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 ‡ Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States S Supporting Information *
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 richness of welldefined activate sites. Herein, we developed a facile approach to prepare highly efficient nonprecious metal and nitrogen-doped carbon-based ORR catalysts based on covalent organic polymers (COPs) synthesized in situ in the nanoconfined space of highly ordered metal organic frameworks (MOFs). The MOF templet ensured the developed electrocatalysts possess a high surface area with homogeneously distributed small metal/nitrogen active sites, as confirmed by X-ray absorption fine structure measurements and first-principles calculations, leading to highly efficient ORR electrocatalytic activity. Notably, the developed COP-TPP(Fe)@MOF-900 exhibits a 16 mV positive halfwave potential compared with the benchmarked Pt/C. KEYWORDS: 2D covalent organic polymers, metal−organic frameworks, nanoconfinement, oxygen reduction reaction, electrocatalysis 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, etc.) between organic linkers. We have recently employed metal porphyrin as monomer to produce well-defined two-dimensional (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 were free from any methanol-crossover/COpoisoning 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
T
he 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 hamper 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 the 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-metalrelated 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 © 2017 American Chemical Society
Received: May 31, 2017 Accepted: July 13, 2017 Published: July 13, 2017 8379
DOI: 10.1021/acsnano.7b03807 ACS Nano 2017, 11, 8379−8386
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Figure 1. Schematic illustration of nanoconfinement-induced synthesis of COP-TPP@MOF catalyst. Top-left picture is the crystal structure of MOF-180, and bottom-left picture is the cavity in crystal MOF-180, i.e., the confined space for growing the COP-TPP@MOF material.
Figure 2. (a) SEM image of the as-synthesized COP-TPP@MOF. (b and c) TEM and HRTEM images of COP-TPP(Fe)@MOF-900. Lattice fringe at 0.206 and 0.340 nm refers to Fe3N (111) and C (002), respectively. (d) SEM image of the as-synthesized COP-TPP. (e and f) TEM and HRTEM images of the COP-TPP(Fe)-900. Lattice fringe at 0.290 and 0.340 nm refers to Fe3N (101) and C (002), respectively.
>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), constructed of a 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 nanoconfined space for in situ growing well-defined COPs. By replicating the MOF-180 template, the 8380
DOI: 10.1021/acsnano.7b03807 ACS Nano 2017, 11, 8379−8386
Article
ACS Nano
Figure 3. ORR electrocatalytic performances of catalysts. (a) CV curves of MOF-900, COP-TPP(Fe)@MOF-900, and COP-TPP(Fe)-900 at a scan rate of 100 mv s−1. (b) LSV curves and (c) corresponding Tafel plots of Pt/C, MOF-900, COP-TPP(Fe)@MOF-900, and COP-TPP(Fe)900 at a scan rate of 5 mv s−1 with 1600 rpm. (d) The electron-transfer number per oxygen molecule on various catalysts at 0.6 V versus RHE: ① for MOF-900, ② for COP-TPP(Fe)-900, ③ for COP-TPP(Fe)@MOF-900, and ④ for Pt/C. (e) LSV curves of COP-TPP(Fe)@MOF-900 before and after 5000 cycles, respectively. Inset shows the corresponding Tafel plots of COP-TPP(Fe)@MOF-900 before and after 5,000 potential cycles. (f) The capacitive current at 1.1 V versus RHE at various scan rates for COP-TPP(Fe)@MOF-900 and COP-TPP(Fe)-900, respectively.
using MOF-180 as the confined space template (Figure 2a), compared with severely stacked morphology for the templatefree COP-TPP (Figure 2d). Moreover, COP-TPP@MOF showed a more porous texture with respect to COP-TPP because of the use of the MOF-180 templet with a high porosity. The BET specific surface area of COP-TPP also increased from 790 to 955 m2 g−1 by growing in the MOF template (Table S3). The as-synthesized COP-TPP@MOF was further incorporated with metal (Fe and/or Co), followed by carbonization to improve its electrocatalytic activity. The prepared electrocatalysts were termed as COP-TPP@MOF (Fe and/or Co)-X (X refers to the carbonization temperature). The reference samples were also prepared free from the MOF templet and designated as COP-TPP (Fe and/or Co)-X (X refers to the carbonization temperature). Transmission electron microscope (TEM) images revealed that COP-TPP(Fe)@MOF-900 contains more uniform and much smaller (
