Mo-Doped Zn, Co Zeolitic Imidazolate Framework-Derived Co9S8 Quantum Dots and MoS2 Embedded in Three-Dimensional Nitrogen-Doped Carbon Nanoflake Arrays as an Efficient Trifunctional Electrocatalysts for the Oxygen Reduction Reaction, Oxygen Evolution Reaction, and Hydrogen Evolution Reaction

Herein, we first propose a facile strategy to synthesize Co₉S₈ and MoS₂ nanocrystals embedded in porous carbon nanoflake arrays supported on carbon nanofibers (Co₉S₈-MoS₂/N-CNAs@CNFs) by the pyrolysis of Mo-doped Zn, Co zeolitic imidazolate framework grown on carbon nanofibers and subsequent sulfuration. The electrocatalyst shows high and stable electrocatalytic performance, with a half-wave potential of 0.82 V for oxygen reduction reaction and an overpotential at 10 mA cm^-2 for oxygen evolution reaction (0.34 V) and hydrogen evolution reaction (0.163 V), which outperform the metal-organic framework-derived transition metal sulfide catalysts reported so far. Furthermore, the Co₉S₈-MoS₂@N-CNAs@CNFs are employed as an air cathode in a liquid-state and all-solid-state zinc-air battery, presenting high power densities of 222 and 96 mW cm^-2, respectively. Such excellent catalytic activities are mainly owing to the unique three-dimensional structure and chemical compositions, optimal electronic conductivity, adequate surface area, and the abundance of active sites. Thus, this work provides an important method for designing other metal-organic framework-derived three-dimensional structural sulfide quantum dot multifunctional electrocatalysts for wider application in highly efficient catalysis and energy storage.

Surface-engineered cobalt nitride composite as efficient bifunctional oxygen electrocatalyst

Efficient and low-cost bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the practical application of rechargeable metal-air batteries. In this work, we developed an efficient cobalt nitride hybrid bifunctional electrocatalyst, which consists of sulfur-doped and mildly oxidized Co_5.47N nanoparticles supported on nitrogen-doped reduced graphene oxide sheet (O-S-Co_5.47N@N-RGO). The composite exhibits good ORR-OER catalytic activity and excellent stability as well. It delivers an ORR half-wave potential of 0.82 V and an over-potential of 380 mV for OER at 10 mA cm^-2 in 0.1 M KOH electrolyte. Density functional theory calculations indicate that the ORR activity of the composite might have originated from the Co-N₄ site in the RGO sheet, whereas the surface Co sites on O-S-Co_5.47N crystal are responsible for its OER activity. The facile preparation method and insight into the ORR-OER active sites reported in this study advances the development of high-performance bifunctional oxygen electrocatalyst.