Research On The Synthesis Of Cobalt Oxide With Morphology Control And Electrochemical Properties Of Electrode Materials

Co3O4 is a promising electrochemical capacitor electrode material that has attracted more and more attentions due to its low cost, high theoretical capacitance(3560F/g) and environmental friendliness. As pseudo-capacitance materials, the application possibilities of other transition metal oxides are greatly restricted. Specifically, the core problems can be summarized as: inability to achieve high capacitance characteristics of materials with current morphologies and structures, comparatively short cycle life, severe capacitance degradation during cycling tests. To solve these key problems, in this paper, simple hydrothermal method was proposed. Co3O4 nanostructures with controllable morphologies have been prepared by incorporating thermodynamics with systematic study of the feature of intermediate product,constructing kinetic model of hydrothermal growth of reaction product, and adjusting critical parameters. This study is of great importance, especially in further enriching the preparation method of nanomaterials, corresponding mechanism research and developing new energy materials. The main work is as follows.1. We tried to introduce cobalt complexes ammonia into the hydrothermal preparation of Co3O4 to reduce the thermodynamic energy barrier in oxidation process of Co(II). To reveal the specific formation conditions of 2D nanofilms with the assistance of nitrate under hydrothermal circumstances, the reconstruction of cobalt complexes ammonia during hydrothermal process was thoroughly investigated. The results indicated that graphene-like Co3O4 nanofilms could be synthesized with one-step hydrothermal method by adopting proper reaction parameters. Superfluous free ammonia was essential, as insufficient NH3 was unable to drive the 2D nucleation and growth of nanocrystals. Only a large number of dendritic structures and a small quantity of nano-slices existed in the final product. In order to get ultrathin nanofilms, appropriate amount of nitrate used in inhibiting the vertical growth of Co3O4 nanocrystals is very crucial. For one thing, if the nitrate were insufficient, not all the vertical growth of Co3O4 could be well restricted due to lack of enough nitrate to insert into Co-O bond. As a result, only Co3O4 nanosheets were obtained rather than nanofilms. For another, excessive insertion resulted from too much nitrate would instead entirely destroy the acceleration of free NH3 to 2D growth, and finally, spherical structures were prepared due to in situ growth of Co3O4 nanocrystals.2. We explored the possibility of low temperature preparation of hollow Co3O4 microspheres by making the best of easily oxidized feature of cobalt(II) complexes ammonia, using O2 resulted from thermal decomposition of H2O2 as bubble template and combining the cladding of nitrate around Co3O4 nanocrystals under hydrothermal conditions. The results indicated that the amount of nitrate was closely related to the microspheres formation. Adjacent Co3O4 nanocrystals could not be separated from each other on the condition that the concentration of nitrate was comparatively low, so no product with good microspheres morphology could be obtained. However, the surface area of Co3O4 nanocrystals encapsuled by nitrate was expanded with high concentration of nitrate, leading to size differentiation of Co3O4 microspheres. Once the dosage of H2O2 and temperature were properly increased, hollow Co3O4 microspheres could be obtained rather than solid Co3O4 microspheres. This could be attributed to the fact that the accelerated decomposition of H2O2 under higher temperature provided more O2 to serve as templates. When the H2O2 was insufficient, limited amount of O2 was unable to provide enough templates for the temporary core-shell structure of Co3O4 nanocrystals. As a result, it greatly affected the hollow rate and inner space size of hollow Co3O4 microspheres. However, when too much H2O2 was introduced into the system, a lot of bubble disturbance would cause tremendous impact to hydrothermal reaction which means the cladding of nitrate were being disturbed.3. Co3O4 nanocubes were synthesized in one-step with surfactant-assisted hydrothermal method. The results indicated that cube-like nanoparticles and hexagonal nanosheets were obtained when the ratio of Co(Ac)2/NaOH was set as 1:0.75 and 1:2, respectively. The ratio of 1:1.25 was most conducive to the synthesis of Co3O4 nanocubes. Co3O4 nanocubes with good shape were more likely to be prepared under comparatively low reactant concentration. And SDBS would accelerate the growth of(111) crystal plane to greatly overpass(001) crystal plane, therefore, appropriate amount of SDBS could tremendously favor the morphology completeness and the uniformity of size distribution.4. The internal relation between structure and property was studied by characterizing the application performance of Co3O4 materials with various morphologies as supercapacitor electrodes. The results indicated that ultrathin graphene-like Co3O4 nanofilms possessed the highest specific capacitance of 1400 F/g among all the as-prepared Co3O4 structures. And only a small decrease of the capacitance(2.9 %) could be observed even after a cycling test of 1500 times which declared the cycling stability was excellent. However, hollow Co3O4 microspheres and solid Co3O4 microspheres possessed maximum specific capacitance of 1227, 850F/g, respectively. The maximum specific capacitance of Co3O4 nanocubes was only 586F/g. The study revealed that Co3O4 materials with nanoscale film or thin-wall structures were more likely to achieve ultrahigh capacitance. These kind of structures also devoted themselves to releasing thermal expansion during electrothermal process and greatly improving the cycling stability of electrodes.