Study On The Electrochemical Properties And Kinetic Analysis Of Co₃O₄ Electrode Materials For Supercapacitors

In recent years,with the development of flexible wearable smart electronic devices and the promotion of electric vehicles,traditional lithium ion batteries cannot meet the actual needs due to some problems such as safety and low power density.There is an urgent need for an energy storage device that is safe and reliable and can be quickly charged and discharged.Supercapacitors have received great attention due to their advantages such as fast charge and discharge,long cycle life,safety and reliability.They are likely to become one of the mainstream energy storage devices.According to the charge storage mechanism,they can be divided into electric double layer capacitor and pseudocapacitor.The pseudocapacitor mainly stores energy by Faraday reaction on the surface or near surface of the electrode material,so it has higher energy than the electric double layer capacitor.Cobaltosic oxide（Co₃O₄）,as an important pseudocapacitor electrode material,has received great attention due to its high theoretical mass ratio capacity(3560 F g^-1)and large price advantage over lithium ion batteries.However,as a metal oxide,Co₃O₄ has poor conductivity,low material utilization,and the actual capacity is much lower than theoretical capacity.Therefore,further study the reasons for its low capacity and improve its capacity is of great significance for improving the practical application of supercapacitors.In the paper,Co₃O₄ was used as the research object,and Co₃O₄ electrode materials with different crystallinity were prepared by strict control variables.The effects of grain size on the electrochemical properties were investigated by kinetic analysis.Then,by further reducing the crystallinity of Co₃O₄,an electrode material having more excellent electrochemical properties is obtained.The main research contents are as follows:（1）The precursor of Co₃O₄ was synthesized by hydrothermal synthesis,and in the subsequent annealing process,Co₃O₄ with different crystallinity was obtained by controlling the annealing temperature.The results show that the grain size of Co₃O₄ will increase with the increase of annealing temperature,but it will not affect the macroscopic morphology of the material.Annealing temperatures will have a greater impact on its electrochemical performance.As the annealing temperature increases,the grain size becomes larger and its electrochemical energy storage performance drops sharply.At a sweep rate of 1 mV s^-1,samples with annealing temperatures of300,350,and 400°C have specific capacitances of 855,642,and 296 F g^-1,respectively.Through the kinetic analysis of its electrochemical data,it can be found that the influence of the annealing temperature on the electrode material has two main aspects.On the one hand,the increased annealing temperature causes the decrease of diffusion contribution capacity,which may be caused by the increase of crystal grains and the decrease in grain boundaries,leading to the immersing of electrolyte into the grain boundaries or grains get more difficulty.On the other hand,an increase in the annealing temperature causes the decrease of the specific surface area of the electrode material and the decrease of surface oxygen vacancies,which resulted the reduce of surface reaction sites and surface redox reaction.Eventually,the decrease in diffusion capacity and surface capacity results the decrease of the overall electrochemical energy storage performance of the electrode material.（2）By adding palladium chloride to the precursor solution,a hydroxide precursor doped with metal palladium particles is obtained after hydrothermal treatment.Then,the annealing is performed under an argon atmosphere to obtain Pd metal particles doped Co₃O₄（Pd-Co₃O₄）.Subsequent XRD characterization,TEM test shows that palladium doping can significantly reduce the grain size of Co₃O₄,resulting in the increase of amorphous regions and the sharp decline of crystallinity.Electrochemical tests show that Co₃O₄ with smaller grain size and more amorphous regions has smaller charge transfer resistance and higher rate performance than Co₃O₄ with larger grain size.Compared to other cobalt-based capacitor materials,it has a higher specific capacity of 1340 F g^-1at 1 A g^-1,and the surface capacitance is also 4.5 F cm^-2.In addition,when used as a positive electrode material for asymmetric devices,the devices show a high energy density of 4.3 mWh cm^-3,and can be used as a power to make an electronic watch work properly,demonstrating a higher practical application and flexibility.