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Design of N-coordinated dual-metal sites: A stable and active Pt-free catalyst for acidic ORR Jing Wang, Zhengqing Huang, Wei Liu, Chun-Ran Chang, Haolin Tang, Zhijun Li, Wenxing Chen, Chunjiang Jia, Tao Yao, Shiqiang Wei, Yuen Wu, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10385 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017
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Journal of the American Chemical Society
Design of N-coordinated dual-metal sites: A stable and active Pt-free catalyst for acidic ORR Jing Wang,1 Zhengqing Huang,5 Wei Liu,3 Chunran Chang,*5 Haolin Tang,4 Zhijun Li, 7 Wenxing Chen,2 Chunjiang Jia,6 Tao Yao,3 Shiqiang Wei,3 Yuen Wu,*1 Yadong Li2 1
Department of Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Technology of China, Hefei 230026, China; 2 Department of Chemistry, Tsinghua University, Beijing 100084, China; 3 Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China; 4 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China; 5 Institute of Industrial Catalysis, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China; 6 Key Laboratory for Colloid and Interface Chemistry, Key Laboratory of Special Aggregated Materials, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China; 7 Department of Chemistry, Queen's University, Kingston, Ontario, K7L 3N6, Canada. Supporting Information Placeholder ABSTRACT: Herein, we develop a host-guest strategy to construct a novel electrocatalyst with Fe-Co dual sites embedded on N-doped porous carbon and demonstrate its activity for the oxygen reduction reaction in acidic electrolyte. Our best catalyst exhibits a superior ORR performance, with comparable onset potential (Eonset, 1.06 V vs. 1.03 V) and half-wave potential (E1/2, 0.863 V vs. 0.858 V) than commercial Pt/C. The fuel cell test reveals this (Fe,Co)/N-C outperform most of the reported Ptfree catalysts both in H2/O2 and H2/air conditions. In addition, this cathode catalyst with dual metal sites is found to be very stable in a long-term operation with 50,000 cycles for electrode measurement and 100 h for H2/air single cell operation. The density functional theory calculations reveal the dual sites is favored for the activation of O-O, which is crucial for fourelectron oxygen reduction process.
Owing to the high energy conversion efficiency and environmentally-friendly features, polymer electrolyte membrane fuel cell (PEMFC) owns a great potential in automobile transportation and other stationary devices1,2. However, the prohibitive cost of platinum (Pt), which is the primary element to catalyze the sluggish cathodic ORR, is responsible for the major cost of PEMFC power system. Considering the power density is the key factor with the largest effect on cost of PEMFC vehicles3,4, finding the non-precious alternatives for the Pt-based catalysts with similar ORR catalytic property is the key challenge to realize the widespread adoption of PEMFC. Regarding to the PEMFC technology, the Pt-based electrodes can be ideally substituted both in alkaline and acidic PEMFC5,6. It is generally believed that alkaline environment is more conducive to the Pt substitution because of the fast kinetics for ORR7. For instance, silver, transition-metal oxide, metal-nitrogen-carbon (M-N-C) materials (M = Co, Fe, Ni, Mn, etc.) and metal-free nitrogen-carbon
materials have achieved the superior or comparable catalytic performance than commercial Pt/C8-11. Nowadays, the acidic PEMFC is more close to large-scale industrialization due to its better power density and stability than alkaline PEMFC12. Hence, developing the substituted materials to Pt-based catalyst in acidic electrolyte is more promising pathway to make zero-emission cars a reality. Despite tremendous efforts have been paid, the ORR activities catalyzed by the reported non-precious catalysts are still significantly sluggish in acidic media due to the severe leaching problem, as manifested by larger overpotentials (40-400 mV) and poor stability than commercial Pt/C. In 1964, Jasinski made the firstly breakthrough on M-N-C complexes that Co-phthalocyanine can serve as a candidate of Pt in electrocatalysis13. Later on, Yeager et al. utilized the combination of low-cost non-N4marcrocycles precursors, transition-metal inorganic salts, and carbons to further reduce the production cost14. Zelenay and Dodelet sequentially applied these non-precious materials to single cell measurements in acid single fuel cell15,16. It is believed that the structure of non-precious catalyst is highly sensitive to the selection of suitable sources and pyrolysis conditions, which apparently determine the ORR performance17,18. The single-atom electrocatalysts would optimize the activity, stability and selectivity through tuning the well-defined metal active centers.1921 Even to date, the actual active sites for O2 activation and the intrinsic reaction mechanism still remain elusive. Moreover, further improvements are needed to meet industrial demands such as high volumetric activity at meaningful voltage, cost reduction of catalyst, and long-term working stability. To achieve the substitution of commercial Pt-based materials, an ideal ORR catalyst should comprise O2-favorable reactive sites for cleavage of O=O bond, optimized geometric structure for water management and O2 diffusion, and well-defined active sites to retain the excellent performance for long periods22-25. Herein,
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we report on a host-guest design of hollow carbon-derived material with porphyrin-like Fe-Co dual sites ((Fe,Co)/N-C) and compared its ORR activity in O2-saturated 0.1 M HClO4 solution against commercial Pt/C. The synthesis is mainly based on the precise control over the bonding between the Co nodes (host) and adsorbed Fe ions (guest) within the confined space of metalorganic frameworks (MOFs). As shown in (Figure 1A), a type of Zn/Co bimetallic MOF (BMOF) comprising similar sodalite coordination of Co2+ and Zn2+ nodes with 2-methylimidazole (ZIF-8 structure) was used as host to encapsulate the FeCl3 molecules within the cavities26,27. To remit the diffusion resistance originated from the narrow aperture of ZIFs (
