Achilles' heel of iron-based catalysts during oxygen reduction in acidic medium

Abstract : For catalyzing dioxygen reduction, iron-nitrogen-carbon (Fe-N-C) materials are today the best candidates to replace platinum in proton-exchange membrane fuel cell (PEMFC) cathodes. Despite tremendous progress in their activity and site-structure understanding, improved durability is critically needed but challenged by insufficient understanding of their degradation mechanisms during operation. Here, we show that FeNxCy moieties in a representative Fe-N-C catalyst are structurally stable but electrochemically unstable when exposed in acidic medium to H2O2, the main oxygen reduction reaction (ORR) byproduct. We reveal that exposure to H2O2 leaves iron-based catalytic sites untouched but decreases their turnover frequency (TOF) via oxidation of the carbon surface, leading to weakened O2-binding on iron-based sites. Their TOF is recovered upon electrochemical reduction of the carbon surface, demonstrating the proposed deactivation mechanism. Our results reveal for the first time a hitherto unsuspected key deactivation mechanism during ORR in acidic medium. This study identifies the N-doped carbon surface as Achilles' heel during ORR catalysis in PEMFCs. Observed in acidic but not in alkaline electrolyte, these insights suggest that durable Fe-N-C catalysts are within reach for PEMFCs if rational strategies minimizing the amount of H2O2 or reactive oxygen species (ROS) produced during ORR are developed.
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Submitted on : Thursday, October 4, 2018 - 1:33:11 PM
Last modification on : Thursday, November 15, 2018 - 2:23:03 PM

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Chang Hyuck Choi, Hyung-Kyu Lim, Min Wook Chung, Gajeon Chon, Nastaran Ranjbar Sahraie, et al.. Achilles' heel of iron-based catalysts during oxygen reduction in acidic medium. Energy & Environmental Science, Royal Society of Chemistry, 2018, 11 (11), pp.3176-3182. ⟨10.1039/C8EE01855C⟩. ⟨hal-01887695⟩

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