Low-cost all-iron flow battery with high performance towards long-duration energy storage

Long duration energy storage(LDES)technologies are vital for wide utilization of renewable energy sources and increasing the penetration of these technologies within energy infrastructures.Herein,we propose a low-cost alkaline all-iron flow battery by coupling ferri/ferro-cyanide redox couple with ferric/ferrous-gluconate complexes redox couple.The designed all-iron flow battery demonstrates a coulombic efficiency of above 99%and an energy efficiency of~83%at a current density of80 m A cm^(-2),which can continuously run for more than 950 cycles.Most importantly,the battery demonstrates a coulombic efficiency of more than 99.0%and an energy efficiency of~83%for a long duration(~12,16 and 20 h per cycle)charge/discharge process.Benefiting from the low cost of iron electrolytes,the overall cost of the all-iron flow battery system can be reached as low as$76.11 per k Wh based on a10 h system with a power of 9.9 k W.This work provides a new option for next-generation cost-effective flow batteries for long duration large scale energy storage.

Opportunities and challenges of organic flow battery for electrochemical energy storage technology

For flow batteries(FBs), the current technologies are still expensive and have relatively low energy density, which limits their large-scale applications. Organic FBs(OFBs) which employ organic molecules as redox-active materials have been considered as one of the promising technologies for achieving lowcost and high-performance. Herein, we present a critical overview of the progress on the OFBs, including the design principles of key components(redox-active molecules, membranes, and electrodes) and the latest achievement in both aqueous and nonaqueous systems. Finally, future directions in explorations of the high-performance OFB for electrochemical energy storage are also highlighted.

KHCO_3 activated carbon microsphere as excellent electrocatalyst for VO^(2+)/VO_2^+ redox couple for vanadium redox flow battery

In this paper,carbon microsphere prepared by hydrothermal treatment was activated by KHCO_3 at high temperature,and employed as the catalyst for VO^(2+)/VO_2^+redox reaction for vanadium redox flow battery(VRFB).Carbon microsphere can be etched by KHCO_3 due to the reaction between the pyrolysis products of KHCO_3 and carbon atoms.Moreover,KHCO_3 activation can bring many oxygen functional groups on carbon microsphere,further improving the wettability of catalyst and increasing the active sites.The electrocatalytic properties of carbon microsphere from hydrothermal treatment are improved by high temperature carbonization,and can further be enhanced by KHCO_3 activation.Among carbon microsphere samples,the VO^(2+)/VO_2^+redox reaction exhibits the highest electrochemical kinetics on KHCO_3 activated sample.The cell using KHCO_3 activated carbon microsphere as the positive catalyst demonstrates higher energy efficiency and larger discharge capacity,especially at high current density.The results reveal that KHCO_3 activated carbon microsphere is an efficient,low-cost carbon-based catalyst for VO^(2+)/VO_2^+redox reaction for VRFB system.

A highly concentrated vanadium protic ionic liquid electrolyte for the vanadium redox flow battery

A protic ionic liquid is designed and implemented for the first time as a solvent for a high energy density vanadium redox flow battery.Despite being less conductive than standa rd aqueous electrolytes,it is thermally stable on a 100 ℃ temperature window,chemically stable for at least 60 days,equally viscous and dense with typical aqueous solvents and most importantly able to solubilize to 6 mol L^(-1) vanadium sulfate,thus increasing the VRFB energy density by a factor of 2.5.Electrochemical measurements revealed quasi-reversible redox transitions for both catholyte and anolyte at 25 ℃ while a proof-of-concept redox flow cell with the proposed electrolyte was tested for a total of 150 cycles at 25 ℃,showing an open circuit potential of 1.39 V and energy and coulombic efficiencies of 65% and 93%,respectively.What’s more,the battery can be equally cycled at 45℃ showing good thermal stability.This study underlines a new route to improve the energy-to-volume ratio of energy storage system.

