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Bottom-Up Construction of Triazine-Based Frameworks as Metal-Free Electrocatalysts for Oxygen Reduction Reaction Long Hao, Shuangshuang Zhang, Rongji Liu, Jing Ning, Guangjin Zhang,* and Linjie Zhi*
Electrochemical oxygen reduction reaction (ORR) is a critical process that limits the commercialization of fuel cells and metal–air batteries.[1] Only when the expensive platinum (Pt)based electrocatalysts for ORR are replaced by more-efficient, cost-effective, and stable catalysts, the corresponding energystorage devices can find their reasonable roles in real life.[2] Over the past few years, tremendous efforts have been made to overcome this problem, among which heteroatom-doped (e.g., N-,[3] B-,[4,5] F-,[6] P-,[7] and S-[8–10] doped) carbon materials have attracted great attention due to their metal-free nature and promising catalytic performance. As a new type of electrochemical catalysts, their catalytic mechanism is highly attractive. Recent studies reveal that the electrochemical activities of these metal-free catalysts can be attributed to the charge polarization, which derives from the difference of electron negativities between carbon and heteroatoms.[4,10] In addition, some heteroatoms have various configurations, such as nitrogen atom which contains pyridinic nitrogen, pyrrolic nitrogen, and quaternary nitrogen,[11] leading to probably different catalytic activities. Meanwhile, ORR is determined by multiple parameters like electron transfer number, onset potential, etc., which can be, respectively, influenced by different heteroatoms.[12] All of these need a deeper understanding of the metal-free catalysts, which are essentially highly dependent on the precise control of the catalyst structures. However, traditional heteroatom-doped carbon materials, like heteroatom-doped carbon nanotubes, reduced graphene oxide, mesoporous carbons, etc., usually have random structures with high uncertainty, which obviously cannot meet the requirement. Consequently, materials with more-accurate structures are required to further study the metal-free catalysis in ORR. The development of porous organic networks (PONs) provides a great opportunity for the controllable construction of
L. Hao, J. Ning, Prof. L. Zhi Key Laboratory of Nanosystem and Hierarchical Fabrication National Center for Nanoscience and Technology 100190 Beijing, China E-mail: [email protected] L. Hao, S. Zhang, J. Ning University of Chinese Academy of Sciences Beijing 100039, China S. Zhang, Dr. R. Liu, Prof. G. Zhang Key Laboratory of Green Process and Engineering Institute of Process Engineering, Chinese Academy of Sciences 100190 Beijing, China E-mail: [email protected]
DOI: 10.1002/adma.201500863
Adv. Mater. 2015, DOI: 10.1002/adma.201500863
such kinds of catalysts.[13] PONs are 2D/3D materials built by the polymerization of rigid organic molecules.[14] Heteroatoms can be controllably introduced into the networks by using heteroatom-containing molecular building blocks.[15,16] Notably, PONs also have high specific surface areas (SSAs) and abundant pore distributions, which are also favorable to increase the catalytic active sites. In addition, our recent studies demonstrate that the conductivity of PONs can be significantly improved by elevating their polymerization temperatures, during which time their basic network skeletons can still be maintained.[17] All these extraordinary properties make PONbased materials a great platform for more accurate studying the metal-free catalysis in ORR and optimizing their performances at the same time. Herein, a novel ORR catalyst, a thermalized triazine-based framework (TTF), is successfully developed by a bottom-up approach using a nitrogen-containing molecule, terephthalonitrile, as the basic building block and through first trimerization into a 2D covalent triazine-based framework (CTF),[16,18] and subsequent thermal transformation to a 3D porous structure with enhanced conductivity (Scheme 1). These TTFs show increasing electrocatalytic activities along with the increase of the thermalized temperatures. More interestingly, this bottom-up strategy affords not only great opportunity to tune the nitrogen configurations of TTFs but possibilities to further dope the nitrogen-containing structure with boron (B) or fluorine (F). With this unique structure flexibility, we can observe the nitrogen configuration effect, which reveals that the ORR activities are dependent on the changes of nitrogen configurations and the quaternary nitrogen (Niii) seems to be the most favorable nitrogen configuration for ORR. In addition, different heteroatoms have disparate influences to the parameters of ORR catalysts: boron doping can improve the catalytic selectivity, while fluorine doping can enhance the catalytic activity. The construction of the triazine-based framework was first carried out at different temperatures from 400 to 700 °C (see details in the Experimental Section), and the as-prepared catalysts are labeled as TTF-400, TTF-500, TTF-600, and TTF-700. The transmission electron microscopy (TEM) images (Figure S1, Supporting Information, and Figure 1a) demonstrate the highly porous nature of the TTFs, and nitrogen atoms are distributed homogeneously in the structure matrix. Elemental analysis (EA) shows that the nitrogen contents of these TTFs are from 12.44 to 5.75 wt% (Figure 1b and Table S1, Supporting Information). The nitrogen adsorption/desorption measurements reveal that their Brunauer–Emmett–Teller (BET) SSAs are from 1175 to 2237 m2 g−1 (Figure 1b and Table S2, Supporting Information), and their pore sizes gradually become bigger with the increase of thermalized temperatures
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Scheme 1. The bottom-up construction of metal-free catalysts for ORR: the terephthalonitrile is trimerized into 2D CTF, and then evolved into 3D TTF at higher temperatures. The N-doped nature and the enhanced conductivity make TTF great candidate for ORR. The ratios of nitrogen configurations (Ni, Nii, and Niii) of TTF can be tuned through adjusting the reaction time, and other heteroatoms (like B and F) can be further doped into the TTF, which are then used to study the effects of nitrogen configurations and different heteroatoms, respectively.
(Figure S2, Supporting Information). Additionally, the X-ray photoelectron spectroscopy (XPS) N1s deconvolution analyses (Figure S3, Supporting Information) show that the TTFs contain three types of nitrogen configurations: pyridinic nitrogen (Ni), pyrrolic nitrogen (Nii), and quaternary nitrogen (Niii),[19] and the ratio of Ni drops with the increase of thermalized temperature, while the ratios of Nii and Niii raise (Figure 1c and Table S3, Supporting Information). More importantly, the conductivities of these TTFs are dramatically increased along with the temperature ascension (see the Nyquist plots, and the inserted equivalent series resistances in Figure 1d).[20] All these inherent unique structures of TTFs coupled with the enhanced conductivities make them great candidates as catalysts for ORR.
