Rational design and construction of nanoporous iron- and nitrogen-doped carbon electrocatalysts for oxygen reduction reaction

Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most sustainable energy conversion systems because of their high energy conversion efficiency and low/zero emissions. Unfortunately, the utilization of highly active but costly platinum (Pt)-based electrocatalysts is necessary to accelerate the sluggish kinetics of cathodic oxygen reduction in PEMFCs for practical applications. Under such circumstance, enormous efforts have been devoted to the exploration of inexpensive and earth-abundant non-noble metal-based electrocatalysts to replace or reduce the usage of Pt-based electrocatalysts in the past decades. Heteroatom-doped carbon materials are among some of the most promising non-noble metal-based electrocatalysts, especially transition metal- and nitrogen-doped carbon materials. According to previous findings, iron- and nitrogen-doped carbon (Fe-N/C) materials derived using various methodologies showed outstanding electrocatalytic activity and impressive durability. Therefore, tremendous progress has been achieved in the synthesis of Fe-N/C and the identification of active sites for oxygen reduction reaction (ORR). Creating ORR active sites, such as Fe-N-x, N/C, and Fe3C@C moieties, increasing the density of active sites and improving the utilization efficiency of ORR active sites are considered as the most effective steps for enhancing the ORR performance of Fe-N/C electrocatalysts. The creation of nanoporous structure of Fe-N/C electrocatalysts plays critical roles in increasing the number of ORR active sites and exposing abundant accessible ORR active sites to electrolytes. In addition, the interconnected nanopores facilitate the mass transfer of reactants and products inside the carbon matrix during the ORR reactions. Therefore, this review pays specific attention to the design and synthetic strategies of Fe-N/C materials with porous structures and their merits toward ORR. Finally, based on the construction of nanoporous structures, the challenges and perspectives with respect to future development of highly active nanoporous Fe-N/C electrocatalysts are discussed.