Surface Engineering Route to Non-pyrolysis MOFs with High-Density Co–N_x 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 three-dimensional (3D) conductive network for oxygen reduction reaction (ORR) catalysis. The surface engineering strategy employs a π-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.