3234 Chem. Mater. 2009, 21, 3234–3241 DOI:10.1021/cm900747t
Cobalt Porphyrin Functionalized Carbon Nanotubes for Oxygen Reduction Wei Zhang,†,§ Ali U. Shaikh,†,‡ Emily Y. Tsui,† and Timothy M. Swager*,† †
Department of Chemistry and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. ‡ Visiting Faculty. Permanent address: Department of Chemistry, University of Arkansas at Little Rock, Little Rock, AR 72204 Received March 18, 2009. Revised Manuscript Received May 15, 2009
Carbon nanotube (CNT) compositions were prepared by covalently grafting a Co(II) porphyrin to functionalized multiwalled carbon nanotubes (MWCNTs) via zwitterionic functionalization of the CNT sidewalls followed by a SN2 substitution reaction. The MWCNT-Co-porphyrin compositions, mixed with Nafion, displayed excellent catalytic performance for oxygen reduction in acidic media (pH range, 0.0-5.0) at room temperature. With low catalyst loading, the oxygen reduction rates achieved are more than 1 order of magnitude higher than previously reported values for similar Coporphyrin catalysts. These results demonstrate the advantages of systems of MWCNTs covalently linked to electrocatalytic molecules. The electrodes are easily fabricated by a drop-casting vacuum drying procedure. Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements revealed the mechanism to be a direct four-proton and four-electron reduction of oxygen to water. These results demonstrate that new MWCNT electrocatalytic systems are potential substitutes for platinum or other metal-based cathode materials in proton conducting membrane fuel cells. Introduction There is great interest in the electrocatalytic reduction of oxygen to water at the cathodes of low temperature fuel cells.1,2 A critical need is lowering the cost of the material while minimizing any overpotential required for cathodic reduction of oxygen, maintaining high proton mobility, and ensuring a low electrical resistance of the electrolytic membrane. The materials that have been investigated include platinum-based alloys,3-5 platinum nanoparticle § Current address: Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309.
(1) Ball, S. C. Platinum Met. Rev. 2005, 49, 27–32. (2) Adzic, R. In Electrocatalysis; Lipkowski, J, Ross, P. N., Eds; 1998; Chapter 5, pp 197-242. (3) Paulus, U. A.; Wakaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 2002, 47, 3787–3798. (4) Mukherjee, S.; Srinivasan, S.; Soriaga, M. P. J. Phys. Chem. 1995, 99, 4577–4589. (5) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B 2005, 56, 9–35. (6) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 495, 134–145. (7) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 508, 41–47. (8) Geneis, L.; Faure, R.; Durand, R. Electrochim. Acta 1998, 44, 1317–1327. (9) Higuchia, E.; Uchidab, H.; Watanabe, M. J. Electroanal. Chem. 2005, 583, 69–76. (10) Weia, Z. D.; Chanb, S. H.; Lia, L. L.; Caja, H. F.; Xiab, Z. T.; Sunc, C. S. Electrochim. Acta 2005, 50, 2279–2287. (11) Okada, T.; Katou, K.; Hirose, T.; Yuasa, M.; Sekine, I. J. Electrochem. Soc. 1999, 146, 2562–2568. (12) Kingsborough, R. P.; Swager, T. M. Chem. Mater. 2000, 12, 872–874. (13) Marcotte, S.; Villers, D.; Guilett, N.; Rou, L.; Dodelet, J. P. Electrochim. Acta 2004, 50, 179–188. (14) Okada, T.; Yoshida, M.; Hirose, T.; Kasuga, K.; Yu, T.; Yuasa, M.; Sekine, I. Electrochim. Acta 2000, 45, 4419–4428.
