Development of a Zn-Mn aqueous redox-flow battery operable at 2.4 V of discharging potential in a hybrid cell with an Ag-decorated carbon-felt electrode

The Zn-Mn redox pair has great potential as a next-generation redox flow battery (RFB) because of its economic strength and capability to conduct safe reactions. However, because of the potential window restriction of the aqueous system, maximizing the performance of the system with a Zn-Mn redox pair is difficult. Hence, in this study, a hybrid RFB cell with three-electrolyte chambers is designed to expand the potential window using different electrolytes with varying pH values in the anolyte and catholyte. In the three-electrolyte chamber cell, a buffer electrolyte is utilized between the anolyte and catholyte, and the system exhibits a high discharge potential of approximately 2.4 V. Despite this success, stability problems still arise because of MnO2 precipitation in the electrolyte. Therefore, the cathode is further modified using an Ag-based metal-organic framework (MOF). The stability of the carbon-felt electrode is also improved when the cathode is decorated with an Ag electrocatalyst. The potential plateau during the discharge sustained at 2.3–2.4 V, while the cell is maintained over 550 mAh/gMnO₂ of specific capacity. Accordingly, the system design with a three-electrolyte chamber cell and cathode modification can enable the Zn-Mn redox reaction to provide stable and superior cell performance during charge-discharge cycles.