Meso-substituted Metalloporphyrin-based Composites for Electrocatalytic Oxygen Reduction Reactions

Metalloporphyrins have been regarded as one of the most promising electrocatalysts for oxygen reduction reactions (ORRs) due to their ease of structure modification and the robust coordinated M−N4 environment. However, the electrocatalytic activity, selectivity and stability of the metalloporphyrin-based composite catalysts are often reported to be much poorer than those of the Pt-based materials, arousing researchers to devote to exploring a thorough understanding of the relationship between the catalyst structures and ORR performance/mechanisms. Here we will review the design of meso-positioned porphyrin structures from the aspects of selection of central metal ion species in both monometallic and bimetallic molecules, and modulation of peripheral functional substituents to introduce beneficial effects, including electron affinity, steric effects, interfacial charge states and proton management. Influences from different carbon materials as the support for composite catalysts as well as the electrolytes on oxygen reduction properties will be briefly illustrated before presenting the perspectives and insights for future works in conclusion remarks. We hope that this Review will serve as a roadmap for advancing the insights of the molecular structural and substrate morphological factors impacting ORR properties with the ultimate goal of developing and improving novel metalloporphyrins as efficient and durable electrocatalysts to champion the precious Pt/C.