The ORR activities of these TTFs obtained at different temperatures were first evaluated by the cyclic voltammetry (CV) in O2- and Ar-saturated 0.1 M KOH solution (details in the Supporting Information). They all exhibit catalytic activities in the O2-saturated electrolyte; particularly, along with the increase of the thermalized temperature, the onset potential and reduction peak potential both shift positively, and the peak current density becomes bigger as well (Figure 1e), which means higher temperature-treated TTFs provide lower overpotential toward ORR and higher catalytic activity. The linear sweep voltammogram (LSV) curves from rotating disk electrode (RDE) analyses further confirm the above observation (Figure 1f); the TTFs prepared at the highest temperature (TTF-700) show the most
Figure 1. The structural and electrochemical characterization results of TTFs prepared at different temperatures (TTF-400, TTF-500, TTF-600, and TTF700): a) typical TEM image of TTF-700 with elemental mapping images inserted; b) nitrogen contents from elemental analysis (EA) and BET specific surface areas; c) the ratios of different nitrogen configurations determined from the XPS N1s deconvolution results; d) Nyquist plots with frequency ranges from 100 000 to 0.01 Hz (the inserted equivalent series resistances are extracted at 10 Hz); e) CV curves in Ar- and O2-saturated 0.1 M KOH solution (scanning rate: 10 mV s−1); f) linear sweep voltammograms (LSV) curves in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1; rotation rate: 1600 rpm).
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positive onset potential (Eonset) of 0.822 V versus RHE, and the biggest diffusion limiting current density (id) of 4.63 mA cm−2 (details in Table S4, Supporting Information). Since the thermal treatment leads to the significant increase of the conductivities, SSAs and obvious decrease of the nitrogen contents in the obtained TTFs, it is reasonable to conclude that the conductivity plays more important role in enhancing the catalytic activities of TTFs in ORR. This is also easy to understand, because the electrochemical active sites (such as the nitrogen-containing structures) cannot perform their function unless the electrons can be smoothly transferred from the active sites to the current collectors, which means a highly conductive media is critically important for the ORR catalysts.[21] The high surface area will further improve the nanoscale channels for ion and charge transport which together with the electron conductive media can afford excellent microenvironment for catalytic reactions of ORR. Additionally, different nitrogen configurations in TTFs may also play different roles in ORR, which triggers the following study of the effect of different nitrogen configurations. Interestingly, the bottom-up strategy developed here provides a great opportunity to further tune the nitrogen configurations of TTF. At the same reaction temperature (700 °C), two extreme reaction times, 1 and 96 h were used to further control the extent of the crosslinking reactions, so as to tune the nitrogen configurations of the corresponding TTFs (details in the Experimental Section), and the as-prepared materials are marked as TTF-700-1 and TTF-700-96, respectively. The XPS N1s deconvolution results (Figure 2a) demonstrate that the TTFs obtained at two different reaction times
(TTF-700-1 and TTF-700-96) both contain the three types of nitrogen configurations: Ni, Nii, and Niii. As the reaction time prolongs, the ratios of Ni and Nii gradually decrease, while the ratio of Niii increases (Figure 2b and Table S3, Supporting Information); meanwhile, the total nitrogen content drops from 9.3 to 4.1 wt% (Figure 2c and Table S1, Supporting Information). Additionally, the nitrogen adsorption/desorption analysis reveals that the increase of the SSA is quite small while the reaction time is prolonged, with 2597 m2 g−1 for TTF-700-1 and 2849 m2 g−1 for TTF-700-96 (Figure 2c), and their pore size distributions are almost the same (Figure S4, Supporting Information). All these structure characterizations indicate that the basic skeletons of the TTFs obtained at different reaction times barely change, however, the nitrogen configurations are changed significantly. It is also very interesting that the conductivities of TTF-700-1 and TTF-700-96 are very close (Figure S5, Supporting Information). The catalytic activities and kinetics of the TTF obtained at different reaction times were then analyzed with a RDE. The obtained LSV curves (Figure 2d) show that TTF-700-96 has more positive Eonset and higher id than TTF-700-1 (details in Table S4, Supporting Information), which means TTF-700-96 has better catalytic activity on ORR. The electron transfer numbers (n) of the catalysts are also calculated based on the RDE tests at different rotating speeds and the corresponding Koutecky–Levich (K–L) plots (Figure 2e and Figure S6, Supporting Information). The bigger electron transfer numbers of TTF700-96 at different potentials than TTF-700-1 (Figure 2f), suggest that TTF-700-96 has higher selectivity toward total oxygen reduction, and the ORR catalyzed by TTF-700-96 is mainly dominated by the one-step, four-electron pathway.
Figure 2. The structural and electrochemical characterization results of TTFs prepared at different reaction times (TTF-700-1 and TTF-700-96): a) XPS N1s deconvolution results; b) the ratios of different nitrogen configurations; c) nitrogen contents from EA and BET specific surface areas; d) LSV curves in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1; rotation rate: 1600 rpm); e) LSV curves of TTF-700-96 obtained from the RDE tests at different rotating rates with the corresponding Koutecky–Levich (K–L) plots inserted; f) the electron transfer numbers calculated based on the K–L equations.
