Elucidating the Structural Composition of a Fe-N-C Catalyst By Nuclear and Electron Resonance Techniques

Abstract

Today, state of the catalysts in proton exchange fuel cells are platinum-based. Cost estimates show that basically the catalyst is one of the most expensive components in the FC stack. While platinum is very active for the hydrogen oxidation reaction, large quantities (approx. 80 % of the overall Pt content) are required on the cathode for the oxygen reduction reaction (ORR). Thus, alternative catalysts are required. Fe-N-C catalysts achieve very promising ORR activities in PEFC applications. The most active ones are prepared from zinc immidazole frameworks in combination with an iron source (and nitrogen precursor), that are then pyrolysed at temperatures of 900 °C or even higher. This makes it very likely that inorganic spectator species are formed. In order to get some more fundamental insight, in this work we focused on the characterization of a Fe-N-C model catalyst. The material was prepared from iron porphyrin supported on carbon with low iron loading at 600 °C followed by acid leaching. For this condition, previous results indicate that still the overall amount of FeN4 centers remain intact, but transforms to slightly different local environments. Herein, we will show by the use of nuclear inelastic scattering (also known as Nuclear Resonance Vibrational Spectroscopy), low temperature Mössbauer spectroscopy and Electron Paramagentic Resonance spectroscopy that the above given conclusion was wrong: even for these mild preparation conditions large amounts of spectator species are found, that overlay in room temperature Mössbauer spectroscopy with the so-called D1 doublet, previously assigned as ORR active site by us and others. Nevertheless, still the catalyst contains about 50 % FeN4 centers, thereof a partial fraction that is coordinated by oxygen molecules. These results underline the importance of in-depth analysis of Fe-N-C catalysts as the amount of spectator species should be decreased in best case to zero, in order to avoid side reactions.