Hollow fibrous NiCo2O4 electrodes with controllable Zn substitution sites for supercapacitors

Abstract This study provides a detailed investigation on using two hollow micron-fibrous Zn-doped NiCo2O4 materials as supercapacitor electrodes. These materials can be easily prepared using a simple hydrothermal, calcination process, employing cotton fiber as a template. By changing the content of metal salts in the precursor, the Zn substitution can be controlled in the octahedral Ni-site (NiOh) and tetrahedral Co-site (CoTd). These results are supported using XRD refinements and XPS fine spectra. Furthermore, the phase structure, morphology and pore characteristics of two Zn-doped samples are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and N2 adsorption-desorption isotherms. The improved intrinsic conductivity and interfacial diffusion are further verified by ultraviolet-visible (UV-vis) spectroscopy, density of states (DOS), and electrochemical impedance spectroscopy (EIS). When used in supercapacitors, both Zn-modified NiCo2O4 materials, despite the different in substitution sites, show a significant improvement in specific capacitance and cyclic retention. Particularly, the tetrahedral Co-site substitution results in the best electrochemical performance. The fabricated electrode at a current density of 2.0 A g−1 exhibits a high specific capacitance of 1344 F g−1 and good capacitance retention of ∼92% after 3000 cycles.