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Progress and Perspectives of Flow Battery Technologies

  • Huamin ZhangEmail author
  • Wenjing Lu
  • Xianfeng Li
Review article
  • 166 Downloads

Abstract

Flow batteries have received increasing attention because of their ability to accelerate the utilization of renewable energy by resolving issues of discontinuity, instability and uncontrollability. Currently, widely studied flow batteries include traditional vanadium and zinc-based flow batteries as well as novel flow battery systems. And although vanadium and zinc-based flow batteries are close to commercialization, relatively low power and energy densities restrict the further commercial and industrial application. To improve power and energy densities, researchers have started to investigate novel flow battery systems, including aqueous and non-aqueous systems. Here, novel non-aqueous flow batteries possess low conductivity and low safety, limiting further application. Therefore, the most promising systems remain vanadium and zinc-based flow batteries as well as novel aqueous flow batteries. Overall, the research of flow batteries should focus on improvements in power and energy density along with cost reductions. In addition, because the design and development of flow battery stacks are vital for industrialization, the structural design and optimization of key materials and stacks of flow batteries are also important. Based on all of this, this review will present in detail the current progress and developmental perspectives of flow batteries with a focus on vanadium flow batteries, zinc-based flow batteries and novel flow battery systems to provide an effective and extensive understanding of the current research and future development of flow batteries.

Graphic Abstract

Keywords

Energy storage Flow battery Vanadium flow battery Zinc-based flow battery Novel flow battery system 

1 Introduction

Energy is the foundation of human survival and provides power for societal development with energy supply capacity and energy security being essential to support the sustainable development of society. Currently, however, improvements to energy supply capacity and energy security face serious challenges because existing fossil fuel-based energy infrastructures cannot meet the requirements for sustainable societal development. As a result, green and effective renewable energy needs to be explored and its use in existing energy supply infrastructures needs to increase. However, renewable energy such as solar and wind power is affected by natural environments (e.g., alternation of day and night, changes in season) and geographical conditions [1], resulting in the discontinuous, unstable and uncontrolled generation of electricity, causing serious impacts on the safe and stable operation of power grids [2, 3, 4]. To resolve these issues and improve the ability of power grids to accommodate the intermittent nature of electricity generated by renewable energy, high capacity energy storage devices are required, especially large-scale energy storage technologies. This is because energy storage devices can provide amplitude modulation and frequency regulation, smoothen power output and regulate power generation, allowing for continuity, stability and controllability of the electricity generated by renewable energy and reducing the challenges of renewable energies. Because of this, high capacity large-scale energy storage technologies are key to the expansion of renewable energy [3, 5, 6, 7]. Batteries can store electric energy generated by renewable energies in the form of chemical energy and can transform chemical energy into electric energy as needed. Due to this, battery technologies can accelerate the utilization of renewable energy. However, battery technologies suitable for large-scale energy storage applications need to be safe, inexpensive and environmentally friendly. Based on this, flow battery energy storage technologies, possessing characteristics such as environmental benignity as well as independently tunable power and energy, are promising for large-scale energy storage systems [8].

2 Overview of the Research and Application of Flow Batteries

L. H. Thaller at National Aeronautics and Space Administration (NASA) first proposed the concept of the dual flow battery in 1974 [9], in which the conversion between electric energy and chemical energy can be achieved based on the reversible redox reaction of active materials in positive and negative electrolytes, respectively (namely the valence state change) (Fig. 1). Different from traditional solid-state batteries, the negative and positive electrolytes of conventional dual flow batteries such as iron-chromium flow batteries, vanadium flow batteries (VFBs), zinc-based flow batteries (ZFBs) and sodium polysulfide-bromine flow batteries are stored in external tanks (Fig. 1) [10, 11, 12, 13, 14] and are pumped inside the battery body, allowing redox reactions to occur on the surface of electrodes. As a result, the power and energy of flow batteries can be independently adjusted [10, 15, 16, 17, 18, 19].
Fig. 1

Schematic of a typical dual flow battery

In theory, ion pairs with valence state changes can serve as active materials in flow batteries, and currently, there are various types of flow batteries, among which inorganic VFBs and ZFBs have demonstrated good market prospects. Here, VFBs possess attractive features such as high energy efficiencies, large storage capacities, lack of geographical condition limitations, deep discharging, long working lifespans, environmental benignity and so forth [20, 21, 22]. ZFBs (mainly including zinc-bromine (Zn-Br) flow batteries, Zn-Br single flow batteries, zinc-nickel (Zn-Ni) single flow batteries, zinc-iron (Zn-Fe) flow batteries, zinc-iodine (Zn-I) flow batteries, etc. [23, 24, 25]) possess highly plentiful active material sources and are inexpensive. In addition, organic flow batteries have also been intensively researched in recent years [26]. Moreover, researchers have recently investigated a series of novel aqueous and non-aqueous flow battery systems based on supporting electrolytes [27] with the purpose of improving power and energy densities and decreasing costs. And up to now, many studies have been conducted and many relevant research papers have been published. Based on this, this review will provide an introduction to traditional VFBs, ZFBs and novel flow battery systems.