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composites,6-10 electropolymerized cobalt salens,11,12 and chemically designed cobalt compounds,13-23 as well as multiple other compostions.21,24-28 Co(II) porphyrins generally catalyze oxygen reduction via the two-electron reduction of oxygen to hydrogen peroxide (H2O2) followed by further two-electron reduction to water.29 (15) Durand, R. C.; Anson, F. C. J. Electroanal. Chem. 1982, 134, 273– 289. (16) Steiger, B.; Anson, F. C. Inog. Chem. 2000, 39, 4579–4585. (17) Vasudevan, P.; Mann, S. N.; Sudha, T. Trans. Met. Chem. 1990, 15, 81. (18) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027–6036. (19) Guilard, R.; Barbe, J.-M.; Stern, C.; Kadish, K. M. In The Porphyrin Handbook; Kadish, K. M., Smith, J. R. L., Guilard, R., Eds.; Academic Press: Boston, 2003; Vol. 18, pp 303-349. (20) Collman, J. P.; Boulatov, R.; Sunderland, C. J. In The Porphyrin Handbook; Kadish, K. M., Smith, J. R. L., Guilard, R., Eds.; Academic Press: Boston, 2003; Vol. 18, pp 1-49. (21) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437– 444. (22) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.; Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 10013–10020. (23) Kadish, K. M.; Fremond, L.; Ou, Z.; Shao, J.; Shi, C.; Anson, F. C.; Burdet, F.; Gros, C. P.; Barbe, J.-M.; Guilard, R. J. Am. Chem. Soc. 2005, 127, 5625–5631. (24) Collman, J. P.; Chang, L. L.; Tyvoll, D. A. Inorg. Chem. 1995, 34, 1311–1324. (25) Zhang, L.; Song, C.; Zhang, J.; Wang, H.; Wilkinson, D. P. J. Electrochem. Soc. 2005, 152, A2421–A2426. (26) Brisard, G.; Bertranda, N.; Ross, P. N.; Markovic, N. M. J. Electroanal. Chem. 2000, 480, 219–28. (27) Duron, S.; Rivera-Noriega, R.; Nkeng, P.; Poillerat, G.; SolorzaFeria, O. J. Electroanal. Chem. 2004, 566, 281–289. (28) Shi, C.; Anson, F. C. Inorg. Chem. 1994, 225, 215–227. (29) Oxygen reduction by use of dual catalysts was also reported, which involved two-electron reduction of oxygen to H2O2 by a cobalt porphyrin catalyst, followed by further two-electron reduction of H2O2 to water by Prussian blue nanoparticles in MWCNT/ionic liquid medium. See Yu, P.; Yan, J.; Zhao, H.; Su, L.; Zhang, J.; Mao, L. J. Phys. Chem. C 2008, 112, 2177–2182.
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r 2009 American Chemical Society
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Some cobalt porphyrins, however, have shown direct four-electron reduction of oxygen to water at more positive potentials (lower overvoltage) comparable to that of a bare platinum electrode,21-23 which is a primary requirement for viable proton conducting membrane fuel cells (PCMFCs). Recently, studies have shown that platinum nanoparticles dispersed in polymers29,30 and carbon nanotube-cobalt-porphyrin composites31-33 enhance the oxygen reduction efficiency. Catalysts are often mixed with Nafion (polytetrafluoroethylene backbone with perfluorosulfonic acid moieties) to achieve uniform dispersion, enhanced proton mobility, and strong binding of the catalyst with the supporting electrode.33 Carbon nanotubes (CNTs), as a result of their conducting properties and high surface areas,34 have great potential as cathode materials for oxygen reduction in PCMFCs. Chemically and electrochemically modified CNTs have been used extensively as electrocatalysts for oxidation and reduction of a variety of chemicals, including oxygen reduction.35,36 CNTs can further enhance mechanical strength and electrical conductivity and provide chemical stability in highly corrosive environments.37-41 Examples include a Pt/single-walled carbon nanotube (SWCNT) composite, which showed a twofold increase in rate constant for oxygen reduction, compared to similar Pt/carbon black composites33 and a multiwalled carbon nanotube (MWCNT)/cobalt porphyrin suspension drop-cast on a glassy carbon electrode, which exhibited increased current density and a more positive reduction potential but a stepwise process for oxygen reduction.32 In the latter, the increased catalytic activity was proposed to result from absorption of cobalt porphyrin catalysts into the cavities formed between MWCNTs cast on the electrode.32 Although this previously reported MWCNT/cobalt porphyrin composite shows improved catalytic performance, it does not catalyze oxygen reduction with the four-electron mechanism (30) Coutanceau, C.; Croissant, M. J.; Napporn, T.; Lamy, C. Electrochim. Acta 2000, 46, 579–588. (31) Zhou, Q.; Li, C. M.; Li, J.; Cui, X.; Gervasio, D. J. Phys. Chem. 2007, 111, 11216–11222. (32) Qu, J.; Shen, Y.; Qu, X.; Dong, S. Electroanalysis 2004, 16, 1444– 1450. (33) Konkanand, A.; Kuwabata, S.; Girishkumar, G.; Kamat, P. Langmuir 2006, 22, 2392–2396. (34) Baughman, R. H.; Zhakidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (35) Wildgoose, G. G.; Banks, C. E.; Leventis, H. C.; Compton, R. G. Microchim. Acta 2006, 152, 187–214. (36) Zagal, J. H.; Griveau, S.; Ozoemena, K. I.; Nyokong, T; Bedioui, F. J. Nanosci. Nanotechnol. 2009, 9, 2201–2214. (37) Iijima, S. Nature 1991, 354, 56. (38) Salvetat, J. P.; Bonard, J. M.; Thompson, N. H.; Kulik, A. J.; Forro, L.; Benoit, W.; Zuppiroli, L. Appl. Phys. A 1999, 69, 255. (39) Smith, B. W.; Benes, Z.; Luzzi, D. E.; Fischer, J. E.; Walters, D. A.; Casavant, M. J.; Schmidt, J.; Smalley, R. E. Appl. Phys. Lett. 2000, 77, 663. (40) Hone, J.; Llaguno, M. C.; Nemes, N. M.; Johnson, A. T.; Fischer, J. E.; Walters, D. A.; Casavant, M. J.; Schmidt, J.; Smalley, R. E. Appl. Phys. Lett. 2000, 77, 666. (41) It has been reported that functionalization remarkably reduces the cytotoxic effects of CNTs, while increasing their biocompatibility, particularly compared to pristine, purified CNTs. See (a) Prato, M.; Kostarelos, K.; Bianco, A. Acc. Chem. Res. 2008, 41, 60. (b) Singh, R.; Pantarotto, D.; Lacerda, L.; Pastorin, G.; Klumpp, C.; Prato, M.; Bianco, A.; Kostarelos, K. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3357.
