Preparation And Properties Of Composite Electrocatalysts Supported By Carbon Dots

In recent years,rechargeable zinc-air batteries（ZABs）have attracted much attention due to their high energy density,safety,environmental protection,low cost and stable performance.However,the rechargeable zinc-air battery still faces serious problems in its application,such as energy conversion efficiency,long-term cycle stability and voltage difference in charge and discharge cycle.Based on the structure and working mechanism of ZABs,the development of efficient electrocatalysts is the key to improve the oxygen catalytic kinetic process and improve the overall performance of batteries.In this paper,new carbon dots（CDs）used as the carrier,via effectively combining the transition metal compound with M-N₄-C single atomic site,an efficient carbon-based composite electrocatalyst was constructed and applied to the cathode of ZABs,and the battery performance was tested and analyzed.The main work is as follows:（1）A Fe single atom seed-mediated approach to synthesize Fe-N-C/Fe₃C composites as powerful bifunctional electrocatalysts for ZABs.Firstly,using CDs prepared by hydrothermal method as the carrier,the Fe-N₄-C/Fe₃C composite electrocatalyst was prepared by using the complexation of Fe ion and glucose,followed by mixed melamine annealing.Then,by changing the amount of Fe source,the ratio of Fe-N₄-C and Fe₃C was optimized to further improve the efficiency of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）of the catalyst,and the best performance Fe-N₄-C/Fe₃C bifunctional electrocatalyst was obtained.The test results show that based on the synergy effects of Fe₃C and Fe-N₄-C,the obtained Fe-N₄-C/Fe₃C electrocatalyst potential difference between（?E）ORR and OER was 0.668 V,significantly lower than most bifunctional electrocatalysts reported in the literature.Furthermore,theoretical analysis revealed that the adsorption-desorption process of intermediate products during the ORR and OER reactions was improved due to the electron-rich character of Fe₃C tuned the electronic state in Fe-N₄-C,and enhanced O₂^*adsorption and OH^*desorption,the energy barrier of the intermediate is reduced.In addition,the rechargeable ZABs using Fe-N₄-C/Fe₃C-op electrocatalyst as the cathode show a high power density was 137.4 m W cm^-2,and maintain the charge-discharge stability of 1150 cycles（450 h）,the battery has excellent comprehensive performance.（2）Co-N₄-C/Co₂N catalyst was constructed based on cascade anchoring strategy as an efficient ORR catalyst.Firstly,CDs prepared by hydrothermal method were ultrasonically dispersed in a mixed solution of glucose and cobalt sources.Glucose molecules were used as chelators to effectively adsorb Co ions and bind CDs through the interaction of oxygen-containing functional groups,Co-N₄-C single atom catalysts（SACs）were then carbonized with melamine.Continue to increase the amount of cobalt,and some cobalt ions react with carbon to form Co₂N.The presence of nanoparticles greatly enhances the Co-N₄-C activity of the catalyst.The test results show that the Co-N₄-C/Co₂N catalyst synthesized by this method has a half-wave potential of ORR of 0.86 V in 0.1 M KOH solution,which is closed to the catalytic activity of 20%Pt/C.In addition,the rechargeable ZABs using Co-N₄-C/Co₂N electrocatalyst as the cathode show a high power density was 138.9 m W cm^-2.The specific capacity and energy density are 800.85 m Ah g_zn^-1 and 993.21 m Wh g_zn^-1,and the charge-discharge test stability is as high as 1000 h.