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This article can be cited before page numbers have been issued, to do this please use: J. H. Lee, M. J. Park, S. J. Yoo, J. H. Jang, H. Kim, S. Nam, C. W. Yoon and J. Y. Kim, Nanoscale, 2015, DOI: 10.1039/C5NR01584G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C5NR01584G
Highly Active and Durable Co-N-C Electrocatalyst Synthesized Using Exfoliated Graphitic Carbon Nitride Nanosheets
Jin Hee Lee,a,+ Min Jung Park,a,b+ Sung Jong Yoo,a Jong Hyun Jang,a Hyoung-Juhn Kim,a Suk Woo
a
Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,
Seongbuk-gu, Seoul 136-791, Republic of Korea b
Department of Energy and Environmental Engineering, Korea University of Science and Technology,
Daejeon, Republic of Korea c
Department of Clean Energy and Chemical Engineering, Korea University of Science and
Technology, Daejeon, Republic of Korea *
Corresponding authors: Dr. J. Y. Kim, Tel.: +82-2-958-5294, E-mail: [email protected] Dr. C. W. Yoon, Tel: +82-2-958-5262, E-mail: [email protected]
+
These authors contributed equally to this work.
Abstract Exfoliated graphitic carbon nitride nanosheets (g-C3N4-NS) were applied for the first time to the preparation of an electrocatalyst for the oxygen reduction reaction (ORR). A less dense structure with increased surface area was observed for g-C3N4-NS compared to bulk g-C3N4 from detailed analyses including TEM, STEM, AFM with depth profiling, XRD, and UV-Vis spectroscopy. The pyrolysis of the prepared g-C3N4-NS with Co and carbon under inert environment provided enhanced accessibility to the N functionalities required for efficient interaction of Co and C with N for the formation of CoN-C networks and produced hollow and interconnected Co-N-C-NS structure responsible for high durability. The Co-N-C-NS electrocatalyst exhibited superior catalytic activity and durability and further displayed fast and selective four electron transfer kinetics for the ORR, as evidenced by various electrochemical experiments. The hollow, interconnected structure of Co-N-C-NS with increased pyridinic and graphitic N species was proposed to play a key role in facilitating the desired ORR reaction.
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Nam,a Chang Won Yoon,a,c,* Jin Young Kima,*
Nanoscale
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DOI: 10.1039/C5NR01584G
The demand for renewable energy resources has been continuing to grow due to the drive to curtail the extensive use of carbon-based fossil fuels that generate large amounts of greenhouse gases. Polymer electrolyte membrane fuel cells (PEMFCs) have long been recognized as promising and sustainable power generators due to their high energy efficiency, reaching up to 60% even in small scale devices, and the advantage that water is released as the only byproduct.1 Despite the obvious price of the Pt-based electrocatalysts.2 To achieve economically viable PEMFC systems, a number of Pt-free electrocatalyts have been developed for the oxygen reduction reaction (ORR), the most feasible of which are non-noble metals supported on N-doped carbon materials. Such catalysts are typically denoted M-N-C, where M most frequently represents Fe or Co.3, 4 In M-N-C catalysts, N-doping of carbon materials has been shown to improve the catalytic activity and durability, presumably because the enhanced π−bonding networks in the carbon materials, improved electron donor ability, and increased active sites may satisfy the requirements of electrocatalysts for catalyzing the ORR. However, there is still no unambiguous explanation of the origin of the superior ORR activity of M-N-C.5, 6 Several synthetic methods have been employed to develop highly active and stable M-N-C electrocatalysts. These include the use of transition metal macrocyclic complexes,7, 8 pyrolysis of Ncontaining polymers and other carbon sources with metals,9, 10 post-treatment with NH3 gas,11, 12 and utilization of molecular precursors containing nitrogen atoms (such as cyanamide or pyridine).13-16 Emerging from these efforts, several Pt-free electrocatalysts that show competitive or even better activity than the traditional Pt/C catalyst have been developed.11, 12, 17 Notwithstanding the significant progress, there are still several problems to overcome for ubiquitous application of such catalysts. The complicated and multi-step preparation methods, high cost and toxicity of starting materials, and the lack of scalable synthetic strategies are such limitations. Graphitic carbon nitride (g-C3N4) is an attractive support for M-N-C catalysts since it contains numerous nitrogen functionalities that may facilitate the formation of M-N-C networks and further stabilize the metal nanoparticles. In addition, g-C3N4 can be readily produced on a large scale at a reasonable price. Moreover, g-C3N4 is highly stable under harsh acidic/basic environments.18-21 For these reasons, numerous electrocatalysts based on g-C3N4 have been developed over the last few years.22, 23 However, like graphite, the low surface area of g-C3N4 (ca. 20 m2·g-1), which results from the tightly stacked structure composed of several dozens of sheets, is prone to inhibit favorable contact between the metals and carbon atoms where the N sites are located at the g-C3N4 layers. For the application of g-C3N4 to ORR electrocatalysts, it is thus necessary to increase the formation of
