Study On Preparation And Properties Of Self-supported NiCo₂O₄ Electrode Materials By Hydrothermal Method

Spinel nickel cobalate（NiCo₂O₄）is the electrode material for pseudocapacitors,which is one of the supercapacitor materials with the highest theoretical specific capacitance.NiCo₂O₄has become the focus of research because of its rich resources,low toxicity and good reversibility.However,NiCo₂O₄ is still limited by its poor electrical conductivity,few active sites and low energy density.In order to further improve the electrical conductivity and electrochemical performance of NiCo₂O₄,this paper intends to achieve the composite with nickel foam（NF）,morphology control and metal ion doping modification.In this paper,cobalt nitrate and nickel nitrate are used as base salts,urea is used as precipitant,ammonium fluoride is used as surfactant,aluminum nitrate and chromium nitrate are used as doping modifiers.Self-supported NiCo₂O₄/NF electrode materials were successfully prepared by hydrothermal method combined with calcination process,and the effects of process parameters and aluminum and chromium doping on the morphology and electrochemical properties of the materials were studied in detail.Valuable results were obtained as follows:1.The hydrothermal synthesized NiCo₂O₄ precursor nucleates on The surface of NF and grows into nano-sheet amorphous particles.Spinel NiCo₂O₄ is completely synthesized by calcination at 300℃,and decomposed into Co₃O₄ and Ni O when the temperature exceeds500℃.The self-supported NiCo₂O₄/NF electrode material obtained by calcination at 300℃is composed of macroporous nickel foam,nanosheets and mesoporous materials formed by their stacking,which indicates that the nanosheets synthesized by hydrothermal synthesis have inheritance property.According to the morphology evolution law and electrochemical test results of hydrothermal synthesis,the optimal process is n（Ni）=0.003 mol/L,n（Ni）:n（Co）:n（CH₄N₂O）:n（NH₄F）=1:2:30:12,hydrothermal temperature 140℃,hydrothermal time 6 h,calcination temperature 300℃.2.The specific capacitance of the electrode material obtained under the optimal conditions decreases with the increase of current density.The specific capacitance is 615.11F/g at 1 A/g,and the ratio is 85%at 10 A/g.At 10 A/g,the specific capacitance retention exceeds 100%after 3000 charges and discharges.The specific capacitance reaches 87.14 F/g when the asymmetric device is assembled at 1 A/g.The energy density is 32.57 Wh/kg at 0.8k W/kg power density.After 10000 charge-discharge cycles at current density of 10 A/g,the specific capacitance remains 100%of the initial value.3.In order to improve the electrochemical properties of NiCo₂O₄/NF samples,the Ni CO_2-XCr_xO₄/NF（0?x≤0.1）electrode material was successfully prepared by doping with Cr³⁺instead of some cobalt ions.Microanalysis showed that the flake particles decreased with the increase of Cr³⁺content.When the doping concentration is 10 mol%,it turns into tightly arranged spherical particles.The electrochemical test results show that the electrochemical performance is the best when the doping concentration of Cr³⁺is 1 mol%.At 1 A/g,the discharge specific capacitance is 757.6 F/g,and the current density is 10 A/g,the rate characteristic is 81%.At 10 A/g,the retention rate of discharge capacity is 86%after 3000cycles.Compared with NiCo₂O₄/NF,the specific capacitance of Ni Co_1.99Cr_0.01O₄/NF is increased by 23.2%at the same current density,but the cycle stability is decreased.This is because after doping,the particle size of the product is reduced,and the contact with the electrolyte is more sufficient,and the initial specific capacitance increases.However,the product particles are densely arranged,and more active sites cannot be stimulated after repeated charge and discharge,so the specific capacitance is attenuated and the cycle performance is reduced.4.Ni_1-xAl_xCo₂O₄/NF electrode materials with different Al³⁺doping were successfully prepared by replacing part of nickel with Al³⁺.Microscopic analysis showed that the morphology of the product was still flake structure after Al³⁺doping,and the flake particles grew thinner and denser with the increase of Al³⁺content.The electrochemical test results showed that the electrochemical performance of the sample prepared was best when the Al3+doping concentration was 1 mol%.At 1 A/g,the specific capacitance is 808 F/g,at 10 A/g,the rate characteristic is 91%,compared with NiCo₂O₄/NF,under the same current density,the specific capacitance increases by 31.4%,the rate characteristic increases by 6%.After3000 cycles at 10 A/g,the initial specific capacitance increases from 704.4 F/g to 832.4 F/g,and the capacitance retention reaches 118%.The specific capacitance of the assembled ASC device is improved by 110.3%at 1 A/g,the energy density is 35.06 Wh/kg at 0.8 k W/kg.After 10000 cycles at 10 A/g,the discharge specific capacity retention rate is 100%.