Redox Flow Batteries: Fundamentals and Applications

A redox flow battery is an electrochemical energy storage device that converts chemical energy into electrical energy through reversible oxidation and reduction of working fluids. The concept was initially conceived in 1970s. Clean and sustainable energy supplied from renewable sources in future requires efficient, reliable and cost-effective energy storage systems. Due to the flexibility in system design and competence in scaling cost, redox flow batteries are promising in stationary storage of energy from intermittent sources such as solar and wind. This chapter covers basic principles of electrochemistry in redox flow batteries and provides an overview of status and future challenges. Recent progress in redox couples, membranes and electrode materials will be discussed. New demonstration and commercial development will be addressed.


Introduction
Global economic growth with the increasing release of carbon dioxide disrupts our ecosphere and causes significant impacts on climate change. An environmentally friendly route to generate electricity from renewable sources such as wind and solar is desirable. To promote the utilization of renewable and sustainable energy and to enhance the stability of grid networks, energy storage systems are needed to store surplus electricity. The stored energy can be then delivered to end customers or to power grids upon need. It is becoming clear that the electrochemical energy storage using rechargeable batteries based on redox chemistry can provide a central solution to tackle such an issue. Through storing energy in recirculating liquid electrolytes, redox flow batteries have merits of decoupled energy density (tank size, electrolyte concentration, cell voltage and number dependent) and power generation capability (electrode size and reaction kinetics dependent). In terms of cost, system flexibility, quick response and safety concerns for large-scale applications, redox flow batteries show great advantages over other types of batteries such as lead-acid and lithium-ion batteries and are expected to have increasing commercial space through technological development in future. Therefore, the redox chemistry and technical fundamentals of flow batteries, which determine the technological success and market penetration, need to be well understood.

Classic vanadium redox flow batteries
Among various flow batteries, vanadium redox flow battery is the most developed one [1]. Large commercial-scale vanadium redox flow batteries are currently in construction. The structure and charge-discharge reactions of vanadium redox flow batteries are schematically shown in Figure 1. During discharging, reduction occurs at the cathode and oxidation occurs at the anode as shown in Eqs. (1)-(3) (discharge: !, charge: ). While these redox reactions occur, proton ions diffuse across the membrane and electrons transfer through an external circuit.
Positive electrode : Overall reaction : The standard cell voltage for the all-vanadium redox flow batteries is 1.26 V. At a given temperature, pH value and given concentrations of vanadium species, the cell voltage can be calculated based on the Nernst equation: where R, T and F are the universal gas constant, absolute temperature and Faraday constant, respectively. The crossover of vanadium ions through the membrane may occur, resulting in self-discharge with the unwanted mixing of vanadium species at both sides of the cell, as following [2]:

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At the negative electrode: At the positive electrode: Side reactions such as hydrogen evolution due to water decomposition and CO 2 evolution due to the oxidation of carbon-based electrode may occur during operation [3]. The battery performance is generally evaluated with three efficiencies: coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE), which are defined as following: The CE reduces because of crossover of vanadium ions during cell operation and side reactions. The VE is related to the operation current density, ionic conductivity of membrane, electrode materials, flow rate and mass transport of electrolyte. As current density increases, the VE reduces due to the increase in polarization.

