The Assembly Of Asymmetric Supercapacitor Based On Graphene/Nickel Hexacyanoferrate Composites

Supercapacitors have attracted great attention because of the high power density and good cycling stability.However,compared with lithium-ion batteries,their energy density is still low.In recent years,how to further improve the energy density on the basis of maintaining the original advantages of supercapacitors become research hotspots.At present,there are two main means,one is to improve the specific capacitance of the electrode material by optimizing the design mode,and the other is to broaden the operating voltage through the assembled asymmetrical supercapacitors based on the rational matching of different electrode materials.Because of the rich microporous structure and additional pseudo-capacitance provided by the coordinated metal ions through redox reaction,metal organic skeleton compound is a great potential electrode material with high theoretical specific capacity.However,some inherent negative properties such as poor conductivity and unstable structure also caused a poor rate capability and cycle stability,and become the main reason limiting its practical applications.In this paper,by adding cationic polymers and other metal ions to the assembled compound of nickel ferricyanide/graphene,the rate capability and cycle stability of the composites were greatly improved.And then,the effects of microstructures on the electrochemical properties of the electrode were also clearly investigated.As a result,the electrode material and activated carbon electrode material were assembled into asymmetric supercapacitor,and the electrochemical energy storage characteristics were studied.The main contents are as follows:1)In this paper,the effects of different carriers(graphene oxide and graphene)on the microstructure and electrochemical properties of the composites synthesized by coprecipitation method were studied.The experimental results showed that the addition of graphene could improve the conductivity of the composites,and lead to good rate characteristics.2)In the graphene/ferricyanide composite,the acting force between carrier and nickel ferric nickel obviously increased and the nickel ferricyanide could homogeneously grow on the surface of graphene due to the introduction of cationic polymer,resulting in an improved conductivity and stable structure of the composites.What’s more,the capacity retention rate is more than 80%after 2000 cycles of charge/discharge process,exhibiting a good cycle stability.3)The microstructure and electrochemical properties of different metal-doped graphene/ferricyanide composites were investigated in detail.It was found that cobalt ferrocyanide had a larger particle size due to faster growing rate than nickel ferricyanide in the nucleation process,consequently could not homogeneously grow on the surface of graphene.However,the bimetallic ferricyanide composites exhibited better electrochemical performance due to the cooperation effect of the two metals in the chare/discharge process.As a result,the asymmetric supercapacitors were assembled with synthesized composites as positive electrode and active carbon as negative electrode,which exhibited excellent electrical performance.When the power density is 600 W·kg-1,the energy density was up to 30 Wh·kg-1.