In-situ Synthesis Of Ni₃S₂@Ni Foam Composite Electrodes For Supercapacitors And Their Electrochemical Properties

With the rapid development of the new energy automobile industry,power supply has become a hot spot of investment in recent years.People are committed to the development of new energy storage devices with high energy and high power density.As rechargeable energy storage devices,supercapacitors have a great expectation for their ultra-long cycle life,high security coefficient and fast charge/discharge applications.Ni3S2,a typical nickel sulfide with a high theoretical capacity,is found able to offer more fluent charge transfer than many other transition metal sulfides due to its high electrical conductivity,thermal stability and especially intrinsic metallic property,thus attracts increasing intention of researchers to develop high-performance supercapacitors by using Ni3S2 as electrode materials.This dissertation focuses on the solvothermal synthesis of Ni3S2@Ni foam composite electrode through the in-situ synthesis of Ni3S2 active materials on Ni foam substrate.Three-dimensional meso/macroporous Ni3S2@Ni foam architectures are achieved by adjusting reaction conditions,such as solvent,reaction time,temperature and initia raw materials,etc.The growth mechanism and Faradaic reaction of Ni3S2@Ni foam composite electrodes in KOH aqueous solution have been proposed reasonably.The results offer the valuable ideas for the controllable growth of Ni3S2 nanocrystals and the evaluation of their electrochemical performances using as active electrode materials for supercapacitors.The major research works can be divided into four parts,which are narrated as follows(1)Ni3S2@Ni foam composite electrode for supercapacitor is synthesized via a one-step solvothermal treatment of Ni foam using Na2S·9H2O as sulfer source dissolved in absolute ethanol.When the reaction temperature is 120?,Ni3S2@Ni foam composite electrode synthesized in absolute ethanol is successfully achieved with a 3D meso/macroporous network structure.The structure contains Ni3S2 nanosheets with 15-20 nm in thickness and meso/macropores with an average pore size of 24.69 nm and a high BET surface area of 50.55 m2/g.At a current density of 5 mA/cm2,the composite electrode demonstrates a high areal capacitance of 5.92 F/cm2,and retains a areal capacitance of 3.31 F/cm2 at a high current density of 60 mA/cm2 which showing a good retention rate of 55.91%.At the current density of 60 mA/cm2,it retains the capacitance retention ratio above 97%after 5000 charge/discharge cycles,exhibiting a good cyclic stability.(2)Growth mechanism of Ni3S2 nanosheets on Ni foam is revealed through adjusting reaction time and solvent,and its formation is regulated through the slow release of S2' ions and the solid/liquid interfacial reactions in absolute ethanol.The oxidation and the sulfuration of NF substrate are considered to occur almost simultaneously during the solvothermal reaction,thus the Ni3S2 nanosheets are in-situ synthesized on Ni foam in absolute ethanol.Furthermore,the reversible Faradaic reactions of Ni3S:in KOH aqueous solution are demonstrated as a two-step oxidation process and three-step reverse reduction process attributing to the valence transitions of Ni0 in Ni3S2 between Ni0 and Ni3+(3)In addition,an asymmetric supercapacitor,Ni3S2@Ni//NF ASC,is assembled to assess the potential application using Ni3S2@Ni foam composite electrode as the positive electrode and Ni foam as the negative electrode.The ASCs deliver an energy density of 55.79 Wh/kg at the power density of 938.98 W/kg.Moreover,the NiS2@Ni//NF ASCs are directly connected with a circuit board which consisted of 71 light-emitting-diodes and light the board lasting more than 10 min after being charged to 4.5 V at a current density of 5 mA/cm2,showing a good performance and being expected to be used as electrochemical energy storage system with high power density and high energy density(4)Ni3S2@Ni foam composite electrode is synthesized through solvothermal treatment of Ni foam using thiacetamide as sulfer source in hydrazine hydrate at 65? for 20 h.When the concentration of thiacetamide is 0.8 mol/L,a composite structure consisted of porous Ni3S2 nanofilms and Ni3S2 microspheres which are formed by porous Ni3S2 nanofilms are obtained.At a current density of 5 mA/cm2.the composite electrode demonstrates a high areal capacitance of 3.78 F/cm2,and retains a areal capacitance of 1.15 F/cm2 at 60 mA/cm2.Furthermore,the composite electrode delivers an energy density of 55.97 Wh/kg at the power density of 540.81 W/kg and an energy density of 13.31 Wh/kg at a high power density of 4990.73 W/kg.