Design Of Cobalt-based Bifunctional Oxygen Catalyst And Study On Performance Of Zn-Air Batteries

As one of the candidates for storage and conversion of new energy devices,secondary zinc-air batteries have great advantages in terms of energy density/power density,safety,environmental protection and cost.And mature water-based alkaline zinc-air batteries can also significantly improve safety while minimizing manufacturing costs.However,despite the great application prospects of this energy storage system,the widespread application of Zn-air batteries is driven by the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）at the liquid-gas-solid three-phase cathode interface on the air cathode.A serious hindrance to slow learning.Therefore,the key measure to further promote the development of this technology is to develop low-cost,highly active bifunctional OER/ORR catalysts.In response to this problem,this dissertation is devoted to designing and preparing bifunctional oxygen electrocatalysts with high ORR/OER active sites,and investigating their performance in Zn-air batteries.Finally,the configuration and charge-discharge mechanism of Zn-air batteries are initially explored.In order to improve the ORR performance of Ni Fe-LDH in Zn-air batteries,a facile hydrothermal method was used to support Co₉S₈@Ni Fe-LDH nanocomposites on carbon cloth current collectors as stand-alone electrodes for Zn-air batteries.The characterization results of the physical properties of the material show that the increased specific surface area of the material can expose more ORR/OER active sites,and the conductive substrate Co₉S₈ hollow tube solves the problem of slow electron transport in Ni Fe-LDH in zinc-air batteries,thus realizing the bifunctional ORR/OER activity of the air cathode.The highly anisotropic hollow tube structure can promote the penetration of the electrolyte,increase the contact surface between the catalyst and the electrolyte,and shorten the diffusion path of ions to speed up the electrocatalytic reaction.Electrochemical methods were used to explore the electrochemical performance of each component nanocomposite in ORR and OER.The electrocatalytic performance test results demonstrate that Co₉S₈@Ni Fe-LDH exhibits bifunctional ORR/OER activity（ΔE=0.88 V）.And Co₉S₈@Ni Fe-LDH greatly enhanced the ORR activity of Ni Fe-LDH.An aqueous zinc-air battery assembled with a high-conductive carbon cloth loaded with Co₉S₈@Ni Fe-LDH was tested as a current collector and an air cathode.The Co₉S₈@Ni Fe-0DH air cathode exhibits a discharge power density of 148 mWcm^-2.The galvanostatic charge-discharge test shows a stable charge（1.93V）and discharge（1.22V）voltage plateau,and can cycle 430 times in 220hours,showing good durability.Co₃O_4-x/C nanocomposite fibers were successfully synthesized by electrospinning and heat treatment methods.And through a physically controllable method,holes are created on the crystal structure to increase the oxygen-deficient active sites.The physical property characterization results of the material explored the formation mechanism of nanofibers and oxygen vacancies.When the oxidation time was 50 min,the oxygen vacancy concentration in the MCB sample was the highest.The highly porous lotus root-like channel structure in this fiber facilitates fast electron/ion transport and accelerates interfacial charge transport.According to the electrocatalytic test results,the effects of different oxidation degrees on ORR and OER were discussed.The small overpotential difference between ORR and OER（ΔE value of 0.693 V）of the MCB sample proved its good bifunctional catalytic activity.Moreover,MCB fibers still maintained good ORR/OER stability after 5000 cycles of testing.It is also demonstrated that MCB fibers can meet or exceed the bifunctional catalytic activity and durability of commercial precious metals.Co₃O_4-x/C nanocomposite fibers were used in the air cathode of Zn-air battery.The galvanostatic discharge test results showed that the capacity of the Zn-air battery could reach 805.7 mAhg _Zn^-1when the discharge current density of the MCB sample was 10 mAcm^-2.The constant current charge-discharge test curve results of MCB samples at a current density of 3 mAcm^-2 show that the Zn-air battery can achieve a stable charge-discharge voltage plateau and maintain good durability within 800 charge-discharge cycles.Finally,the overall performance of the zinc-air battery is initially explored from the overall configuration and charge-discharge mechanism of the zinc-air battery.Firstly,the effect of periodic electrolyte replacement on the performance of zinc-air batteries and the effect of electrolyte flow rate on the performance of zinc-air batteries were analyzed.By regularly replacing the electrolyte,the ions consumed during the charging and discharging process of the battery are replenished,the oxygen bubbles generated during the charging process are taken away,and the concentration polarization caused by the high current charging and discharging is reduced at the same time.Then,under the condition of periodically replacing the electrolyte,the traditional galvanostatic charge-discharge mechanism was changed,and the optimal galvanostatic charge-discharge scheme was determined according to the discharge power and cycle stability test results.