Studies On Synthesis And Supercapacitive Property Of M_xO_y-based(M=Mn Or Co)Composite Electrode Materials

As energy and environmental issues become more and more intensified,environmentally friendly and high-performance energy storage devices have drawn widespread attention.Supercapacitors have become one of the most promising electrochemical energy storage devices due to their high power density,long life,safe operation and fast charge-discharge.However,it has some drawbacks such as low working voltage and relatively small energy density.One of the important approaches to improve the performance of supercapacitors is to develop high-performance electrode materials.Since single materials used as electrode materials show some shortcomings,herein,we have combined the conducting materials(carbon material,conductive polymer materials,etc.)with metal oxides to prepare pseudocapacitive composite electrode materials with good performance.The main contents are as follows:1.A binder-free supercapacitor electrode of polypyrrole(PPy)/MnO2/Ni foam was fabricated by two-step approach.Well-aligned MnO2 nanosheets arrays(MnO2 NSAs)are prepared by hydrothermal method and grown on the 3D Ni foam,and followed by the deposition of in-situ polymerized PPy onto each MnO2 nanosheet surface.Specific capacitance of the electrode is 282 mF cm-2 at 1 mA cm-2,and 95%of the capacitance is kept after 6000 charge-discharge cycles at 4 mA cm-2.A button-type symmetric supercapacitor device(SSC)was fabricated using PPy/MnO2/Ni foam electrodes.It exhibits the outstanding cycleability(90%retention even after 6000 cycles)and also sucessfully lights a LED.The enhanced pseudocapacitive performance is mainly due to the well-designed 3D hierarchical structure of the composite electrode which can provide a large number of active sites,fast ion and electron transfer rate,and good strain accommodation of the ultrathin core-shell structure.The impressive electrochemical performance and the low cost property suggest its great promise in energy storage applications.2.The unique self-assembled ZnO-CoO@NC heterogeneous mesoporous microspherical composite(ZnO-CoO@NC)was synthesized for supercapacitors with the long-term cycling stability,trough a facile hydrothermal method followed by a post-annealing treatment,using polyvinyl pyrrolidone(PVP)as a structure-directing agent and precursor of N-doped carbon(NC).The oxygen vacancy defects and nitrogen-doped carbon were proved by XPS.The effect of PVP concentration on the morphology and electrochemical properties of ZnO-CoO@NC was investigated.The result shows that ZnO-CoO@NC electrode in 2 M KOH reaches a specific capacitance of 301 F g-1 at 0.5 A g-1,and the capacitance retention is 77.2%from 0.5 to10 A g-1.Moveover,it exhibits an excellent cycle stability with 92%retention of the initial capacitance after 40000 constant current charge-discharge(GCD)cycles at 2 A g-1.The assembled asymmetric supercapacitor(ASC),ZnO-CoO@NC//graphene(GE),also performs the outstanding cyclic stability with 94%capacitance retention after 10000 cycles at 2 A g-1.The remarkable cycle stability and rate performance of the self-assembled ZnO-CoO@NC composite benefits from the well-designed hierarchical mesoporous structure of microspheres,rich oxygen vacancy defects in the surface of metal oxides,the possible synergestic effect among these components,as well as the high conductivity of ZnO scaffold nitrogen-doped carbon layer.3.A flexible electrode material of graphene/manganese dioxide(rGO/MnO2)composite paper was prepared by a simple combination method of filtration and post-reduction.Firstly,graphene oxide(GO)/MnO2 was obtained by vacuum filtration of the mixture of GO and MnO2 prepared in advance.Then it was calcined and reduced to form the flexible electrode materials of rGO/MnO2 paper.The structure and morphology were characterized by XRD,TEM and SEM.The electrochemical properties of this binderless,flexible supercapacitorelectrode material were measured in KOH solution.It is found that the specific capacitance of the rGO/MnO2 electrode was 107.9 F g-1 at 0.5 A g-1,and when the current density increased from 0.5 to 6 A g-1,the capacity retention was 71.6%.These excellent electrochemical properties show that it can be a promising supercapacitor electrode material.