Unravelling the Influence of Oxygen on the Degradation Mechanisms of Fe-N-C Oxygen Reduction Reaction Catalysts

Abstract

Pyrolyzed iron-nitrogen-carbon materials (Fe-N-C) comprising mostly atomic Fe-Nx moieties are the most mature class of noble metal free catalysts for the oxygen reduction reaction (ORR) in acidic media. Such catalysts show excellent performance in proton exchange membrane fuel cell (PEMFC), approaching that of platinum-based catalysts. [1] However, an important performance loss is observed during operation, in conditions mimicking those of a PEMFC. [2] To shed fundamental light onto the degradation mechanisms at stake, two different Fe-N-C materials were prepared, one using a metal organic framework (MOF) and the other a sacrificial support method (SSM), and are labelled Fe-N-C_MOF and Fe-N-C_SSM, respectively. These materials were characterized before and after two different accelerated stress tests (ASTs: 10 k cycles, 0.6-1.0 V vs. RHE, 3s-3s, 0.1 M H2SO4, 80°C) under Ar or O2 atmosphere. Stronger degradation and higher ORR mass activity loss were observed when the AST is performed in O2 vs. Ar-saturated acidic electrolyte. For AST under Ar condition, physicochemical characterisations revealed a demetallation process and the eventual occurrence of a clustering mechanism whereas for AST performed in O2 atmosphere, a decrease of the Fe content and the formation of Fe oxide particles was observed (Figure 1).Keywords: Electrocatalysis, Fe-N-C, Oxygen Reduction Reaction, Durability, PEMFC Acknowledgements These studies were financed by the French National Research Agency in the frame of the CAT2CAT (grant number n°ANR-16-CE05-0007) and the ANIMA (grant number n°ANR-19-CE05-0039) projects.