217). References [1] P. Navalpotro et al., Angew. Chem. Int. Ed. 56 (2017), 1–6. [2] P. Navalpotro et al., Energy Storage Materials. 26 (2020) 400-407. [3] International patent WO 2021/209585 A1. Redox Flow Battery with Immiscible Electrolyte and Flow Through Electrode.

Lately, the field of redox flow batteries is flourishing because of the emergence of new redox chemistries, including organic compounds, new electrolytes, and innovative designs. Recently, we reported an original membrane-free battery concept based on the mutual immiscibility of an aqueous catholyte containing hydroquinone and an ionic liquid anolyte containing para-benzoquinone as redox species. Here, we investigate the versatility of this concept exploring the electrochemical performance of 10 redox electrolytes based on different solvents, such as propylene carbonate, 2-butanone, or neutral-pH media, and containing different redox organic molecules, such as 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-hydroxy- 2,2,6,6-tetramethylpiperidine1-oxyl (OH-TEMPO), or substituted anthraquinones. The most representative electrolytes were paired and used as immiscible anolyte−catholyte in 5 different membrane-free batteries. Those batteries with substituted anthraquinones in the anolyte exhibited up to 50% improved open-circuit voltage (2.1 V), an operating voltage of 1.75 V, and 62% higher power density compared with our previous work. On the other hand, the partition coefficient of redox molecules between the two immiscible phases and the inherent self-discharge occurring at the interphase are revealed as intrinsic features affecting the performance of this type of membrane-free battery. It was successfully demonstrated that the functionalization of redox molecules is an interesting strategy to tune the partition coefficients mitigating the crossover that provokes low battery efficiency. As a result, the cycling life of a battery having OH-TEMPO as active species in the catholyte and containing propylene carbonate-based anolyte was evaluated over 300 cycles, achieving 85% capacity retention. These results demonstrated the huge versatility and countless possibilities of this new membrane-free battery concept.

Flexible and scalable energy storage solutions are necessary for mitigating fluctuations of renewable energy sources. The main advantage of redox flow batteries is their ability to decouple power and energy. However, they present some limitations including poor performance, short‐lifetimes, and expensive ion‐selective membranes as well as high price, toxicity, and scarcity of vanadium compounds. We report a membrane‐free battery that relies on the immiscibility of redox electrolytes and where vanadium is replaced by organic molecules. We show that the biphasic system formed by one acidic solution and one ionic liquid, both containing quinoyl species, behaves as a reversible battery without any membrane. This proof‐of‐concept of a membrane‐free battery has an open circuit voltage of 1.4 V with a high theoretical energy density of 22.5 Wh L−1, and is able to deliver 90 % of its theoretical capacity while showing excellent long‐term performance (coulombic efficiency of 100 % and energy efficiency of 70 %).

Highly water-dispersible, redox-active 1D coordination polymers (CPs) have been synthesized using low-cost precursors. These CPs, containing chloranilic acid as organic ligand and a transition element, such as Fe and Mn as a metal center, form long-term stable slurries containing up to 100 g/L solid particles in aqueous media (0.5 M NaCl, 1 mg carbon nanotubes). Voltammetry studies showed that the iron-based particulate slurries exhibited three different redox stages with no metal plating. However, the suspensions with manganese-based coordination polymers experienced a metal plating process in the same potential window range as for the iron-based CPs. Moreover, the particulate suspension of iron-CPs shown longer-term stability than their isostructural analogs based on manganese. The 1D Fe-CPs were used as catholyte and anolyte in a symmetrical cell with a low-cost size exclusion cellulose membrane acting as a separator. The cell experienced a reversible capacity value of 45 mAh/g (225 mAh/L) at a current density value of 20 mA/g for 50 cycles (~12 days) at neutral pH. This study opens the possibility of using inexpensive coordination polymers as single bifunctional electrolyte material in aqueous batteries and other sustainable energy storage-related systems., J.M. greatly acknowledge the Funds of his doctoral grant, Materialsfor Health, Environment and Energy, from the University of Rome TorVergata (Italy). J.C.G. acknowledges support from the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO) througha Ramon y Cajal Fellowship (RYC-2015-17722), the Retos Project(MAT2017-86796-R, AEI/FEDER/UE) and the PTI-FLOWBAT project(Ref. N°: 201980E101) from the Spanish Research Council (CSIC). J.M., B.M., and A.D. acknowledge the Regione Lazio for funding theresearch through the project POR FESR LAZIO 2014–2020, NoA0375-2020-36492.

