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

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 degrees C (Co9S8@CT-800) showed highly bifunctional catalytic activity for O-2 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.