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State-of-art of Flow Batteries: A Brief overview

Energy storage technologies may be based on electrochemical, electromagnetic, thermodynamic, and mechanical systems [1]. Energy production and distribution in the electrochemical energy storage technologies, Flow batteries, commonly known as Redox Flow Batteries (RFBs) are major contenders.

Components of RFBs RFB is the battery system in which all the electroactive materials are dissolved in a liquid electrolyte. A typical RFB consists of energy storage tanks, stack of electrochemical cells and flow system. Liquid electrolytes are stored in the external tanks as catholyte, positive electrolyte, and anolyte as negative electrolytes [2]. The membrane between two stacks provides the path for ions movement. The electrolytes pump into the stack for electrochemical reaction and circulate back to their respective tanks.

Carbon papers or carbon felts are used to construct porous carbon electrodes to provide the path for decent electronic conduction during the operation of the electrochemical cell. A porous membrane generally made of Nafion is placed between two electrodes to ease the exchange of ions. It prevents the flow of electrolytes and electrons but

2030 sometimes crossover of electrolytes occurs leading to degradation of the capacity of the cell. The arrangement of electrodes and the porous membrane is sandwiched between two graphite plates to build a cell. Several cells are stacked in series combinations to scale up the voltage. This assembly is held together by using metal end plates and tie rods to form a flow battery stack which is then connected with electrolyte tanks, pumps, and electronics to form an operational flow battery system [3].

Flow BatteryTechnologies RFBs have been investigated and produced during the past few decades using various chemistries. Among them the commercialized deployment of all vanadium RFB began in the 1980s.

Various flow battery systems have been investigated based on different chemistries. Based on the electro-active materials used in the system, the more successful pair of electrodes are liquid/gas-metal and liquid-liquid electrode systems. The commercialized flow battery system Zn/Br falls under the liquid/gas-metal electrode pair category whereas All-Vanadium Redox Flow Battery (VRFB) contains liquid-liquid electrodes. Some other systems are under development like the Zn/V system. Similarly, there are some technologies investigated in the laboratory prototype stage like V-Br.

High reactivity and large stable voltage windows are the fundamental prerequisites of the RFBs.To achieve these requirements, the redox couple which has a large difference in voltage should be picked. After choosing the proper chemistries, we need to find the other components and stacked them together. Then the system needs to integrate with power electronics and control.

All-Vanadium Redox Flow Battery(VRFBs)

In this flow battery system Vanadium electrolytes, 1.6-1.7 M vanadium sulfate dissolved in 2M Sulfuric acid, are used as both catholyte and anolyte. Among the four available oxidation states of Vanadium, V2+/V3+ pair acts as a negative electrode whereas V5+/V4+ pair serves as a positive electrode. During discharge, penta-valent Vanadium is reduced to yield tetra-valent Vanadium and water at the positive electrode generating +1.00 V with respect to Standard Hydrogen Electrode (SHE). Similarly, bivalent Vanadium is oxidized to form tri-valent Vanadium at a negative electrode with -0.26 V vs SHE. The overall redox reaction is as follows:

Advantages: · Absence of membrane cross-over risk. · Stable battery system. · Nocatalyst required for redox reaction. Disadvantages: · Low energy and power density. · Fluctuation in the price of electrolytes.

Zinc Bromine Flow Battery (ZBFB)

In this flow battery system 1-1.7 M Zinc Bromide aqueous solutions are used as both catholyte and anolyte. Bromine dissolved in solution serves as a positive electrode whereas solid zinc deposited on a carbon electrode serves as a negative electrode. Hence ZBFB is also referred to as a hybrid flow battery. The redox reaction and voltage generated with respect to SHE is given below:

Advantages · Low-cost electrolyte. · Obtained high energy density. · Generation of high voltage. Disadvantages: · Poor lifetime of the battery system. · Safety concern due to zinc dendrites. · Takes time while recharging. · Excess Br2 evolution causes a fall in the capacity of the battery. Iron – Chromium Flow Battery (Fe-CrFB)

In this flow battery system, 1 M Chromium Chloride aqueous solution is used as an anolyte and Ferrous Chloride in 2M Hydrochloric acid serves as a catholyte.

The redox reaction and voltage generated with respect to SHE is given below:

Advantages: · Low-cost flow battery system. Disadvantages: · Low energy density · Slow exchange of Chromium ions · Evolution of hydrogen at the anode · High chance of crossover.

Aqueous OrganicRedox Flow Batteries (AORFBs)

The structural components of AORFBs and VRFBs are the same, with the only difference being the kind of electrolytes. The redox active materials in this flow battery system include organic molecules consisting of the elements C, H, O, N, and S, which are common on Earth. The organic electro-active solutions that have thus far been studied include quinones, quinoxalines, bipyridines, and nitroxyl radicals [5].

Advantages: · Wider Cell Voltage. · Fast electrodereactions · Minimal cross-over · Higher electrochemical Disadvantages: · Degradation of organic electrolytes · Frequent electrolyte replacement

Metal Air Flow Batteries (MAFBs)

In this flow battery system, the cathode is air (Oxygen), the anode is a metal, and the separator is immersed in a liquid electrolyte. In both aqueous and non-aqueous media, zinc, aluminum, and lithium metals have so far been investigated. Slurry or solution flow forms and single or double circulation cell designs have resulted in the development of several

Advantages: · High power density · Metal slurry checks the dendrite formation

Disadvantages: · Clog problem · Use of metal particles inefficiently

Semi-Solid Flow Batteries (SSFBs)

Two current collectors, a separator, and gaskets are the typical components of a cell reactor for the SSFBs system. Instead of using an electrolyte comprising soluble electro­active components, a semi-solid electrolyte is used. An external reservoir houses the solid-containing slurry, which is then pumped into the electrochemical cell. Working examples of SSFB prototypes that used flow-able suspensions with up to a 12 M concentration have been displayed [5].

