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2009, 113, 20148–20151 Published on Web 10/28/2009
Carbon Nitride as a Nonprecious Catalyst for Electrochemical Oxygen Reduction Stephen M. Lyth,*,† Yuta Nabae,† Shogo Moriya,† Shigeki Kuroki,† Masa-aki Kakimoto,† Jun-ichi Ozaki,†,‡ and Seizo Miyata†,§ Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 S5-20, Ookayama, Tokyo 152-8552, Japan, Department of Chemical and EnVironmental Engineering, Graduate School of Engineering, Gunma UniVersity, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515, Japan, and New Energy and Industrial Technology DeVelopment Organization, 310 Omiya-cho, Saiwa-ku, Kawasaki, Kanagawa 212-8554, Japan ReceiVed: August 17, 2009; ReVised Manuscript ReceiVed: October 19, 2009
Nitrogen-doped carbon-based catalysts are increasingly being studied as Pt-free electrodes for oxygen reduction in polymer electrolyte membrane fuel cells. Here, we study the oxygen reduction activity of stoichiometric carbon nitride, which has much higher nitrogen content and is synthesized at lower temperatures, without using ionic or metallic iron. Carbon nitride was studied and characterized via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, BET specific surface area analysis, and thermogravimetric analysis. Rotating electrode voltammetry in oxygen-saturated sulfuric acid was used to determine the catalytic activity. The onset potential for oxygen reduction by carbon nitride electrodes was 0.69 V (vs NHE) compared to 0.45 V for a carbon black reference electrode. However, the current density was low, possibly due to the low surface area of the material. Blending the carbon nitride with a high surface area carbon black support resulted in a significant improvement in current density and in an increase in onset potential to 0.76 V. The role of surface area was elucidated via cyclic voltammetry. This work confirms that stoichiometric carbon nitride has improved inherent oxygen reduction activity compared to pure carbon, and suggests that Fe coordination sites are not essential for electrochemical oxygen reduction in nitrogen-containing carbon materials. Introduction Tetrahedral carbon nitride (β-C3N4) was predicted in 1985 and estimated to have a bulk modulus comparable to that of diamond.1,2 However, this elusive material has yet to be observed in significant quantities. The planar phase (g-C3N4) is relatively stable3 and has been synthesized in large quantities, first fabricated via thermal decomposition of carbon- and nitrogencontaining chemical precursors.4 The planar phase is analogous to graphite; however, unlike graphite, g-C3N4 has both 3-fold coordinated (graphite-like) and 2-fold coordinated (pyridinelike) nitrogen atoms, and every carbon atom is bonded to three nitrogen atoms (Figure 1). There are two commonly mentioned structural isomers of g-C3N4. The first (Figure 1a) comprises condensed melamine units, and contains a periodic array of single carbon vacancies. The second is made up of condensed melem (2,5,8,-triamino-tri-s-triazine) subunits, and contains larger periodic vacancies in the lattice (Figure 1b). The melem isomer is predicted to be slightly more stable;5 however, both are commonly cited in the literature. Incidentally, the vacancyfree direct analogue of graphite is predicted to be energetically unstable.6 Since the first observance of stoichiometric g-C3N4, many other fabrication methods have been reported, for example, rapid thermal decomposition (of trichloromelamine),7 elec* To whom correspondence should be addressed. E-mail: stephenlyth@ physics.org. Phone/Fax: +81 35734 2581. † Tokyo Institute of Technology. ‡ Gunma University. § New Energy and Industrial Technology Development Organization.
10.1021/jp907928j CCC: $40.75
Figure 1. Two predicted structures of g-C3N4 made up of (a) condensed melamine subunits and (b) condensed tri-s-triazine subunits.
