Enhancing Four-Electron Selective Oxygen Reduction via Fluorine-Doped Multiwalled Carbon Nanotubes: Synergistic Interaction with Nickel Tetraphenylporphyrin

Tuning the catalytic efficiency and selectivity of metal macrocycles via functionalization and/or π-extension is a recent strategy in electrocatalysis. However, these approaches often require complex syntheses and specialized expertise. To address this, we investigated the use of heteroatom incorporation in carbon supports to modulate the electronic nature and oxygen reduction reaction (ORR) performance of a metal macrocyclic catalyst. Nickel tetraphenylporphyrin (5,10,15,20-tetraphenyl-porphine nickel(II), NiTPP) has been integrated with both pristine multiwalled carbon nanotubes (MWCNTs) and fluorinated MWCNTs (F-MWCNTs) to form NiTPP@MWCNT and NiTPP@F-MWCNT composites, respectively. These materials were characterized by using a range of physicochemical and structural techniques to assess their morphology, crystallinity, and surface chemistry. In alkaline media, NiTPP@F-MWCNT exhibits an enhanced onset potential of 0.86 V vs RHE relative to 0.72 V vs RHE for NiTPP@MWCNTs. Rotating ring-disk electrode analyses show that NiTPP@F-MWCNT favors a four-electron pathway, as evidenced by its higher electron transfer number and reduced H₂O₂ yield. Stability tests confirmed high durability, showing minimal alterations in current and ORR onset potential after 10,800 s and 3000 cyclic voltammetry cycles. These findings suggest a strong synergistic interaction between NiTPP and F-MWCNT in the NiTPP@F-MWCNT composite. Fluorine doping effectively modulates the electronic environment of NiTPP, enhancing both the efficiency and selectivity for the ORR. This study demonstrates the potential of integrating macrocyclic catalysts with heteroatom-doped carbon supports to advance energy conversion technologies.