© Copyright 2004 American Chemical Society
AUGUST 17, 2004 VOLUME 20, NUMBER 17
Letters Surface Observation of Solvent-Impregnated Nafion Membrane with Atomic Force Microscopy Abed M. Affoune, Akifumi Yamada, and Minoru Umeda* Department of Chemistry, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka, Niigata 940-2188, Japan Received December 10, 2003. In Final Form: June 19, 2004 Surface morphology modification of the Nafion 117 membrane has been investigated by atomic force microscopy. The effects of water and methanol on the topography and structure of the surface were studied using the contact and tapping atomic force microscopy modes. Nafion topography considerably changes when samples absorb water. However, samples stored in methanol are characterized by flat surfaces. Surface modification was linked to an expansion phenomenon during the swelling of Nafion by solvents. Tapping-mode phase images showed that ionic and cluster domains are distinguishable from the surface of samples impregnated either in water or methanol.
Introduction Nafion has been the focus of a large number of experimental studies using different techniques, particularly X-ray, NMR, IR, and electron microscopy, in order to explain the microstructure of the membrane.1 Many models were given, and still suggestions for new ones continue to be proposed, and the real structure of Nafion membranes still remains to be clarified. Using scanning probe microscopy, especially atomic force microscopy (AFM), Nafion membrane in different forms was investigated under different conditions.2-7 Results particularly showed that tapping-mode phase *
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(1) Pourcelly, G.; Gavach, C. In Proton Conductors; Colomban, P., Ed.; Cambridge University Press: Cambridge, 1992; p 294. (2) Chomakova-Haefke, M.; Nyffenegger, R.; Schmidt, E. Appl. Phys. A 1994, 59, 151-153. (3) Lehmani, A.; Durand-Vidal, S.; Turq, P. J. Appl. Polym. Sci. 1998, 68, 503-508. (4) James, P. J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. Polymer 2000, 41, 4223-4231. (5) James, P. J.; Elliott, J. A.; McMaster, T. J.; Newton, J. M.; Elliott, A. M. S.; Hanna, S.; Miles, M. J. J. Mater. Sci. 2000, 35, 5111-5119. (6) McLean, R. S.; Doyle, M.; Sauer, B. B. Macromolecules 2000, 33, 6541-6550. (7) James, P. J.; Antognozzi, M.; Tamayo, J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. Langmuir 2001, 17, 349-360.
imaging could be used to distinguish between hydrophilic and hydrophobic domains. Nafion films prepared by spincoating technique from Nafion solution show an irregularly shaped microstructure when observed by AFM.8 However, the microstructure shifts toward an apparently homogeneous structure after samples are exposed to methanol vapor. Previously, Fan and Bard9 imaged Nafion thin films spin-coated using scanning electrochemical microscopy (SECM) and reported that SECM can distinguish between zones of different ionic conductivity in a sample. Recently, Kanamura et al.10 have combined AFM and surface potential measurement (SPoM) and reported that bright spots in potential images can be assigned to ion channels in the Nafion membrane. The use of Nafion as a solid electrolyte in direct methanol fuel cells (DMFCs)11 is an important application of the membrane. This type of fuel cell appears to be the most promising for portable applications such as cellular phones and laptop computers. In DMFCs, the anodic compartment (8) Umeda, M.; Ojima, H.; Mohamedi, M.; Uchida, I. J. Polym. Sci. B 2002, 40, 1103-1109. (9) Fan, F. F.; Bard, A. J. Science 1995, 270, 1849-1852. (10) Kanamura, K.; Morikawa, H.; Umegaki, T. J. Electrochem. Soc. 2003, 150, A193-A198. (11) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.; Leger, J. M. J. Power Sources 2002, 105, 283-296.
