Synthesis And Electrochemical Performance Of Two-dimensional Porous Zinc Based Vanadates

With the ever-increasing energy depletion and environmental deterioration issues and the continuous consumption of renewable energy,the effective storage and utilization of energy and the development of new energy are urgently needed.In recent years,a new generation of energy strorage devices,including batteries and supercapacitors,was developed.Compared to batteries,supercapacitors have attracted more attention due to their advantages of high power density,long cycle stability,and fast charge and discharge rates.The study found that the selection and design of electrode materials have became the key to improving the performance of supercapacitors.Vanadium in transition metals is a trace element widely distributed on the earth.Vanadium has been extensively studied due to its various valences and low prices.For the binary compounds of vanadium oxide(VO₂,V₂O₃,V₂O₅,etc.),vanadium nitride(VN),vanadium sulfide(VS₂ and VS₄)and their corresponding composite materials,a lot of research has been done in supercapacitors.There are relatively few studies on metal vanadates.Metal vanadate materials be considered as one of ideal storage energy materials due to its advantage characteristics of strong conductivity,multiple oxidation states and low cost.Therefore,we choose metal vanadate materials as the research object.In paper,transition metal vanadates(Zn₃(OH)₂V₂O₇(H₂O)₂ composites and Co(VO₃)₂(H₂O)₄/Zn₃(OH)₂V₂O₇(H₂O)₂ composites heterojunction)was synthesized by the simple liquid-phase method.The two dimensional nanosheets structure exhibited by the as-prepared samples can fully contact with electrolytes,thus improving the electrochemical performance.The crystal structure,element composition and micro-morphology of the as-obtained samples are analyzed by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FITR),Specific surface area and pore size distribution testing technology(BET),Scanning electron microscope(SEM),Transmission electron microscope(TEM)and other technology.The electrochemical performance of materials was tested by utlizing cyclic voltammetry(CV),constant current charge-discharge test(GCD),cyclic stability test and electrochemical impedance spectroscopy(EIS).In addition,the energy storage performance of materials was also explored.The main work of this paper is as follows:Firstly,Two-dimensional porous zinc vanadate(Zn₃(OH)₂V₂O₇(H₂O)₂)nanosheets for the high performance supercapacitors synthesized by the co-precipitation and hydrothermal method.The prepared Zn₃(OH)₂V₂O₇(H₂O)₂ thin sheets by co-precipitation was stacked neatly,and there are many mesopores on the surface of the thin sheets.And the synthesized Zn₃(OH)₂V₂O₇(H₂O)₂ nanosheets by hydrothermal was disorderly arranged,a single nanosheets consist of cross-linked nanosheets.The specific surface area of the former and latter was calculated to be 10.27 and 124.63 m²/g,respectively.When assemmbled by the three-electrode systems,the latter nanosheets exhibited the specific capacity of 889.75 C/g at 1 A/g and the cycle stability of 94%,and former thin sheets showed 398.53 C/g of the specific capacity at the current density of 1 A/g and 104%of capacity rentention.When prepared by the hybrid supercapacitor(HSC)device with the synthesized Zn₃(OH)₂V₂O₇(H₂O)₂ nanosheets by hydrothermal as the positive electrode and the actived carbon(AC)as negative electrode,its specific capacity is 113.1 C/g at 1 A/g and the excellent cycle performance.And the energy density of the device is 25.13 Wh/kg at the power density of 800 W/kg.Secondly,two-dimensional porous Co(VO₃)₂(H₂O)₄/Zn₃(OH)₂V₂O₇(H₂O)₂(CVH/ZHVH)nanosheets composites heterojunction for high performance electrochemical supercapacitor.In experiment,two-dimensional porous Co(VO₃)₂(H₂O)₄/Zn₃(OH)₂V₂O₇(H₂O)₂ nanosheets composites was prepared by co-precipitation method with two metal compounds in different molar ratios and sodium metavanadate.All materials synthesized at different molar ratios are nanosheets structures.According to different molar ratios,there are nanosheets with different structures,and the distribution of mesopores on the nanosheet surfaces is different.During electrochemical performance process,CVH/ZHVH(the molar ratio:Co/Zn=1)presents 1191.17 C/g of the specific capacity at 1 A/g,and the ultrihgh cycle perfomance.The HSC device was assembled with CVH/ZHVH nanosheets as positive electrode and AC as negative electrode.The device displays a specific capacity of 125.3C/g at 1 A/g,,the energy density of 27.84 Wh/kg at the power density of 800 W/kg,with a long cycle life(102%of capacitance retention after 10 000 cycles).