The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium
The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium
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DOI：
10.1039/c8ee01855c
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发表时间：
2018-11-01
影响因子：
32.5
通讯作者：
Jaouen, Frederic
中科院分区：
文献类型：
--
作者：
Choi, Chang Hyuck;Lim, Hyung-Kyu;Jaouen, Frederic
对于催化分子氧还原，铁-氮-碳（Fe-N-C）材料是当今质子交换膜燃料电池（PEMFC）阴极中替代铂的最佳候选材料。尽管他们的活动和网站结构的理解取得了巨大的进步，提高耐久性是迫切需要的，但在操作过程中，他们的降解机制的理解不足的挑战。在这里，我们表明，在一个代表性的Fe-N-C催化剂的FeNxCy部分是结构稳定的，但电化学不稳定时，暴露在酸性介质中的H2 O2，主要的氧还原反应（ORR）的副产物。我们发现，暴露于H2 O2留下铁基催化位点不变，但降低其周转频率（TOF）通过氧化的碳表面，导致削弱O-2结合铁基网站。它们的TOF在碳表面的电化学还原后恢复，证明了所提出的失活机制。我们的研究结果首次揭示了一个迄今为止未被怀疑的关键失活机制在ORR在酸性介质中。这项研究确定了氮掺杂的碳表面的致命弱点，在ORR催化PEMFC。在酸性而不是碱性电解质中观察到，这些见解表明，持久的Fe-N-C催化剂对于PEMFC来说是触手可及的，如果开发出合理的策略，最大限度地减少ORR期间产生的H2 O2或活性氧物质（ROS）的量。
For catalysing 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 an 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 O-2-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 the ORR in an acidic medium. This study identifies the N-doped carbon surface as the Achilles' heel during ORR catalysis in PEMFCs. Observed in acidic but not in alkaline electrolytes, 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 the ORR are developed.