Surface bonding of MN4 macrocyclic metal complexes with pyridine-functionalized multi-walled carbon nanotubes for non-aqueous Li-O2 batteries

Abstract

It is essential to develop bifunctional catalysts with high activity and stability for reversible oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) in lithium-oxygen (Li-O2) batteries. In this work, pyridine (Py) functionalized multi-walled carbon nanotubes (MWCNTs) were prepared to immobilize various solid MN4 macrocyclic metal complexes (MN4-MC) as cathode electrocatalysts for Li-O2 batteries. Three types of MN4-MC molecules, including iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and iron protoporphyrin IX (Heme) were examined to evaluate the influence of central metal atoms and ligand substituents found in MN4-MC molecules on the electrocatalytic performance of the study samples. The order of the ORR/OER catalytic activity of the bifunctional catalysts is FePc > Heme > CoPc. The central metal atom in FePc molecule has the highest occupied molecular orbital (HOMO) energy than the corresponding metal atoms in CoPc and Heme molecules. This made the molecule to have better dioxygen-binding ability and higher catalytic activity in the ORR process; it also made it to easily lose electrons that were oxidized in the OER process. This study proposed a simplified scheme of the electrode surface route to assist in understanding the diverse ORR/OER performances of MN4-MC. It is discovered that the positive core of the MN5 coordination sphere in MN4-MC/Py/MWCNTs composite is the primary active site that can influence the formation of MN5···O2* and MN5-LOOLi cluster in the ORR process. The interfacial electron could be easily delivered between MWCNTs and MN5 active site through the Py bridge. This facilitated the formation and decomposition of MN5-LOOLi species during the ORRs/OERs, leading to the enhancement of its catalytic performance. This work provides a new insight into the effects of the molecular structure and organization of MN4-MC on the catalytic activity of O2 electrodes in Li-O2 batteries.