| With the increasing of energy consumption and environmental pollution,people's demands for sustainable and renewable new energy are getting stronger.As a new type of electrochemical energy storage device,supercapacitors have attracted much attention due to its high energy,rapid charge and discharge capacity,long period stability,environmental friendliness and safety.As an important component of supercapacitors,the electrode materials play a decisive role in the overall performance of supercapacitors.Currently,there are three main types of electrode materials assembled into supercapacitors:carbon-based materials,conductive polymer materials and metal oxide/hydroxide materials.Although metal oxide/hydroxide electrode materials have higher theoretical capacitance and power density than carbon-based electrode materials,and higher stability than conductive polymer electrode materials,their relatively low practical specific capacity and conductivity limit the practical applications.In this paper,metal oxide/hydroxide electrode materials were grown in situ with copper foam substrate.Using the advantages of large specific surface area of copper foam and low contact resistance of the products grown in situ,further constructing a high electron mobility Zinc Oxide layer between copper foam and metal oxide/hydroxide as hierarchical structure to reduce equivalent series resistance and improve inherent cycle stability and rate performance of metal oxide/hydroxide electrode materials,further improve its actual specific capacity.In addition to constructing the Zinc Oxide layer,we also tried to build the core shell structure with copper foam as the spontaneous source substrate to enhance the electrochemical performance of the hydroxide electrode material.The research contents are as follows:?1?In order to reduce the equivalent series internal resistance and improve the electrode material properties,the Co3O4 electrode materials with book-like microstructure were grown in situ on the copper foam substrate by hydrothermal method,and its electrochemical properties were studied.The electrochemical test results show that the specific capacitance of the book-like Co3O4 electrode material is554.70 mF cm-2(504.27 F g-1)at the current density of 2 mA cm-2.When the ratio of current density is up to 15 times,the rate performance is down to 81.6%.The equivalent series internal resistance is 0.89?cm-2.After 5000 cycles of charging and discharging,the specific capacitance maintains the initial capacitance value of 78%.It can be seen that the in-situ growth of Co3O4 electrode materials on the three-dimensional network substrate and controlling the surface morphology can reduce the equivalent series internal resistance and improve the rate performance to a certain extent.?2?ZnO has good electron transport,thermal stability and chemical stability.Growing ZnO layer between the Co3O4 and copper foam substrate may be an effective way to reduce the series internal resistance of the electrode material and improve cycle stability due to its excellent electron transporting ability.Therefore,we have synthesized the hierarchical structure ZnO@Co3O4 electrode material by one-step hydrothermal method on the copper foam substrate,discussed the synthesis mechanism and energy storage mechanism and studied its electrochemical properties.The results of SEM and TEM showed that the prepared ZnO@Co3O4 completely coated the foam copper substrate,and the Co3O4 uniformly covered the surface of ZnO.The electrochemical test results showed that the ZnO@Co3O4 electrode material has presented a specific capacitance of 1.5345 F cm-2 at the current density of 2 mA cm-2.When the current density increased 10 times,the specific capacitance maintains 75.8%of the initial specific capacitance.The equivalent series internal resistance is only 0.83?cm-2.The specific capacitance maintains 92%of the initial value after 5000 cycles.Assembling it into an asymmetric supercapacitor,when the power density is 8.085 W cm-3,the energy density is 0.6958 W h cm-3 and the capacitance retention 96%after3000 cycles.These above results prove that build a hierarchical structure by constructing ZnO layer can reduce the equivalent series internal resistance of the copper foam based Co3O4 electrode material,improve the cycling stability and the specific capacitance of the electrode materials and obtain excellent performance of supercapacitor electrode materials.?3?In order to improve the poor conductivity of Ni?OH?2 electrode material and enhance its overall electrochemical performance,we used copper foam as the substrate,introduced ZnO layer to construct a hierarchical structure of ZnO@Ni?OH?2 electrode material for supercapacitor,discussed the synthesis mechanism and energy storage mechanism and studied its electrochemical properties.Firstly,the flower-like structure of ZnO was prepared on the copper foam substrate by hydrothermal method,and then Ni?OH?2 was deposited on the ZnO surface by electrochemical deposition,so that the Ni?OH?2 electrode material was uniformly coated on the surface of ZnO,and finally the flower-like hierarchical structure was formed.The electrochemical test results showed that the specific capacitance of electrode material is 3301 F g-1(3.962 F cm-2)at a current density of 1 A g-1.When the current density increased 20 times,the specific capacitance keeps 80%of the initial value;The equivalent series internal resistance is0.75?cm-2.The specific capacitance maintains 94%of the initial value after 5000cycles of charging and discharging.Assembling the ZnO@Ni?OH?2 electrode material into an asymmetric supercapacitor,when the power density is 9.722 W cm-3,the energy density is 1.6473 W h cm-3.The specific capacitance still maintains 90%of the initial capacitance after 3000 cycles of charging and discharging.These above results indicate that using copper foam as substrate and introducing a high electron mobility ZnO layer can improve the poor conductivity of the Ni?OH?2 electrode material,its cycling stability and actual specific capacitance.?4?In order to improve the conductivity and cycle stability of the layered double hydroxide?LDH?supercapacitor electrode materials,we designed a CuO@Ni-Fe LDH core-shell structure by a two-steps method via using copper foam as the substrate and spontaneous source,discussed the synthesis mechanism and energy storage mechanism and studied its electrochemical properties.Firstly,the Cu?OH?2 nanorod arrays were prepared by electrochemical anodization using copper foam as the spontaneous source.After the high temperature treatment,the stable CuO nanorod arrays were formed.Then,the core-shell electrode materials of CuO@Ni-Fe LDH were obtained by electrochemical deposition of Ni-Fe LDH nanosheets with the CuO nanorod arrays as core.The results of electrochemical measurement showed that the specific capacity of CuO@Ni-Fe LDH core-shell structure electrode material was 2.682 F cm-2at the sweep rate of 2 mV s-1.As the current density is 2 mA cm-2,the coulombic efficiency is 82.7%.When the current density is increased by 10 times,the rate performance maintains the initial value of 72%.The series internal resistance is 1.19?cm-2.The specific capacitance retains 86%of its initial capacitance after 5000 cycles.Those above results showed that the CuO@Ni-Fe LDH core-shell electrode material prepared by using copper foam as spontaneous source has a small equivalent series internal resistance and excellent cycle stability,and exhibiting that it is a potential supercapacitor electrode material.In summary,four kinds of hierarchical structure supercapacitor electrode materials,Co3O4,ZnO@Co3O4,ZnO@Ni?OH?2and CuO@Ni-Fe LDH,were prepared based on three-dimensional network structure copper foam substrate.The effects of different hierarchical structures on the electrochemical properties of these electrode materials were discussed and it provided new ideas for reducing the equivalent series internal resistance of metal oxide/hydroxide electrode materials and improving their cycle stability. |
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