Synthesis And Electrochemical Properties Of Graphene Hydrogel Composites

Graphene has become a hot material in the field of supercapacitor because of its high specific surface area and conductivity.However,its sheet is easy to agglomerate and the capacity of charge storage in the double-layer capacitor is limited,which greatly hinders the application of graphene.The design of two-dimensional graphene to a three-dimensional graphene hydrogel(3DGH),elemental doping and pseudocapacitive material composite are effective manners to improve the electrochemical performance of graphene.This paper is devoted to the preparations of heteroatom doped 3DGH and 3DGH composite materials for supercapacitor electrode materials by simple,rapid and efficient methods,and the following work has been carried out:Firstly,the modified Hummers'method was used to prepare graphene oxide(GO).Depending on the improved reduction of thiourea in the acid and alkali medium,GO was reduced by thioure with adjusting the p H value of solution to obtain the three dimensional N/S co-doped graphene hydrogel(3DNS-GHs)at low temperature and atmospheric pressure,which leads to the great decrease of time and energy consumption.Through the structure analysis and characterization,it is found that with the change of p H value of the solution,the heteroatom doping amount,reduction degree,defect degree and nano pore structure of3DNS-GHs will also be different.It is the structural gap that result in the discrepancy of electrochemical performance.In the electrochemical performance test,under the optimal condition(p H=12),the specific capacitance of as-prepared 3DNS-GH-12 was 259.2 F/g at a current density of 1 A/g in 2 MKOH electrolyte solution,and the initial capacitance remained96.0%after 10000 cycles at a current density of 10 A/g,showing excellent charge storage capacity and cycle stability.Then,the water-soluble sodium anthraquinone-2-sulfonate and the GO solution prepared by the improved Hummers method were mixed at different mass ratios(0.4:1,1:1,2:1).The reduced graphene oxide(AQS/r GO)hydrogels modified by sodium anthraquinone-2-sulfonate were obtained directly through chemical reduction.The whole process was carried out at low temperature and normal pressure,and the time consumption was short,which greatly promoted the sustainability and efficiency of material preparation.Through the characterization analysis of composite materials,it can be seen that the addition of sodium anthraquinone-2-sulfonate enhances the layer spacing of graphene and reduces the agglomeration of graphene sheets.The specific surface area of the composite is significantly improved,compared with that of pure graphene.Besides,in the analysis of electrochemical properties,not only the pseudocapacitive is successfully introduced into the AQS/r GO hydrogels,but also the wettability and charge transfer efficiency of them are also obviously stronger in 1 M H₂SO₄ electrolyte solution.When the mass ratio of sodium anthraquinone-2-sulfonate and graphene oxide is 1:1,the specific capacitance of AQS/r GO-1 hydrogel reaches 387.43 F/g at 1 A/g current density.After 10000 cycles at a current density of 10 A/g,the specific capacitance of the hydrogel is maintained at 91.11%.Finally,we assembled those electrode materials into supercapacitors to further observe the effects of its practical application.The symmetrical supercapacitor assembled by 3DNS-GH-12 shows an energy density of 6.08 W h/kg and a high power density of 7.51 k W/kg.The asymmetric supercapacitors fabricated by AQS/r GO-1 hydrogel and pure graphene hydrogel exhibits a maximum energy density of 15.98 W h/kg,and the initial capacitance of 84.8%can be still maintained at 5 A/g current density after 10000 cycles.