Fe–N/C catalysts with tunable mesoporous structures and carbon layer numbers reveal the role of interlayer O2 activation

Abstract

A class of Fe,N-codoped carbon (Fe–N/C) electrocatalysts has made remarkable advances as highly promising non-Pt group metal catalysts for the oxygen reduction reaction (ORR). However, the design of Fe–N/C catalysts whose active site structure and O₂ activation mode mimic those of an enzymatic ORR catalyst still remains a challenge. Herein, we report the preparation of mesoporous Fe–N/C catalysts with tunable tube- or rod-like frameworks and carbon layer numbers via solid-state nanocasting of mesoporous silica with an iron–phenanthroline complex. The tube-type Fe–N/C exhibited a larger surface area and active site density than the rod-type Fe–N/C. Unexpectedly, the rod-type Fe–N/C showed superior ORR activity to the tube-type Fe–N/C, with a smaller overpotential, greater turnover frequency (TOF), and lower Tafel slope. Temperature-programmed desorption studies revealed a weaker binding strength of the rod-type Fe–N/C with O₂. The rod-type Fe–N/C consisting of multiple carbon layers is likely to activate O₂ in the interlayer between the Fe–N_x-containing carbon layers, reminiscent of the enzymatic catalyst, whereas in the tube-type Fe–N/C with 3–4 carbon layers, O₂ is mostly activated at a single Fe center on the external carbon layer. As a result, the TOF of the rod-type Fe–N/C exceeded those of reported catalysts. We envisage that our work can provide a new insight into the design of highly active Fe–N/C catalysts.