Controllable synthesis of NiCo2O4, NiCo2O4/graphene composite and their electrochemical application in supercapacitors

As traditional energy storage devices, supercapacitors exhibit the advantage of low cost, fast charge/discharge, long-term cycling stability and high powder density. The development of electrode materials is a major task for constructing high-performance supercapacitors. Among existing transition metal oxides, NiCo2O4 has a high theoretical capacitance, which has been received much more attention. In view of the excellent electrochemical performance of NiCo2O4, herein, we highlight the research advance of NiCo2O4 and NiCo2O4/graphene composites in supercapacitors. The controllable synthesis of NiCo2O4 and the electrochemical performance are introduced firstly, including commonly used methods, such as, hydrothermal/ solvothermal method, coprecipitation, chemical bath deposition, electrochemical deposition, and template method. According to the dimension feature of graphene matrix, we classify NiCo2O4/graphene composite into two-dimensional (2D) and threedimensional (3D) composite, and summarize the design principle, microstructure and their application performance in supercapacitors. Finally, we summarize the existing problems about NiCo2O4 and NiCo2O4/graphene composite, and propose the solving strategies and future perspectives. The major purpose is to provide theoretical guidance for related researchers in supercapacitors.