2
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advantages of PEMFCs, only limited commercialization has been accomplished because of the high
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catalytically active sites by maximizing the interactions of the metals and carbon with N functionalities in g-C3N4. Herein, we report the ORR electrocatalytic properties of Co-N-C synthesized using exfoliated graphitic carbon nitride nanosheets (g-C3N4-NS) as a nitrogen source and stabilizer for metal nanoparticles (Fig. 1). To the best of our knowledge, there are no reports in which g-C3N4-NS has various ways.24-27 In comparison with bulk-g-C3N4, exfoliated g-C3N4-NS was found to have lower thickness with much fewer stacking layers, resulting in increased surface area and pore volume. These physical changes further led to the formation of catalytic interfaces that allowed efficient interactions between both Co and C with N, ultimately improving the ORR activity and durability. Exfoliation of bulk g-C3N4 was accomplished by treatment with concentrated sulfuric acid followed by addition of water under ultrasonication. Upon exfoliation, a sudden color change of gC3N4 from yellow to colorless was observed. Depth profiling studies using an atomic force microscope (AFM) confirmed that the colorless material obtained after filtration and drying was exfoliated g-C3N4-NS (Fig. 2a), and the height of g-C3N4 decreased from 20-40 nm to 5-10 nm after exfoliation. Based on the interlayer distance of g-C3N4 (0.326 nm),18 it is inferable that g-C3Ns-NS consists of 15-20 single layers of g-C3N4. Moreover, the average particle size of g-C3Ns-NS was found to decrease following exfoliation. X-ray diffraction (XRD) further substantiates the exfoliated structure of g-C3N4-NS (Fig. 2b). The XRD pattern of bulk g-C3N4 typically shows a major peak centered at 27.5° that corresponds to the interlayer spacing between the g-C3N4 planes, whereas the profile of g-C3N4-NS displays clear broadening and weakening of the peak. This signature indicates weakening of the interlayer stacking of g-C3N4. In addition, the absence of peak shift indicates that the interlayer distance between g-C3N4-NS remained unchanged even after exfoliation. UV-Vis spectroscopic studies further corroborate exfoliation of bulk g-C3N4, and as depicted in Fig. 2c, exfoliation led to a blue shift of the electronic transition for bulk g-C3N4 along with an increase in the bandgap from 2.8 to 3.0 eV (Fig. S1). This observation is attributed to a decrease in the conjugation units of g-C3N4 due to exfoliation, which results in substantial quantum confinement effects.27 In addition, compared with bulk g-C3N4, g-C3N4-NS exhibits enhanced gas adsorption ability (Fig. 2d). Consistent with these observations, the BET surface area of g-C3N4-NS increased considerably from 20 to 140 m2·g-1. The enhanced surface area increases the probability for the N sites to interact with metal ions and carbon atoms, ultimately facilitating the formation of efficient M-N-C networks. Notably, despite such physical changes of g-C3N4, infrared spectroscopy indicates that the exfoliation process involving H2SO4-treatment did not affect the nature of chemical bonding for the conjugated melm units in g-C3N4.
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been used for the synthesis of ORR electrocatalysts, although g-C3N4 exfoliation has been achieved in
Nanoscale
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After successful synthesis of the g-C3N4-NS support, the Co-N-C nanocomposites were synthesized by mixing Co, g-C3N4-NS, and C, followed by pyrolysis at 700 °C under N2. Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) studies of Co-N-C-NS show interconnected hollow structures (Fig. 3a and 3b), obtained from Coassisted decomposition of C as well as rearrangement of the N atoms during pyrolysis, as typically
