Study On Preparation And Electrochemical Capacitance Properties Of Cobalt-nickel Based Nano-ordered Arrays

In recent years,the limited reserves of traditional fossil energies have forced people to turn to research on new energy and high-performance energy storage devices.As a new type of high-power electrochemical energy storage devices,electrochemical supercapacitors have broad application prospects due to the advantages of high-power density,high-energy density,and long length of life.Electrode materials are the most critical parts that determine the performance of energy storage devices.Among them,nickel-cobalt-based electrode materials have attracted much attention owing to their high theoretical capacity,simple morphology control,large abundance,reasonable cost,and low biological toxicity.However,some pseudo-capacitance electrode materials represented by Ni-Co-based materials are restricted their further development and utilization by their inherent defects.On account of their relatively low electrical conductivity,the high-rate performances of Ni-Co-based electrode materials are limited.Meanwhile,the structure of active materials collapses easily during charging and discharging due to the poor structural stability,which results in poor long-term cycle stability.In view of the above problems,cobalt-nickel-based electrode materials in this dissertation are studied from two aspects:material composition control and structural design.Here,series of Ni-Co based nano-ordered arrays with different structure are constructed on conductive substrates by introducing defective active sites,carbon material coating,and other active material composites.The morphological layered nickel-cobalt-based nano-ordered array structure not only improves the electrical conductivity of electrode but also increases the contact area between the electrode and the electrolyte,leading to the high-rate performance,excellent structural stability of electrode materials,and the excellent long-term cycling.Thus,the synergistic improvement of the performance of the energy storage device is realized.The main points of this dissertation are presented as follows:(1)The nickel foam was used as the substrate,and the NiCo₂O₄ ordered nanoarrays were grown on the nickel foam after hydrothermal method followed by annealing in an air atmosphere.Through simple Na BH ₄solution immersion treatment,oxygen vacancy defects were introduced to the NiCo ₂O₄ nanoarrays.Thus,the reduced NiCo ₂O₄ nanoarrays containing oxygen vacancies(r-NiCo ₂O₄)were prepared successfully by avoiding the complicated high-temperature oxygen-deficient atmosphere treatment process.By adjusting the concentration of the Na BH ₄ solution,the concentration of the introduced oxygen vacancies was also changed,so as to realize the control of the electrochemical energy storage performance of the r-NiCo ₂O₄ ordered nanoarrays.After the optimal oxygen-vacancy treatment,the specific capacitance,rate characteristics,cycle stability,et al of the r-NiCo ₂O₄ NWs electrode were all improved.In the galvanostatic charge/discharge(GCD)test,the optimized r-NiCo ₂O₄ NWs electrode showed a high specific capacity of 1564.4 F g^-1at 1 A g^-1.When the current density is increased to 20 A g^-1,the specific capacity of the electrode was still 859.2 F g^-1.In addition,the r-NiCo ₂O₄NWs electrode exhibited excellent cycle stability.At a current density of 10 A g^-1,the electrode still had 82.5%of the initial capacitance value after 5000 cycles.(2)The NiCo₂O₄ ordered nanoarrays were grown on the nickel foam after hydrothermal method followed by annealing in an air atmosphere.Then,nickel-cobalt oxide composite nanoarrays coated with a nitrogen-doped carbon layer(NiCo ₂O₄@NC)were prepared successfully by soaking in a dopamine solution and annealing treatment in N ₂ atmosphere.The dopamine here was used as the N-doped carbon source.The dopamine coating treatment process was simple,and the immersion treatment in the aqueous solution at room temperature would not cause secondary damage to the electrode microscopic morphology.At the same time,dopamine as a carbon source could achieve nitrogen doping into the carbon layer and improve the wettability of the electrode in the electrolyte.By adjusting the soaking time,the thickness of the coated carbon layer could be adjusted and changed to optimize the electrochemical