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as the predominant process. These studies were also conducted at pH 3.8, which is less acidic than the practical PCMFC working condition (pH ∼ 0), and the performance of the composite in a strong acidic medium was not evaluated. To facilitate the practical applications of PCMFCs, improved systems that efficiently incorporate CNTs and Co-porphyrin catalysts are deserving of investigation. The catalytic system should ideally be able to catalyze oxygen reduction exclusively through a four-electron mechanism in a strong acidic medium (e.g., pH∼0). Recently, we have developed a highly efficient modular zwitterion-mediated approach to functionalize fullerenes and CNTs (both single- and multiwalled).42 This approach enables CNT functionalization under mild conditions and on large-scales (10+g).43 In this report, we utilized this zwitterion method to functionalize MWCNTs with a cobalt(II)-porphyrin. The covalently linked composition (MWCNT-Co-porphyrin) is readily dispersible in DMF and can be drop-cast on a glassy carbon surface for oxygen reduction studies in aqueous acidic media (pH=0). The composition was also mixed with Nafion to produce a smooth thin film that was electroactive in a strong acidic medium for prolonged periods. Carbon nanotubes are stable to Brønstead acids at elevated temperatures,44 and therefore composites may be designed that are suitable for high temperature fuel cells. The synthesis and electrochemical characteristics of the MWCNT-Co-porphyrin catalyst at room temperature are reported herein. Results and Discussion Functionalized MWCNT 2 was easily prepared by adapting our previously reported zwitterion approach.42,43 The reaction was conducted in p-dioxane at 95 °C, with dichloroethyl acetylenedicarboxylate45 as the electrophile and 4-dimethylaminopyridine (DMAP) as the nucleophile. The product was characterized by X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) (Figures S1 and S2, Supporting Information). XPS displays signals for carbon, oxygen, and chlorine atoms, indicating successful installment of chloroethyl groups on the CNT surface. The peaks at 198, 270, 283, and 530 eV correspond to the chlorine 2p and 2s and carbon and oxygen 1s core-level energies. TGA showed a weight loss of 30% (compared to pristine MWCNT), corresponding to 1 functional group for 46 CNT carbon atoms. An SN2 substitution reaction between MWCNT 2 and Co-porphyrin compound 3 in THF at 90 °C provided the target Co-porphyrin-MWCNT composition 4 (Scheme 1). XPS of such composition clearly shows that the successful incorporation of Co centers (Figure S3, Supporting Information) with signals at 780 and 795 eV (42) Zhang, W.; Swager, T. M. J. Am. Chem. Soc. 2007, 129, 7714–7715. (43) Zhang, W.; Sprafke, J. K.; Ma, M.; Tsui, E. Y.; Sydlik, S. A.; Rutledge, G. C.; Swager, T. M. 2009, 131, 8446-8454. (44) Li, L.; Xing, Y. J. Power Sources 2008, 178, 75–79. :: (45) Tom, D. H.; Munz, C.; Muller, C. J. J. Organomet. Chem. 1990, 384, 243.
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Scheme 1. Synthesis of Co-Porphyrin Functionalized MWCNT 4
corresponds to the Co 2p3/2 and Co 2p1/2 core-level energies, respectively.46 Composition 4 was also characterized by SEM (Figure S4, Supporting Information). The oxygen reduction catalyzed by composition 4 was initially conducted in 1.0 M sulfuric acid, mimicking PCMFC working conditions. Composition 4 was mixed with Nafion (0.5 composition 4/Nafion, mass ratio), drop-cast onto a carbon electrode, and dried under high vacuum for 3 min before use. This MWCNT-Co-porphyrin catalytic system showed a reduction peak potential of 0.25 V (vs Ag/AgCl electrode). This potential is far more positive (less overvoltage) than that on a bare glassy carbon electrode (-0.6 V vs Ag/AgCl electrode). The onset of oxygen reduction on the platinum electrode exhibits a much lower overpotential and is observed at approximately +0.68 V vs Ag/AgCl. Composite 4 maintains constant catalytic activity in a strongly acidic medium (1.0 M sulfuric acid) for months, thus exhibiting the desired characteristics of an efficient electrocatalyst. Figure 1a shows the cyclic voltammogram of drop-cast composition 4 as a function of the number of layers (mixed with Nafion) in air-saturated 1.0 M H2SO4 solution. As can be seen from the response in deaerated solution (Figure 1b), at 100 mV/s scan rate, the redox peaks of the catalyst are very small. At much higher scan rates (e.g., 1000 mV/s), the catalyst displays more prominent redox signals. In contrast, in air-saturated solutions higher currents are readily observed at 100 mV/s scan rate, thereby indicating the catalytic activity of the MWCNT-Co-porphyrin composite. As the number of thickness of composition 4/Nafion coating is increased with sequential drop casting of layers, the peak potential for oxygen reduction gradually shifts to more positive values, along with an increased current, but eventually stabilizes after seven layers. Similar behavior was (46) On the basis of the integration between Co and O atom peaks (Co/ O, 1/134), the Co loading was estimated to be 1 Co per 33 functional sites. The Co amount may in fact be greater, considering the possible interference from some oxygen containing contaminants.
