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100 Natl Sci Rev, 2017, Vol. 4, No. 1 REVIEW is not ideal for large-scale applications. Some of the solvents and salts such as acetonitrile, TFSI– or PF6– will also significantly increase the cost. In ad- dition, non-aqueous RFBs are normally operated at lower current densities because of lower electrolyte conductivity, which significantly affects the cell ef- ficiency and power density, and thus increases the cost. The low current density (therefore low power) inherently leads to increased stack size, thereby af- fecting the overall cost. Recent calculations have shown that all the organic-chemistry-based battery systems studied to date may not be promising for low E/P applications because the lower power densities do not allow for a small stack area. However, organic-chemistry-based systems show good promise for high E/P applica- tions provided the concentration can be increased and divalent or more active species are used. The cost of the solvent and the active materials would still be a key factor, however. In a high E/P case of 48 hours of storage with optimistic component costs, 2 M concentration and two electrons per mol, the total cost comes to $80/kWh—a path- way to a system that costs less than the $100/kWh goal. One possible direction for future research for both aqueous and non-aqueous redox systems could come from the experience in solid organic electrode materials for Li-ion batteries [77,78]. For inorganic materials, the redox reaction is based on the va- lence change of the transition-metal or elemental substance, while, for organic materials, the redox re- action is related to the charge state change of the electroactive organic group or moiety. Generally, or- ganic materials potentially used for batteries could be divided into several groups: conjugated hydrocar- bon, conjugated amine, conjugated thioether, organ- odisulfide, thioether, nitroxyl radical and conjugated carbonyl. Their reaction mechanisms are shown in Table 2 [78]. Among the seven types of structures, organodisulfide and conjugated carbonyl belong to n-type organic materials (electron transport), con- jugated amine and conjugated thioether belong to p-type organics (hole-transport), while conjugated hydrocarbons and nitroxyl radicals belong to bipolar organic materials. A reversible redox reaction usually takes place in an organic group or moiety with con- jugated structure and atoms with lone-pair electrons, such as O, N and S, because a conjugated structure is beneficial to the electron transportation and charge delocalization, and lone-pair electrons usually have a higher reaction activity. Based on each structure listed in Table 2, the re- dox molecules could be rationally designed. Some basic requirements for these organic candidates in- clude the following factors: r Electrochemical reaction reversibility. This factor can determine cell polarization. Among the struc- tures shown in Table 2, nitroxyl radical presents the highest reaction kinetics. On the contrary, organodisulfide and thioether show slow reaction kinetics because the bond breaking/forming of the S–S or S = O requires a high-activation en- ergy. r Redox potential. This factor determines the side (anode or cathode) on which the organics can be used. Generally, p-type organics (e.g. nitroxyl radical) have a higher redox potential than n-type organics (e.g. conjugated carbonyl). In addition, as discussed above for quinone-based organ- ics, electron-withdrawing and electron-donating groups can effectively tune the redox potential. r Solubility. Small organic molecules tend to dis- solve in either aqueous or non-aqueous elec- trolytes. In addition, a proper substituent (e.g. hy- drophilic groups for aqueous systems) or a change of structure symmetricity can effectively increase the solubility of organic materials. r Chemical stability. This factor is very important for organic materials used in long-term applica- tions. Degradation of some radical compounds (e.g. FL–) is closely associated with the nature of solvents and salts. For example, FL– fades faster in acetonitrile solvents and salts containing BF4– be- cause of side reactions [76]. In addition, a lack of suitable membranes is one of the major technical hurdles impeding develop- ment of non-aqueous flow batteries. For porous separators, crossover of redox materials could cause irreversible performance degradation. Two size exclusion-based strategies have been attempted to overcome this limitation. The first strategy involves reducing the membrane pore size to increase selec- tivity. Helms et al. fabricated a porous membrane from polymers of intrinsic microporosity (PIM) with 0.8-nm pore size [79]. This PIM membrane yielded a 500-fold increase of polysulfide-blocking ability in a Li/S battery compared to Celgard (∼17-nm pore). The second strategy is to put redox moieties onto polymeric backbones. Moore et al. demonstrated that increased size of viologen- bearing polymers greatly improves the selectivity of Celgard [80]. This method also was tried with water-soluble polymers in an aqueous flow battery [81]. However, the conductivity of PIM membranes and the solubility of redox polymers still remain a concern. In some non-aqueous and hybrid systems, Li- ion-conducting ceramic membranes also are used; however, such membranes have limited ion con- ductivity and are difficult to manufacture and scale Downloaded from https://academic.oup.com/nsr/article/4/1/91/2866462 by guest on 11 January 2023PDF Image | Progress in low cost redox flow batteries energy storage
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