Progress and Perspectives of Flow Battery Technologies
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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.
KeywordsEnergy storage Flow battery Vanadium flow battery Zinc-based flow battery Novel flow battery system
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 , 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 .
2 Overview of the Research and Application of Flow Batteries
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 . Moreover, researchers have recently investigated a series of novel aqueous and non-aqueous flow battery systems based on supporting electrolytes  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  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 , 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 . 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 . Therefore, it is facile for electrolytes of VFB energy storage systems to be recycled.
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 . 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  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 . 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 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.  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.  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.
2.2.2 Zn-Ni Single Flow Batteries
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  (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  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 . 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 .
2.2.4 Zn-I Flow Batteries
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 . Here, novel aqueous flow battery systems utilize aqueous solutions as their supporting electrolytes, whereas novel non-aqueous systems use organic solvents . 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  as caused by side reactions from hydrogen evolution reactions . 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
Yu et al. also proposed a Li/ferrocene (Li/Fc) flow battery system in 2015 . 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.  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 , 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
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 . 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 . 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 . 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 . 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.
3.2 ZFB Energy Storage Technologies
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 . 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.
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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