Recent Insights into the Oxygen-Reduction Electrocatalysis of Fe/N/C Materials

Owing to the high activity toward the oxygen reduction reaction (ORR) and the selectivity toward the four-electron pathway, single atom Fe/N/C electrocatalysts have been considered as the most promising low-cost candidates to replace Pt in fuel cells. Despite the significant progress achieved in the past decade, the performance and durability of Fe/N/C catalysts remain far behind that of the Pt-based materials in practical devices such as proton exchange membrane fuel cells (PEMFCs) in light-duty vehicles. Recent progress in Fe/N/C electrocatalysis has been mainly based on the empirical approach with rather random combinational synthesis conditions and precursors. For rational design of applicable Fe/N/C catalysts, an insightful understanding of fundamental electrocatalysis of Fe/N/C is required. In this critical review, we will focus on the mechanisms of the ORR catalysis of Fe/N/C, the categories of active sites in Fe/N/C catalysts including the electronic and structural properties of the catalytic centers, the assessment and quantification of the active sites, pH effect on the activity, and the degradation mechanisms. We hope the comprehensive and thorough discussions of Fe/N/C electrocatalysis will help to guide the design and development of high-performance Fe/N/C electrocatalysts toward the application of PEMFCs.