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Gas-Phase Methanol Electrooxidation Using a Nafion-Coated Integrated Ultramicroelectrode Minoru Umeda, Mohamed Mohamedi, and Isamu Uchida* Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aramaki-Aoba 07, Aoba-ku, Sendai 980-8579, Japan Received June 4, 2001 A novel electrochemical technique based on an integrated ultramicroelectrode (IUME) was developed to study the gas-phase methanol electrooxidation. The IUME consisted of a Pt working, a reference, and a counter electrode, all integrated at a capillary tip and coated by a Nafion thin film. The ultramicroelectrode enabled measurement of the methanol-vapor electrooxidation even without the presence of water molecules. We observed for the first time that the peak potential of methanol oxidation monotonically shifts depending on the water-vapor concentration. Moreover, ac conductivity measurements showed that water-vapor concentration had no effect on the resistance of the Nafion film to make a change in the iR drop. It is, therefore, concluded that the presence of a water molecule strongly affects the methanol oxidation mechanism resulting in a decrease in the overpotential at the Pt electrode.
Introduction Ultramicroelectrodes are attractive to conduct electrochemical measurements because of their small size and low current responses.1,2 The ordinary-size electrodes are incapable of such performances, especially in a high resistive medium and/or in a limited space. Gas-phase electrochemistry has been the center of interest in several fundamental research fields,3 since it contributes to many applications, such as fuel cell gaseous electrode reactions,4,5 especially at the three-phase interface,6 and gas sensors.7,8 Brina et al. showed that a two-electrode-based microelectrode system can detect various organic solvents in the gas phase. This system has been employed as a gas chromatography detector.7,8 However, the electrodes used are unable to provide the electrode potential because of the absence of an incorporated reference electrode. In some direct methanol fuel cells (DMFCs), gasified methanol is supplied to the anode and subsequently electrooxidized, where the water molecule is indispensable to make a six-electron reaction for a complete oxidation, as follows: 9
CH3OH + H2O f CO2 + 6H+ + 6e-
(1)
Thus far, methanol electrooxidation studies at Pt electrode have been mainly restricted to aqueous solutions;10-17 therefore, it is difficult to understand the role of water * To whom correspondence should be addressed. Fax: +81-22214-8646. E-mail:
[email protected]. (1) Fleischmann, M.; Pons, S.; Robinson, D.; Schmidt, P. P. Ultramicroelectrodes; Datatech Systems: Morganton, 1987. (2) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1988; Vol. 15, p 267. (3) Heinze, J. Angew. Chem. 1993, 32, 1268. (4) Appleby, A. J.; Foulkes, F. R. Fuel Cell Handbook; Van Nostrand Reinhold: New York, 1989. (5) Larminie, J.; Dicks, A. Fuel Cell Systems Explained; Wiley: Chichester, 2000. (6) Fuel Cell Handbook, 5th ed.; EG&G Services, Persons Inc., Science Applications International Corporation: Morgantown, 2000; Chapter 1. (7) Brina, R.; Pons S.; Fleischmann, M. J. Electroanal. Chem. 1988, 244, 81. (8) Brina, R.; Pons S. J. Electroanal. Chem. 1989, 264, 121. (9) Randin, J.-P. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 7, Chapter 1. (10) Kunimatsu, K. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1025. (11) Aramata, A.; Veerasai, W. Electrochim. Acta 1991, 36, 1043.
molecules in a condition of an abundant amount of water. Gas-phase electrochemistry seems to be an ideal method to solve the problem since one can easily change the methanol/water ratio, even including water-free methanol vapor. The gas-phase electrochemistry can hardly be investigated using classical electrodes in highly resistive media. This work reports electrochemical measurements of the gas-phase methanol oxidation by employing a newly developed ultramicroelectrode.18 This technique consists of a working, a reference, and a counter electrode. All the electrodes are integrated in a limited area and coated by a polymer electrolyte film to respond to gaseous reactants. Herein, gasified methanol electrooxidation was studied in the presence and in the absence of water molecules. Experimental Section An integrated ultramicroelectrode for the gas-phase measurement was prepared as follows. Three glass tubes of 1.2 mm outer diameter were bundled together, and the tip was formed to a thin capillary with a gas burner. Two Pt wires, 25 and 100 µm in diameter, each connected to a Cu lead wire were separately inserted into the glass tubes to the inside of the tip to form a working and a counter electrode, respectively. Another Pt wire of 100 µm in diameter and 3 mm in length was inserted to the inside of the tip of the remaining glass tube for a liquid junction to the reference electrode.19 Subsequently, the tip of the assembled tubes was heat-sealed, cutoff, and then polished to a mirror finish. Finally, the tube for the reference electrode was filled with saturated KCl aqueous solution and a silver wire covered with AgCl on the surface. The tip of the thus-assembled ultramicroelectrode was immersed in Milli-Q water for 24 h; no ions from the reference electrode were detected in the water by using an inductively (12) Me´li, G.; Le´ger, J.-M.; Lamy, C.; Durand, R. J. Appl. Electrochem. 1993, 23, 197. (13) Wilde, C. P.; Zhang, M. Electrochim. Acta 1994, 39, 347. (14) Frelink, T.; Visscher, W.; Cox, A. P.; van Veen, J. A. R. Electrochim. Acta 1995, 40, 1537. (15) Xia, X. H.; Iwasita, T.; Ge, F.; Vielstich, W. Electrochim. Acta 1996, 41, 711. (16) Wang, K.; Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., Jr. Electrochim. Acta 1996, 41, 2587. (17) Dinh, H. N.; Ren, X.; Garzon, F. H.; Zelenay, P.; Gottesfeld, S. J. Electroanal. Chem. 2000, 491, 222. (18) Umeda, M.; Dokko, K.; Mohamedi, M.; Itoh, T.; Uchida, I. Chem. Lett. 2001, 508. (19) Hills, G. J.; Ives, D. J. G. In Reference Electrodes; Ives, D. J. G., Janz, G. J., Eds.; Academic: New York, 1961; Chapter 2.
