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.