Earthworm-like N, S-Doped carbon tube-encapsulated Co9S8 nanocomposites derived from nanoscaled metal–organic frameworks for highly efficient bifunctional oxygen catalysis

Abstract

Herein, a novel earthworm-like N, S-doped carbon nanotube-encapsulated Co₉S₈ hybrid was developed using the metal–organic framework (MOF) UiO-66-NH₂ as a nanoscaled template via an immiscible two-phase incorporation. Highly efficient heteroatom-doping in graphene stacking generated a number of joints in the nanotubes, which harvested numerous defects as active sites for oxygen catalysis. Moreover, the Co₉S₈ nanoparticles were found to be tightly defined in the graphitic carbon shell, which were triggered by the spatial confinement of UiO-66-NH₂. This nanostructure greatly enhanced the reaction kinetics and the stability of the catalyst. The carbon nanotube-Co₉S₈ composites via pyrolysis at 800 °C (Co₉S₈@CT-800) showed highly bifunctional catalytic activity for O₂ catalysis processes. The low overpotential for ORR obtained for Co₉S₈@CT-800 was comparable to that of commercial Pt/C, and the small Tafel slope of 72 mV dec⁻¹ for OER was better than that obtained for conventional RuO₂. Density functional theory calculations revealed that the high activity of the developed nanocomposites for oxygen electrocatalysis in an alkaline medium was attributed to the synergistic effects of Co₉S₈ and the N-doped graphitic shell.