Nitrogen-doped Carbon Dot-derived Transition Metal Hybrid Materials For Zinc-air Batteries

Rechargeable Zinc-air batteries(ZABs)have the advantages of high theoretical energy density(1086 Wh kgzn-1),low price and safety,and are expected to replace lithium/sodium-ion batteries and become the power source of future electric vehicles.However,the key problem faced for ZABs is the slow kinetic rate of cathode oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)during discharging/charging,and the significant reduction of battery output power and efficiency caused by high polarization overpotential.At present,Pt-based and Ir/Ru-based noble catalysts are still the most effective electrocatalysts for ORR and OER,but their scarcity,poor stability,high price,and susceptibility to CO poisoning limit the practical applications of ZABs.Therefore,the key to the current research is the preparation of low-cost non-noble metal-based ORR/OER electrocatalysts for high-performance ZABs.Carbon dots(CDs)are zero-dimensional carbon materials with extremely small dimensions.Compared with other carbon nanomaterials,CDs have the advantages of simple synthesis,safety,and good electrical conductivity,so they are widely used as ideal materials for conductive carbon supports for electrocatalysts.Based on the above,this work mainly discusses the synthesis method of transition metal hybrid materials supported by N-doped carbon dots as carbon supports and their applications in electrocatalytic ORR and ZABs.It mainly includes the following three aspects:1.Carbon dot-derived three-dimensional carbon nanoflowers loaded with Co single atoms and nanoparticles for high-energy zine-air batteriesThe citric acid-melamine carbon dots(CM-CDs)and metal salts were mixed and annealed at high temperature to obtain a three-dimensional nanoflower-structured electrocatalyst loaded with Co single atoms and nanoparticles(Co SAs/NPs CNF).The effects of various components on the morphology and performance of the catalysts were explored experimentally,and the theoretical calculations proved that Co single stoms and nanoparticles synergistically promoted the catalytic performance of the catalysts.Thanks to the 3D porous network of electrocatalyst,the Co SAs/NPs CNFs-based ZAB exhibit high energy density up to 1000.86 mWh gzn-1 and stability up to 450 h.2.Fe&Fe2O3@C composites for electrocatalytic oxygen reduction and zinc-air batteriesIn order to improve the ORR performance of the catalyst,reducing the battery charge-discharge potential gap,we synthesized carbon-supported Fe&Fe2O3 electrocatalysts(Fe&Fe2O3@C)with excellent ORR performance.Firstly,N-doped carbon dots(NCDs)were prepared hydrothermally using ethanol as solvent,citric acid as carbon source,and ethylenediamine as nitrogen source.Nextly,NCDs were mixed with melamine and Fe salt to obtain Fe&Fe2O3@C.Electrochemical tests explored the effect of each component in Fe&Fe2O3@C on the catalytic performance.The ZAB assembled with Fe&Fe2O3@C exhibited an open circuit voltage of 1.49 V and achieved a stability of 625 h.The solvent effect of CDs can effectively improve the ORR performance and make up for the OER performance of catalyst.Therefore,the battery charge-discharge potential gap is reduced and the energy efficiency of the battery is improved.3.Fe-CoO/Co2O3@C composites for electrocatalytic oxygen reduction/oxygen evolution and zinc-air batteriesIn order to further reduce the battery charge-discharge potential gap and improve the energy efficiency of the battery,we synthesized carbon-supported Fe-CoO/Co2O3 catalyst(Fe-CoO/Co2O3@C)with ORR/OER bifunctionality.The Fe-CoO/Co2O3@C electrocatalyst was obtained by mixing CE-CDs with transition metal salts(Fe and Co)and pyrolysis.The synergistic effect between the two metal compounds makes the catalyst have excellent ORR/OER performance,and the charging performance of the battery is also improved.The charge-discharge potential gap of battery is reduced to 0.84 V,which further improves the energy efficiency of the battery.