1. 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

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Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

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.

#### 2. 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.

$$\text{Negative electrode}: \text{V}^{2+} \leftrightarrow \text{V}^{3+} + \text{ e}^- \tag{1}$$

$$\text{Positive electrode}: \text{VO}\_2^{\cdot +} + 2\text{H}^+ + \text{e}^- \leftrightarrow \text{VO}^{2+} + \text{H}\_2\text{O} \tag{2}$$

$$\text{Overall reaction}: \text{V}^{2+} + \text{VO}\_2^{+} + 2\text{H}^{+} \leftrightarrow \text{VO}^{2+} + \text{V}^{3+} + \text{H}\_2\text{O} \tag{3}$$

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:

$$E = \text{ 1.26 V - RT/F} \ln\left( \left[ \text{VO}^{2+} \right] \cdot \left[ \text{V}^{3+} \right] \right) / \left( \left[ \text{VO}\_2^{+} \right] \cdot \left[ \text{H}^{+} \right]^{2} \cdot \left[ \text{V}^{2+} \right] \right) \tag{4}$$

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]:

Figure 1. A schematic of a vanadium redox flow battery: (a) charge reaction and (b) discharge reaction.

At the negative electrode:

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 technolog-

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

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

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

Negative electrode : <sup>V</sup><sup>2</sup><sup>þ</sup> \$ <sup>V</sup><sup>3</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� <sup>ð</sup>1<sup>Þ</sup>

<sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� \$ VO2<sup>þ</sup> <sup>þ</sup> H2O <sup>ð</sup>2<sup>Þ</sup>

<sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> \$ VO2<sup>þ</sup> <sup>þ</sup> V3<sup>þ</sup> <sup>þ</sup> H2O <sup>ð</sup>3<sup>Þ</sup>

<sup>þ</sup> ½ �� <sup>H</sup><sup>þ</sup> ½ �<sup>2</sup> � V2<sup>þ</sup>

ð4Þ

ions diffuse across the membrane and electrons transfer through an external circuit.

<sup>E</sup> <sup>¼</sup> <sup>1</sup>:26 V –RT=<sup>F</sup> ln VO2<sup>þ</sup> � V3<sup>þ</sup> <sup>=</sup> VO2

Figure 1. A schematic of a vanadium redox flow battery: (a) charge reaction and (b) discharge reaction.

ical success and market penetration, need to be well understood.

Positive electrode : VO2

Overall reaction : <sup>V</sup><sup>2</sup><sup>þ</sup> <sup>þ</sup> VO2

calculated based on the Nernst equation:

following [2]:

2. Classic vanadium redox flow batteries

104 Redox - Principles and Advanced Applications

$$\text{V}^{2+} + \text{VO}^{2+} + 2\text{H}^{+} \rightarrow 2\text{V}^{3+} + \text{H}\_{2}\text{O} \tag{5}$$

$$\rm{2V^{2+}} + \rm{VO\_2^{+}} + 4\rm{H^{+}} \rightarrow \rm{3V^{3+}} + \rm{2H\_2O} \tag{6}$$

$$\text{V}^{3+} + \text{VO}\_2^{+} \rightarrow 2\text{VO}^{2+} \tag{7}$$

At the positive electrode:

$$\text{V}^{2+} + 2\text{VO}\_2^{+} + 2\text{H}^{+} \rightarrow 3\text{VO}^{2+} + \text{H}\_2\text{O} \tag{8}$$

$$\text{V}^{3+} + \text{VO}\_{2}^{+} \rightarrow 2\text{VO}^{2+} \tag{9}$$

$$\rm V^{2+} + VO^{2+} + 2H^{+} \to 2V^{3+} + H\_{2}O \tag{10}$$

Side reactions such as hydrogen evolution due to water decomposition and CO2 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:

$$\text{CE} = \frac{\text{discharge capacity}}{\text{charge capacity}} \times 100\% \tag{11}$$

$$\text{VE} = \frac{\text{average discharge voltage}}{\text{average charge voltage}} \times 100\% \tag{12}$$

$$\text{EE} = \text{CE} \times \text{VE} \tag{13}$$

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.

#### 3. 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<sup>þ</sup> (349.8 S cm2 mol�<sup>1</sup> ) and OH� (198.0 S cm2 mol�<sup>1</sup> ), have the highest limiting molar conductivity among all known cations and anions, respectively [4]. All-vanadium redox flow batteries, for instance, have V3þ/V2<sup>þ</sup> redox reactions on the negative side (anolyte) and VO2 <sup>þ</sup>/VO2<sup>þ</sup> 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 Fe3þ/Fe2<sup>þ</sup> and Cr3þ/Cr2<sup>þ</sup> in an iron-chromium flow battery, the cross-contamination via ion penetration may cause irreversible performance loss.

Figure 2. Configurations of (a) a conventional redox flow battery with two divided compartments containing dissolved active species, (b) a hybrid redox flow battery with gas supply at one electrode, (c) a redox flow battery with membraneless structure and (d) a redox flow battery with solid particle suspension as flowing media.

Hybrid redox flow batteries such as zinc-bromine and zinc-cerium systems use metal stripping/plating reactions (Zn2þ/Zn, �0.76 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 Liþ/Li couple (�3.04 V vs. SHE). During charging, zinc or lithium is deposited from the electrolyte and during discharging, Zn2<sup>þ</sup> or Li<sup>þ</sup> 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 Cl2, O2 and H2 as the reaction medium or with gas evolution reaction at the cathode or anode (Figure 2b). For instance, oxygen reduction reaction (O2 þ 4H<sup>þ</sup> þ 4e� ⇄ 2H2O) 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: O2 þ 2Li<sup>þ</sup> þ 2e� ⇄ Li2O2. 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 Li2O2 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 (2V2<sup>þ</sup> <sup>þ</sup> 2H<sup>þ</sup> ! H2 <sup>þ</sup> 2V3<sup>þ</sup>) 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 [8–14]. 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. H2 (flowing across anode with pumped liquid hydrobromic acid) aqueous bromine laminar flow


Table 1. Membrane-free redox flow batteries.

Hybrid redox flow batteries such as zinc-bromine and zinc-cerium systems use metal stripping/plating reactions (Zn2þ/Zn, �0.76 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 Liþ/Li couple (�3.04 V vs. SHE). During charging, zinc or lithium is deposited from the electrolyte and during discharging, Zn2<sup>þ</sup> or Li<sup>þ</sup> dissolves into the solution again. A shortcoming of such hybrid redox flow battery is that the energy storage capability is

Figure 2. Configurations of (a) a conventional redox flow battery with two divided compartments containing dissolved active species, (b) a hybrid redox flow battery with gas supply at one electrode, (c) a redox flow battery with membrane-

A second-type hybrid redox flow batteries use gas such as Cl2, O2 and H2 as the reaction medium or with gas evolution reaction at the cathode or anode (Figure 2b). For instance, oxygen reduction reaction (O2 þ 4H<sup>þ</sup> þ 4e� ⇄ 2H2O) 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: O2 þ 2Li<sup>þ</sup> þ 2e� ⇄ Li2O2. 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 Li2O2 proceed at porous matrix, which can be held statically in a gas diffusion tank over charging/discharging. Such a concept

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.

limited by the free space inside the cell accommodating the metal deposits.

less structure and (d) a redox flow battery with solid particle suspension as flowing media.

106 Redox - Principles and Advanced Applications

may maintain the character of decoupled energy and power for flow batteries.

battery demonstrated herewith allows high concentration reactants, fast reaction rates and a high peak power density (0.795 W cm�<sup>2</sup> ) [13].

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

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 electronics and electric vehicles.
