Preparation And Supercapacitive Properties Of Transition Metals(Co,Ni) Nanocomposites

The oxide,hydroxide and sulfide,which are composed of transition metals,have excellent optoelectronic,electrical and mechanical properties,and so on.Therefore,they always the emphasis of research in the fields of photocatalysis,photo-electrocatalysis,lithium-ion batteries and supercapacitors.Among them,supercapacitors have drawn increasing attention in recent years due to their high power density,long cycle life,ability to bridge the power/energy gap existing between conventional capacitors and batteries/fuel cells.However,the lower energy density of the supercapacitor relative to the battery always restricts its widely application.From its energy formula:E = 1/2CV2,it can be seen that the specific capacitance of the electrode material and the voltage window of the device are important factors in determine the energy density of the supercapacitor.Therefore,Therefore,it is an effective way to increase the energy density by selecting or constructing electrode materials with high capacitance and wide voltage window.In this article.The electrode materials such as oxides,hydroxides and sulfides with high specific capacitance were prepared by selecting transition metals such as Co and Ni,and the following three aspects were described:1.Reduced CoNi2S4 nanosheets with enhanced conductivity for high-performance supercapacitorsDefect engineering on transition metal dichalcogenides has been regarded as an effective method to improve electrochemical properties in terms of generating active sites and enhancing the intrinsic conductivity.This study reports a new high-performance electrochemical supercapacitor made of reduced CoNi2S4(r-CoNi2S4)nanosheets,which were synthesized via a facile moderate-reduction process.The sulfur-deficient r-CoNi2S4 nanosheets exhibit significantly enhanced conductivity which is induced by abundant sulfur vacancies formed in the reduction reaction.Compared with the pristine CoNi2S4 nanosheets,the r-CoNi2S4 nanosheets are characterized with a higher specific capacitance(1117 C g-1 at current density of 2 A g-1)as well as excellent rate capability and stable cycling performance.First-principle analysis confirms that the sulfur vacancies originating from the reduction lead to improved hybridization between the Ni and Co d states and the S p states close to the fermi level,and consequently enhanced conductivity with the CoNi2S4 nanostructure.Moreover,an ultrahigh specific energy density of 55.4 Wh kg-1 at the power density of 8 kW kg-1 is obtained in an asymmetric supercapacitor configuration,and 80%capacitance of the supercapacitor remains even after 10000 cycles.2.Carbon-coated MoS2@NiCo2S4 core-shell hollow-sphere structure for high performance supercapacitorsIn this chapter,hierarchical carbon-coated MoS2@NiCo2S4(MoS2@NiCo2S4@C)core-shell hollow sphere structure was prepared with a mild hydrothermal method by using glycerate-molybdenum spheres as precursors and templates.Due to the protective carbon layer and the cross-linked NiCo2S4 shell providing more active sites to contact with the electrolyte.When the MoS2@NiCo2S4@C was applied to a supercapacitor electrode material,which shows a high specific capacitance of 901 C g-1 at 2 A g-1.Meanwhile,the specific capacitance remained 87%after 5000 cycles at a current density of 10 A g'1,indicating excellent cycle stability of the electrode material.3.Synthesis and supercapacitor properties of FeCo2O4@NiCo-LDH core-shell structuresThis work mainly int:roduces the core-shell structure of FeCo2O4@NiCo layered double hydroxide(LDH)prepared by a simple hydrothermal method,which FeCo2O4 nanorods as the core and NiCo-LDH nanosheets as the shell.The supercapacitor performance of the prepared FeCo2O4@NiCo-LDH composite was found to have an excellent specific capacitance of 1050 C g-1 at a current density of 2 A g-1,even though the current density was increased to 40 A g'1,this core-shell composite still retains a significant 886 C g-1 capacitance,showing superior rate performance.In addition,the specific capacitance remains 90%after 5000 cycles at a current density of 10 A g'1.