2.1 VFB Energy Storage Technologies

M. Skyllas at the University of New South Wales (UNSW) was the first to propose the concept of VFBs in 1985 [28, 29, 30], and currently, VFB energy storage technologies are at the stage of industrial demonstration [8] with organizations such as Sumitomo Electric Industries (Japan), Fraunhofer UMSICHT (Germany), Pacific Northwest National Laboratory (USA), UniEnergy Technologies (USA) and Reat (UK) working on the research, development and industrialization of VFB energy storage technologies. Among these organizations, Pacific Northwest National Laboratory utilized mixed acids as supporting electrolytes [31], which has also been adopted by UET, whereas Sumitomo Electric has studied flow battery technologies since 1980s and has accumulated rich engineering and industrial experiences. In addition, Sumitomo Electric has also implemented various demonstration projects for engineering applications in which their as-built 15 MW/60 MWh VFB energy storage power station has run stably for over two years, highly evaluated by Hokkaido Electric Power Company [32]. In China, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (DICP, CAS), Central South University, China Academy of Engineering Physics and various other organizations have also carried out the research and development of flow battery technologies. Their investigations mainly concentrate on VFBs and ZFBs with promising prospects for stationary energy storage applications. Furthermore, the research group at DICP has established a spin-off company called Dalian Rongke Power Technology Development Co. Ltd. (RKP), and in recent years, the DICP-RKP cooperation group has achieved rapid technological advancements in flow battery materials and stack designs [6, 17, 33, 34, 35, 36, 37, 38, 39, 40]. Flow battery materials mainly include electrolytes, membranes, electrodes and bipolar plates, and up to now, the DICP-RKP cooperation group has developed ion conducting membranes with high selectivity and conductivity as well as carbon–plastic composite bipolar plates with high conductivity and mechanical stability [15, 22, 39, 40]. At the same time, this group has also developed the design of high power density stacks based on numerical simulations and experimental verifications [16, 17, 18, 36, 41]. The ohmic polarization of stacks has been significantly decreased, increasing working current densities of stacks from 80 to 180 mA cm−2 if the energy efficiency of batteries is kept at over 80%, elevating the power density of stacks and significantly reducing costs. Furthermore, a VFB single cell developed by the DICP-RKP group achieved a working current density of 300 mA cm−2 if the energy efficiency of the battery remained at over 80%. And a 20 kW VFB stack achieved a rated working current density of more than 220 mA cm−2 through innovations in battery material and stack structure. Besides, based on a vanadium price of less than 70,000 $ t−1, the cost of a 1 MW/5 MWh VFB energy storage system developed by the DICP-RKP group can decrease to 350 $ kWh−1. The cost of the electrolyte in VFB energy storage systems accounts for over 50% of the overall cost. And although vanadium electrolytes do not degrade, minor side reactions can accumulate over extended use and cause imbalances in ion valence states, resulting in capacity fading. However, this capacity can be restored by in-suit or ex-suit recovery depending on valence adjustment technologies [42]. Therefore, it is facile for electrolytes of VFB energy storage systems to be recycled.

In terms of application demonstration, RKP ran the world’s largest 5 MW/10 MWh VFB energy storage system on a 50 MW wind farm in Liaoning Province belonging to Longyuan Electric Power Company in 2012 (Fig. 2) [42], which has achieved smooth output and generation scheme tracking of the wind farm with continuous and stable operation for more than 6 years, confirming the safety and reliability of this VFB energy storage system. Furthermore, RKP succeeded in building a VFB energy storage equipment manufacturing factory in 2016 with a capacity of 300 MW per year and the products of RKP have been exported to Germany, USA, Japan, Italy and other countries. RKP has also implemented nearly 30 application demonstration projects, including distributed generation systems, smart grids, off-grid power supplies, renewable energy source generation and so forth [43, 44, 45, 46]. Currently, the DICP-RKP group is also working on a 200 MW/800 MWh national VFB demonstration project authorized by the National Energy Administration [47].
Fig. 2

5 MW/10 MWh VFB energy storage system developed by RKP

2.2 ZFB Energy Storage Technologies

The cost of active materials in vanadium electrolytes is relatively high and is easily affected by fluctuations in vanadium market prices. Alternatively, zinc is earth abundant, inexpensive and possesses high energy density and good reversibility in redox reactions [48]. Owing to this, ZFBs utilizing zinc as the anode material such as Zn-Br flow batteries, Zn-Br single flow batteries, Zn-Ni single flow batteries, Zn-Fe flow batteries and Zn-I flow batteries are particularly promising [23, 24, 25, 49, 50]. Currently, companies developing Zn-Br flow batteries include Redflow (Australia), EnSync Energy Systems (USA), Primus Power (USA) and LOTTE Chemical (Korea) and the progress of Zn-Br flow batteries has reached the industrialized promotion stage [51] in which in 2018, Redflow launched a domestic 10 kWh Zn-Br flow battery module together with a 600 kWh battery system for smart grids [51]. As for Zn-Ni single flow batteries and Zn-Br single flow batteries, these are currently at the fundamental research and engineering amplification stage [23, 52, 53, 54, 55]. Recently, DICP has also conducted fundamental and practical research into ZFBs and has obtained a series of significant achievements concerning battery key materials as well as stack integration and design. Here, the research has been concentrated on electrode materials, stack structural designs, zinc deposition morphology, formation and regulatory mechanisms of zinc dendrites, etc. [56, 57, 58].

