Recently, a new type of ORR catalyst composed of carbon-coated iron carbide nanoparticles has been synthesized . It exhibits only slightly lower activity than, and comparable durability to platinum in acidic solutions. It has been suggested that a new ORR catalytic site may have been discovered . In this work we study the catalyst by means of the Density Functional Theory to identify its active sites.
Spin-polarized DFT calculations are performed using the Vienna Ab Initio Simulation Package (VASP) with PAW potentials, BEEF-vdW exchange-correlation functional , plane wave cutoff of 400 eV and vacuum layer of 14 Å. The model system consists of Fe3C/carbon interface and a few explicit water molecules to model the liquid environment. The following factors influencing the activity are taken into account: (i) thickness of the carbon layer (single- or multilayer graphene), (ii) defects in the carbon structure, (iii) nitrogen doping, (iv) adsorbate coverage, (v) presence of spectator adsorbates. A few other catalyst structures, which are independent of the presence of iron carbide, but still may exist in the real system, are also studied. These include catalysts with porphyrin-iron moiety embedded in the graphene structure and defective graphene with or without nitrogen doping. As a result, a few possible structures of catalytic sites in the carbon-coated Fe3C catalyst are proposed.
 H.A. Gasteiger, D.R. Baker, R.N. Carter, W. Gu, Y. Liu, F.T. Wagner, P.T. Yu, in Hydrogen and Fuel Cells: Fundamentals, Technologies and Applications (D. Stolten, Ed.) Ch. 1 p. 3, Wiley-VCH, Weinheim, 2010.
 Y. Hu, J. O. Jensen, W. Zhang, L. N. Cleemann, W. Xing, N. J. Bjerrum, Q. Li, Angew. Chem. Int. Ed., 2014, 53, 3675.
 J.-P. Dodelet, R. Chenitz, L. Yang, M. Lefèvre, ChemCatChem, 2014, 6, 1866.
 J. Wellendorff, K.T. Lundgaard, A. Møgelhøj, V. Petzold, D.D. Landis, J.K. Nørskov, T. Bligaard, K.W. Jacobsen, Phys. Rev. B, 2012, 85, 235149.