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108 Redox - Principles and Advanced Applications battery demonstrated herewith allows high concentration reactants, fast reaction rates and a high peak power density (0.795 W cm2) [13]. Among various electrical energy storage technologies, redox flow batteries generally have relatively low energy density (for instance about 30 Wh L1 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 consid- ered to be used in stationary energy storage. To increase the energy density, highly water- soluble 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 theoret- ical energy density of 322 Wh L1 [15], which may even rival batteries based on lithium-ion chemistry (LiFePO4 cathode, 223 Wh L1). 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 L1 have been achieved [16]). Thus, redox flow batteries may find applications even in portable electronics and electric vehicles. 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 temper- ature should be reached for selecting a suitable electrolyte. Ce4þ/Ce3þ 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 anPDF Image | Redox Flow Batteries Fundamentals and Applications
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