Mitigating capacity fading in aqueous organic redox flow batteries through a simple electrochemical charge balancing protocol

Aqueous organic redox flow batteries (AORFBs) have recently been attracting much attention due to their potential utilization as a sustainable solution for stationary energy storage. However, AORFBs have still to face various challenges to become a competitive technology to other mature redox flow batteries. Fading of the energy storage capacity upon cycling leading to insufficient lifetime is likely the most pressing issue. Several processes are contributing to this issue. Among the capacity fading promoters, the existence of side reactions such as water splitting and reactions related to oxygen reduction triggers an imbalanced state of charge for the catholyte and anolyte. Herein, a simple electrochemical balancing procedure is proposed and successfully demonstrated through the restoration of the oxidation states of the two half-cell solutions. The results reveal that it is possible to mitigate and even revert the effects of such side reactions, developing a useful method for mitigating the capacity fading and prolonging the cycling performance of AORFBs. In the two case studies, the implementation of this simple charging procedure leads to a remarkable 20-fold reduction of capacity fading (% h−1). The protocol is a general approach for redox flow batteries, easily implementable and inexpensive.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. The authors also acknowledge the financial support by the Spanish Government through the Research Challenges Programme (Grant RTI2018-099228-A-I00). E.V. thanks the MINECO for the financial support (RYC2018-026086-I). A.M.-C. thanks the MINECO for the financial support (RYC2017-22700)

Organic batteries based on just redox polymers

[EN]Redox-active polymers have gained interest as environmentally friendly alternative to inorganic materials in applications such as electrodes in lithium-ion batteries. All-polymer batteries were first disregarded with respect to other technologies due to their lower energy densities. However, the inherent benefits of redox polymers such as processability, flexibility, recyclability, high-rate performance and the perspective to prepare batteries from renewable resources has re-ignited interest in recent years. This review article aims to provide a comprehensive overview on the state of the art of batteries in which the active material is a redox polymer; including "static" all-polymer batteries and polymer-air batteries but also "flowing" systems such as polymer based redox-flow batteries (pRFB). First, a succinct overview of the recent developments of redox polymers will be given, summarizing the historic trends and developments. Second, an exhaustive discussion of the various battery prototypes will be provided, considering all steps in the development of organic batteries just based in redox polymers. Finally, future perspectives on all-polymer batteries will be discussed, summarizing the major challenges that are still to be overcome to unlock their commercial implementation.Authors thank POLYSTORAGE ETN project, this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No 860403. RM and NP thank the Spanish MCI through the SUSBAT project (Ref. RTI2018-101049-B-I0 0) and Juan de la Cierva fellowship [FJC2018-037781-I] (MCI-AEI/FEDER, UE) . NC would like to thank the University of the Basque Country forfunding through a specialization of research staff fellowship (ES-PDOC 19/99) . NG acknowledges the funding from the European Union's Horizon 2020 framework programme under the Marie Skodowska-Curie Agreement No. 101028682

A significantly improved polymer||Ni(OH)2 alkaline rechargeable battery using anthraquinone-based conjugated microporous polymer anode

Alkaline rechargeable batteries (ARBs) are predicted to be an attractive solution for large-scale electrochemical energy storage applications. However, their advancement is greatly hindered by the lack of high-performance and sustainable anode that can stably operate in less-corroding, low electrolyte concentration. Herein, we report the first example of polymer ARB able to operate in low concentrate electrolyte (1м potassium hydroxide [KOH]) due to the employment of a robust anthraquinone-based conjugated microporous polymer (IEP-11) as anode. The assembled IEP-11||Ni(OH)2 achieves high cell voltage (0.98 V), high gravimetric/areal capacities (150 mAh/g/7.2 mAh/cm2 at 3.5 and 65 mg/cm2, respectively), long cycle life (22,730 cycles, 960 h, 75% capacity retention at 20C), excellent rate performance (75 mAh/g at 50C) and low temperature operativity (75 mAh/g at −10 \ub0C). Furthermore, rate capability, low-temperature performance and ability to prepare high mass loading anodes, along with low self-discharge is improved compared to conventional linear poly (anthraquinone sulfide) (PAQS) in commonly used 10 м KOH. This overall performance for IEP-11||Ni(OH)2 is not only far superior to that of PAQS||Ni(OH)2 owing to porous polymer\u27s high specific surface area, combined micro-/mesoporosity and robust and mechanically stable three-dimensional (3D) architecture compared to the linear PAQS, but also surpass most of the reported organic||nickel [Ni]/cobalt [Co]/manganese [Mn] alkaline rechargeable batteries (ARBs)

