Nonaqueous organic redox flow batteries (NAORFBs) show great promise for grid energy storage but are currently facing key challenges such as high electroactive material cost and low energy density. Herein, we report the electrochemical properties and the potential application of a series of cost-effective electroactive nitrobenzene molecules in NAORFBs. Pairing the low-cost miscible liquid nitrobenzene (NB) with 2,5-di-tert-butyl-1-methoxy-4-(20-methoxyethoxy)-benzene (DBMMB) resulted in a flow battery that provides a high theoretical cell voltage of 2.2 V and a calculated energy density of 192 Wh L −1 . In the charge−discharge testing, this battery delivers a stable cycling capacity retention of 99.5% per cycle over 100 cycles and a 70% energy efficiency at 40 mA cm −2 operation current density, verifying that liquid nitrobenzene is a promising low-cost electroactive anode molecule for NAORFBs.

Aqueous organic redox flow batteries (AORFBs) employing synthetically tailorable organic electroactive compounds have received significant attention for energy storage technologies. There have been many efforts in developing electroactive materials for AORFBs with anion-exchange membranes. On the contrary, electroactive compounds that are compatible with cationexchange membranes in AORFBs are less studied. Here, we report an electroactive 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO 2 Na-TEMPO) molecule for neutral AORFBs. The compound exhibits a good solubility of 1.5 M in an aqueous sodium-based solution, which is 3 times more than that of the pristine 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO). When paired with a 1,10-bis(3-sulfonatopropyl)-4,4′-bipyridinium (SPr) 2 V anolyte, the resulting RFB operating through a cation-exchange membrane achieved an open-circuit voltage of 1.19 V and a high energy density of 14.7 W h L −1 . In the long-term cycling study, the RFB features a stable capacity retention of 99.94% per cycle over 400 cycles with nearly 100% Coulombic efficiency.

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We synthesize a covalently linked bipolar molecule 1-(2-(4methoxy-2,5-dimethylphenoxy) ethyl)-1′-methyl-[4,4′-bipyridine]-1,1′-diium hexafluorophosphate (VIODAMB) and apply this new compound to a nonaqueous organic redox flow battery (NAORFB) as both the anolyte and catholyte. We demonstrate that this symmetrical electrolyte can mitigate the cross-contamination issue in the flow battery. The compound exhibits an enhanced solubility of 0.66 M in acetonitrile. The flow cell delivers a capacity retention rate of 80% and an energy efficiency of 85% over 35 cycles at a current density of 1.5 mA cm −2 in the cycling test.

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Redox flow batteries (RFBs) that employ dissolved electroactive materials to store energy have attracted growing attention because of their great potential to store renewable but intermittent solar and wind energy. The unique technological merits of decoupled energy and power leads to high flexibility and scalability in design for the RFBs to satisfy a wide range of energy/power applications[1,2]. In established traditional RFBs, the electroactive species are aqueous solutions of redox-active inorganic transition-metal salts including all-vanadium, iron-chromium and zinc-bromine RFBs. Despite immense progress, the attainable cell voltages in these aqueous systems are largely limited by the narrow electrochemical stability window of water (1.5V). As a result, aqueous RFBs suffer from low energy densities and in turn require more electroactive materials and supporting salts to achieve high storage capacity[3,4]. To address these limitations, recent research is shifting to non-aqueous RFBs (NARFBs) that utilize non-aqueous solvents, taking advantage of the wider electrochemical windows over ~5 V depending on the organic solvent employed. A variety of electro-active species including organic redox-active molecules and organometallic complexes have been used in NARFBs[5,6]. Here, we will introduce our research efforts on the development of boron-based redox active anode molecules for non-aqueous redox flow batteries. We have investigated the electrochemical properties of boron-based anode molecules with different substitution groups and the cyclic voltammetry (CV) results indicate that the boron-based redox active molecule with di-tert-butyl substituted (DTBF2) exhibits superior electrochemical reversibility and stability and low half wave potential of -1.82V. When paired with the catholyte 2,5-ditert-butyl-1-methoxy-4-[2′-methoxyethoxy]benzene (DBMMB), the cell voltage can reach up to 2.56V. The built redox flow cell demonstrates an energy efficiency of 80% and capacity retention of 99.3% per cycle, as shown in figure 2. These initial results proved that this readily synthesized boron-based redox active anode is promising anolyte for NARFBs. More experimental data and results will be disclosed during the meeting. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) References: [1] M. Park, J. Ryu, W. Wang, J. Cho, Nat. Rev. Mater. 2016, 2, 16080. [2] D. Larcher, J. M. Tarascon, Nat. Chem. 2015, 7, 19-29. [3] J. Winsberg, T. Hagemann, T. Janoschka, M. D. Hager, U. S. Schubert, Angew Chem Int Ed Engl 2017, 56, 686-711. [4] J. Zhang, G. Jiang, P. Xu, A. Ghorbani Kashkooli, M. Mousavi, A. Yu, Z. Chen, Energy Environ. Sci. 2018, 11, 2010-2015. [5] Y. Ding, C. Zhang, L. Zhang, Y. Zhou, G. Yu, Chem Soc Rev 2018, 47, 69-103. [6] X. Wei, W. Pan, W. Duan, A. Hollas, Z. Yang, B. Li, Z. Nie, J. Liu, D. Reed, W. Wang, et al., ACS Energy Lett. 2017, 2, 2187-2204. Figure 1