Preparation And Electrochemical Properties Of Cobalt Doped Nickel Hydroxide Based Composite Materials

With the development of technology,the new green energy technology is raised more attention.Ni（OH）₂ is a pseudocapacitive electrode material commonly applied in supercapacitors,which present high specific capacitance.As a transition metal hydroxide,the disadvantages of Ni（OH）₂,are poor electrical conductivity and low rate performance.Therefore,the morphology,composition,and structure of Ni（OH）₂ are designed to improve the energy storage performance as electrode materials.Here,cobalt doping Ni（OH）₂,Ni（OH）₂@nickel foam,and Ni（OH）₂@carbon cloth were synthesized with different compositions and morphology by hydrothermal in-situ growth method,respectively.The specific research are as follows:（1）The Co-doped Ni（OH）₂ electrode materials were prepared by a hydrothermal method.The doping of Co improves the conductivity of the electrode and increases the number of defects and active sites.The morphology of Ni_0.83Co_0.17（OH）₂ electrode prepared under the condition of 1.2 mmol hexamethylenetetramine,hydrothermal temperature of 90℃,and 5 wt.%cobalt acetate was thin petal-like.And the phase composition wasγ-Ni OOH andβ-Ni（OH）₂.The experimental results showed that the specific capacitance of Ni_0.83Co_0.17（OH）₂was1730.37 F g^-1 at a current density of 1 A g^-1,and for 5000 cycles of constant current charging and discharging,which had high specific capacitance and cycling stability.The Ni_0.83Co_0.17（OH）₂ and activated carbon YP-50 assembled asymmetric capacitors,the specific capacitance retention rate for 10,000 cycles of constant current charging and discharging is87.37%.（2）To improve the electrochemical performance,the Co-doped Ni（OH）₂ electrodes were grown in-situ by a suspension hydrothermal method on a nickel foam substrate.The suspension growth method optimizes the array-like morphology of Co-Ni（OH）₂ and avoids the non-uniform caused by the deposition method.The morphology of Ni_0.83Co_0.17（OH）₂@v NF-1.5-12,which was synthesized under hydrothermal time of 12 h and the amount of nickel acetate was 1.5 mmol by suspension growth,was uniform array-like.The array-like morphology of Ni_0.83Co_0.17（OH）₂@v NF-1.5-12 improves the diffusion rate of ions,increases the contact area with the electrolyte and provides a certain buffer space,and takes advantage of the high electrical conductivity of nickel foam itself.The experimental results showed that the specific capacitance of Ni_0.83Co_0.17（OH）₂@v NF-1.5-12 electrode reached 2352.25 F g^-1 at a current density of 1 A g^-1,the initial specific capacitance retention was 69.3%when the current density was increased from 1 A g^-1 to 50 A g^-1,and the initial specific capacitance retention was 90.14%after 5000 cycles of constant current charging and discharging,thus,it has high specific capacitance and rate performance.The asymmetric capacitor was assembled with Ni_0.83Co_0.17（OH）₂@v NF-1.5-12 and activated carbon YP-50,which the power density was 2774.38 W kg^-1 at an energy density of 62.58 Wh kg^-1,and the initial specific capacitance retention was 89.63%after 10,000 cycles of constant current charging and discharging.（3）Co-doped Ni（OH）₂ was grown suspension in-situ on carbon cloth substrate by a hydrothermal method.The Ni_0.83Co_0.17（OH）₂@v CC-1.5-6 with vertical array-like morphology was obtained at nickel acetate 1.5 mmol and hydrothermal time of 6 h.The results show that the Ni_0.83Co_0.17（OH）₂@v CC-1.5-6 electrode has a high electrochemical energy storage capacity.The specific capacitance was 1516.5 F g^-1 at a current density of 1 A g^-1,and an initial specific capacitance retention of 70.8%when the current density is increased from1 A g^-1 to 20 A g^-1.The Cyclic voltammetry,Galvanostatic charge-discharge and Electrochemical impedance spectroscopy curves of the electrode material at each bending angle basically overlap,proving its excellent mechanical properties.The asymmetric capacitor was assembled with Ni_0.83Co_0.17（OH）₂@v CC-1.5-6 and activated carbon YP-50,the power density was 1956.47 W kg^-1 at an energy density of 38.15 Wh kg^-1,and the initial specific capacitance retention was 90.11%after 10,000 cycles of constant current charging and discharging.