further indicates uniform distribution of N and Co over the carbon support (Fig. 3c-f). Based on this analysis, it is anticipated that the resulting Co-N-C-NS nanocomposite should have the following advantages for ORR: (i) the exposed N sites and increased surface area of g-C3N4-NS are beneficial for preparing of Co-N-C by initially stabilizing the Co sites via Co-N bond formation. (ii) The nanosheet structure could be preferentially assembled onto the carbon surface by strong π-π stacking between the aromatic carbon units and carbon nitride units. (iii) The unique interconnected structure may offer enhanced catalytic activity by providing high electrical conductivity with exposed active sites, as well as enhanced durability by stabilizing the catalyst through the interconnected carbon skeleton (vide infra).29, 30 To evaluate the ORR activity of the prepared catalysts, rotating disk electrode (RDE) experiments were carried out using 0.1 M KOH solution (Fig. 4). The Co-N-C-NS catalyst showed better activity than normal Co-N-C. Moreover, the high activity of Co-N-C-NS was also confirmed by the difference of only 20 mV between the half wave potential of the synthesized catalyst and that of the commercial Pt/C catalyst (Fig. 4a, 0.82 V and 0.84 V for Co-N-C-NS and Pt/C, respectively). No ORR activity was observed when N-C was used as the catalyst, confirming that Co metal played a critical role in catalyzing the ORR (Fig. 4a, blue line). Rotating ring disk electrode (RRDE) experiments further suggest that Co-N-C-NS favorably and selectively promotes the four electron transfer mechanism of the ORR; 3.9 over the entire potential range employed (Fig. 4b). With the use of Co-N-C, ca. 3-10% HO2- was produced, and 3.7 electron transfer kinetics was observed. The slopes of the Tafel plots of both catalysts were calculated as 41 mV·dec-1, indicating that both catalysts operate by a similar reaction pathway involving analogous active sites comprising Co, N, and C (Fig. 4c). Furthermore, the closeness of the Tafel slope values to 2.303(2RT/3F) (R and F represent the universal gas constant and Faraday constant, respectively) indicates that the reaction steps for H2Omediated protonation of adsorbed O2- species is a rate determining step in the current catalytic system.17, 31 Superior stability of the catalysts was also demonstrated by an attenuated durability test (ADT) in which a decrease in the half wave potential of only 18 mV was observed for Co-N-C-NS even after 10,000 repeated cycles (Fig. 4d). 4
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observed in the synthesis of carbon nanotubes.28 Electron energy loss spectroscopy (EELS) mapping
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The superior ORR activity of Co-N-C-NS relative to Co-N-C is derived from the effective incorporation of N sites into the matrix containing Co and C, and stems from the increased surface area and the accessibility of Co and C to the N functionalities of the exfoliated nanosheet structure. The higher N content of Co-N-C-NS (2.3 wt%) relative to Co-N-C (1.7 wt%), determined by elemental analysis (EA), demonstrates the advantageous roles of g-C3N4-NS in the formation of the spectra, which showed that both Co-N-C and Co-N-C-NS have various types of N-functionalities including graphitic, pyrrolic, nitrile, and pyridinic N species (Fig. 5a). However, the relative quantities of each N-functionality in the respective catalysts were quite different (Fig. 5b). Compared to Co-N-C, Co-N-C-NS was found to possess more graphitic and pyridinic N sites that are known to play a significant role in boosting the ORR owing to their low barriers for electron transfer and high selectivity toward the desirable four electron pathway.32, 33 These observations are consistent with the obtained ORR activity. The incorporation of efficient N sites and generation of catalytically active N moieties by g-C3N4-NS possibly led to the improved activity and selectivity in the ORR.
Conclusions In summary, the Co-N-C-NS electrocatalyst, prepared for the first time using g-C3N4-NS as a Ndoping material, exhibited excellent catalytic performance for ORR. Exfoliation was successfully achieved by acid treatment of g-C3N4, and the resulting g-C3N4-NS was revealed to have a decrease in particle size and thickness, but an increase in the surface area compared to the properties of bulk-gC3N4. The enhanced accessibility of the N sites on g-C3N4-NS resulted in efficient incorporation of N as well as Co metal onto the carbon support to generate the Co-N-C network; this accounts for the fine distribution of Co and N on the carbon matrix, high nitrogen content, and extensive formation of pyridinic and graphitic N species in the resulting catalyst. These properties contribute to the superior activity and durability along with the fast and selective four electron transfer kinetics of the Co-N-CNS catalyzed ORR. This novel catalytic system provides insight for the development of high performance of ORR catalysts via a low cost, scalable and facile synthesis procedure.
Acknowledgements This work was supported by Korean Government through the New & Renewable Energy Core Technology Program of the KETEP funded by MOTIE (No. 20133030011320) and Center of Excellence (COE) program of the Korea Institute of Science and Technology. 5
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Co-N-C structure. The positive effect of the nanosheet structure was also evidenced by the N1s XPS
Nanoscale
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1. 2. 3.
