Electrodeposition Preparation And Electrochemical Properties Of Perovskite LaMnO₃-based Composite Electrode Materials

Supercapacitors are electrochemical energy storage devices characterized by large capacity,high power density,maintenance-free,less environmental pollution,long cycle life,and wide operating temperature range.Hence,supercapacitors are widely used in backup power systems,portable electronic equipment,electric vehicles,and other fields.Perovskite is a transition metal oxide with a unique ABO₃structure.Previous studies have found that the perovskite-structured lanthanide metal oxide La Mn O₃has good electrochemical performance,and its"oxygen anion intercalation"energy storage mechanism has also been confirmed.Since then,perovskite materials have attracted great interest in the field of supercapacitors.La Mn O₃has been studied and found to have good thermal stability,wide potential window and excellent ionic conductivity.However,perovskite materials still suffer from some problems such as poor electronic conductivity,insufficient theoretical capacity and poor cyclic stability,which hinder its practical application.These problems are related to the characteristics of the material itself,and are also limited by the commonly used synthetic methods.The common methods for the preparation of La Mn O₃include solid phase reaction method,hydrothermal method and sol-gel method,etc.,which have problems of large particle size,many pollutants and sample agglomeration respectively.We urgently need to expand the preparation methods of perovskite oxides.Electrochemical deposition has been widely used in the synthesis of electrode materials.The samples prepared by electrochemical deposition usually have small particle size and can grow regularly and orderly on the substrate.In addition,the method is environmentally friendly,and the equipment used is also commonly used for electrochemical testing.Therefore,electrochemical deposition is a good choice for the preparation of La Mn O₃.On the other hand,in order to solve the problems caused by the characteristics of La Mn O₃itself,it can be solved by combining it with materials with high specific surface area,high conductivity and high capacity.Composite materials can often make up for the insufficiency of a single component,play a synergistic effect between the components,and finally obtain a new type of electrode with good performance.Based on the above investigation,La Mn O₃-based composite electrode will be prepared by electrochemical deposition.Firstly,La Mn O₃nanoparticles were grown in situ on conductive carbon cloth(CC)to obtain La Mn O₃@CC electrode.After that,the high performance materials polypyrrole(PPy)and Ni Co₂O₄were grown on the surface of La Mn O₃@CC by secondary electrodeposition.La Mn O₃@CC-PPy with coaxial cable structure and La Mn O₃@Ni Co₂O₄/CC with multilayer nanoarray structure were obtained,respectively.PPy and Ni Co₂O₄were used to improve the conductivity,specific capacitance and cycling stability of La Mn O₃composite electrode.At the same time,the rational composite structure designed by electrochemical deposition will also give full play to the synergistic effect between the components and improve the electrochemical performance of the composite electrode.The main research contents of this thesis are as follows:1.We prepared the composite electrode La Mn O₃@CC-PPy by two-step electrodeposition.The PPy film wraps the uniformly dispersed La Mn O₃nanoparticles,forming a multi-layer nanostructure system.Through this design,we constructed a coaxial cable-like morphology.This enables composites with good electron and ion transfer rates.The specific capacitance of La Mn O₃@CC-PPy can reach up to 862 F g^-1at 1 A g^-1,which is much higher than 143 F g^-1of La Mn O₃@CC,and still has a retention rate of 75%at 10 A g^-1.It proves that the rate performance of the material is also very good.Consequently,La Mn O₃@CC-PPy was used as anode material to assemble an asymmetric supercapacitor(ASC)with the as-prepared cathode Ni Co₂O₄@CC,the ASC provides an ultra-high energy density of73 Wh kg^-1at a power density of 800 W kg^-1.At the same time,it exhibits an excellent energy density of 34 Wh kg^-1at an extremely high power of 24000 W kg^-1.ASC also has good cycling stability.After 3000 charge-discharge cycles,the capacitance retention rate remained at 66%.2.The Ni Co₂O₄nanosheets were coated on La Mn O₃@CC by electrodeposition,and the in-situ multilayer nanoarray structure was formed.This structure shortens the electron transfer path and promotes the electrochemical reaction process.The composite La Mn O₃@Ni Co₂O₄/CC has both the large capacity of spinel and the wide potential window of perovskite,and has a high specific capacitance of 941 F g^-1in the potential range of-0.5-0.6V.Higher than 672 F g^-1of Ni Co₂O₄@CC and 182 F g^-1of La Mn O₃@CC.Moreover,the multilayer nanoarray structure grown in situ can effectively improve the cyclic stability of the material,and the capacitance retention is more than 90%after 8000 cycles.Considering the general application of material selection,La Mn O₃@Ni Co₂O₄/CC and the common negative material activated carbon(AC)were assembled into asymmetric supercapacitor,and the maximum energy and power density of the device reached 51 Wh kg^-1and 22,500 W kg^-1,respectively.After 10000 cycles,the performance is 106%of the initial capacitance,showing ultra-high cycle stability.The above results show that the devices have high energy density and long service life,and have very good application prospects.