A low-cost all-iron hybrid redox flow batteries enabled by deep eutectic solvents

Redox flow batteries (RFBs) emerge as highly promising candidates for grid-scale energy storage, demonstrating exceptional scalability and effectively decoupling energy and power attributes. Nevertheless, the high cost of vanadium metal hinders the continued commercialization of vanadium redox flow batteries (VRFBs), prompting the exploration of low-cost all-iron RFBs as a viable alternative. In this context, we propose an innovative deep eutectic-based all-iron hybrid RFBs. By synthesizing a deep eutectic solvent through the mixture of choline chloride, ethylene glycol, and an appropriate amount of deionized water, the deficiencies in mass transport performance of the deep eutectic electrolyte have been successfully addressed, while enhancing its conductivity. The deep eutectic solvents facilitate the restructuring of the solvent shell surrounding the negative electrolyte Fe2+, effectively suppressing its hydrolysis. Additionally, the synergistic interaction between the restructured solvation structure and the chelating ring structure formed between Fe2+ and glycine improves the deposition morphology of iron. Simultaneously, utilizing a eutectic-based positive electrolyte improves the solubility of active substances while maintaining excellent cycling stability and mass transport performance. The entire battery system exhibits an average coulombic efficiency exceeding 98 % in a 360-hour charge-discharge cycle at 10 mA/cm2. In the first 66 cycles, the coulombic efficiency remains at 100 %, and the energy efficiency approaches 54 %. This indicates that the battery system developed using deep eutectic solvents demonstrates promising applications within the same category of eutectic-based batteries.