Redox Flow Batteries (RFB) have been recognized as a viable technology for the integration of renewable energies into the electricity grids due to their ability to decouple power and energy and the possibility of storing large amount of energy at competitive cost. However, wide-spread implementation of conventional RFBs is limited by some obstacles related to low abundance, toxicity and high cost of vanadium redox compounds, and the poor-performing and expensive ion exchange membranes used to separate catholyte and anolyte compartments. This key element can account for more than 30 % of the cost of the battery (2) and may limit the long-term battery performance due to unavoidable cross-contamination and insufficient mechanical, chemical and electrochemical stability. Here, we present an innovative concept of Membrane-Free Battery which proposes to eliminate any separator or membrane by using immiscible redox electrolytes (3). These electrolytes spontaneously form a biphasic system whose interphase functions as a “natural” barrier making the use of any membrane superfluous. Moreover, the vanadium redox species are replaced by cheap and abundant organic redox molecules that can be specifically designed to exhibit high solubility, adequate redox potentials and suitable partition coefficient between the two immiscible phases. It will be demonstrated that this disruptive technology is hugely versatile with respect to the type of organic redox species as well as the nature of immiscible electrolytes forming the biphasic system. We report on the electrochemical performance of Membrane Free batteries based on aqueous/non-aqueous immiscible electrolytes (4) but also a Total Aqueous Membrane-free Battery in which the two phases are aqueous (5), which brings some advantages in terms of cost, environmental issues and battery performance. New challenges such as the inherent selfdischarge in the liquid-liquid interface, the need of developing new reactors designs to keep the two immiscible liquids separated, and future opportunities of this innovative technology making use of thermodynamics and computational chemistry will be also discussed. References: a) Nagarjuna, G.; Hui, J.; Cheng, KJ.; Lichtenstein, T.; Shen, M.; Moore, JS.; Rodríguez-Lopez, J.; Am. Chem. Soc. 2014, 136, 16309−16316, b) Janoschka, T.; Martin, N.; Martin, U.; Friebe, C.; Morgenstern, S.; Hiller, H.; Hager, M. D.; Schubert, U. S. Nature 2015, 527 (7576), 78–81. Zhang, M.; Moore, M.; Watson, J. S.; Zawodzinski, T. A.; Counce, R. M. J. Electrochem. Soc. 2012, 159 (8), A1183–A1188. Navalpotro, J. Palma, M. Anderson, R. Marcilla. Angew. Chem. Int. Ed. 56, 12460 –12465. 2017 Navalpotro, P.; Sierra, N.; Trujillo, C.; Montes, I.; Palma, J.; Marcilla, R.. ACS Appl. Mater. Interfaces, 2018, 10, 48, 41246-41256. P. Navalpotro, C. Trujillo, I. Montes, C.M.S.S. Neves, J. Palma, M.G. Freire, J.A.P. Coutinho, R. Marcilla, Energy Storage Mater. 26 (2020) 400–407. Acknowledgments MFreeB project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 726217). The results reflect only the authors’ view and the Agency is not responsible for any use that may be made of the information they contain. Figure 1

In this communication, a novel two-dimensional polymer network (DAPT-TFP) has been synthesized based on pyrenetetraone moieties connected through robust enamine functional groups. Results show that DAPT-TFP exhibit high thermal and chemical stability, even after subjecting the material to a basic or acidic media for certain time (1 M HSO or 1 M NaOH, respectively). An application to conductive additive-free electrode is detailed, as a proof of the ideal interaction between the ions of the neutral aqueous electrolyte and the molecular polymer nanostructure activated by simple acidic treatment. Thus, the study indicates that the redox properties of new organic electrode materials exhibiting limited electroactive properties can be boosted by a straightforward acidic treatment., This work was financially supported by MINECO (MAT2016-77608- C3-2-P and FIS2017-82415-R), MICINN (PID2019-106268GB-C33) and the UCM (INV.GR.00.1819.10759). JCG acknowledges the Ramon y Cajal Fellow (RYC-2015-17722) from the Spanish Ministry of Science, Innovation and Universities. PGA is gratefully acknowledged to Comunidad de Madrid for a predoctoral contract.

