Reversible metal ionic catalysts for high-voltage aqueous hybrid zinc-manganese redox flow batteries

Aqueous zinc-manganese redox flow batteries are promising candidates for next-generation electrical energy storage systems, but the low voltage and inherent limitations hinder their practical use. Here, the aqueous hybrid zinc-manganese redox flow battery (ZMFB) is reported with an acidic manganese redox couple (Mn2+/MnO2(s)) and an alkaline zinc redox couple (Zn/[Zn(OH)4]2-), showing a high operating voltage of 2.75 V at a state of charge 100%. We found that the reversibility and kinetics of the Mn2+/MnO2 redox couple can be enhanced by 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 at 20 mA cm-2 and an energy density of 25.2 Wh L-1, as well as high reversibility of Mn2+/MnO2 redox couple with the reversible reaction of catalysts. Notably, the hybrid ZMFB with BCF-MIC electrode shows an energy efficiency of 89.8% at 15 mA cm-2. This study opens a new opportunity for the development of zinc-manganese redox flow batteries and should be of immediate benefit for large-scale energy storage systems.