Fe-N-C Catalysts for Oxygen Reduction Reaction: A Computational Analysis of Electronic Structure and Spectroscopic Properties

Abstract

In proton exchange fuel cells (PEFCs), the electrochemical reduction of oxygen (ORR) plays a key role as the complementary half-cell reaction to the oxidation of hydrogen. It requires the transport of four electrons per oxygen molecule associated with high activation barriers and slow kinetics and thus requires high amounts of catalyst. [1] State-of-the art catalysts for both reactions are based on platinum, an expensive metal.Iron and nitrogen doped carbon (Fe-N-C) catalysts show high ORR activity and are thus a very promising alternative to Pt catalysts. It is believed that in Fe-N-C catalysts, iron is atomically dispersed as pseudo-molecular active centres with four- or fivefold nitrogen coordination spheres which are embedded in a graphene matrix. However, the exact nature of the active site in terms of Fe spin and oxidation states, precise coordination environment and spatial extent of the graphene sheets are still debated, as is the influence of these factors on the ORR mechanism in Fe-N-C catalysts. [2-4]Unlike other spectroscopies, Mössbauer spectroscopy, which provides direct insights on iron spin and oxidation states, is used successfully to characterise the amorphous FeNC catalysts. Until recently, the interpretation of Mössbauer spectra was limited to comparisons with small reference complexes [2] which lack the extended π-systems of Fe-N-C catalysts. Recently, with increasing use of in situ and operando experiments on these catalysts, the interest in computational models for the interpretation of experimental Mössbauer spectra has grown. [4-6]. Using density functional theory, we show for different FeN4 environments how the electronic structure is influenced by the size and symmetry of the π-system. The influence is quantified in terms of structural parameters, relative spin state energies, spin distributions and spectroscopic properties.