Preparation Of Graphene Hydrogel And CoNi(Mn)-layered Hydroxides And Their Electrochemical Properties

Supercapacitor,a new energy storage device,embraces high power density,long life-span and fast charge and discharge rate.It is seen as a complement to battery and thus has an extensive application.Traditional supercapacitor electrode materials can be categorized into two main types on the basis of their charge storage mechanisms : carbon-based nano-materials and pseudocapacitive materials.In recent years,the engergy storage material has been studied intensively and applied widely in many field.Graphene,as one of advanced carbon materials,is a hotspot in many research fields.Moreover,how to design new techniques for the fabrication of three-dimensional(3D)architectures of graphene has been the interest of researchers.This paper,therefore,is focused on the preparation method of 3D graphene structure.Additionally,battery-like materials have been widely applied in supercapacitors area recently.For one thing,we prepared three-dimensional cobalt/nickel layered double hydroxides microspheres and explored the application value in hybrid energy device.For another,we explored the electrochemical properties of ternary metal hydroxides and further improved their performance deficiency.The main results are as follows:(1)Three-dimensional(3D)interconnected graphene hydrogel(GH)was successfully prepared by using glucose and ammonia solution for reducing graphene oxide(GO)under mild hydrothermal conditions.During the reaction process,GO sheets were self-assembled into a well-defined 3D GH,and the GH was nitrogen(N)doped because of the incorporation of ammonia solution.Besides,it was found that compared to GH-1,GH-2 with higher N content presents more orderly interconnected structures and better electrochemical properties.(2)Three-dimensional nanostructured cobalt/nickel layered double hydroxides(Co1-xNix LDHs)microspheres were synthetized by a solvothermal method using ethanol as solvent and urea as alkali source.By simply tailoring the molar ratio of nickel and cobalt,the Co0.2Ni0.8 LDH with optimal electrochemical property is determined which delivered a high capacity of 855.4 C g-1 at 1 A g-1,and still retained a high capacitance of 777.9 C g-1 as the current density increased up to 10 A g-1.Moreover,a hybrid device was successfully fabricated by using the Co0.2Ni0.8 LDH as the positive electrode and activated carbon(AC)as the negative electrode.The hybrid Co0.2Ni0.8 LDH//AC device delivered a high energy density and power density with an operational voltage of 1.6 V.(3)Cobalt/nickel/manganese layered hydroxide compounds(CoNiMn-LHCs)were prepared and fabricated on the surface of Ni foam(NF)substrate via an in situ growth method.We found that the electrode of 2-CoNiMn/NF exhibited highest specific capacity by regulating the relative molar ratio of initial reactants.However,the capacity of this electrode decreased to 30.4% of its initial capacity after 1000 cycles at 10 A g-1.Therefore,we designed two composite electrode materials to improve the cycle performance.First,the NF substrate was immersed into 3 mg mL-1 GO sulution,and the NF would be wrapped with RGO film in the process of hydrothermal reaction.This composite electrode is named after RGO/NF.Second,we fabricated Co3O4 arrays on the surface of NF(Co3O4/NF).And then CoNiMn-LHCs materials were growed on the surface of RGO/NF and Co3O4/NF in the same solvothermal condition.Thus we obtained two composite electrodes——CoNiMn/RGO/NF and CoNiMn/Co3O4/NF.The electrochemical tests indicated that both composite electrode materials showed better cycle performance.The CoNiMn/Co3O4/NF electrode,especially,displayed very high specific capacity,illustrating its great research value and potential applications in energy storage area.