Nitrogen-, Oxygen-and Sulfur-Doped Carbon-Encapsulated Ni3S2 and NiS Core Shell Architectures: Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Reduction Reactions

Developing cost-effective and energy-efficient noble-metal-free bifunctional electrocatalysts for clean and renewable energy conversion systems, such as fuel cells and water splitting devices, has been highly desirable nowadays. Herein, we report the successful synthesis of N-, O-, and S-doped carbon encapsulated Ni,S, and NiS core-shell architectures (Ni3S2/NiS/NOSCs) by pyrolysis of S- and Ni(II)-containing polypyrrole solid precursors, producing carbon-encapsulated Ni3S2 composites (Ni3S2/NOSCs), followed by the conversion of the Ni3S2 core's surface into NiS shell with concentrated HCl solution. The materials are proven to serve as bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in alkaline media. Notably, Ni3S2/NiS/NOSC-900, pyrolyzed at 900 degrees C, exhibits remarkable electrocatalytic performance toward HER with a low overpotential of 180 mV at a current density of 10 mA cm(-2), a small Tafel slope of 83 mV dec(-1), and a good long-term stability of 15 h. Moreover, it can also efficiently electrolyze ORR with good performance, affording positive onset and half-wave potentials and high electron transfer number (similar to 4), as well as robust stability and methanol crossover tolerance. The materials' excellent catalytic activities might be attributed to the synergistic effect between NOSC layers and Ni3S2/NiS core-shell nanostructures, as well as the interface effect between the NiS and Ni3S2 phases.