Using a Fe-doping MOFs strategy to effectively improve the electrochemical activity of N-doped C materials for oxygen reduction reaction in alkaline medium

For the rational design of heteroatom-doping carbon catalysts toward the oxygen reduction reaction (ORR), various structure-wise and porous metal-organic-frameworks (MOFs) were preferred especially the transition metal imidazoles frameworks (ZIFs) with abundant N and C elements. Herein, taking advantage of the volatile nature of Zn in ZIF-Zn at high temperatures, we synthesized the bimetallic ZIFs with Zn/Fe molar ratio of 10:1 firstly, and then obtained a structure-define and porous Fe- and N-doped C electrocatalyst by calcining at 900 °C in Ar flow. With the well-defined polyhedrons morphology, high surface area, and evenly distributed C-Nx and Fe-Nx active sites, the as-synthesized Fe-N-C catalysts including the acid-leached Fe-N-C gave superior ORR capability, selectivity, and stability in 0.1 M KOH, and can be also applied in acidic electrolyte. Inspiringly, Fe-N-C exhibited excellent ORR activity exceeding Pt/C benchmark in 0.1 M KOH with a half-wave potential (E1/2) of 0.878 V and the kinetic current density (jk at 0.85 V) of 13.2 mA cm−2, and the single N-C material. And after acid leaching, the obtained Fe-N-C(L) catalyst still maintained the excellent ORR activity (E1/2 = 0.855 V, jk = 6.8 mA cm−2). Additionally, though their ORR performance in acid was inferior to that of Pt/C, the gap was also significantly narrowed by Fe-introduction into N-C structures. Experiments demonstrated that Fe-Nx structure in N-doped C matrix and cooperated with C-Nx centers were very essential to deliver the significantly enhanced ORR activity, in addition to good structural characteristics, i.e., the high pore volume, abundant defects, and good conductivity. This synthesis method based on M-doped MOFs was expected to be transplanted to obtain other energy-relative non-precious metal catalysts.