The Preparation And Performance Of Pyrolysis-free COP-based Oxygen Reduction And Oxygen Evolution Electrodes

The growing energy crisis has increased the demand for new energy devices,such as fuel cells and metal-air batteries,which are the next generation of clean energy conversion and storage devices.In recent years,pyrolytic carbon based single-atom catalysts have made a great contribution to the development of clean energy devices in the fields of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）.However,as the research progresses,the precursors of pyrolysis-based catalysts inevitably undergo disorderly and irregular collapse during the high temperature process,which eventually leads to random and irreducible sample structures and severely limits their performance in cathodic reactions（ORR&OER）and the scientific design of catalytic sites.In this article,a series of pyrolysis-free catalysts with well-defined active sites were designed and prepared by drawing on the synthetic idea of pyrolysis-free.The effects of the coordination environment of the active center Fe atoms on the oxygen reduction under acidic conditions and the enhancement effect of the three-dimensional heterogeneous structure on the performance of the dual-effect catalysts in the oxygen reduction reaction and oxygen precipitation reaction in alkaline environment were investigated.（1）The regulation pattern of the electronic properties of the axial group on the ORR activity of the Fe N₄ site was revealed.A series of axial five-ligand catalysts were designed and synthesized,and the effect of axial electron-withdrawing/donating groups for oxygen reduction performance under acidic environment was investigated.The aminopyridine（modified by Cl,Br,H,CH₃,O-CH₃,respectively）was directly grafted onto carbon nanotubes through covalent bonding.Further,Fe-N₄ sites of the COP_BTC network are riveted to aminopyridine on carbon tubes,marked as COP_BTC@X-CNTs.The introduction of axial electron-absorbing groups greatly improves the oxygen reduction performance of Fe-N₄ in acidic environments.The COP_BTC@Cl-CNTs exhibited a half-wave potential of 751 m V in 0.1 M HCl O₄,which was significantly higher than the donating electron group-modified COP_BTC@O-CH₃-CNTs（630 m V）.In addition,COP_BTC@Cl-CNTs exhibited a power density of 170 m W cm^-2 in the proton exchange membrane fuel cell,which was also superior to COP_BTC@O-CH₃-CNTs(119 m W cm^-2).The control of the ORR activity of Fe-N₄ sites from the axial direction was successfully achieved by introducing groups with different electronic properties into the axial direction of Fe sites.（2）A new method for the preparation of high-performance multifunctional heterogeneous catalysts was developed.The three-dimensional nanoflower structure of 3D-LDH was synthesized by using the alcohol phase and successfully compounded with COP-Fe to obtain the heterogeneous material COP-Fe@3D-LDH.This heterogeneous structure promoted the OER and ORR performance of 3D-LDH and COP-Fe,which exhibited a half-wave potential of900 m V for COP-Fe@3D-LDH in a 0.1 M KOH environment and an overpotential of 410 m V(at 10 m A cm^-2).Moreover,the COP-Fe@3D-LDH also shows excellent stability,with 3.7%decay in ORR performance and almost no decay in OER performance after 75000 s at a potential of 0.7 V.The development of this pyrolysis-free 3D heterostructure provides a new idea to achieve a simple synthesis of highly efficient multifunctional catalysts.