Surface Engineering Route to Non-pyrolysis MOFs with High-Density Co-NX Sites and 3D Conductive Networks for Efficient Oxygen Reduction

The poor electronic conductivity of metal-organic framework (MOF) materials hinders their direct application in the field of electrocatalysis in fuel cells and metal-air batteries. Herein, we present an effective and scalable surface engineering strategy to produce a non-pyrolysis metal-organic framework (MOFs)-based catalyst with high-density Co-Nx active sites and a threedimensional (3D) conductive network for oxygen reduction reaction (ORR) catalysis. The surface engineering strategy employs a pi-conjugated amino-rich hexaaminotriphenylene (HITP) ligand to modify the surface of the zeolite imidazolate skeleton material (ZIF-67) through the coordination of Co in ZIF-67 with N in HITP to construct robust Co coordination sites. The results show that the HITP ligand not only modifies the surface of a single ZIF polyhedron but also connects two or multiple ZIF polyhedrons, constructing a 3D electronic conductive network that is beneficial to facilitate electron transfer during ORR catalysis and thus increase the accessible contact between electrons and Co-Nx sites. The electronic conductivity of the hybrid catalyst was increased by six orders of magnitude than that of pure ZIF-67. The optimized catalyst shows an outstanding electrocatalytic activity for ORR with a half-wave potential of 0.82 V even comparable to commercial Pt/C-and excellent electrochemical durability. Density functional theory (DFT) calculations indicate that the HITP ligand can not only coordinate with unsaturated Co sites on ZIF-67 using its abundant N atoms but also exchange thermodynamically with the 2-methylimidazole ligand in ZIF-67. The modification of HITP also reduces the free energy barrier of the rate-determining step toward ORR catalysis, leading to an improved ORR activity for the HITP-modified ZIF-67 catalyst. This scalable surface engineering strategy represents a breakthrough in development of nonpyrolysis conductive MOF materials for ORR catalysis.