One-Step Conversion from Core-Shell Metal-Organic Framework Materials to Cobalt and Nitrogen Codoped Carbon Nanopolyhedra with Hierarchically Porous Structure for Highly Efficient Oxygen Reduction.

Rational design of porous structure is an effective way to fabricate the nonprecious metal electrocatalysts (NPMCs) toward oxygen reduction reaction (ORR) with high activity comparable or even superior to Pt-based electrocatalysts. Herein, we demonstrate a facile synthetic route to fabricate cobalt and nitrogen codoped carbon nanopolyhedra with hierarchically porous structure (Co,N-HCNP) by one-step carbonization of core-shell structured ZIF-8@ZIF-67 crystals. The resultant Co,N-HCNP electrocatalyst possesses a unique hierarchically micro/mesoporous structure with internal micropores and external mesopores, of which sufficient exposure and accessibility of ORR active sites can be achieved due to the large specific surface area and efficient transport pathway. More importantly, the existence of ZIF-8 core in the core-shell structured ZIF-8@ZIF-67 can promote the homogeneous pyrolysis of ZIF-67 shell, leading to a uniform distribution of Co-N active sites for Co,N-HCNP. As a result, the well-designed Co,N-HCNP electrocatalyst exhibits remarkable ORR activity with a high onset potential comparable to the commercial Pt/C, a half-wave potential of 0.855 V (9 mV more positive than that of Pt/C), and a kinetic current density of 63.84 mA cm at 0.8 V (2.3-fold enhancement compared with that of Pt/C) in alkaline electrolyte. Furthermore, the Co,N-HCNP electrocatalyst also presents outstanding electrochemical durability and methanol tolerance in comparison with Pt/C. The unique hierarchically porous structure of Co,N-HCNP achieved in this work provides a new insight into the design and synthesis of nanoarchitecture with targeted pore structure and opens a new avenue for the synthesis of highly efficient NPMCs for ORR.