2.2.1 Zn-Br Single Flow Batteries and Zn-Br Flow Batteries

The redox reaction of bromide ions and bromine occurs at the positive side of Zn-Br flow batteries, whereas deposition and dissolution reactions occur at the negative side [59, 60] in which both positive and negative electrolytes utilize zinc bromide as the active material. As for Zn-Br single flow batteries, the cathode structure is completely sealed and no electrolyte circulation devices are required at the positive side (Fig. 3a). Alternatively, active materials in negative electrolytes need to be pumped into the anode by using a circulation device [23].
Fig. 3

a Schematic of a Zn-Br single flow battery [23]. Adapted/Reprinted with permission from Ref. [23]. Copyright © 2013, Elsevier. b Principle of the cage-like porous carbon applied in bromine-based batteries (Q: complexing agent, BrnQ: Br2 complex) [49]. Adapted/Reprinted with permission from Ref. [49]. Copyright © 2017, John Wiley and Sons

The low activity of the Br2/Br redox couple at the positive side can lead to relatively low working current densities for Zn-Br flow batteries. And in order to improve the catalytic activity of cathode materials used in Zn-Br flow batteries, Zhang et al. [60] designed and fabricated bimodal highly ordered mesostructured carbons with excellent activity to Br2/Br as the cathode material using a solvent evaporation-induced self-assembled method. Here, the bimodal pores of this cathode material were distributed at 2 nm and 5 nm in which the 2 nm pores enhanced reactant adsorption, whereas the 5 nm pores enhanced mass transfer. Furthermore, the high specific surface area of the resulting material provided more active sites for Br2/Br reactions, allowing the energy efficiency of a Zn-Br flow battery stack using the bimodal highly ordered mesostructured carbon as the cathode material to reach 80.15% at 80 mA cm−2. In another study, Wang et al. [49] attempted to prevent Br2 diffusion and improve the reaction activity of Br2/Br by designing and fabricating a cage-like porous carbon with a specific pore structure that combined super-high activity with Br2-complex-entrapping capabilities (Fig. 3b). Here, the pore size of the cage-like porous carbon was exactly between the sizes of Br, the complexing agent and the Br2 complex. Consequently, as Br oxidizes to Br2, it can react with the complexing agent to immediately form a Br2 complex and become trapped in the cage based on pore size exclusion, effectively inhibiting bromine crossover.

As an example, DICP developed the first international 5 kW Zn-Br single flow battery system in 2017 in which energy efficiencies reached more than 78% (Fig. 4) [61]. This demonstration system has also been applied in an optical storage power supply system for Shanxi Huayin Technology Co. Ltd. (Ankang, Shanxi Province).
Fig. 4

5 kW/5 kWh Zn-Br single flow battery demonstration system developed by DICP [61]

2.2.2 Zn-Ni Single Flow Batteries

Zn-Ni single flow batteries achieve the conversion between electric energy and chemical energy based on the electrochemical reaction between zinc ions and nickel ions [52, 53, 54, 55]. Zn-Ni single flow batteries use nickel oxide or nickel hydroxide as the cathode and zinc as the anode. And both the anolyte and the catholyte are the same in which zinc oxide dissolves in alkaline solutions to form zincate. Similar to Zn-Br single flow batteries, only the negative electrolyte requires an electrolyte circulatory system (Fig. 5a).
Fig. 5

a Cell structure schematic of a Zn-Ni single flow battery [52]. Adapted/Reprinted with permission from Ref. [52]. Copyright © 2013, Elsevier. b Single cell structure schematics for (up) traditional and (down) novel structures [62]. Adapted/Reprinted with permission from Ref. [62]. Copyright © 2013, Elsevier

To address the issues of low power density and short cycle lifespans in Zn-Ni single flow batteries, researchers from DICP have constructed ion transport channels on nickel hydroxide cathodes by adding a serpentine flow field [62] (Fig. 5b). As a result, the mass transfer of reactant ions across the solid/liquid interface can be enhanced and cathode polarization can be decreased. In addition, researchers have also used 3D porous nickel foam to replace 2D nickel sheets as anodes [52] to decrease the actual current density of electrodes and lessen anode polarization. And based on these developments, the current density of Zn-Ni single flow batteries has increased from 20 to 80 mA cm−2 with energy efficiencies reaching over 80%, dramatically improving battery power densities [52, 62]. Moreover, through electrode structural designs and optimizations, researchers report that anode and cathode columbic efficiencies can be equalized, successfully resolving the issue of zinc accumulation and significantly enhancing the stability of battery cycling performances in which at a current density of 80 mA cm−2, researchers report no obvious fading in battery performance after 3500 charge/discharge cycles [24]. And to meet commercialization and battery upscaling requirements for flow battery technologies, high capacity porous carbon-based nickel cathodes have also been developed in which battery power densities have been further improved through optimizations of electrode structures. Here, charging areal capacities of Zn-Ni single flow battery systems can reach 80 mAh cm−2 with an energy efficiency of 85% and no clear efficiency decay after 500 charge/discharge cycles.

