Synthesis Of Two-Dimensional Porous Cobalt Nickel-Based Oxides/Hydroxides Thin Sheets And Their Electrochemical Performances

Supercapacitors have become a hot topic because of their unique properties such as short charge and discharge time,high power density and long service life.However,the low energy density of supercapacitors limits their practical application.As reported previously,hybrid supercapacitor(HSC)made of battery-type electrode material(such as transition metal oxide/hydroxide)and capacitive electrode material can effectively improve the energy density of the device.Therefore,the development of battery-type electrode materials with larger capacity,wider working voltage window and lower price is the key to solve the problem of supercapacitors.Transition metal oxide/hydroxide electrode materials have become the most widely studied battery-type materials due to their rich redox chemistry and low cost,but the complex synthesis methods limit their development.As far as the nanostructure of electrode material is concerned,two-dimensional porous material can provide more channels for ion transfer and increasing the number of active sites by using large specific surface area,so as to obtain enhanced supercapacitive performance.Therefore,the synthesis of two-dimensional porous transition metal oxides/hydroxides via simple method would be of great research value and practical significance.In this paper,simple liquid-phase method without using any templates and surfactants were designed to synthesize two-dimensional porous cobalt nickel-based oxides/hydroxides thin sheet with hierarchical structure.X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Specific surface area analysis(BET),Scanning electron microscopy(SEM),Transmission electron microscopy(TEM)and other methods were conducted to study the crystal structure,element composition and morphology of the as-obtained materials.Cyclic voltammetry(CV),galvanostatic charge discharge(GCD),cyclic stability test and electrochemical impedance spectroscopy(EIS)were used to test the energy storage performance of the material,and the possible charge storage mechanism of the material was also investigated.The research content of this paper mainly includes the following three aspects:Firstly,we prepared two-dimensional honeycomb-like Co Ni-LDH thin sheets with three-dimensional porous structure via one-step co-precipitation method under mild conditions.The products present a unique two-dimensional(2D)thin sheet structure assembled by honeycomb-like nanosheet array.At the same time,the preparation of Co(OH)₂ and Ni(OH)₂ thin sheets with similar structure was realized by using similar method.Due to the synergistic effect,the bimetallic Co Ni-LDH has better performance than Co(OH)₂ and Ni(OH)₂,with larger specific capacity(394.5 C·g^–1)and better cycle performance(92.3%capacity retention over 10000 cycles).HSC device with Co Ni-LDH thin sheets as positive electrode and active carbon(AC)as negative electrode shows a energy density of 20.38 Wh·kg^–1 at the power density of 800 W·kg^–1.Secondly,we report the synthesis and electrochemical energy storage properties of two-dimensional mesoporous Co₃V₂O₈,Ni₃V₂O₈,and Co_1.5Ni_1.5V₂O₈.The synthesis of two-dimensional mesoporous Co_1.5Ni_1.5V₂O₈ with hierarchical structure was realized by simple hydrothermal reaction.The effect of reaction time on the morphology of the product was investigated.It was found that the product under reaction time of 10 hours showed the optimum nanostructure.The three-electrode system test results show that the Co_1.5Ni_1.5V₂O₈ thin sheets delivered a higher specific capacity(848.5 C·g^–1)and better cycle stability(91.4%capacity retention over 10000 cycles)than the single metal vanadate due to the synergistic effect between the two metal ions.A hybrid supercapacitor was assembled with Co_1.5Ni_1.5V₂O₈ thin sheets as positive electrode and AC as negative electrode.When the power density is 850 W·kg^–1,the energy density of the device can reach 51.66 Wh·kg^–1,and the energy density can be kept at 38.01 Wh·kg^–1 when the power density is increased by 10 times,with a good cycle stability(only 7.2%capacitance loss after 10000 cycles).Thirdly,study on the synthesis and electrochemical energy storage performance of two-dimensional porous Co_0.5Ni_0.5WO₄ thin sheet.The two-dimensional porous Co_0.5Ni_0.5WO₄ thin sheets assembled by nanosheet array was prepared by one-step hydrothermal method.Meanwhile,the products with different Co/Ni ratio were prepared by controlling the proportion of metal salt solution.when Co:Ni=1:1,the electrochemical performance of the cobalt nickel tungstate reached the peak,which showed a high specific capacity of 626.4 C·g^–1 and better cycle stability(105.27%)at 1A·g^–1.The HSC device assembled by Co_0.5Ni_0.5WO₄ and AC displayed a specific capacitance of 90.56 F·g^–1 at 1 A·g^–1.In addition,when the power density is 800 W·kg^–1,the energy density of the device is 32.2 Wh·kg^–1,with a long cycle life(only 6.48% capacitance loss after 15000 cycles).