Synthesis and growth mechanism of bamboo like N-doped CNT/Graphene nanostructure incorporated with hybrid metal nanoparticles for overall water splitting

Herein, we report a melamine and metal-salt based pyrolysis technique for synthesizing metal encapsulated N-doped carbon nanotube (CNTs) in form of bamboo-like CNTs and multi walled CNTs (MWCNT). Sulfur doping during synthesis greatly influenced the physio-chemical properties of the material formed. X-ray diffraction (XRD) analysis confirms NiCo alloy (NiCo@CNT) formation that transformed into a hybrid NiCo/Co₃Ni₆S₈/Co₃O₄ nanocomposite (NiCoS@CNT) in presence of sulfur. A detailed study was conducted on the mechanism of the formation of metal-encapsulated N-doped CNT structures from the polymerization of melamine. The unique NiCoS@CNT structure renders high specific surface area (232.2 m²/g), large pore volume (0.92 cm²/g), and high lattice defect with abundant oxygen vacancies resulting in excellent performance for OER and HER in alkaline medium. The hybrid catalyst requires over-potentials of 198 mV and 295 mV to deliver a current-density of 10 mAcm⁻², respectively for HER and OER. A cell voltage of only 1.53 V was required to deliver a long-term stable current-density of 10 mAcm⁻² for water splitting when NiCoS@CNT was used as both anode and cathode. Superior performance of NiCoS@CNT could be ascribed to high surface area, abundant active sites, fast charge-transfer rate, high pyridinic-N content and the presence of highly conductive CNT architecture.