Mathematical modelling of a membrane-less redox flow battery based on immiscible electrolytes

We present a mathematical model to study the steady-state performance of a membrane-less reversible redox flow battery formed by two immiscible electrolytes that spontaneously form a liquid-liquid system separated by a well defined interface. The model assumes a two-dimensional battery with two coflowing electrolytes and flat electrodes at the channel walls. In this configuration, the analysis of the far downstream solution indicates that the interface remains stable in all the parameter range covered by this study. To simplify the description of the problem, we use the dilute solution theory to decouple the calculation of the velocity and species concentration fields. Once the velocity field is known, we obtain the distribution of the mobile ionic species along with the current and the electric potential field of the flowing electrolyte solution. The numerical integration of the problem provides the variation of the battery current density Iapp with the State of Charge (SoC) for different applied cell voltages Vcell. A detailed analysis of the concentration density plots indicates that the normal operation of the battery is interrupted when reactant depletion is achieved near the negative electrode both during charge and discharge. The effect of the electrolyte flow on the performance of the system is studied by varying the Reynolds, Re and Péclet, Pe, numbers. As expected, the flow velocity only affects the polarization curve in the concentration polarization region, when is well below the equilibrium potential, resulting in limiting current densities that grow with Re......This work has been partially funded by the Spanish Agencia Estatal de Investigación under projects (PID2019-106740RB- I00 and PID2019-108592RB-C41/AEI/10.13039/50110 0 011033), by Grant IND2019/AMB-17273 of the Comunidad de Madrid and 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). D. Ruiz-Martín acknowledges the support of an FPI predoctoral fellowship (BES-2016-078629) under project ENE2015-68703-C2-1-R (MINECO/FEDER, UE) and the insigh- ful conversations with professor Mark Blyth during her research visit at the University of East Anglia (UK)

Aging Effect of Catechol Redox Polymer Nanoparticles for Hybrid Supercapacitors

[EN] Redox-polymer nanoparticles are a promising solution to avoid the detrimental dissolution of organic electrode materials while showing discrete redox processes. In this work, catechol-based redox-active polymer nanoparticles (cRPNs) were synthesized through one-step emulsion polymerization with a tunable size from 25 to 150 nm. The fresh cRPNs were characterized and showed a reversible redox process centered at 0.50 V (vs. Ag/AgCl) in 1 M H2SO4. Unexpectedly, the cRPN latex aged after days passing from white to pink. This aging resulted in a shift of its redox potential toward higher values, which could be associated to autoxidation of the catechol groups and subsequent crosslinking of NPs due to catechol dimer formation. Finally, we compared the performance of fresh and aged cRPNs in a hybrid supercapacitor device, proving how the aging effect had some benefits such as an increase in the voltage output, specific capacitance, cyclability and Coulombic efficiencies of the device.The authors thank for technical and human support provided by IZO-SGI SGIker of UPV/EHU. Technical and human support provided by IZO-SGI, SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged for assistance and generous allocation of computational resources. The authors would like to thank the European Commission for financial support through funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 823989. N.P. appreciates Spanish MINECO for the Juan de la Cierva-formation fellowship (FJC2018-037781-I). R.M. thanks the Spanish Ministry of Science, Innovation and Universities through the SUSBAT project (Ref.RTI2018-101049-B-I00) (MINECO/FEDER, UE) for financial support

Critical aspects of membrane-free aqueous battery based on two immiscible neutral electrolytes

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.publishe