trodeposition,8 and pyrolysis (of melamine).9 Most fabrication techniques involve chemical synthesis; usually, condensation of s-triazine derivatives reacted at high temperature and/or pressure with a catalyst.10,11 This route has also led to nanostructured carbon nitrides.12-16 In this work, carbon nitride 2009 American Chemical Society
Letters synthesis is reproduced from the literature.17 The technique was chosen because it does not use sodium metal as a catalyst and does not include HCl in the reaction products, which would damage the steel reaction chamber. Fuel cells were originally described by Christian Scho¨nbein in 1839, and the first practical demonstration was William Grove’s “gas voltaic battery” soon afterward.18,19 Today, polymer electrolyte membrane fuel cells (PEMFCs) utilizing the oxygen reduction reaction (ORR) have been intensely studied as power sources, due to their simplicity, viability, and quick start-up.20 One of the limiting factors in the wide-scale acceptance of PEMFCs as clean, green energy sources is the high cost, partially driven by the price and scarcity of platinum.21 This has led to numerous attempts to find alternative nonprecious catalysts.22 It has been suggested that the presence of nitrogen in carbon based catalysts can lead to improved ORR activity.23-28 Computer simulations have strengthened this hypothesis, showing that the barrier to ORR at a carbon atom can be reduced in the presence of an adjoining nitrogen atom.29 Other work suggests that Fe coordinated to carbon and nitrogen is an important active site for ORR.30 In this work, we extend the idea of using nitrogen-doped carbons by manufacturing stoichiometric carbon nitride materials with much higher nitrogen content (by an order of magnitude or more), fabricated at relatively low temperatures, and testing them for their ORR activity. This should clarify if nitrogen-containing carbons are inherently catalytic active, or if Fe coordination is necessary for catalytic activity, since no Fe was used in synthesis of this material. Carbon nitride has already proven to be effective as a catalyst for water splitting under visible light,31,32 Friedel-Crafts acylation,33 the splitting of CO2,34 and cyclotrimerization of various nitriles.35 As such, g-C3N4 is worth investigating for its potential use as a catalyst for oxygen reduction. Experimental Methods Carbon nitride was fabricated in a stainless steel high pressure reactor. The synthesis involved reacting cyanuric chloride and sodium azide (in benzene) at 220 °C for 22 h, according to the scheme C3N3Cl3 + 3NaN3 f g-C3N4 + 3NaCl + 4N2.17 The product was washed with benzene and deionized water and then heated overnight at 80 °C under vacuum, to remove adsorbed water. The elemental composition and bonding were investigated via X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer 5500-MT). Fourier transform infrared spectroscopy (FTIR) (Jasco FT/IR-6100) was used to investigate the structure of the material. The ORR activity was measured via rotating disk electrode voltammetry at room temperature in an oxygensaturated 0.5 M H2SO4 electrolyte solution. Electrodes were prepared by mixing 5 mg of catalyst (or 2.5 mg of catalyst and 2.5 mg of carbon black, XC-72R, Cabot) and 50 µL of Nafion solution (5 wt %) (Aldrich) with Millipore water (150 µL) and ethanol (150 µL). This mixture was sonicated for 10 min, resulting in a uniform, well dispersed catalyst ink. Subsequently, 4 µL of the catalyst ink was carefully deposited on a circular glassy carbon electrode and dried at room temperature. ORR voltammograms were recorded using electrochemical equipment (ALS 700C), by rotating the electrode at 1500 rpm and sweeping from 1.2 to 0 V (vs NHE) at 1 mV/s. The onset potential is defined in this work as the voltage at which a current density of 2 µA/cm2 was recorded. Cyclic voltammetry was conducted in nitrogen and oxygen, sweeping between -0.25 and 1.1 V (vs NHE) at a scan rate of 0.1 V/s. BET specific surface area was measured via nitrogen adsorption (Belsorp II mini, Bel Japan, Inc.).
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Figure 2. (a) C1s and (b) N1s XPS signals for g-C3N4. (c) FTIR spectrum for g-C3N4.
Results and Discussion XPS data agreed well with published data.17 A C/N ratio of 0.73 was measured, which is very close to the ideal theoretical value of 0.75. Oxygen was also detected (17%), which is likely to be due to oxygen-containing terminal groups as well as adsorbed water. XPS analysis of g-C3N4 materials is complicated by the large number of conflicting reports of the assignments of bond types to binding energies. The C1s signal (Figure 2a) has two components. The peak at 284.7 eV is almost invariably assigned to sp2 C-C bonds in the literature (carbon in a carbon environment); however, the origin of such a strong C-C signal in any stoichiometric C3N4 material is unclear, since there are no carbon-carbon bonds in the predicted structure. To eliminate the possibility of carbon contamination in the sample, thermogravimetric analysis (Rigaku) was conducted in helium (not shown). The material completely decomposed between 600 and 900 °C, whereas graphitic carbon would be stable well above this temperature. Additionally, if significant contamination were present, the atomic percentage of carbon would not be as low as it is. Considering that C-C bonds are unlikely to be present in these materials, one must arrive at a different peak assignment. The natural contender is carbon, sp2 bonded to nitrogen, which, looking at the chemical structure of g-C3N4 (Figure 1), is the most abundant bond in these materials. Confusion may arise because the binding energies of these two different bond types overlap, and the sp2 C-N peak is only observed at sufficiently high nitrogen concentrations.36 The peak at 287.2 eV can be attributed to CdO bonding.37 These oxygencontaining bonds probably represent surface functionalities. The N1s signal (Figure 2b) can be deconvoluted into four peaks. Those at 398.3 and 400.0 eV are attributed to pyridinelike and graphite-like nitrogen atoms, respectively, and the peak at 402.7 eV may be attributed to amino terminal groups, or the
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Letters generated. If the surface area can be made comparable to existing catalysts, the current density is expected to increase significantly. To attempt to improve the performance of the g-C3N4 ink, it was blended with carbon black (50 wt %) which is well-known as a high surface area catalyst support. The g-C3N4/carbon black blend outperforms the pure g-C3N4 sample by some way, with an onset potential of 0.76 V and a current density of 2.21 mA/ cm2 (Figure 3a). Figure 3b shows typical cyclic voltammograms in N2 for g-C3N4 (black line). A distinctive oxidation/reduction peak is observed on either side of the cycle. This may be due to the oxygen groups associated with surface functionalities.39 However, comparing the CVs in nitrogen and in oxygen (inset), it is clear that this redox pair is not involved in ORR. Comparing pure g-C3N4 to that mixed with carbon black (red line), an obvious increase in the double layer capacitance is observed, suggesting an increase in the effective surface area, corresponding to the increased catalytic activity for ORR. This surface area increase is attributed directly to the use of the carbon black support. These results show that carbon nitride has an inherent catalytic activity for ORR, that carbon black can be used to increase the effective surface area resulting in an improvement in ORR, and that Fe coordination is not necessary for ORR in nitrogencontaining carbons.