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Considering the different nitrogen configurations of the TTFs, it can be recognized that the catalyst with the higher ratio of quaternary nitrogen (Niii), TTF-700-96 has better overall performances for ORR, although its nitrogen content is much lower (Figure 2c), which means that the nitrogen configuration is a critical influence factor to the electrocatalytic performance, and in this case, the Niii is the most favorable nitrogen configuration for ORR. The above study is based on the TTFs with only N-doped structures. Actually, different heteroatoms may have different effects on ORR. Consequently, another two kinds of heteroatoms with different electronegativity (χ), boron (χ = 2.04) and fluorine (χ = 3.98) were selected to further dope the TTF. TTF-700-96 which had the best electrochemical performance was used as the raw material and comparison at the same time. The as-prepared materials were labeled as TTF-B and TTF-F, respectively (see details in the Experimental Section). The XPS analyses of TTF-B and TTF-F indicate that apart from the N atoms, the B atoms and F atoms can be feasibly doped into the TTF, and the B content of TTF-B is 1.28 at% and the F content of TTF-F is 1.62 at%. The dominant peak at about 189.6 eV in the B1s XPS spectrum is related to BC3, and the other two peaks at about 191.2 and 192.2 eV are corresponding to BC2O and BCO2, respectively;[4] the peak at about 686.04 eV in the F1s spectrum is corresponding to the ionic C–F (deconvolution XPS spectra in Figure 3a).[6] The doping of B/F also leads to the changes of nitrogen configurations: the ratios of Niii in TTF-B and TTF-F are both higher than that in TTF-700-96, and the ratios of Ni and Nii are also changed (Figure 3b); however, their total contents of nitrogen
keep almost constant (Figure 3c). The TEM mapping analyses indicate that all the heteroatoms are distributed homogeneously in the corresponding materials (Figure S7, Supporting Information). The nitrogen adsorption/desorption measurements reveal that the pore-size distributions of TTF-B and TTF-F are almost the same as that of TTF-700-96 (Figure S4b, Supporting Information), and the SSA of TTF-F (2570 m2 g−1) is comparable to that of TTF-700-96 (2849 m2 g−1), however, the SSA of TTF-B (1694 m2 g−1) is much lower (Table S2, Supporting Information). In addition, the conductivities of TTF-B and TTF-F are at the same level of TTF-700-96 (Figure S5, Supporting Information). The catalytic performances of these heteroatom-doped TTFs were first tested by a RDE, then the rotating ring-disk electrode (RRDE) was used to further verify the ORR pathway by monitoring the formation of intermediate peroxide species. As the LSV curves shown in Figure 3d, the TTF-B exhibits a slight enhancement of the ORR catalytic activity with almost the same Eonset and id in comparison to that of TTF-700-96, which may be caused by the decrease of the SSA after the B-doping (Figure 3c). However, a significant drop of HO2− yield is observed on TTF-B (7–10%) than on TTF-700-96 (15–30%) (Figure 3e); the corresponding electron transfer number of TTF-B is also bigger than that of TTF-700-96 (Figure 3f), which are very close to that of the commercial 20% Pt/C, indicating that B-doping has remarkably increased the four-electron selectivity of the catalysts. As for the F-doped catalyst, a significant enhancement of the ORR catalytic activity is observed. As shown in Figure 3d, the half-wave potential of TTF-F reaches 0.767 V which is very close to 0.78 V of the commercial 20% Pt/C catalyst; and the
Figure 3. The structural and electrochemical characterizations of further doped TTFs (TTF-B and TTF-F) with TTF-700-96 as a comparison: a) XPS deconvolution results of the heteroatoms; b) the ratios of different nitrogen configurations; c) nitrogen contents from EA and BET specific surface area; d) LSV curves in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1; rotation rate: 1600 rpm); e,f) the HO2− yields and electron transfer numbers obtained through the rotating ring-disk electrode measurements.
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id is 6.15 mA cm−2 which has surpassed that of Pt/C catalyst, 5.4 mA cm−2. Additionally, the HO2− yield of TTF-F also decreases to 15% (Figure 3e), and the corresponding electron transfer number increases to 3.7 (Figure 3f). These catalytic performances of TTF-F are comparable to most of the advanced catalysts reported in the literature (Table S5, Supporting Information).[4,22] These electrochemical testing results of the heteroatomdoped TTFs demonstrate that different heteroatoms have disparate effects to the ORR performance: the B-doped catalyst (TTF-B) has higher catalytic selectivity with lower HO2− yield and better four-electron selectivity; while the F-doped catalyst (TTF-F) shows better catalytic activity with more positive Eonset, half-wave potential, and bigger id. All of these can be caused by the changes of the chemical environments around the active sites of the ORR catalysts, such as the most commonly believed carbon-atom active sites.[4,8,12,23] In other words, herein the doping of B/F causes the altering of the chemical states of the carbon atoms in TTF-700-96, including the transformation of the nitrogen configurations (Figure 3b), finally leading to the enhancement of the catalytic performances. This also suggests that in order to optimize the performances of metalfree catalysts, the effects of different heteroatoms including the principles behind them should be clarified, which is highly dependent on the precise control of the chemical structures of the catalysts, and the bottom-up approach has shown its unique advantages. Furthermore, the stability and poison tolerance of the catalysts are also very important for practical applications. Consequently, the stability of the TTF-F-based catalyst was first assessed with the current–time (i–t) chronoamperometric responses,[10,24] and commercial 20% Pt/C catalyst was used as the reference. It reveals that the TTF-F shows no apparent attenuation during a 40 000 s test; and in contrast, the commercial Pt/C was reduced to ≈70% (Figure 4a). The catalyst was then exposed to methanol to test its tolerance to methanol. When 3 M methanol was added, almost no response was observed for TTF-F, but the catalytic activity of 20% Pt/C severely drops. These results indicate that the TTF-F catalyst has better stability and poison tolerance for ORR than the commercial Pt/C catalyst. In summary, an efficient metal-free electrocatalyst for ORR has been developed successfully with a bottom-up strategy through trimerization of terephthalonitrile and subsequent thermalizing. The as-prepared TTF holds not only highly conductive frameworks and nanoscale channels but nitrogen-rich structures and tunable nitrogen configurations. More interestingly, multi-heteroatom doping can be feasibly realized by this approach, resulting in an ideal platform for deeper studying the structure–property relationships of metal-free catalysts for ORR. On this novel platform, we observe that the nitrogen configurations can greatly influence the electrocatalytic performances of the catalysts, and quaternary nitrogen (Niii) can be the most efficient nitrogen configuration for ORR. In addition, different heteroatoms have disparate influences to the parameters of ORR catalysts: boron doping can improve the catalytic selectivity, while fluorine doping can enhance the catalytic activity. Obviously, the rational design of catalysts via bottom-up strategies developed in this work opens up a new avenue for
Figure 4. a) The stability and b) methanol-tolerance evaluation of TTF-F tested by the current–time chronoamperometric responses at 0.6 V versus RHE in O2-saturated 0.1 M KOH solution (commercial 20% Pt/C is used for comparison). The arrow in (b) represents the addition of 3 M methanol into the electrolyte.
detailed exploration of the electrochemical mechanisms of metal-free catalysts, as well as developing high-performance catalysts for practical applications of ORR.
