Phosphating Of Graphene/cobalt Tetroxide Composite Electrode And Its Effect On Capacitance Properties

Supercapacitors have attracted considerable attention from researchers and governments all over the world due to their unique properties such as simple construction principle,low maintenance cost,high power density,long cycling life and good reversibility.The supercapacitor is mainly composed of four parts:cell,electrode material,electrolyte and separator,in which the electrode material is the key component determining the specific capacity,the rate capability,the cycle life,self-discharge and impedance performance of device.Graphene is a special two-dimensional carbon material with large specific surface area,high carrier mobility and good thermal conductivity as a new electrode material candidate for the next generation supercapacitor.However,the van der Waals interaction and the ?-?force between the graphene layers lead to the easy stacking of the layers,the reduction of the specific surface area contacted by the electrolyte ions,and the limitation of the ions transport,causing the lower specific capacitance of the double-layer eventually,thus making it difficult to achieve ideal device performance.Cobalt oxide(C03O4)has the advantages of natural abundance,environmental-friendly feature,corrosion resistance and high theoretical capacitance.However,its low conductivity and unsatisfying cycle stability caused by its semiconductor characteristics seriously restrict its further application as electrode materials.In certain conditions,Co3O4 can be composited with the graphene and it is expected to combine good conductivity of graphene and high pseudo-capacitance of Co3O4 to achieve complementarity of their performance.Nevertheless,the intrinsic chemical reaction activity of Co3O4 can not be significantly improved by compositing,which still results in unsatisfying energy storage performance.In this work,the sodium hypophosphite(NaH2PO2·H2O)is used as phosphorus source to improve the electrochemical properties of graphene/cobalt oxide hybrid electrode materials by heating the phosphorus source to phosphate the hybrid electrode materials with different morphologies.The effects of phosphating conditions on morphology,structure,composition and electrochemical properties of phosphating samples are systematically explored by changing the parameters such as phosphating temperature,time and phosphating mass ratios.The main research contents are as follows:(1)The precursor GO/Co(OH)2 nanoparticles are prepared by low-temperature liquid-phase method.RGO/Co3O4 nanoparticles composites with good crystallinity are prepared by calcining the precursor GO/Co(OH)2 nanoparticles at 275? in air atmosphere for 2 h,then followed by another calcination at 500? for 90 min in nitrogen atmosphere.By analyzing the morphology and structure of the calcined products,the optimized preparation conditions of RGO/Co3O4 nanoparticles composites with good crystallinity are explored.It is expected that GO can be reduced to RGO effectively by the thermal reduction,and Co3O4 nanoparticles can uniformly supported by the graphene layers.On the basis of high crystallized RGO/Co3O4 nanoparticles,NaH2PO2·H2O is reacted with RGO/Co3O4 nanoparticles by pyrolysis at different mass ratios between them at different treatment temperature and treatment time.The optimized conditions of phosphating RGO/Co3O4 nanoparticles composites are studied.Using sample electrode prepared at optimized phosphating treatment conditions as the working electrode,the cyclic voltammetry curves(CV)and the constant current charge and discharge curves(GCD)of the sample electrode are measured in the three electrode system with 6 M KOH as aqueous electrolyte.The improved mechanism of electrochemical properties of sample electrode by phosphating treatment is explored.The crystalline phases of the phosphating samples are greatly influenced by the phosphating parameters.Higher phosphating temperature and phosphating mass ratios will result in the formation of CoP or CoO.The optimized phosphating sample electrode shows the improved specific capacitance and rate capability.The mass specific capacitance of the optimized phosphating sample electrode PIF-RGO/Co-275-500 is 330 F g-1 at a current density of 1 A g-I.When the current density is increased to 20 A g-1,the capacitance retention is up to 71.2%.The capacitance performance and rate capability of the composite is improved by phosphating.(2)Precursor GO/Co(OH)2 nanosheets are prepared by low-temperature liquid-phase method using surfactant P123 as the morphology guide reagent.Precursor GO/Co(OH)2 nanosheets are calcined at 275? for 2 h in air atmosphere.Subsequently,RGO/Co3O4 nanosheets are prepared by calcination at 500? for 90 min in N2 atmosphere.The porous structure is expected to be formed on the surface of Co3O4 nanosheets because of the evaporation of water and the volume shrinkage caused by the change of crystalline phase during calcination.Meanwhile,Co3O4 nanosheets with length size of 1?m in can be uniformly supported on the surface of graphene layers,and GO in the composites is effectively reduced to RGO by the thermal reduction.By changing the phosphating temperature,time and phosphating mass ratios,the optimized phosphating treatment conditions of RGO/Co3O4 nanosheets are investigated.The optimized conditions of phosphating treatment is treated at 225? for 30 min under phosphating mass ratio of 10 in a tube furnace with the inert atmosphere.We expect that specific capacitance and rate capability of the phosphating sample electrode can be further improved.When the current density is 1 A g-1,the mass specific capacitance of the PIF-RGO/Co3O4-35 sample electrode is up to 448 F g-1.When the current density is increased to 10 A g-1,the capacitance retention rate is 78%,which is significantly higher than that of the sample electrode without phosphating.Meanwhile,the capacitance retention rate of the phosphating sample electrode PIF-RGO/Co3O4-35 is 117%after 5000 cycles at a current density of 10 A g-1,showing good cycle stability.The results demonstrate that phosphating treatment can effectively improve the capacitance performance and rate capability of composites.The main reasons for improving the electrochemical performance of the materials electrode are that(H2PO4)-and(PO3)-species formed on the surface of Co3O4 increase the diffusion rate of electrolyte ions and decrease the charge transfer resistance.At the same time,intermediate phosphating products formed during the phosphating reaction have higher reactive activity than Co3O4,which makes the redox reaction of phosphating samples faster and more effectively in the electrolyte.Phosphating technology provides a new idea to improve the performance of cobalt oxide-based electrode materials.