Molecular Design To Prepare Transition Metal Copper/cobalt-nitrogen-carbon Catalyst And Its Structure-activity Relationship

Hydrogen-oxygen fuel cells and metal-air batteries are important new energy conversion devices.However,the oxygen reduction reaction(ORR)kinetics of the cathode of these two batteries is slower than that of the anode,and more platinum(Pt)-based catalysts are needed to promote the cathode ORR reaction.The high price of Pt-based catalysts which is caused by the scarcity of platinum resources restricts the large-scale application of these two batteries.Therefore,the development of high-performance and inexpensive non-precious metal catalysts is of great significance to the commercialization of hydrogen-oxygen fuel cells and metal-air batteries.Among the various non-precious metal catalysts,transition metal(M)-nitrogen(N)-carbon(C)catalysts(M=Fe,Co,Cu,etc.)with high ORR activity,abundant resources and cheap resources are the most promising to replace platinum-based catalysts.However,the activity and durability of M-N-C catalysts still cannot meet the commercial requirements of hydrogen-oxygen fuel cells and metal-air batteries.On the one hand,the preparation of high-performance M-N-C catalysts almost requires high-temperature pyrolysis of precursors containing carbon sources,nitrogen sources and metal salts.Upon the pyrolysis process,the prepared catalysts not only contains M-Nx active sites but also varieties of metal species due to the structure of the precursors inevitably decomposes and recombines,which makes it difficult to characterize the active site structure and hinders the construction of the structure-activity relationship.On the other hand,although the non-pyrolysis M-N-C catalysts inspired by biological enzymes have the well-defined M-N_xstructure,its ORR activity is lower.Furthermore,it is difficult to further improve its ORR activity by regulating its structure,due to the structural characteristics of the macromolecular ligand in the non-pyrolysis M-N-C catalysts.In this work,the M-N-C catalysts were chosen as the research object,proposed a molecular design to prepare high-performance M-N-C catalysts and investigated their structure activity relationships.Based on the fact that the ORR active sites of both pyrolytic and non-pyrolytic M-N-C catalysts are related to the M-N_xcoordination structure,small molecules containing Cu-N_xor Co-N_xcoordination structures are synthesized through molecular design.Then,the Cu-N_xor Co-N_xmoieties in the molecules are covalently grafted to the carbon-based surface,thereby prepared high-performance Cu-N-C or Co-N-C catalysts with a well-defined active site structure.Combined with first-principles calculations,their interaction with O₂were explored,to promote the rational design and preparation of M-N-C catalysts.The main content and results of the research in this paper are summarized as follows:(i)The transition metal small molecules precursors were synthesized by the molecular design,grounding the foundation for the catalysts preparation and structural control to obtain high-performance M-N-C;(ii)Based on the molecular design,the Cu-N₄coordination structures were bottom-up constructed into graphene to form an active sites to prepare a high-performance Cu-N₄-C catalyst.The Cu-N₄-C catalyst shows a good catalytic performance,with a half-wave potential of 0.84 V,a kinetic mass activity of 50.7 A·g^-1at 0.80 V and a relative current only loss 5%after10,000 s.By tunning the structure of the Cu-N₄-C catalyst,we confirmed that the Cu-N₄embedded to the edges of the graphene sheet is the most effective ORR active site,where the electronic structure of the Cu center has the right symmetry for the degenerate?*orbital of the O₂molecule;(iii)The prepared molecules containing Co-N₄coordination structures were inserted into the interlayers of multilayer graphene to obtain high-performance Co-N₄-C catalyst.The Co-N₄-C catalyst exhibits wide graphite interlayer spacing(9.2?),low work function,and new electronic states.Due to these unique crystal structure and electronic structure,the Co-N₄-C catalyst exhibits a good catalytic performance,with a half-wave potential of 0.88 V,a kinetic activity of 37.90 A·g^-1at 0.85 V and a good durability.The Co-N₄-C catalyst was used as the air electrode of the aqueous sodium-air battery.The peak discharge power density of the battery reaches of 10.40 m W cm^-2,about 2.76 times that of the battery using the Pt/C-JM as cathode catalyst(3.77 m W cm^-2).Furthermore,the Co-N₄-C catalyst also has good oxygen evolution reaction(OER)performance,the overpotential of 340 m V at 10 m A cm^-2,which is less 50 m V than that of the commercial Ir/C-Premetek catalyst.The rechargeable aqueous sodium-air battery assembled with the Co-N₄-C catalyst as the air electrode shows a charge-discharge voltage gap of 0.21 V,a round trip efficiency of 93.29%at a current density of 0.1 m A cm^-2and excellent cycling durability of charging and discharging.