Figure 1. Cyclic voltammograms of MWCNT-Co-porphyrin (4, mixed with Nafion) in 1.0 M sulfuric acid at 22 °C (a) drop-casted with different numbers of layers in air-saturated solution with a scan rate of 100 mV/s; (b) the electrocatalyst response (8 layers) with different scan rates in deaerated solution.
reported for a multilayered composite of Co(II) porphyrin with multi-walled carbon nanotubes.32 The increased activity of the multilayer catalyst toward oxygen reduction is attributed to increased Co(II) loading to give higher density of catalytic sites on the electrode surface.47 In contrast to the high catalytic activity of composition 4/ Nafion, drop cast films (single or multilayer) of Coporphyrin compound 3 or functionalized MWCNTs 2 showed little activity toward oxygen reduction. Drop cast films from mixtures of 2 and 3 gave poorly defined oxygen reduction peaks (Figure S6, Supporting Information), presumably due to the aggregation of free Co(II)-porphyrin compounds. These results demonstrate the importance of covalently anchoring Co-porphyrin centers onto the MWCNT surface in the catalytic system, which minimizes the aggregation of Co-porphyrin complexes and facilitates oxygen reduction. Our drop-cast/vacuum drying approach to electrode modification also offers advantages relative to other absorption/slow dry processes,32 which are more time-consuming and can lead to difficulties in dealing with metastable dispersions. SEM characterization (Figure 2) of the drop-cast materials reveals that the MWCNTs are covered in the Nafion matrix (for an SEM of composition 4 without Nafion see (47) Increased cobalt loading on the electrode surface was verified by cyclic voltammogram of the drop-cast material with different numbers of layers (Figure S5, Supporting Information). The data was acquired in a deaerated, 1 M HClO4 solution. The redox signal of the cobalt center shows significant increase from one layer to two layers, but after four to five layers, the redox signal only shows small changes. The thickness of each drop-cast layer is estimated to be 1.2 μm (density of the composite F=0.85 g/cm3; diameter of the electrode d=0.62 cm; mass of each composite layer m=3 10-5 g).
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Figure 2. SEM of MWCNT-Co-porphyrin composition 4 mixed with Nafion in 1/2 (compound 4/Nafion) mass ratio.
Figure S4, Supporting Information). The concentration of the electroactive cobalt centers per area of the electrode was found to be Γ=1.210-11 mol cm-2 as determined from the area under the cyclic voltammogram recorded in dioxygen-free 1 M HClO4.23 From simple molecular modeling the area of the Co-porphyrin is approximately 660 A˚2, and hence a dense packed monolayer with the plane of the porphyrin parallel to a flat surface would give a coverage of Γ=2.510-11 mol cm-2. Considering that the surface is also largely covered with Nafion, suggests that the underlying MWCNT network gives an expanded active electrocatalytic area. In contrast to the highly efficient oxygen reduction behavior of MWCNT-Co-porphyrin compositions, SWCNT-Co-porphyrin48 materials showed very low activity, as depicted in Figure 3. Although pristine SWCNT films can exhibit high electrical conductivity, the covalent modification of the sidewalls dramatically reduces their ability to transport charge between the Coporphyrin and the electrode. In the case of MWCNTs only the outer walls are damaged by the covalent modification and the inner tubes maintain their high conductivity.49 Given the higher catalytic activity of MWCNTCo-porphyrin compositions, an eight-layer drop-cast of the composite of 4/Nafion was utilized for all of the electrochemical studies described hereafter. The durability and catalytic performance of composite 4/Nafion was studied by exposing the modified electrode in 1.0 M sulfuric acid solution for prolonged periods (48) The SWNT-Co-porphyrin composition was prepared by using the same reaction condition as for preparing MWNT-Co-porphyrin composition. (49) Li, Z.; Kandel, H. R.; Dervishi, E.; Saini, V.; Xu, Y.; Biris, A. R.; Lupu, D; Salamo, G. J.; Biris, A. S. Langmuir 2008, 2655–2662. (50) A small shift to less positive potentials was observed in the oxygen reduction after the exposure to sulfuric acid for a prolonged period of time. (51) The “stability” claimed means the catalytic performance of the MWCNT-Co-porphyrin/Nafion composite is stable and repeatable. It is possible that after a prolonged period of time of exposure to the 1 M H2SO4 solution, the ester bonds in the composite are cleaved via hydrolysis. However, because of the poor solubility of the free Co-porphyrin compounds and their strong noncovalent interactions with MWCNTs, they likely remain in the film and continue to act as the catalytic centers.
