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

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 .