Preparation Of ZIF-8 Derived Hierarchical Porous Carbon-based Composites And Their Energy Storage Properties

With the consumption of energy and environmental pollution,there is an urgent need to develop high-performance energy storage systems to meet the development of society and economy.In numerous energy storage/conversion technologies,supercapacitors are extensively used in wind turbines,new energy vehicles,portable electronic devices and other fields because of their advantages of fast charging and discharging processes,high power density,and excellent cycling stability.ZIF-8 derived carbon materials are promising electrode materials due to their designable structure,nitrogen self-doping structure,and simple preparation,resulting in their broad application prospects in chemical energy storage,electrocatalysis,and wastewater treatment.However,the microporous structure of ZIF-8 after carbonization will greatly limit the transport of electrolyte ions inside the materials,thus limiting its application scope.In order to obtain better energy storage properties,the methods to improve the performance of ZIF-8-derived carbon materials are mainly divided into two types according to the energy storage mechanism:one is to increase the pseudocapacitance;the other is to improve the electric double layer capacitance by optimizing the pore structure.Based on this,the ZIF-8-derived nitrogen-doped porous carbon（N-PC）material was used as the starting point for this study,and the advantages of heteroatom doping strategy and hierarchical porous structure were combined to successfully obtain a hierarchical porous structure that facilitates ion transport.On this basis,the nitrogen-doped hierarchical porous carbon material was used as the substrate,and the carbon-based composites were further prepared by a simple hydrothermal method to grow uniformly nickel oxide nanosheets with high theoretical capacitance in situ on the substrate.The main studies are as follows:（1）Preparation of B,N co-doped hierarchical porous carbon（BN-HPC）material and its supercapacitance propertiesThe single-crystal ordered macroporous ZIF-8（SOM ZIF-8）was prepared by a two-solvent-assisted method using polystyrene microspheres as hard template.Then,SOM ZIF-8was as the carbon and nitrogen sources to prepare the nitrogen-doped hierarchical porous carbon（N-HPC）material.BN-HPC was successfully prepared by annealing the mixture of boric acid and N-HPC.Through a series of electrochemical tests,it was confirmed that the BN-HPC electrode has the best electrochemical performance.In a three-electrode system with1 M H₂SO₄ as the electrolyte,the specific capacitance of BN-HPC electrode at 1 A g^-1 was236.9 F g^-1,which is much higher than those of N-HPC(221.8 F g^-1)and N-PC(152.2 F g^-1).In addition,the symmetrical supercapacitor assembled with BN-HPC has an energy density of33.3 Wh kg^-1 at a power density of 212.5 W kg^-1.（2）Preparation of S,N co-doped hierarchical porous carbon（SN-HPC）material and its supercapacitance propertiesSN-HPC was successfully prepared by annealing the mixture of thiourea and N-HPC.Through a series of electrochemical tests,it was confirmed that the co-doping of sulfur and nitrogen would improve the electrochemical properties of the material.In a three-electrode system with 1 M H₂SO₄ as the electrolyte,the specific capacitance of SN-HPC at 1 A g^-1 was262.3 F g^-1,higher than that of N-HPC(207.4 F g^-1).In addition,the symmetrical supercapacitor assembled with SN-HPC has an energy density of 29.1 Wh kg^-1 at a power density of 212.5 W kg^-1.（3）Preparation of nickel oxide nanosheets/nitrogen-doped hierarchical porous carbon（NiO/N-HPC）composites and its propertiesNiO/N-HPC was obtained by growing nickel oxide nanosheets on N-HPC through a simple hydrothermal and annealing treatment.The uniform distribution of NiO nanosheets and the hierarchical porous structure on the surface of composites can provide more effective specific surface area and generate more abundant active interfaces,which can improve the capacitive contribution of the electrode.In a three-electrode system with 0.5 M K₂SO₄ as the electrolyte,the specific capacitance of NiO/N-HPC at 0.5 A g^-1 was 126.4 F g^-1,higher than that of NiO(46.5 F g^-1).In addition,the assembled NiO/N-HPC//N-HPC aqueous potassium ion hybrid capacitor has an energy density of 22.0 Wh kg^-1 and a power density of 9000 W kg^-1.