Preparation And Electrochemical Performance Of Self-Supporting Cathode For Zn-Air Battery Based On Co₃O₄

The development of new green energy storage technologies is of great significance for protecting the environment and optimizing the energy structure of our country.As a kind of environmental friendly and safe new energy storage technology,zinc-air battery has a bright prospect of development in recent years.As an important part of the zinc-air battery,the development of self-supporting air cathodes with high catalytic performance and without carbon and binder at the same time is particularly rapid.The main research this thesis is the preparation,characterization and electrochemical performance of self-supporting air cathode based on Co3O4.Co3O4 nanowire arrays were firstly grown directly on the stainless steel mesh by hydrothermal method combined with high-temperature calcination process to obtain well-crystallized Co3O4-based self-supporting air cathodes.The X-ray diffraction(XRD),scanning electron microscope(SEM)and field emission transmission electron microscopy(TEM)was employed to investigate the change regulation of phase,morphology and composition of the Co3O4-based self-supporting air cathode systematically.In addition,silver nanoparticles were introduced by seed method to modify Co3O4 nanowire arrays to further improve the electrochemical properties of Co3O4-based self-supporting air cathodes.The following conclusions can be obtained:(1)Hydrothermal reaction and high-temperature calcination are the two processes for the preparation of self-supporting Co3O4 nanowire arrays.The XRD,SEM and TEM characterization show that the Co(CO3)0.5OH0.11H2O and Co(OH)F nano wires with smooth surface and needle-like morphology can be obtained at the surface of stainless steel mesh after the hydrothermal reaction.After calcined at high temperature,a highly porous Co3O4 nanowire arrays can be obtained.By treating the steel mesh with concentrated nitric acid,the density of nucleation sites increased,which can significantly reduce the growth difficulty of Co3O4 nanowires and increase the distribution density of Co3O4 nanowire arrays.The electrochemical measurements show that the oxygen reduction catalytic performance of Co3O4 self-supporting cathode is obviously stronger than that of the traditional powder Co3O4 cathode under the same mass loading,which confirmed that the morphology of Co3O4 nanomaterials has great influence on its catalytic activity.(2)The optimized synthetic process of the Co3O4-based self-supporting air cathode with large specific surface area and excellent crystallinity in this paper are as follows:the use of 0.3 7g NH4F precursor solution,120? hydrothermal reaction temperature,6 hours holding time and 300? calcination temperature together with 2 hours holding time.The XRD and SEM measurements show that the hydrothermal temperature affects the morphology and crystallinity of Co3O4 nanoparticles.When the hydrothermal temperature is set to 80?,the crystallinity of Co3O4 nanoparticles is poor,showing a spherical and hexagon mixed morphology.When the hydrothermal temperature is 100-140?,Co3O4 with wire morphology and good crystallinity can be obtained.When the hydrothermal temperature reaches 160?,Co3O4 change to irregular shape,and impurities of Fe2O3 can be found in the product.The hydrothermal reaction time also affects the morphology of Co3O4.When the holding time was 2 hours,Co3O4 nanosphere with the diameter of 3 ?m can be obtained after calcination.When the holding time extended to 4 hours,Co3O4 changed to wire morphology.The length of nanowires increased with time extended.Furthermore,the calcination temperature affects the phase composition and crystallinity of Co3O4.When the calcination temperature is lower than 300?,the product contains more impurities and the crystallinity is poor.When the calcination temperature rises to 500?,the Co3O4 nanowires break severely and have the tendency of agglomeration.(3)Ag nanoparticles were decorated on the Co3O4 nanowire array by seed growth method to obtain Ag@Co3O4 composite self-supporting air cathode.The optimized synthesis parameters of Ag@Co3O4 composite cathode were as follows:10 mmol·L-1 concentration ofAg seeds for 10 min growth.Before and after the decoration of Ag particles,the number of oxygen transfer reactions on the air cathodes increased from 3.63 to 3.76,which is closer to the 4e-pathway.The results indicated that the modified Ag nanoparticles could significantly enhance the catalytic activity of Co3O4-based self-supporting air cathodes and increase the proportion of the 4e-pathway in the oxygen reduction reaction at the same time.The preparation cost of Ag@Co3O4 composite self-supporting air cathode is much lower than that of the air cathode with commercial Pt/C catalyst,while its oxygen reduction catalytic performance can reach up to 86%of the latter.