Triphenylamine Effects On Interfacial Charge Transfer Mechanism In Cobalt Porphyrin Electrocatalytic Oxygen Reduction Reaction

Triphenylamine（TPA）is often used to construct functional organic materials.Its propeller-like three-dimensional spatial structure endows it with excellent charge transfer dynamics in photoelectric chemistry.However,the structural effect of TPA in electrocatalytic system has not been investigated yet.To this end,seven symmetric cobalt porphyrin derivatives containing different substitutuents were synthesized to study the effect of TPA on the catalytic performance of Oxygen Reduction Reaction（ORR）,particularly in three conditions,i.e.,electron-donating group-substituted cobalt porphyrins,electron-withdrawing group-substituted cobalt porphyrins and different electrolytes.Meanwhile,the effect of the charge states at the catalytic interface on the charge transfer mechanism and catalytic performance of ORR was further analyzed,and the feasibility of triphenylamine and intermolecular forces in synergistic catalysis were proposed.Seven cobalt porphyrins were synthesized and their photochemical properties were tested.The intermediates were synthesized following the protocols of Vilsmeier reaction,Suzuki reaction,etc,and the target molecules were produced using the Macdonald module method.NMR spectroscopy and mass spectrometry were recorded to characterize the structures of the synthesized materials,suggesting the successful synthesis of 7 metalloporphyrins.In addition,the yields of 7 free porphyrins were generally found to be between 8-15%,where that of the porphyrin3e with largest steric hindrance experienced the lowest value at about 5%.The yields of target metalloporphyrins were more than 90%.UV-Vis spectra and differential pulse voltammetry（DPV）were used to study the photochemical properties of cobalt porphyrins,and the frontier molecular orbital energy levels of metalloporphyrins were calculated from the electrochemical results.The energy band gaps of the investigated porphyrins spanned from 1.7 e V to nearly 2.0 e V,with CP1possessing the largest（1.96 e V）while XCP4 showing the smallest value（1.7 e V）.Five cobalt porphyrins CP1&XCP1-4 with different electron-donating substituents at meso positions were selected to investigate the effect of triphenylamine on ORR performance.Among them,the meso-5,15 substituents of CP1&XCP1-4 were methylbenzene,methylbenzene-carbazole,carbazole,triphenylamine,and carbazole-triphenylamine,respectively.Cyclic voltammetry（CV）,linear sweep voltammetry（LSV）,rotating disk electrode（RDE）and rotating ring-disk electrode（RRDE）methods were employed to measure the ORR performance of five cobalt porphyrins in acidic electrolyte.The results showed that XCP4 containing carbazole-triphenylamine derivatives exhibited the highest reduction current density and the largest electron transfer number.The related mechanism was discussed by carrying out LSV experiments in varied proton concentrations in electrolyte.Two main findings were proposed:1)the dipole in XCP4induces partial negative charges on the triphenylamine group,which can attract more protons from solution;2)the more exposed molecular orbitals of triphenylamine can promote the collision frequency between the porphyrin and protons,thus accelerating the interfacial charge exchange and improving the ORR performance.This was the initial work to explore the utility of intermolecular forces and more exposed molecular orbitals in electrocatalysis.The ORR performance of the electron-donating-substituted porphyrins was found to be quite moderest,encouraging us to design two electron-withdrawing substituted cobalt porphyrins to check the TPA effect on ORR performance in acidic solution.The meso-5,15 substituents of BCP1and BCP2 were 2,1,3-benzothiadiazole（BTD）and BTD-triphenylamine,respectively.Compared with BCP2/C,BCP1/C showed a more positive reduction potential and a smaller limiting current density.Detailed electrochemical tests and analysis suggested that the more negative potential of BCP2/C towards ORR might originate from the reduced adsorption affinity of cobalt center to oxygen;the adsorption affinity has a strong relationship with the substituents,and the linkage of TPA with strong electron-donating and BTD with intense electron-deficient ability may result in a significance change of the force.On the other hand,positive partial charges should be localized on TPA group in BCP2,which is different from the partial charge states in XCP series with electron-donating substituents,however,the TPA effect still plays a role in accelerating the reaction since a greater current density were found by BCP2/C than BCP1/C.In addition,the H₂O₂ generation rate of BCP2/C in acidic solution was more than 80%,which was much higher than that of BCP1/C（49%）.The results implied that the substituents can change the intramolecular induced charge state and thus affecting the ORR performance.This work provides insights in the structure design of organic electrocatalysts and the interfacial charge transfer mechanism in ORR.Different electrolytes could change the state of interfacial charges and thus alter the triphenylamine effect on ORR.KOH electrolyte and three cobalt porphyrins CP1,BCP1 and BCP2 were therefore selected to test the TPA effect on ORR in alkali condition.The charge transfer mechanism in alkaline solution is different from that in acid solution,so that the catalytic performance of carbon carrier was also studied and analyzed in the experiment.Compared with that of CP1/C,the ORR potentials of BCP1-2/C underwent a negative shift,but the limited current densities were increased greatly,which was even close to that of Pt/C.The results of the RRDE test showed that the electron transfer numbers of the three porphyrin catalysts in alkaline solution ranged from 2.8 to 3.0,and the HO₂^-yields were 48-60%.Based on the effect of molecular structure and the partial charge state induced by intramolecular dipole,the Outer-Sphere Electron Transfer（OSET）mechanism and Inner-Sphere Electron Transfer（ISET）of different porphyrin catalysts were also discussed.