Ultrasmall 2 D Cox Zn2-x (Benzimidazole)4 Metal-Organic Framework Nanosheets and their Derived Co Nanodots@Co,N-Codoped Graphene for Efficient Oxygen Reduction Reaction.

The development of nonprecious metal-nitrogen-carbon (M-N-C) materials with efficient metal utilization and abundant active sites for the oxygen reduction reaction (ORR) is of great significance for fuel cells and metal-air batteries. Ultrasmall 2 D Cox Zn2-x (benzimidazole)4 [Cox Zn2-x (bim)4 ] bimetallic metal-organic framework (MOF) nanosheets (≈2 nm thick) are synthesized by a novel bottom-up strategy and then thermally converted into a core-shell structure of sub-5 nm Co nanodots (NDs) wrapped with 2 to 5 layers of Co,N-codoped graphene (Co@FLG). The size of the Co NDs in Co@FLG could be precisely controlled by the Co/Zn ratio in the Cox Zn2-x (bim)4 nanosheet. As an ORR electrocatalyst, the optimized Co@FLG exhibits an excellent half-wave potential of 0.841 V (vs. RHE), a high limiting current density of 6.42 mA cm-2 , and great stability in alkaline electrolyte. Co@FLG also has great ORR performance in neutral electrolyte, as well as in Mg-air batteries. The experimental studies and DFT calculations reveal that the high performance of Co@FLG is mainly attributed to its great O2 absorptivity, which is endowed by the abundant Co-Nx and pyridinic-N in the FLG shell and the strong electron-donating ability from the Co ND core to the FLG shell. This elevates the eg orbital energy of CoII and lowers the activation energy for breaking the O=O/O-O bonds. This work sheds light on the design and fabrication of 2 D MOFs and MOF-derived M-N-C materials for energy storage and conversion applications.