Ultrathin Carbon Shell Entrapped Metal Co/Coo for High-Performance Electrochemical Supercapacitor

Abstract

Nowdays, a series of transition-metal oxides (TMOs) such as MnO2, MoO3, Co3O4, CoO, and NiO have been widely studied as supercapacitive materials because of their low cost and high theoretical specific capacitance. As a low-oxidized cobalt compound, to date, CoO has been successfully applied as Li-ion battery anodes and showed satisfactory Li-ion storage capacities. The theoretical specific capacitance of CoO reaches 4292 F g-1. This value is higher than that of NiO (2573 F g-1), MnO2 (1370 F g-1), and Co3O4 (3560 F g-1). However, the practical application of CoO is severely hindered by its easily oxidized character (CoO is oxidized to Co3O4 after exposure in air) and the poor electrical conductivity (intrinsic semiconducting or insulating property). The poor conductivity is also one of the main reasons that limit the application of other TMOs in constructing high performance supercapacitors. To overcome these challgenges, we report a metallic Co doped CoO-based energy storage material by elaborately tailoring both the electrical conductivity and material configuration. The cobalt-doped CoO (denoted as CoO/Co) heterostructure is designed to improve the electrical conductivity. The metallic Co, acting as a current “expressway,” effectively improves the electrical conductivity of the composite, thus accelerating the electron transport. Additonally, an ultrathin layer of amorphous carbon is generated on the surface of heterostructure to prevent further oxidation of CoO/Co. This amorphous carbon shell serves as a surface stabilizer to hinder further oxidation of Co/CoO, thus sustaining good stability of supercapacitor. As a result, the as-prepared composite exhibits an excellent supercapacitor performance of 2165.7 F g-1 at a scanning rate of 10 mV s-1. An asymmetric supercapacitor cell fabricated using the c and active carbon achieves a maximum energy density of 146.3 Wh kg-1 at a power density of 1800 W kg-1, and the maximum of 27000 W kg-1 can be obtained with a remaining energy density of 63.0 Wh kg−1. Even more importantly, such a composite also showed a good cycling ability and improved long-term storage stability. As this supercapacitive material can be prepared with a high yield by a facial low-cost route, it is highly expected that it could be a promising material for electrochemical energy storage in practical applications.