Preparation And Electrochemical Performance Of Transition Metal Electrode Materials For Asymmetric Supercapacitor

Supercapacitor is a new-type of energy storage device,which has high specific capacitance and excellent cycle performance.In this paper,the liquid phase coprecipitation method and hydro-thermal method were used to prepare the manganese dioxide electrode materials and nanometer cobalt oxide materials successfully.The properties of manganese oxide electrode materials were investigated by adding polyvinylpyrrolidone（PVP）,and the effects of phosphide modification on the electrochemical properties of the cobalt oxide materials were discussed.The metal oxide/resin activity carbon（RAC）composite electrode materials with two dimensional layered structure were prepared by the method of lattice doping,the composite electrode materials are assembled into asymmetric capacitors to increase energy density.Then the nanoparticles of Co_nNi_1-nLDHs were prepared by PVP assisted chemical coprecipitation,and the asymmetric supercapacitors were constructed.The details are summarized as follows:By adding PVP,the crystallinity of manganese dioxide electrode materials was reduced,and the particles were smaller and more dispersed,which increased the effective area of the materials and greatly improved the properties of the materials.The manganese dioxide electrode materials were optimized by heat treatment.At a constant temperature of 350℃,heat treatment weakens the aggregation of material functional groups,and makes the particles of the particle reduced,thus increasing the specific surface area,and improving the performance of the electrode materials.Through electrochemical tests,the material has shown high specific capacitance and great cycle performance.After circulating 1000times at the current density of 0.5 A g^-1,the specific capacitance of the sample is437 F g^-1,which can still be maintained at around 85%.After the test of Electrochemical Impedance Spectroscopy,it is found that the optimized sample has low internal resistance and strong conductivity,which is suitable for the electrode material of supercapacitor.Cobalt tetroxide was prepared by hydrothermal method,the crystallinity was decreased,the particles were smaller and more dispersed,and the effective area of the material was increased.Through heat treatment for 2 hours at temperature of 350℃,the material performance obtained huge improvement.Afterwards,with sodium hypophosphite as a source of phosphorus at 350℃,the precursor of cobalt and Co₃O₄ electrode materials is optimized.After phosphatization,the capacitance of cobalt tetroxide is obviously greater than that of cobaltous hydroxide,and its symmetry is also optimal.At the current density of 1A g^-1,the specific capacitance of phosphate ion functionalized Co₃O₄ can reach 439 F g^-1.After 1000 charge-discharge cycles at the high current density of 5 A g^-1,the capacitance has no obvious attenuation,which proves that phosphate ion functionalized Co₃O₄ is an ideal electrode material.The Co₃O₄/RAC composite with a two-dimensional layer structure was obtained after heat treatment by assembling two-dimensional cobalt acetate nano-sheet and phenolic resin with opposite charge by bulk phase doping method.Co₃O₄ manifests great dispersion in RAC,providing a channel for the generation of electron conduction in the pseudocapacitor reaction,and enabling the composite to show excellent electrochemical properties.The mass ratio of Co₃O₄and resin carbon has an important impact on the electrochemical performance of the composite.The experiment shows that Co₃O₄/RAC-50%has the best electrochemical performance.As can be seen,with ultrahigh performance of pseudocapacitor,specific capacitance of Co₃O₄/RAC-50%reached the maximum of 1037 F g^-1 at 0.5 A g^-1 current density,and maintained 93.5%capacitance after 10,000 cycles at the current density of 5 A g^-1.Co₃O₄/RAC-50%//RAC asymmetric capacitors are obtained by using Co₃O₄/RAC-50%composite material as positive pole and RAC as negative pole.When the power density is reduced from 3750 W kg^-1 to 24.9 W kg^-1,the energy density is increased from 45.4 W h kg^-1 to 49.8 W h kg^-1,which is about 3 times that of the Symmetrical capacitor.A series of Co_xNi_1-x LDHs composites were prepared by PVP assisted chemical coprecipitation.Co_xNi_1-x LDHs composite is made up ofα-Co（OH）₂ andα-Ni（OH）₂.The proportion of its composition has an important influence on the capacitance performance.Co_0.51Ni_0.49LDHs has the best capacitance performance.When the current density is 1 A g^-1,specific capacitance rises to 2221 F g^-1.When the constant current density is 10 A g^-1,the specific capacitance only decays by14.2%after circulating 5000 times.The asymmetric capacitors were constructed with the optimized Co_0.51Ni_0.49LDHs composite as the positive pole and activated carbon dioxide as the negative pole.The specific capacity reached its maximum when the RAC/Co_0.51Ni_0.49LDHs mass ratio was 4:1.When current density is120 m A g^-1,specific capacitance can reach 192.21 F g^-1.The energy density of an asymmetric capacitor is 64.24 Wh kg^-1 when the power density is 374.42 W kg^-1.However,at the huge energy density of 6240.38 W kg^-1,power density remained at 58.57 Wh kg^-1.