Enhanced Intrinsic Catalytic Activity of λ-MnO2 by Electrochemical Tuning and Oxygen Vacancy Generation

Chemically prepared lambda-MnO2 has not been intensively studied as a material for metal-air batteries, fuel cells, or supercapacitors because of their relatively poor electrochemical properties compared to alpha- and delta-MnO2. Herein, through the electrochemical removal of lithium from LiMn2O4, highly crystalline lambda-MnO2 was prepared as an efficient electrocatalyst for the oxygen reduction reaction (ORR). The ORR activity of the material was further improved by introducing oxygen vacancies (OVs) that could be achieved by increasing the calcination temperature during LiMn2O4 synthesis; a concentration of oxygen vacancies in LiMn2O4 could be characterized by its voltage profile as the cathode in a lithiun-metal half-cell. lambda-MnO2-z prepared with the highest OV exhibited the highest diffusion-limited ORR current (5.5 mAcm(-2)) among a series of lambda-MnO2-z electrocatalysts. Furthermore, the number of transferred electrons (n) involved in the ORR was > 3.8, indicating a dominant quasi-4-electron pathway. Interestingly, the catalytic performances of the samples were not a function of their surface areas, and instead depended on the concentration of OVs, indicating enhancement in the intrinsic catalytic activity of lambda-MnO2 by the generation of OVs. This study demonstrates that differences in the electrochemical behavior of lambda-MnO2 depend on the preparation method and provides a mechanism for a unique catalytic behavior of cubic lambda-MnO2.