Types and configurations of redox flow batteries
Conventional redox flow batteries have two divided electrolyte reservoirs (Figure 2a). Catholyte and anolyte are separated by a membrane, which permits ions to pass through it. The most common working ions in aqueous media, H þ (349.8 S cm 2 mol À1 ) and OH À (198.0 S cm 2 mol À1 ), have the highest limiting molar conductivity among all known cations and anions, respectively [4]. All-vanadium redox flow batteries, for instance, have V 3þ /V 2þ redox reactions on the negative side (anolyte) and VO 2 þ /VO 2þ on the positive side (catholyte). Such battery uses the same metal ions on both sides. Crossover of metal ions through the membrane will then not cause contamination of the electrolyte. In contrast, for redox flow batteries with different metal ions such as Fe 3þ /Fe 2þ and Cr 3þ /Cr 2þ in an iron-chromium flow battery, the cross-contamination via ion penetration may cause irreversible performance loss.
Hybrid redox flow batteries such as zinc-bromine and zinc-cerium systems use metal stripping/plating reactions (Zn 2þ /Zn, À0.76 V vs. [standard hydrogen electrode] SHE) on one of the electrodes inside the cell and the other side with normal soluble flowing electrolyte. Similarly, redox flow lithium batteries in non-aqueous electrolytes have been explored to make use of the low redox potential of Li þ /Li couple (À3.04 V vs. SHE). During charging, zinc or lithium is deposited from the electrolyte and during discharging, Zn 2þ or Li þ dissolves into the solution again. A shortcoming of such hybrid redox flow battery is that the energy storage capability is limited by the free space inside the cell accommodating the metal deposits.
A second-type hybrid redox flow batteries use gas such as Cl 2 ,O 2 and H 2 as the reaction medium or with gas evolution reaction at the cathode or anode (Figure 2b). For instance, oxygen reduction reaction (O 2 þ 4H þ þ 4e À ⇄ 2H 2 O) with high positive potential can be used as the cathode. The cell capacity is then only determined by the capacity of anolyte. Oxygen reduction reaction in non-aqueous electrolytes with the presence of lithium ions can proceed through: O 2 þ 2Li þ þ 2e À ⇄ Li 2 O 2 . Moreover, oxygen reduction and oxidation during discharging and charging can be catalysed chemically with redox mediators [5]. Interestingly, the use of electrocatalysts for the oxygen reduction and oxidation as in a conventional system can be avoided. Note that the formation and deposition of Li 2 O 2 proceed at porous matrix, which can be held statically in a gas diffusion tank over charging/discharging. Such a concept may maintain the character of decoupled energy and power for flow batteries.
For aqueous electrolytes, oxygen and hydrogen gas evolution reactions by electrolysis of water take place during charging at very positive and negative electrode potentials, respectively. Hydrogen evolution reaction has been observed as a parasitic side reaction at the anode for some flow battery systems. Such behaviour has been used to store electricity and to generate hydrogen simultaneously (2V 2þ þ 2H þ ! H 2 þ 2V 3þ ) as demonstrated in a vanadium-cerium flow battery [6]. Hydrogen generated can be then used to produce electricity in fuel cells.
The ionic conductivity and selectivity of membranes often significantly affect the overall cell performance for many redox flow batteries. High area resistance of membrane restricts the practical operation only at low current densities. Crossover of active species through membrane leads to performance loss over cycling. Redox chemistry of active species with formation of electrodeposits leads to another type of cell configuration without membranes and with only one electrolyte reservoir [7] (Figure 2c). Some selected membrane-free redox flow batteries are listed in Table 1 [8][9][10][11][12][13][14]. Reasonable energy efficiencies and cycling stability have been observed. Considering the high cost of most commercial ion exchange membranes, such membrane-free cell configuration could enable simple operation and cost-effective applications.
Deposited anodic species should have slow dissolution rate in the presence of oxidized catholyte species as a self-discharge reaction. A direct reaction between the deposited metal and the other electroactive species in the electrolyte should be negligible or inhibited. Selfdischarge effects must be minimized compared to a targeted rapid charging/discharging reaction. Acidic-supporting electrolyte is not suitable for anodic metal deposition. Solid-phase reactions in general have poor kinetics, in comparison with those in liquid electrolytes. The voltage efficiencies in most of the membrane-free flow batteries are relatively low (60-80%) restricted by mass transport and charge transfer kinetics. Compared to the flow-by configuration, an undivided battery with flow-through electrodes may assure enhanced mass transport. However, the flow rate will be largely limited.
A laminar flow battery using two-liquid flowing media, pumped through a slim channel without lateral mixing or with very little mixing, enables membrane-free operation. H 2 (flowing across anode with pumped liquid hydrobromic acid) aqueous bromine laminar flow

Flow batteries
Energy efficiencies Cycling stability Ref.
Among various electrical energy storage technologies, redox flow batteries generally have relatively low energy density (for instance about 30 Wh L À1 for all-vanadium redox flow batteries). Thus, although recharging the electrolyte can be done by replacing the depleted one within a few minutes of transportation applications, redox flow batteries are only considered to be used in stationary energy storage. To increase the energy density, highly watersoluble species for instance LiI (solubility up to 8.2 M) and ZnI (7 M) can potentially enhance the volumetric energy density. The use of concentrated ZnI electrolyte leads to a high theoretical energy density of 322 Wh L À1 [15], which may even rival batteries based on lithium-ion chemistry (LiFePO 4 cathode, 223 Wh L À1 ).

