Preparation Of Co-based Spinel Metal Oxides As Catalysts And Its Application In Zinc-air Batteries

In the context of excessive consumption of fossil fuels and environmental protection,research on new green energy conversion and storage equipment is concerned.Among them,zinc-air batteries have been widely studied because of their safety,low cost and high theoretical power density.However,during the charging and discharging process,the oxygen evolution reaction（denoted as OER）and the oxygen reaction（denoted as ORR）that occur on the air surface have slow dynamics,which limits the practical application of rechargeable zinc-air batteries.At present,the commercial catalysts are still noble metal oxides（such as OER catalyst Ru O₂and Ir O₂,ORR catalyst Pt/C）,but their scarcity,high cost,and low stability limit their wide commercial application.Therefore,a cheap and efficient OER/ORR bifunctional electrocatalysis is needed.Spinel-based metal oxides have high OER activity,and have large reserves,low price and easy regulation,which is generally regarded as a substitute for precious metal catalysts.Electronic structure and catalytic performance can be improved by oxygen vacancy adjustment,non-metallic element doping,and composite carbon materials.The main research contents and conclusions of this paper are as follows:First of all,the spinel oxide Co₃O₄is treated with Na BH₄,which have strong reducibility.It can produce oxygen vacancies in the structure of Co₃O₄.After the introduction of oxygen vacancies,the catalytic activity of Co₃O₄was significantly improved and the sample treated with 2 M Na BH₄has the best catalytic performance.The presence of oxygen vacancies is beneficial to the transport of oxygen molecules within the catalyst,accelerates oxygen contact with active sites,and thus completes the transformation of oxygen to reaction reduction.The enhanced OER performance can be attributed to the increase of oxygen vacancies concentration,the improvement of conductivity and the increase of surface area.It provides a direction for a new type of high-efficiency non-noble metal-based catalyst.Secondly,we study the effect of non-metal S element doping on the bi-functional catalytic performance of spinel metal oxides,and prepare S-doped Ni Co₂O₄（denoted as NCS-60,NCS-120,NCS-180 and NCS-240）.Among them,NCS-180 catalyst has the best OER/ORR dual-function catalytic properties.The electronical difference between sulfur and oxygen elements causes the reduction the density of the surrounding oxygen price electron cloud,weaken the bond energy between metal and oxygen,and cause lattice oxygen to escape,resulting in more oxygen vacancies and Co²⁺.This facilitates the intermediate product-OH adsorption and the transmission of the reactor O₂.Zinc-air battery based on NCS-180 catalyst has a high-power density of 80 m W cm^-2,a specific capacity of 700 m Ah/g _Zn,and good charge-discharge stability.Finally,we prepared Ni Co₂O₄/CNTs composite electrocatalysts（denoted as NCO-0.5CNTs,NCO-1CNTs and NCO-2CNTs）by hydrothermal method.The porous structure of carbon nanotubes improves the electrochemical surface area of composite catalysts,exposing more reaction active sites.The excellent conductivity of carbon nanotubes makes composite catalysts less resistant and accelerates electron transfer during catalysis.In addition,there is synergy between carbon nanotubes and spinel oxides,which is conducive to the occurrence of OER and ORR reactions.When Ni Co₂O₄has a mass ratio of 1:1to carbon nanotubes（NCO-1CNTs）,the composite catalysts exhibit optimal OER and ORR catalytic activity.Given the excellent dual-function catalytic activity of the NCO-1CNTs catalyst,we assemble it as an air electrode for zinc-air battery with a power density of 65 m W cm^-2and excellent cycle stability.This work provides a new way to design efficient dual-function catalysts and zinc-air batteries.