Figure 3. (a) Oxygen reduction voltammograms of carbon black (black solid line), platinum (black dotted line), g-C3N4 (red line), and g-C3N4 blended with carbon black (CB) (blue line). The inset shows the same data magnified and smoothed. (b) Cyclic voltammograms for g-C3N4 (black line) and g-C3N4 blended with carbon black (red line). The inset shows the voltammograms for g-C3N4 in nitrogen (black line) and in oxygen (red line).
π conjugation.17 The peak at 404.4 eV is attributed to NO2 terminal groups, another source of oxygen contributing to the O1s peak.37 This N1s peak provides further evidence for the sp2 C-N assignment of the C1s signal at 284.8 eV; if nitrogen bonded to carbon is present in the N1s spectra, then the corresponding carbon atoms should be observed in the C1s signal. Overall, these results confirm that the material under study is indeed stoichiometric g-C3N4, albeit with fairly high oxygen content, probably arising from surface functionalities. FTIR (Figure 2c) corresponds well with published results.17 The broad, low intensity peaks between 3500 and 3000 cm-1 are assigned to N-H stretches, suggesting hydrogenation of some of the nitrogen atoms. The peak at 2150 cm-1 is attributed to CtN cyano terminal groups. The broad set of peaks between ∼1700 and 900 cm-1 are characteristic of s-triazine derivatives. This region is remarkably similar to the spectrum for melem,38 suggesting relation to the g-C3N4 structural isomer shown in Figure 1b, rather than the condensed melamine subunit structure in Figure 1a. Voltammograms for ORR are shown in Figure 3a. The carbon black reference (XC-72R, Cabot) has an onset potential of 0.45 V, and the current density steadily increases to 0.91 mA/cm2 at 0 V. The g-C3N4 has a much better onset potential of 0.69 V, but the current density increases at a slower rate up to 0.72 mA/cm2. The onset potential of g-C3N4 is much better than that of carbon black. This confirms that stoichiometric g-C3N4 has greater inherent catalytic activity than pure carbon. The poor current density may arise due to low surface area, measured as just 5 m2/g (2 orders of magnitude less than that of typical electrocatalysts). Such a low surface area would limit the number of available active sites and therefore limit the amount of current
Conclusions In summary, g-C3N4 has been used as a catalyst for oxygen reduction. The fundamental catalytic activity of g-C3N4 was better than that of pure carbon; however, the current densities achieved were low. This was resolved by blending carbon nitride with carbon black, utilizing the catalytic activity of carbon nitride and the high surface area of carbon black. Presently, these materials fall short of the formidable platinum benchmark. However, if the surface area of g-C3N4 can be significantly improved, it may become a promising material for inclusion in fuel cells as a nonprecious alternative to platinum, synthesized at a relatively low temperature. As such, this class of materials is currently under further investigation. Acknowledgment. The authors thank NEDO (New Energy and Industrial Technology Development Organization) for financial support and the other members of our team for their contributions. References and Notes (1) Cohen, M. L. Phys. ReV. B 1985, 32, 7988. (2) Liu, A. Y.; Cohen, M. L. Science 1989, 245, 841. (3) Ortega, J.; Sankey, O. F. Phys. ReV. B 1995, 51, 2624. (4) Kouvetakis, J.; Bandari, A.; Todd, M.; Wilkens, B.; Cave, N. Chem. Mater. 1994, 6, 811. (5) Kroke, E.; Schwarz, M.; Horath-Bordon, E.; Kroll, P.; Noll, B.; Norman, A. D. New J. Chem. 2002, 26, 508. (6) Deifallah, M.; McMillan, P. F.; Cor, F. J. Phys. Chem. C 2008, 112, 5447. (7) Miller, D. R.; Wang, J.; Gillan, E. G. J. Mater. Chem. 2002, 12, 2463. (8) Li, C.; Cao, C.-B.; Zhu, H.-S. Mater. Lett. 2004, 58, 1903. (9) Zhao, Y. C.; Yu, D. L.; Zhou, H. W.; Tian, Y. J. J. Mater. Sci. 2005, 40, 2645. (10) Montigaud, H.; Tanguy, B.; Demazeau, G.; Alves, I.; Courjault, S. J. Mater. Sci. 2000, 35, 2547. (11) Khabashesku, V. N.; Zimmerman, J. L.; Margrave, J. L. Chem. Mater. 2000, 12, 3264. (12) Guo, Q.; Xie, Y.; Wang, X.; Lv, S.; Hou, T.; Liu, X. Chem. Phys. Lett. 2003, 380, 84. (13) Li, C.; Yang, X.; Yang, B.; Yan, Y.; Qian, Y. Mater. Chem. Phys. 2007, 103, 427.
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