Controlling the Oxygen Reduction Selectivity of Asymmetric Cobalt Porphyrins by Using Local Electrostatic Interactions

Abstract

The development and improvement of electrocatalysts for the 4H + /4e – reduction of O 2 to H 2 O is an ongoing challenge. The addition of ancillary groups (e.g., hydrogen bonding, Brønsted acid/base) near the active site of metal-containing catalysts is an effective way to improve selectivity and kinetics of the oxygen reduction reaction (ORR). In this regard, iron porphyrins are among the most researched ORR catalysts. Closely related cobalt porphyrin ORR catalysts can function closer to the O 2 /H 2 O thermodynamic potential, but they tend to be less selective and follow a different mechanism than for the iron porphyrins. Herein, we explore strategies to extend the ideas about ancillary groups that have been developed for iron porphyrin ORR electrocatalysts to improve the performance of the corresponding cobalt complexes. We describe a series of porphyrin electrocatalysts that are modified versions of Co(5,10,15,20-tetraphenylporphyrin), where the 2-position of one of the phenyl groups contains -NH 2, -N(CH 3 ) 2, and -N(CH 3 ) 3 + . Investigations using cyclic voltammetry and hydrodynamic electrochemistry show that the presence of a cationic ancillary group gives rise to a catalyst that is selective for the conversion of O 2 to H 2 O across a wide pH range. In contrast, the other catalysts are selective for reduction of O 2 to H 2 O at pH 0, but produce H 2 O 2 at higher pH. The ORR rate (∼10 6 M –1 s –1 ) and selectivity of the -N(CH 3 ) 3 + -modified catalyst are invariant between pH 0 and 7. Quantum chemical calculations support the hypothesis that the enhancement of selectivity can be attributed to the distinct mechanism of O 2 reduction by Co-porphyrins. Specifically, the mechanism relies on anionic, peroxide-bound intermediates. While protic ancillary groups are important in the performance of iron porphyrin ORR catalysts, we suggest that electrostatic stabilizers of O 2 -bound intermediates are more crucial for cobalt porphyrin ORR catalysts.