Identification and Understanding of Active Sites of Non-Noble Iron-Nitrogen-Carbon Catalysts for Oxygen Reduction Electrocatalysis

Non-noble iron-nitrogen-carbon (Fe-N-C) catalysts have been explored as one type of the most promising alternatives of precious platinum (Pt) in catalyzing the oxygen reduction reaction (ORR). However, their catalytic ORR activity and stability still cannot meet the requirement of practical applications. Active sites in such catalysts are the key factors determining the catalytic performance. This review gives a critical overview on identification and understanding of active sties of non-pyrolytic and pyrolytic Fe-N-C catalysts in terms of design strategies, synthesis, characterization, functional mechanisms and performance validation. The diversity and complexity of active sites that greatly dominate the progress of Fe-N-C catalysts include Fe-containing sites (Fe-based nanoparticles and single-atom Fe-species) and metal-free sites (heteroatoms doping and defects). Meanwhile, synergistic effects are also discussed in this review with emphasis on the interaction among multiple active sites. Although substantial endeavors have been devoted to develop the efficient Fe-N-C catalysts, some challenges still remain. To facilitate further research on Fe-N-C catalysts toward practical applications, some research perspectives are prospected in the aspects of innovative synthesis methods, active-sites modulation strategies, high-resolution ex situ/in situ/operando characterization techniques, theoretical calculations, and so on. This review may provide a guideline for identifying and understanding active-sites for developing high-performance Fe-N-C catalysts.

Automated summary

Non-noble iron-nitrogen-carbon (Fe-N-C) catalysts have been explored as promising alternatives to precious platinum (Pt) in the oxygen reduction reaction (ORR), but their catalytic activity and stability still fall short of practical applications. This review critically examines the identification and understanding of active sites in non-pyrolytic and pyrolytic Fe-N-C catalysts in terms of design strategies, synthesis, characterization, functional mechanisms, and performance validation. The complexity of Fe-N-C catalysts is attributed to diverse active sites, including Fe-containing sites and metal-free sites, and the review also discusses synergistic effects among multiple active sites. Despite significant efforts, challenges still exist, and the review suggests research perspectives in innovative synthesis methods, active-site modulation strategies, advanced characterization techniques, theoretical calculations, etc., to further facilitate the development of efficient Fe-N-C catalysts. This review serves as a guideline for identifying and understanding active sites to develop high-performance Fe-N-C catalysts.