Recently, redox flow batteries (RFBs) have attracted attention as a large-scale energy storage technology. To improve their energy density, we investigated organic-based active materials with high water solubility, synthesized regular dendritic structures comprising viologen molecular assemblies, and applied them to RFBs. The compounds containing 3, 5, and 13 viologen molecular units showed electrolysis activity corresponding to the number of units, and it was found that the redox reaction progressed quantitatively. Furthermore, the charge and discharge characteristics confirmed that the energy efficiency was improved compared to methyl viologen batteries. These compounds have high solubility, and the viologen derivatives can function in acidic conditions, in which methyl viologen does not work as a redox active material, due to reduced interaction between molecules and the cation exchange membrane. Finally, molecules having 5 and 13 viologen units can be prepared to concentrations of 1 and 0.5 M, theoretically reaching a capacity of 134 and 174 Ah/L, respectively.

Summary The application of hydrocarbon‐based polymer electrolyte membranes as an alternative to Nafion membranes for vanadium redox‐flow batteries (VRFBs) has been investigated. Ionic conductivity measurements and the crossover characteristics of vanadyl ions (VO2+), as well as durability testing against V5+ in H2SO4 solution, were performed to investigate the applicability of sulfonated poly(ethersulfone) (S‐PES) membranes with high molecular weight to VRFB. The diffusion coefficient of V4+ in S‐PES was lower than that in Nafion, and the ionic conductivity was comparable to that of Nafion in S‐PES with an ion‐exchange capacity (IEC) of 1.5 meq/g. A durability evaluation of V5+ with high oxidizing power indicated that it was extremely stable compared to the sulfonated poly(etheretherketone). It was clarified that by setting the IEC to an appropriately high value and reducing the membrane thickness, the membrane characteristics can be made comparable to those of Nafion. Finally, a VRFB test was performed using S‐PES with controlled IEC and membrane thickness, whereby the high molecular weight S‐PES was shown to have relatively good performance comparable to Nafion.

Large-scale storage batteries include lithium-ion batteries, sodium-sulfur batteries, lead-acid batteries, and redox flow batteries (RFBs),1-3 and these storage batteries have been used for distributed power systems in various places depending on the size and application. Above all, although RFBs4,5 have technical issues, including energy density lower than that of other storage batteries, and their vanadium content makes them expensive, they have long life and high design flexibility. In addition, they operate at normal temperature and pose no danger of thermal runaway or explosion. Therefore, the advantages of RFBs have been attracting attention as one way to achieve power leveling for renewable energy. More recently, research on alternative technology, such as organic, Ti-Mn,6 or hybrid RFBs,7-9 has become active, and in terms of performance, some alternative RFBs, especially organic RFBs, have become comparable to high-cost vanadium RFBs. One of the greatest features of organic RFBs is that it is possible to increase the energy density by controlling the solubility and redox potential based on molecular design, and pioneering and unique research has been reported so far.10-14 The electron transfer reaction rate of the active material in an organic RFB is larger than that in a vanadium RFB, and the reactivity with the carbon electrode is relatively good. For this reason, continued development of RFBs is expected to provide a new power storage technology that can flexibly cope with being combined with other secondary batteries and hydrogen production technologies. Currently, we are focusing on viologen units and have newly synthesized an assemblage of viologen molecules with relatively high symmetry and a regular structure. We are adopting that concept for active material design and aim at improving RFB performance by increasing both the solubility of the designed viologen assembly and the redox response of the individual introduced viologen molecules. Here, we report the application of this idea to aqueous RFBs using newly synthesized viologen molecular units for an anolyte with a relatively high symmetry and regular structure. [1] B. Zakeri, and S. Syri, Renewable and Sustainable Energy Reviews, 42, 569 (2015). [2] F. Shi, Reactor and Process Design in Sutainable Energy Technology, Elsevier (2016). [3] B. Dunn, H. Kamath, and J. -M. Tarascon, Science, 334, 928 (2011). [4] J. Noack, N. Roznyatovskaya, T. Herr, P. Fischer, Angew. Chem., Int. Ed., 54, 9776 (2015). [5] G. L. Soloveichik, Chem. Rev., 115, 11533 (2015). [6] Y. R. Dong, H. Kaku, K. Hanafusa, , K. Moriuchi, T. Shigematsu, ECS Trans., 69, 59 (2015). [7] Y. Xu, Y. Wen, J. Cheng, G. Cao, Y. Yang, Electrochem. Commun., 11, 1422 (2009). [8] X. Wei, W. Xu, M. Vijayakumar, L. Cosimbescu, T. Liu, V. Sprenkle, W. Wang, T. Adv. Mater., 26, 7649 (2014). [9] B. Huskinson, M. P. Marshak, C. Suh, S. Er, M. R. Gerhardt, C. J. Galvin, X. Chen, A. Aspuru-Guzik, R. G. Gordon, M. J. Aziz, Nature, 505, 195 (2014). [10] B. Yang, L. Hoober-Burkhardt, F. Wang, G. K. S. Prakash, S. R. Narayanan, J. Electrochem. Soc. 161, A1371 (2014). [11] J. Winsberg, C. Stolze, S. Muench, F. Liedl, Martin D. Hager, U. S. Schubert, ACS Energy Lett. 1, 976 (2016). [12] K. Lin, R. Gómez-Bombarelli, E. S. Beh, L. Tong, Q. Chen, A. Valle, A. Aspuru-Guzik, M. J. Aziz, R. G. Gordon, Nat. Energy, 1, 16102 (2016). [13] A. Hollas, X. Wei, V. Murugesan, Z. Nie, B. Li, D. Reed, J. Liu, V. Sprenkle, W. A Wang, Nat. Energy, 3, 508 (2018). [14] T. Janoschka, N. Martin, U. Martin, C. Friebe, S. Morgenstern, H. Hiller, M. D. Hager, U. S. Schubert, Nature 527, 78 (2015).

To suppress CO2 emissions, technologies that incorporate CO2 capture, utilization, and storage are being actively developed. Particularly, batteries using CO2 redox reactions are one of the most promising systems that combine energy conversion and storage. However, the stoichiometric reactions of CO2 and metal restrict the CO2 storage capacity of previously proposed systems. Applying catalyst-mediated CO2 redox is expected to provide flexible control of the CO2 storage capacity without dependence on the metal content. Herein, we report novel aqueous flow battery using CO2–formate redox with a bifunctional homogeneous Ir catalyst. Using of an Ir catalyst bearing a 4-hydroxy-N-methylpicolinamidate ligand was demonstrated for 50 cycles, with a maximum discharge capacity of 1.5 Ah L-1, capacity decay of 0.20% cycle-1, and total turnover number of 3,500. Furthermore, as the design concept, the CO2 storage capacity of the catalyst-based flow battery was improved more than 1000 times compared to the previously proposed system.