Effect of β-fluorinated porphyrin in changing selectivity for electrochemical O2 reduction

Abstract

The development of catalytic systems that selectively convert O2 to water is required to progress fuel cell technology. As an alternative to platinum catalysts, derivatives of iron and cobalt porphyrin molecular catalysts provide one benchmark for catalyst design. However, the inclusion of these catalysts into homogeneous platforms remains a difficulty. Co-porphyrins have been studied as heterogeneous O2 reduction catalysts; however, they have not been explored much in homogeneous systems. Moreover, they suffer from poor selectivity for the desired four-electron reduction of O2 to H2O. Herein, we present two cobalt-based β-fluorinated porphyrin complexes (CoTPF8(OH)2 and CoTPF8(OH)4) and demonstrate applicability as effective catalysts for the oxygen reduction reaction. Using rotating ring-disk electrochemistry, the catalysts, CoTPF8(OH)2 and CoTPF8(OH)4, showed maximum Faradaic efficiency for H2O of 92 % and 97 %, respectively. DFT calculations suggest that the formation of a phlorin intermediate could occur before O2 reduction and that a stronger H2O2 binding in the cobalt-based β-fluorinated porphyrin species compared to the unsubstituted parent compound, CoTP(OH)2, was responsible for the observed experimental selectivity for H2O. These results reveal that the β-fluorinated porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions.

Keywords

• O2 reduction reaction (ORR)
• Homogeneous electrocatalysis
• Cobalt
• Fluorination
• Porphyrins
• Density functional theory