High performance of zinc-ferrum redox flow battery with Ac^-/HAc buffer solution

A green low-cost redox flow battery using Zn/Znredox couple in HAc/NaAc medium and Fe/Feredox couple in HSOmedium was first proposed and investigated for potential stationary energy storage applications. The presence of HAc/NaAc in the negative electrolyte can keep the pH between 2.0 and 6.0even when a large amount of Hions move into negative electrolyte from positive electrolyte through ion exchange membrane. In the pH range of 2.0–6.0, the chemical reaction of Zn species with Hspecies is very insignificant; furthermore, the electroreduction of Hion on the negative electrode is significantly suppressed at this pH range. The zinc-ferrum redox flow battery（Zn/Fe RFB） operated within a voltage window of 0.5–2.0 V with a nearly 90% utilization ratio, and its energy efficiency is around 71.1% at room temperature. These results show that Zn/Fe RFB is a promising option as a stationary energy storage equipment.

A bipolar verdazyl radical for a symmetric all-organic redox flow-type battery

A symmetric all-organic non-aqueous redox flow-type battery was investigated employing the neutral small molecule radical 3-phenyl-1,5-di-p-tolylverdazyl,which can be reversibly oxidized and reduced in one-electron processes,as the sole charge storage material.Cyclic voltammetry of the verdazyl radical in 0.5 M tetrabutylammonium hexa fluoro phosphate(TBAPF6)in acetonitrile revealed redox couples at-0.17 V and-1.15 V vs.Ag+/Ag,leading to a theoretical cell voltage of 0.98 V.From the dependence of peak currents on the square root of the scan rate,diffusion coefficients on the order of 4 x 10 6 cm2 s-1 were demonstrated.Cycling performance was assessed in a static cell employing a Tokoyuma AHA anion exchange membrane,with 0.04 M verdazyl as catholyte and anolyte in 0.5 M TBAPF6 in acetonitrile at a current density of 0.12 mA cm-2.Although coulombic efficiencies were good(94%-97%)throughout the experiment,the capacity faded gradually from high initial values of 93%of the theoretical discharge capacity to 35%by the 50th cycle.Voltage and energy efficiencies were 68%and 65%,respectively.Postcycling analysis by cyclic voltammetry revealed that decomposition of the active material with cycling is a leading cause of cell degradation.

Performance of redox flow battery systems in Japan

Renewable energies, such as solar and wind power, are increasingly being introduced as alternative energy sources on a glosbal scale toward a low-carbon society. For the next generation power network, which uses a large number of these distributed power generation sources, energy storage technologies will be indispensable. Among these technologies, battery energy storage technology is considered to be most viable. Sumitomo Electric Industries, Ltd. has developed a redox flow battery system suitable for large scale energy storage, and carried out several demonstration projects on the stabilization of renewable energy output using the redox flow battery system. This paper describes the advantages of the redox flow battery and reviews the demonstration projects.

Wind Power Flow Optimization and Control System Based on Rapid Energy Storage

风电功率的间歇与波动致使电场容量可信度低、可调度性差；同时易引起局部电网的电压不稳、频率波动，影响了系统的电能质量及稳定性。针对此现象，将超级电容器与蓄电池组成快速储能装置，用于风电的潮流优化控制。采用三重双向直流变换电路控制储能元件间的功率流动；采用四象限交直流变换电路控制储能与电网间的能量交换。提出基于超级电容器电压低频波动抑制的功率分配方法，可显著减少蓄电池的充放次数；提出基于储能元件荷电状态的储能能量调整规则，可避免储能元件的过充和频繁深度放电，以优化其功率调节能力。实验结果表明，系统可实现2种储能元件的优势互补，能有效平滑调节风电注入电网的有功功率，并实时补偿控制风电接入点的无功功率。

Towards an all-vanadium redox flow battery with higher theoretical volumetric capacities by utilizing the VO^2＋/V^3＋ couple

An all-vanadium redox flow battery with V（IV） as the sole parent active species is developed by accessing the VO2＋/V3＋ redox couple. These batteries, referred to as V4RBs, possess a higher theoretical volumetric capacity than traditional VRBs. Copper ions were identified as an effective additive to boost the battery performance.

