title: A General Approach to Preferential Formation of Active Fe-NxSites in Fe-N/C Electrocatalysts for Efficient Oxygen Reduction Reaction.
abstract: 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-Nxcoordination. However, the preparation of high-performance Fe-N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only catalytically active Fe-Nxsites, 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-Nxsites 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-N4sites. The CNT/PC catalyst contained higher density of active Fe-Nxsites 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-3in acidic proton exchange membrane fuel cell. We further demonstrated the generality of this synthetic strategy to other carbon supports.
authors.forename: Young Jin
authors.initials: YJ
authors.lastname: Sa
authors.affiliation: Hydrogen and Fuel Cell Center, Korea Institute of Energy Research (KIER) , Jellabuk-do 56332, Republic of Korea.
authors.forename: Dong-Jun
authors.initials: DJ
authors.lastname: Seo
authors.forename: Jinwoo
authors.initials: J
authors.lastname: Woo
authors.affiliation: Department of Physics, Kookmin University , Seoul 02707, Republic of Korea.
authors.forename: Jung Tae
authors.initials: JT
authors.lastname: Lim
authors.forename: Jae Yeong
authors.initials: JY
authors.lastname: Cheon
authors.affiliation: Hydrogen and Fuel Cell Center, Korea Institute of Energy Research (KIER) , Jellabuk-do 56332, Republic of Korea.
authors.forename: Seung Yong
authors.initials: SY
authors.lastname: Yang
authors.affiliation: Hydrogen and Fuel Cell Center, Korea Institute of Energy Research (KIER) , Jellabuk-do 56332, Republic of Korea.
authors.forename: Jae Myeong
authors.initials: JM
authors.lastname: Lee
authors.forename: Dongwoo
authors.initials: D
authors.lastname: Kang
authors.affiliation: UNIST Central Research Facility, Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.
authors.forename: Tae Joo
authors.initials: TJ
authors.lastname: Shin
authors.forename: Hyeon Suk
authors.initials: HS
authors.lastname: Shin
authors.affiliation: UNIST Central Research Facility, Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.
authors.forename: Hu Young
authors.initials: HY
authors.lastname: Jeong
authors.affiliation: Department of Physics, Kookmin University , Seoul 02707, Republic of Korea.
authors.forename: Chul Sung
authors.initials: CS
authors.lastname: Kim
authors.affiliation: Beamline Division, Pohang Accelerator Laboratory , Pohang, Kyungbuk 37673, Republic of Korea.
authors.forename: Min Gyu
authors.initials: MG
authors.lastname: Kim
authors.affiliation: Hydrogen and Fuel Cell Center, Korea Institute of Energy Research (KIER) , Jellabuk-do 56332, Republic of Korea.
authors.forename: Tae-Young
authors.initials: TY
authors.lastname: Kim
authors.forename: Sang Hoon
authors.initials: SH
authors.lastname: Joo