Preparation Of Two-dimensional Composites Based On Graphitic Carbon Nitride And Their Energy Storage Applications

Recently,the continuous consumption of traditional non-renewable fossil energy has caused serious environmental problems and restricted economic development.However,solar energy,tidal energy,and wind energy,which depend on natural conditions,are limited by time,region,and seasonality,and cannot be converted into electricity stably for period.Therefore,there is an urgent demand to develop efficient energy storage devices to achieve the sustainability of renewable energy.Among the energy storage devices,supercapacitors have attracted more and more attention due to their fast charge and discharge rates,high energy and power densities,and long cycle life.Carbon materials are considered as promising industrial electrode.Among them,graphitic carbon nitride（g-C₃N₄）,as a two-dimensional graphite-like structure,is especially attractive due to its good structural stability,unique electronic structure,environmental friendly,simple synthesis,and more importantly the high nitrogen content.As an electrode material,g-C₃N₄ can provide more active reaction sites and enhance the wettability between the electrode and the electrolyte,which is considered to be a potential electrode material for supercapacitors.Therefore,g-C₃N₄ is selected as the research object to in this thesis:（1）The melamine as the nitrogen-rich precursor was calcined at different temperatures to synthesize g-C₃N₄ by thermal polycondensation to study the effect of calcination temperature on its structure and electrochemical properties.The results showed that the obtained g-C₃N₄ demonstrated the optimal crystal structure with the temperature of550℃.When it was used as the electrode material,the supercapacitor gave the specific capacitance of 75.6 F g^-1 in 3 mol/L KOH solution at a scan rate of 5 m V/s and the capacitance retention rate of 83.0%for 10000 cycles at a scan rate of 50 m V/s.（2）The effect of sulfuric acid on the structure and electrochemical performance of g-C₃N₄ was investigated.The results showed the concentration of sulfuric acid was the key parameter,and 6 mol/L sulfuric acid can promise the best performance for supercapacitors.Too high concentration,the sulfuric acid will deteriorate the crystal structure of g-C₃N₄ and thus compromise its electrochemical performance.The supercapacitor with g-C₃N₄ as the electrode material treated with 6 mol/L sulfuric acid,referred as SCN,demonstrated the specific capacitance of 155.9 F g^-1 in 3 mol/L KOH electrolyte at a scan rate of 5 m V/s and the capacitance retention rate of 87.2%for 10000 consecutive cyclic voltammetry tests at a scan rate of 50 m V/s.（3）The composite of SCN and polyaniline（SCN/PANI）was investigated as the electrode materials in supercapacitors.Here,SCN/PANI was prepared by in-situ polymerization.It was found that the PANI fibers with network structure were successfully grown on the surface of the SCN lamellar structure.When SCN/PANI was used as the electrode material in supercapacitors,the device showed the best specific capacitance of234 F g^-1 in 3 mol/L KOH solution at a scan rate of 5 m V/s,which was significantly larger than that of the devices with SCN and g-C₃N₄ as electrode materials.In addition the supercapacitor showed the capacitance retention rate of 86.2%for 10000 cycles at the scan rate of 50 m V/s,which was not much different from that of SCN.The results indicated that introduction of PANI fibers can effectively improve the specific capacitance of SCN without affecting the stability.