Construction Of Cobalt-based Composite Electrode Materials And Study On Supercapacitor Performance

Developing high-performance green energy storage devices is one of the effective measures to alleviate energy shortage and environmental pollution.As a new type of green energy storage equipment,supercapacitor plays an important role in the field of electrochemical energy conversion and storage because of its environment-friendly,ultra-long cycle stability and ultra-high electrochemical capacity.However,its low energy density greatly limits its large-scale practical application.The selection of electrode material is the key to determine the performance of electrode.In recent years,transition metal cobalt-based electrode materials have been favored by researchers because of their low price and high specific capacity.However,as a transition metal material,it is still limited by the problems of narrow potential window and long-term cyclic instability,which are the core problems of cobalt-based materials.In order to further improve the electrochemical performance of cobalt-based materials,this paper overcome the problem of structural instability faced by single cobalt-based materials by rational construction of cobalt-based composite electrode materials,and solved the problem of low energy density of supercapacitors by constructing asymmetric supercapacitors.The micro-nano structure and energy storage behavior of the prepared electrode materials were studied and analyzed by multiple structural characterization methods and subsequent electrochemical performance tests.The specific research contents are as follows:1.Through reasonable construction,Fe Co₂O₄@Mn O₂ core-shell composite nanosheet array structure was prepared on flexible carbon fibers（CFs）,which was directly used as the integrated electrode of supercapacitor.The amount of Mn O₂in the outer layer was controlled by different reaction time of KMn O₄ solution,and the morphology change and influence on the performance were investigated.The electrochemical performance of Fe Co₂O₄@Mn O₂ composite electrode was optimized.The results showed that the Mn O₂ nanosheets were uniformly coated on the surface of two-dimensional Fe Co₂O₄ nanosheets,and the composite cobalt electrode material exhibited a high area specific capacity of 4.8 F cm^-2 at the current density of 1 m A cm^-2.In addition,the constructed asymmetric supercapacitor Fe Co₂O₄@Mn O₂//AC showed high energy density/power density(22.68 Wh kg^-1 at 406.01 W kg^-1,7.06 W h kg^-1 at 1802.5 W kg^-1)and can maintain 90.1%of the initial capacitance after 5000cycles.These performances indicate that the designed asymmetric supercapacitor has great potential in the field of energy storage due to its excellent electrochemical performance.2.A novel hierarchical Co₉S₈@Ni（OH）₂ core-shell nanotube array was prepared on CFs by simple hydrothermal synthesis and electrodeposition.One-dimensional Co₉S₈ nanotubes were an ideal framework for improving the electrical conductivity of Ni（OH）₂ nanosheets.The ultra-thin Ni（OH）₂ nanosheets with redox activity deposited on Co₉S₈ can greatly enhance the specific surface area and provide more electrically active sites for Faraday reaction.The specific capacity of the optimized Co₉S₈@Ni（OH）₂electrode can reach 149.44 m Ah g^-1 at the current density of 1 A g^-1,and 75 m Ah g^-1 even at the current density of 10 A g^-1.The assembled asymmetric supercapacitor Co₉S₈@Ni（OH）₂//AC showed high energy density(31.35 Wh kg^-1 at252.8 W kg^-1),high power density(2500 W Kg^-1 at 12.5 Wh kg^-1)and 97.3%initial capacitance retention after 5000 cycles.3.Because binary cobalt oxide has higher electrochemical performance than single cobalt oxide,in this study,a self-supporting core-shell Co₉S₈@Ni Co₂O₄ hollow nanoneedles composite electrode material with high performance was prepared on CFs by simple hydrothermal,electrodeposition and subsequent calcination methods.Among the hybrid nanostructures,Co₉S₈ hollow nanoneedles with high conductivity can be used as the framework for anchoring high capacity Ni Co₂O₄ nanosheets.Thus,the hierarchical hybrid structure showed a high specific capacitance of 1022.5 F g^-1(1 A g^-1).In addition,the constructed asymmetric supercapacitor Co₉S₈@Ni Co₂O₄//AC showed a high energy density of 24.85 Wh kg^-1(222 W kg^-1)and an initial capacitance retention of 88.9%after 6000 cycles.4.By self-templating effect of Zn O nanorods and ZIF-8 nanoshells,CNTs@Ni Co-LDH hierarchical nanotube composite arrays with porous diffusion interfaces were prepared in situ on carbon fibers.In this unique structure,carbon nanotubes with solid growth and good arrangement on carbon fiber had unique stability as carbon material at various potentials,and can improve ion mass transfer on Ni Co-LDHs surface by using the porous diffusion interface on the surface,and served as a good skeleton for the loading of subsequent high-capacity Ni Co LDHs nanosheets.The optimized core-shell electrode exhibited an excellent capacity of 176.33 m Ah g^-1,and the prepared asymmetric supercapacitor CNTs@Ni Co-LDH//ZIF-8 derived carbon showed a high energy density of 37.38 Wh kg^-1 and excellent cycle stability（90.22%retention after5200 cycles）at 800 W kg^-1.5.A Co-MOF derived P-Co₃O₄@NC@O_v-Ni Mn LDH composite was synthesized by P-doping and vacancy strategy.The synergistic effect of dual defects was utilized to achieve the high performance of supercapacitor,while the trade-off effect caused by single defect was avoided.The introduction of oxygen vacancy in cobalt-based hydrotalcite like materials had a positive effect on improving its electrochemical performance.The test results showed that the specific capacity of the optimized electrode was 285.56 m Ah g^-1 at 1 A g^-1.In addition,the constructed P-Co₃O₄@NC@O_v-Ni Mn LDH//CNT@ZIF-8 derived C showed superior capacity characteristics(61.33 m Ah g^-1 at 1 A g^-1).When the power density was 800 W kg^-1,energy density can be as high as 49.06 Wh kg^-1 and excellent cycle stability up to 91.92%after 12000 cycles.