Quinones for redox flow batteries

Quinones are electroactive species that have shown great promise for redox flow batteries due to the ability to tune their properties and to act as both negative and positive electrolytes. The following review outlines highlights of work in the last couple of years working to provide materials with higher stability, solubility, and performance. Developments toward stable negolytes have provided opportunities for potential commercial opportunities when paired with alternate chemistries. However, the stability of quinones in high potential electrolytes is still not sufficient and the number of potential quinones limited.     

Metal coordination complexes in nonaqueous redox flow batteries

The ongoing search for new electroactive materials for next-generation redox flow batteries has within the last decade encompassed metal–ligand coordination chemistry. Here, we review the handful of metal coordination complexes proposed as redox flow battery electrolytes. We highlight examples with careful ligand design, driving research towards higher energy density storage materials. Metal coordination complexes designed to be highly soluble not only in the initial redox state but also in all charged states accessed during the battery cycle give particularly impressive performances. Optimisation of flow cell conditions for metal coordination complexes remains largely unexplored, with most complexes screened in nonideal H-cell experiments with little investigation into membrane and electrode materials.     

Membranes and separators for redox flow batteries

Ion-exchange membranes are performance- and cost-relevant components of redox flow batteries. Currently used materials are largely ‘borrowed’ from other applications that have different functional requirements. The trend toward higher current densities and the complex transport phenomena of the different species in flow batteries need to be taken into consideration for the design of next-generation membrane/separator materials. In this article, the key requirements and current development trends for membranes and separators for the vanadium redox flow battery are highlighted and discussed.     

Non-aqueous vanadium acetylacetonate electrolyte for redox flow batteries

The electrochemistry of a single-component redox flow battery employing vanadium(III) acetylacetonate in acetonitrile and tetraethylammonium tetrafluoroborate has been investigated. The electrode kinetics of the anodic and cathodic reactions were studied using cyclic voltammetry. The V(II)/V(III) and V(III)/V(IV) couples were quasi-reversible and together yielded a cell potential of 2.2 V. The diffusion coefficient for vanadium acetylacetonate was estimated to be in the range of 1.8–2.9 × 10^?6 cm² s^?1 at room temperature. The charge–discharge characteristics of this system were evaluated in an H-type glass cell, and coulombic efficiencies near 50% were achieved.     

Non-aqueous chromium acetylacetonate electrolyte for redox flow batteries

A single-metal redox flow battery employing chromium(III) acetylacetonate in tetraethylammonium tetrafluoroborate and acetonitrile has been investigated using electrochemical techniques. Cyclic voltammetry was used to evaluate electrode kinetics. Four redox couples were observed in the stable potential window. The Cr^II/Cr^III, Cr^I/Cr^II, Cr^III/Cr^IV and Cr^IV/Cr^V redox couples all appeared to be quasi-reversible, with the Cr^III/Cr^IV couple exhibiting comparatively slow kinetics. A cell potential of 3.4 V was measured for the one-electron disproportionation of the neutral Cr^III complex. The diffusion coefficient for chromium acetylacetonate in the supporting electrolyte solution was estimated to be in the range of 5.0–6.2 × 10^?7 cm² s^?1 at room temperature. The charge–discharge characteristics of this system were evaluated in an H-type glass cell, and coulombic and energy efficiencies of approximately 55% and 20%, respectively, were obtained.     

Chloride supporting electrolytes for all-vanadium redox flow batteries

This paper examines vanadium chloride solutions as electrolytes for an all-vanadium redox flow battery. The chloride solutions were capable of dissolving more than 2.3 M vanadium at varied valence states and remained stable at 0-50 °C. The improved stability appeared due to the formation of a vanadium dinuclear [V(2)O(3)·4H(2)O](4+) or a dinuclear-chloro complex [V(2)O(3)Cl·3H(2)O](3+) in the solutions over a wide temperature range. The all-vanadium redox flow batteries with the chloride electrolytes demonstrated excellent reversibility and fairly high efficiencies. Only negligible, if any, gas evolution was observed. The improved energy capacity and good performance, along with the ease in heat management, would lead to substantial reduction in capital cost and life-cycle cost, making the vanadium chloride redox flow battery a promising candidate for stationary applications.     