10.1021/la036329q CCC: $27.50 © 2004 American Chemical Society Published on Web 07/16/2004
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Figure 1. AFM surface topography images of Nafion stored in pure water (a) and in methanol (b). Images are contact mode and the corresponding tapping-mode images are shown in Figure 2.
is fed by an alcohol-water mixture. The alcohol part of the mixture is intended to be oxidized on the anode while the water part is intended to participate in the anode reaction and also to hydrate Nafion. However, the crossover of methanol and the diffusion of water occur while the fuel cell is in operation. We are interested in the modification of the Nafion surface induced by the interaction with water and methanol under these conditions. We present here an AFM study on the effect of water and methanol on the surface properties of the Nafion membrane. Investigation was carried out using the contact and tapping AFM modes. Experimental Procedure Nafion 117 perfluorinated membrane films, manufactured by DuPont, were obtained from Aldrich Chemical Co., Inc. The membrane has a nominal equivalent weight of 1100 and a thickness of 178 µm. High-purity methanol was purchased from Nacalai Tesque, Kyoto, Japan. The Nafion membranes were pretreated by boiling in 0.5 M H2SO4 for 1 h then boiled in deionized water for 1 h and rinsed thoroughly using deionized water. After that, samples were stored in Milli-Q pure water and methanol. The films were examined by AFM using the SPM-9500 J3 model from Shimadzu. It is equipped with a 55 µm scanning head and is operated in the contact and tapping modes. For the contact mode, we used silicon nitride probes (Olympus), with a spring constant of 0.57 N m-1 and a resonance frequency of 73 kHz. For the tapping mode, we used silicon probes (Olympus), with a spring constant of 42 N m-1 and a resonance frequency of 300 kHz. The Nafion morphologies were imaged with at a scan rate of 1 Hz and 512 × 512 pixel resolution. The calibration of the piezo scanner of the atomic force microscope was carried out by imaging a Gold grating sample from Shimadzu Company.
Results and Discussion The main advantage of scanning probe microscopy compared with electron microscopy is the possibility to study samples under ambient conditions without any further preparation or restrictions. Samples in dry or wet states can be investigated in air. Contact mode topography images of Nafion samples stored in water and methanol are shown in Figure 1. There is a large difference in topography between the two samples. The surface of waterstored samples is characterized by a high roughness, while the surface of the methanol-stored samples is very flat. The same features were observed using both the contact mode and the tapping mode. For example, the images in Figure 1 were taken in the contact mode, while those in Figure 2 were taken in the tapping mode. We have also verified that the as-received samples, without any pretreatment, when observed by AFM, generally had the same roughness as the samples stored in water. Pretreated
Table 1. Roughness Parameters and Volume Increase of Nafion Samples solvent water methanol
Ra, nm
rms, nm
27.150 (Fig. 1) 26.772 (Fig. 2) 2.957 (Fig. 1) 3.052 (Fig. 2)
34.680 (Fig. 1) 32.595 (Fig. 2) 4.073 (Fig. 1) 4.059 (Fig. 2)
Volume increase ∆V/V, %12 43 209
samples subsequently observed in AFM also had the same roughness. However, samples dried in a vacuum oven at 130 °C for 1 h had smooth surfaces. This indicates that pretreatment itself under boiling conditions is not directly responsible for the observed features, but the absorption and interaction of water with Nafion surface is the main reason for that. The surface water layer, which probably formed on the surface of the sample, and tip convolution can both influence the image and result in an image that does not truly reflect all the surface’s features. However, It is unlikely that contributions of these phenomena are the only ones responsible for the shape of the features in the images obtained. The quantitative analysis of the roughness parameters is presented in Table 1, including the values of the arithmetic mean value, Ra, and rootmean-square, rms, parameters, which are typically used to quantify roughness in surface analysis. The comparison shows that the roughness of Nafion in water is several times higher than in methanol. Figure 2 shows the tapping-mode AFM topography and phase images acquired simultaneously of Nafion previously immersed in water and methanol. This figure reveals that the topographic features observed with water immersion for the height images are not reproduced in the phase images. It is well known that material properties should affect the magnitude of the phase-shift signal and phase contrast can be used to distinguish between different materials on the surface. The features observed in the height images did not produce a strong phase-shift signal, indicating that phase contrast does not depend on the roughness of the surface. It also indicates that the roughness is related to the membrane material and not related to other foreign materials. Comparison between the phase images acquired in water and methanol indicates that the surface structure of the membrane is not affected by the roughness observed in the topographic images. It is known that the Nafion membrane absorbs water and other solvents and consequently expands to large dimensions. Gebel et al.12 measured the expansion of (12) Gebel, G.; Aldebert, P.; Pineri, M. Polymer 1993, 34, 333-339.
