1State Key Laboratory of Mechanics and Control of Mechanical Structures & College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, China
2Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, China
3Institute of Nanoscience and Nanotechnology, Department of Physics, Central China Normal University, Wuhan, China
Correspondence: Professor HB Zeng, Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China. E-mail: firstname.lastname@example.org
4These authors contributed equally to this work.
Received 9 October 2014; Revised 29 December 2014; Accepted 6 January 2015
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 115.2m2g−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 718Fg−1, was achieved at a current density of 1Ag−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 34.8Whkg−1 was obtained at a power density of 464Wkg−1.