Ultrasmall 2 D CoxZn2-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 CoxZn2-x(benzimidazole)(4) [CoxZn2-x(bim)(4)] bimetallic metal-organic framework (MOF) nanosheets (approximate to 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 CoxZn2-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 O-2 absorptivity, which is endowed by the abundant Co-N-x 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 e(g) orbital energy of Co-II 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.