2.2.3 Zn-Fe Flow Batteries

Zn-Fe flow batteries are inexpensive, environmentally friendly and provide high performances. These batteries rely on the electrochemical reaction between zincate ions and iron ions to achieve the storage and release of electric energy [25, 63]. Here, alkaline Zn-Fe flow batteries utilize alkaline solutions (i.e., KOH and NaOH solutions) as supporting electrolytes and generally utilize ferrocyanide or ferricyanide as the active material at the positive side, with zinc deposition and dissolution occurring at the negative side [25, 63]. Alternatively, neutral Zn-Fe flow batteries utilize neutral solutions (i.e., KCl solution) as supporting electrolytes [64].

So far, optimal operating current densities of Zn-Fe flow batteries have only reached 35 mA cm−2, whereas VFBs can achieve 120 mA cm−2. As a result, the practical application of Zn-Fe flow batteries is significantly restricted. However, by replacing Nafion membranes possessing low ion selectivity and high cost with the polybenzimidazole custom membranes with high mechanical stability and high conductivity, and utilizing 3D porous carbon felt to guide the zinc stripping/plating, alkaline Zn-Fe flow batteries can potentially operate at a current density ranging from 60 to 160 mA cm−2 and simultaneously achieve a battery energy efficiency of over 80%. And the energy efficiency of alkaline Zn-Fe flow batteries can reach 89.59% at a current density of 80 mA cm−2 (Fig. 6) [25].
Fig. 6

Performance of recently reported flow batteries [25]. Adapted/Reprinted with permission from Ref. [25]. Copyright © 2018, Elsevier

Zinc accumulation and zinc dendrite formation on anode surfaces can significantly affect cycling stability and are also key issues restricting the development of Zn-Fe flow batteries. To resolve these issues, researchers have combined the use of porous electrodes [25] and highly mechanical negatively charged nanoporous membranes [63] with the research of electrolyte chemistry [64] to reduce zinc accumulation and zinc dendrite formation in Zn-Fe flow batteries (Fig. 7a). Negatively charged groups on the surface and pore walls of membranes could effectively prevent negatively charged zincate ions based on the Donnan exclusion effect. Thus, even if zinc dendrites exist, they will grow back onto membranes, avoiding the penetration of membranes by zinc dendrites and increasing the battery lifespan (Fig. 7a). As a result, alkaline Zn-Fe flow batteries using negatively charged nanoporous membranes can run continuously for ~ 240 cycles at current densities ranging from 80 to 160 mA cm−2 with no distinct fading in energy efficiency (Fig. 7b) [63].
Fig. 7

a Schematic of a dendrite-free alkaline Zn-Fe flow battery; b cycling performance of an alkaline Zn-Fe flow battery using an optimized membrane. Insets: representative charge and discharge profiles [63]. Copyright © 2018, Springer Nature

2.2.4 Zn-I Flow Batteries

Zn-I flow batteries have been investigated by Li et al. [65] at Pacific Northwest National Laboratory with the purpose of improving flow battery power and energy densities. Compared with traditional Zn-Br flow batteries, as-utilized Zn-polyiodide electrolytes in Zn-I flow batteries possess higher solubility, are more environmentally benign and can achieve higher energy densities. For example, a Zn-polyiodide flow battery can provide a discharge energy density of 166.7 Wh L posolyte −1 by using 5.0 M ZnI2 electrolyte [concentration is close to the saturation concentration of ZnI2 (5.6 M)], which is almost 7 times higher than that of current aqueous flow batteries [66]. In another study, Lu et al. [67] proposed a novel zinc/iodine-bromide battery (ZIBB) using Br as the complexing agent (Fig. 8a) in which Br can stabilize free iodine to form iodine-bromide ions (I2Br) and subsequently release iodide ions for charge storage, contributing to higher energy densities (Fig. 8a, b). Here, the introduction of Br prevented released iodide ions from acting as a complexing agent to stabilize iodine, which can lead to reductions in capacity (Fig. 8c). As a result, the introduction of Br as a complexing agent allowed the as-demonstrated ZIBB in this study to achieve an ultrahigh energy density of 202 Wh L posolyte −1 , which is currently the highest energy density reported in the literature for aqueous flow batteries.
Fig. 8

a Concept illustration of Br as a complexing agent to stabilize iodine; b reaction equations with Br as the complexing agent; c reaction equations without Br as the complexing agent [67]. Adapted/Reprinted with permission from Ref. [67]. Copyright © 2017, Royal Society of Chemistry

