Preparation And Electrochemical Properties Of Graphene/Manganese Dioxide Composite Films

Supercapacitor,a new type of environment-friendly energy storage device,has the characteristics of fast charging/discharging rate,good cycle stability and high power density.However,its energy density?especially the volume energy density?is still low,which limits its application.So far,the carbon material is the most widely used electrode material because of its high specific surface area,rich hole structure and excellent conductivity.Due to the structure,the charge is mainly stored in the form of a double layer structure.With the low apparent density,the specific capacity of carbon materials?especially the volume ratio capacity?is also very low.How to improve the specific capacity of carbon materials by regulating the structure and improving the technology has become one of the research hotspots in this field.In this paper,the packed graphene electrode material with high volume energy density has been researched.It's a wally idear to improve the diffusion of electrolyte ions inside the materials by inserting supported materials between graphene layers and creating holes at the surfaces of the graphene.The pseudocapacitor is also introduced by metal oxide doping,which can enhance the electrochemical ratio capacity of the material.Specific research contains the following two sides:Firstly,uniform holes are formed on the surfaces of the graphene by chemical etching method,on the basis of which the porous graphene and manganese dioxide nanowires are assembled in three-dimensional space to prepare the packed porous graphene/manganese dioxide nanowires composite membrane material?PGNs-MnO₂NWs?.The manganese dioxide nanowires supporting the graphene layer play the role of inhibiting the agglomeration of graphene layers and increasing the specific capacity of the composite films.And the pore structure of graphene surfaces effectively converts the two-dimensional ion diffusion mode between graphene layers into three-dimensional diffusion mode,which significantly shortens the diffusion distance of electrolyte ions between graphene layers.The problem of increasing diffusion resistance between ionic layers of electrolyte caused by graphene densification is solved.The results show that the PGNs-MnO₂ NWs composite membrane electrode material has fast ion diffusion channels and a packed spatial structure(density of 1.25g cm^-3).When the scan rate is 5 mV s^-1,the mass ratio capacity of the PGNs-MnO₂NWs composite film is 117 F g^-1,and its volume ratio capacity is 147 F cm^-3 in 1 mol L^-1 Na₂SO₄ litmusless electrolyte.On the basis of the PGNs-MnO₂ NWs composite membrane,the porous graphene and the carbon nanotubes with growed manganese dioxide nanoparticles,are assembled in three-dimensional space,which comes into being the packed porous graphene/carbon nanotubes&manganese dioxide nanoparticles composite membrane materials?PGNs-CNT&MnO₂ NPs?.The carbon nanotubes supporting the graphene layers plays the role of inhibiting the agglomeration of graphene layers and enhancing the mechanical properties and the conductivity.Manganese dioxide nanoparticles growing on the carbon nanotubes can produce pseudocapacitors by redox reaction,which improves the specific capacity of composite films.The holes of the graphene surface still shortens the diffusion distance of electrolyte ions between graphene layers.The results show that the PGNs-CNT&MnO₂ NPs composite membrane electrode material has fast ion diffusion channels and a packed spatial structure(density of 0.78 g cm^-3).When the current density is 1 A g^-1,the mass ratio capacity of the PGNs-CNT&MnO₂ NPs composite membrane is 320 F g^-1,and its volume ratio capacity is 275 F cm^-3 in 1 mol L^-1 Na₂SO₄ litmusless electrolyte.In addition,the porous graphene and the carbon nanotubes are assembled to prepare the porous graphene/carbon nanotubes composite film?PGNs-CNT?.An asymmetric supercapacitor are assembled in 1 mol L^-1 Na₂SO₄ litmusless electrolyte with the PGNs-CNT&MnO2 NPs composite membrane as the anode and the PGNs-CNT composite film as the cathode.Its mass energy density has the maximum of 26.43 Wh kg^-1 of itself,and its volume energy density has the maximum of 22.73 Wh L^-1 of itself.