DFT Study of the Oxygen Reduction Reaction on Carbon-Coated Iron and Iron Carbide

Iron carbide encapsulated in graphitic layers has recently been recognized as an active oxygen reduction reaction (ORR) catalyst made of earth-abundant elements. Here, the ORR activity of graphene (G) and N-doped graphene (NG) supported on Fe3C(010) and Fe(110) is studied computationally by means of density functional theory calculations. The calculations show higher activity of the Fe3C-supported model system than the Fe-supported one, as well as the importance of N-doping in achieving high ORR activity, in agreement with experimental observations. We find the most active sites on a single N-doped graphitic layer placed on the Fe3C surface. Like in the case of unsupported NG, the reaction on the Fe3C/NG model interface proceeds at the atomic oxygen coverage between 0.5 <θO <1.0. The charge on O adsorbate caused by the presence of support is found to correlate with the oxygen binding strength. In the case of the Fe/NG system, this results in a surface poisoning by oxygen. On the basis of these findings, we propose that a heterostructure consisting of a NG overlayer and a support with stronger electron-donating properties than Fe3C and weaker than Fe may approach or even exceed the ORR activity of the Pt(111) surface.