A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for Efficient Oxygen Reduction Reaction
A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for Efficient Oxygen Reduction Reaction
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DOI：
10.1021/jacs.6b09470
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发表时间：
2016-11-16
影响因子：
15
通讯作者：
Joo, Sang Hoon
中科院分区：
文献类型：
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作者：
Sa, Young Jin;Seo, Dong-Jun;Joo, Sang Hoon
碳上铁氮（Fe-N/C）催化剂是一种很有前途的非贵金属催化剂，可用于能量转换和存储装置中的氧还原反应（ORR）。人们普遍认为，Fe-N/C催化剂的活性位结构含有Fe-N-x配位。然而，高性能Fe-N/C催化剂的制备大多涉及高温热解步骤，该步骤不仅生成具有催化活性的Fe-N-x位，而且生成活性较低的大铁基颗粒。在此，我们报告了一种通用的“二氧化硅保护层辅助”方法，该方法可以优先在Fe-N/C催化剂中产生具有催化活性的Fe-N-x位点，同时抑制大铁基颗粒的形成。该催化剂的制备由铁卟啉前驱体在碳纳米管（CNT）上吸附、二氧化硅层包覆、高温热解、二氧化硅层蚀刻等步骤组成，得到了包覆有薄层卟啉碳（CNT/PC）催化剂的CNTs。在制备CNT/PC催化剂的过程中，原位x射线吸收光谱的温度控制揭示了二氧化硅层的配位作用以稳定Fe-N-4位点。与未涂覆二氧化硅的CNT/PC相比，碳纳米管/PC催化剂含有更高密度的Fe-N-x活性位点。CNT/PC在碱性介质中表现出很高的ORR活性和良好的稳定性。重要的是，具有CNT/ pc基阴极的碱性阴离子交换膜燃料电池（AEMFC）在npmc基AEMFC中表现出创纪录的高电流和功率密度。此外，碳纳米管/ pc基阴极在酸性质子交换膜燃料电池中表现出320 a cm(3)的高体积电流密度。我们进一步证明了这种合成策略对其他碳载体的通用性。
Iron-nitrogen on carbon (Fe-N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR) in energy conversion and storage devices. It has been widely suggested that an active site structure for Fe-N/C catalysts contains Fe-N-x coordination. However, the preparation of high-performance Fe-N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only catalytically active Fe-N-x sites, but also less active large iron-based particles. Herein, we report a general "silica-protective-layer-assisted" approach that can preferentially generate the catalytically active Fe-N-x sites in Fe-N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst preparation consisted of an adsorption of iron porphyrin precursor on carbon nanotube (CNT), silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. Temperature-controlled in situ X-ray absorption spectroscopy during the preparation of CNT/PC catalyst revealed the coordination of silica layer to stabilize the Fe-N-4 sites. The CNT/PC catalyst contained higher density of active Fe-N-x sites compared to the CNT/PC prepared without silica coating. The CNT/PC showed very high ORR activity and excellent stability in alkaline media. Importantly, an alkaline anion exchange membrane fuel cell (AEMFC) with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. In addition, a CNT/PC-based cathode exhibited a high volumetric current density of 320 A cm(3) in acidic proton exchange membrane fuel cell. We further demonstrated the generality of this synthetic strategy to other carbon supports.