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Figure 2. AFM topography and corresponding phase image of Nafion samples stored in water (a, b) and in methanol (c, d), respectively. Z scale in phase images is 13.32° for (b) and 18.18° for (d).
Nafion in numerous solvents and found that, in methanol, the membrane expands considerably more than in water, as shown in Table 1. Elliott et al.13 reported from the SAXS study of swelling Nafion by ethanol-water mixtures that a less polar solvent than water can affect the behavior of a fluorocarbon matrix. Yeo14 also reported that the solvent uptakes for Nafion are 21% in water and 54% in methanol. From these uptakes, one can find by calculation that volume increase is approximately 42% in water and 54% in methanol. The volume increase is the ratio between the volume of the solvent uptaken by the membrane and the volume of Nafion. The densities of Nafion, water, and methanol used in the calculation are 2, 1, and 0.79 g cm-3, respectively. Comparatively with Gebel’s experimental values, this indicates that the Nafion structure undergoes high modification due to the swelling of methanol. Our samples are pretreated before being stored in methanol, so their surfaces are already rough before introducing them into methanol solvent. During equilibration, water is removed and replaced by methanol. Because it interacts strongly with the fluorocarbon matrix, methanol makes Nafion expand much more than water, and our AFM topography images clearly reflect this expansion leading to the flat surfaces. The Nafion surface is an open system versus the bulk, therefore, it is expected that modification inside the membrane is less important than on the surface. Very-high-magnification phase images are presented in Figure 3. Images from samples stored in water (Figure 3a) are found independent of height area. This means (13) Elliott, J. A.; Hanna, S.; Elliott, A. M. S.; Cooley, G. E. Polymer 2001, 42, 2251-2253. (14) Yeo, R. S. Polymer 1980, 42, 432-435.
that the same images could be obtained from either higher or lower zones in topographic images, which reconfirms that the surface structure is not affected by roughness due to the absorption of water. Other authors who studied phase images for Nafion 115 acid form4,5 and Nafion 117 potassium form6 concluded that bright spots in very-highmagnification phase images are attributed to ionic clusters, and dark spots correspond to fluorocarbon domains because the intensity of bright spots are dependent on water content. The average size of ionic clusters (Figure 3a) is approximately 7-15 nm, which is in good agreement with data reported by McLean et al.6 In the phase images corresponding to samples stored in methanol (Figure 3b), ionic clusters and fluorocarbon domains can also be distinguished, as in the case of the water images. The shape of the ionic domain and the intensity of the phase signal in the methanol images are slightly different from those in the water images. These observations may support the recent report of Rivin et al.,15 which indicates that water interacts more strongly with the sulfonic acid groups, while the alcohols preferentially solvate the fluoroether side chain and cause structural change. It seems that the phase image of methanol is consistent with this report and may indicate the absorption of methanol in the interfacial region of Nafion. Conclusion Nafion topography considerably changes when samples absorb water. However, samples stored in methanol are characterized by flat surfaces. The surface modification (15) Rivin, D.; Kendrick, C. E.; Gibson, P. W.; Schneider, N. S. Polymer 2001, 42, 623-635.
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Figure 3. AFM phase images of Nafion samples stored in water (a) and in methanol (b). Z scale in phase images is 1.44°.
was linked to an expansion phenomenon and a swelling mechanism of Nafion in solvents. The interaction between the Nafion surface and methanol is different from that with water, which is due to the presence of hydrophilic and hydrophobic zones in the Nafion structure. It seems that methanol cannot only remove and replace water but may also interact with perfluorinated vinyl ether chains. The present study provided new data on the surface properties of the Nafion membrane. This investigation could be extended to other alcohols used in so-called direct alcohol fuel cells (DAFCs) like ethanol and 2-propanol. In-situ electrochemical AFM allows the observation of surface property changes in an electrochemical environment. Our conclusions would be very useful in investigating methanol oxidation in more detail by in-situ electro-
chemical AFM using the tip as an electrode to obtain an electrical response during imaging of the Nafion membrane. Acknowledgment. The present work was financially supported by the research and development of polymer electrolyte fuel cells from the New Energy and Industrial Technology Development Organization (NEDO), Japan. This work was also supported by Grant-in-Aids for Scientific Research (B) (No. 14350450) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. LA036329Q
