Restructured Fe-Mn Alloys Encapsulated by N-doped Carbon Nanotube Catalysts Derived from Bimetallic MOF for Enhanced Oxygen Reduction Reaction

A set of novel catalysts FeMn/N-CNTs that partly maintain the core-shell structure have been prepared successfully by calcination of analogous MOF-74, which has bimetallic species (Fe and Mn) and a cheap organic ligand (2, 5-dihydroxylbenzoic acid, DHBA) with melamine as additional nitrogen source. These catalysts exhibit a distinctive microstructure of Fe-Mn alloys surrounded by N-doped carbon nanotubes (CNTs). Electrochemical methods have been employed to investigate their activity in oxygen reduction reaction (ORR) in alkaline solution. The highest ORR performance of Fe3Mn1/N-CNTs-100 shows that the half wave potential is at 0.865V and the kinetic current density (at 0.9V) is 1.447 mA cm(-2), which are higher than those of commercial Pt/C (0.855V, 0.946 mA cm(-2)). In addition, Fe3Mn1/N-CNTs-100 is much more durable than commercial Pt/C under the conditions tested. The highly efficient ORR performance may be attributed to the unique microstructure and large surface area with appropriate pore size, as well as to the synergistic effects between the pyridinic N species and the Fe-N-x species that play important roles in ORR in alkaline solution. However, in acid medium, only Fe-N-x species catalyze ORR and pyridinic N species are limited to work as the active sites. This study may prompt others to explore the development of heteroatom-doped CNTs surrounding particles as efficient catalyst for ORR and fuel cell applications.