| In recent years,with the rapid development of the economy and the growing population,the continuous combustion of fossil fuels has intensified the global energy crisis and caused a series of environmental pollution problems.In order to cope with the environmental problems caused by the burning of these fossil fuels,the society is striving for the use of renewable resources such as solar energy,wind power,hydroelectric power,and geothermal energy to provide sustainable energy without damaging the environment.However,these renewable energy sources rely heavily on natural conditions?such as sunshine,wind,water and terrain?;therefore,there is a need for technologies that will convert off-peak hours of electricity into different forms for storage to compensate for energy shortages during peak periods.In order to strike a balance between energy demand and ecological issues,it is particularly important to invest in clean,safe,and efficient electrochemical energy sources,especially lithium-ion batteries?LIBs?and supercapacitors?SCs?.In contrast,the supercapacitor has a higher specific power than the lithium ion battery,and is a more promising energy storage and conversion device.The supercapacitors can be divided into two types according to the storage mechanism:one is an electric double layer capacitor?EDLC?and the other is the pseudocapacitor.In contrast,the double-layer supercapacitor charge storage relies only on the electrostatic charge of the surface charge,which is not faradiac process;and the pseudocapacitor electrochemical process occurs on the surface and in the body near the surface of the solid electrode.Therefore,pseudocapacitor has become the focus of research because of their higher specific capacity and specific energy compared to double layer supercapacitors.For pseudocapacitor,their performance depends mainly on the electrode material.The pseudocapacitor materials mainly include conductive polymers,metal oxides or hydroxides,and metal sulfides.Conductive polymers have large changes in volume during continuous charge/discharge,affecting their cycle stability;transition metal sulfides have higher electrical conductivity,mechanical stability,and thermal stability than metal oxides or hydroxides.Therefore,transition metal sulphide has become a hot spot in the current study.In this paper,the transition metal sulphides were synthesized by unipolar pulse electrodeposition?UPED?method using sulfur sublimed as the sulfur source and studied in the application of supercapacitors.The details are as follows:?1?Synthesis of Three-Dimensional Foamed Cobalt Sulfide Active Composites and Supercapacitor PerformanceThis chapter uses polypyrrole?PPy?as a sulfur source and PPy loaded with elemental sulfur as a working electrode,graphite paper as a counter electrode,and a saturated calomel electrode as a reference electrode for a three-electrode system.Cobalt nitrate in ethyl alcohol absolute solution is an electrolyte.A different reduction potential is applied to the working electrode to reduce the sulfur sublimed to polysulfide ions and react with the cobalt ions in the electrolyte to obtain a cobalt sulfide active material.Cobalt sulfide composites were synthesized by applying different reduction potentials to the working electrode,and their morphology,structure characterization and electrochemical performance are tested.Scanning electron microscopy shows that the morphology of synthetic sulfides is a three-dimensional foam network.Further enlargement of the low magnification electron microscope scans clearly shows that the foam network is composed of nanosheets.Electrochemicalexperimentaldatashowthatthe electrochemical performance of cobalt sulfide is best at the potential of-0.85 V.The area specific capacity is 0.41 F cm-22 at a current density of 1mA·cm-2;the current density increases from 1 mA·cm-22 to 20 mA·cm-2,and the area specific capacitance of the active material maintains 84.8%,showing good rate performance.The current density of 10 mA·cm-2maintains 86%of the initial capacity after 10000 charge-discharge cycles,showing superior cycle performance.In addition,an active material is used as a positive electrode at a potential of-0.85 V,and an activated carbon is used as a negative electrode to assemble an asymmetric supercapacitor device,the device has an area specific capacity of 0.05F·cm-22 at a current density of 1 mA·cm-22 and an area specific capacity of0.03 F·cm-22 at a current density of 20 mA·cm-2,showing good rate performance.The power density corresponding to the energy density of0.05 to 0.17 Wh·m-22 is 8.14 to 212.75 W·m-2.Test data shows that the device has good electrochemical performances.?2?Synthesis of binary Ni-Co Sulfides composites and performance of supercapacitorsThis chapter synthesizes different Ni/Co ratio sulphides at a potential of-0.85 V using an unipolar pulse electrodeposition method with sulfur sublimed as the sulfur source and different Ni/Co ratios as the metal source.By scanning electron microscopy?SEM?,the synthesized active material was nanosheets.Electrochemical test data show the best performance at Ni/Co ratio of 3:3,and the area specific capacitance can reach 4.0 F·cm-22 at the current density of 2 mA·cm-2;After 1000 cycles at a constant current density of 10 mA·cm-2,the capacity retention rate exceeds 90%,showing good cycle performance.In addition,an asymmetrical supercapacitor assembled with an active material of positive electrode and activated carbon as a negative electrode has an area specific capacitance of 0.33 F·cm-22 at a current density of 5 mA·cm-2;the current density increases from 5 mA·cm-22 to 60 mA·cm-22 The area specific capacitance retention rate was 61.3%,showing good rate performance;The device exhibits excellent cycle stability with 10000cycles at a constant current density of 25 mA·cm-2.Therefore,binary cobalt nickel sulfide has bright application prospects. |
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