Challenges and advances in redox flow batteries utilizing sustainable and cost-effective non-vanadium active materials

The key parameters for grid-scale energy storage systems (ESSs) are safety, longevity, and cost-effectiveness. Aqueous redox flow batteries (RFBs) are good candidates for grid-scale ESSs because of the prospect of long-term stability while offering cost-effectiveness due to the use of cheap, non-volatile aqueous electrolytes. The currently most widely used RFB, the vanadium RFB (VRFB), is limited for even wider commercialization by its high acquisition cost and the temperature sensitivity of vanadium electrolytes. To overcome those limitations, RFBs, utilizing redox-active materials that can be cheaply prepared and have strong resistance to temperature, are a viable option. As new redox-active materials, current trends in low-cost redox active materials, quinones, iron-based complexes, and iodide are highlighted. Their pros and cons are summarized, and appropriate preparation methods for their electrolytes are summarized, while the performance and stability of RFBs utilizing these electrolytes are scrutinized.