However, the existence of zinc dendrites in Zn-I flow batteries can lead to limited cycling lifespans because zinc dendrites can impale membranes to shorten batteries [48]. In addition, the use of expensive Nafion membranes restricts the practical application. To resolve these issues, Li et al. [50] presented a Zn-I flow battery using a cost-effective porous polyolefin membrane with high ion conductivity in which during battery operation, oxidized I3 ions can fill the pores of the porous membrane and react with zinc dendrites to eliminate the adverse effects of zinc dendrites. As a result, the Zn-I flow battery exhibited high cycling stability with more than 300 cycles at 80 mA cm−2. Subsequently, Li et al. [68] designed a novel Zn-I single flow battery (ZISFB) with super-high energy density, ultrahigh efficiency and excellent stability (Fig. 9) in which analogous to Zn-Ni single flow batteries, the researchers sealed a high concentration electrolyte (7.5 M KI and 3.75 M ZnBr2) at the positive side of the Zn-I single flow battery (Fig. 9a). Here, the researchers reported that the high solubility of KI allowed the battery to be able to completely charge to nearly 100% state of charge (SOC) and the iodide ions can be fully converted to solid-state iodine, achieving maximum energy density. And as a result, the novel Zn-I single flow battery provided a high energy density of 205 Wh L−1 (theoretical energy density is ~ 240 Wh L−1), which is the highest cycling energy density reported in the literature (Fig. 9b). Furthermore, the researchers also reported that the battery ran continuously for more than 500 cycles at 40 mA cm−2, which was much longer than that of Zn-I flow batteries reported previously (Fig. 9b). Overall, the outstanding performances obtained in this study demonstrate the excellent potential of this type of Zn-I single flow batteries for large-scale energy storage and power battery applications.
Fig. 9

a Schematic of a Zn-I single flow battery during charging; b comparison of a Zn-I single flow battery system with other systems in terms of cycling energy density, cycling stability and current density [68]. Adapted/Reprinted with permission from Ref. [68]. Copyright © 2019, Royal Society of Chemistry

2.3 Novel Flow Battery Systems

Recently, researchers have explored many types of novel flow battery systems in an attempt to address the low power and energy density of traditional flow battery systems such as VFBs and ZFBs. And dependent on the features of supporting electrolytes, novel flow battery systems can be divided into aqueous and non-aqueous systems [27]. Here, novel aqueous flow battery systems utilize aqueous solutions as their supporting electrolytes, whereas novel non-aqueous systems use organic solvents [26]. Similar to VFBs and ZFBs, novel aqueous flow battery systems commonly possess lower electrolyte resistances, higher safety and better environmental benignity but require improvements to their relatively low operating voltages and energy densities [10] as caused by side reactions from hydrogen evolution reactions [69]. Alternatively, the voltage of novel non-aqueous flow battery systems is not restricted by hydrogen evolution reactions and can provide more extensive working temperature windows, higher voltages and higher theoretical energy densities [27, 69]. However, the active species of non-aqueous flow batteries are normally unstable and the solvents used are generally flammable, causing poor stability [10, 27, 70].

2.3.1 Novel Non-aqueous Flow Battery Systems

Singh et al. [71] were the first to propose the concept of a non-aqueous flow battery system in which the active species were coordination compounds based on metals such as V and Ru and organic ligands. However, the working current density of non-aqueous flow battery systems is low due to the low ionic conductivity of supporting electrolytes, membranes, non-aqueous solvents and so on [27]. To address this, Wang et al. [72] developed a lithium/2,2,6,6-tetramethylpiperidine-1-oxyl (Li/TEMPO) flow battery system in which TEMPO served as the electroactive material at the positive side, lithium hexafluorophosphate (LiPF6) acted as the supporting charge carrier (Fig. 10a), Li was used as the anode and a mixture of ethylene carbonate (EC), propylene carbonate (PC) and ethyl methyl carbonate (EMC) was used as the solvent. Here, the cathode reaction was the radical reaction of TEMPO, whereas the anode reaction was the deposition and dissolution of Li (Fig. 10a). However, despite the materials used, the comparably low electrical conductivity of the organic system led to low working current densities for the Li/TEMPO flow battery system with only 5 mA cm−2.
Fig. 10

a Redox reactions at the negative and positive sides during charge/discharge in the voltage range of 2.5–4.0 V versus Li/Li+; the counter anion is PF6 [72]. Adapted/Reprinted with permission from Ref. [72]. Copyright © 2014, John Wiley and Sons. b Synthesis of Fc1N112-TFSI [73]. Adapted/Reprinted with permission from Ref. [73]. Copyright © 2014, John Wiley and Sons

Yu et al. also proposed a Li/ferrocene (Li/Fc) flow battery system in 2015 [74]. However, the solubility of the positive electrolyte in this system was low and Li dendrite formation on the Li anode compromised battery safety. In another study, Wang et al. [73] modified Fc by introducing quaternary ammonium groups to prepare ferrocenylmethyl dimethyl ethyl ammonium bis(trifl uoromethanesulfonyl)imide (Fc1N112-TFSI) (Fig. 10b). Here, the solubility of Fc in the supporting electrolyte of 1.2 M LiTFSI in EC/PC/EMC increased from 0.04 to 0.85 mol L−1. However, the researchers reported that the main challenge of this Li/modified Fc flow battery system was the low working current density in which at an electrolyte concentration of 0.1 mol L−1, a working current density of only 3.5 mA cm−2 can be achieved for this battery. Furthermore, at an electrolyte concentration of 0.8 mol L−1, the working current density reached only 1.5 mA cm−2. Moreover, increased electrolyte concentrations can result in relatively fast capacity fading.