Room temperature ionic liquid electrolytes for redox flow batteries

Redox flow batteries (RFBs) usually contain aqueous or organic electrolytes. The aim of this communication is to explore the suitability of room temperature ionic liquids (RTILs) as solvents for RFBs containing metal complexes. Towards this aim, the electrochemistry of the metal acetylacetonate (acac) complexes Mn(acac)₃, Cr(acac)₃, and V(acac)₃ was studied in imidazolium-based RTILs. The V²⁺/V³⁺, V³⁺/V⁴⁺, and V⁴⁺/V⁵⁺ redox couples are quasi-reversible in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C₂C₁Im][N(Tf₂)]. The Mn(acac)₃ and Cr(acac)₃ voltammetry, on the other hand, is irreversible in [C₂C₁Im][N(Tf₂)] at glassy carbon (GC) but the rate of the Mn²⁺/Mn³⁺ reaction increases if Au electrodes are used. Charge–discharge measurements show that a coulombic efficiency of 72% is achievable using a V(acac)₃/[C₂C₁Im][N(Tf₂)]/GC cell.     

Degradation of electrochemical active compounds in aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) represent a promising energy storage technology that may enable the grid-scale integration of intermittent renewable energy. The water-soluble, redox-active organic species that are utilized to reversibly store electricity are the most critical performance-determining components in AORFBs. To ensure affordability and competitiveness in practical installations, it is of vital importance to enhance the structural stability and long-term durability of organic electrolytes, ultimately decreasing their levelized cost. Herein, we summarize the proposed decomposition mechanisms for representative organic electrolytes, including quinones, viologens, nitroxide radicals, and ferrocene derivatives. By reviewing the influence of molecular engineering on the side reactions of electrolytes, we intend to provide a better understanding of the decisive factors and inspire further attempts to design structurally robust and cycling-stable electrolytes for AORFB. Finally, we provide possible directions and prospects for future AORFB research.     

3D printing for aqueous and non-aqueous redox flow batteries

Additive manufacturing technologies, generally grouped under the name of 3D printing, are experiencing an explosion of interest during the last few years. The possibility of fast prototyping enabled by 3D printing has been recognized as a crucial booster for device fabrication and general scientific advancements. In this review, attention is focused on the latest developments in the field of redox flow batteries which are, similar to other energy related devices, characterized by the recent adoption of 3D printing methods for the fabrication of key components. Whether simply to investigate flow phenomena, test new designs or fabricate final-product components with custom features, the use of 3D printing can critically drive this field of research towards better performing energy-storage systems. The latest and most representative examples of redox flow battery studies will be discussed, categorized in relation to the electrolyte used and whether the devices are employed in aqueous or non-aqueous applications.     

Aqueous organic and redox-mediated redox flow batteries: a review

Redox flow batteries (RFBs) are among the most investigated technologies for large-scale energy storage applications. Since the first commercialization of all-vanadium RFB (in the early 90s), the technology has evolved towards the development of new systems. This review focuses on three innovative concepts including aqueous organic RFB (AO-RFB), dual-circuit RFB and redox solid booster–based RFB. We will highlight the recent advances in the last five years and discuss the main challenges encountered. Particularly, we focused on the use of redox-mediated process to reach higher energy density than conventional RFB.     

Ionic liquid-mediated aqueous redox flow batteries for high voltage applications

Aqueous redox flow batteries with high cell voltages represent a promising approach for low-cost, high safety and high energy density applications. However, water breakdown is a major concern because it limits cell voltage. For the first time, we report the use of a highly concentrated aqueous ionic liquid electrolyte, 1-butyl-3-methylimidazolium chloride (BMImCl)-H₂O, in an aqueous flow battery operating with a broad electrochemical stability window of 3 V. The proof-of-concept was demonstrated using 2 V redox couples of metal acetylacetonates and a hybrid Zn/Ce flow battery.     

