Asymmetric Cobalt Porphyrins for Oxygen Reduction Reactions: Boosted Limiting Current Via Facilitating Interfacial Charge Transfer

Abstract

Asymmetric metalloporphyrins inherently possess the feature of facilitated intramolecular charge separation and ease of controlling electron density near the metal core as compared to their symmetric analogs, while such molecules are seldom investigated in oxygen reduction reaction (ORR). Two asymmetric porphyrins A-Cb-CoPor and A-TPA-CoPor are designed by substituting one of the meso -positions with carbazole and triphenylamine derivatives, respectively. A symmetric tetraphenyl porphyrin, CoPor1 , is also synthesized and compared as the reference. The electronic properties of the molecules are calculated using density functional theory, and the composite catalysts are characterized by a series of electrochemical measurements. A-TPA-CoPor/C attains an onset potential of 0.56 V vs. RHE, an electron transfer number of 3.6 and hydrogen peroxide yield of 22%, whose ORR activity is better than A-Cb-CoPor/C but similar to CoPor1/C . Impressively, both asymmetric porphyrin-based composites exhibit greater limiting current densities than CoPor1/C , which may originate from the reduced charge transfer resistance. Our work demonstrates the promise of the asymmetric molecular approach as a general design strategy for efficient ORR catalysts.

Keywords: Cobalt porphyrin, oxygen reduction reaction, Asymmetric porphyrin, Substituent effect, electrocatalysts