Facile Preparation And Cobalt Doping Performance Optimization Of Ni₃（NO₃）₂（OH）₄ Porous Nanosheets

Nickel hydroxidenitrate（Ni₃（NO₃）₂（OH）₄）has a hexagonal structure similar to nickel hydroxide（Ni（OH）₂）,but the layer spacing is far higher than that of Ni（OH）₂（c:6.9?vs 4.6?）,which is favorable for the rapid insertion and removal of electrolyte ions.Researches showed that,Ni₃（NO₃）₂（OH）₄ as pseudocapacitance material has excellent specific capacitance and rate capability,and it likely make the supercapacitor realize the compatibility of energy density and power density.At present,the preparation of Ni₃（NO₃）₂（OH）₄ generally adopts solvent thermal method,this method has disadvantages such as:long reaction time,low yield,high cost and waste solvent pollution,which is not suitable for mass preparation and production.In view of this problem,a simple and efficient self-template method was adopted to prepare Ni₃（NO₃）₂（OH）₄ porous nanosheets electrode material,taking advantage of porous structure to increase the contact areas between electrode material and electrolyte and shorten the distance of ions migration.It has been shown that cobalt doping can effectively improve the electron and ion conductivity of pseudocapacitance material,thus improving the rate capability of material.Therefore,the cobalt doping Ni₃（NO₃）₂（OH）₄ electrode material was prepared through one-step method.Then,we used different characterization methods to confirm the existence form of cobalt element,and systematacially studied the influence of cobalt doping on the electrochemical properties of Ni₃（NO₃）₂（OH）₄.The detailed research contents of this paper are as follows:1.The Ni₃（NO₃）₂（OH）₄ porous nanosheets were obtained through heating a mixed solution under 250oC for 30 min,which dissolved certain mole ratio of Ni（NO₃）₂·6H₂O and CO（NH₂）₂.In the reaction,a large number of mesoporous structure were formed in the material when the by-product NH₄NO₃ was cleared by centrifuging,which acted as self-template and pore forming agent.The results showed that the specific surface area and pore volume of materials respectively increased 180 times and 90 times after the NH₄NO₃ was removed.The electrochemical test showed that Ni₃（NO₃）₂（OH）₄ porous nanosheets can obtain a specific capacitance of 1579 F/g（loading mass is 2.15 mg/cm²）at the current density of 1 A/g.This material still obtained a specific capacitance of720 A/g,when the current density increased to 20 A/g.The specific capacitance retention rate was 71.9%after 5000 cycles at the current density of 10 A/g.Even under the high loading mass of 17.55 mg/cm²,the specific capacitance and areal capacitance of this material can be reached 1094 F/g and 19.2 F/cm².The Ni₃（NO₃）₂（OH）₄//AC asymmetric supercapacitor assembled by Ni₃（NO₃）₂（OH）₄porous nanosheets and super capacitance activated carbon can provide an energy density of 33.3 Wh/Kg at the power density of 190.5 W/Kg.2.For further optimizing the electrochemical performance of Ni₃（NO₃）₂（OH）₄,different mole ratios of Co（NO₃）₂·6H₂O were added to the mixed solution of Ni（NO₃）₂·6H₂O and CO（NH₂）₂.The cobalt doping Ni₃（NO₃）₂（OH）₄ electrode material was synthesized by one-step method through heating the mixed solution 30 min under 250?C.The results showed that the cobalt element existed as Co²⁺after doping,and the different dosages of doped cobalt has a great influence on the morphology and specific surface area of the product.The test of CV and GCD showed that cobalt doping can obviously improve the rate capability of Ni₃（NO₃）₂（OH）₄.Among which,the most obvious improvement was Sample 15,whose dosage of cobalt doping is 15%.The specific capacitance of Sample 15 decreased from 1492.7 F/g to 1168.9 F/g,when the current density increased from 1 A/g to 20 A/g.The specific capacitance retention rate of Sample 15 was 78.3%,which increased by 32.7%compared with the value（45.6%）of un-doped sample.The test of EIS showed that the internal resistance and ion diffusion resistance of Ni₃（NO₃）₂（OH）₄ had decreased after doping cobalt.Even under high loading mass of 15.05 mg/cm²,the specific capacitance and areal capacitance of Sample 15 can be reached1062 F/g and 16 F/cm².The asymmetric supercapacitor assembled by Sample15 and super capacitance activated carbon can provided an energy density of 26Wh/Kg at the power density of 2766 W/Kg,whose performance was superior to Ni₃（NO₃）₂（OH）₄//AC capacitor.