A review of electrolyte additives and impurities in vanadium redox flow batteries

As one of the most important components of the vanadium redox flow battery(VRFB), the electrolyte can impose a significant impact on cell properties, performance and capital cost. In particular, the electrolyte composition will influence energy density, operating temperature range and the practical applications of the VRFB. Various approaches to increase the energy density and operating temperature range have been proposed. The presence of electrolyte impurities, or the addition of a small amount of other chemical species into the vanadium solution can alter the stability of the electrolyte and influence cell performance, operating temperature range, energy density, electrochemical kinetics and cost effectiveness. This review provides a detailed overview of research on electrolyte additives including stabilizing agents, immobilizing agents, kinetic enhancers, as well as electrolyte impurities and chemical reductants that can be used for different purposes in the VRFBs.     

Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries

Fabricating of high performance electrodes by a sustainable and cost effective method is essential to the development of vanadium redox flow batteries(VRFBs).In this work,an effective strategy is proposed to deposit carbon nanoparticles on graphite felts by hydrothermal carbonization method.This in-situ method minimizes the drop off and aggregation of carbon nanoparticles during electrochemical testing.Such integration of felts and hydrothermal carbons(HTC)produces a new electrode that combines the outstanding electrical conductivity of felts with the effective redox active sites provided by the HTC coating layer.The presence of the amorphous carbon layers on the felts is found to be able to promote the mass/charge transfer,and create oxygenated/nitrogenated active sites and hence enhances wettability.Consequently,the most optimized electrode based on a rational approach delivers an impressive electrochemical performance toward VRFBs in wide range of current densities from 200 to 500 mAcm^-2.The voltage efficiency(VE)of GFs-HTC is much higher than the VEs of the pristine GFs,especially at high current densities.It exhibits a 4.18 times increase in discharge capacity over the pristine graphite felt respectively,at a high current density of 400 mAcm^-2.The enhanced performance is attributed to the abundant active sites from amorphous hydrothermal carbon,which facilitates the fast electrochemical kinetics of vanadium redox reactions.This work evidences that the glucose-derived hydrothermal carbons as energy storage booster hold great promise in practical VRFBs application.     

Recent progress in organic redox flow batteries:Active materials,electrolytes and membranes

Redox flow batteries(RFBs) have great potentials in the future applications of both large scale energy storage and powering the electrical vehicle. Critical challenges including low volumetric energy density,high cost and maintenance greatly impede the wide application of conventional RFBs based on inorganic materials. Redox-active organic molecules have shown promising prospect in the application of RFBs,benefited from their low cost, vast abundance, and high tunability of both potential and solubility. In this review, we discuss the advantages of redox active organic materials over their inorganic compart and the recent progress of organic based aqueous and non-aqueous RFBs. Design considerations in active materials, choice of electrolytes and membrane selection in both aqueous and non-aqueous RFBs are discussed.Finally, we discuss remaining critical challenges and suggest future directions for improving organic based RFBs.     

Engineering porous electrodes for next-generation redox flow batteries: recent progress and opportunities

Redox flow batteries are a promising electrochemical technology for energy-intensive grid storage applications, but further cost reductions are needed for universal adoption. As porous electrodes are responsible for functions within the flow cell that impact charge transfer, ohmics, and mass transport, improvements in electrode materials and design may yield significant performance and economic benefits. This mini-review summarizes recent developments in the design and characterization of porous electrodes with a focus on understanding and controlling both the microstructure and surface chemistry, which broadly align with mass transport and reaction kinetics. Key opportunities and challenges in the science and engineering of these materials are also presented with the goal of engaging the broader community and accelerating progress towards chemistry-specific flow battery electrodes.     

Novel catalytic effects of Mn3O4 for all vanadium redox flow batteries

A new approach for enhancing the electrochemical performance of carbon felt electrodes by employing non-precious metal oxides is designed. The outstanding electro-catalytic activity and mechanical stability of Mn(3)O(4) are advantageous in facilitating the redox reaction of vanadium ions, leading to efficient operation of a vanadium redox flow battery.     