10.1021/la010824+ CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001
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Figure 1. Photographs of the integrated ultramicroelectrode: (a) sideview, scale is 1 mm per division; (b) bottom view, scale bar is 200 µm. WE, RE, and CE denote working, reference, and counter electrodes, respectively.
Figure 2. A schematic illustration of the experimental setup. coupled plasma spectrometer (Jarrell Ash, IRIS1000). Then, a Nafion thin layer (proton exchange membrane) was applied on the three-electrode-integrated tip by a dip-coating technique using a Nafion 117 solution (Sigma-Aldrich Japan) resulting in a polymer layer about 2 µm thick. The thickness of the layer was estimated with a profile measurement microscope (Keyence, VF7500). The thus prepared integrated ultramicroelectrode shown in Figure 1 is different from that for simple gas sensing both in structure and in concept.20-22 The ultramicroelectrode was successively immersed in a 3% H2O2 solution for 1 min and in a 1 N H2SO4 solution for 5 min to activate the Nafion layer right before the electrochemical measurements. Figure 2 illustrates the experimental setup. The ultramicroelectrode was inserted to settle in one port of a ‘T’shaped Pyrex cell with a screw thread. Methanol-saturated nitrogen gas (97.5 mmHg methanol partial pressure)23 and watersaturated nitrogen gas (17.4 mmHg water partial pressure)23 were obtained by bubbling N2 dry gas through wash bottles separately filled with methanol (Wako Pure Chemical Industries Japan, HPLC grade, 99.7% purity) and Milli-Q water. The two gases were then introduced individually or mixed into the cell with a total flow rate of 0.4 dm3/min by using a flow controller (20) Kuwata, S.; Miura, N.; Yamazoe, N. Chem. Lett. 1988, 1197. (21) Narayanan, S. R.; Valdez, T. I.; Chun, W. Electrochem. SolidState Lett. 2000, 3, 117. (22) Umeda, M.; Kabasawa, A.; Kokubo, M.; Mohamedi, M.; Itoh, T.; Uchida, I. Jpn. J. Appl. Phys., 2001, 40, 5141. (23) Lange’s Handbook of Chemistry, 12th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1979; Chapter 10.
Figure 3. Cyclic voltammograms of the gas-phase methanol oxidation at the integrated ultramicroelectrode. Flow rate ratios of H2O-saturated N2/methanol-saturated N2 are 0.4:0 (B), 0.3: 0.1 (C), 0.1:0.3 (D), and 0:0.4 (E) in dm3‚min-1. Background (A) was measured in dry N2 atmosphere. Scan rate was 5 mV s-1. Numbers in the figure mean the first and second potential sweeps. (Nippon Tylan, model SG-3). Electrochemical measurements were then conducted in the given gas atmosphere with a potentiostat (Hokuto Denko HA-150). During measurements, the gas flow was stopped. All experiments were carried out at room temperature (20 ( 2 °C).
