Preparation Of Defective Metal Oxides And Their Applications In Supercapacitors

Supercapacitors are a type of high-efficiency energy storage devices.As a crucial segment of the supercapacitor,the electrode material greatly determines the performance of the device.Compared to electric double layer capacitor electrode material,electrode materials based on the pseudocapacitor energy storage mechanism can obtain higher specific capacity,and get more attention.Mn O₂,CeO₂pseudocapacitive electrode materials have high theoretical specific capacity and environmental compatibility,and have gained much attention recently.However,Mn O₂and CeO₂have the problems of poor electrical conductivity and bad cycle stability,which limits their further applications.In order to solve the above-mentioned problems,this paper introduced oxygen vacancies in Mn O₂ and CeO₂ materials through Na BH₄ solution reduction treatment,thereby preparing Mn O_2-x,CeO_2-x and composite materials CeO_2-x@Mn CO_3.The morphology,structure and composition of the electrode materials were measured by field emission scanning electron microscopy(FESEM),X-ray diffraction(XRD),Raman spectroscopy(Raman),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS)methods.The electrochemical test methods of cyclic voltammetry(CV),constant current charge-discharge method(GCD)and alternating current impedance method(EIS)were used to study the supercapacitor performance of electrode materials.Further assembling into asymmetric supercapacitor devices,the practical application ability of the electrode materials was investigated,and the reasons for the excellent performance of the electrode materials and the charging and discharging mechanism were discussed based on theoretical calculation.The main contents are as follows:(1)A network structure composed of tetragonal?-Mn O₂ nanosheets with a thickness of about 13 nm was prepared on the activated graphite foil current collector by electrodeposition method,and then oxygen vacancies were introduced by the Na BH₄solution reduction treatment to prepare Mn O_2-x.The XPS results show that the oxygen vacancy content in Mn O_2-x is about 7.8%.The electrochemical test results show that Mn O_2-x has a capacitance of 704.6 F/g at 1 A/g,the specific volume retention rate after1000 cycles is 92.5%,and the equivalent internal resistance is about 1.94?.The performance of Mn O_2-x is better than the Mn O₂ electrode without introducing oxygen vacancies,indicating that the introduction of oxygen vacancies improves the conductivity,capacitance performance and cycle stability of the material.Theoretical calculation results show that the introduction of oxygen vacancies in?-Mn O₂ can enhance the material's ability to adsorb electrolyte ions and improve the conductivity of the material.(2)A two-step method of electrodeposition-Na BH₄ solution reduction was used to prepare a close-packed cubic CeO_2-x electrode material of nanoparticles on the activated graphite foil.The XPS results show that material the introduction of oxygen vacancies is about 6.2%.The electrochemical test results show that the specific volume of CeO_2-xat 1 A/g is 781.0 F/g,85.9%of the initial specific volume is retained after 1000 cycles,and the equivalent internal resistance of the material is 0.68?,showing good capacitance performance and conductivity.Furthermore,CeO_2-x and Mn O_2-x electrodes are assembled into an asymmetric supercapacitor device.The device has a capacitance of 145.3 F/g at 1 A/g,and a specific volume retention rate of 91.8%after 5000 cycles at a current density of 10 A/g,indicating that it has excellent capacitance performance and cycling stability.The device has an energy density of 45.4 Wh/kg at a power density of750.2 W/kg,which can light up six parallel red LED lights,reflecting the potential practical application value.(3)The CeO_2-x@Mn CO₃ electrode material was successfully synthesized on the foamed nickel current collector by the method of hydrothermal-Na BH₄ solution reduction-calcination.The material has a nanoparticle-sheet composite structure similar to cactus fleshy stems,with an oxygen vacancy content of about 6.0%,and is composed of hexagonal phase Mn CO₃ and cubic phase CeO_2-x with a mass ratio of 3:1.The asymmetric supercapacitor device is assembled with CeO_2-x@Mn CO₃ as the positive electrode and activated carbon as the negative electrode.The device has a capacitance of 135.3 F/g at 1 A/g,and an energy density of 42.3 Wh/kg at a power density of 750.2W/kg,which is enough to light up commercial LED lights.In addition,the specific volume retention rate of the device after 5000 cycles at 5 A/g is as high as 95.6%,showing excellent cycle stability.Therefore,the composite CeO_2-xcontaining oxygen vacancies can effectively improve the properties of Mn CO₃ and significantly improve the electrochemical performance of the composite material.