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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
Proceedings of the 2000 U.S. DOE Hydrogen Program Review. M. Kim, J.-N. Park, H. Kim, S. Song and W.-H. Lee, J. Power Source, 2006, 163, 93-97. F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, H. T. Chung, C. M. Johnston and P. Zelenay, Energy Environ. Sci., 2011, 4, 114-130. C. W. B. Bezerra, L. Zhang, K. Lee, H. Liu, A. L. B. Marques, E. P. Marques, H. Wang and J. Zhang, Electrochim. Acta, 2008, 53, 4937-4951. M. Li, L. Zhang, Q. Xu, J. Niu and Z. Xia, J. Catal., 2014, 314, 66-72. M. D. Esrafili, Comput. Theor. Chem., 2013, 1015, 1-7. C. Zhang, R. Hao, H. Yin, F. Liu and Y. Hou, Nanoscale, 2012, 4, 7326-7329. A. Morozan, S. Campidelli, A. Filoramo, B. Jousselme and S. Palacin, Carbon, 2011, 49, 4839-4847. H. Peng, F. Liu, X. Liu, S. Liao, C. You, X. Tian, H. Nan, F. Luo, H. Song, Z. Fu and P. Huang, ACS Catal., 2014, 4, 3797-3805. G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 2011, 332, 443-447. E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz and J.-P. Dodelet, Nat. Commun., 2011, 2, 416. U. I. Kramm, M. Lefèvre, N. Larouche, D. Schmeisser and J.-P. Dodelet, J. Am. Chem. Soc., 2013, 136, 978-985. Z.-Y. Yang, Y.-X. Zhang, L. Jing, Y.-F. Zhao, Y.-M. Yan and K.-N. Sun, J. Mater. Chem. A, 2014, 2, 2623-2627. J. H. Lee, M. J. Park, J. Jung, J. Ryu, E. Cho, S.-W. Nam, J. Y. Kim and C. W. Yoon, Inorg. Chim. Acta, 2014, 422, 3-7. Z. Chen, D. Higgins, H. Tao, R. S. Hsu and Z. Chen, J. Phys. Chem. C, 2009, 113, 2100821013. H. T. Chung, J. H. Won and P. Zelenay, Nat. Commun., 2013, 4, 1922. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780-786. Y. Zheng, J. Liu, J. Liang, M. Jaroniec and S. Z. Qiao, Energy. Environ. Sci., 2012, 5, 67176731. Y. Wang, X. Wang and M. Antonietti, Angew. Chem. Int. Ed., 2012, 51, 68-89. A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Muller, R. Schlogl and J. M. Carlsson, J. Mater. Chem., 2008, 18, 4893-4908. J. H. Lee, J. Ryu, J. Y. Kim, S.-W. Nam, J. H. Han, T.-H. Lim, S. Gautam, K. H. Chae and C. W. Yoon, J. Mater. Chem. A, 2014, 2, 9490-9495. K. Qiu and Z. X. Guo, J. Mater. Chem. A, 2014, 2, 3209-3215. N. Mansor, A. B. Jorge, F. Corà, C. Gibbs, R. Jervis, P. F. McMillan, X. Wang and D. J. L. Brett, J. Phys. Chem. C, 2014, 118, 6831-6838. F. Zhao, H. Cheng, Y. Hu, L. Song, Z. Zhang, L. Jiang and L. Qu, Sci. Rep., 2014, 4, 5882. S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang and P. M. Ajayan, Adv. Mater., 2013, 25, 2452-2456. J. Xu, L. Zhang, R. Shi and Y. Zhu, J. Mater. Chem. A, 2013, 1, 14766-14772. P. Niu, L. Zhang, G. Liu and H.-M. Cheng, Adv. Func. Mater., 2012, 22, 4763-4770. J. Prasek, J. Drbohlavova, J. Chomoucka, J. Hubalek, O. Jasek, V. Adam and R. Kizek, J. Mater. Chem., 2011, 21, 15872-15884. G. Zheng, S. W. Lee, Z. Liang, H.-W. Lee, K. Yan, H. Yao, H. Wang, W. Li, S. Chu and Y. Cui, Nat. Nano., 2014, 9, 618-623. D. Li, L.-X. Ding, H. Chen, S. Wang, Z. Li, M. Zhu and H. Wang, J. Mater. Chem. A, 2014, 2, 16617-16622. M. De Koninck and B. Marsan, Electrochim. Acta, 2008, 53, 7012-7021. 6
Nanoscale Accepted Manuscript
Notes and References
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H. Kim, K. Lee, S. I. Woo and Y. Jung, Phys. Chem. Chem. Phys., 2011, 13, 17505-17510. K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 323, 760-764.
Nanoscale Accepted Manuscript
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32. 33.
7
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Fig. 1 A schematic illustration of the synthetic procedure for Co-N-C-NS.
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Fig. 2 Characterization data for g-C3N4 and g-C3N4-NS: a) AFM images and depth profiles (scale bars, 100 nm), b) XRD spectra, c) UV-vis spectra, and d) N2 adsorption/desorption isotherm plots.
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Fig. 3 a) TEM image of Co-N-C-NS, b) HAADF STEM image of Co-N-C-NS, EELS-mapping images of Co-N-C-NS; c) C K edge, d) N K edge, e) Co L edge, and f) integrated image of C, N and Co.
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Fig. 4 a) ORR polarization curves of the catalysts in O2-saturated 0.1 M KOH, b) plots for the number of electron transferred and HO2- yields obtained by RRDE measurements, c) Tafel plots, and d) ORR polarization curves after ADT by Co-N-C-NS.
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Fig. 5 a) N1s XPS spectra of Co-N-C and Co-N-C-NS, and b) relative amounts of N species in Co-NC and Co-N-C-NS calculated from fitted N1s spectra.