Figure 3. Cyclic voltammograms of MWCNT-Co-porphyrin (a) and SWCNT-Co-porphyrin (b) in 1.0 M sulfuric acid at 22 °C (air-saturated).
Figure 4. Cyclic voltammograms of MWCNT-Co-porphyrin (4, mixed with Nafion) after 30 s (green line), 48 h (purple line), and 120 h (blue line) exposure to air-saturated 1.0 M sulfuric acid at 22 °C.
(48-120 h) followed by the CV analysis. We observed reproducible cyclic voltammograms for oxygen reduction after an initial small shift and loss of current (Figure 4),50 thereby indicating reasonable long-term stability for this catalytic system.51 Modified electrodes that had been stored in air for four months also did not exhibit any noticeable change in the shape and height of the oxygen reduction current. To better understand the MWCNT-Co-porphyrin/ Nafion catalytic system, we studied its catalytic activity by rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) experiments. The rotating disk electrode reduction curves at different rotation rates in airsaturated 1.0 M perchloric acid52 are shown in Figure 5a. Representative ring current and disk current responses at 2500 rpm (Figure 5b) show a lower ring current consistent (52) Perchloric acid was used instead of sulfuric acid in our mechanism study for the purpose of comparing the catalytic behavior of our system to other previously reported systems, which were also studied in perchloric acid solution; see refs 23, 25, and 32.
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A plot of jlim-1 versus ω-1/2 gives a slope of 1/0.62nFCD2/3υ-1/6, and the resultant Koutecky-Levich plots for our experimental results are compared to theoretical (n = 2 and 4) data in Figure 6. The slopes of the experimental data closely match the theoretical n=4 plot with corresponding slopes of 12.456 and 12.376, thereby indicating that we observe four-electron oxygen reduction in a single step. We also calculated the number of electrons (n) involved based on RRDE data (Figure 5b) by using the equation: n ¼ 4I disk =ðI disk þ I ring =N Þ
Figure 5. (a) RRDE curves for the reduction of oxygen in 1.0 M perchloric acid (22 °C) at various electrode rotating speeds; (b) the diskring current at 2500 rpm.
with minimal production of H2O2. Our results compare favorably with those of Kadish et al.23 and Qu et al.32 (Table 1) and we observe current densities per electroactive Co center that were about 80 times higher than the graphite bound Co(III) porphyrin-corrole dyad at pH 0.0, and 200 times higher than MWCNT/cobalt porphyrin composite mixture at pH 3.8. As a result, covalently grafting of cobalt porphyrin catalysts to CNTs has merit in generating electrocatalytic systems. The limiting current densities (jlim) for the oxygen reduction at the disk electrode can be determined from the disk currents (current density=limiting current/electrode surface area) using the RDE plots (Figure 5a). The Levich current (jlev) and the kinetic current ( jk) are related to the measured current-limited chemical reaction and can be calculated from the Koutecky-Levich equation (eq 1):53,54 1=jlim ¼ 1=jLev þ 1=jk
ð1Þ
where jLev = 0.62nFCD2/3υ-1/6ω1/2 (n= number of electrons, F = Faraday = 96 486.4 Coulombs, C = molar concentration of oxygen=2.4 10-4 M at 22 °C, D= diffusion coefficient of oxygen in water at 22 °C=1.7 10-5 cm2/s, υ = kinematic viscosity of the solution at 22 °C=0.01 cm2/s, and ω=angular velocity of the disk= 2πN, where N is the linear rotation speed); jk = rate of kinetically limited reaction=103nFkCΓ (k=rate constant for the oxygen reduction, Γ=1.2 10-11 mol/cm2). (53) Koutecky, J.; Levich, V. G. Zh. Fiz. Khim. 1958, 32, 1565. (54) Treimer, S.; Tang, A.; Johnson, D. C. Electroanalysis 2002, 14, 165–171.
