Fine Co Nanoparticles Encapsulated in a N-Doped Porous Carbon Matrix with Superficial N-Doped Porous Carbon Nanofibers for Efficient Oxygen Reduction.

Abstract

Herein, we develop a novel method to synthesize evenly dispersed fine Co nanoparticles (CoNPs) (particle size of ∼42 nm) encapsulated in a N-doped porous carbon matrix (NPCM) with superficial N-doped porous carbon nanofibers (NPCNF) (denoted as Co@NPCM/CNF-850) as an oxygen reduction reaction (ORR) electrocatalyst. Such an electrocatalyst is the direct pyrolysis product of the novel pine needle-like ZIF-67-based metal-organic framework nanowire array (MOFNWA) prepared using an inorganic cobalt carbonate hydroxide (Co(CO3)0.5(OH)·0.11H2O) nanowire array as a linear sacrificial template, which is totally different from the traditional method, that is, using inorganic salts to synthesize MOF particles. Because of the high dispersibility of the effective fine N-doped carbon-wrapped CoNPs (rather than the overlarge CoNP aggregates); the unique linear MOF-derived assemblies, which are beneficial to electronic transmission; the high degree of graphitization, which is attributed to the superficial NPCNF and carbon layers wrapping the CoNPs; as well as the high porosity, our catalyst showed remarkable ORR activity (Eonset of 1.033 V vs the reversible hydrogen electrode) in alkaline solution. Besides, our catalyst revealed excellent stability and tolerance of methanol. Furthermore, on the basis of the X-ray absorption near-edge structure, extended X-ray absorption fine structure, and linear sweep voltammetry data, we first provided proof that a catalyst devoid of obvious Co-Nx can have superior ORR activity.