MOFs derived metallic cobalt-zinc oxide@nitrogen-doped carbon/carbon nanotubes as a highly-efficient electrocatalyst for oxygen reduction reaction

Abstract

In this work, a series of metallic Co-ZnO encapsulated in N-doped carbon/carbon nanotubes (Co-ZnO@NC/CNT-T) for the electrochemical oxygen reduction reaction (ORR) were synthesized by annealing bimetallic metal-organic frameworks (MOFs) containing Co and Zn in N2 atmosphere. Within the hybrid, metallic Co was integrated with ZnO and encapsulated by NC, while coral-like CNTs grew out of the obtained Co-ZnO@NC nanoparticles. When applied in ORR catalysis, the optimized Co-ZnO@NC/CNT-700 obtained in 700 °C displayed a half-wave potential of ~0.86 V and a limiting current density of ~−5.98 mA cm−2, which were even superior to 20 wt% Pt/C. Further results demonstrated that the presence of ZnO was closely related with the high ORR catalytic activity from two aspects. (i) ZnO catalyzed the formation of Co0, Co2+, pyridinic and graphitic N as the main active species for ORR. (ii) ZnO promoted the growth of CNTs and micro/mesoporous structure, which improved the electrochemical active surface area (ECSA) and mass transfer during the ORR process.