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by dimethyl ethyl ammonium bis(trifl uoromethanesulfonyl)imide group, the ionic derivative of Fc has a more than 20 times higher solubility in the supporting electro- lyte (Figure 4F). Moreover, two methyl substituents are also found to lower the sym- metry of Fc and thus significantly reduce the melting point of Fc from 172C–174C to 37C–40C for 1,1-dimethylferrocene (DMFc) (Figure 4G).33 However, DMFc still has low solubility in the presence of LiTFSI, and the Li/DMFc hybrid cell can only be operated at high temperature.33 Replacement of LiTFSI with a more compatible sup- porting salt of tetraethylammonium tetrafluoroborate (TEABF4) gives rise to a higher concentration (3–4 M in 1.0 M LiTFSI in ACN) and hence a larger volumetric capacity. Large-Size Organic Redox Species Crossover of redox species not only causes low Coulombic efficiency of RFBs but also in- duces degraded cycling performance. Via fine molecular tuning, the size of electroactive molecule can be increased without affecting its electrochemical activity, which opens a new avenue to alleviate crossover issues in RFBs. Currently, the most widely used ion-ex- change membranes, such as Nafion and Fumasep, are costly and account for $40% of the stack cost. Alternative cost-effective membranes include microporous separators, nanofiltration membranes, and dialysis membranes, all of which are in demand of large-size redox molecules based on physical blocking mechanisms (Figure 4H). Hen- driks et al. recently reported an oligomeric catholytes based on tris(dialkylamino)cyclo- propenium (CP) salts that are specifically tailored for pairing with size-exclusion mem- branes composed of polymers with intrinsic microporosity (PIMs). Initial evaluation of a series of dimeric derivatives revealed that an n-butyl spacer is required between the redox-active moieties in order to achieve electrochemical stability (Figure 4I). Further studies indicate that increasing the oligomer size from monomers to tetramers imposed no penalty on the theoretical charge capacity while coinciding with only a small reduction in the solution diffusion and electron self-exchange rates. Pairing with cost-effective size- exclusion membranes, a proof-of-concept RFB demonstrated good capacity retention through the first 17 cycles (<5% loss) with an average Coulombic efficiency of 95% and an energy efficiency of 79%.34,35 Moreover, to address the problems associated with cross contamination in an organic RFB, a bipolar redox species has been designed as both positive and negative redox- active materials. Such a strategy is similar with traditional vanadium RFBs in which only vanadium-based species take part in redox reactions. Moreover, it also simplifies the preparation and production techniques of chemicals for RFBs. Schubert and co- workers connected two nitronyl-nitroxide-based redox-active units by a tetraethylene glycol chain to form a bipolar an organic molecule.36 The theoretical capacity and the solubility of the two units were twice as high as those of the single unit. The static solu- tion-based battery exhibits charge and discharge plateaus at 1.67 and 1.45 V, respec- tively. After 75 charge-discharge cycles, a capacity decay of $20% was observed.36 In addition to nitronyl nitroxide, diaminoanthraquinones (DAAQs),37 2,3,7,8-tetracyano- 1,4,5,6,9,10-hexazaanthracene (TCAA),38 3-phenyl-1,5-di-p-tolylverdazyl,39 and boron dipyrrin (BODIPY) derivative40 molecules have already been investigated in the symmet- ric nonaqueous RFBs as well. However, most of these redox species have low solubility and exhibit rapid decomposition during cycling. Further effort should be devoted to tuning the molecular structure to ensure good solubility and stability of the redox-active materials in different charge states. EUTECTIC-BASED RFBs In addition to molecular engineering of organic species, the concept of eutectic- type redox species in the field of RFBs has been explored as a potential strategy Chem 5, 1964–1987, August 8, 2019 1971PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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