Interfacial charge transfer mechanism of oxygen reduction reaction in alkali media: Effects of molecular charge states and triphenylamine substituent on cobalt porphyrin electrocatalysts

Abstract

Substituents can affect the partial charge distribution on porphyrin, and in tandem with the ion states in electrolyte, the charge transfer mechanism can be altered at catalytic interface. Three cobalt porphyrins with methyl phenyl, 2,1,3-benzothiadiazole (BTD) and BTD-triphenylamine (TPA) substituents at the 5,15-positions, nominated as CoPor, BTD-CoPor and TPA-BTD-CoPor, respectively, have been examined in oxygen reduction reactions (ORR) under alkaline condition. Reduced reduction potentials and enhanced response current densities were observed on the two substituent porphyrin-coated catalysts as compared to CoPor/C. Importantly, TPA-BTD-CoPor/C obtained slightly larger ORR current with less hydroperoxide anion generation yields than BTD-CoPor/C. We proposed that the electronic structure along with the induced partial charge effect caused the difference in ORR potential, while the electron-withdrawing property of BTD contributed to the increased response current. Though TPA unit still benefits the ORR performance, the more exposed cation orbitals on TPA that merit the acidic ORR activity was suggested to be ineffective in alkali condition. These findings demonstrate the importance of the induced intramolecular partial charges in affecting the catalytic process in alkali medium and point out the limitation in utilizing the structural merit of TPA in accelerating the interfacial charge transfer.