Regulating The Oxygen Electroreduction Performance Of Single-Atom Catalysts By Isomer Engineering And Strong Electron Donor/Acceptor Interactions

The depletion of fossil fuels,energy security and environmental pollution have prompted researchers from all over the world to turn their research focus to the development and use of new energy storage and conversion systems.Fuel cells and rechargeable zinc-air batteries have become hot research topics in this field due to their low emissions,safety,and high efficiency.As the key cathodic reaction of fuel cells and zinc-air batteries,oxygen reduction reaction（ORR）requires efficient catalysts to overcome slow kinetics and excessive overpotentials.Currently,noble metal catalysts,such as commercial Pt/C,Pt-based alloys,etc.,are considered to be the most effective ORR catalysts,however,the high price,scarce reserves,and sensitivity to impurities limit the large-scale application of this material.Therefore,the development of highly active and cost-effective electrocatalysts for the ORR is urgent.Single-atom catalysts（SACs）have attracted much attention due to their excellent reactivity and 100%atom utilization efficiency.The most common preparation method for SACs is pyrolysis,however,due to the complexity of the reaction at high temperature,the activity of the catalyst can only be maximized by a“trial and error”method.In addition,the pyrolysis method cannot accurately control the structure of the target product,and there may be a variety of active structures in the prepared catalyst.Based on this,this thesis focuses on the construction of the relationship between the precursor structure and catalyst performance and the development and design of new pyrolysis-free single-atom ORR catalysts to synthesize porous nanoflowers,two-dimensional nanosheets and other materials,and explore their microstructures,ORR activity and zinc-air battery（all-solid flexible and liquid）performance.The main contents are as follows:1.A Fe-N-C single-atom catalyst（Fe-PpPD-800）with high ORR activity was developed using phenylenediamine-based azo polymer,Fe Cl₃and 4,4’-bipyridine as precursors.The modulation of different structures,micro morphology,and catalytic activities can be easily achieved by isomer engineering of phenylenediamines,thereby establishing the correlation of ORR performance with the structure of nitrogen/carbon precursors.The Fe–Pp PD-800 derived from p-phenylenediamine shows the best ORR activity with a half-wave potential(E_1/2)reaches 0.892 V vs reversible hydrogen electrode（RHE）,which is better than the counterparts derived from o-phenylenediamine（Fe–Po PD-800）and m-phenylenediamine（Fe–Pm PD-800）,even surpassing commercial Pt/C(E_1/2=0.881 V vs RHE).Furthermore,the self-made zinc–air battery based on Fe–Pp PD-800 achieves high power density and specific capacity up to 242 m W·cm^-2and 873 m Ah·g _Zn^-1,respectively,a stable open circuit voltage of 1.45 V,and excellent cycling stability.2.Metal tetraaza annulene polymers were designed and developed by changing the type ofβsubstituents and loading the polymers onto graphene,a series of nanocomposites（Co TAA-R@GR,R=H,Py,Qy,Ph,Ph（Cl）,Cl and NO₂）were obtained.Due to the substituent effect,the Co TAA-R@GR catalysts exhibit different catalytic performances.With the enhancement of the electron-withdrawing ability of the substituents,the electronic structure of the metal center changes,resulting in different affinities for oxygen molecules and reaction intermediates.Co TAA-R@GR has the best ORR activity when the substituent is-Cl.Co TAA-Cl@GR catalyst shows a Tafel slope of 157 m V·dec^-1,an initial potential of 0.903 V,an E_1/2of 0.818V and a kinetic current density of 12.3 m A·cm^-2in alkaline medium.In addition,when used as cathode catalysts to assemble a zinc-air battery,it also exhibits good device performance.The peak power density of the zinc-air battery based on Co TAA-Cl@GR is 127 m W·cm^-2and the specific capacity is as high as 816.0mAh·g^-1.