Porous polybenzimidazole membranes with positive charges enable an excellent anti-fouling ability for vanadium-methylene blue flow battery

A cost-effective, high-performance and highly stable membrane has always been in intensively needed in aqueous organic-based flow batteries. Here we present a porous polybenzimidazole(PBI) membrane with positive charges that endow the membrane with a high rejection and an excellent anti-fouling ability for target organic molecule and asymmetric structure that affords a high conductivity for vanadiummethylene blue flow battery(V-MB FB). The morphologies and thickness of separating layer in particular of the porous PBI can be well adjusted by simply altering the polymer concentration in the cast solution and further afford the membrane with a controllable property in terms of both ion selectivity and ion conductivity. As a result, a V-MB FB assembled with a porous PBI membrane delivers a coulombic efficiency(CE) of 99.45% and an energy efficiency(EE) of 86.10% at a current density of 40 mA cm^(-2), which is 12% higher than that afforded by a Nafion 212 membrane. Most importantly, the V-MB FB demonstrates a methylene blue(MB) utilization of 97.55% at a theoretical capacity of 32.16 Ah L^(-1)(based on the concentration of MB in the electrolyte) because of the high ion conductivity of the membrane, which favors reducing the cost of a battery. The results suggest that the designed porous PBI membranes exhibit a very promising prospect for methylene blue-vanadium flow battery.

Systematic approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries

Polyoxometalates have been explored as multi-electron active species in both aqueous and non-aqueous redox flow batteries. Although non-aqueous systems in principle offer a wider voltage window for redox flow battery operation, realization of this potential requires a judicious choice of solvent as well as polyoxometalate properties. We demonstrate here the superior performance of N,N-dimethylformamide(DMF)compared to acetonitrile as a solvent for redox flow batteries based on Li3PMo12O40. This compound displays two 1-electron transfers in acetonitrile but can access an extra quasi-reversible 2-electron redox process in DMF. A cell containing 10 mM solution of Li3PMo12O40 in DMF produced a cell voltage of 0.7 V with 2-electron transfers(State of Charge = 60%) and showed a good cyclability. As a means to boost energy density, operation of the redox flow battery at a higher concentration of 0.1 M Li3PMo12O40 produced cells with cell voltage of 0.6 V in acetonitrile and a cell voltage of 1.0 V in DMF;both showed excellent coulombic efficiencies of more than 90% over the course of 30 cycles. Energy density was also increased by employing an asymmetric cell with different polyoxometalates on each side to extend cell voltage.Li6P2W18O62 exhibited 3 quasi-reversible 2-electron transfers in the potential range between-2.05 V and-0.5 V vs. Ag/Ag+. 10 mM Li6P2W18O62/Li3PMo12O40 in DMF produced a cell with cell voltage of 1.3 V involving 4-electron transfers(State of Charge = 50%) with coulombic efficiency of nearly 100% and energy efficiency of nearly 70% throughout the test with more than 20 cycles. These promising results demonstrate proof-of-concept approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries.

Two electron utilization of methyl viologen anolyte in nonaqueous organic redox flow battery

Methyl viologen （MV） as a bench-mark anolyte material has been frequently applied in aqueous organic redox flow batteries （AORFBs） towards large-scale renewable energy storage. However, only the first re- duction of MV was utilized in aqueous electrolytes because of the insoluble MV0generated from the second reduction of MV. Herein, we report that methyl viologen with bis（trifluoromethane）sulfonamide counter anion, MVTFSI, can achieve two reversible reductions in a nonaqueous supporting elec- trolyte. Paired with （Ferrocenylmethyl）trimethylammonium bis（trifluoromethanesulfonyl）imide, FcNFFSI, as catholyte, the MVTFS/FcNTFSI nonaqueous organic redox flow battery （NOARFB） can take advantage of either one electron or two electron storage of the methyl viologen moiety and provide theoretical energy density of 24.9Wh/L and a cell voltage of up to 1.5V. Using a highly conductive LiTFSI/CH_3CN supporting electrolyte and a porous Daramic separator, the NOARFB displayed excellent cycling performance, includ- ing up to a 68.3g energy efficiency at 40 mA/cm2, and more than 88g total capacity retention after 100 cycles.

