Exploring The Interfacial Charge Transfer Mechanism And Catalytic Performance In Electrocatalytic Oxygen Reduction Reaction Using Cobalt Porphyrin-Based Composites

Cobalt porphyrins are considered as one type of the most promising electrocatalysts for oxygen reduction reaction（ORR）due to their easily modified structures and the stable coordination environment.However,the catalytic activity,selectivity and stability of most reported cobalt porphyrin catalysts are still somewhat inferior to those of Pt-based materials,prompting researchers to devote themselves to understanding the structure-activity relationship between catalyst structure and ORR performance.This work has mainly designed and synthesized seven cobalt porphyrin molecules with different substituent groups,and decorated them onto different carbon supports to study the ORR performance of composite catalysts in acidic and alkaline solutions,and analyzed the relevant catalytic interfacial charge transfer mechanisms.Three symmetric cobalt porphyrins with different electron-donating substituents benzyl,carbazole and triphenylamine（TPA）-carbazole at their meso-position,nominated as Bz-CoPor,Cb-CoPor and TPACb-CoPor,respectively,were selected to dope onto graphene oxide（GO）as the electrocatalysts to check the substituent effects on ORR.The spectroscopic characterization of the composites suggests the strong electronic coupling between porphyrin and GO.Through a series of electrochemical tests,the influence of substituent effect in acid electrolyte on ORR performance of porphyrins was investigated.TPACb-CoPor/GO exhibited the largest electron transfer number of 3.8 with the lowest H₂O₂ generation yield of 9%within the three porphyrin/GO models.This is mainly due to the fact that the three-dimensional structure of TPA groups exposes more molecular orbital,which increases the collision frequency of porphyrin molecules with protons in solution.The findings suggest that the substituent effects of porphyrins on ORR can still function at theπ-πinteraction interfaces,and these symmetric cobalt porphyrins can undergo nearly 4-electron transfer path upon using GO as the substrate.Asymmetric metalloporphyrins have rarely been studied in oxygen reduction reactions.However,the larger dipole moments of these molecules are conducive to charge separation and electron density regulation.Based on such facts,we designed and synthesized two asymmetric molecules A-Cb-CoPor and A-TPA-CoPor containing carbazole and triphenylamine derivatives,respectively,and doped them in carbon black as ORR electrocatalysts.Compared with a symmetric porphyrin-based catalyst CoPor1/C,the ORR performance of the two asymmetric catalysts were much improved due to the structural advantages of TPA,and A-TPA CoPor/C attained better ORR activity and selectivity than A-Cb-CoPor/C.However,A-TPA-CoPor/C and CoPor1/C achieved the same electron transfer number of 3.6,which is probably due to a trade-off of the structural merits and the unfavorable electron donating ability of TPA unit.The conclusions demonstrate the feasibility of asymmetric molecules as efficient ORR catalysts.A deep understanding of the interfacial charge transfer process is crucial for revealing the catalytic mechanism and developing high-performance electrocatalysts.In order to study the influence of support materials and electrolytes on metalloporphyrin-based ORR,and based on the previous findings on molecular structural effects,we designed and synthesized a symmetric cobalt porphyrin s-TPA-CoP,and selected carbon black and multi-walled carbon nanotubes（CNTs）as support materials to prepare composite catalysts for ORR applications in both acid and alkaline solutions respectively.The results indicate that s-TPA-CoP has stronger electron coupling on CNT than on carbon black,resulting in higher limit current densities and greater electron transfer numbers of s-TPA-CoP/CNT than s-TPA-CoP/C in both electrolytes.In addition,it is found that the two composite materials exhibited better ORR catalytic activity in alkali than in acid,while the former conditions showed poor 4-electron selectivity.Importantly,s-TPA-CoP/CNT exhibited an electron transfer number of3.8 in acidic electrolyte,demonstrating that the introduction of TPA groups and the optimization of porphyrin structure are viable to achieve 4-electron transfer in the electrocatalytic ORR with single metal cobalt porphyrins.