Preparation Of G-C₃N₄ With Metal Cobalt Sulfide Composites And Their Application In Supercapacitors

Increasing energy shortages and environmental pollution make the development of energy technology extremely urgent.Energy storage technology is the basis for the development and utilization of new energy sources.The most commonly used energy storage devices are lithium-ion batteries,supercapacitors,and traditional batteries,etc.Among them,supercapacitor is a revolutionary development in the field of energy storage technology.Due to its high power density and long cycle life,research on supercapacitors has been widely concerned.The performance of supercapacitors usually depends on the electrode materials.Therefore,currently research on supercapacitors is mainly focused on the preparation of high performance electrode materials.The electrode materials can be generally divided into two types:electric double layer capacitance and pseudocapacitance materials.As a typical two-dimensional layered semiconductor material,g-C₃N₄ has good thermal stability and chemical stability,and it is environmentally friendly without secondary pollution.g-C₃N₄ is usually used in the photocatalytic field,but has less application in energy storage.It has to be modified to enhance its conductivity and increase the capacitance.Since the sufficient nitrogen pore structure on the surface of g-C₃N₄ can generate strong chemical absorption with metal ions,the metal ions can be uniformly bonded to the surface of g-C₃N₄,so it is possible to improve its capacitance performance by introducing some transition metal compounds.And among the transition metal composite,the ternary cobalt sulfide showed excellent electrochemical performance.Therefore,in this thsis,g-C₃N₄ was modified by two methods.Firstly,a certain proportion of urea and glucose were used as precursors.The carbon-coated g-C₃N₄ nanotube composites were prepared by one-step double in-situ method.Then the g-C₃N₄/transition metal cobalt sulfide composites were prepared via introducing a plurality of peseudocapacitance materials on the g-C₃N₄ surface by one-step hydrothermal method,then we explored their electrochemical performance.At last,an electrode material,the carbon-coated tubular g-C₃N₄/cobalt sulfide composite,was prepared by the combination of carbon-coated tubular nitride with the best-performing ternary compound via one-step hydrothermal method,which had high specific capacity and long cycle life.The specific research contents are as follows:（1）Preparation and characterization of glucose-modified g-C₃N₄ composites.Carbon-coated g-C₃N₄ nanotube composites were prepared by one-step double in-situ method using urea and glucose as precursors.Various products CN-X（X=50,100,200,300,X represent the milligram of glucose at 6 g of urea）were prepared via changing the amount of glucose added.The structure was characterized by SEM,XRD,etc.And it was found that CN-200 formed a relatively regular tubular shape.As an electrode material,the electrochemical performance of CN-X was studied by electrochemical test methods such as CV,GCD and EIS.The results indicated that tubular CN-200 had the highest specific capacitance of 1094 F/g at 1 A/g,which was 1.4 times of the bulk g-C₃N₄（794 F/g）.However,after 2000 times GCD cycles at 20 A/g,the capacitance retention rate was only 64.3%.（2）Preparation,condition exploration and characterization of g-C₃N₄ and metal sulfide composites.Firstly,during the preparation of g-C₃N₄/CoNi₂S₄ composite by one-step hydrothermal method,the influence of the amount of g-C₃N₄,the ratio of mixed solvent water to ethanolamine and reaction time on product structure and properties were separately explored according to the orthogonal experimental method.When the amount of g-C₃N₄ was 30 mg,the ratio of solvent water to ethanolamine was1:1,and the reaction time was 14 h,the specific capacitance of g-C₃N₄/CoNi₂S₄ was maximum.Under this condition,g-C₃N₄/FeCo₂S₄,g-C₃N₄/ZnCo₂S₄,g-C₃N₄/CuCo₂S₄and g-C₃N₄/CoMn₂S₄ composites were prepared separately.And their capacitance properties were tested by GCD.It was found that g-C₃N₄/CoNi₂S₄ had the highest specific capacitance of 2058 F/g at 1 A/g,which was 2.6 times of the bulk g-C₃N₄.And the capacitance retention rate was 72.2%after 5000 cycles at 20 A/g,performed good cycle stability.In addition,the asymmetric supercapacitor g-C₃N₄/CoNi₂S₄//AC fabricated by using activated carbon（AC）as the negative electrode,displayed an excellent capacitance of 181 F/g,a maximum energy density and power density of 71.9Wh/kg and 6.9 kW/kg,respectively.And after 5000 cycles（10 A/g）,the capacitance retention rate was 72.2%.（3）Preparation and performance characterization of CN-200/CoNi₂S₄ electrode materials.The CN-200/CoNi₂S₄ composites were prepared by the same synthesis method as g-C₃N₄/CoNi₂S₄.Its structure was characterized by SEM,XRD and BET,etc.The electrochemical test methods such as CV,GCD and EIS were used to explore the electrochemical performance of CN-200/CoNi₂S₄.CN-200/CoNi₂S₄ showed a specific capacitance of 1678 F/g at 1 A/g,which was smaller than that of g-C₃N₄/CoNi₂S₄.However,the cycle stability had been greatly improved,exhibited an excellent cycle stability with 81%specific capacity retention at a current density of 10 A/g after 5000 cycles.Further,an asymmetric capacitor CN-200/CoNi₂S₄//AC composed of CN-200/CoNi₂S₄ and AC,exhibited a favourable capacitance performance specific of117.6 F/g,and its maximum energy density and power density were 45.6 Wh/kg and5.7 kW/kg,respectively.After 5000 cycles at 10 A/g,the capacitance retention rate of CN-200/CoNi₂S₄//AC was 83%.