Reversible Metal Ionic Catalysts for High-Voltage Aqueous Hybrid Zinc-Manganese Redox Flow Batteries

Redox flow batteries (RFB) have been considered as a promising candidate for large-scale electrochemical energy storage systems (EES) due to excellent scalability, high efficiency, and long lifetime. Among the various types of redox flow batteries, the aqueous zinc-manganese batteries have attracted tremendous attention due to its intrinsically abundant resources, low-cost and environmentally friendliness. The development of the rechargeable zinc-manganese batteries has focused on the reversible Zn ions insertion and extraction into MnO₂ structures. However, the aqueous zinc-based batteries have fundamental problems such as the dissolution of manganese ions, which usually causes the structure collapse of MnO₂, eventually resulting in a potential degradation with low electrochemical performances. Accordingly, the aqueous-based Mn²⁺/MnO₂ redox couple has been studied to avoid the insertion and deintercalation of zinc ions into crystalline MnO₂, which could prevent the undesirable phase transformation of crystalline MnO₂ structures. Moreover, the acidic and alkaline electrolytes in the aqueous hybrid batteries have been reported to overcome the low operation voltage and effectively use the Mn²⁺/MnO₂ redox couple with a high redox potential (<i>E</i> =1.228 V vs. standard hydrogen electrode (SHE)).<br/>The critical issue for the aqueous-based Mn²⁺/MnO₂ redox couple is low reversibility because of the incomplete dissolution of MnO₂ with the exfoliated dead MnO₂ species. The dead MnO₂ species eventually block the ion transport channel of the membrane and are considered an irreversible species with the main reason for the decreased active areas. Herein, we report the aqueous hybrid zinc manganese redox flow battery (ZMFB) with double-membrane-based three-electrolytes, using an alkaline zinc redox couple (Zn/[Zn(OH)₄]²⁻) and an acidic manganese redox couple (Mn²⁺/MnO₂(s)), showing a high operating voltage of 2.75 V at the state of charge 100%. To improve the manganese redox couple kinetics, we utilized the bismuth nanoparticle embedded carbon felt (BCF) electrode and metal ionic catalysts (MIC) consisting of nickel and magnesium ions. This unique strategy demonstrated the outstanding performance in the aqueous hybrid ZMFB for 150 cycles corresponding to 35 hours at 20 mA cm⁻². It also demonstrated an energy density of 25.2 Wh L⁻¹ with respect to catholyte volume at a concentration of 1 M manganese. Notably, the hybrid ZMFB with BCF-MIC electrode shows an energy efficiency of 89.8% at 15 mA cm⁻². The BCF improved the electrodeposition of MnO₂ by creating atomically reconstructed Bi nanocrystals from dissolved Bi³⁺ ions. Simultaneously, the use of Ni²⁺ ionic catalysts in catholyte help to maintain the high oxidation state of manganese, resulting in high energy efficiency and long cycle performances with the reversible reaction of metal ionic catalysts. We believe that our approach for the high-voltage aqueous hybrid ZMFB is advantageous for the development of stable and large-scale energy storage systems.<br/>Acknowledgments<br/>This research was supported by Pusan National University BK21 Four Education and Research Division for Energy Convergence Technology. Minsoo Kim is grateful financial support from Hyundai Motor Chung Mong-Koo Foundation. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2021R1C1C1008349, NRF-2021R1A4A1022198)