Hierachical design of Fe-N-C catalysts for the electroreduction of oxygen

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The proton exchange membrane fuel cell (PEM-FC) is a promising technology for sustainable propulsion in vehicles. The overall reaction is described by the reaction of hydrogen with oxygen under the formation of water. Both half-cell reactions are accelerated by platinum-based catalysts. Caused by the limited kinetics of Pt/C for the oxygen reduction reaction (ORR) there are several scientific affords to replace platinum on the cathode by cheap alternatives. In this respect, Fe-N-C catalysts are todays most promising alternative for the ORR. The label as Fe-N-C is related to the fact that for the formation of active materials iron and nitrogen have to be utilized in the precursor composition or during the heat treatment and that the active sites are embedded in carbon. A mayor difference in comparison to Pt/C catalysts is given in the molecular nature of active sites. Therefore, several techniques that were found as rather useful for the description of Pt/C catalysts cannot be applied or a strongly limited for Fe-N-C. Within this proposal hierarchically designed Fe-N-C catalysts are fabricated in order to investigate the role of specific structure units on the catalytic activity and selectivity. Two different polymer-based synthesis and processing strategies are applied for the fabrication. In this approach the composition and molecular structure of the polymers is varied in such a way that the properties of the obtained catalysts can be modulated (porosity, density of FeN4 sites). Hence, the influence of different pore classes (as defined by IUPAC) on the catalytic activity of these non-precious metal catalysts and its selectivity can be investigated. In addition, one can estimate to what extend the reduction mechanism is affected by distinct structure units or e.g. the concentration of iron in Fe-N-C catalysts. Structural characterization of the polymer precursors, the model and reference catalysts is made by scanning and tunneling electron microscopy (SEM/TEM), Raman and Mößbauer spectroscopy, as well as photoelectron and x-ray absorption spectroscopy. For the electrochemical characterization of the catalysts especially the selectivity and activity play a crucial role in order to correlate them with the catalytic properties. Based on this an integral concept of the catalyst from its molecular structure of active sites and of the carbon towards the macroscopic dimension is visualized.