Cycling performance and efficiency of sulfonated poly(sulfone) membranes in vanadium redox flow batteries

As an alternative to Nafion® ion exchange membrane, an inexpensive commercially-available Radel® polymer was sulfonated, fabricated into a thin membrane, and evaluated for its performance in a vanadium redox flow battery (VRFB). The sulfonated Radel (S-Radel) membrane showed almost an order of magnitude lower permeability of VO²⁺ ions (2.07 × 10^?7 cm²/min), compared to Nafion 117 (1.29 × 10^?6 cm²/min), resulting in better coulombic efficiency (~ 98% vs. 95% at 50 mA/cm²) and lower capacity loss per cycle. Even though the S-Radel membrane had a slightly higher membrane resistance, the energy efficiency of the VRFB with the S-Radel membrane was comparable to that of Nafion because of its better coulombic efficiency resulting from the lower vanadium ion crossover. The S-Radel membrane exhibited good performance up to 40 cycles, but a decline in performance at later cycles was observed, likely as a result of membrane degradation.     

Modified carbon cloth as positive electrode with high electrochemical performance for vanadium redox flow batteries

Carbon cloth modified by hydrothermal treatment in ammonia water is developed as the positive electrode with high electrochemical performance for vanadium redox flow batteries.The SEM shows that the treatment has no obvious influence on the morphology of carbon cloth.XPS measurements indicate that the nitrogenous functional groups can be introduced on the surface of carbon cloth successfully.The electrochemical performance of V（IV）/V（V） redox couple on the prepared electrode is evaluated with cyclic voltammetry and linear sweep voltammetry measurements.The N-doped carbon cloth exhibits outstanding electrochemical activity and reversibility toward V（IV）/V（V） redox couple.The rate constant of V（IV）/V（V） redox reaction on carbon cloth can increase to 2.27 × 10^-4cm/s from 1.47 × 10^-4cm/s after nitrogen doping.The cell using N-doped carbon cloth as positive electrode has larger discharge capacity and higher energy efficiency compared with the cell using pristine carbon cloth.The average energy efficiency of the cell using N-doped carbon cloth for 50 cycles at 30 m A/cm² is 87.8%,4.3% larger than that of the cell using pristine carbon cloth.It indicates that the N-doped carbon cloth has a promise application prospect in vanadium redox flow batteries.     

How anthraquinones can enable aqueous organic redox flow batteries to meet the needs of industrialization

Owing to the importance of storage and its hybridization with renewable energy technologies for the energy transition, a high attention has been paid towards the development of redox flow batteries. Among all different emerging technologies, aqueous organic redox flow batteries (AORFBs) are particularly attractive since the objectives in terms of sustainability, cost and safety issues can be achieved owing to the high possibilities offered by molecular engineering, organometallic and coordination chemistry. Thus, AORFBs based on anthraquinones paired with ferrocyanide in basic medium have been widely developed and are close to reach the performances required for industrial processes. This review aims to focus on the main parameters making possible the integration of anthraquinone derivatives as negolyte in AORFB with a special attention for their implementation in industrial process.     

High electro-catalytic graphite felt/Mn O2composite electrodes for vanadium redox flow batteries

A mild and simple synthesis process for large-scale vanadium redox flow batteries(VRFBs)energy storage systems is desirable.A graphite felt/Mn O_2(GF-MNO)composite electrode with excellent electrocatalytic activity towards VO~(2+)/VO_2~+redox couples in a VRFB was synthesized by a one-step hydrothermal process.The resulting GF-MNO electrodes possess improved electrochemical kinetic reversibility of the vanadium redox reactions compared to pristine GF electrodes,and the corresponding energy efficiency and discharge capacity at 150 m A cm~(-2)are increased by 12.5%and 40%,respectively.The discharge capacity is maintained at 4.8 A h L~(-1)at the ultrahigh current density of 250 m A cm~(-2).Above all,80%of the energy efficiency of the GF-MNO composite electrodes is retained after 120 charge-discharge cycles at 150 m A cm~(-2).Furthermore,these electrodes demonstrated that more evenly distributed catalytic active sites were obtained from the Mn O_2particles under acidic conditions.The proposed synthetic route is facile,and the raw materials are low cost and environmentally friendly.Therefore,these novel GF-MNO electrodes hold great promise in large-scale vanadium redox flow battery energy storage systems.