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the cathode and anode materials, respectively.80 In such a design, two pairs of redox species, namely dibromoferrocene (FcBr2)/Fc and cobaltocene (CoCp2)/bis(penta- methylcyclopentadienyl)cobalt [CoCp*2], are adopted as the mediators for cathode and anode separately. Cyclic voltammograms show that the potentials of electrodes lie in between the mediators, which enables the chemical reactions between the electrodes and mediators in charge and discharge processes. It has been calculated that the energy density can reach nearly 500 Wh L1, which is ten times larger than that of conventional vanadium RFBs. To simplify the molecule selection and eliminate the voltage loss, Wang et al. took several strategies to achieve the target reactions of electrode materials solely by us- ing one redox mediator. It is known that iodide undergoes two successive oxidation reactions in nonaqueous electrolytes based on I/I3 ($3.15 V versus Li/Li+) and I3/ I2 ($3.70 V versus Li/Li+), and the redox potential of LiFePO4 sits right between, mak- ing it applicable to drive both the delithiation and lithiation of LiFePO4 (Figure 8D).81 Though an energy density up to 670 Wh L1 is anticipated, the power density only reaches 0.5 mW cm2 because of the poor ionic conductivity of the membrane and low concentration of target molecules. Likewise, Wang and coworkers found that an organic mediator, 2,3,5,6-tetramethyl-p-phenylenediamine (TMPD), exhibits similar bifunctional properties with two pairs of peaks (Figure 8E), making it possible to construct the LiFePO4-based RFBs.82 Moreover, considering the activity change of the redox molecules upon reducing and oxidizing, the mediator of a Fc-function- alized ionic liquid species possesses a wide Nernstian potential difference, which can drive both the charging-discharge processes of LiFePO4 as well.83 Figure 8F shows the energy diagram with detailed electron transfer processes, in which elec- trons will transport from the valence band of LiFePO4 to the lowest unoccupied mo- lecular orbital of the Fc+-based redox molecule (RM+) to extract Li ions from LiFePO4 and vice versa in the discharging procedure. It has been calculated that the voltage efficiency can reach 95% with an energy density of 330 Wh L1. In addition to nonaqueous RFBs, the redox-targeted strategy can also be applied in aqueous systems. Wang et al. found that the redox potential of [Fe(CN)6]4/ [Fe(CN)6]3 exactly matches that of LiFePO4 in an aqueous electrolyte, and so is the case of S2/S22 with LiTi2(PO4)3.84 In light of the Nernstian potential-driven redox targeting reactions introduced above, a full RFB was built, and it delivered a volumetric capacity up to 305 Ah L1 based on LiFePO4 and 207 Ah L1 based on LiTi2(PO4)3. Compared with the nonaqueous counterparts, the aqueous systems show much lower expenses, higher ionic conductivity, and more convenient cell as- sembly procedures. PHOTOCHARGEABLE RFBs In addition to redox species, the cell structure has witnessed some great progress as well, especially integrated energy conversion and storage systems. Unlike nonre- newable fossil fuels, solar energy as a renewable source of energy is the most impor- tant energy source for all life forms in the world. The vast amounts of solar energy available on earth make it a highly appealing source of electricity for human beings. Given the intermittent nature of solar energy, simultaneous conversion and storage of the electricity generated are important to deliver dispatchable power on demand, especially in remote sites. RFBs can be charged through photoelectrochemical reac- tions over photoelectrodes, and these are known as photochargeable RFBs. Photo- chargeable RFBs can simultaneously convert and store solar energy into electrical energy, which is a promising alternative to tackle the intermittency problem of 1980 Chem 5, 1964–1987, August 8, 2019PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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