Comparison of in-Situ 57 Fe Mössbauer Spectroscopic Data for Differently Prepared Fe-N-C Catalysts

Abstract

Electrochemical energy conversion in proton exchange fuel cells (PEFCs) is very promising device for world-wide energy demand, especially in the transportation sector [1]. Fe-N-C catalysts are promising materials as alternative for platinum-based catalyst because of the low cost and high activities especially for the ORR. However, a fundamental understanding of the nature of active site structures and the ORR reaction mechanism is missing. Especially also considering the question, to what extent differently prepared catalysts obtain similar trends or not. 57Fe Mössbauer Spectroscopy (MS) is a very good technique in distinguishing FeN4 centers with changing local coordination environment [2,3]. So far, the electronic and geometric structure have been investigated under in-situ / operando conditions by spectroscopic techniques, like XAS [4], XES [5]. However, only once Mössbauer spectroscopy was applied quasi in-situ by Bouwkamp-Wijnoltz et al. on frozen electrodes [6] and there are only a rare number of publications on carbon supported macrocycles [7,8]. In this work, we have designed a feasible electrochemical cell for in-situ Mössbauer spectroscopy. Two different types of Fe-N-C catalysts were studied under in-situ conditions and we will present the structural changes induced by the effect of applied potential. By comparing different catalysts, we are able to show to what extent Mössbauer sites remain the same, independent of the preparation route or are changing. The data will be correlated with selectivity measurements of the catalysts. Based on this, further fundamental insights on the nature of the active site structure in Fe-N-C catalysts can be gained which could guide the preparation for highly active non precious metal catalyst for commercialization.