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 400F/g at current densities of 1 and 10A/g, respectively, with good stability (80% of the initial capacitance retained after 1000 cycles). Moreover, an ED as high as 65.8Wh/kg was achieved at a power density of 900W/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.