Redox electrochemistry of flow batteries
The overall system performance and cost for redox flow batteries depend largely on the flow cell redox electrochemistry. Great efforts have been made in search of alternative battery chemistry from electrolytes to electrodes [4,17,18]. The possible cell voltage depends on the selected redox couples ( Table 2) and is limited by the electrochemical window of a given solvent-electrode system, stability of the supporting cation or anion and stability of the bipolar plate materials (Figure 3).  (Figure 3), needs special electrodes such as catalyst-coated titanium plate or mesh.
Many anodic reactions have low negative potential; the applications in aqueous batteries can be hindered by H 2 evolution due to the electrolysis of water with unwanted energy loss and an imbalance in the state of charge between two sides of the batteries. Through using concentrated electrolytes of water-in-ionic liquid (water in 1-butyl-3-methylimidazolium chloride, BMImCl) [19,20] or water-in-salt (water in lithium, bis(trifluoromethylsulphonyl)imide, LiTFSI) [21], the onset of oxygen evolution and hydrogen reactions can be shifted to more positive and negative potentials, respectively. Broad electrochemical window of about 3 V has been achieved accordingly ( Figure 3). It is considered that the amount of free water molecules reduces at such Cathode redox reactions Anode redox reactions Cell OCV/V Br 2 þ 2e À ⇄ 2Br À or Br 3 À þ 2e À ⇄ 3Br À AQDSH ⇄ AQDS À þ H þ þ e À 0.86 [Mn(acac) 3 ] þ þ e À ⇄ Mn(acac) 3 [Mn(acac) 3 ] À ⇄ Mn(acac) 3 þ e À 1.10 Fe 3þ þ e À ⇄ Fe 2þ Cr 2þ ⇄ Cr 3þ þ e À 1.19 ClBr 2 2À þ e À ⇄ 2Br À þ Cl À VBr 2 þ Br À ⇄ VBr 3 þ e À 1.35 3 [Ru(acac) 3 ] À ⇄ Ru(acac) 3 þ e À 1.77 Cl 2 þ 2e À ⇄ 2Cl À Zn ⇄ Zn 2þ þ 2e À 2.12 Cell is chemically discharged [6]  In contrast to the electrochemical stability of the redox species and solvents, chemical stability of electroactive species and cell components is also critical for long-term operation. Vanadium electrolytes form solid precipitates at a temperature above 40 or below 10 C at concentrations above 1.6 M for all-vanadium redox flow batteries. Oxidizing V 5þ and Ce 4þ may cause degradation of membrane and the graphite electrode materials. Complexing agents are needed to store bromine, whereas phase separation (formation of water-insoluble emulsion) occurs for bromine complexes during charging for bromine-based flow batteries. Cross-contamination in bromine-polysulphide batteries may generate heat and release toxic Br 2 and H 2 S.
High rate performance of redox flow batteries means high power generation capability. Ideally, two active species at both sides of the cell are expected to have close rate constants. However, mismatches in reaction rates are often observed. For many electrode reactions with sluggish kinetics, catalysts are needed to reduce the polarization (i.e. to improve the voltage efficiencies) and to improve the reaction rate ( Table 3) [23]. Catalysts are generally applied onto a porous material, which offers high contact area for electrolytes. The supporting materials should have high electrical conductivity, mechanical stability, reasonable cost and high levels of oxygen and hydrogen evolution overpotential for aqueous system. Carbon-based materials are commonly used for this purpose [24].

