Preparation Of Pyrolysis-free COP-based Electrocatalysts And Studies Of Their Electrocatalytic Properties

In recent years,transition metal-nitrogen-carbon（M-N-C）electrocatalysts have shown great promise and application potential in oxygen reduction reaction（ORR）,but such catalysts usually require high-temperature pyrolysis above 700°C for synthesis.Its components and structures tend to be complicated,and the active center is not clearly understood;the catalyst activity is highly sensitive to the structure of the active site,which brings great challenges to the study of catalytic mechanism and controllable preparation and restricts the further performance improvement and scale-up preparation of catalysts.It is an ideal solution to get rid of the pyrolysis preparation process and construct a‘pyrolysis-free’non-precious metal catalyst with precise arrangement of active centers.Covalent organic polymer（COP）materials are a class of geometric structures connected by strong covalent bonds through irreversible reactions under the guidance of network chemistry.The completely uniform and periodic structural arrangement in the framework endows each metal atom with equal reaction availability and maximum atom utilization in the ORR process.Based on the challenges and requirements of M-N-C electrocatalysts in the current research,this thesis clarifies the necessity of pyrolysis-free COP materials for the application in proton exchange membrane fuel cells（PEMFCs）and zinc-air batteries（ZABs）devices.The specific research content and innovation points in thisis are as follows:（1）Four kinds of fullyπ-conjugated quasi-phthalocyanine COP materials with clear topological framework were prepared by adopting a pyrolysis-free strategy;the structure-activity relationship between the carbon coordination environment and ORR catalytic activity was explored;based on this,the acidic ORR performance of carbon-based catalysts was improved.In view of the complex active sites of traditional pyrolytic Fe-N-C catalysts and the difficulty in studying the regulation of the carbon atom coordination environment on the catalytic activity of active sites,thesis thesis designed and synthesized a series of COP materials using a pyrolysis-free strategy.Their molecular structures contain clear Fe N4 active sites and well-organized carbon matrix adjacent to Fe N4 moieties.A series of related experiments and density functional theory（DFT）finally elucidated the structure-activity relationship between the carbon coordination environment and catalytic activity.（2）The COP_BTC-F catalyst was prepared by utilizing the F-decorated edge strategy,and its molecular weight was as high as 522,557 g mol^-1,realizing the application of pyrolysis-free catalysts in PEMFCs devices.Aiming at the problem that pyrolysis-type M-N-C catalysts generally exhibit low acidic ORR performance,this thesis doped strong electron-withdrawing F functional groups into the fullyπ-conjugated intrinsic COPBTC framework via bottom-up manner as closed F edges（denoted as COPBTC-F）.The as-synthesized COPBTC-F catalyst exhibits excellent catalytic activity in acidic ORR,with an intrinsic performance five times higher than that of the undoped F-functional counterpart（termed COPBTC）and enables efficient operation of PEMFCs devices.（3）The axial coordination strategy was utilized to explore the effect mechanism of axial side chains with electron-withdrawing/donating properties on Fe N₄ active sites,and an order of magnitude performance leap was achieved in PEMFCs energy devices(peak power of 231.5 m W cm^-2).In this thesis,aiming at the difficulty of studying the structure-activity relationship of the active sites in pyrolytic M-N-C catalysts and improving their acidity performance,a series of molecular structures that naturally take into account Fe N4 active sites and axial side-chain with different electron-withdrawing/donating properties are designed.A novel descriptorξwas innovatively proposed to quantitatively demonstrate the volcano-regulating mechanism of the catalytic activity of Fe N4 sites decorated by different electron-withdrawing/donating side-chain groups.Finally,it was directly assembled into PEMFCs,achieving an order of magnitude performance improvement compared with COPBTC-CNTs without functional group decoration.（4）A series of COP materials that can be simultaneously dissolved and stripped by Lewis acid or alkaline solution were designed and synthesized through the electron donor-acceptor（D-A）regulation strategy,and the assembly of energy devices was realized by utilizing the solution-processability of COPs.Aiming at the difficulty of solution-processability caused by the cross-linked structure in COP,this thesis designed and synthesized a series of COP materials that can be dissolved and exfoliated by Lewis acid or alkaline solution at the same time through the electron D-A regulation strategy.Taking advantage of their solubility properties,they could be assembled into perovskite quantum dot light emitting diodes（Pe QLEDs）to achieve stable electroluminescence（EL）.On the other hand,it is directly dissolved in the alkaline electrolyte of ZAFBs for corresponding charge and discharge reactions,avoiding the problems of the catalyst being continuously washed by the electrolyte,easy to fall off and overflowing in the traditional spraying process,greatly improving the durability and stability of ZAFBs.（5）A bifunctional oxygen electrocatalyst with both ORR and OER catalytic activity was developed through a pyrolytic-free synthesis strategy,which achieved the excellent charge-discharge performance and long-term cycle stability of ZABs and the preparation of a single batch of catalyst at a kilogram scale.Aiming at the problem that pyrolysis-type M-N-C catalysts are difficult to scale up,this thesis constructs a new bifunctional oxygen catalyst based on a pyrolysis-free strategy.The catalyst maintain a clear structure and simple synthesis routine.The synthesized catalysts are not only easier to achieve large-scale production,but also can maintain high bifunctional catalytic activity and long-term cycle stability after large-scale synthesis（ΔE[Ej10–E1/2]=591 m V;1200 cycles After that,the energy efficiency drops only by 2.02%）.