Promoting oxygen reduction reaction by tuning externally doped nitrogen around atomically dispersed iron

As potential alternatives to Pt-based catalysts for oxygen reduction reaction (ORR), iron (Fe)-based single-atomic site catalysts (SACs) have shown superior performances compared with others. For Fe SACs, the effect of directly coordinating N atoms has received intensive discussion, yet by contrast, the effect of secondary coordinating atoms has been left largely unexplored. Here, we developed a route to tuning externally doped N around Fe SACs to boost ORR activity. Density functional theory (DFT) calculations established the four different models for FeN4 for ORR, indicating that the FeN4-PR-GN (pyrrolic-N for the first coordination shell and graphitic-N for the second coordination shell) can effectively improve the ORR performance. On the basis of theoretical calculations, we developed a two-step strategy including interior encapsulation and surface adsorption, which helped to regulate the different external coordination N of Fe SACs. The best-performing catalyst, FeN4-PR-GN, displays excellent ORR performances in alkaline electrolytes; in particular, the catalyst gives a half-wave potential of 0.92 V (vs. RHE) and high stability, which is much better than those of commercial Pt/C. Our work here not only demonstrates an efficient catalyst to boost ORR performances, but also develops a facile strategy for the preparation of SACs for advanced fuel cells.