Experimental Section Synthesis of TTF-400, TTF-500, TTF-600, and TTF-700: Terephthalonitrile (1 g, 7.8 mmol) and anhydrous zinc chloride (5.32 g, 39 mmol) were mixed in a glove box (argon with 0.1 ppm oxygen and 0.1 ppm water) and transferred into a quartz ampoule (Ø 17 × 180 mm), sealed and heated at the designated temperatures (400, 500, 600, and 700 °C) for 40 h. Then the ampule was cooled to the room temperature and opened. The black complex inside was washed thoroughly with 5% HCl solution, deionized water, and then dried under vacuum at 120 °C for 12 h to get TTF-400, TTF-500, TTF-600, and TTF-700. Synthesis of TTF-700-1 and TTF-700-96: TTF-700-1 and TTF-700-96 were synthesized under a similar procedure as above, but heated at different times: 400 °C for 20 h and then 700 °C for 1 h (for TTF-700-1); 400 °C for 20 h and then 700 °C for 96 h (for TTF-700-96). Synthesis of TTF-B, TTF-F: TTF-700-96 was used as the raw material to synthesize TTF-B and TTF-F. For TTF-B: TTF-700-96 (100 mg), anhydrous zinc chloride (1 g), and boron oxide (64 mg) were mixed in an agate mortar in a glove box and transferred into a quartz boat, then heated at 600 °C under an argon atmosphere for 5 h. After cooling down to room temperature, the complex was washed thoroughly with 5% HCl solution, deionized water, and then dried under vacuum at 120 °C for 12 h to get
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www.MaterialsViews.com TTF-B. For TTF-F: TTF-700-96 (100 mg) and ammonium fluoride (2 g) were put into a beaker with 20 mL of deionized water, and sonicated for 2 h. Then the deposition in the beaker was collected by centrifugation, dried under vacuum at 40 °C for 12 h, transferred into a quartz boat, heated at 600 °C under argon atmosphere for 2 h, then cooled down to room temperature to obtain TTF-F. Electrochemical Measurements: The electrochemical measurements were carried out on an electrochemical biopotentiostat (CHI 760e, CH Instrument, Shanghai, China) with a typical three-electrode system. A Ag/AgCl electrode (+0.949 V vs RHE) was used as the reference electrode, a platinum wire as the counter electrode, and the TTFmodified glassy carbon electrode (GCE) as the working electrode. The working electrodes were prepared by dropping each of the catalyst inks onto a prepolished GCE. Typically, the TTF was dispersed into ethanol and sonicated for 30 min to form a uniform catalyst ink (5 mg mL−1). A total of 7.5 µL of the catalyst ink (containing 37.5 µg of catalyst) was loaded onto a mirror polished GCE (4 mm in diameter), giving a loading density of 0.3 mg cm–2. After the solvent was evaporated, a thin layer of Nafion solution (0.5 wt%) was coated onto the electrode surface. The prepared electrode was dried at room temperature overnight before electrochemical tests. For comparison, GCE coated with Pt/C (20 wt% Pt on Vulcan XC-72, purchased from Alfa Aesar) was also fabricated with the same procedure, and the loading density is 50 µg cm−2.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements L.H. and S.Z. contributed equally to this work. The authors acknowledge the support from the Ministry of Science and Technology of China (Nos. 2012CB933403 and 2012CB932504), the National Natural Science Foundation of China (Grant Nos. 51425302, 21371173, and 21173057), the Sino-German Centre for Research Promotion (GZ879), and the Chinese Academy of Sciences. Received: February 18, 2015 Revised: March 14, 2015 Published online:
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[6] X. Sun, Y. Zhang, P. Song, J. Pan, L. Zhuang, W. Xu, W. Xing, ACS Catal. 2013, 3, 1726. [7] a) C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Adv. Mater. 2013, 25, 4932; b) C. H. Choi, S. H. Park, S. I. Woo, ACS Nano 2012, 6, 7084; c) Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano 2014, 8, 5290. [8] J. Liang, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2012, 51, 11496. [9] a) I. Y. Jeon, H. J. Choi, S. M. Jung, J. M. Seo, M. J. Kim, L. Dai, J. B. Baek, J. Am. Chem. Soc. 2013, 135, 1386; b) Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. A. Chen, S. Huang, ACS Nano 2011, 6, 205. [10] I. Y. Jeon, S. Zhang, L. Zhang, H. J. Choi, J. M. Seo, Z. Xia, L. Dai, J. B. Baek, Adv. Mater. 2013, 25, 6138. [11] a) Z. S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. Feng, K. Mullen, Adv. Mater. 2012, 24, 5130; b) X. Zhuang, F. Zhang, D. Wu, X. Feng, Adv. Mater. 2014, 26, 3081; c) S. Yang, X. Feng, X. Wang, K. Müllen, Angew. Chem. Int. Ed. 2011, 50, 5339; d) H. W. Liang, X. Zhuang, S. Bruller, X. Feng, K. Mullen, Nat. Commun. 2014, 5, 4973. [12] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, J. Am. Chem. Soc. 2014, 136, 4394. [13] Z. Xiang, D. Cao, L. Huang, J. Shui, M. Wang, L. Dai, Adv. Mater. 2014, 26, 3315. [14] a) R. Dawson, A. I. Cooper, D. J. Adams, Prog. Polym. Sci. 2012, 37, 530; b) A. Thomas, Angew. Chem. Int. Ed. 2010, 49, 8328; c) Y. Xu, S. Jin, H. Xu, A. Nagai, D. Jiang, Chem. Soc. Rev. 2013, 42, 8012. [15] a) J. Schmidt, J. Weber, J. D. Epping, M. Antonietti, A. Thomas, Adv. Mater. 2009, 21, 702; b) S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452; c) A. P. Cote, A. I. Benin, N. W. Ockwig, M. O’Keeffe, A. J. Matzger, O. M. Yaghi, Science 2005, 310, 1166; d) L. Hao, X. Li, L. Zhi, Adv. Mater. 2013, 25, 3899; e) Y. Kou, Y. Xu, Z. Guo, D. Jiang, Angew. Chem. Int. Ed. 2011, 50, 8753. [16] P. Kuhn, M. Antonietti, A. Thomas, Angew. Chem. Int. Ed. 2008, 47, 3450. [17] L. Hao, B. Luo, X. L. Li, M. H. Jin, Y. Fang, Z. H. Tang, Y. Y. Jia, M. H. Liang, A. Thomas, J. H. Yang, L. J. Zhi, Energy Environ. Sci. 2012, 5, 9747. [18] P. Kuhn, A. Thomas, M. Antonietti, Macromolecules 2008, 42, 319. [19] a) H. M. Jeong, J. W. Lee, W. H. Shin, Y. J. Choi, H. J. Shin, J. K. Kang, J. W. Choi, Nano Lett. 2011, 11, 2472; b) L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin, R. S. Ruoff, Energy Environ. Sci. 2012, 5, 7936; c) W. Ding, Z. Wei, S. Chen, X. Qi, T. Yang, J. Hu, D. Wang, L. J. Wan, S. F. Alvi, L. Li, Angew. Chem. Int. Ed. 2013, 52, 11755. [20] E. J. Ra, E. Raymundo-Piñero, Y. H. Lee, F. Béguin, Carbon 2009, 47, 2984. [21] C. Wang, Z. Guo, W. Shen, Q. Xu, H. Liu, Y. Wang, Adv. Funct. Mater. 2014, 24, 5511. [22] a) W. Ai, Z. Luo, J. Jiang, J. Zhu, Z. Du, Z. Fan, L. Xie, H. Zhang, W. Huang, T. Yu, Adv. Mater. 2014, 26, 6186; b) R. Silva, D. Voiry, M. Chhowalla, T. Asefa, J. Am. Chem. Soc. 2013, 135, 7823; c) H. Zhang, Y. Wang, D. Wang, Y. Li, X. Liu, P. Liu, H. Yang, T. An, Z. Tang, H. Zhao, Small 2014, 10, 3371; d) J. Wu, C. Jin, Z. Yang, J. Tian, R. Yang, Carbon 2015, 82, 562; e) G.-L. Tian, Q. Zhang, B. Zhang, Y.-G. Jin, J.-Q. Huang, D. S. Su, F. Wei, Adv. Funct. Mater. 2014, 24, 5956; f) S. Dou, A. Shen, L. Tao, S. Wang, Chem. Commun. 2014, 50, 10672; g) M. Favaro, L. Ferrighi, G. Fazio, L. Colazzo, C. Di Valentin, C. Durante, F. Sedona, A. Gennaro, S. Agnoli, G. Granozzi, ACS Catal. 2015, 5, 129. [23] a) S. Wang, L. Zhang, Z. Xia, A. Roy, D. W. Chang, J. B. Baek, L. Dai, Angew. Chem. Int. Ed. 2012, 51, 4209; b) J. Y. Cheon, J. H. Kim, J. H. Kim, K. C. Goddeti, J. Y. Park, S. H. Joo, J. Am. Chem. Soc. 2014, 136, 8875. [24] T. N. Ye, L. B. Lv, X. H. Li, M. Xu, J. S. Chen, Angew. Chem. Int. Ed. 2014, 53, 6905.
