Axial Ligand Coordination Tuning of Electrocatalytic Activity of Iron Porphyrin Electrografted on Carbon Nanotubes for Oxygen Reduction Reaction.

Oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. Inspired by enzymatic oxygen activation/reduction processes, it is desirable to design transition metal molecular catalysts as an alternative to noble-metal-Pt-based ORR electrocatalysts in view point of fuel cell commercialization. Herein, we fabricate bio-inspired molecular catalysts electrografted on multiwalled carbon nanotubes (MWCNTs), where iron porphyrin FeF 20 TPP  is coordinated with covalently electrografted axial ligand on the MWCNTs' surface varying from thiophene to imidazole. With the change of axial coordination environment, the catalysts' electrocatalytic activity varied accordingly, and the imidazole coordinated catalyst MWCNTs-Im-FeF 20 TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNTs-Im-FeF 20 TPP was loaded on the cathode of a zinc-air battery assembly, open-cell voltage (OCV) of 1.35 V and the maximum power density ( P max ) of 110 mW cm -2 were achieved, which was higher than those of MWCNTs-Thi-FeF 20 TPP (OCV = 1.30 V, P max = 100 mW cm -2 ) and MWCNTs-Ox-FeF 20 TPP (OCV = 1.28 V, P max = 86 mW cm -2 ) and comparable with that loaded with a Pt/C catalyst (OCV = 1.45 V, P max = 120 mW cm -2 ) under similar experimental conditions. This study provides a time-saving method to prepare covalently immobilized molecular electrocatalysts on carbon-based materials with structure-performance correlation, which is also applicable to the design of other electrografted catalysts for energy conversion.