The effect of tuning the coordination sphere of iron complexes for the oxygen reduction reaction in acidic media

Abstract

As a type of important non-precious catalyst for the oxygen reduction reaction (ORR), the regulating role of a metal centre in metal-macrocycles and other complexes for activity has been extensively studied. However, a common guideline to explain the effect of peripheral coordinated-ligands has not been reached. Herein, a series of organic iron complexes (denoted as FeL, L = TAA, Pc, TPP, Corrole, Tim and Salen) were synthesized as ORR catalysts and an explicit relationship of structure-activity was constructed. The kinetic current density for these compounds was identified to follow the order of FeTAA > FePc > FeTPP > FeCorrole > FeTim > FeSalen. An electron-transfer number close to 4 was derived for all these complexes except for FeTim and FeSalen, implying a near complete reduction of oxygen to water. X-ray absorption near edge structure spectroscopy (XANES) and Mössbauer spectroscopy were used to probe the nature of the distinct activities by investigating the iron-centre electron structures. Density function theory (DFT) calculations were carried out to study the charge redistribution across the iron complexes. Novel activity descriptors including the charge and spin densities on the Fe site were proposed and validated by available experimental data, presenting a strategy to design highly-active nonprecious metal complex catalysts with specific supporting ligands.