Highly active sites of low spin Fe^ⅡN₄ species: The identification and the ORR performance

Over recent years, catalytic materials of Fe-N-C species have been recognized being active for oxygen reduction reaction (ORR). However, the identification of active site remains challenging as it generally involves a pyrolysis process and mixed components being obtained. Herein Fe₃C/C and Fe₂N/C samples were synthesized by temperature programmed reduction of Fe precursors in 15% CH₄/H₂ and pure NH₃, respectively. By acid leaching of Fe₂N/C sample, only single sites of FeN₄ species were presented, providing an ideal model for identification of catalytic functions of the single sites of FeN₄ in ORR. A correlation was conducted between the concentration of Fe^ⅡN₄ in low spin state by Mössbauer spectra and the kinetic current density at 0.8 V in alkaline media, and such a structure-performance correlation assures the catalytic roles of low spin Fe^ⅡN₄ species as highly active sites for the ORR.