Enhancing the Stability of FeNC Catalysts in PEMFCs by Improved Purification

FeNC catalysts are very promising candidates for the oxygen reduction reaction in proton exchange fuel cells (FCs). Their preparation is dominated by pyrolysis approaches, which however lead also to the formation of inorganic impurity species. While a subsequent removal of iron access is thus desirable, even after acid leaching (AL), some side phases still remain in the catalyst. Such species might leach out during operation and cause severe degradation by Fenton's reaction and subsequent formation of reactive oxygen species. However, it remains under debate if iron from the catalytically active FeN4 centers also contributes in this degradation mechanism. To get further insights, different strategies to improve the AL are compared with respect to structural (XAS, XES, and ICP) and morphological (TEM, Raman, and hydrophobicity) changes and their impact on the activity and more important stability of FeNC catalysts. In all cases, the iron content was significantly reduced. While the positive impact of purification on the performance in half-cell measurements is not obvious, almost doubling of the FC activity is observed, which goes hand in hand with an improved stability. Thus, this work underlines the importance of thorough ex situ leaching to avoid severe degradation.