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DOI: 10.1039/C5NR01584G
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. H. Lee, M. J. Park, S. J. Yoo, J. H. Jang, H. Kim, S. Nam, C. W. Yoon and J. Y. Kim, Nanoscale, 2015, DOI: 10.1039/C5NR01584G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/nanoscale
Page 1 of 12
Nanoscale View Article Online
DOI: 10.1039/C5NR01584G
Highly Active and Durable Co-N-C Electrocatalyst Synthesized Using Exfoliated Graphitic Carbon Nitride Nanosheets
Jin Hee Lee,a,+ Min Jung Park,a,b+ Sung Jong Yoo,a Jong Hyun Jang,a Hyoung-Juhn Kim,a Suk Woo
a
Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,
Seongbuk-gu, Seoul 136-791, Republic of Korea b
Department of Energy and Environmental Engineering, Korea University of Science and Technology,
Daejeon, Republic of Korea c
Department of Clean Energy and Chemical Engineering, Korea University of Science and
Technology, Daejeon, Republic of Korea *
Corresponding authors: Dr. J. Y. Kim, Tel.: +82-2-958-5294, E-mail: [email protected] Dr. C. W. Yoon, Tel: +82-2-958-5262, E-mail: [email protected]
+
These authors contributed equally to this work.
Abstract Exfoliated graphitic carbon nitride nanosheets (g-C3N4-NS) were applied for the first time to the preparation of an electrocatalyst for the oxygen reduction reaction (ORR). A less dense structure with increased surface area was observed for g-C3N4-NS compared to bulk g-C3N4 from detailed analyses including TEM, STEM, AFM with depth profiling, XRD, and UV-Vis spectroscopy. The pyrolysis of the prepared g-C3N4-NS with Co and carbon under inert environment provided enhanced accessibility to the N functionalities required for efficient interaction of Co and C with N for the formation of CoN-C networks and produced hollow and interconnected Co-N-C-NS structure responsible for high durability. The Co-N-C-NS electrocatalyst exhibited superior catalytic activity and durability and further displayed fast and selective four electron transfer kinetics for the ORR, as evidenced by various electrochemical experiments. The hollow, interconnected structure of Co-N-C-NS with increased pyridinic and graphitic N species was proposed to play a key role in facilitating the desired ORR reaction.
1
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Nam,a Chang Won Yoon,a,c,* Jin Young Kima,*
Nanoscale
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DOI: 10.1039/C5NR01584G
The demand for renewable energy resources has been continuing to grow due to the drive to curtail the extensive use of carbon-based fossil fuels that generate large amounts of greenhouse gases. Polymer electrolyte membrane fuel cells (PEMFCs) have long been recognized as promising and sustainable power generators due to their high energy efficiency, reaching up to 60% even in small scale devices, and the advantage that water is released as the only byproduct.1 Despite the obvious price of the Pt-based electrocatalysts.2 To achieve economically viable PEMFC systems, a number of Pt-free electrocatalyts have been developed for the oxygen reduction reaction (ORR), the most feasible of which are non-noble metals supported on N-doped carbon materials. Such catalysts are typically denoted M-N-C, where M most frequently represents Fe or Co.3, 4 In M-N-C catalysts, N-doping of carbon materials has been shown to improve the catalytic activity and durability, presumably because the enhanced π−bonding networks in the carbon materials, improved electron donor ability, and increased active sites may satisfy the requirements of electrocatalysts for catalyzing the ORR. However, there is still no unambiguous explanation of the origin of the superior ORR activity of M-N-C.5, 6 Several synthetic methods have been employed to develop highly active and stable M-N-C electrocatalysts. These include the use of transition metal macrocyclic complexes,7, 8 pyrolysis of Ncontaining polymers and other carbon sources with metals,9, 10 post-treatment with NH3 gas,11, 12 and utilization of molecular precursors containing nitrogen atoms (such as cyanamide or pyridine).13-16 Emerging from these efforts, several Pt-free electrocatalysts that show competitive or even better activity than the traditional Pt/C catalyst have been developed.11, 12, 17 Notwithstanding the significant progress, there are still several problems to overcome for ubiquitous application of such catalysts. The complicated and multi-step preparation methods, high cost and toxicity of starting materials, and the lack of scalable synthetic strategies are such limitations. Graphitic carbon nitride (g-C3N4) is an attractive support for M-N-C catalysts since it contains numerous nitrogen functionalities that may facilitate the formation of M-N-C networks and further stabilize the metal nanoparticles. In addition, g-C3N4 can be readily produced on a large scale at a reasonable price. Moreover, g-C3N4 is highly stable under harsh acidic/basic environments.18-21 For these reasons, numerous electrocatalysts based on g-C3N4 have been developed over the last few years.22, 23 However, like graphite, the low surface area of g-C3N4 (ca. 20 m2·g-1), which results from the tightly stacked structure composed of several dozens of sheets, is prone to inhibit favorable contact between the metals and carbon atoms where the N sites are located at the g-C3N4 layers. For the application of g-C3N4 to ORR electrocatalysts, it is thus necessary to increase the formation of
