Metal Sulfides For High Performance Asymmetric Supercapacitors Of Study

In the face of the growing shortage of traditional fossil fuels and the resulting environmental problems,scientists are making every effort to explore clean,renewable new energy materials and develop new technologies for energy conversion and storage devices.At present,among many high-energy storage and conversion devices that have attracted much attention,supercapacitors have become a research hotspot of researchers because of their unique properties,such as high power density,fast charge and discharge speed,good cycle performance,safety and environmental friendliness.However,the energy density of supercapacitors is still lower than that of lithium ion batteries?LIB?,thus limiting its further wide application.According to the supercapacitor energy density formula E=¹?₂ CV²,to increase its energy density,one strategy is increase the specific capacitance?C?of the electrode material,and another is enlarge the electrochemical window?V?of the device.It is also possible to assemble a device with a new structure.By analyzing the factors affecting supercapacitors,the metal sulfides with different morphology were designed and prepared as the cathode material for the supercapacitor.Moreover,the biomass with special structure is used as the raw material to prepare the biomass-based carbon materials with rich pore structure by simple activated carbonization method and further used as a negative materials for supercapacitors.In addition,the asymmetric supercapacitor devices are assembled based on above two materials to increase the energy density of the device.The main research contents are as follows:1.The flower-like Zn-Ni-S cathode electrode material was synthesis by the two-step hydrothermal method using zinc nitrate hexahydrate,nickel nitrate hexahydrate and sodium sulfide nonahydrate as raw materials,which the specific capacity can reach 246.7 mAh g^-1 at a current density of 1 A g^-1 in the three-electrode system.The pitaya mesocarps-based carbon nanosheet?PMNC?was prepared by simple high-temperature carbonization method using the pitaya peel as the carbon source,CaCl₂ and urea as the activator,which specific surface area was up to 1356 m²g^-1.In a three-electrode system,the specific capacitance of PMNC could reach 227.5F g^-1 at the current density of 0.5 A g^-1.The asymmetric supercapacitor?ASC?constructed with flower-like Zn-Ni-S as the positive electrode material and PMNC carbon nanosheet as the negative electrode material could extend the potential window to 1.65 V.Furthermore,the energy density of Zn-Ni-S//PMNC ASC can reach 52.4 Wh kg^-1 at a power density of 551.7 W kg^-1,as well as high capacitance retention rate of 93.5%was obtained after 6,000 charge and discharge cycles.2.Nickel-aluminum double hydroxide?NA LDH?was synthesized by low-temperature hydrothermal method using nickel nitrate hexahydrate and aluminum nitrate nonahydrate as raw materials.Then thiourea was used as the sulfur source to vulcanize it to obtain nickel aluminum bimetallic sulfide?NAS?nanosheets.When it was used as a cathode material for the supercapacitor,the specific capacity can reach240.8 mAh g^-1 at a current density of 1 A g^-1 in the three-electrode system.The NAS nanosheet was futher used as a positive electrode material and a dragon skin carbon?PMNC?nanosheet was used as a negative electrode material to construct a novel NAS//PMNC asymmetric supercapacitor,which the potential window can be extended to 1.7 V and has almost no loss of capacitance after 6000 cycles.3.The uniform cobalt manganese hydroxide?CMOH?nanoneedle arrays was firstly prepared via low-temperature hydrothermal process using nickel?Ni?foam as the nanoneedle growth support framework.Subsequently,the stable and integrated heterostructures of cobalt-manganese sulfides?CMS?with porous structure can be simultaneously formed and grown through the reaction of CMOH with Na₂S at hydrothermal anion-exchange reaction process while retaining their intrinsic nanoneedle arrays structure.The as-prepared CMS-8 nanoneedle arrays as a binder-free electrode for supercapacitor exhibits high area capacity of 351 mAh g^-1(0.53 mAh cm^-2)at 1 A g^-1(2 mAh cm^-2).Based on the unique architecture and excellent electrochemical performance of CMS-8 nanoneedle arrays,a novel CMS-8//PMNC asymmetric supercapacitor with large operating voltage of 1.7 V was fabricated using CMS nanoneedle arrays as the positive electrode and pitragon skin carbon nanosheet?PMNC?as the negative electrode,respectively,and provided a maximum specific energy of 56.5 Wh kg^-1 and good capacitor retention rate of 90.4%after 6,000 cycles.