Fe–N bonding in a carbon nanotube–graphene complex for oxygen reduction: an XAS study

Abstract

The electronic structure study of carbon nanotube–graphene complexes has been performed using comprehensive X-ray absorption spectroscopy (XAS) at Fe L- and K-edges, along with C, N and O K-edges. The results obtained from the study of an iron-containing carbon nanotube–graphene complex (NT–G) have been compared in great detail with those of an iron-free carbon nanotube–graphene complex (pNT–G) and iron phthalocyanine (FePc). It has been confirmed that complex-like Fe³⁺ in a high spin state is the major iron component in NT–G. The C and N K-edge XANES further confirmed that Fe is very likely to be bonded to N in NT–G. This Fe–N species should be the active site for enhanced oxygen reduction reaction (ORR) activity in NT–G. A unique O K-edge X-ray absorption spectroscopic feature has been observed in NT–G, which might be caused by chemisorbed O₂ on the Fe–N site. Such knowledge is important for the understanding of this specific complex, and the knowledge should benefit the rational design of other carbon/metal/nitrogen-containing ORR catalysts with further improved performance.