2
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advantages of PEMFCs, only limited commercialization has been accomplished because of the high
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catalytically active sites by maximizing the interactions of the metals and carbon with N functionalities in g-C3N4. Herein, we report the ORR electrocatalytic properties of Co-N-C synthesized using exfoliated graphitic carbon nitride nanosheets (g-C3N4-NS) as a nitrogen source and stabilizer for metal nanoparticles (Fig. 1). To the best of our knowledge, there are no reports in which g-C3N4-NS has various ways.24-27 In comparison with bulk-g-C3N4, exfoliated g-C3N4-NS was found to have lower thickness with much fewer stacking layers, resulting in increased surface area and pore volume. These physical changes further led to the formation of catalytic interfaces that allowed efficient interactions between both Co and C with N, ultimately improving the ORR activity and durability. Exfoliation of bulk g-C3N4 was accomplished by treatment with concentrated sulfuric acid followed by addition of water under ultrasonication. Upon exfoliation, a sudden color change of gC3N4 from yellow to colorless was observed. Depth profiling studies using an atomic force microscope (AFM) confirmed that the colorless material obtained after filtration and drying was exfoliated g-C3N4-NS (Fig. 2a), and the height of g-C3N4 decreased from 20-40 nm to 5-10 nm after exfoliation. Based on the interlayer distance of g-C3N4 (0.326 nm),18 it is inferable that g-C3Ns-NS consists of 15-20 single layers of g-C3N4. Moreover, the average particle size of g-C3Ns-NS was found to decrease following exfoliation. X-ray diffraction (XRD) further substantiates the exfoliated structure of g-C3N4-NS (Fig. 2b). The XRD pattern of bulk g-C3N4 typically shows a major peak centered at 27.5° that corresponds to the interlayer spacing between the g-C3N4 planes, whereas the profile of g-C3N4-NS displays clear broadening and weakening of the peak. This signature indicates weakening of the interlayer stacking of g-C3N4. In addition, the absence of peak shift indicates that the interlayer distance between g-C3N4-NS remained unchanged even after exfoliation. UV-Vis spectroscopic studies further corroborate exfoliation of bulk g-C3N4, and as depicted in Fig. 2c, exfoliation led to a blue shift of the electronic transition for bulk g-C3N4 along with an increase in the bandgap from 2.8 to 3.0 eV (Fig. S1). This observation is attributed to a decrease in the conjugation units of g-C3N4 due to exfoliation, which results in substantial quantum confinement effects.27 In addition, compared with bulk g-C3N4, g-C3N4-NS exhibits enhanced gas adsorption ability (Fig. 2d). Consistent with these observations, the BET surface area of g-C3N4-NS increased considerably from 20 to 140 m2·g-1. The enhanced surface area increases the probability for the N sites to interact with metal ions and carbon atoms, ultimately facilitating the formation of efficient M-N-C networks. Notably, despite such physical changes of g-C3N4, infrared spectroscopy indicates that the exfoliation process involving H2SO4-treatment did not affect the nature of chemical bonding for the conjugated melm units in g-C3N4.
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been used for the synthesis of ORR electrocatalysts, although g-C3N4 exfoliation has been achieved in
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After successful synthesis of the g-C3N4-NS support, the Co-N-C nanocomposites were synthesized by mixing Co, g-C3N4-NS, and C, followed by pyrolysis at 700 °C under N2. Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) studies of Co-N-C-NS show interconnected hollow structures (Fig. 3a and 3b), obtained from Coassisted decomposition of C as well as rearrangement of the N atoms during pyrolysis, as typically
