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

Abstract

Owing to rich-redox chemistry, metal-N4-molecular catalysts have been found as promising electrocatalysts for oxygen reduction reaction (ORR), however, de-metalation of the metal atom from N4-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 π∗ orbital of an O2 molecule, resulting in optimal coupling between HOMO and π∗ 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 π∗ orbital of O2, weakening the O–O bond during ORR. In addition to developing low-cost ORR electrocatalysts, this work will contribute to defining their activity descriptors.