Molecular MnN4-Complex immobilized on carbon black as efficient electrocatalyst for oxygen reduction reaction

Owing to rich-redox chemistry, metal-N-4-molecular catalysts have been found as promising electrocatalysts for oxygen reduction reaction (ORR), however, de-metalation of the metal atom from N-4-cavity in the electrolyte limits their activity and endurance. Herein, a MnN4-macrocyclic complex was prepared and characterized via various techniques. The results of analytical and theoretical studies suggest that the MnN4-macrocyclic complex has saddle-shaped geometry. Further, this complex was incorporated with black carbon (MnN4@CB) and studied for electrocatalytic ORR in alkaline media. The MnN4@CB composite displayed comparable ORR formal potential (-0.14 V vs Ag/AgCl) to the traditional 20% Pt/C catalyst (-0.13 V vs Ag/AgCl) and remarkable methanol tolerance. Theoretical studies revealed that the highest occupied molecular orbital (HOMO) of MnN4 has almost the same energy as the pi* orbital of an O-2 molecule, resulting in optimal coupling between HOMO and pi* orbital, leading to efficient ORR. The charge distribution and 2D Fourier optimization of the MnN4-complex suggested that the N-donor atoms increased the electron density on Mn-atom, and transferred into the pi* orbital of O-2, weakening the O-O bond during ORR. In addition to developing low-cost ORR electrocatalysts, this work will contribute to defining their activity descriptors. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

A MnN4-macrocyclic complex combined with black carbon was studied as an electrocatalyst for ORR in alkaline media. The composite exhibited comparable catalytic performance to traditional Pt/C catalyst and remarkable methanol tolerance. Theoretical studies explained the efficient catalytic mechanism based on charge distribution and molecular orbital structure.