The Construction Of Sulfide Heterojunctions And The Application In Supercapacitors

The consumption of fossil fuels leads to serious environmental pollution and energy crisis,promoting the demand for sustainable clean energy.Supercapacitors have become an alternative in the field of electrochemical energy storage,which is featured with high power density,fast charging and discharging rate and long cycle life.However,its energy density is much lower than that of batteries.The energy density is determined by the capacitor’s capacity and potential window.Therefore,it is very important to improve the specific capacitance of capacitor through electrode material design to optimize the capacitor material.Transition metal sulfides are considered to be one of the most promising electrode materials due to their excellent theoretical specific capacity,excellent electrical conductivity and low price.However,transition metal sulfide with unitary structure and composition exhibited poor capacitance and rate performance,and the serious volume expansion during charging and discharging also limits their common application.In order to solve this problem,bimetallic sulfide heterojunction is constructed due to electron regulation derived from the heterogeneous interface.Moreover,the diffusion of anions in charge and discharge process is optimized by micro-structure design,so as to reduce the polarization in electrochemical reaction,improve the rate performance and promote the improvement of capacitor cycle stability.The contents of this paper are summarized as follows:（1）Ni-Cobimetallic sulfide nanoparticles were synthesized by one-step hydrothermal method.The molar ratio of nickel/cobalt（Ni:Co）precursor and hydrothermal time on the morphology and structure of the products were investigated.The results show that the molar ratio of nickel/cobalt（Ni:Co）affects the phase composition of nanoparticles.With the increase of cobalt dosage,the content of CoS_xincreases gradually.Moreover,the binding energy of Co2p energy level shift to higher value with the binding energy of Ni2p energy level shift to lower value,indicating the enhance electron interaction between the metallic atoms.The electrochemical tests show that NiS_x/CoS_y exhibit the optimal performance when the molar ratio of Ni/Co（Ni:Co）is 1:1.At the current density of 1 A g^-1,the specific capacitance reaches 1277F g^-1.（2）Three-dimensional flower-like Ni-doped cobalt sulfide nanoflakes on Prussian blue cobalt analogs(Ni_xCo_3-xS₄/PBA)were prepared by one-step hydrothermal method.The Ni_xCo_3-xS₄ nanoflakes supported on the highly porous cubic PBA exhibit a flower-like morphology,which not only exhibit highly exposed active sites,but also promotes the effective diffusion of ions in the electrolyte.It was found that thiourea,as a precursor,is a key component in the synthesis of PBA and Ni_xCo_3-xS₄.With altering the reaction times of Ni_xCo_3-xS₄/PBA,the morphology of Ni_xCo_3-xS₄/PBA evolves from cubic to nanoflowers.Further spectroscopic analysis shows that the-CN formed by the decomposition of thiourea was first coordinated with Coion to form PBA,and then excess Ni_xCo_3-xS₄ nanoflakes grows via surface vulcanization.Because the crystalline coefficient of PBA（220）crystal plane and Co₃S₄（220）crystal plane are similar,Ni_xCo_3-xS₄ nanoflakes could grow epitaxially from PBA,which is beneficial for enhancing interface interactions.Electrochemical analysis shows that the specific capacitance of Ni_xCo_3-xS₄/PBA-7h is 964 F g^-1 at the current density of 1 A g^-1.The asymmetric capacitor assembled with Ni_xCo_3-xS₄/PBA-7h delivers an energy density of 52.2 Wh kg^-1 at power density of 414.8 W kg^-1.In addition,the capacitance of the asymmetric capacitor retains 79.8%of the initial capacitance after 8000 cycles of charge-discharge test.（3）The binder-free Sulfide-coated LDH（CoMn-LDH@CoMnS）was directly grown on nickel foam by hydrothermal and ion exchange method in order to reduce the influence of binder on capacitance performance.In this chapter,the effects of Co/Mnmolar ratio on the composition and morphology of LDH have been discussed in detail.The results showed that the morphology of CoMn-LDH varies from nanoparticles to sea urchin-like morphology with the increase of cobalt contents,and its density increased gradually.When the cobalt/manganese molar ratio is 10:1,the synthesized CoMn-LDH-10 has a specific capacitance of 4.6 F cm^-2 at the current density is 10 mA cm^-2.Further vulcanization was conducted to construct heterogeneous junction.TEM images clearly showed CoMnS uniformly coated on the surface of LDH nanoneedles,which form a core-shell structure The vulcanization time and concentration of Na₂S on the structure and properties of CoMn-LDH@CoMnS were investigated.The CoMn-LDH@CoMnS material obtained after reacting for 6 h in 0.2 M Na₂S exhibit the highest specific capacitance of 7.5 F cm^-2 at the current density of 10 mA cm^-2.