Asymmetric cobalt porphyrins for pH-universal oxygen reduction reactions: Benzoic acid advances phenyl as substituents

Abstract

Asymmetric metalloporphyrins are potential electrocatalysts to construct composites for efficient oxygen reduction reactions (ORRs) due to their inherent large dipole moment, which facilitates the charge separation. To find more effective molecular structures towards the ORRs, in particular their performance and electrolyte relationship, we rationally designed two A3B-type asymmetric cobalt porphyrin aPh-TCoP and aBA-TCoP with phenyl and benzoic acid as substituents, respectively. The ORR performance of the porphyrin-decorated carbon black composites was examined in five electrolytes at different pH values spanning from 0.7 to 13.7. It was found that aBA-TCoP/C exhibited better ORR activity and selectivity than aPh-TCoP/C in all solutions. Interestingly, both composites demonstrated their highest ORR activity in the pH13.7 electrolyte and exhibited 4-electron selectivity in the pH3.7 solution. This conclusion is drawn from a comprehensive analysis of various electrochemical properties, encompassing current density, reduction potentials, electron transfer numbers, and intermediate yields. The results illustrate the regulation of ORR properties by altering the electrolyte pH and a notable improvement in the ORR performance through the modification of porphyrin substituents. These findings offer a foundational understanding for the design of efficient metalloporphyrins and the attainment of controlled oxygen reduction in electrochemical devices.