Synthesis And Electrochemical Properties Of Cobalt-Nickel-Based Two-Dimensional Porous Materials

With the development of economy,the aggravation of environmental pollution and the continuous consumption of non-renewable energy resources,there is an urgent need to improve the efficiency of energy utilization,develop new energy sources,especially renewable energy,and ensure sustainable development,which cannot be separated from electrochemical energy storage equipments.Among which,supercapacitors（SCs）have the advantages of high power density,long cycle life,short charging time,wide temperature range,economy and environmental protection,and have broad application prospects in energy,power,communications,national defense and other fields.Electrode material is the key of SCs and determines their main performance index.The core of SCs development is the development of their electrode materials.Compared with carbon materials and conductive polymers,transition metal-based materials（such as oxides,hydroxides,oxalates,etc）have higher energy density and higher electrochemical stability,making them the most ideal electrode materials for SCs.Compared with bulk materials,two-dimensional（2D）porous materials have some superior characteristics:（i）The large specific surface area can make full use of the electroactive sites of electrode materials to generate redox reactions,high specific capacity of electrode materials and good cycle stability.（ii）The porous structure is more favorable for electrolyte ions to rapidly enter and exit the electrode surface channel and is favorable for cyclic stability.（iii）The self-assembly structure of interdigitate nanosheets or nanosheet bundle is beneficial to electron transmission and maintains the stability of the structure.In this thesis,2D porous cobaltate MCo₂O₄（M=Co and Mg）,Ni C₂O₄ nanoflakes and Mn-doped Co（OH）₂ nanosheets are designed and synthesized with high performance.X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,energy dispersive X-ray spectrometer（EDX）,specific surface area tester（BET）,field emission scanning electron microscope（FE-SEM）,and transmission electron microscope（TEM）were used to analyze the crystal structure,elemental composition,microstructure and size of as-prepared 2D materials.The electrochemical performance of as-prepared 2D materialsl was investigated by cyclic voltammetry（CV）,constant current charge-discharge test（GCD）,cyclic performance test,electrochemical impedance spectroscopy（EIS）and other electrochemical characterization.The content of this paper mainly includes the following three aspects:1.The controlled synthesis of 2D porous Co₃O₄ and Mg Co₂O₄ nanosheets was realized by direct hydrothermal decomposition of two or three mixed aqueous solutions of cobalt chloride（Co Cl₂）solution,magnesium chloride（Mg Cl₂）solution and cobalt chloride（Co Cl₂）solution and simplest monocarboxylic acid of formic acid（HCOOH,FA）solution respectively.In the aspect of electrochemical performance,the comparison of three-electrode system test shows that Mg Co₂O₄ nanosheets have larger specific capacitance(1431.2 F·g^–1 at 1.0 A·g^–1)and excellent cycle performance(2.4%loss after5000 cycles at 10 A·g^–1).The study of two-electrode system found that the specific capacitance of the asymmetric supercapacitor（ASC）device assembled by Mg Co₂O₄sheet as positive electrode and activated carbon（AC）as negative electrode can reach85.3 F·g^–1 at 1 A·g^–1 and 57.5 F·g^–1 at 10 A·g^–1.When the power density is 800 W·kg^–1,the energy density is as high as 32 Wh·kg^–1.And high cycle stability of only 6.3%specific capacitance loss in 10,000 charge-discharge cycle tests at a current density of10 A·g^–1.2.The controlled synthesis of 2D porous Ni C₂O₄ thin sheets was realized by direct hydrothermal decomposition of a mixed solution of nickel nitrate（Ni（NO₃）₂）solution and the simplest dicarboxylic acid of oxalic acid（HOOC-COOH,OA）solution.The microstructure of as-prepared products can be controlled by altering the hydrothermal reaction time.Compared with the products of 12 and 24 h,the as-prepared products at200°C for 6 h have higher specific surface area(136.52 m²·g^–1),larger pore size（7.36nm）and higher pore volume(0.26 cm³·g^–1).In terms of electrochemical performance:The comparison of three-electrode system tests shows that Ni-OA thin sheest at 200°C for 6 h have the advantages of larger specific capacitance(2835.06 F·g^–1)and higher cycle stability（94.3%）.The two-electrode system study found that the maximum specific capacitance of ASC device assembled by Ni-OA sheet at 200°C for 6 h as positive electrode and AC as negative electrode can reach 56.38 F·g^–1.When the power density is 900 W·kg^–1,the energy densityof the device is as high as 35.7 Wh·kg^–1.And it also displays excellent cycle stability of only 7.5%specific capacitance loss in 10,000charge and discharge cycle tests at 10 A·g^–1.3.The controlled synthesis of 2D porous Co（OH）₂ and Mn-doped Co（OH）₂nanosheets was realized by direct hydrothermal decomposition of manganese chloride（Mn Cl₂）solution and mixed aqueous solution of cobalt chloride（Co Cl₂）solution and urea（CO（NH₂）₂）solution,respectively.In the aspect of electrochemical performance,three-electrode system tests show that Mn doped Co（OH）₂ nanosheets have larger specific capacitance of 1915.88 F·g^–1 at 1.0 A·g^–1 and excellent cycle performance of6.5%loss after 5,000 cycles at 10 A·g^–1.Mn²⁺is added into the hydrothermal reaction to obtain a doped product.Not only the morphology of the product is changed,but also some oxygen vacancies are generated,which improves the conductivity and thus the electrochemical behavior and achieve higher specific capacitance.