Rare Earth Metal Lanthanum And Cerium Derived Nanocomposite Materials For Energy Storage

As an environmentally friendly energy storage device,supercapacitors are widely used in all walks of life due to their low cost and high efficiency.Compared with batteries and fuel cells,supercapacitors not only gain better stability and greater power density but also have the advantage of fast charge/discharge process,which have been applied to electric vehicles,portable electronic devices and so on.However,the low energy density and the narrow working voltage of supercapacitors are still the main factor to hinder their development.According to E=1/2CV2,the high specific capacitance(C)and large operating voltage(V)are the fundametal reasons for effectively improving the energy density of supercapacitors.Therefore,development of new electrode materials has become one of the important means to increase the specific capacitance of the device,and subsequently increase the energy density of supercapacitors.In this thesis,the rare earth metals or their derived electrode materials with low cost and excellent performance are prepared based on the structural advantages of the rare earth metals lanthanum and cerium.Meanwhile,based on the unique structure of the material and excellent electrochemical energy storage characteristics,the energy density of the symmetrical and asymmetrical supercapacitors devices are improved.This has developed and enriched the research of rare earth metal-based electrode materials in supercapacitors.The main research contents and results are as follows:(1)A new three-dimensional(3D)mesoporous interconnected carbon nanosheet(LMCN)material with large surface area and high graphitization degree was prepared by the method of simultaneous activation and catalytic carbonization using the rare earth metal lanthanum-organic framework as a premise.During the synthesis process,ZnCl2 and FeCl3 act as activation and catalyst,respectively,effectively promoting the formation of mesoporous carbon nanosheets with developed pore structures.The LMCN-1 prepared with the optimal ratio of ZnCl2 and FeCl3 not only possesses ultra-thin nanosheets that are cross-linked,but also has abundant porosity.In the three-electrode tests,LMCN-1 as the electrode material has a specific capacitance of 251 F g-1 at a current density of 0.5 A g-1.In addition,the symmetric supercapacitor assembled based on the LMCN-1 electrode can reach 1.8 V in 0.5 M Na2SO4 aqueous solution,and the energy density is as high as 20.7 W h kg-1 at a power density of 228 W kg-1.After 10,000 cycles,it still has a capacity retention rate of 90.7%.(2)A rare-earth metal cerium(Ce)prussian blue analog nanosheets(Ce-PBA)was prepared by co-precipitation,followed by a one-step high-temperature calcination process to obtain CeO2 and nitrogen-doped carbon(CO/NC)nanosheet composites.Among them,the CO/NC-700 nanosheet composites prepared at the most suitable carbonization temperature gain the largest specific surface area and excellent electrochemical performance.When it is used as a supercapacitor electrode material,the pseudocapacitance effect of CeO2 and doping nitrogen make the composite material have a high specific capacitance value(the specific capacitance value reaches 236.5 F g-1 at 0.5 A g-1).Based on above,the optimized CO/NC-700 was used as the electrode material with the aqueous KOH was used as the electrolyte,and a symmetric supercapacitor device was assembled.At a power density of 125 W Kg-1,the energy density was 5.63 Wh Kg-1.Even at a high power density(2700 W kg-1)the energy density is also maintained at 4.23 Wh Kg-1.After 10,000 charge and discharge tests,having a capacity retention rate of 85%(3)The simple one-step solvothermal growth method doped a typical rare earth element lanthanum(La)into NiHN(La-NiHN)nanomaterials.For the constructed La-NiHN nanospheres,large-diameter lanthanum ions can be as a pillar directly increase the c-axis spacing and lattice volume lead to successfully improve the conductivity of NiHN nanosheets,which will cause a high specific capacity(at 1 A g-1 to 154 mAh g-1)and high-capacity positive electrode performance.In addition,Fe2O3(Fe2O3/GNS)loaded on graphene nanosheets with high specific capacity and excellent stability was selected as the negative electrode to match the La-NiHN nanoflower positive electrode to assemble a new La-NiHN//Fe2O3/GNS asymmetric supercapacitor(ASC).Due to excellent electrochemical performance and complementary working voltage,the assembled ASC has a wide working voltage of 1.65 V and a high energy density of 51 Wh kg-1 at a power density of 825 W kg-1.At the same time,after 5000 cycles,it still has 87%cycle stability.