Alternatively, Yu’s research group at the University of Texas has achieved many outstanding achievements in non-aqueous flow battery systems in recent years [75, 76, 77, 78]. For example, these researchers developed a non-aqueous all-metallocene-based lithium-based flow battery that uses ferrocene (FeCp2) and cobaltocene (CoCp2) as the redox-active cathode and the anode, respectively [75], and reported that this all-metallocene-based Li-based flow battery can benefit from the greater reaction rate constants of metallocenes to allow for higher working potentials and higher capacity retention of over 99% per cycle. As a result, their fabricated full cell achieved a coulombic efficiency of more than 95% and an energy efficiency of over 85% and was powerful enough to light up a yellow LED bulb. Based on this, this study can provide a generic design route for the design of high-performance non-aqueous Li-based flow batteries through the use of electroactive organometallic species.

2.3.2 Novel Aqueous Flow Battery Systems

As an example of a novel aqueous flow battery system, Aziz et al. [79] proposed a quinone/bromide flow battery system that used bromine and 9,10-anthraquinone-2,7-disulphonic acid (AQDS) as the active species at the positive and negative sides, respectively. Here, the positive supporting electrolyte was hydrobromic acid and the negative supporting electrolyte was sulfuric acid, allowing the working current density of this quinone/bromide flow battery to reach 500 mA cm−2, achieving a relatively high power density. However, the researchers reported that the bromine in this quinone/bromide flow battery system was easily oxidized and not resistant to corrosion. In addition, the open-circuit voltage of the system was low at only 0.7 V and the cycle life of the system was short. To resolve this, Aziz et al. [80] developed a quinone/iron flow battery system in 2015 that replaced bromide with nontoxic ferricyanide ions (Fig. 11a) in which the cathode and the anode active species of the system was K4Fe(CN)6 and 2,6-dihydroxyanthraquinone (2,6-DHAQ), respectively (Fig. 11b). Here, the researchers reported an open-circuit voltage of 1.2 V and steady operations of over 100 charge/discharge cycles at a current density of 100 mA cm−2 with an energy efficiency that remained around 84%. In addition, the researchers also reported a capacity loss of only 0.1% per cycle. However, the Nafion membrane used in this battery was expensive and the concentration of the electrolytes was low (the catholyte: 0.4 mol L−1, the anolyte: 0.5 mol L−1), which caused low energy densities.
Fig. 11

a Cyclic voltammetry of the electrolyte and cell schematic; b cyclic voltammogram of 2 mmol L−1 2,6-DHAQ (dark cyan curve) and ferrocyanide (gold curve) scanned at 100 mV s−1 on a glassy carbon electrode; arrows indicate scan direction; dotted line represents the CV of 1 mol L−1 KOH background scanned at 100 mV s−1 on a graphite foil electrode [80]. Adapted/Reprinted with permission from Ref. [80]. Copyright © 2015, Science

In summary, organic solvents used in non-aqueous flow battery systems are generally flammable and the stability of their active materials requires further improvement. In addition, the working current density of non-aqueous flow battery systems is comparably low and cannot meet the requirements of practical application. Compared with non-aqueous flow battery systems, the lower electrolyte resistance, higher power density, lower costs, higher safety and better environmental friendliness of aqueous flow battery systems make them more promising for industrial applications. However, the power and energy density of aqueous flow battery systems still require improvement.

3 Development and Perspective of Flow Battery Energy Storage Technologies

3.1 VFB Energy Storage Technologies

The investigation of the numerical simulation of stack structural designs for VFBs needs to be reinforced. In addition, the uniformity of electrolyte diffusion inside VFB stacks needs to be improved and the internal resistance of VFB stacks needs to be decreased, allowing the ohmic polarization of VFB batteries to lessen, further creating design integration methods for VFB battery stacks with higher power densities (or rated working current densities). As for VFB battery stacks charged/discharged at a constant power mode, their average rated working current density should reach 300 mA cm−2, where the energy efficiencies of stacks remain over 80%. Therefore, the main research and development of VFBs should focus on the following aspects.

3.1.1 Research and Development of Novel High-Performance, Low-Cost Key Materials for VFBs

High-performance and low-cost key materials (electrolytes, ion conducting membranes, electrodes and bipolar plates) are the key to inexpensive and high performing flow batteries. Here, electrolytes should possess high solubility and excellent stability to improve the energy density of flow batteries and ideal ion conducting membranes used in flow batteries should be highly selective, highly conductive, chemically stable and inexpensive. In addition, the high reaction activity, high stability and high thickness uniformity of electrode materials (such as carbon fiber) are important and bipolar plates should possess high conductivity and high tenacity.