Herein, we present a multigram scale-up route for the preparation of novel polymer composite nanoparticles as potential multifunctional rechargeable material for future, sustainable batteries. The nanoparticles (20 nm) comprise three innocuous yet functional interpenetrated macromolecular networks: polypyrrole, methylcellulose, and lignin. They are uniquely assembled in strands or chains (~200 nm) such as necklace beads and show long-term stability as water dispersion. We find that an aqueous suspension of this hierarchical nanomaterial shows two sets of reversible redox peaks, separated by ~600 mV, originating from the catechol moieties present in the lignin biopolymer. Remarkably, the addition of carbon dioxide increased the capacity of one of the redox processes by 500%. Importantly, the three redox stages occur in the presence of the same nanostructured polymer so being a potentially bifunctional material to be used in advanced electrochemical systems. The new properties are attributed to an intrinsic chemical and electronic coupling at the nanoscale among the different building blocks of the metal-free polymer composite and the structural rearrangement of the interpenetrated polymer network by the incorporation of CO. We have provided both a new electrochemically multifunctional hierarchically structured material and a facile route that could lead to novel sustainable energy applications., J.C.G. acknowledges support from the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO) through a Ramon y Cajal Fellowship (RYC-2015-17722) and the Retos Project (MAT2017-86796-R, AEI/FEDER/UE). D.A.E. and D.A.B acknowledge the Retos Project (MAT2017-84385-R, AEI/FEDER/UE). P.N. acknowledges the postdoctoral contract from the Government of the Comunidad de Madrid (CAM, PEJD-2018-POST/AMB-9248). M.L.M. and J.C.G. acknowledge the PTI-FLOWBAT project (Ref. Nº: 201980E101) from the Spanish Research Council (CSIC). We would also like to thank the Biomaterials Group from the ICTP-CSIC for letting us use the DLS equipment.

A solution of 0.4M para-Benzoquinone (p-BQ) in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR 14 TFSI) was used as a redox electrolyte in hybrid supercapacitors. Two carbons with very different textural properties, Pica carbon and Vulcan carbon, were used as electrode material. Electrochemical performance of these energy storage systems was investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (CD). Unlike SCs with pure IL electrolyte, new battery-like features appeared in the CV curves and CD profiles. This electrochemical performance, associated with the faradaic contribution of the redox electrolyte, results in a significant improvement of the electrochemical performance of the hybrid system. For Vulcan carbon with low specific surface area (S BET = 240 m 2 g -1 ), specific capacitance (C s ) and specific real energy (E real ) values as high as 70 Fg -1 and 10.3 WhKg −1 were obtained at 5 mAcm −2 with hybrid SC operating at 3 V. This represents an increment of 300% in C s and E real with respect to the SC based on pure PYR 14 TFSI. For high surface area carbon such as Pica (S BET = 2410 m 2 g -1 ), the addition of the redox quinone molecule resulted in a moderate enhancement reaching values of 156 Fg -1 and 30 WhKg −1 under the same experimental conditions (36% and 10% increment, respectively).

