Search

The implementation of sustainable batteries coupled to renewable energy conversion systems in off-grid, poor and remote communities will overcome some of the technological challenges for electrification, such as the difficult geographical and cost accessibility, improving their modest technical capabilities and making the full system ecofriendly. Besides, their application to power off-grid "structures and facilities"requiring a large amount of energy (>50 kW) such as heavyweight carriers (e.g. boats and trucks) and for instance those located far away from the main grid such as small-size islands and mines, will have an immediate impact on the decrease of greenhouse gas emissions to the atmosphere. Herein we present an overview of recent efforts to develop green and advanced electrode and electrolyte materials for future sustainable batteries. Further, we will also highlight both current second-life and end-of-life battery disposal sustainable strategies that are applied towards a zero-carbon future. This journal is, P. N. thanks the Spanish Ministry of Science and Innovation through the SUSBAT project (RTI2018-101049-B-I00, MCI-AEI/ FEDER, UE). E. C. M. acknowledges financial support through a H2020 Marie Skłodowska Curie Individual Fellowship (H2020- MSCA-IF-2016 747449) and the Spanish Ministry of Science and Innovation through the ANOKIB project (RTI2018-094550-A- I00). J. C. G. acknowledges support from the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO) through a Ramon y Cajal Fellowship (RYC-2015-17722), the Retos Project (MAT2017-86796-R, AEI/FEDER/UE), the PTI-FLOWBAT project (Ref. No.: 201980E101) from the Spanish Research Council (CSIC) and the AESCA-CYTED project (Ref. No.: 201980E101). Finally, we thank Vincent Castillo-Carretero for being an inspiration and motivation.

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.

Recently, new electrode materials and electrolytes for hybrid supercapacitors with enhanced performance have been extensively studied. However, the energy storage mechanism of these hybrid devices has not been assessed in detail. In this sense, Electrochemical Impedance Spectroscopy is one of the most powerful techniques to better understand the physicochemical phenomena involved in this new kind of supercapacitors. In this work, Impedance Spectroscopy was used for the first time to analyse the energy storage mechanism in supercapacitors with non-aqueous electrolytes with organic redox molecules dissolved, as a function of the frequency and bias voltage. In addition, a simple electrical model has been developed to describe the behavior of the supercapacitor with a very good correlation with the experimental results. This investigation provides useful information delving into the behavior of the devices which is fundamental for their future practical applications.

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.

New polymeric materials such as polymer electrolytes or redox polymers are actively being searched for in order to increase the performance and security of electrochemical energy storage devices such as batteries. Poly(ionic liquid)s are very popular materials in energy nowadays finding applications as ion conducting polymer electrolytes and electrode binders for batteries and supercapacitors. In this work, the incorporation of redox-active counter anions (anthraquinone and nitroxide molecules) into poly(ionic liquid)s has broadened the scope of applications as redox-active materials in different energy storage technologies. Polymers having those known redox-active molecules usually involve challenging synthetic routes and yield insoluble materials very difficult to handle. In this paper, we show that the synthesis of the redox-active poly(ionic liquid)s can be achieved through a straightforward and simple anion exchange reaction. We also show that this new family of redox-active poly(ionic liquid)s can be applied in several electrochemical energy storage technologies such as lithium batteries, as electrocatalysts in fuel cells and metal–air batteries or as electrolytes in organic redox flow batteries.

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.

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.

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.