Study On The Application Of Cobaltbased Catalysts With Atomic-Level Active Centers In Zinc-Air Batteries

Energy scarcity and environmental pollution are driving the urgent need for efficient renewable energy storage devices.Rechargeable zinc-air batteries（ZABs）,as an advanced renewable energy conversion and storage system,offer advantages such as high theoretical capacity density,high safety,cost effectiveness,and environmental friendliness.However,the performance of ZABs has been limited by the slow reaction rate and excessive reaction barrier of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）in the air electrode,resulting in low energy conversion efficiency and poor cycle life.Rational modification of non-precious metal catalysts is an effective way to solve the above problems and improve the performance of ZABs.Among many non-precious metal catalysts,metallic cobalt has become a topical issue in the field of catalysis due to its abundant reserves,diverse electronic valence states and tunable electronic structure,especially the atomically dispersed catalysts with Co-N_x-C as the active center have attracted much attention in recent years.However,the catalytic performance of Co-N_x-C still lags behind that of noble metal catalysts.Therefore,in this thesis,Co CN catalysts are investigated to modulate the electronic structure of the Co-N_x-C active center by developing different synthetic methods to introduce metal particles or heteroatomic coordination,aiming to improve their bifunctional catalytic performance and apply to rechargeable ZABs.The CoNi@CoCN composite catalyst is prepared using a vapor migration strategy to achieve uniformly fine Co Ni nanoparticles（10 nm）as"solid ligands"to modify the Co-N_x-C active sites.Compared with the conventional wet impregnation,the vapor migration strategy allows the Ni-containing gaseous precursors to be uniformly trapped and effectively stored by the Co/N co-doped carbon substrate,and ensures the fine size and uniform distribution of Co Ni alloy nanoparticles in the subsequent processing,which facilitates their effective coupling with the adjacent atomically dispersed Co-N_x-C active sites.The Co Ni@Co CN composite catalyst exhibits excellent bifunctional oxygen electrocatalytic performance（ΔE=0.71 V）and satisfactory durability in the accelerated aging test（only 1 m V shift of half-wave potential after 5000 cycles）.The ZAB assembled with the Co Ni@Co CN catalyst as the oxygen electrode demonstrates a high power density of 157.1 m W cm^-2,a high specific capacity,an long cycle life of over 200 h（400cycles）,and a high voltage-ampere efficiency.To address the problem that the nanoscale particles can not sufficiently regulate the atomically dispersed active sites,a Co N₄-O/MX catalyst with five-coordinated Co active centers is prepared by developing an electrostatic self-assembly method to precisely introduce O atomic ligands into the axial position of Co N₄,realizing further regulation of the active sites with a single atomic ligand.The catalysts exhibit a sheet-like morphology with a three-dimensional wrinkled structure,which can effectively suppress the repacking of delaminated MXene nanosheets and ensure the full exposure of the active sites.DFT theoretical calculations show that the axial coordination of O-Co N₄ can affect the electron localization between Co-O bonds and break the symmetry of Co electron distribution,leading to better desorption ability of OH*intermediate as well as higher ORR/OER activity than symmetric Co N₄ sites.On this basis,it is demonstrated that the use of melamine as a protective ammonium salt and a suitable temperature（700°C）can effectively improve the performance of Co N₄-O/MX.Ultimately,the prepared Co N₄-O/MX catalysts exhibits bifunctional catalytic activity（ΔE=0.72 V）and significant durability in accelerated degradation tests,significantly outperforming Co CN catalysts without axial O coordination and noble metal catalysts.Impressively,the Co N₄-O/MX exhibits excellent conversion efficiency（TOF）,which is five times higher than that of commercial Pt/C.The ZAB assembled with the Co N₄-O/MX as the oxygen electrode shows a high power density of 182.8 m W cm^-2,a high specific capacity（92.5%of the theoretical capacity）,an ultra-long cycle life of over 250 h（500 cycles）,and a high voltage-ampere efficiency.To address the insufficient stability,atomically dispersed Co-N_x-C sites anchored on Cr₂O₃ substrate are prepared by a Na Cl molten salt template-assisted method as a novel super-stable bifunctional oxygen catalyst.The Co CN@Cr₂O₃ is an ultrathin nanosheet composed of many interconnected nanocrystals,which allows the electrolyte and ion transport through the sheet to improve the mass transfer of the catalyst due to the large number of pores between the crystals.More uniquely,the introduction of Cr₂O₃effectively inhibits the 2e^-reaction pathway of ORR,which results in less H₂O₂production.The reduced yield of by-product H₂O₂ can effectively attenuate the Fenton effect and avoid the shedding of active sites.Finally,the Co CN@Cr₂O₃ provides satisfactory bifunctional catalytic activity（ΔE=0.71 V）and surprising catalytic durability.The Co CN@Cr₂O₃ can maintain 100%of the initial current value after 10 h of chronoamperometry test,and even remained at more than 90%of the initial current after50 h,far exceeding the Co CN catalyst without the introduction of Cr₂O₃.Also,the ZAB assembled with the Co CN@Cr₂O₃ catalyst as the oxygen electrode exhibits an extremely long cycle life of over 1500 h.