ð2Þ
where Idisk and Iring are the limiting currents for disk and ring electrodes, respectively (N is the collection efficiency of the electrode). The calculated n value is 3.7, which is slightly lower than the value determined from the RDE experiment. The effect of pH on the MWCNT-Co-porphyrin/ Nafion catalytic system was also investigated. The calculated values of n from the Koutecky-Levich plots are close to 4.0 for acid solutions with pH values ranging from 0.0 to 3.75. As expected, at higher pH the calculated values are significantly lower (n=3.3 for pH 5.0) because fewer of the required protons (eq 3)21 are available. In contrast to the relatively consistent n values obtained from RDE data at low pH, n values calculated from the RRDE data (eq 2, using both disk and ring currents) show significant variation as the rotation rate of the electrode was varied. With increasing rpm, the calculated n values were higher. Nevertheless, the values obtained at 3600 rpm (see Table 2) are considerably lower than those obtained from the Koutecky-Levich plots. At lower rpm, other convectional factors may interfere with the electrode measurements,55 and the results from the Koutecky-Levich plots are likely more reliable. On the basis of the RDE data acquired in solutions of various pH, values of n, jk, and k were calculated from the Koutecky-Levich equation (eq 1), and the results are shown in Table 2. The rate constant appears to be slightly higher at lower pH. It should be noted that even with very low active cobalt loading (Γ = 1.2 10-11 mol cm-2 compared to Γ = 1.1 10-9 mol cm-2 as previously reported23), the oxygen reduction rate constant is one to two orders of magnitude higher than other Co-porphyrins bound to graphite (1.8 106 M-1 s-1 vs 0.2 105 to 3.0105 M-1 s-1).23,56 The oxygen reduction capability of the electrocatalyst varies significantly with pH of the acidic medium. With lowering pH, the reduction potential shifts to more positive value (Figure 7), indicating greater ease for oxygen reduction to occur.21 The reduction reaction is a four-electron process: O2 ðgÞ þ 4Hþ ðaqÞ þ 4e - f 2H2 OðlÞ
ð3Þ
(55) Adams, R. N. Marcel Dekker: New York, 1969; p 92. (56) Durand, R. R.Jr.; Bencosme, C. S.; Collman, J. P.; Anson, F. C. J. Am. Chem. Soc. 1983, 105, 2710.
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Table 1. Rotating Disk Current Response to Various Types of Electrodes Attached to Glassy Carbon Electrodea type of catalyst (attached to glassy carbon electrode)
source Kadish et al.23 Qu et al.32 present work
Co(III) porphyrin-corrole dyad (adsorbed by dip coating) MWCNT/ Co(III) porphyrin (adsorbed by dip coating) MWCNT covalently attached to Co(II) porphyrin
electrode surface area (AS), cm2
catalyst surface concentration (CS), mol/cm2
0.282
1.10 10-9
0.071 0.247
rotating disk current (ID) at 2500 rpm
current sensitivity ratio [ID/ASCS] 7.4 1011
8.5 10-9
230 (estimated) (pH = 0.0) 85 (estimated) (pH = 3.8)
1.2 10-11
175 (pH = 0.0)
5.9 1013
85 (pH = 3.8)
2.9 1013
1.4 1011
a Current sensitivity ratio was calculated on the basis of direct proportionality of current with electrode surface area and surface concentration on a solid electrode.53
Figure 6. Koutecky-Levich plot of RDE studies of oxygen reduction at pH 0 (1.0 M perchloric acid).
Figure 7. Cyclic voltammograms of MWCNT-Co-porphyrin (4, mixed with Nafion) in air-saturated solutions of various pH at 22 °C.
Table 2. Calculated Values of n, jk, and k for Oxygen Reduction in Acidic Solutions with Different pH Values
density on the MWCNTs.
pH 0.0 1.6 2.5 3.75 5.0
n (KouteckyLevich plot) 4.2 4.0 4.0 4.2 2.9
n (RRDE data at 3600 rpm) 3.7 3.8 3.7 3.2 3.4
jk(KouteckyLevich plot)
k (mol-1 s-1)
2.141 1.774 1.422 1.285 1.073
1.8 10 1.6 106 1.3 106 1.1 106 1.3 106
MWCNT-CoðIIÞ þ O2 f MWCNT-CoðIIIÞ-O-O
ð4Þ
MWCNT-CoðIIIÞ-O-O þ 4Hþ þ 4e - f MWCNT-CoðIIÞ þ 2H2 O
ð5Þ
6
Our experimental data yield a slope of -0.066 V per pH unit for the plot of E1/2 (half-wave potential57 for oxygen reduction) vs pH, which is close to the expected slope of -0.059 V per pH unit, which is also consistent with a clean oxygen reduction to H2O in a direct four-proton and four-electron mechanism from an initial cobalt oxygen complex (eqs 4 and 5).58 It is important to note that we are not sure if the oxygen is bound to a single cobalt center or if bridged Co-O-O-Co species are also present. Future studies will seek to correlate catalytic activity with catalyst (57) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. The half-wave potential is estimated to be the potential corresponding to 85% of the cathodic peak current (preceding the reduction) in cyclic voltammetric studies at the stationary solid electrode. (58) The mechanism shown in eqs 4 and 5 is a simple illustration, and other possible intermediates during the reduction process are not taken into consideration. There is the possibility of multiple cobalt centers participating in the oxygen reduction. Further studies are in progress to elucidate a detailed mechanism of the reduction process.
