Integrating large specific surface area and high conductivity in hydrogenated NiCo2O4 double-shell hollow spheres to improve supercapacitors

Abstract

Increasing specific surface area and electrical conductivity are two crucial ways to improve the capacitive performance of electrode materials. Nanostructure usually enlarges the former but reduces the later; thus, it is still a great challenge to overcome such contradiction. Here, we report hydrogenated NiCo2O4 double-shell hollow spheres, combining large specific surface area and high conductivity to improve the capacitive performance of supercapacitors. The specific surface area of NiCo2O4 hollow spheres, fabricated via programmed coating of carbon spheres, was enlarged 50% (from 76.6 to 115.2 m2 g−1) when their structure was transformed from single-shell to double-shell. Furthermore, activated carbon impedance measurements demonstrated that the low-temperature hydrogenation greatly decreased both the internal resistance and the Warburg impedance. Consequently, a specific capacitance increase of >62%, from 445 to 718 F g−1, was achieved at a current density of 1 A g−1. Underlying such great improvement, the evolution of chemical valence and defect states with co-increase of these two factors was explored through X-ray photoelectron spectroscopy. Moreover, a full cell combined with NiCo2O4 and AC was assembled, and an energy density of 34.8 Wh kg−1 was obtained at a power density of 464 W kg−1. Hydrogenated hollow spheres with two shells rather than one are more suitable for use in supercapacitors, show scientists in China. Hollow microspheres are attractive electrodes for supercapacitors, which are promising power sources for hybrid electric vehicles. However, increasing the specific surface area of hollow microspheres tends to reduce their conductivity, whereas it is desirable to maximize both parameters for supercapacitors. Now, researchers have devised a two-pronged strategy to achieve this goal — using double-shell hollow spheres to increase the specific surface area and employing low-temperature hydrogenation to enhance the conductivity. By making both single-shell and double-shell NiCo2O4 hollow spheres, they showed that this strategy increased the specific surface area by 50% and greatly improved the conductivity, resulting in a 62% increase in the specific capacitance of the spheres. Hydrogenated NiCo2O4 double-shell hollow spheres, combining large specific surface area and high conductivity, are prepared. A specific capacitance increase of >62%, from 445 to 718 F g−1, is achieved at a current density of 1 A g−1. A full cell combined with NiCo2O4 and activated carbon is assembled, and an energy density of 34.8 Wh kg−1 is obtained at a power density of 464 W kg−1.