Nano-CO₃O₄ Composite Electrodes:Preparation And Properties

Asymmetric capacitor is an electrochemical energy storage device with electrochemical performance between supercapacitors and batteries.Compared with supercapacitors,it has higher energy density and is widely used in the energy storage fields such as electric vehicles,rail transit and intelligent instruments.Co₃O₄ is a typical battery-type electrode material with high theoretical specific capacity and great environmental friendliness.However,the practical application of Co₃O₄ has been limited owing to some issues such as particle agglomeration and poor stability of single component.In this paper,nano-Co₃O₄/CoS electrode,nano-Co₃O₄/Co（OH）₂ electrode and Co₃O₄/MnO₂ composite electrode have been synthesized successfully.We have explored the relationship between electrochemical performance and morphology of these electrodes.The research of this paper is mainly divided into the following three parts:1.Nano-Co₃O₄/CoS electrode with special structure was prepared by hydrothermal and electrodeposition methods.Co₃O₄ nanoneedle arrays have been prepared on the Ni foam by hydrothermal synthesis,which avoids the use of bingers and conductive agents.CoS nanosheets were grown on the surface of Co₃O₄ nanoneedle arrays,the special nanostructure can reduce the electrode resistance and facilitate adequate contact between the electrolyte and active materials.After the optimization of hydrothermal time,the energy density of 1.4 V asymmetric capacitor with nano-Co₃O₄/CoS electrode can reach 22 Wh kg^-1 at 1 A g^-1,and the energy density retention rate is 90.9%after cycling 1000 times at 2 A g^-1.2.The hierarchical Co₃O₄/Co（OH）₂ nanostructure was prepared as a binder-free electrode by hydrothermal and constant voltage electrodeposition methods.The 3D Co₃O₄/Co（OH）₂ nanostructure is composed of Co₃O₄ nanoneedle arrays and ultrathin Co（OH）₂nanosheets.The special structure increases the specific surface area of the electrode.The energy density of the asymmetric capacitor with nano-Co₃O₄/Co（OH）₂electrode can reach 39 Wh kg^-1 at 1 A g^-1,which is 3 times higher than nano-Co₃O₄.And the energy density retention rate can reach 96.2% after cycling 1000 times.3.MnO₂ nanosheets were coated on the surface of Co₃O₄ nanospheres by hydrothermal method,which can increase the active area of electrode.The asymmetric capacitor with Co₃O₄/MnO₂ electrode has been assembled to explore the practicality of synthetic product.It has poor rate capability in Na₂SO₄ electrolyte but the energy density can reach 17 Wh kg^-1.The energy density of this asymmetric capacitor in KOH is 11.5 Wh kg^-1,and the energy density retention rate can reach 92.8%after 1000 cycles,showing the excellent cycling stability of Co₃O₄/MnO₂composite electrode.