Pairing nitroxyl radical and phenazine with electron-withdrawing/-donating substituents in "water-in-ionic liquid" for high-voltage aqueous redox flow batteries

Aqueous redox-active organic materials-base electrolytes are sustainable alternatives to vanadium-based electrolyte for redox flow batteries (RFBs) due to the advantages of high ionic conductivity, environmentally benign, safety and low cost. However, the underexplored redox properties of organic materials and the narrow thermodynamic electrolysis window of water (1.23 V) hinder their wide applications. Therefore, seeking suitable organic redox couples and aqueous electrolytes with a high output voltage is highly suggested for advancing the aqueous organic RFBs. In this work, the functionalized phenazine and nitroxyl radical with electron-donating and electron-withdrawing group exhibit redox potential of -0.88 V and 0.78 V vs. Ag, respectively, in "water-in-ionic liquid" supporting electrolytes. Raman spectra reveal that the activity of water is largely suppressed in "water-in-ionic liquid" due to the enhanced hydrogen bond interactions between ionic liquid and water, enabling an electrochemical stability window above 3 V. "Water-in-ionic liquid" supporting electrolytes help to shift redox potential of nitroxyl radical and enable the redox activity of functionalized phenazine. The assembled aqueous RFB allows a theoretical cell voltage of 1.66 V and shows a practical discharge voltage of 1.5 V in the "water-in-ionic liquid" electrolytes. Meanwhile, capacity retention of 99.91% per cycle is achieved over 500 charge/discharge cycles. A power density of 112 mW cm-2 is obtained at a current density of 30 mA cm-2. This work highlights the importance of rationally combining supporting electrolytes and organic molecules to achieve high-voltage aqueous RFBs. (c) 2022 Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).