further indicates uniform distribution of N and Co over the carbon support (Fig. 3c-f). Based on this analysis, it is anticipated that the resulting Co-N-C-NS nanocomposite should have the following advantages for ORR: (i) the exposed N sites and increased surface area of g-C3N4-NS are beneficial for preparing of Co-N-C by initially stabilizing the Co sites via Co-N bond formation. (ii) The nanosheet structure could be preferentially assembled onto the carbon surface by strong π-π stacking between the aromatic carbon units and carbon nitride units. (iii) The unique interconnected structure may offer enhanced catalytic activity by providing high electrical conductivity with exposed active sites, as well as enhanced durability by stabilizing the catalyst through the interconnected carbon skeleton (vide infra).29, 30 To evaluate the ORR activity of the prepared catalysts, rotating disk electrode (RDE) experiments were carried out using 0.1 M KOH solution (Fig. 4). The Co-N-C-NS catalyst showed better activity than normal Co-N-C. Moreover, the high activity of Co-N-C-NS was also confirmed by the difference of only 20 mV between the half wave potential of the synthesized catalyst and that of the commercial Pt/C catalyst (Fig. 4a, 0.82 V and 0.84 V for Co-N-C-NS and Pt/C, respectively). No ORR activity was observed when N-C was used as the catalyst, confirming that Co metal played a critical role in catalyzing the ORR (Fig. 4a, blue line). Rotating ring disk electrode (RRDE) experiments further suggest that Co-N-C-NS favorably and selectively promotes the four electron transfer mechanism of the ORR; 3.9 over the entire potential range employed (Fig. 4b). With the use of Co-N-C, ca. 3-10% HO2- was produced, and 3.7 electron transfer kinetics was observed. The slopes of the Tafel plots of both catalysts were calculated as 41 mV·dec-1, indicating that both catalysts operate by a similar reaction pathway involving analogous active sites comprising Co, N, and C (Fig. 4c). Furthermore, the closeness of the Tafel slope values to 2.303(2RT/3F) (R and F represent the universal gas constant and Faraday constant, respectively) indicates that the reaction steps for H2Omediated protonation of adsorbed O2- species is a rate determining step in the current catalytic system.17, 31 Superior stability of the catalysts was also demonstrated by an attenuated durability test (ADT) in which a decrease in the half wave potential of only 18 mV was observed for Co-N-C-NS even after 10,000 repeated cycles (Fig. 4d). 4
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observed in the synthesis of carbon nanotubes.28 Electron energy loss spectroscopy (EELS) mapping
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The superior ORR activity of Co-N-C-NS relative to Co-N-C is derived from the effective incorporation of N sites into the matrix containing Co and C, and stems from the increased surface area and the accessibility of Co and C to the N functionalities of the exfoliated nanosheet structure. The higher N content of Co-N-C-NS (2.3 wt%) relative to Co-N-C (1.7 wt%), determined by elemental analysis (EA), demonstrates the advantageous roles of g-C3N4-NS in the formation of the spectra, which showed that both Co-N-C and Co-N-C-NS have various types of N-functionalities including graphitic, pyrrolic, nitrile, and pyridinic N species (Fig. 5a). However, the relative quantities of each N-functionality in the respective catalysts were quite different (Fig. 5b). Compared to Co-N-C, Co-N-C-NS was found to possess more graphitic and pyridinic N sites that are known to play a significant role in boosting the ORR owing to their low barriers for electron transfer and high selectivity toward the desirable four electron pathway.32, 33 These observations are consistent with the obtained ORR activity. The incorporation of efficient N sites and generation of catalytically active N moieties by g-C3N4-NS possibly led to the improved activity and selectivity in the ORR.
Conclusions In summary, the Co-N-C-NS electrocatalyst, prepared for the first time using g-C3N4-NS as a Ndoping material, exhibited excellent catalytic performance for ORR. Exfoliation was successfully achieved by acid treatment of g-C3N4, and the resulting g-C3N4-NS was revealed to have a decrease in particle size and thickness, but an increase in the surface area compared to the properties of bulk-gC3N4. The enhanced accessibility of the N sites on g-C3N4-NS resulted in efficient incorporation of N as well as Co metal onto the carbon support to generate the Co-N-C network; this accounts for the fine distribution of Co and N on the carbon matrix, high nitrogen content, and extensive formation of pyridinic and graphitic N species in the resulting catalyst. These properties contribute to the superior activity and durability along with the fast and selective four electron transfer kinetics of the Co-N-CNS catalyzed ORR. This novel catalytic system provides insight for the development of high performance of ORR catalysts via a low cost, scalable and facile synthesis procedure.
