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Organic-Based Eutectic Redox Species Organic electroactive molecules are currently attracting much attention as a prom- ising redox species for RFBs because of the vast molecular diversity and elemental abundance.17,51 Although significant efforts have been devoted to improving the solubility of organic-based species by functionalization, it remains a big challenge to achieve high concentration. Organic synthesis for molecular engineering may be time consuming, complicated, and technically unaffordable.17 Distinguished from the laborious and expensive organic synthesis approaches to tailor the chem- ical and physical properties, the preparation of eutectic solvents represents another promising avenue to modify organic molecules especially in terms of solubility. Moreover, the mechanism behind this approach can be fundamentally elucidated in light of the molecular interactions and coordination environments, which provides guidelines for building broad organic-based eutectic electrolytes. Therefore, as a promising alternative, the adoption of eutectic concept is emerging to elevate the molar fraction of active species in organic-based electrolytes. The adoption of eutectic concept on highly concentrated organic redox species was demonstrated by Hase and coworkers.52 By mixing 4-methoxy-2,2,6,6-tetra-methy- piperidine 1-oxyl (MT) with LiTFSI, they obtained the orange-colored catholyte with a high concentration above 2 M at room temperature (Figure 5E). Furthermore, a small amount of water can break up MT-TFSI- and MT-Li+ associations, and thus the ions can move more freely. Therefore, with the increase in water molar ratio, both the utilization of MT active species and the discharge voltage were increased. For the anolyte design, Yu and coworkers recently utilized the eutectic concept to enhance the reversibility and solubility of phthalimide (Ph) derivatives.53 Interacting with both urea and LiTFSI salt, N-methylphthalimide (NMePh) achieved a nearly 6- fold enhanced solubility. The strong interaction between electrophilic C=O groups of NMePh and Li+ cations from LiTFSI was proved to be the driving force for the for- mation of eutectic liquid (Figure 5F). In addition, urea could help stabilize the coor- dination geometry in LiTFSI-NMePh-urea eutectic electrolyte and decrease the vis- cosity. In aqueous systems, the improved solubility of electroactive materials interacting with urea has been also observed.54 By changing the hydrogen-bonding environments, the solubility of hydroquinone (H2BQ) in the hydrotropic solution was enhanced three times (Figure 5G). Coupled with the Al DES anolyte, the hybrid bat- tery showed an elevated energy density of 25.3 Wh L1 with the adoption of hydro- trope-enhanced catholyte. It is noted that this hydrotropic effect of urea could be further extended to other promising organic electroactive materials. ALKALI-METAL-BASED RFBs Alkali metals are widely considered as one of the most promising anode candidates because of their high theoretical capacities and low redox potentials.55 A typical configuration of RFBs with alkali-metal anodes is shown in Figure 6A, in which organic electrolytes or buffer layers are usually applied to prevent the corrosion from alkali metals. Thus, integrating the advantages of typical flow and alkali-metal batteries, this hybrid RFB design offers a great flexibility for the development of high-performance energy storage technologies. Compared with traditional aqueous RFBs with limited operating voltage, the application of alkali-metal anodes can high- ly increase the working voltage to the level of Li-ion batteries. Currently, different types of alkali metals, including Li, Na, K, and related bimetallic alloys (Li-Na or Na-K), have been reported for RFBs. The possible positive redox species for alkali- metal-based RFBs are summarized in Figure 6B. It is noted that the combination of alkali-metal anodes with aqueous catholytes can give rise to some potential safety 1974 Chem 5, 1964–1987, August 8, 2019PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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