pH Effect on the H₂O₂-Induced Deactivation of Fe-N-C Catalysts

Abstract

Despite the promising activity of Fe-N-C catalysts at the beginning of life in proton-exchange membrane fuel cells (PEMFCs), their poor durability in operating PEMFCs remains a great challenge for the successful replacement of commercial Pt-based catalysts. One of the key reasons for this poor operando durability is the surface oxidation of carbonaceous supports via Fenton(-like) reactions between the Fe centers and the intermediate product of the oxygen reduction reaction (ORR) in an acidic medium, H2O2. In the present study, we have investigated the pH effect on the chemical deactivation of Fe-N-C catalysts by contacting them with a controlled amount of H2O2. Covering the entire pH range 0-14, we reveal a strong pH dependence of the H2O2-induced deactivation. Especially, acidic H2O2 treatment leads to a severe decrease in ORR activity while almost negligible deactivation is found after a treatment in a sufficiently strong alkaline electrolyte. An electron paramagnetic resonance (EPR) study reveals a positive correlation between the magnitude of Fe-N-C activity decrease and the signal intensity of the hydroxyl radical spin adduct after H2O2 treatment at a given pH. A reactive oxygen species (ROS) such as the hydroxyl radical is identified as a key deactivating agent of Fe-N-C catalysts operating from acidic to neutral pH environments. This result suggests that controlling the formation and lifetime of ROS at such pH is crucial to secure durable fuel cell operation with Fe-N-C cathodes. Alternatively, fuel cell operation under highly alkaline environment could also be considered to improve the catalytic durability, by virtue of a different Fenton(-like) reaction pathway at such pH. Copyright © 2020 American Chemical Society.