energy storage performance of the final product.The optimized NiCo ₂O₄@NC nanoarrays electrode had excellent electrochemical performance.The NiCo ₂O₄@NC arrays showed a high specific capacity of 1132 F g^-1(645.2 C g^-1)at 5A g^-1.As increased to 50 A g^-1,the specific capacity of the electrode was still 474 F g^-1(270.2 C g^-1).Moreover,the NiCo ₂O₄@NC arrays electrode demonstrated outstanding cycle stability,and still had 91.3%of the initial capacitance value after 5000 cycles at10 A g^-1.In the two-electrode test,the NiCo ₂O₄@NC arrays electrode and the activated carbon loaded on the nickel foam were respectively used as the positive electrode and the negative electrode to assemble an asymmetric supercapacitor(ASC).The stable working voltage window of the assembled ASC was 0-1.4 V.After 15000cycles at a current density of 5 A g^-1,the capacitance retention rate reached 97.3%of the initial capacitance value.At the same time,it had a high energy density of 29.4 Wh kg^-1 at a power density of 349 W kg^-1,and it still had an energy density of 18.5 Wh kg^-1 when the power density was 3502 W kg^-1.(3)Taking nickel foam as the substrate,the NiCo₂O₄ ordered nanoarrays were grown through hydrothermal method followed by annealing in an air atmosphere.After that,NiCo ₂O₄@MnO₂composite nanoarrays were prepared successfully by coating of MnO₂nanosheets through hydrothermal method.By adjusting the concentration of potassium permanganate,the thickness of the MnO₂nanosheets could be changed.The optimized NiCo ₂O₄@MnO₂ composite nanoarrays electrode had excellent electrochemical performance.The NiCo ₂O₄@MnO₂arrays electrode displayed a high specific capacity of 1686.4 F g^-1 at 1 A g^-1.As increased to 20 A g^-1,the specific capacity of the electrode was still 984.4 F g^-1.Furthermore,the NiCo ₂O₄@MnO₂composite nanoarrays electrode showed excellent cycle stability,and still had 93.5%of the initial capacitance value after 5000 cycles at 10 A g^-1.In the two-electrode test,the NiCo ₂O₄@MnO₂ array electrode was used as the positive electrode and activated carbon was loaded on the nickel foam as the negative electrode to assemble an ASC.The stable working voltage window of the assembled ASC was as high as 0-1.6 V.After 10000 cycles at 4 A g^-1,the capacitance maintained 86.6%of the initial capacitance value.At the same time,it had a high energy density of 29.1 Wh kg^-1 at a power density of 650 W kg^-1,and it still had an energy density of 17.5 Wh kg^-1 when the power density was 6499.6 W kg^-1.(4)The NiCo₂O₄ ordered nano-arrays were grown on the nickel foam after hydrothermal method followed by annealing in an air atmosphere.Then,NiCo P nanoarrays with well-maintained nanostructure morphology were obtained after low-temperature phosphating treatment by using sodium hypophosphite as the phosphorus source.Finally,NiCoP@CoS composite nanoarrays were prepared successfully by coating the Co S nanosheets on NiCo ₂O₄ arrays with an electrodeposition treatment.In this composite structure,the NiCo P nanowire arrays as the core material supported the overall structure of the electrode material,which had advantageous to accelerate charge transfer and to improve the electronic conductivity of the material.As the outer shell material,Co S nanosheets could expand the specific surface area of the electrode,which could improve the contact between the electrode and the electrolyte,and provide more redox reaction surface active sites.The combination of two active materials showed good synergistic effect,leading to the improvement of the energy storage performance and outstanding cycle stability.The optimized 3-dimensional hierarchical NiCoP@CoS core-shell nanoarrays electrode had high specific capacity,good rate performance,and greatly improved cycle stability.The prepared NiCoP@CoS core-shell nanoarrays were a battery-type electrode material with excellent performance.At a current density of 2 A g^-1,the mass specific capacity could achieve as high as 1796 F g^-1.After 5000 cycles at 10 A g^-1 current density,the capacitance retention rate was as high as 91.4%.In the two-electrode test,the NiCoP@CoS arrays electrode was used as the positive electrode and activated carbon was loaded on the nickel foam as the negative electrode to assemble an ASC.When the power density was 748.9 W kg^-1,the energy density of the ASC was as high as 35.8 Wh kg^-1.