Fe-N-C single-atom catalysts with an axial structure prepared by a new design and synthesis method for ORR

Fe-N-C single-atom catalysts usually exhibit poor ORR activity due to their unsatisfactory O-2 adsorption and activation. Here, a new design idea and tailored self-assembly synthesis method are reported to improve their ORR performance. DFT calculations indicate that the ORR electrocatalytic activity of Fe-N-C single-atom catalysts with an axial structure is superior to that of Fe-N-C single-atom catalysts with a Fe-N-4 active site. In order to experimentally demonstrate the difference, Fe-N-C single-atom catalysts with a Fe-N-5 active site were successfully synthesized on the surface of monolayer graphene. XANES, SEM, HRTEM, XRD, Raman and XPS analyses indicate that the synthesized Fe-N-C catalyst possessed nanofibre morphology and a curved layer-like crystal structure. For comparison, FePc powder was used as the FePc(Fe-N-4) catalyst as its molecular structure involves a Fe-N-4 active site embedded in carbon six-membered rings. The current density of the synthesized Fe-N-5/C@G catalyst at a potential of 0.88 V vs. RHE is 1.65 mA cm(-2), which is much higher than that of the FePc(Fe-N-4) catalyst (1.04 mA cm(-2)) and even higher than that of commercial Pt/C catalyst (1.54 mA cm(-2)). The results are very well consistent with the DFT calculations, verifying the dependability and accuracy of DFT calculations. This work reports a new synthetic idea to obtain better performance and proposes a formation mechanism to explain the process of the synthesis method.