Insights into the Electrochemical Behavior of Manganese Oxides as Catalysts for the Oxygen Reduction and Evolution Reactions: Monometallic Core-Shell Mn/Mn 3 O 4 .

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/Mn 3 O 4 nanoparticles in comparison with other manganese dioxides (β- and γ-MnO 2 ), and benchmarked against Pt/C and Pt/C-IrO 2 . Under reversible operation in O 2 -saturated 5 M KOH at 22 °C, the early stage activity of core-shell Mn/Mn 3 O 4 shows two times higher ORR and OER current density compared to the other MnO 2 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/Mn 3 O 4 activity loss during ORR/OER potential cycling. To address it, an electrochemical activation method using Co(II) is proposed. By incorporating Co(II) into MnO x , 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/Mn 3 O 4 has superior activity and durability. Furthermore, it also surpassed the activity of Pt/C-IrO 2 with similar durability. This study demonstrates that cost-effective ORR/OER catalysis is possible.