4. 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).

Table 2 summarises the electrochemical redox reactions at cathode and anode and cell open circuit voltage (OCV) for various reported redox flow batteries. For aqueous electrolytes, the typical cell voltage is below 1.5 V. To achieve high cell voltage, organic solvents with a broad electrochemical window such as acetonitrile (6.1 V) and propylene carbonate (6.6 V) are needed [4]. However, most of the used active species have poor solubility in organic solvents. High cell voltage in this case comes at the expense of low concentration of active species. A compromise among the solubility, cell voltage, reaction kinetics and suitable working temperature should be reached for selecting a suitable electrolyte. Ce4þ/Ce3<sup>þ</sup> redox reaction (from 1.44 to 1.70 V vs. SHE, depending on the type of supporting acidic electrolyte), occurring at potential beyond the stability limit of bipolar plate (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 H2 evolution due to the electrolysis of water with unwanted energy loss and an


Table 2. Selected redox reactions and cell OCV for redox flow batteries.

battery demonstrated herewith allows high concentration reactants, fast reaction rates and a

Among various electrical energy storage technologies, redox flow batteries generally have relatively low energy density (for instance about 30 Wh L�<sup>1</sup> 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�<sup>1</sup> [15], which may even rival batteries based on lithium-ion

Another successful development is the redox flow lithium batteries. Pulverized energy-dense solid electrode materials such as LiCoO2 and LiFePO4 can be suspended in a flowable slurry, which is then circulated like a liquid-soluble electrolyte (Figure 2d). Due to the high molar concentration of lithium in the solid materials (for instance about 51.2 M for LiCoO2 and 22.8 M for LiFePO4, compared to about 1.6 M for vanadium species in conventional vanadium redox flow batteries), such flow batteries allow high volumetric energy density (about 580 Wh L�<sup>1</sup> have been achieved [16]). Thus, redox flow batteries may find applications even in portable

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

Table 2 summarises the electrochemical redox reactions at cathode and anode and cell open circuit voltage (OCV) for various reported redox flow batteries. For aqueous electrolytes, the typical cell voltage is below 1.5 V. To achieve high cell voltage, organic solvents with a broad electrochemical window such as acetonitrile (6.1 V) and propylene carbonate (6.6 V) are needed [4]. However, most of the used active species have poor solubility in organic solvents. High cell voltage in this case comes at the expense of low concentration of active species. A compromise among the solubility, cell voltage, reaction kinetics and suitable working temperature should be reached for selecting a suitable electrolyte. Ce4þ/Ce3<sup>þ</sup> redox reaction (from 1.44 to 1.70 V vs. SHE, depending on the type of supporting acidic electrolyte), occurring at potential beyond the stability limit of bipolar plate (Figure 3), needs special electrodes such as

Many anodic reactions have low negative potential; the applications in aqueous batteries can be hindered by H2 evolution due to the electrolysis of water with unwanted energy loss and an

) [13].

).

high peak power density (0.795 W cm�<sup>2</sup>

108 Redox - Principles and Advanced Applications

chemistry (LiFePO4 cathode, 223 Wh L�<sup>1</sup>

electronics and electric vehicles.

plate materials (Figure 3).

catalyst-coated titanium plate or mesh.

4. Redox electrochemistry of flow batteries

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

Figure 3. Potentials and relative solubility of selected inorganic and organic redox couples for redox flow batteries. Dotted lines show the electrochemical stability limit of typical aqueous electrolytes. Dashed lines show the possibility to extend the stability limit for aqueous electrolytes using concentrated electrolytes [19, 21].

concentrated mixtures. The inner Helmholtz layer close to the electrode surface is mostly occupied by the [BMIm]þ cation or TFSI� anion, respectively. Water decomposition is then largely inhibited. The redox potentials for hydrogen and oxygen evolution reactions are pH dependent. Individual control in the pH values of the anolyte and catholyte with a multimembrane system leads to high cell operation voltage of about 3 V [22].

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 V5<sup>þ</sup> and Ce4<sup>þ</sup> 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 Br2 and H2S.

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].


Table 3. Catalysts used for redox couple reactions.

concentrated mixtures. The inner Helmholtz layer close to the electrode surface is mostly occupied by the [BMIm]þ cation or TFSI� anion, respectively. Water decomposition is then largely inhibited. The redox potentials for hydrogen and oxygen evolution reactions are pH dependent. Individual control in the pH values of the anolyte and catholyte with a multi-

Figure 3. Potentials and relative solubility of selected inorganic and organic redox couples for redox flow batteries. Dotted lines show the electrochemical stability limit of typical aqueous electrolytes. Dashed lines show the possibility to

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 V5<sup>þ</sup> and Ce4<sup>þ</sup> 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

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

membrane system leads to high cell operation voltage of about 3 V [22].

extend the stability limit for aqueous electrolytes using concentrated electrolytes [19, 21].

bromine-polysulphide batteries may generate heat and release toxic Br2 and H2S.

used for this purpose [24].

110 Redox - Principles and Advanced Applications

## 5. 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�<sup>1</sup> vs. \$27 kg�<sup>1</sup> 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 (�OH, �NH2) or -accepting (�SO3H, �NO2, �PO3H2) 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) [25–30].

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�<sup>2</sup> has been observed for a 9,10-anthraquinone-2,7 disulfonic acid (AQDS)-bromide system [31], which is close to a reported peak power density of 1.34 W cm�<sup>2</sup> 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


Table 4. Selected redox active organic compounds used for redox flow batteries.

of organic active materials for redox flow batteries holds great promise for stationary electrochemical energy storage.

#### 6. 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 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) [33–37]. 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.


Table 5. Selected examples for semi-solid redox flow batteries.

of organic active materials for redox flow batteries holds great promise for stationary electro-

Organodisulfide About �1 V [30]

Reaction mechanisms Redox reactions Redox potential/V vs. SHE Ref. 2e�, 2Hþ redox reactions pH dependent, ranging from 0.56 to 0.75 [25]

2e�, 2Hþ redox reactions 0.2 [26]

2e�, 2H<sup>þ</sup> redox reactions �0.73 for R ¼ OH [27]

Organic radicals About 0.5 V in carbonate electrolyte [28]

Organic radicals �0.45 V [29]

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

chemical energy storage.

112 Redox - Principles and Advanced Applications

6. Semi-solid redox flow batteries

Table 4. Selected redox active organic compounds used for redox flow batteries.

Figure 4. An illustration of a semi-solid redox flow battery with solid materials stored statically in the tank, and redox shuttle molecule (SM) circulated with electrolyte.