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Bottom-Up Construction of Triazine-Based Frameworks as Metal-Free Electrocatalysts for Oxygen Reduction Reaction Long Hao, Shuangshuang Zhang, Rongji Liu, Jing Ning, Guangjin Zhang,* and Linjie Zhi*
Electrochemical oxygen reduction reaction (ORR) is a critical process that limits the commercialization of fuel cells and metal–air batteries.[1] Only when the expensive platinum (Pt)based electrocatalysts for ORR are replaced by more-efficient, cost-effective, and stable catalysts, the corresponding energystorage devices can find their reasonable roles in real life.[2] Over the past few years, tremendous efforts have been made to overcome this problem, among which heteroatom-doped (e.g., N-,[3] B-,[4,5] F-,[6] P-,[7] and S-[8–10] doped) carbon materials have attracted great attention due to their metal-free nature and promising catalytic performance. As a new type of electrochemical catalysts, their catalytic mechanism is highly attractive. Recent studies reveal that the electrochemical activities of these metal-free catalysts can be attributed to the charge polarization, which derives from the difference of electron negativities between carbon and heteroatoms.[4,10] In addition, some heteroatoms have various configurations, such as nitrogen atom which contains pyridinic nitrogen, pyrrolic nitrogen, and quaternary nitrogen,[11] leading to probably different catalytic activities. Meanwhile, ORR is determined by multiple parameters like electron transfer number, onset potential, etc., which can be, respectively, influenced by different heteroatoms.[12] All of these need a deeper understanding of the metal-free catalysts, which are essentially highly dependent on the precise control of the catalyst structures. However, traditional heteroatom-doped carbon materials, like heteroatom-doped carbon nanotubes, reduced graphene oxide, mesoporous carbons, etc., usually have random structures with high uncertainty, which obviously cannot meet the requirement. Consequently, materials with more-accurate structures are required to further study the metal-free catalysis in ORR. The development of porous organic networks (PONs) provides a great opportunity for the controllable construction of
L. Hao, J. Ning, Prof. L. Zhi Key Laboratory of Nanosystem and Hierarchical Fabrication National Center for Nanoscience and Technology 100190 Beijing, China E-mail: [email protected] L. Hao, S. Zhang, J. Ning University of Chinese Academy of Sciences Beijing 100039, China S. Zhang, Dr. R. Liu, Prof. G. Zhang Key Laboratory of Green Process and Engineering Institute of Process Engineering, Chinese Academy of Sciences 100190 Beijing, China E-mail: [email protected]
DOI: 10.1002/adma.201500863
Adv. Mater. 2015, DOI: 10.1002/adma.201500863
such kinds of catalysts.[13] PONs are 2D/3D materials built by the polymerization of rigid organic molecules.[14] Heteroatoms can be controllably introduced into the networks by using heteroatom-containing molecular building blocks.[15,16] Notably, PONs also have high specific surface areas (SSAs) and abundant pore distributions, which are also favorable to increase the catalytic active sites. In addition, our recent studies demonstrate that the conductivity of PONs can be significantly improved by elevating their polymerization temperatures, during which time their basic network skeletons can still be maintained.[17] All these extraordinary properties make PONbased materials a great platform for more accurate studying the metal-free catalysis in ORR and optimizing their performances at the same time. Herein, a novel ORR catalyst, a thermalized triazine-based framework (TTF), is successfully developed by a bottom-up approach using a nitrogen-containing molecule, terephthalonitrile, as the basic building block and through first trimerization into a 2D covalent triazine-based framework (CTF),[16,18] and subsequent thermal transformation to a 3D porous structure with enhanced conductivity (Scheme 1). These TTFs show increasing electrocatalytic activities along with the increase of the thermalized temperatures. More interestingly, this bottom-up strategy affords not only great opportunity to tune the nitrogen configurations of TTFs but possibilities to further dope the nitrogen-containing structure with boron (B) or fluorine (F). With this unique structure flexibility, we can observe the nitrogen configuration effect, which reveals that the ORR activities are dependent on the changes of nitrogen configurations and the quaternary nitrogen (Niii) seems to be the most favorable nitrogen configuration for ORR. In addition, different heteroatoms have disparate influences to the parameters of ORR catalysts: boron doping can improve the catalytic selectivity, while fluorine doping can enhance the catalytic activity. The construction of the triazine-based framework was first carried out at different temperatures from 400 to 700 °C (see details in the Experimental Section), and the as-prepared catalysts are labeled as TTF-400, TTF-500, TTF-600, and TTF-700. The transmission electron microscopy (TEM) images (Figure S1, Supporting Information, and Figure 1a) demonstrate the highly porous nature of the TTFs, and nitrogen atoms are distributed homogeneously in the structure matrix. Elemental analysis (EA) shows that the nitrogen contents of these TTFs are from 12.44 to 5.75 wt% (Figure 1b and Table S1, Supporting Information). The nitrogen adsorption/desorption measurements reveal that their Brunauer–Emmett–Teller (BET) SSAs are from 1175 to 2237 m2 g−1 (Figure 1b and Table S2, Supporting Information), and their pore sizes gradually become bigger with the increase of thermalized temperatures
