Synthesis Of Pseudocapacitive Composite Materials And Their Application For Supercapacitors

The development of technology and economic prosperity has inevitably resulted in the consumption of large amounts of fuel.Environmental pollution,energy crisis and depletion have begun to threaten human society.To solve these problems,renewable and clean energy sources have been explored.Since these energy sources are generally scattered and fluctuating,we need to store them for reuse.Therefore,it is necessary to develop energy storage devices with green,good performance and low cost on a large scale.As an energy storage device,supercapacitors have been extensively studied due to the advantages of high power density,long cycle life,wide operating temperature range,safety and environmental friendly.In particular,electrode materials are critical to the performance of supercapacitors.Compared with electric double layer capacitor materials,pseudocapacitor electrode materials have greater specific capacitance.Nevertheless,there are problems of low power density and poor cycle performance in its application.Therefore,this thesis combines pseudocapacitive electrode materials(transition metal compounds,conducting polymers and metal organic frameworks)with other materials to optimize their electrochemical properties.The main research contents are as follows:(1)Transition metal oxides have the advantages of fast redox reactions,excellent energy storage properties,and long cycle stability.However,the weak conductivity and structural instability of these materials limit their effective capacitance.To overcome these drawbacks,one strategy is to combine transition metal oxide materials with carbon materials that have excellent electrical conductivity and large specific surface area.Based on this,we developed a feasible hybridization of nickel oxide with porous carbon,which has a layered porous structure and exhibits good electrochemical properties with a specific capacitance of 580 F g^-1 at 1 A g^-1 as well as a capacitance retention of more than 90%after 8000 cycles at 5 A g^-1.(2)Conductive polymers have the advantages of high conductivity and environmental friendliness,but changes such as expansion and fracture during charging and discharging affect their cycling stability.Metal organic frameworks have ordered porous structure,high chemical stability,and large specific surface area.However,the poor electrical conductivity of metal organic frameworks limits their capacitance and rate performance.Combining metal organic frameworks with conductive polymers can facilitate electron and ion transport,increase the specific capacitance of supercapacitors and enhance cycling stability.Based on this,we developed a polypyrrole and ZIF-67 composites by a one-pot synthesis strategy,which forms a three dimensional interpenetrating network structure and exhibits a high specific capacitance of 1241 F g^-1 at 1 A g^-1 and a retention rate of 91%after10,000 cycles at 10 A g^-1.(3)Metal hydroxides have the advantages of high theoretical specific capacitance,fast redox activity,and low cost.However,they also have the disadvantages of low conductivity and poor stability during charging and discharging cycles.Compared to crystalline metal organic frameworks,amorphous metal organic frameworks are mechanically more stable,have higher conductivity and retain useful functionality in their structure.Herein,we developed a solvothermal method to synthesize composites of Ni-p PD amorphous metal organic frameworks with nickel hydroxide nitrate.The material forms an irregular honeycomb structure and exhibits high capacitance(2058F g^-1 at 1 A g^-1)and low resistance of 0.15?in a three-electrode system.In addition,the assembled hybrid supercapacitor devices also exhibit excellent cycling stability(99%after 5000 cycles at 4 A g^-1),achieving a high energy density of 35.6 Wh kg^-1and a power density of 7882 W kg^-1.