Results and Discussion Figure 3 shows the gas-phase cyclic voltammograms (CVs) measured at the integrated ultramicroelectrode. All CVs demonstrated a good reproducibility. CVs A and B were recorded under a dried nitrogen gas and a watersaturated nitrogen gas atmosphere, respectively. The profile of CV B well agrees with that obtained in aqueous solution using a conventional three-electrodes system,11-14 demonstrating that the integrated ultramicroelectrode enables one to perform electrochemical measurement under controlled electrode potential in the gas phase. In the case of a dry condition (Figure 3A), a negligible background current is observed, which also means that the electrode potential is regulated in the presence of a Nafion coating even in the dry condition. On the contrary, in the absence of a Nafion coating we observed that the electrode potential was not controlled. It is reported that gas-phase electrode reactions take place without an electrolyte when the electrode gap is smaller than 10
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µm.7,8,22 This is because an ionic conduction occurs on the surface of the narrow insulator with a small iR drop. The integrated ultramicroelectrode having an electrode gap of about 150 µm (see Figure 1) is unable to control the electrode potential and thus requires the use of an electrolyte. In Figure 3, CV C was measured after a 1-h mixed-gas flow of methanol-saturated nitrogen (0.1 dm3/min) and water-saturated nitrogen (0.3 dm3/min). The gas-phase voltammogram exhibits methanol oxidation current peaks at around 0.77-0.79 and 0.65 V vs Ag/AgCl during anodic and cathodic potential scans, respectively. The peak shapes were not affected by successive cycling as seen in the figure. The CV profile is in agreement with those observed in aqueous solutions.10-17 This demonstrates that the integrated ultramicroelectrode can directly measure the electrooxidization of methanol vapor, which takes place at the Pt/Nafion interface, in the presence of water vapor, just like in aqueous solution. Thus, it can be said that the CVs obtained with the integrated ultramicroelectrode represent the three-phase interface of fuel cell electrodes. Voltammograms were also measured by changing the gas mixture ratio and are reported in Figure 3D, E. On comparison of CVs B-E, it is clearly seen that magnitude of the current peaks during anodic scan increases according to the amount of methanol content, while in waterfree methanol vapor (CV E) the apparent peak is not seen. The magnitude of the current peak within 0.77-0.79 V is almost the same as that in CV D. Moreover, the peak potential shifted toward the anodic region with a decrease in water content. The same trend was seen for the current peak during cathodic potential sweep. Therefore, we examined the ac conductivity of the Nafion thin film using an interdigitated microarray (IDA) electrode, which was fabricated on a thermally oxidized Si wafer so as to have two sets of comb-type Pt arrays. Each array had 25 electrode elements, 10 µm width and 2.4 mm length, separated by 10 µm from its adjacent elements.24 The Nafion film of 2 µm thickness was prepared on the IDA electrode by a cast-coat technique. The electrode was placed in the same Pyrex cell in order to evaluate the ac conductivity under similar gas conditions as in Figure 3. The conductivity was measured with an LCR meter (Hewlett-Packard, model 4332A; 1 mV root mean square, 1 kHz). The results are summarized in Table 1. It is seen that the conductivity of the Nafion film is not sensitive to the gas mixture ratio. The ac conductivity data show a good reproducibility; hence, the data are believed to be at equilibrium condition. Thus, the result in Table 1 can be explained by a solvation of sulfonic group with methanol molecules which dissociates the -SO3H into -SO3- and H+ for ionic conduction as well as water.25 According to the ac conductivity result, the aboveobserved peak-potential shift for methanol electrooxidation is considered not to be due to a change in the Nafion(24) Nishizawa, M.; Koshika, H.; Uchida, I. J. Phys. Chem. B 1999, 103, 192. (25) Handbook of Chemistry, 3rd ed.; The Chemical Society of Japan, Ed.; Maruzen: Tokyo, 1984; p II-345.
Letters Table 1. ac Conductivity of Nafion Film Prepared on an IDA Electrodea gas flow rate (dm3‚min-1) H2O saturated N2
methanol saturated N2
ac conductivity (S‚cm-1)
0.4 0.3 0.1 0.0
0.0 0.1 0.3 0.4
2.84 × 10-4 3.05 × 10-4 3.16 × 10-4 3.05 × 10-4
a
The film thickness was 2 µm.
film resistance that might cause iR drop in voltammetry. If we assume that a small amount of water molecules remaining in the membrane participate in the reaction under water-free methanol vapor supply, the CV profile will change according to the consumption of water molecule by the reaction. One would see a shift in the peak potential toward negative regions. However, the CV never changed by successive potential sweeps. It is likely that the potential shift noticed in Figure 3 is attributed to a change in the methanol electrooxidation mechanism. In other words, a lack of water or small amount of water contribution to methanol oxidation generates a reaction mechanism different from eq 1 in which the water molecule hardly participates.9,26-28 Consequently, with the thus-established integrated ultramicroelectrode technique, the effect of water molecules on gaseous methanol electrooxidation has been established, which never undergoes in aqueous solutions. In the future, we plan to further analyze in detail changes in the mechanism of the gas-phase methanol oxidation employing the integrated ultramicroelectrode technique and electrochemical impedance spectroscopy. Conclusion Successful observation of methanol electrooxidation directly in the gas phase has been achieved by constructing a Nafion-coated integrated ultramicroelectrode. This system enabled us to measure the methanol oxidation with and without the influence of water molecules. The oxidation peak potential was found to be water-concentration dependent. This behavior was ascribed to change in the reaction mechanism of methanol oxidation. It is, therefore, deduced that the presence of water molecules strongly affects the methanol oxidation mechanism at the Pt electrode to reduce the overpotential of the reaction. Acknowledgment. The present work was supported by Grant-in-Aids for Scientific Research (B) (No. 13450349) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. LA010824+ (26) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker: New York, 1970; Chapter 8. (27) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995, 99, 10423. (28) Zhu, Y.; Uchida, H.; Yajima, T.; Watanabe, M. Langmuir 2001, 17, 146.