Redox active organic electrolytes
Compared to the metal-based electrolytes for redox flow batteries with limited number and resource, organic molecules with unlimited chemical space allow low-cost (for instance, from $5-10 kg À1 vs. $27 kg À1 for vanadium) and high-performance operation. Fast reaction kinetics of organic compounds permit high power generation. High solubility can be realized by controlling the solubilizing functional groups. Redox potentials can be adjusted by varying the electron-donating (ÀOH, ÀNH 2 ) or -accepting (ÀSO 3 H, ÀNO 2 , ÀPO 3 H 2 ) properties of the functional groups. By tuning the molecule size or grafting polymer chains, low membrane crossover can be obtained. High-performance organic-based aqueous redox flow batteries have been demonstrated recently ( Table 4) [25][26][27][28][29][30].
Quinone-based organic compounds have received great attention, ranging from simple hydroquinone to large anthraquinone. These materials have merits of low cost and fast reaction rates. A peak power density of 1 W cm À2 has been observed for a 9,10-anthraquinone-2,7disulfonic acid (AQDS)-bromide system [31], which is close to a reported peak power density of 1.34 W cm À2 for vanadium redox flow batteries. Compared to the relative small molecules such as hydroquinone and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), large molecules such as AQDS and methyl viologen (MV) are expected to have low-membrane crossover rates. Functionalization of these active organic compounds with polymer backbone chains further enables the battery operation with low-cost size-exclusion membranes [32]. The development  of organic active materials for redox flow batteries holds great promise for stationary electrochemical energy storage.

Semi-solid redox flow batteries
To overcome the restrictions in solubility of active species in liquid electrolytes, suspensions with energy-dense solid materials have been introduced for redox flow batteries. The concept Reaction mechanisms Redox reactions Redox potential/V vs. SHE Ref.

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was first demonstrated with intercalation materials by Chiang et al. [33], which are typically used for lithium ion batteries. Such semi-solid lithium redox flow batteries combine the merits of high energy density for lithium ion batteries and the decoupled character of conventional redox flow batteries. In order to form a percolation network for charge transfer, several strategies have been proposed: (i) dispersing conductive additive such as carbon into the electrolytes, (ii) adding redox mediators and (iii) inserting a metal wire as a current collector [34]. It has been found that the conductive electrolytes encounter the issue of shunt current between cells in a stack.
Energy-dense batteries, based on lithiation chemistry and intercalation chemistry of abundant elements (such as Na, Mg and Al etc.), contribute significantly to the transportable applications of various electronic devices and revolution of our modern societies. The successful development in these materials raises opportunities in new applications for flow batteries. Li-, Na-and organic molecule-based semi-solid redox flow batteries have been developed recently ( Table 5) [33][34][35][36][37]. For a pumping system with solid suspension, the rheological properties of suspension need to be considered.
In contrast to the flow batteries with both (de)lithiation and electron transfer reactions occurring inside the electrochemical cells (Figure 2d), a new concept using redox shuttle molecules has been introduced [38], wherein solid active materials are kept statically in the tank and only the shuttle molecules are circulated in the electrochemical cell ( Figure 4). Electrochemical redox reactions of the shuttle molecules go on at the electrode inside the cell, whereas chemical (de)lithiation of the active solid materials in the tank occurs through the reactions between the solid materials and the shuttle molecules. Since the active solid materials are not involved in the electrochemical reaction, conductive additives (such as carbon black) are not necessary in such a system. In addition, low concentration shuttle molecules of only several mM are sufficient to induce the (de)lithiation reaction of a large amount of solid materials. Symmetric battery with polythiophene Polythiophene (8.41 g L À1 ), Ketjenblack (2 g L À1 ) Low current density (<1 mA cm À2 ), energy efficiency of 60.9% at 0.5 mA cm À2 [36] Zn/polyaniline 10 wt% polyaniline powder in suspension 0.28 V overpotential at 20 mA cm À2 [37]

Conclusions and perspectives
Redox flow battery technology is relatively new and not yet well-developed. Rational electrolyte management and cell design can lead to the enhancement of energy storage capability and a reduction in construction cost. Novel electrolyte chemistry and development of a new configuration of flow batteries will create high system flexibility. Physiochemical and electrochemical redox properties of active couples, stability window of supporting electrolyte, selection of supporting ions, stability of electrode materials and cell components are key factors for successful applications. Future market penetration of flow batteries needs low cost, high energy density and high power density. The pace of recent development in the active organic molecules as electrolytes opens new strategies of cost-effective and sustainable solutions for large-scale stationary energy storage. The application of energy-dense solid materials in suspension for redox flow batteries may largely enhance the energy density of flow battery systems.