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coworkers reported a membrane-free Zn-Br cell97 in which the generated Br2 was localized at the bottom of the chamber because of its higher density and low water miscibility; therefore, the parasitic side reactions between Zn and Br2 could be mini- mized. The color change of the electrolyte in discharged and charged state also proves the self-segregation effect during the cell operation. To further isolate the positive and negative redox species, two immiscible aqueous- nonaqueous electrolytes were investigated as well. In order to design such a mem- brane-free battery, the charge carriers should be soluble in both electrolyte phases. Yan et al. firstly proposed such a novel design by employing Zn anode in an aqueous electrolyte paired with Fc/Fc+ in a nonaqueous solvent of butyl acetate; meanwhile, a hydrophobic ionic liquid was added as the supporting salt (Figure 10D).98 The nonaqueous catholyte is thermodynamically immiscible with the aqueous anolyte; therefore, the expensive membranes can be removed. When the cell works, chloride ions will commute between aqueous and nonaqueous phases to balance the charge. Moreover, Marcilla and coworkers reported a membrane-free RFB by using redox species dissolved in two immiscible electrolytes (Figure 10E).99 They found that an interphase barrier was formed between a hydroquinone aqueous solution and a hy- drophobic ionic liquid solution of BQ (Figure 10F). In contrast to hybrid RFBs using one solid electrode based on stripping-deposition reactions, such a system with all redox species dissolved in solutions maintains higher scalability. The energy density was estimated to be 22.5 Wh L1, and the operating voltage was 1.2 V. Nevertheless, it should be pointed out that the power density still lags behind in comparison with commercial aqueous RFBs.17 Some basic chemistry questions—including the parti- tion coefficients of positive and negative redox species, the potential self-discharge reactions at the interface, and the diffusion processes of charge carriers in both of the two immiscible solvents—should be elucidated clearly in the future research. CONCLUSION AND OUTLOOK To date, various types of redox species and cell architectures have been designed for next-generation RFBs. Despite tremendous advances made, more effort is still needed to implement those devices for widespread applications. Given the advan- tages of potential low cost, minimal environmental footprint, and highly tailorable properties, organic molecules are regarded as a promising type of electroactive ma- terials to replace conventional metal-based redox species. However, the demon- strated energy and power density have yet to be improved to rival vanadium RFBs, and sometimes the complicated synthesis processes will complicate increased chemical cost.17 Compared with conventional solute molecules requiring large amounts of solvents to accomplish the solvation process, the eutectic redox species can be formed via eutectic mixture of ionic components, removing the need for sol- vent molecules. On the one hand, highly concentrated eutectic electrolytes offer an opportunity to achieve an ultrahigh energy density. On the other hand, the super- concentrated solution is accompanied by high viscosity, leading to considerable po- tential hysteresis and degraded energy efficiency. By adopting alkali-metal anodes in RFBs, such a novel design combines the concept and merits of alkali-ion batteries (high energy density) and RFBs (great scalability). However, the uncontrollable dendrite growth and continuous parasitic reactions should be addressed before the practical applications of alkali metals. In suspension-based RFBs, the well-stud- ied Li-ion battery electrodes can be applied in RFBs, and this approach greatly stretches the research boundaries and hybridizes the technological features of various types of energy storage systems. The photochargeable RFBs can achieve simultaneous energy harvesting and storage, and such an integrated device 1984 Chem 5, 1964–1987, August 8, 2019PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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