A comparative study of water-contained deep eutectic solvent electrolyte with different active species for enhancing the performance of redox flow batteries

To improve the performance of redox flow batteries (RFBs) using deep eutectic solvent (DES) electrolytes, this study systematically investigates the effect of water content on the physicochemical and electrochemical properties of aqueous DES electrolytes containing different active species. The redox couples studied include organic species-tetramethylpiperidine oxide (TEMPO) and quinoxaline-as well as metal-based species (Fe and V ions). The experimental results show that the strategic addition of water effectively reduces viscosity and density, while significantly enhancing the ionic conductivity of DES electrolytes. Electrochemical analysis determines the optimal water content to be 10 wt% for inorganic species and 20 wt% for organic species. Furthermore, this work evaluates the performance of aqueous DES-based RFBs employing the aforementioned active species. Full-cell tests demonstrate that a TEMPO-VCl 3 hybrid RFB with 10 wt% water exhibits the optimal overall performance, achieving 90% coulombic efficiency, 77% energy efficiency (at 5 mA cm −2 ), and excellent cycling stability. In contrast, the all-organic TEMPO-quinoxaline RFB, despite its high discharge power density, suffers from poor cycling stability due to material degradation and unavoidable side reactions. This study confirms the critical role of water content regulation in optimizing DES-based electrolytes and suggests the promising application of organic-inorganic hybrid RFBs with aqueous DES electrolytes.