Advantages: · High energy density

Disadvantages: · Solid electrolyte interface (SEI) formation · New system architecture is required · Low currentdensity

Solar RedoxFlow batteries (SRFBs)

Photoelectron converting electrodes are incorporated into redox flow batteries in the SRFBs. Research and development of SRFBs are still in their infancy. There have been some prototype SRFBs with organic compound, metal-organic, and vanadium-metal as the electro-active materials. The picture below Fig (10) depicts the two distinct architectures that have been suggested for this system:

Advantages: · Cost effective · Reduction in operational over potentials Disadvantages: · Low capacity · Insufficient photo-voltage

Solid Mediated Flow batteries (SMFBs)

The flow battery systems incorporate redox mediators as charge carriers between the electrochemical reactor and external reservoirs. With the addition of solid active materials in the external tanks, SMFBs have been successfully shown to be compatible with a traditional RFB. The redox potential of the redox mediator and the solid active materials must be well-matched to gain higher energy density [5].

Advantages: · Higher energy density · Low energy cost Disadvantages: · Low voltage · Mechanical degradation

Li-Ion Batteries (LIBs) vs Redox Flow Batteries (RFBs)

Li-Ion Batteries (LIBs) and Redox Flow Batteries (RFBs) are popular battery system in electrical energy storage technology. Currently, LIBs have dominated the energy storage market being power sources for portable electronic devices, electric vehicles and even for small capacity grid systems (8.8 GWh) [5]. Due to high maintenance cost, safety limitations of LIBs, RFBs are considered as effective alternatives for the energy storage system.

In the conventional LIBs, the electrodes present inside the cell serve as the electro-active materials as well as the structural unit of the battery. The power density and energy density both depend upon the nature and characteristics of electrode materials. On the other hand, electro-active materials are stored externally in a flow battery and the electrodes as the structural units act as passive source/sink of electrons. The power density of RFBs depends upon the size of the external storage tanks and energy density is determined by the mass of the electro-active materials present in the tank. Flexible modular design and operation, high stability, average maintenance cost and long-life cyclability are some special features of RFBs which make them promising candidates in the sustainable energy generation and storage system [5].


Due to the world's growing population, expanding economy, and technological advancements, there is now a massive increase in the demand for energy. Global energy consumption has reached approximately 580 million terajoules per year, according to The World Count [6]. A 77 percent rise in global energy use is predicted between 2000 and 2040.

About 83 percent of this energy demand is being fulfilled by fossil fuels.The excess burning of oil and gases is causing detrimental effects on nature and environment contributing to global warming. So, renewable energy innovation is in demand in the present scenario of global energy generation mix. In 2018,energy supply from renewable sources increased by 14.5%. In general, the use of solar energy and wind energy to generate electricity has been increasing per year. By 2050, it is projected that solar energy and wind energy will jointly contribute 56 % of electricity generation worldwide and 69 % in combined by all renewable energy sources [7].

The intermittent, inflexible, and wasteful nature of renewable energy sources like sun and wind mean that the electricity generated needs to be stored with the proper equipment to maintain balance and smooth energy distribution in accordance with demand. Energy storage systems that are dependable, affordable, and scalable are crucial for accelerating the switch from fossil fuels to sustaining renewable energy sources.Redox-flow batteries are excellent candidates for cost-effective stationary storage, especially in the case of long discharges and extended storage times, due to their unique capacity to decouple power and energy. The integration of renewable energy sources and the resulting requirement for energy storage are encouraging work on the development of the redox-flow batteries technology.


[1] Ahmed Zayed AL Shaqsi a , Kamaruzzaman Sopian , Amer Al-Hinai; Review of energy storage services, applications, limitations, and benefits;

[2] Grigorii L. Soloveichik , Flow Batteries: Current Status and Trends, GE Global Research, 1 Research Circle, Niskayuna, New York 12309, United States.

[3] Rhodri Jervis, Leon D Brown, Tobias P Neville, JasonMillichamp, DonalP Finegan, Thomas M M Heenan, Dan J L Brett, and Paul R Shearing; Design of a miniature flow cell for in situ x-ray imaging of redox flow batteries; J. Phys. D: Appl. Phys. 49 (2016) 434002 (9pp).

[4] Babu R. Chalamala, Thiagarajan Soundappan, GrahamR. Fisher, MitchellR. Anstey, Vilayanur V. Viswanathan, and Michael L. Perry; Redox Flow Batteries: An Engineering Perspective

[5] Eduardo Sanchez-Díez ´ , TEdgar Ventosa , Massimo Guarnieri , Andrea Trovo` , Cristina Flox , Rebeca Marcilla , Francesca Soavi , Petr Mazuri , Estibaliz Aranzabe , RaquelFerret; Redox flow batteries: Statusand perspective towardssustainable stationary energy storage; Journalof Power Sources, 481,


[7] Bloomberg NEF New Energy Outlook2021.


About the Author

Amrit Kafle is a PhD candidate in Materials Science division, department of Physics at the Catholic University of America , Washington DC, USA, working as a Research Assistant in Vitreous State Laboratory. BatteryMBA graduate. Amrit is associated with National Institute of Standards and Technology (NIST), Gaithersburg, Maryland, as a International Guest Researcher. Amrit research includes structure-property relationship of garnet type ceramics and oxide based glass as prospective electrolytes for Lithium/Sodium ion batteries. Fabrication and characterization of batteries and batteries materials is another area of study.

The views expressed in this article are those of the authors alone and not Battery Associates.

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