Oxygen Reduction Reactions of Fe-N-C Catalysts: Current Status and the Way Forward

Currently, Fe-N-C materials are considered to be among the most important oxygen reduction reaction (ORR) catalysts, because they are potential substitutes for Pt-based catalysts and are therefore promising in the development of non-noble metal-based catalysts. However, challenges such as electron transfer kinetics still exist and need to be improved upon. From a chemical stand point, improvements can be made through the better understanding of mechanisms in Fe-N-C-based ORR catalysis along with a deeper understanding of the chemical origin of active sites on Fe-N-C catalyst surfaces. Based on these, this comprehensive review will focus on the energy conversion, transformation kinetics and electron transfer of the ORR process as catalyzed by Fe-N-C catalysts. And by taking these and other relevant analytical results for Fe-N-C materials into consideration, primary strategies in the improvement in Fe-N-C catalyst activity will be presented. As the promising Pt substrate for oxygen reduction catalysis, the Fe-N-C materials are active toward the four-electron reduction of O2 to H2O. This review focuses on the profound understanding of heterogeneous oxygen reduction reaction on Fe-N-C materials from the following aspects: (1) thermodynamics of energy conversion in ORR processes, (2) kinetics of ORR processes based on Fe-N-C catalysts, (3) the textural features of Fe-N-C and analytic results known as far, (4) fundamental principle for Fe-N-C materials synthesis and (5) practical application for fuel cell and metal–air batteries.