Conclusion We have prepared CNT-Co-porphyrin compositions via zwitterionic functionalization of the CNT sidewalls followed by SN2 substitution reactions. Covalently grafting the Co(II) porphyrin compound 3 to functionalized multiwalled carbon nanotubes (2) provides the MWCNT-Co-porphyrin composition 4, which showed excellent catalytic performance for oxygen reduction in acidic media (with pH ranging from 0.0 to 5.0) at room temperature (22 °C). With low cobalt loading, significant oxygen reduction rates were achieved that are more than one order of magnitude higher than previously reported values for similar Co-porphyrin catalysts. These results demonstrate the advantages of systems of MWCNTs covalently linked to electrocatalytic molecules. The electrodes are easily fabricated by a drop-cast/vacuum drying procedure. A direct four-proton and four-electron reduction of oxygen to water was experimentally verified through RDE and RRDE measurements. Our results
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demonstrate that new MWCNT electrocatalytic systems may be possible substitutes for platinum or other metalbased cathode materials in proton conducting membrane fuel cells. Experimental Section Materials. MWCNTs were obtained from Bayer Group (BaytubesC 150 P, >95% purity) and used as received. Nafion perfluorinated resin solution (5 wt % in water) was obtained from Aldrich and used as received. All solvents were of spectroscopic grade unless otherwise noted. Anhydrous tetrahydrofuran was obtained using a solvent purification system (Innovative Technologies). All other chemicals were of reagent grade and used as received. General Information. Nuclear Magnetic Resonance (NMR) spectra were recorded on Inova-500 NMR Spectrometer. Chemical shifts are referenced to residual solvent. Mass spectra were obtained on Bruker Daltonics APEXIV 4.7 T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). The spectra in the UV-vis-NIR range were obtained using a Cary 6000i UV-vis-NIR spectrometer. TGA analyses were performed with a TGA Q50 apparatus (TA Instruments). Experiments were carried out under nitrogen. Samples were heated at 5 °C/min from 30 to 800 °C. XPS spectra were recorded on a Kratos AXIS Ultra X-ray photoelectron spectrometer. Electrochemical experiments (cyclic voltammetry, rotating disk voltammetry, and rotating ring-disk voltammetry) were conducted using an Autolab Model PGStat30 electrochemical analyzer. All synthetic manipulations were carried out under an argon atmosphere using standard Schlenk techniques unless otherwise noted. Glassware was oven-baked and cooled under N2 atmosphere. a. Synthesis of MWCNT-Co-Porphyrin Composition. Synthesis of Chloroethyl-Substituted MWCNTs (2)42,43. A suspension of as-received MWCNTs (20.0 mg, 1.67 mmol of carbon) in p-dioxane (40 mL) was sonicated for 3 min using an ultrasonic probe (Branson Sonifier 450, 60 W, 20 kHz). The heterogeneous solution was heated at 95 °C. To the MWCNT suspension were added simultaneously a solution of dichloroethyl acetylenedicarboxylate (0.996 g, 4.17 mmol)45 in THF (10 mL) and a solution of 4-dimethylaminopyridine (0.611 g, 5.00 mmol) in THF (10 mL) via syringe pump within 36 h. The resulting mixture was stirred at 95 °C for another 6 h. The reaction mixture was cooled to room temperature, then centrifuged at 5000 rpm for 5 min. The supernatant was discarded, and the residue was dispersed in DMF for 5 min using an ultrasonic bath. The mixture was centrifuged (5000 rpm, 5 min), and the supernatant was discarded. The same sequence was repeated twice with DMF and acetone used as solvents. Functionalized MWCNT 2 was dried under vacuum overnight. See Figure S1 and S2 (Supporting Information) for XPS and TGA characterization data. 5-(4-Hydroxyphenyl)-10,15,20-triphenylporphyrin59. Pyrrole (13.9 mL, 200 mmol), benzaldehyde (15.3 mL, 150 mmol), and p-hydroxybenzaldehyde (6.11 g, 50 mmol) were added to refluxing propionic acid (500 mL). The reaction mixture was refluxed for 40 min. After cooling to room temperature, the solvent was removed in vacuo. The black residue was washed with methanol, and the solid was purified by column chromatography with chloroform and further washed with methanol to yield the (59) D’Souza, F.; Gadde, S.; Zandler, M. E.; Arkady, K.; EI-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. A 2002, 106, 12393.
