Lattice Defect and N, S-Co-Doped Engineering : Improved Commercial Carbon Nanotubes for Hydrogen and Oxygen Evolution, Oxygen Reduction, and Zinc-Air Batteries

Multifunctional electrocatalysts are desirable for overall water splitting in the energy conversion and storage fields. In this work, N, S-co-doped carbon nanotubes (CNT-NS) were obtained using a superior cost-effective method. The low-cost multi-walled CNTs were activated under a standard air atmosphere at 550°C; and ammonium chloride, ammonium bicarbonate, thiamine hydrochloride, and thiocarbamide were used to carbonize the CNT-NS. Ammonium chloride and ammonium bicarbonate provided NH3 and HCl atmospheres, which etched the carbon nanotubes, forming defects, while thiamine hydrochloride and thiocarbamide polymerized the N and S dopants onto the CNT surfaces. Due to the synergistic effects of numerous defects on the CNT surfaces and the biatom (N and S) co-doping, CNT-NS exhibited superior behavior for electrocatalysis of HER, OER, and ORR in alkaline media. Operando Raman spectroscopy measurements indicated that the S atoms served as the intrinsic catalytic sites for HER and OER, and the defective CNT structures enhanced the electron-transfer process and promoted catalytic kinetics. Density functional theory (DFT) results also indicated the contribution of binary atom co-doping for low ΔGH* values. Therefore, this work provides reference values for future rational design and construction of metal-free catalysts for overall water splitting