A High-Voltage Aqueous Zinc-Based Redox Flow Batteries with Reversible Metal Ionic Catalysts and Bi-Modal Tin and Copper Clusters

With the increasing interest of renewable energy sources, the electrical energy storage (EES) system has received great attention to solve uncertainty of power input and output. Among many EES systems, a redox flow battery is the most promising candidate due to design flexibility for large scale. Recently, low cost and earth abundant transition metal based redox couples have attracted significant attention to replace the expensive vanadium redox couples. Conventional Mn-based aqueous zinc-ion battery is highly safe energy storage system, but low operating voltage (&lt;1.5V) and low energy density limits its broad application in large-scale energy storage system. Recently, a novel aqueous zinc-manganese dioxide redox flow battery is reported. This battery is composed of Mn²⁺ based acidic catholyte and Zn²⁺ based alkaline anolyte, respectively, exhibiting an open circuit voltage of 2.66 V. Also, a neutral electrolyte was located between two different membranes to prevent the cross contamination of acidic and alkaline electrolytes, maintaining the charge balance. However, Mn³⁺ ions suffer from the disproportionation side reaction, lowering battery performance. Thus, the low reversibility of Mn/MnO₂ redox couple should be improved for broad application of Zn-Mn redox flow batteries. In addition, the formation of zinc dendrite should be suppressed when the zinc-based anolyte was used, which could deteriorate the cell performance.<br/>In this work, we report a high voltage aqueous Zn−Mn hybrid redox flow battery with a high operating voltage of 2.75 V at the 100% state of charge. This work is a significant step forward by exploring the pH differential hybrid flow cell with the double-membrane, three-electrolyte configuration. The manganese cathode was deposited on carbon felt electrode with bismuth using carbothermal reduction process. Moreover, metal ionic catalysts are added as solution catalysts. We found that the reversibility of Mn ions was improved, and side reactions were suppressed by adding the metal ionic catalysts, maintaining a higher oxidation state of Mn ions. As a result, our works suggest that the zinc-manganese dioxide redox flow battery can be operated at high voltage with the excellent reversibility. Considering zinc dendrite issues, we introduced the bi-modal sized tin and copper (Sn, Cu) incorporated carbon felt in the negative electrode. The zinc nucleation overpotential was lowered by surface modified carbon felt electrode, also minimizing hydrogen evolution.<br/><br/><b>Acknowledgements</b><br/>This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2021R1C1C1008349, NRF-2021R1A4A1022198). This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE)(20214000000140, Graduate School of Convergence for Clean Energy Integrated Power Generation). This work was supported by BK21 FOUR Program by Pusan National University Research Grant, 2021.