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Scheme 1. The bottom-up construction of metal-free catalysts for ORR: the terephthalonitrile is trimerized into 2D CTF, and then evolved into 3D TTF at higher temperatures. The N-doped nature and the enhanced conductivity make TTF great candidate for ORR. The ratios of nitrogen configurations (Ni, Nii, and Niii) of TTF can be tuned through adjusting the reaction time, and other heteroatoms (like B and F) can be further doped into the TTF, which are then used to study the effects of nitrogen configurations and different heteroatoms, respectively.
(Figure S2, Supporting Information). Additionally, the X-ray photoelectron spectroscopy (XPS) N1s deconvolution analyses (Figure S3, Supporting Information) show that the TTFs contain three types of nitrogen configurations: pyridinic nitrogen (Ni), pyrrolic nitrogen (Nii), and quaternary nitrogen (Niii),[19] and the ratio of Ni drops with the increase of thermalized temperature, while the ratios of Nii and Niii raise (Figure 1c and Table S3, Supporting Information). More importantly, the conductivities of these TTFs are dramatically increased along with the temperature ascension (see the Nyquist plots, and the inserted equivalent series resistances in Figure 1d).[20] All these inherent unique structures of TTFs coupled with the enhanced conductivities make them great candidates as catalysts for ORR.
The ORR activities of these TTFs obtained at different temperatures were first evaluated by the cyclic voltammetry (CV) in O2- and Ar-saturated 0.1 M KOH solution (details in the Supporting Information). They all exhibit catalytic activities in the O2-saturated electrolyte; particularly, along with the increase of the thermalized temperature, the onset potential and reduction peak potential both shift positively, and the peak current density becomes bigger as well (Figure 1e), which means higher temperature-treated TTFs provide lower overpotential toward ORR and higher catalytic activity. The linear sweep voltammogram (LSV) curves from rotating disk electrode (RDE) analyses further confirm the above observation (Figure 1f); the TTFs prepared at the highest temperature (TTF-700) show the most
Figure 1. The structural and electrochemical characterization results of TTFs prepared at different temperatures (TTF-400, TTF-500, TTF-600, and TTF700): a) typical TEM image of TTF-700 with elemental mapping images inserted; b) nitrogen contents from elemental analysis (EA) and BET specific surface areas; c) the ratios of different nitrogen configurations determined from the XPS N1s deconvolution results; d) Nyquist plots with frequency ranges from 100 000 to 0.01 Hz (the inserted equivalent series resistances are extracted at 10 Hz); e) CV curves in Ar- and O2-saturated 0.1 M KOH solution (scanning rate: 10 mV s−1); f) linear sweep voltammograms (LSV) curves in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1; rotation rate: 1600 rpm).
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positive onset potential (Eonset) of 0.822 V versus RHE, and the biggest diffusion limiting current density (id) of 4.63 mA cm−2 (details in Table S4, Supporting Information). Since the thermal treatment leads to the significant increase of the conductivities, SSAs and obvious decrease of the nitrogen contents in the obtained TTFs, it is reasonable to conclude that the conductivity plays more important role in enhancing the catalytic activities of TTFs in ORR. This is also easy to understand, because the electrochemical active sites (such as the nitrogen-containing structures) cannot perform their function unless the electrons can be smoothly transferred from the active sites to the current collectors, which means a highly conductive media is critically important for the ORR catalysts.[21] The high surface area will further improve the nanoscale channels for ion and charge transport which together with the electron conductive media can afford excellent microenvironment for catalytic reactions of ORR. Additionally, different nitrogen configurations in TTFs may also play different roles in ORR, which triggers the following study of the effect of different nitrogen configurations. Interestingly, the bottom-up strategy developed here provides a great opportunity to further tune the nitrogen configurations of TTF. At the same reaction temperature (700 °C), two extreme reaction times, 1 and 96 h were used to further control the extent of the crosslinking reactions, so as to tune the nitrogen configurations of the corresponding TTFs (details in the Experimental Section), and the as-prepared materials are marked as TTF-700-1 and TTF-700-96, respectively. The XPS N1s deconvolution results (Figure 2a) demonstrate that the TTFs obtained at two different reaction times
(TTF-700-1 and TTF-700-96) both contain the three types of nitrogen configurations: Ni, Nii, and Niii. As the reaction time prolongs, the ratios of Ni and Nii gradually decrease, while the ratio of Niii increases (Figure 2b and Table S3, Supporting Information); meanwhile, the total nitrogen content drops from 9.3 to 4.1 wt% (Figure 2c and Table S1, Supporting Information). Additionally, the nitrogen adsorption/desorption analysis reveals that the increase of the SSA is quite small while the reaction time is prolonged, with 2597 m2 g−1 for TTF-700-1 and 2849 m2 g−1 for TTF-700-96 (Figure 2c), and their pore size distributions are almost the same (Figure S4, Supporting Information). All these structure characterizations indicate that the basic skeletons of the TTFs obtained at different reaction times barely change, however, the nitrogen configurations are changed significantly. It is also very interesting that the conductivities of TTF-700-1 and TTF-700-96 are very close (Figure S5, Supporting Information). The catalytic activities and kinetics of the TTF obtained at different reaction times were then analyzed with a RDE. The obtained LSV curves (Figure 2d) show that TTF-700-96 has more positive Eonset and higher id than TTF-700-1 (details in Table S4, Supporting Information), which means TTF-700-96 has better catalytic activity on ORR. The electron transfer numbers (n) of the catalysts are also calculated based on the RDE tests at different rotating speeds and the corresponding Koutecky–Levich (K–L) plots (Figure 2e and Figure S6, Supporting Information). The bigger electron transfer numbers of TTF700-96 at different potentials than TTF-700-1 (Figure 2f), suggest that TTF-700-96 has higher selectivity toward total oxygen reduction, and the ORR catalyzed by TTF-700-96 is mainly dominated by the one-step, four-electron pathway.
