Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs

Abstract

PERSPECTIVE This journal isAs a family member of redox-flow batteries (RFBs), nonaqueous RFBs can offer wide working temperature, high cell voltage, and potentially high energy density. These key features make nonaqueous RFBs an important complement of aqueous RFBs, broadening the spectrum of RFB applications. The development of nonaqueous RFBs is still at its early research stage and great challenges remain to be addressed before successful for practical applications. As such, it is essential to understand the major components in order to advance the nonaqueous RFB technology. In this perspective, three key major components of nonaqueous RFBs: organic solvent, supporting electrolyte, and redox pairs are selectively focused and discussed, with the emphases on providing an overview for those components and on highlighting the relationship between structure and property. Urgent challenges are also discussed. To advance nonaqueous RFBs, the understanding of both components and systems is critically needed and it calls for inter-disciplinary collaborations across expertise including electrochemistry, organic chemistry, physical chemistry, cell design, and system engineering. In order to demonstrate key features of nonaqueous RFBs, herein we also present an example of designing a 4.5 V ultrahigh-voltage nonaqueous RFB by combining BP/BP• − redox pair and ONF• + /OFN redox pair. PERSPECTIVEThis journal is less negative charge. When ions that do not follow those assumptions are used, the working principles are still applicable with minor alterations. (Reproduced with permission from Ref. 22) Figure 3. The BP-OFN nonaqueous RFB concept and its working principles. The negative electrolyte containing the BP/BP•redox pair and the positive electrolyte containing the OFN• + /OFN redox pair are separated by a Li + -conducting ceramic membrane (e.g., LiSICON). 1 M LiPF 6 is used as an example of supporting electrolyte. When the cell is being charged, BP molecules are reduced to form BP•radical anions in negative electrolyte (i.e., BP + e -= BP• -), and OFN molecules are oxidized to form OFN• + radical cations in positive electrolyte (i.e., OFN = OFN• + + e -). Meanwhile, Li + ions pass through Li + -conducting ceramic membrane from positive electrolyte to negative electrolyte. The discharging process is in reverse.

Na-β-Alumina as a Separator in the Development of All-Vanadium Non-Aqueous Tubular Redox Flow Batteries: An Electrochemical and Charging-Discharging Examination Using a Prototype Tubular Redox Flow Cell

“…The Na-β-Al 2 O 3 membrane in a 0.1 TBAP supporting electrolyte showed a slightly elongated semicircle with an increased resistance (Fig. 2B curve b) of 12.881 cm 2 , which suggests that the charge carrier could be via a standard vacancy mechanism or correlated movement of interstitial Na + ion pairs 25 or the charge balance might have occurred by size exclusion. At the same time, the cell potential between the divided (by Na-β-Al 2 O 3 ) and undivided cell containing the 0.1 M NaClO 4 supporting electrolyte in AN showed a noticeable difference.…”

“…5,7 On the other hand, the two vanadium redox couples exhibited quasi-reversible behavior under these given conditions, particularly in the TBAP and NaClO 4 -supporting electrolytes. In addition, the V(acac) 3 redox behavior varied according to the supporting electrolyte used, 31 which means that V(acac) 3 shows sluggish electron transfer in the both the TBAP and NaClO 4 supporting electrolytes, which might have influenced the cell potential variations. Importantly, the redox peak potentials in the presence of the NaClO 4 supporting electrolyte was 100 mV less than the TBAP supporting electrolyte, either in the oxidation or reduction region, support that the NaClO 4 supporting electrolyte facilitates the V(acac) 3 redox process.…”

“…In the N-ARFB system, tetra ethylammonium tetrafluoroborate (TEAB) showed the maximum suitability, particularly on reversibility, than the other electrolytes due to the small radii and high limiting oxidation potential. 31 In the present work, the cyclic voltammetry (CV) behavior of V(acac) 3 using the Na-β-Al 2 O 3 membranedivided electrochemical cell in 0.1 M tetra butylammonium perchlorate (TBAP) and NaClO 4 in AN at the glassy carbon (GC) electrode were compared, as shown in Fig. 3.…”

“…1 Although the development of N-ARFBs has attracted considerable interest, such as redox active species selection, 2 electrolyte development, 2,3 and solvent selection, 4 the identification of a suitable membrane is one of the crucial factors because of the crossover of redox active species. [4][5][6] The oxidation state of most redox active metal organic complexes (RAMOCs) is negative, which means that there is theoretically less migration on membranes with the same polarity but the transport of oppositely charged negative ions is allowed.…”

Redox Flow Batteries: Fundamentals and Applications

“…Great efforts have been made in search of alternative battery chemistry from electrolytes to electrodes [4,17,18]. The possible cell voltage depends on the selected redox couples ( Table 2) and is limited by the electrochemical window of a given solvent-electrode system, stability of the supporting cation or anion and stability of the bipolar plate materials (Figure 3).…”