Study On The Properties Of Transition Metal Oxide As Electrode Materials For Supercapacitors

Supercapacitors developed rapidly in recent years.Transition metal oxides,hydroxide and LDHs based on Faraday pseudocapacitive have become the most widely used electrode materials,because of their high theoretical capacitance and low cost.However,the actual capacity is much lower than the theoretical capacity due to the limit of charge transmission rate.To solve these problems,we try to build three dimensional structure and form a directionally ordered nano array architecture in order to improve their specific surface area and electronic transmission rate,so that their electrochemical performance can improve greatly.In this paper,ZnO@MnO2 core-shell arrays,MgAI-LDHs films on aluminum substrate and NiAl-LDHs nanosheet arrays on nickel foam were prepared.The morphology and electrochemical properties of the materials were tested by various characterization methods.The details of the work are as follows:ZnO@MnO2 core-shell column arrays on nickel foam have been synthesized by a two-step approach,which includes hydrothermal synthesis of ZnO column arrays and electrodeposition of MnO2.And the core-shell column array structurewas confirmed by various characterization methods.In addition,the hierarchical ZnO@MnO2 core-shell column arrays on nickel foam are directly used as binder-free electrodes for supercapacitors.The electrode exhibits high specific capacitance of 586.8 F g-1 at a current density of 2 A g-1,which is much higher than the capacitance of pure MnO2.Furthermore,the ZnO@MnO2 electrode only loses 7.2 % of the initial capacitance after 2000 charge-discharge cycles,which demonstrates its excellent cycling stability.It is proved that the three dimensional core-shell arrays structure can improve the electrochemical performance of materials.The Mg AI-LDHs film perpendicular to the substrate was grown on the surface of pure aluminum by the method of urea hydrolysis.The morphology test shows that the film is a typical LDHs material,and the flake particles are uniform and interlaced,forming a very dense film.And The film has good smoothness.The scratch test shows that the binding force between the film and the substrate is very strong.After a week of immersion in 4% Na Cl Solution,the morphology of the films did not change significantly,and the lamellar structure remained clear and complete,which proves its good anticorrosion property.The electrochemical impedance spectroscopy(EIS)test shows that the film has strong corrosion resistance,and the corrosion resistance enhances with the increase of film thickness.The mechanism can be elucidated that the dense film can block the charge transfer between the metal substrate and the solution,thus inhibiting the corrosion of the ions in the solution.The barrier effect is shown in the form of impedance in the EIS curve.The porous Ni Al-LDHs nanosheet arrays on nickel foam have been synthesized by urea hydrolysis.The structure and morphology of Ni Al-LDHs nanosheet arrays are characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and energy dispersive X-ray spectroscopy(EDS).The results show that the material has a porous honeycomb structure,it is the special structure and morphology of material that can provide great active surface area and good electron transfer ability,so as to improve the specific capacitance.The electrochemical test results show that the electrode material exhibits good electrochemical performance.The electrode exhibits high specific capacitance of 684.67 Fg-1 at a current density of 2 Ag-1.Furthermore,the NiAl-LDHs electrode only loses 7.2 % of the initial capacitance after 2000 charge-discharge cycles,which demonstrates its excellent cycling stability.With the discharge current density increased from 1 Ag-1 to 20 Ag-1,the specific capacitance decreased to 55.6%.It can be seen that the rate capability of the material still needs to be improved.