Structural Regulation Of Ni/Co Bimetallic Oxides And Application Of Electrode Materials For Supercapacitors

As an important energy storage device,supercapacitor has high power density and excellent cycle stability,but also has the problem of relatively low energy density and poor mechanical stability.Bimetallic oxide NiCo₂O₄ is one of the most promising materials for supercapacitors due to its good synergistic and complementary effects in redox reactions.However,due to the influence of crystallinity and grain size,the actual capacitance of NiCo₂O₄ material is much lower than the theoretical value.The grain size of NiCo₂O₄ material prepared by traditional methods is not controllable,which becomes the fundamental problem in improving the specific capacity.In this paper,NiCo₂O₄ was utilized as the research object to explore the key factors that determine the grain growth and nucleation.Then hydrothermal and electrochemical deposition methods were used to prepare NiCo₂O₄with an ultra-thin nanosheet structure.In addition,NiCo₂O₄/MnO₂ composites were obtained by in-situ deposition of MnO₂ on NiCo₂O₄ nanostructure to further improve the specific capacity and achieve the purpose of increasing energy density.The details are as follows:（1）Effect of solvent environment on grain size of NiCo₂O₄ nanostructures were prepared by hydrothermal method.The adsorbability of ethylene glycol,ethanol and deionized water on the surface of NiCo₂O₄ and their effects on nucleation and growth were investigated.The results show that the glycol solvent environment was conducive to the uniform growth of NiCo₂O₄ grains.The prepared NiCo₂O₄ nanostructures appear as nanoneedles,and the capacitance retention rate was 86.85%when the specific capacity reached 1082 F g^-1,2500 cycles at the current density of 1 A g^-1.（2）To study on migration and nucleation of transition metal ions.,the factors influencing the nucleation rate of NiCo₂O₄ crystal synthesis were explored.Since the nucleation of NiCo₂O₄ requires the simultaneous migration of Ni²⁺and Co²⁺ions,the electrodeposition rates of Ni²⁺and Co²⁺ions were studied at different temperatures,and NiCo₂O₄ ultra-thin nanosheets were prepared by electrochemical deposition.It is found that at 10℃,the deposition rate ratio of Co²⁺and Ni²⁺was close to 2.5∶1,the nucleation rate was the highest,and the grain size was the smallest.The prepared electrodes can provide ultra-high specific capacitance of 1590 F g^-1 and 1080 F g^-1 under the current density of 1 and 20 A g^-1.（3）NiCo₂O₄/MnO₂ composite was prepared by in-situ deposition of MnO₂ on NiCo₂O₄ electrode material on the basis of fully mastering the technical parameters of NiCo₂O₄ electrode material,in order to further improve the specific capacity.The effects of concentration ratio of KMnO₄,Mn SO₄ and deposition time of MnO₂ on the morphology and electrochemical properties of NiCo₂O₄/MnO₂ composites were investigated.The results show that the NiCo₂O₄/MnO₂ composite electrode obtained by in-situ deposition at KMnO₄:Mn SO₄=1:2 for 30min presents spherical nanosheets with optimal specific capacity,reaching 2416 F g^-1 at the current density of 1 A g^-1.