Acknowledgements This work was supported by Korean Government through the New & Renewable Energy Core Technology Program of the KETEP funded by MOTIE (No. 20133030011320) and Center of Excellence (COE) program of the Korea Institute of Science and Technology. 5
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Co-N-C structure. The positive effect of the nanosheet structure was also evidenced by the N1s XPS
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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
Proceedings of the 2000 U.S. DOE Hydrogen Program Review. M. Kim, J.-N. Park, H. Kim, S. Song and W.-H. Lee, J. Power Source, 2006, 163, 93-97. F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, H. T. Chung, C. M. Johnston and P. Zelenay, Energy Environ. Sci., 2011, 4, 114-130. C. W. B. Bezerra, L. Zhang, K. Lee, H. Liu, A. L. B. Marques, E. P. Marques, H. Wang and J. Zhang, Electrochim. Acta, 2008, 53, 4937-4951. M. Li, L. Zhang, Q. Xu, J. Niu and Z. Xia, J. Catal., 2014, 314, 66-72. M. D. Esrafili, Comput. Theor. Chem., 2013, 1015, 1-7. C. Zhang, R. Hao, H. Yin, F. Liu and Y. Hou, Nanoscale, 2012, 4, 7326-7329. A. Morozan, S. Campidelli, A. Filoramo, B. Jousselme and S. Palacin, Carbon, 2011, 49, 4839-4847. H. Peng, F. Liu, X. Liu, S. Liao, C. You, X. Tian, H. Nan, F. Luo, H. Song, Z. Fu and P. Huang, ACS Catal., 2014, 4, 3797-3805. G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 2011, 332, 443-447. E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz and J.-P. Dodelet, Nat. Commun., 2011, 2, 416. U. I. Kramm, M. Lefèvre, N. Larouche, D. Schmeisser and J.-P. Dodelet, J. Am. Chem. Soc., 2013, 136, 978-985. Z.-Y. Yang, Y.-X. Zhang, L. Jing, Y.-F. Zhao, Y.-M. Yan and K.-N. Sun, J. Mater. Chem. A, 2014, 2, 2623-2627. J. H. Lee, M. J. Park, J. Jung, J. Ryu, E. Cho, S.-W. Nam, J. Y. Kim and C. W. Yoon, Inorg. Chim. Acta, 2014, 422, 3-7. Z. Chen, D. Higgins, H. Tao, R. S. Hsu and Z. Chen, J. Phys. Chem. C, 2009, 113, 2100821013. H. T. Chung, J. H. Won and P. Zelenay, Nat. Commun., 2013, 4, 1922. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780-786. Y. Zheng, J. Liu, J. Liang, M. Jaroniec and S. Z. Qiao, Energy. Environ. Sci., 2012, 5, 67176731. Y. Wang, X. Wang and M. Antonietti, Angew. Chem. Int. Ed., 2012, 51, 68-89. A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Muller, R. Schlogl and J. M. Carlsson, J. Mater. Chem., 2008, 18, 4893-4908. J. H. Lee, J. Ryu, J. Y. Kim, S.-W. Nam, J. H. Han, T.-H. Lim, S. Gautam, K. H. Chae and C. W. Yoon, J. Mater. Chem. A, 2014, 2, 9490-9495. K. Qiu and Z. X. Guo, J. Mater. Chem. A, 2014, 2, 3209-3215. N. Mansor, A. B. Jorge, F. Corà, C. Gibbs, R. Jervis, P. F. McMillan, X. Wang and D. J. L. Brett, J. Phys. Chem. C, 2014, 118, 6831-6838. F. Zhao, H. Cheng, Y. Hu, L. Song, Z. Zhang, L. Jiang and L. Qu, Sci. Rep., 2014, 4, 5882. S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang and P. M. Ajayan, Adv. Mater., 2013, 25, 2452-2456. J. Xu, L. Zhang, R. Shi and Y. Zhu, J. Mater. Chem. A, 2013, 1, 14766-14772. P. Niu, L. Zhang, G. Liu and H.-M. Cheng, Adv. Func. Mater., 2012, 22, 4763-4770. J. Prasek, J. Drbohlavova, J. Chomoucka, J. Hubalek, O. Jasek, V. Adam and R. Kizek, J. Mater. Chem., 2011, 21, 15872-15884. G. Zheng, S. W. Lee, Z. Liang, H.-W. Lee, K. Yan, H. Yao, H. Wang, W. Li, S. Chu and Y. Cui, Nat. Nano., 2014, 9, 618-623. D. Li, L.-X. Ding, H. Chen, S. Wang, Z. Li, M. Zhu and H. Wang, J. Mater. Chem. A, 2014, 2, 16617-16622. M. De Koninck and B. Marsan, Electrochim. Acta, 2008, 53, 7012-7021. 6
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Notes and References
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H. Kim, K. Lee, S. I. Woo and Y. Jung, Phys. Chem. Chem. Phys., 2011, 13, 17505-17510. K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 323, 760-764.
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32. 33.
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Fig. 1 A schematic illustration of the synthetic procedure for Co-N-C-NS.
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Fig. 2 Characterization data for g-C3N4 and g-C3N4-NS: a) AFM images and depth profiles (scale bars, 100 nm), b) XRD spectra, c) UV-vis spectra, and d) N2 adsorption/desorption isotherm plots.
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Fig. 3 a) TEM image of Co-N-C-NS, b) HAADF STEM image of Co-N-C-NS, EELS-mapping images of Co-N-C-NS; c) C K edge, d) N K edge, e) Co L edge, and f) integrated image of C, N and Co.
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Fig. 4 a) ORR polarization curves of the catalysts in O2-saturated 0.1 M KOH, b) plots for the number of electron transferred and HO2- yields obtained by RRDE measurements, c) Tafel plots, and d) ORR polarization curves after ADT by Co-N-C-NS.
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Fig. 5 a) N1s XPS spectra of Co-N-C and Co-N-C-NS, and b) relative amounts of N species in Co-NC and Co-N-C-NS calculated from fitted N1s spectra.
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