Sulfuration of an Fe–N–C Catalyst Containing FexC/Fe Species to Enhance the Catalysis of Oxygen Reduction in Acidic Media and for Use in Flexible Zn–Air Batteries

During the preparation of atomically dispersed Fe-N-C catalysts, it is difficult to avoid the formation of iron-carbide-containing iron clusters ("Fe_x C/Fe"), along with the desired carbon matrix containing dispersed FeN_x sites. As a result, an uncertain amount of the oxygen reduction reaction (ORR) occurs, making it difficult to maximize the catalytic efficiency. Herein, sulfuration is used to boost the activity of Fe_x C/Fe, forming an improved system, "FeNC-S-Fe_x C/Fe", for catalysis involving oxygen. Various spectroscopic techniques are used to define the composition of the active sites, which include Fe-S bonds at the interface of the now-S-doped carbon matrix and the Fe_x C/Fe clusters. In addition to outstanding activity in basic media, FeNC-S-Fe_x C/Fe exhibits improved ORR activity and durability in acidic media; its half-wave potential of 0.821 V outperforms the commercial Pt/C catalyst (20%), and its activity does not decay even after 10 000 cycles. Interestingly, the catalytic activity for the oxygen evolution reaction (OER) simultaneously improves. Thus, FeNC-S-Fe_x C/Fe can be used as a high-performance bifunctional catalyst in Zn-air batteries. Theoretical calculations and control experiments show that the original FeN_x active centers are enhanced by the Fe_x C/Fe clusters and the Fe-S and C-S-C bonds.