Figure 2. The structural and electrochemical characterization results of TTFs prepared at different reaction times (TTF-700-1 and TTF-700-96): a) XPS N1s deconvolution results; b) the ratios of different nitrogen configurations; c) nitrogen contents from EA and BET specific surface areas; d) LSV curves in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1; rotation rate: 1600 rpm); e) LSV curves of TTF-700-96 obtained from the RDE tests at different rotating rates with the corresponding Koutecky–Levich (K–L) plots inserted; f) the electron transfer numbers calculated based on the K–L equations.
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Considering the different nitrogen configurations of the TTFs, it can be recognized that the catalyst with the higher ratio of quaternary nitrogen (Niii), TTF-700-96 has better overall performances for ORR, although its nitrogen content is much lower (Figure 2c), which means that the nitrogen configuration is a critical influence factor to the electrocatalytic performance, and in this case, the Niii is the most favorable nitrogen configuration for ORR. The above study is based on the TTFs with only N-doped structures. Actually, different heteroatoms may have different effects on ORR. Consequently, another two kinds of heteroatoms with different electronegativity (χ), boron (χ = 2.04) and fluorine (χ = 3.98) were selected to further dope the TTF. TTF-700-96 which had the best electrochemical performance was used as the raw material and comparison at the same time. The as-prepared materials were labeled as TTF-B and TTF-F, respectively (see details in the Experimental Section). The XPS analyses of TTF-B and TTF-F indicate that apart from the N atoms, the B atoms and F atoms can be feasibly doped into the TTF, and the B content of TTF-B is 1.28 at% and the F content of TTF-F is 1.62 at%. The dominant peak at about 189.6 eV in the B1s XPS spectrum is related to BC3, and the other two peaks at about 191.2 and 192.2 eV are corresponding to BC2O and BCO2, respectively;[4] the peak at about 686.04 eV in the F1s spectrum is corresponding to the ionic C–F (deconvolution XPS spectra in Figure 3a).[6] The doping of B/F also leads to the changes of nitrogen configurations: the ratios of Niii in TTF-B and TTF-F are both higher than that in TTF-700-96, and the ratios of Ni and Nii are also changed (Figure 3b); however, their total contents of nitrogen
keep almost constant (Figure 3c). The TEM mapping analyses indicate that all the heteroatoms are distributed homogeneously in the corresponding materials (Figure S7, Supporting Information). The nitrogen adsorption/desorption measurements reveal that the pore-size distributions of TTF-B and TTF-F are almost the same as that of TTF-700-96 (Figure S4b, Supporting Information), and the SSA of TTF-F (2570 m2 g−1) is comparable to that of TTF-700-96 (2849 m2 g−1), however, the SSA of TTF-B (1694 m2 g−1) is much lower (Table S2, Supporting Information). In addition, the conductivities of TTF-B and TTF-F are at the same level of TTF-700-96 (Figure S5, Supporting Information). The catalytic performances of these heteroatom-doped TTFs were first tested by a RDE, then the rotating ring-disk electrode (RRDE) was used to further verify the ORR pathway by monitoring the formation of intermediate peroxide species. As the LSV curves shown in Figure 3d, the TTF-B exhibits a slight enhancement of the ORR catalytic activity with almost the same Eonset and id in comparison to that of TTF-700-96, which may be caused by the decrease of the SSA after the B-doping (Figure 3c). However, a significant drop of HO2− yield is observed on TTF-B (7–10%) than on TTF-700-96 (15–30%) (Figure 3e); the corresponding electron transfer number of TTF-B is also bigger than that of TTF-700-96 (Figure 3f), which are very close to that of the commercial 20% Pt/C, indicating that B-doping has remarkably increased the four-electron selectivity of the catalysts. As for the F-doped catalyst, a significant enhancement of the ORR catalytic activity is observed. As shown in Figure 3d, the half-wave potential of TTF-F reaches 0.767 V which is very close to 0.78 V of the commercial 20% Pt/C catalyst; and the
Figure 3. The structural and electrochemical characterizations of further doped TTFs (TTF-B and TTF-F) with TTF-700-96 as a comparison: a) XPS deconvolution results of the heteroatoms; b) the ratios of different nitrogen configurations; c) nitrogen contents from EA and BET specific surface area; d) LSV curves in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1; rotation rate: 1600 rpm); e,f) the HO2− yields and electron transfer numbers obtained through the rotating ring-disk electrode measurements.
