A review of Fe–N/C and Co–N/C catalysts for the oxygen reduction reaction

This paper reviews over 100 articles related to heat-treated Fe– and Co–N/C catalysts for the oxygen reduction reaction. The literature shows that through several decades’ effort in the development of non-noble catalysts such as heat-treated Fe– and Co–N/C catalysts, tremendous progress has been made in catalyst synthesis methodologies and the understanding of the mechanism. A heat-treatment step has been identified as necessary for catalyst activity and stability improvement. The enhanced performance of the catalysts is strongly dependent on the carbon support, the source of metal and nitrogen, and the thermal treatment conditions. The metal content in these catalysts also plays an important role in their activity and stability. A saturated metal content has been identified as a major limiting factor for further improvement of catalyst activity. The nitrogen content and the presence of a disordered or heterogeneous phase on the carbon-support surface seem to be the main requirements for an effective catalyst. The mechanisms by which activity and stability are enhanced after the heat treatment of these Fe– and Co–N/C catalysts are not fully understood yet. It is necessary to answer the question of whether or not the metal is part of the active catalytic site, as well as to identify the nature of the catalytic site. A more fundamental understanding will be of great help in designing alternative and innovative routes for catalyst synthesis. In general, the catalytic activity and stability of Fe– and Co–N/C catalysts are still below those of a Pt-based catalyst. However, under the strong driving force of fuel cell commercialization, Pt-free cathode catalysts with methanol tolerance, such as Fe– and Co–N/C, are attractive candidates for solving the problem of the cost of fuel cell catalysts.