Chemical Structure Control of Metal-Reduced Organic Framework-Supported Nitrogen-Doped Graphene Catalyst for Oxygen Reduction Reaction

Abstract

Nitrogen-doped graphene (N-G) has a huge potential in many electrochemical systems such as fuel cells due to the high raw material cost of platinum group metal (PGM) catalysts. However, there is a significant gap between the current level of electrochemical performance of N-G catalysts and that of PGM catalysts. Here we report an advanced metal-reduced organic framework-supported N-G catalyst (N-G/MOF-x) prepared by functionalizing ZIF-8 and N-G using nanoscale high energy wet ball milling method. The chemical structure control of the N-G/MOF was studied by characterizing the variation of elemental composition and chemical bonding composition of synthesized samples throughout targeted grinding speeds. The results proved that the chemical interaction between ZIF-8 and N-G caused the reduction of nitrogen, oxygen and zinc atoms, and also the variation of chemical bonding composition in N-G/MOF-x. The reduction rate of zinc was gradually increased with the increasing grinding speed, and reached to 82% of zinc loss at 650 RPM. The characterization of carbon and nitrogen bonding composition confirmed that the loss of nitrogen, oxygen and zinc atoms was caused by the decomposition of the C-N-Zn heteroatom contents in ZIF-8 and the O-containing functional groups in N-G, and influenced by the grinding speed. The decomposition ZIF-8 not only influenced the pore structure, but also modified the chemical structure and the surface distribution of N functional groups-constituted active sites in N-G/MOF-x. The N-G/MOF catalyst has comparable electrochemical performance to 10 wt% Pt/C catalyst. The successful accomplishment of the N-G/MOF catalysts will provide the substantial way to the cost-effective and fuel-efficient energy conversion system.