Insights into the Electrochemical Behavior of Manganese Oxides as Catalysts for the Oxygen Reduction and Evolution Reactions: Monometallic Core-Shell Mn/Mn3O4

Abstract

Overcoming the sluggish electrode kinetics of both oxygen reduction and evolution reactions (ORR/OER) with non-precious metal electrocatalysts will accelerate the development of rechargeable metal-air batteries and regenerative fuel cells. The authors investigated the electrochemical behavior and ORR/OER catalytic activity of core-porous shell Mn/Mn3O4 nanoparticles in comparison with other manganese dioxides (β- and γ-MnO2), and benchmarked against Pt/C and Pt/C-IrO2. Under reversible operation in O2-saturated 5 M KOH at 22 °C, the early stage activity of core-shell Mn/Mn3O4 shows two times higher ORR and OER current density compared to the other MnO2 structures at 0.32 and 1.62 V versus RHE, respectively. It is revealed that Mn(III) oxidation to Mn(IV) is the primary cause of Mn/Mn3O4 activity loss during ORR/OER potential cycling. To address it, an electrochemical activation method using Co(II) is proposed. By incorporating Co(II) into MnOx, new active sites are introduced and the content of Mn(II) is increased, which can stabilize the Mn(III) sites through comproportionation with Mn(IV). The Co-incorporated Mn/Mn3O4 has superior activity and durability. Furthermore, it also surpassed the activity of Pt/C-IrO2 with similar durability. This study demonstrates that cost-effective ORR/OER catalysis is possible.