Study On Design,Synthesis,and Performance Of Iron Porphyrin MOFs-based Electrocatalysts For Oxygen Reduction Reaction

Porphyrin-based MOFs,a class of composite crystalline porous materials,may have attractive applications in the field of adsorption/separation/storage,photochemistry,drug delivery,and so on.In particular,atomically dispersed Fe-N-C electrocatalysts derived from pyrolytic porphyrinic MOFs turn out to be highly promising toward ORR in both acidic and alkaline media.Nevertheless,porphyrinic MOFs have been limited to skeleton-or the pore-modified ones mostly by Fe porphyrin（Fe P）,which can only provide insufficient ORR active sites.To the best of our knowledge,controlled modification to both the skeleton and the pore of MOFs with Fe P for the synthesis of Fe-N-C electrocatalysts has not been reported prior to this study,and the main conclusions as following:（1）Metal-organic frameworks with skeletal and pore-filling Fe P were synthesized by co-assembly of Hemin,meso-tetra（4-carboxyphenyl）porphine（TCPP）,Fe^IIITCPP,and Zr₆clusters.After pyrolysis and HF etching,Fe P-modified nanorod electrocatalyst was synthesized.The optimized nanorod electrocatalyst 20Hemin@10Fe-PCN-222-750-HF exhibits excellent ORR activity in both acidic and alkaline media with an E_1/2 of 0.791 V（vs.RHE）and 0.886 V（vs.RHE）,respectively.Moreover,the nanorod electrocatalyst shows efficient durability with only 43 and 20 m V decay of the E_1/2after accelerated durability test in both acidic and alkaline media,respectively(the negative shift of the E_1/2for commercial 20 wt%Pt/C is 34 and 106 m V in acidic and alkaline media,respectively).Based on the analysis results of X-ray absorption fine structure spectra（XAFS）,aberration-corrected high-angle annular dark-field scanning transmission electron microscope（HAADF-STEM）,and X-ray photoelectron spectroscopy（XPS）,the nature of ORR active sites for the nanorod electrocatalyst may be atomically dispersed（O/N）₂-Fe^III-N₄ structure.（2）Ce-Zr bimetallic nanocylinders PCN-222 MOFs were prepared by co-self-assembly of Ce₆ and Zr₆ metal clusters with TCPP and Hemin.After pyrolysis,the nanocylinders of PCN-222 MOFs all regularly convert into six angular nanoprisms.The Ce-Zr bimetallic nanoprisms electrocatalysts exhibit better ORR activity with an E_1/2 of 0.866 V（vs.RHE）in alkaline media and an E_1/2 of 0.700 V（vs.RHE）in acidic media,superior than that of single Zr or Ce metal clusters MOFs-based electrocatalysts.Moreover,the nanoprisms electrocatalysts show excellent durability with only 15 m V and 32 m V degradation of the E_1/2 was observed after accelerated durability test in alkaline and acidic media,respectively.The increase of oxygen vacancy sites,defect sites,and the electronic regulation interaction between Ce and Fe in the nanoprisms electrocatalyst should be beneficial for the ORR process.Moreover,the acid corrosion resistance and anti-potential pulse effect of inert Zr-related species could improve the durability of the bimetallic nanoprisms electrocatalysts.