Aqueous biphasic systems (ABS) formed by water, ionic liquids (ILs), and salts, in which the two phases are water rich, are demonstrated here to act as potential membrane‐free batteries. This concept is feasible due to the selective enrichment of redox organic molecules in each aqueous phase of ABS, which spontaneously form two liquid‐phases above given concentrations of salt and IL. Therefore, the required separation of electrolytes in the battery is not driven by an expensive membrane that hampers mass transfer, but instead, by the intrinsic immiscibility of the two liquid phases. Moreover, the crosscontamination typically occurring through the ineffective membranes is determined by the partition coefficients of the active molecules between the two phases. The phase diagrams of a series of IL‐based ABS are characterized, the partition coefficients of several redox organic molecules are determined, and the electrochemistry of these redox‐active immiscible phases is evaluated, allowing appraisal of the battery performance. Several redox ABS that may be used in total aqueous membrane‐free batteries with theoretical battery voltages as high as 1.6 V are identified. The viability of a membrane‐free battery composed of an IL‐based ABS containing methyl viologen and 2,2,6,6‐tetramethyl‐1‐piperidinyloxy as active species is demonstrated.

Electrochemical energy storage devices are destined to be key technologies to alleviating the intermittent and fluctuating nature of renewable energy sources (1). In particular, Redox Flow Batteries (RFB) have been recognized as a viable technology for the integration and balancing electricity grids due to their ability to decouple power and energy. However, wide-spread implementation of conventional RFBs is limited by some obstacles related to low abundance, toxicity and high cost of vanadium redox compounds, and the poor-performing and expensive ion exchange membranes used to separate catholyte and anolyte compartments (2). Here, we present an innovative concept of Membrane-Free Battery which proposes to eliminate any separator or membrane in the cell by using immiscible redox electrolytes (Fig. 1). Moreover, the metal-based redox species are replaced by cheap and abundant organic redox molecules that can be specifically designed to provide high solubility and adequate redox potentials. As result, this disruptive technology is hugely versatile with respect to the nature of the electrolytes (aqueous / non-aqueous) as well as the type of organic redox species dissolved in each electrolyte (Fig. 1). To demonstrate the feasibility of this concept, we report here a spontaneous biphasic system formed by an acidic solution and an ionic liquid both containing dissolved quinoyl species that behaves as a reversible battery without any membrane or separator (3). The electrochemical performance of this Membrane-Free battery and the new challenges and future opportunities of the technology will be discussed. Figure 1 . a) Scheme of our Membrane-Free Battery concept. b) Versatility of this concept by using different combinations of immiscible electrolytes and their corresponding characteristics. References: Z. Yang et al., Chem. Rev. 111, 3577–3613 (2011). a) H. Prifti, A. Parasuraman, S. Winardi, T. M. Lim, M. Skyllas-Kazacos, Membranes (Basel). 2, 275–306 (2012). b) W. Wang, V. Sprenkle, Nat. Chem. 8, 204–206 (2016). c) J. Winsberg, T. Hagemann, T. Janoschka, M. D. Hager, U. S. Schubert, Angew. Chemie - Int. Ed. 55, 2–28 (2016). P. Navalpotro, J. Palma, M. Anderson, R. Marcilla. Angew. Chem. Int. Ed. 56, 12460 –12465. 2017 Figure 1

Redox Flow Batteries (RFB) stand out as a promising energy storage technology to mitigate the irregular energy generation from renewable sources. However, some hurdles limit their massive implementation including high cost of vanadium and the poor-performance of ion-selective membranes. Recently, we presented a revolutionary Membrane-Free Battery based on organic aqueous/nonaqueous immiscible electrolytes that eludes both separators and vanadium compounds. Here, we demonstrate the feasible application of this archetype in Aqueous Biphasic Systems (ABS) acting as an unprecedented Total Aqueous Membrane-Free Battery. After evaluating several organic molecules, methylviologen (MV) and 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) were selected as active species due to their optimum electrochemical behavior and selective partitioning between the phases. When connected electrically, this redox-active ABS becomes a Membrane-Free Battery with an open circuit voltage (OCV) of 1.23 V, high peak power density (23 mWcm-2) and excellent long-cycling performance (99.99% capacity retention over 550 cycles). Moreover, essential aspects of this technology such as the crossover, controlled here by partition coefficients, and the inherent self-discharge phenomena were addressed for the first time. These results point out the potential of this pioneering Total Aqueous Membrane-Free Battery as a new energy storage technology.