Effects of electrolyte pH on oxygen reduction properties using a cobalt porphyrin-coated carbon composite.

• Tunable oxygen reduction reaction properties is achieved by varying the electrolyte pH. • An A3B-type cobalt porphyrin-coated carbon black is designed as the catalyst. • Five electrolytes with pH values spanning from 0.7 to 13.7 are studied. • The composite exhibits the most positive potentials and the lowest Tafel slope at pH13.7. • Poor activity but high selectivity for 4-electron ORR is obtained at pH3.7. Understanding the pH-dependent mechanism of metalloporphyrin-based composite electrocatalystic oxygen reduction reactions (ORRs) can provide the basis for rational porphyrin molecular design. Tunable ORR properties of a cobalt porphyrin-based composite catalyst aFP-TCoP/C is accomplished by adjusting the electrolyte pH values spanning from 0.7 to 13.7. The composite performs the best ORR reactivity in the pH13.7 electrolyte, exhibiting the most positive characterized potentials (the ORR reduction potential E ORR of 0.76 V and the half-wave potential E 1/2 of 0.80 V) and the smallest Tafel slope of 43.1 mV dec−1. Whilst the sample with the pH3.7 electrolyte displays quite negative reduction potentials and a large Tafel slope, it exhibits an electron transfer number of 3.84, which indicates a nearly 4-electron transfer selectivity. The findings provide a practical protocol for regulating the catalytic reactivity and selectivity of ORRs by adjusting the components of electrolytes. [Display omitted] [ABSTRACT FROM AUTHOR]