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id is 6.15 mA cm−2 which has surpassed that of Pt/C catalyst, 5.4 mA cm−2. Additionally, the HO2− yield of TTF-F also decreases to 15% (Figure 3e), and the corresponding electron transfer number increases to 3.7 (Figure 3f). These catalytic performances of TTF-F are comparable to most of the advanced catalysts reported in the literature (Table S5, Supporting Information).[4,22] These electrochemical testing results of the heteroatomdoped TTFs demonstrate that different heteroatoms have disparate effects to the ORR performance: the B-doped catalyst (TTF-B) has higher catalytic selectivity with lower HO2− yield and better four-electron selectivity; while the F-doped catalyst (TTF-F) shows better catalytic activity with more positive Eonset, half-wave potential, and bigger id. All of these can be caused by the changes of the chemical environments around the active sites of the ORR catalysts, such as the most commonly believed carbon-atom active sites.[4,8,12,23] In other words, herein the doping of B/F causes the altering of the chemical states of the carbon atoms in TTF-700-96, including the transformation of the nitrogen configurations (Figure 3b), finally leading to the enhancement of the catalytic performances. This also suggests that in order to optimize the performances of metalfree catalysts, the effects of different heteroatoms including the principles behind them should be clarified, which is highly dependent on the precise control of the chemical structures of the catalysts, and the bottom-up approach has shown its unique advantages. Furthermore, the stability and poison tolerance of the catalysts are also very important for practical applications. Consequently, the stability of the TTF-F-based catalyst was first assessed with the current–time (i–t) chronoamperometric responses,[10,24] and commercial 20% Pt/C catalyst was used as the reference. It reveals that the TTF-F shows no apparent attenuation during a 40 000 s test; and in contrast, the commercial Pt/C was reduced to ≈70% (Figure 4a). The catalyst was then exposed to methanol to test its tolerance to methanol. When 3 M methanol was added, almost no response was observed for TTF-F, but the catalytic activity of 20% Pt/C severely drops. These results indicate that the TTF-F catalyst has better stability and poison tolerance for ORR than the commercial Pt/C catalyst. In summary, an efficient metal-free electrocatalyst for ORR has been developed successfully with a bottom-up strategy through trimerization of terephthalonitrile and subsequent thermalizing. The as-prepared TTF holds not only highly conductive frameworks and nanoscale channels but nitrogen-rich structures and tunable nitrogen configurations. More interestingly, multi-heteroatom doping can be feasibly realized by this approach, resulting in an ideal platform for deeper studying the structure–property relationships of metal-free catalysts for ORR. On this novel platform, we observe that the nitrogen configurations can greatly influence the electrocatalytic performances of the catalysts, and quaternary nitrogen (Niii) can be the most efficient nitrogen configuration for ORR. In addition, different heteroatoms have disparate influences to the parameters of ORR catalysts: boron doping can improve the catalytic selectivity, while fluorine doping can enhance the catalytic activity. Obviously, the rational design of catalysts via bottom-up strategies developed in this work opens up a new avenue for
Figure 4. a) The stability and b) methanol-tolerance evaluation of TTF-F tested by the current–time chronoamperometric responses at 0.6 V versus RHE in O2-saturated 0.1 M KOH solution (commercial 20% Pt/C is used for comparison). The arrow in (b) represents the addition of 3 M methanol into the electrolyte.
detailed exploration of the electrochemical mechanisms of metal-free catalysts, as well as developing high-performance catalysts for practical applications of ORR.
Experimental Section Synthesis of TTF-400, TTF-500, TTF-600, and TTF-700: Terephthalonitrile (1 g, 7.8 mmol) and anhydrous zinc chloride (5.32 g, 39 mmol) were mixed in a glove box (argon with 0.1 ppm oxygen and 0.1 ppm water) and transferred into a quartz ampoule (Ø 17 × 180 mm), sealed and heated at the designated temperatures (400, 500, 600, and 700 °C) for 40 h. Then the ampule was cooled to the room temperature and opened. The black complex inside was washed thoroughly with 5% HCl solution, deionized water, and then dried under vacuum at 120 °C for 12 h to get TTF-400, TTF-500, TTF-600, and TTF-700. Synthesis of TTF-700-1 and TTF-700-96: TTF-700-1 and TTF-700-96 were synthesized under a similar procedure as above, but heated at different times: 400 °C for 20 h and then 700 °C for 1 h (for TTF-700-1); 400 °C for 20 h and then 700 °C for 96 h (for TTF-700-96). Synthesis of TTF-B, TTF-F: TTF-700-96 was used as the raw material to synthesize TTF-B and TTF-F. For TTF-B: TTF-700-96 (100 mg), anhydrous zinc chloride (1 g), and boron oxide (64 mg) were mixed in an agate mortar in a glove box and transferred into a quartz boat, then heated at 600 °C under an argon atmosphere for 5 h. After cooling down to room temperature, the complex was washed thoroughly with 5% HCl solution, deionized water, and then dried under vacuum at 120 °C for 12 h to get
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www.MaterialsViews.com TTF-B. For TTF-F: TTF-700-96 (100 mg) and ammonium fluoride (2 g) were put into a beaker with 20 mL of deionized water, and sonicated for 2 h. Then the deposition in the beaker was collected by centrifugation, dried under vacuum at 40 °C for 12 h, transferred into a quartz boat, heated at 600 °C under argon atmosphere for 2 h, then cooled down to room temperature to obtain TTF-F. Electrochemical Measurements: The electrochemical measurements were carried out on an electrochemical biopotentiostat (CHI 760e, CH Instrument, Shanghai, China) with a typical three-electrode system. A Ag/AgCl electrode (+0.949 V vs RHE) was used as the reference electrode, a platinum wire as the counter electrode, and the TTFmodified glassy carbon electrode (GCE) as the working electrode. The working electrodes were prepared by dropping each of the catalyst inks onto a prepolished GCE. Typically, the TTF was dispersed into ethanol and sonicated for 30 min to form a uniform catalyst ink (5 mg mL−1). A total of 7.5 µL of the catalyst ink (containing 37.5 µg of catalyst) was loaded onto a mirror polished GCE (4 mm in diameter), giving a loading density of 0.3 mg cm–2. After the solvent was evaporated, a thin layer of Nafion solution (0.5 wt%) was coated onto the electrode surface. The prepared electrode was dried at room temperature overnight before electrochemical tests. For comparison, GCE coated with Pt/C (20 wt% Pt on Vulcan XC-72, purchased from Alfa Aesar) was also fabricated with the same procedure, and the loading density is 50 µg cm−2.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements L.H. and S.Z. contributed equally to this work. The authors acknowledge the support from the Ministry of Science and Technology of China (Nos. 2012CB933403 and 2012CB932504), the National Natural Science Foundation of China (Grant Nos. 51425302, 21371173, and 21173057), the Sino-German Centre for Research Promotion (GZ879), and the Chinese Academy of Sciences. Received: February 18, 2015 Revised: March 14, 2015 Published online:
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Adv. Mater. 2015, DOI: 10.1002/adma.201500863