Zhang et al. product as a purple solid (1.8 g, 6%). 1H NMR (500 MHz, CDCl3): δ 8.89-8.85 (m, 8H), 8.23 (d, J=7.5 Hz, 6 H), 8.08 (d, J=8.4 Hz, 2 H), 7.80-7.74 (m, 9 H), 7.19 (d, J=8.4 Hz, 2 H), 5.09 (bs, 1 H), -2.76 (bs, 2 H). 13C NMR (125.8 MHz, CDCl3): δ 155.59, 142.40, 142.38, 135.93, 134.97, 134.77, 127.91, 126.89, 120.30, 120.22, 120.04, 113.89. m/z (ESI) [M + H]+=631.2513, calculated for (C44H30N4O) = 631.2492. UV-vis (CH2Cl2), λmax/nm (ε): 418 (421203), 515 (16446), 550 (7664), 591 (4725), 648 (4825). 5-(4-Hydroxyphenyl)-10,15,20-triphenylporphyrin cobalt(II) (3). 5-(4-Hydroxyphenyl)-10,15,20-triphenylporphyrin (200 mg, 0.313 mmol), and Co(OAc)2 3 4H2O (79 mg, 0.313 mmol) were dissolved in DMF (20 mL). The reaction mixture was refluxed for 25 min and then quenched with 20 mL of H2O. The mixture was cooled overnight at 0 °C, and the solid was collected by filtration, washed with H2O, and dried to yield 3 as a red solid (190 mg, 87%). m/z (ESI) [M + H]+=688.1640, calculated for (C44H28CoN4O)=688.1668. UV-vis (CH2Cl2), λmax/nm (ε): 411 (102172), 538 (11134). Synthesis of MWCNT-Co-porphyrin Composition (4). To a solution of Co-porphyrin complex 3 (13.4 mg, 0.0195 mmol) in dry THF (2.5 mL) was added NaH (14.8 mg, 0.617 mmol) under nitrogen purge. The resulting mixture was stirred at 60 °C for 5 h, followed by addition of the suspension of functionalized MWCNT 2 (7.5 mg) in THF (2.5 mL). The mixture was stirred at 90 °C for 36 h. After cooling down to room temperature, 8 mL of water was added to the mixture. After centrifuge, the collected black solids were washed with water (10 mL) twice. The purified MWCNT product was then dried under high vacuum overnight. See Figure S3 and S4 (Supporting Information) for XPS and SEM characterization data. b. Electrochemical Studies. A 1.0 M sulfuric acid (VWR Scientific) solution was prepared by dilution of the concentrated acid. Solutions of 1.0 and 0.10 M perchloric acid (Aldrich) were prepared by dilution of the concentrated acid and then titrated with previously standardized (against potassium hydrogen phthalate) 1.0 and 0.10 M sodium hydroxide solutions to accurately determine their concentrations. The pH values were then calculated as 0.00 and 1.00, respectively. Solutions of pH 1.60, 2.00, 2.50, 3.00, and 3.50 (Britton-Robinson buffers) were prepared using a mixture containing 0.040 M each of phosphoric acid, boric acid, acetic acid, and sodium hydroxide (VWR Scientific) that measured a pH of 1.60. Solid sodium hydroxide was then added to the mixture to produce the solutions of higher pH. Solutions of pH 3.75 and 5.00 (acetate buffers) were prepared by titration of 1.0 M acetic acid with 1.0 M NaOH solution until the desired pH was reached. All solutions were made using distilled deionized water. Appropriate amounts of solid KCl were dissolved in each solution to reach a uniform ionic strength of 1.50. Air-saturated buffer solutions were used to measure oxygen reduction. To determine the electroactivity of the MWCNT-Co-porphyrin catalyst, the solutions were purged with high-purity nitrogen (Airgas) to remove oxygen. Electrochemical experiments (cyclic voltammetry and rotating ring-disk voltammetry) were conducted using an Autolab Model PGStat30 electrochemical analyzer using three-electrode and four-electrode probe systems. The graphite disk-platinum ring electrode (RRDE) and a model AFMSR rotor were obtained from the Pine instruments (Grove City, PA), and silversilver chloride reference and platinum wire counter electrodes were purchased from Bioanalytical Systems (W. Lafayette, IN). For ring-disk voltammetric studies, the disk electrode was the primary working electrode, whereas the ring electrode (dual working electrode) was connected to a built-in bipotentiostat
Article (available in PGStat30) to achieve a four-electrode system. For cyclic voltammetric studies, the ring electrode was disconnected to obtain a standard three-electrode system. The electrochemical cell was a 50-mL glass container with a plastic cap through which holes had been cut to insert the electrodes. The experiments were conducted at room temperature (22 °C). The collection efficiency of the RRDE was determined using a ferrocyanide/ferricyanide redox couple in 1.0 M KCl solution and was found to be 0.37 (which is in agreement with the manufacturer of the RRDE). Coating the Disk Electrode with MWCNT-Co-Porphyrin (4) Electrocatalyst. To a dispersion of composition 4 (0.5 mg) in DMF (0.5 mL) was added a solution of Nafion (20 μL, 5 wt %) in water. The resulting mixture was sonicated for 2 min to achieve a homogeneous dispersion. Then 10 μL of the composite solution was added onto the disk electrode to completely cover
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it, followed by drying under high vacuum. The same sequence of drop casting and vacuum-drying was repeated multiple times when necessary to achieve the multiple layers of electrocatalyst (4) coating onto the disk electrode.
Acknowledgment. This work was supported by the U.S. Army through the Institute for Soldier Nanotechnologies (DAAD-19-02-0002) and the National Science Foundation DMR-0706408. Authors thank Dr. Steven Kooi for the help with SEM characterization. Supporting Information Available: XPS, TGA, SEM, and CV characterization of CNT composites (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.
