Enhanced Intrinsic Catalytic Activity of λ-MnO₂by Electrochemical Tuning and Oxygen Vacancy Generation

Chemically prepared λ-MnO₂has not been intensively studied as a material for metal–air batteries, fuel cells, or supercapacitors because of their relatively poor electrochemical properties compared to α- and δ-MnO₂. Herein, through the electrochemical removal of lithium from LiMn₂O₄, highly crystalline λ-MnO₂was 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 LiMn₂O₄synthesis; a concentration of oxygen vacancies in LiMn₂O₄could be characterized by its voltage profile as the cathode in a lithiun–metal half-cell. λ-MnO_2−zprepared with the highest OV exhibited the highest diffusion-limited ORR current (5.5 mA cm⁻²) among a series of λ-MnO_2−zelectrocatalysts. 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 λ-MnO₂by the generation of OVs. This study demonstrates that differences in the electrochemical behavior of λ-MnO₂depend on the preparation method and provides a mechanism for a unique catalytic behavior of cubic λ-MnO₂.