Synthesis And Electrochemical Performances Of Co₃O₄@Cobalt Phosphate Core-shell Composite Materials As Anode For Supercapacitor

Energy shortage has become a huge challenge for the human society after the oil crisis in 1970s.The verity of energy issues have had a huge impact on the global political situation,economic operation and human life.Therefore,the development of efficient energy storage systems which are safe,energy-saving and environmentally friendly become the key to development.In recent years,as a new kind of energy storage,supercapacitor was paid much attention because of its numerous advantages such as safe and reliable,high energy storage efficiency,long life and low pollution.The last few years,supercapacitors based on transition metal oxides with high power density and good cycle stability become the focus of research,of which the cobalt oxide(?,?)(Co3O4)has become a research hotspot in the family of transition metal oxides due to its high theoretical specific capacitance,abundant reserve,lowcost and environmentally friendly.However,the reported capacitances(200-700 F g-1)of Co3O4-based electrodes are still far lower than their theoretical values.Therefore,it is necessary to find novel strategy to improve its performance.Co3O4 arrays can act as not only a popular electrode material but also an ideal skeleton for other electrode materials.So designing rational Co3O4-core/shell hybrid composit is an effective way to enhance the electrochemical performance.In addition,the transition metal phosphate namomaterials are also good candidate for supercapacitor electrode.Compared with corresponding crystalline counterpart,amorphous materials usually possess a large number of defects,which can be served as reversible active sites,and its large channels can facilitate the diffusion and reaction of electrolyte ions.Based on the above analysis of Co3O4 and amorphous cobalt phosphate(Co-Pi),Co3O4@Co-Pi core-shell nanoarray composite with good electrochemical performance is successfully synthesized,and the electrochemical performance of the samples obtained under different conditions is discussed.The main contents of this thesis are as follows:(1)The basic carbonate cobalt nanowire arrays precursor on nickel foam substrates is synthesized via hydrothermal method.Then the synthesis of Co3O4 nanowire arrays is achieved via annealing in N2 atmosphere.Finally using sodium hypophosphite(NaH2PO2-H2O)(2.0g)as P source,Co3O4@Co-Pi core-shell structure naterial is synthesized at different temperatures(200 ?,250 ? and 300 ?)and different time(1 h,2 h,3 h).The crystal structure,morphology and electrochemical properties of the material were studied via XRD,SEM,TEM,HRTEM,HAADF-STEM-EDS,RAMAN,ATR-FTIR,XPS and electrochemical measurement.The resulting core-shell Co3O4@Co-Pi nanowires array exhibits superior(1692 F g-1 at a current density of 1A g-1)compared to pure Co3O4 nanowires array.And remain 86%of capacitance after 6,000 cycles,which shows good charge retentions.An asymmetric supercapacitor(ASC)is assembled by using the optimized Co3O4@Co-Pi as the positive electrode and activated carbon(AC)as the negative electrode.Remarkably,at the power density of 558 W kg-1,the supercapacitor device delivers a better energy density of 35.69 Wh kg-1.And it exhibits super-long cycle stability(82%charge retentions after 40,000 cycles).Co3O4@Co-Pi core-shell nano materials have the following advantages:? The good contact of two compounds promotes electron transfer in the process of redox.? The morphous phase with high structure of disorder has unique electrochemical properties.? The core-shell structure has the ability to mitigate the destruction of the core structure,which can make the core-shell nanomaterial have good cycle stability.(2)In order to explore the difference of electrochemical performance between crystals and non-crystals,the amorphous structure is transformed into crystal structure via high-temperature annealing on the basis of the first part of the study,and the whole crystal Co3O4@CO2P4O12 core-shell composite material is successfully obtained.The composites are characterized by SEM,XRD and TEM.And then the influence of different annealing temperatures on the electrochemical performance of Co3O4@CO2P4O12 core-shell composites are investigated.The results show that the Co3O4@Co2P4O12 core-shell composite which is annealed the at temperature of 500 ? has the best electrochemical performance,whose capacitance reaches up to 1000 F g1 at a current density of 1 A g-1.Compared with Co3O4 nanomaterials,the electrochemical properties of the as-annealed Co3O4@CO2P4O12 core-shell composites are superior to Co3O4.But compared with Co3O4@Co-Pi core-shell composites,the power of Co3O4@CO2P4O12 electrochemical properties are in a weak state.In summary,the results is that the electrochemical properties of the amorphous composites are better than crystalline material,and the electrochemical properties of the composites are superior to the monomeric nanomaterials.