Moderated surface defects of Ni particles encapsulated with NiO nanofibers as supercapacitor with high capacitance and energy density

Nickel oxide is a promising material for supercapacitors owing to its high theoretical specific capacitance; however, its practical capacitance is far below the theoretical limit. In this work, we report a novel Ni/NiO composite supported by carbon nanofibers as a pseudocapacitor electrode. Characterization of this sample by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analysis, and contact angle measurements revealed that Ni nanoparticles were uniformly dispersed on the surface of the nanofibers, leading to strong metal–metal oxide interactions and the formation of oxygen vacancies. Such three dimensional hetero-Ni/NiO components afford high conductivity owing to efficient electron transport and abundant surface defects (oxygen vacancies), which result in enhanced supercapacitor performance and energy density (ED). A moderate concentration of oxygen vacancies is crucial for achieving optimized electrochemical activity. As-prepared Ni/NiO-3 nanofibers generated high capacitances of 526 and 400 F/g at current densities of 1 and 10 A/g, respectively, with good stability (80% of the initial capacitance retained after 1000 cycles). Moreover, an ED as high as 65.8 Wh/kg was achieved at a power density of 900 W/kg, which is higher than those of NiO-based supercapacitors. This work provides a strategy for improving the potential of metal oxides for energy storage applications.