Redox-Modulated Host–Guest Complex Realizing Stable Two-Electron Storage Viologen for Flow Battery
Yufeng Liu, Xianzhi Yuan, Mingbao Huang, Zhipeng Xiang, Shuzhi Hu, Zhiyong Fu, Xuhong Guo, Zhenxing Liang
Abstract
Viologen is investigated as the anolyte in pH-neutral aqueous flow batteries. The full utilization of two electrons is essential to achieve a high specific capacity, which is, however, seriously limited by the proton attack of the two-electron reduced viologen. In this work, a redox-modulated host–guest complex strategy is developed to address the above issue by introducing hydroxyethyl-β-CD (HE-β-CD) in the 1-methyl-1′-[3-(trimethylammonio)propyl]-4,4′-bipyridinium trichloride ((MAPVi)Cl3) solution. The oxidation state (MAPVi3+, MAPVi2+) stays in water and is electrochemically active in the redox transition. The two-electron reduced state (MAPVi+) turns hydrophobic and self-adapts in HE-β-CD’s cavity via a host–guest interaction; therefore, MAPVi+ can be well-protected against the proton attack. To demonstrate the effect, an aqueous flow battery is tested with (MAPVi)Cl3 as the anolyte and 2,2,6,6-tetramethylpiperidin-1-oxy derivative as the catholyte. A 500 cycle test shows that the capacity decay rate is 0.22% per cycle for a mere 0.10 M (MAPVi)Cl3 and 0.05% per cycle for 0.10 M (MAPVi)Cl3 + HE-β-CD. A higher concentration of 0.30 M (MAPVi)Cl3 + HE-β-CD delivers a specific capacity of 14.1 Ah L–1 and an energy efficiency of 82% at 40 mA cm–2 with superior cycling stability over 100 cycles. This work provides a facile supramolecular chemistry strategy for stabilizing electroactive compounds in aqueous flow batteries.