Novel copper redox-based cathode materials for room-temperature sodium-ion batteries

Layered oxides of P2-type Nao.68Cuo.34Mno.6602, P2-type Nao.68Cuo.34Mno.50Tio.1602, and O＇3-type NaCuo.67Sbo.3302 were synthesized and evaluated as cathode materials for room-temperature sodium-ion batteries. The first two materials can deliver a capacity of around 70 mAh/g. The Cu2＋ is oxidized to Cu3＋ during charging, and the Cu3＋ goes back to Cu2＋ upon discharging. This is the first demonstration of the highly reversible change of the redox couple of Cu2＋/Cu3＋ with high storage potential in secondary batteries.

Eu-based anolytes for high-voltage and long-lifetime aqueous flow batteries

Aqueous flow batteries(AFBs) are among the most promising electrochemical energy storage solutions for the massive-scale adoption of renewable electricity because of decoupled energy and power, design flexibility, improved safety and low cost. The development of high-voltage AFB is, however, limited by the lack of stable anolytes that have low redox potential. Here we report Eu-based anolytes for high-voltage p H-neutral AFB applications. Eu^(3+) has a reduction potential of -0.39 V vs. SHE, which can be dramatically lowered when forming stable complex with inexpensive organic chelates. A typical complex, Eu DTPA,features a low redox potential of -1.09 V vs. SHE, fast redox kinetics, and a high water solubility of 1.5 M. When paired with ferrocyanide, the battery had an open-circuit voltage of 1.56 V and demonstrated stable cell cycling performance, including a capacity retention rate of 99.997% per cycle over500 cycles at 40 m A cm^(-2), a current efficiency of >99.9%, and an energy efficiency of >83.3%. A high concentration anolyte at 1.5 M exhibited a volumetric capacity of 40.2 Ah L^(-1), which is one of the highest known for p H-neutral AFBs, promising a potent solution for the grid-scale storage of renewable electricity.

A highly stable membrane with hierarchical structure for wide pH range flow batteries

A membrane with high stability and ion conductivity in wide pH range is essential for energy storage devices.Here,we report a novel membrane with hierarchical core-shell structure,which demonstrates high stability and ion conductivity,simultaneously under a wide pH range applications.Spectral characterizations and theoretical calculation indicate that the non-solvent induces the chain segment configuration and eventually leads to polymer-polymer phase separation,thus forming hierarchical porous core-shell structure.Benefiting from this structure,an acidic vanadium flow battery(VFB)with such a membrane shows excellent performance over 400 cycles with an energy efficiency(EE)of above 81%at current density of 120 mA cm^(-2) and an alkaline zinc-iron flow battery(AZIFB)delivers a cycling stability for more than 200 cycles at 160 mA cm^(-2),along with an EE of above 82%.This paper provides a cost-effective and simple way to fabricate membranes with high performance for variety of energyrelated devices.

A Hydrogen Iron Flow Battery with High Current Density and Long Cyclability Enabled Through Circular Water Management

The hydrogen-iron(HyFe)flow cell has great potential for long-duration energy storage by capitalizing on the advantages of both electrolyzers and flow batteries.However,its operation at high current density(high power)and over continuous cycling testing has yet to be demonstrated.In this article,we discuss our design and demonstration of a water-management strategy that supports high current and long-cycling performance of a HyFe flow cell.Water molecules associated with the movement of protons from the iron electrode to the hydrogen electrode are sufficient to hydrate the membrane and electrode at a low current density of 100 mA cm^(-2)during the charge process.At higher charge current density,more aggressive measures must be taken to counter back-diffusion driven by the acid concentration gradient between the iron and hydrogen electrodes.Our water-management approach is based on water vapor feeding in the hydrogen electrode and water evaporation in the iron electrode,thus enabling high current density operation of 300 mA cm^(-2).

