Development of Co/Co9S8 metallic nanowire anchored on N-doped CNTs through the pyrolysis of melamine for overall water splitting

Highlights: • Co/Co₉S₈ nanowires encapsulated in MWCNT were synthesized using melamine pyrolysis. • Melamine act as the source of both carbon and nitrogen. • Presence of sulfur in Co dramatically improves the electrocatalytic activity. • Carbon bamboo nanotubes (CBNT) and MWCNT structures evolve during the pyrolysis. • Co/Co₉S₈/CNT generated 10 mAcm⁻² water splitting current at 1.5 V cell voltage. -- Abstract: Herein, we report the successful synthesis of Co₉S₈ metal-sulfide nanowire trapped in multi walled carbon nanotube (MWCNT), which was subsequently found to be an effective catalyst for water splitting. Melamine pyrolysis together with a Co precursor results in MWCNTs, with the addition of sulfur during synthesis enhancing the surface area, pore size, and oxygen vacancy defects in the nanotubes. The hierarchical structure of Co/Co₉S₈/CNT products boosted the electron mobility and mass transport for both the oxygen evolution (OER) and hydrogen evolution reaction (HER) in alkaline medium. Doping the catalyst surface with Pyridinic-N atoms, Graphitic-N atoms and thioamide S-atoms dramatically improved the bifunctional electrocatalytic performance by lowering the overpotentials for OER and HER reactions. The Co/Co₉S₈/CNT generated a current density of 10 mAcm⁻² water-splitting current by applying a cell voltage of only 1.5 V. Further, Co/Co₉S₈/CNT showed excellent stability. The mechanism of Co₈S₉ nanowire formation in the MWCNT was also investigated.