Enhanced Oxygen Reduction Reaction Performance Using Intermolecular Forces Coupled with More Exposed Molecular Orbitals of Triphenylamine in Co-porphyrin Electrocatalysts.

Abstract

Triphenylamine (TPA) has often been used as a building block to construct functional organic materials yet is rarely employed in oxygen reduction reaction (ORR) due to its strong electron-donating ability. This versatile segment bears a three-dimensional spatial structure whose effect has not been fully explored in catalytic systems. To this end, five symmetric cobalt porphyrins with carbazole and TPA derivatives have been synthesized and their ORR performance has been evaluated in acid medium. It was found that all compounds produced mainly hydrogen peroxide in oxygen reduction, with CP1 attaching benzyl derivatives and XCP4 possessing TPA-carbazole substituents at the meso-position of porphyrin, showing similar but more positive ORR potential as compared to the other analogues. Importantly, XCP4 achieved the greatest response current and the largest electron transfer numbers and H2O2 yields among the investigated molecules. Detailed electrochemical measurements suggested that the dipole-induced partial charges on the porphyrin in tandem with the more exposed molecular orbitals on TPA contributed to this enhancement, with the former attracting more protons to the affinity of reactive sites and the latter increasing the collision frequency between the electrocatalyst and H+ in solution. This is the first attempt to integrate the intermolecular forces with more exposed molecular orbitals in altering the electrochemical process.