Supramolecular polymerization-assisted synthesis of nitrogen and sulfur dual-doped porous graphene networks from petroleum coke as efficient metal-free electrocatalysts for the oxygen reduction reaction

Abstract

Efficient metal-free carbon electrocatalysts for the oxygen reduction reaction (ORR) represent a promising alternative to the scarce and costly noble metal-based counterparts, and their performance is good enough to fulfill the requirements of real-life applications. Herein, we describe the rational design and synthesis of nitrogen/sulfur-codoped porous graphene networks (N,S-PGN) as a highly efficient ORR catalyst. Assisted by supramolecular polymerization, petroleum coke, a byproduct produced by the oil refinery industry in million tons every year, can be converted into N/S-codoped graphene nanosheets with a highly porous architecture. Such a hierarchical and doped structure favors the exposure of the active sites and facilitates electron transport. As a result, N,S-PGN exhibits a remarkable electrocatalytic activity for the ORR and, in particular, outperforms the Pt/C electrode in terms of a superior diffusion-limiting current density and stability when operated under the same conditions. Furthermore, the increased number of carbon active sites and enhanced electron transport were confirmed by theoretical calculations. It was found that codoping porous graphene increases the charge density on the carbon active sites and increases the HOMO energy, thus favoring the ORR. The present study demonstrates a novel and feasible route for preparing heteroatom-codoped porous graphene for energy applications as well as a new strategy for the value-added utilization of petroleum coke.