Theoretical insights on the hydration of quinones as catholytes in aqueous redox flow batteries

Quinones have been widely studied as a potential catholyte in water-based redox flow batteries(RFBs)due to their ability to carry both electrons and protons in aqueous solutions.The wide variety of quinones and derivatives offers exciting opportunities to optimize the device performance while poses theoretical challenges to quantify their electrochemical behavior as required for molecular design.Computational screening of target quinones with high performance is far from satisfactory.While solvation of quinones affects their potential application in RFBs in terms of both electrochemical windows,stability,and charge transport,experimental data for the solvation structure and solvation free energies are rarely available if not incomplete.Besides,conventional thermodynamic models are mostly unreliable to estimate the properties of direct interest for electrochemical applications.Here,we analyze the hydration free energies of more than 1,400 quinones by combining the first-principles calculations and the classical density functional theory.In order to attain chemical insights and possible trends,special attention is placed on the effects of"backbones"and functional groups on the solvation behavior.The theoretical results provide a thermodynamic basis for the design,synthesis,and screening of high-performance catholytes for electrical energy storage.

Soft Template-Induced Porous Polyvinylidene Fluoride Membrane for Vanadium Flow Batteries

Vanadium flow batteries(VFBs)are considered ideal for grid-sc ale,long-duration energy storage applications owing to their decoupled output power and storage capacity,high safety,efficiency,and long cycle life.However,the widespread adoption of VFB s is hindered by the use of expensive Nafion membranes.Herein,we report a soft template-induced method to develop a porous polyvinylidene fluoride(PVDF)membrane for VFB applications.By incorporating water-soluble and flexible polyethylene glycol(PEG 400)as a soft template,we induced the aggregation of hydrophilic sulfonated poly(ether ether ketone),resulting in phase separation from the hydrophobic PVDF polymer during membrane formation.This process led to the creation of a porous PVDF membrane with controllable morphologies determined by the polyethylene glycol content in the cast solution.The optimized porous PVDF membrane enabled a stable VFB performance for 200 cycles at a current density of 80 mA/cm^(2),and the VFB exhibited a Coulombic efficiency of 95.2%and a voltage efficiency of 87.8%.These findings provide valuable insights for the development of highly stable membranes for VFB applications.

System Energy and Efficiency Analysis of 12.5 W VRFB with Different Flow Rates

Vanadium redox flow battery(VRFB)is considered one of the most potential large-scale energy storage technolo-gies in the future,and its electrolyte flow rate is an important factor affecting the performance of VRFB.To study the effect of electrolyte flow rate on the performance of VRFB,the hydrodynamic model is established and a VRFB system is developed.The results show that under constant current density,with the increase of electrolyte flow rate,not only the coulombic efficiency,energy efficiency,and voltage efficiency will increase,but also the capacity and energy discharged by VRFB will also increase.But on the other hand,as the flow rate increases,the power of the pump also increases,resulting in a decrease in system efficiency.The energy discharged by the system does not increase with the increase in flow rate.Considering the balance between efficiency and pump power loss,it is experimentally proved that 120 mL·min-1 is the optimal working flow rate of the VRFB system,which can maximize the battery performance and discharge more energy.

Catalytically altering the redox pathway of sulfur in propylene carbonate electrolyte using dual-nitrogen/oxygen-containing carbon

Carbonate electrolytes are one of the most desirable electrolytes for high-energy lithium-sulfur batteries(LSBs)because of their successful implementation in commercial Li-ion batteries.The low-polysulfide-solubility feature of some carbonate solvents also makes them very promising for overcoming the shuttle effects of LSBs.However,regular sulfur electrodes experience undesired electrochemical mechanisms in carbonate electrolytes due to side reactions.In this study,we report a catalytic redox mechanism of sulfur in propylene carbonate(PC)electrolyte based on a compari-son study.The catalytic mechanism is characterized by the interactions between polysulfides and dual N/O functional groups on the host carbon,which largely prevents side reactions between polysulfides and the carbonate electrolyte.Such a mechanism coupled with the low-polysulfide-solubility feature leads to stable cycling of LSBs in PC electrolyte.Favorable dual N/O functional groups are identified via a density functional theory study.This work provides an alternative route for enabling LSBs in carbonate electrolytes.