Supporting electrolytes of VFBs can be categorized into aqueous sulfuric acid solutions, aqueous acid solutions mixing sulfuric acid and hydrochloric acid. The low vapor pressure of aqueous sulfuric acid solutions can weaken the causticity of sulfuric acid-based electrolytes. However, the solubility of vanadium in sulfuric acid solutions is relatively low (around 1.5–1.8 mol/L), causing low energy densities and narrow working temperature windows. Alternatively, the solubility of vanadium in mixed acid solutions is comparably high (around 2.5–3.0 mol L−1) and the stability of electrolytes is also relatively better [31]. Furthermore, the upper operating temperature limit of mixed acid solutions can reach 50 °C, which is much higher than that of batteries using sulfuric acid solutions (35 °C), allowing batteries using mixed acid-based electrolytes to possess higher energy densities and wider working temperature windows. However, hydrochloric acid possesses high vapor pressures and is strongly corrosive. In addition, toxic chlorine is generated during the charging process [34]. Therefore, batteries using mixed acid-based electrolytes exert high demands on stack and pipeline materials, increasing costs. Therefore, the research and development of VFB electrolytes with high concentration, high stability and low cost are required to enhance performance over a wider operating temperature range.

Currently, commonly used membranes in VFBs are per-fluorinated sulfonated ion exchange membranes (like DuPont Nafion). This kind of membranes has good chemical stability and high ion conductivity [21, 81, 82]. But then, ion clusters containing ion exchange groups can easily swell in aqueous acidic solutions and contribute to larger-sized ion transport channels, which can allow hydrated vanadium ions to cross through the membrane, leading to poor ion selectivity and decreased coulombic efficiency and storage capacity of VFBs [21]. In addition, the cost of per-fluorinated sulfonated ion exchange membranes is high. For example, the market price of Nafion 115 as manufactured by DuPont is currently around 700 $ m−2, significantly limiting the commercialization and industrialization of VFBs. Therefore, the development of inexpensive and environmentally friendly non-per-fluorinated ion conducting membranes with high ion selectivity, conductivity and chemical stability is vital for accelerating the commercialization of VFBs.

Electrode performance is closely related to active polarization, ohmic polarization and concentration polarization of flow battery stacks [6, 34]. To minimize polarization, the research of high-performance electrode materials should focus on enhancements to catalytic activity, conductivity, regularity of density distribution and uniformity of thickness. In addition, the high compactness, high mechanical strength and high toughness of bipolar plates must be assured. Furthermore, increasing the conductivity of bipolar plates can effectively decrease internal resistances and improve the working current density and power density of batteries. Based on this, the exploration of optimal bipolar plate materials is urgently needed.

3.1.2 Development of VFB Stacks with High Power Density

Cell stacks are the kernel of flow battery energy storage systems in which redox reactions occur for the conversion between electric energy and chemical energy. Here, the performance and reliability of stacks directly affect the performance and reliability of flow battery energy storage systems. However, the working current density of flow battery stacks is currently relatively low (around 80 mA cm−2), resulting in comparatively low power densities, increased consumption of materials and high costs, limiting the large-scale application of flow battery systems. Therefore, the reduction in stack polarization and the improvement in voltage efficiency and power density are pivotal. Here, promising methods include stack design optimization, enhanced uniformity of active material spatial distribution, minimized battery component contact resistances, etc. And based on the design and optimization of stack structures as well as the exploration of structural design techniques for stacks with high power densities, the working current density of current VFB stacks has been increased to over 300 mA cm−2. In addition, improvements to the utilization of electrolytes to enhance energy density are an important goal in the research of structural designs and integration of flow battery systems.

3.1.3 Smart Control Technologies of VFB Systems

VFB systems are complex systems that include stacks, electrolyte storage and supply subsystems, battery management subsystems and so forth [34]. In addition, the exploitation of flow batteries with high power, high reliability, high stability and low costs as well as the design and integration of flow battery modules in VFB systems is vital. Furthermore, the integration of 100-MW-scale VFB systems with high power, smart controls and management policies and associated energy management technologies can propel the application of VFB energy storage technologies.

3.1.4 The Technique Prospect of VFB Energy Storage Technology

The key to popularizing VFB energy storage technologies is to decrease costs and establish novel business models. Currently, the rated output power (P) of VFBs in the constant power model is defined by the product of the working voltage (V), average working current density (I) and electrode area (A) (P = V × I × A) in which during charge/discharge, the average working voltage of batteries is almost consistent. Therefore, the key to decreasing VFB energy storage technology costs is to improve rated working current densities, which can effectively decrease the usage of battery materials.

