Oxygen Reduction Reaction Catalytic Sites on Carbon-Coated Fe3C Catalyst

Abstract

Proton-exchange membrane fuel cells (PEMFC) are promising chemical-to-electrical energy conversion devices which can potentially replace combustion engine in automobiles. Their commercialization is currently suppressed primarily due to the price of the platinum catalyst for the cathodic oxygen reduction reaction (ORR). Required platinum loading per fuel-cell car is about 10 times higher than for the conventional one [1], which results in a significant contribution to the car price. Therefore, either the platinum loading needs to be drastically decreased, or a stable and active non-precious metal catalyst has to be developed. Recently, a new type of ORR catalyst composed of carbon-coated iron carbide nanoparticles has been synthesized [2]. 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 [3]. 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 [4], 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. [1] 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. [2] Y. Hu, J. O. Jensen, W. Zhang, L. N. Cleemann, W. Xing, N. J. Bjerrum, Q. Li, Angew. Chem. Int. Ed., 2014, 53, 3675. [3] J.-P. Dodelet, R. Chenitz, L. Yang, M. Lefèvre, ChemCatChem, 2014, 6, 1866. [4] 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.