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Highly Efficient Oxygen Reduction Reaction Electrocatalysts Synthesized under Nanospace Confinement of Metal Organic Framework Jianing Guo, Yang Li, Yuanhui Cheng, Liming Dai, and Zhonghua Xiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b03807 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017
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ACS Nano
Highly Efficient Oxygen Reduction Reaction Electrocatalysts Synthesized under Nanospace Confinement of Metal Organic Framework Jianing Guo†, Yang Li†, Yuanhui Cheng†, Liming Dai*†‡ and Zhonghua Xiang*† †State Key Lab of Organic-Inorganic Composites, College of Chemical Engineering, College of Energy, Beijing University of Chemical Technology, Beijing 100029, P.R. China. ‡Centre of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106 (USA). Email:
[email protected] or
[email protected] KEYWORDS. 2D covalent organic polymers, metal organic frameworks, nanoconfinement, oxygen reduction reaction, electrocatalysis
ABSTRACT The output energy capacity of green electrochemical devices, e.g., fuel cells, depends strongly on the sluggish oxygen reduction reaction (ORR), which requires catalysts. One of the desired features for highly efficient ORR electrocatalytic materials is the rich of well-defined activate sites. Herein, we developed a facile approach to prepare highly efficient non-precious metal and nitrogen-doped carbon-based ORR catalysts basing on covalent organic polymers (COPs) synthesized 1
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in-situ in the nanoconfined space of highly order metal organic frameworks (MOFs). The MOF templet ensured the developed electrocatalysts to possess a high surface area with homogeneously distributed small metal/nitrogen active sites, as confirmed by X-ray absorption fine structure (EXAFS) measurements and first-principles calculations, leading to highly efficient ORR electrocatalytic activity. Notably, the developed COP-TPP(Fe)@MOF-900 exhibits 16 mV positive half-wave potential comparing with the benchmarked Pt/C.
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ACS Nano
Fuel cell has been regarded as a promising technology to meet the energy requirements for future electric vehicles and/or as stationary power sources.1-4 However, the performance of fuel cells depends strongly on the sluggish oxygen reduction reaction (ORR) at the cathode.2,5 Although certain precious metals (e.g., Pt-based catalysts) have achieved unbeatable electrocatalytic activities for ORR, they are still subjected to multiple disadvantages, including their high cost, scarcity, limited stability, and fast deactivation by CO, which still hampered fuel cells for practical applications.6-8 Therefore, it is critical to develop efficient and durable ORR electrocatalysts at low cost.9-11 Recently, carbon supported transition metal-nitrogen complexes have been recognized as one of promising candidates for substituting Pt-based electrocatalysts for ORR.12-16 Although the active sites in these nitrogen-rich transition metal/nanocarbon catalysts have not been understood clearly, pydinic-type and graphitic-type nitrogen sites in a nanocarbon matrix and transition metal-related compounds (e.g., FeN, Fe3C, and CoO) have been generally considered as effective active sites for the eletrocatalysis of oxygen reduction to hydroxyl (OH-) in alkaline solution.11,17,18 Meanwhile, Fe (or Co)-Nx species in a nanocarbon matrix are favorite active sites for ORR in acidic solution.18-20 More recently, highly efficient ORR catalysts have also been prepared through carbonization of precursor polymers containing metal-porphyrin moieties.21-23 Covalent organic polymers (COPs)24-30 are a class of multidimensional and multifunctional porous organic networks consisting of covalent bonds (B-O, C-C, C-H, C-N et al.) between organic linkers. We have recently employed metal porphyrin 3
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as monomer to produce well-defined 2D porphyrin-based COPs, which, after carbonization, exhibited good electrocatalytic activities toward 4e oxygen reduction in both alkaline and acid media with an excellent stability and free from any methanol-crossover/CO-poisoning effect.21 During carbonization, however, the pyrolyzed COP (COP-P) showed a strong stacking effect, which leads to the aggregation of metal species (particle size > 30 nm) and subsequently reduces density of active sites and limits electrocatalytic activity. To further enhance the ORR electrocatalytic activity, it is critical to produce small particles with uniform distribution. Metal-organic frameworks (MOFs), constructing of coordinate bond between metal ions and organic linkers, are a class of tunable porous materials with a high surface area and free volume.31-34 The coordinated bond in MOFs can be easily removed in acid media to generate well-defined voids. In this study, we used a well-defined MOF-18035 with an ultrahigh free volume as a template of nano-confined space for in situ growing well-defined COPs. By replicating the MOF-180 template, the resultant COPs possess a high electrocatalytic surface area, containing uniform small metal nanoparticles (
