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and Li anode with a high operating voltage of 3.4 V.57 Initially, the Fe(NO3)3 aqueous solution was used as the catholyte, but the hydrolysis of Fe3+ gave rise to the degraded cycling stability, which could be effectively suppressed by the addition of functional additives (glycine) in a recent study.58 Since then, a variety of redox- active species have been explored as the potential positive material. Yu and co- workers employed the Br-based catholyte in designing the heavy-metal-free RFB, which could be operated in moderate acidity. Thanks to the remarkable potential discrepancy between catholyte and anolyte, such a battery was able to deliver a re- cord high voltage above 4 V.59 In addition to halides, another promising positive alternative is sulfur-based species because of their high capacity and low cost. Different from traditional static Li-S batteries, the Li-S RFB requires a high solubility of different polysulfide species during the whole charging and discharging process to ensure the high reversibility. Liu et al. proposed a new dimethyl sulfoxide (DMSO)- based electrolyte containing Li2S8 as active species to demonstrate the Li-S RFB.60 Since the Celgard membrane was used as the separator in this work, a high concen- tration of LiTFSI electrolyte was applied to alleviate the parasitic side reactions on Li anode. Additionally, to further increase the cathode capacity, a strategy of mixed liquid- and solid-based active materials was proposed and demonstrated by Lu and coworkers.61 Combining liquid LiI electrolytes with solid sulfur/carbon (S/C) composite, the mixed catholyte achieved a high volumetric capacity of 550 Ah L1 when paired with a Li anode. Up to date, a number of organic redox species are also utilized as positive molecules because of their potentially low cost, elemental abundance, environmental benig- nity, and high molecular tunability. Yu and coworkers demonstrated a high-perfor- mance Li-Fc RFB showing around 95% energy efficiency and 90 % capacity retention after 250 deep charge and discharge cycles (Figure 6C).32 The environmentally friendly and low-cost Fc/Fc+ redox couple showed the best electrochemical perfor- mance in the DMF electrolyte. Additionally, the solubility of the pristine Fc in organic electrolytes could be improved by molecular engineering.31 Except for Fc, the other metallocene molecules, such as cobaltocene, were also employed recently for nonaqueous RFB designs.62 Nevertheless, redox reactions still rely on metal centers, and to get rid of the metal-based redox species, Yu and coworkers first designed a liquid battery by using hydroquinone aqueous solutions as the catholyte and graphite as the anode.63 They further explored the potential of quinone families for the design of nonaqueous RFBs (Figure 6D).64 The electrochemical properties of quinones were comprehensively investigated via the combination of experiments and density functional theory (DFT) calculations, which were found to be strongly dependent on the molecular aromaticity and electronic structures. As a result of the stabilized effect of the adjacent aromatic ring, NQ presented the most promising performance. Another widely used organic cathode-active species is TEMPO-based active materials. In 2014, Wang et al. first reported the TEMPO-based catholyte for nonaqueous RFBs.65 The pristine TEMPO molecule showed a high concentration (2 M) in the aprotic electrolyte with the Li supporting salts (LiPF6), which gave rise to a high energy density of 126 Wh L1 (Figure 6E), which far exceeds that of most state-of-the-art aqueous and nonaqueous RFBs. Na- or K-Based Hybrid RFBs Na metal, which has similar electrochemical properties to Li, a wide distribution of sources, and low cost (Na: 250 $/ton; Li: 5800 $/ton), is regarded as a suitable alter- native to replace Li-metal anodes.66 Most designs of Na-based RFBs are based on ceramic membranes to separate the Na-metal anodes from aqueous and nonaqueous catholytes. Following the previous work on the LijFe(CN)63 RFB, Kim 1976 Chem 5, 1964–1987, August 8, 2019PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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