1. ¹State Key Laboratory of Mechanics and Control of Mechanical Structures & College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, China


2. ²Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, China


3. 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: zeng.haibo@njust.edu.cn

⁴These authors contributed equally to this work.

Received 9 October 2014; Revised 29 December 2014; Accepted 6 January 2015

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.