Holey 2D Nanosheets of Low-Valent Manganese Oxides with an Excellent Oxygen Catalytic Activity and a High Functionality as a Catalyst for Li–O₂ Batteries

Holey 2D nanosheets of low-valent Mn₂O₃ can be synthesized by thermally induced phase transition of exfoliated layered MnO₂ nanosheets. The heat treatment of layered MnO₂ nanosheets at elevated temperatures leads not only to transitions to low-valent manganese oxides but also to the creation of surface hole in the 2D nanosheet crystallites. Despite distinct phase transitions, highly anisotropic 2D morphology of the precursor MnO₂ material remains intact upon the heat treatment whereas the diameter of surface hole becomes larger with increasing heating temperature. The obtained holey 2D Mn₂O₃ nanosheets show promising electrocatalyst performances for oxygen evolution reaction, which are much superior to that of nonporous Mn₂O₃ crystal. Among the present materials, the holey Mn₂O₃ nanosheet calcined at 500 °C displays the best electrocatalyst functionality with markedly decreased overpotential, indicating the importance of heating condition in optimizing the electrocatalytic activity. Of prime importance is that this material shows much better catalytic activity for Li–O₂ batteries than does nonporous Mn₂O₃, underscoring the critical role of porous 2D morphology in this functionality. This study clearly demonstrates the unique advantage of holey 2D nanosheet morphology in exploring economically feasible transition metal oxide-based electrocatalysts and electrodes for Li–O₂ batteries.