Noncovalent composites of meso-substituted A2B2-porphyrins and carbon nanotubes for enhanced electrocatalytic oxygen reduction

Abstract

Abstract Exploring highly active oxygen reduction electrocatalysts with low precious metals content is imperative but remains a considerable challenge. Herein, a series of heterobimetallic multi-walled carbon nanotubes (MWCNTs) electrocatalysts based on metal complexes are presented. These electrocatalysts feature diverse transition metals (M=Mn, Fe, Co, Ni) 5,15-bromophenyl-10, 20-methoxyphenyl porphyrin (MBMP) and tetrakis(triphenylphosphine)palladium (0) (Pd[P(Ph3)4]) anchored non-covalently on its surface. The resulting NiBMP-based MWCNTs with Pd[P(Ph3)4] (PdNiN4/MWCNTs) display outstanding electrocatalytic oxygen reduction activity (onset potential, 0.941 V; half wave potential, 0.830 V) and robust long-term durability in alkaline electrolyte. While in neutral condition, the MnBMP-based MWCNTs with Pd[P(Ph3)4] (PdMnN4/MWCNTs) are the most active heterobimetallic ORR catalyst and produce ultra-low concentration hydrogen peroxide (H2O2 yield, 1.2%–1.3%). Synergistically tuning the ORR electrocatalytic activity and electron transfer pathway is achieved by the formation of NiBMP/MnBMP-Pd[P(Ph3)4] active sites. This work indicates such metalloporphyrin-Pd[P(Ph3)4] active sites on MWCNTs have significantly positive influence on electrocatalytic ORR systems and provides facile and mild strategy for designing highly efficient ORR electrocatalysts with ultra-low loading precious metal.