In VFB energy storage systems, electrolytes are recycled between the external tank and the stack through electrolyte transport pipelines. However, slight side reactions will accumulate after long-term operations and cause an imbalance in valence states of the positive and negative electrolytes. In addition, minor crossover of electrolytes can lead to capacity fading in batteries. This imbalance in valance states and capacity fading can be restored, however, by in-suit or ex-suit recovery technologies, allowing VFB electrolytes to be circularly utilized. For example, for a 1 MW/5 MWh VFB energy storage system, electrolytes per kWh need ~ 10 kg V2O5, in which if the price of V2O5 is lower than 10,000 $ t−1, the price of the electrolyte is ~ 190 $ kWh−1. And as the rated working current density of the stacks reaches 300 mA cm−2, the cost of the VFB energy storage system reaches ~ 350 $ kW−1 and the cost of the 5 kWh electrolyte reaches 950 $, resulting in the initial cost of the 1 MW/5 MWh VFB energy storage system to be ~ 260 $ kWh−1. And if the residual rate of the electrolytes remained at 70%, the actual cost is ~ 130 $ kWh−1 (perspectives of VFB energy storage technologies shown in Fig. 12).
Fig. 12

Perspectives of VFB energy storage systems

3.2 ZFB Energy Storage Technologies

ZFBs, especially Zn-Br flow batteries, Zn-Br single flow batteries, Zn-Fe flow batteries, etc., are inexpensive and safe, and the materials used are abundant. Because of this, ZFBs possess broad application prospects in the field of energy storage at users’ side. The emphasis of ZFB research is to study the mechanisms of zinc deposition on electrode surfaces along with the dissolution and regulatory mechanisms of zinc, in which deeper understandings of these mechanisms can improve the reversible deposition of zinc (namely areal capacity), resolve issues of zinc dendrites and increase the energy density of batteries. In addition, breakthroughs in structural design and integration technologies along with the technological development of low-cost ZFB stacks are research focuses for the future. The cost of ZFBs is largely influenced by the power density of stacks due to the relatively low price of electrolytes used in ZFBs. Currently, the working current density of ZFBs in demonstration is ~ 20 mA cm−2. And through innovations in battery materials (including electrodes and ion conducting membranes) together with stack structural designs, the charge/discharge working current density of ZFBs has been gradually increased to 80 mA cm−2. And if the areal capacity of deposited zinc can reach 160 mAh cm−2, ZFBs will become competitive in the market. Furthermore, ZFBs can play an important role in commercial energy storage at users’ side and potentially occupy large market shares (the technological perspectives of ZFBs are illustrated in Fig. 13).
Fig. 13

Perspectives of ZFB energy storage systems

3.3 Novel Flow Battery Energy Storage Technologies

Recently, researchers have explored different types of novel flow battery systems, including aqueous and non-aqueous systems. The purpose of studying novel non-aqueous flow batteries is to improve the voltage of flow batteries, and the purpose of studying novel aqueous flow batteries is to decrease costs and improve energy density. Here, the main challenges of novel non-aqueous flow battery systems are their low power density and poor cycling performance, whereas the main challenges of novel aqueous flow battery systems are their low energy density and their high costs.

4 Summary and Outlook

Although ZFBs have reached the demonstration stage, VFB energy storage technologies are still closer to successful industrial and commercial application [61]. ZFB stacks face issues regarding the uniformity of the charge/discharge process that can cause low energy conversion efficiency and reduced long-term cycling stability of stacks. This reduced cycling lifespan for stacks makes ZFB systems unable to meet the requirements of large-scale energy storage applications. In addition, different types of ZFBs possess different drawbacks. For example, the strong corrosivity, oxidizability and diffusivity of bromine make Zn-Br flow batteries unsafe to use and the relatively low energy density of alkaline Zn-Fe flow batteries requires comparatively large amounts of electrolytes, all of which are not favorable for the industrial and commercial utilization of batteries. As for VFBs, VFB energy storage technologies have been applied in many MW and above MW-scale demonstration projects and the successful demonstration of these projects verifies the reliability and stability of VFB energy storage systems, and currently, VFB energy storage technologies have reached the initial stages of commercialization. Overall, VFB systems are more suitable for energy storage at the power generation side, whereas high energy density ZFB systems (i.e., Zn-Br flow batteries) are more suitable for energy storage at the user’s side due to smaller volumes and lower costs. In short, ZFB energy storage technologies cannot replace VFB energy storage technologies and vice versa.

Currently, compared with lithium ion batteries and fuel cells, funding support from government agencies and enterprises for the research and development of flow battery technologies is insufficient. In addition, the number of institutions participating in the research and development of the industrial chain of flow battery technologies is also insufficient. As a result, great developmental potential for flow battery energy storage technologies remains. And in order to propel the wide utilization of flow battery energy storage technologies and realize their industrialization, the integration of basic, applied and translational research is necessary. Moreover, significant improvements to power density, increases in stability and reliability, decreases in costs and the creation of feasible business models are also urgently needed to allow flow battery energy storage technologies to satisfy the requirements of practical and industrial utilization.

Notes

Acknowledgements

This work was supported by the CAS-DOE program, CAS (QYZDB-SSW-JSC032), the China Natural Science Foundation (Grant Nos. U1808209), DICP funding (ZZBS201707) and the Key R&D project of Dalian (2018YF17GX020).

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Copyright information

© Shanghai University and Periodicals Agency of Shanghai University 2019

Authors and Affiliations

  1. 1.Division of Energy StorageDalian Institute of Chemical Physics, Chinese Academy of SciencesDalianChina
  2. 2.Collaborative Innovation Center of Chemistry for Energy MaterialsDalianChina
  3. 3.University of Chinese Academy of SciencesBeijingChina

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