Controlled synthesis of metal-organic frameworks with skeletal and pore-filling iron(III) porphyrins for electrochemical oxygen reduction

Abstract

Electrocatalysts derived from porphyrinic metal-organic frameworks (MOFs) have exhibited very promising electrochemical performances toward oxygen reduction reaction (ORR). Nevertheless, porphyrinic MOFs have been limited to skeleton- or the pore-modified ones mostly by Fe porphyrin (FeP), which only provide insufficient ORR active sites. Herein, we report controlled synthesis of PCN-222 decorated by both pore-filling Hemin and skeletal iron(III) meso-tetra(4-carboxyphenyl) porphyrin (Fe[Formula: see text]TCPP) that partially substitutes original backbone TCPP. Subsequent pyrolysis of the composite PCN-222 led to the synthesis of nanorod electrocatalysts with atomically dispersed Fe-N-C sites, which exhibit efficient activity and durability toward ORR in both alkaline and acidic media. Moreover, it appears that the atomically dispersed Fe-N-C sites might possess a distorted octahedral configuration of (O/N)2-Fe[Formula: see text]-N4 as evidenced by extended X-ray absorption fine structure spectra (EXAFS), aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). To the best of our knowledge, controlled modification to both the skeleton and the pore of MOFs with FeP for the synthesis of Fe-N-C electrocatalysts has not been reported prior to this study. This study offers a new avenue to manipulate the density of Fe-N-C sites of electrocatalysts, which may be applied to other composite MOFs with various functionalities.