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98 Natl Sci Rev, 2017, Vol. 4, No. 1 REVIEW should be studied, particularly those that can be produced from natural products. For example, Liu et al. investigated an aqueous organic flow battery using methyl viologen as the anolyte and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4- HO-TEMPO) as the catholyte [58]. The OCV is 1.25 V, which is similar to that of the VRB. Both organic compounds are highly soluble in water, with solubility over 2 M. The use of an anion-exchange membrane significantly reduces crossover of the cationic redox species, leading to high cycling sta- bility. Similar studies were performed by Schubert’s group [59]. NON-AQUEOUS RFBs In aqueous RFBs, the cell potential is constrained by water electrolysis. Therefore, some recent efforts have shifted to developing non-aqueous RFBs be- cause of the potential to achieve high energy den- sities due to their wider electrochemical windows. Current non-aqueous RFB research is primarily fo- cused on finding viable redox materials for various flow chemistries, such as metal-ligand complexes and organic redox molecules. Figure 5a shows the re- dox potentials (vs. Li/Li+) of representative redox- active organic and organometallic compounds [60]. The organic and organometallic compounds have a wide potential window in the range of 0.7–4 V. The redox potential (vs. Li/Li+ ) of promising candidates used as anode and cathode materials should be lower than 2 V and close to or higher than 4 V, respectively, to achieve high cell voltage (>2 V). Some of the or- ganic materials in Fig. 5a, with proper modifications to tailor redox potential, solubility and electrochem- ical activity, have potential for non-aqueous RFBs. Organometallic flow battery The first study of an RFB with a metal-ligand com- plex ([Ru(bpy)3 ]2+ ) was performed by Matsuda et al. [61]. This flow chemistry exhibited an OCV of 2.6 V and low efficiency of 40%. Thereafter, other metal-ligand (with the metals being Ru, V, Cr, Mn, Fe and Co, and the ligands being bpy, acac, aca- cen, phen, mnt) non-aqueous RFBs were studied [62–69]. However, most metal-ligand, non-aqueous RFBs suffer from low CE due to chemical degrada- tion of the redox molecules and poor solubility. To date, the most soluble transition-metal complexes examined for non-aqueous RFB applications involv- ing multiple electron transfers reach saturation at 0.8 M in acetonitrile. Single-electron transfer com- plexes reach 1.8 M in carbonate solvents. Their en- ergy densities are much lower than those of aqueous systems, although cell potential is larger (>2.0 V). The complicated synthetic procedures, the inherent low solubility and poor chemical stability of metal- ligand complexes have gradually shifted the atten- tion of the research community to all-organic redox molecules. All-organic flow battery All-organic flow batteries can take advantage of wide structural diversity and availability of redox molecules from natural abundant resources. The first all-organic RFB was built by Li et al., which used 2,2,6,6,-teramethyl-1-piperidinylxy (TEMPO) and N-methylphthalimide in acetonitrile as catholytes and anolytes, respectively [70]. NaClO4 was added as a conductive salt. Brushett et al. at Argonne National Laboratory (ANL) proposed another all-organic, non-aqueous RFB with 2,5-Di-tert- butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB) and 2,3,6-trimethylquinoxaline as the catholyte and anolyte, respectively [71]. The proof-of-concept static cell showed a low CE (70%) and EE (37%). In addition, the theoretical energy density was in the 12–16 Wh/L range and was limited by the low solubility of DBBB (0.4 M in propylene carbonate). Odom et al. assembled an all-organic flow cell that replaced DBBB with 3,7-bis(trifluoromethyl)-N- ethylphenothiazine (BCF3EPT) at the positive side [72]. BCF3EPT has a similar solubility and redox potential to DBBB. UV-vis absorption spectra measurements revealed that its radical cation was more stable than that of DBBB. Similarly to aqueous organic systems, the solu- bility of insoluble species such as AQ and ferrocene are significantly improved by incorporating oligo ethylene oxide or quaternary ammonium moieties because of their strong solvation with organic sol- vents [36,73,74]. Even fine-tuning the molecular symmetry can dramatically change the form of redox materials. In dialkoxy-di-tert-butylbenzene deriva- tives, the redox center symmetricity is maintained to keep the electrochemical stability, while the in- corporated poly ethylene oxide (PEO) chains help to improve the solubility in carbonated based po- lar electrolyte solutions [75]. These molecules show similar electrochemical reversibility with the redox potentials around 4.0 V vs. Li/Li+. Owing to the asymmetric incorporation of PEO in ANL-8 and ANL-9 and corresponding additional intramolecu- lar dipole moments in them, they have higher sol- ubility to symmetric DBBB and ANL-10. As shown in Fig. 5b, unsymmetrical ANL-8 and ANL-9 are liq- uid at room temperature, while symmetrical DBBB and ANL-10 have much a lower solubility. Among them, ANL-8 appears to be the best candidate 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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