Configuration Changes of Conducting Channel Network in Nafion

Oct 28, 2010 - We have investigated changes of proton channel network in Nafion membranes annealed at elevated temperatures using current sensing atom...
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J. Phys. Chem. B 2010, 114, 14989–14994

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Configuration Changes of Conducting Channel Network in Nafion Membranes due to Thermal Annealing Osung Kwon,† Shijie Wu,‡ and Da-Ming Zhu*,† Department of Physics, UniVersity of Missouri - Kansas City, Kansas City, Missouri 64110, United States, and Agilent Technologies, Inc., 4330 W. Chandler BlVd., Chandler, Arizona 85282, United States ReceiVed: August 27, 2010

We have investigated changes of proton channel network in Nafion membranes annealed at elevated temperatures using current sensing atomic force microscopy aimed at understanding the aging process of the membranes. The results reveal that the changes of proton channel network undergo two steps: First, the configuration of the ionic domains on the membrane surface changes from cluster-like to chain-like structure, accompanied by an increase of the proton conductivity of the membrane. As the annealing continues, the chain-like configuration for the proton channels persists but the conductance of the membranes decreases. The time constant of the conductivity decay decreases with the annealing temperature. The observed changes can be explained in terms of reorientation of proton channels near the membrane surface from perpendicular to parallel to the surface as the annealing temperature approaches the glass transition of the membranes. Polymer electrolyte membrane (PEM) fuel cells are among the most promising environmental friendly power generation and energy conversion/storage devices for a wide range of applications.1-3 However, improving the stability and the durability of PEM fuel cells that operate under a wide variety of conditions remain a challenge that needs to be met before the technology becomes commercially viable.4-9 Among various parts that a PEM fuel cell is made of, the ion exchange membrane is a key component that directly affects the performance and the durability of the fuel cell. For this reason, studying the aging behaviors of various ion exchange membranes has been a subject of intense interests in recent years.4-8 The function of an ion exchange membrane in a PEM fuel cell is to provide high ionic conductivity and impermeability to the reaction gases and liquids.1-3 Currently the most widely used and that being considered as the state-of-the art ion exchange membrane in fuel cell application is Nafion, which represents a family of comb-shaped ionomers that consist of polytetrafluoroethylene (PTFE) hydrophobic backbone with perfluorinated pendant terminated by hydrophilic sulfonic acid head groups (-SO3H).5,10,11 The microscopic structure of a Nafion membrane is complex and has been actively studied but is still a subject under intensive debate.10-28 A commonly referenced model describes its structure as the PTFE backbone matrix embedded with ionic clusters, which are approximately spherical in shape with an inverted micelle structure formed by aggregated hydrophilic head groups in the membranes, connected by narrow hydrophilic channels allowing ionic conductivity.22,23 A recent work based on computer simulation and the reanalysis of the currently available small-angle X-ray and neutron scattering data finds that the ionic structure of Nafion membranes can be better described as parallel cylindrical nanometer sized hydrophilic channels with directions predominantly perpendicular to the membrane’s surface.26 Protons or ions can transport along the hydrophilic channels, producing * Corresponding author, [email protected]; (816)235-5326; (816)2355221(fax). † University of Missouri - Kansas City. ‡ Agilent Technologies, Inc.

excellent ionic conductivity of the membranes. This model explains many important features of the membranes including fast diffusion of water and protons through the membranes and its persistence at low temperatures.26 However, there is still lack of direct evidence that proves the existence of the ionic channels in Nafion membranes. Also, it is not clear how the ionic channel structure changes in an aging process as the changes in the chemical composition and microstructure of Nafion membranes associated with the aging process have been identified using techniques such as microscopy, optical spectroscopy, nuclear magnetic resonance (NMR).4-8,10-28 In this work, we have used current sensing atomic force microscopy to reveal nanometer scaled proton channels in Nafion membranes and the changes in the channel network configuration due to thermal annealing at different temperatures. We show that in pristine membranes proton channels mostly clump together and align in the direction perpendicular to the membrane surface. After the membrane was annealed at a temperature close to its glass transition temperature, the alignment of proton channels on the membrane surface becomes mostly parallel to the membrane surface, accompanied by an increase of proton conductivity of the membrane. As the annealing continues, the proton conductivity decays following an approximate exponential behavior, and the decay constant increases with the annealing temperature. Current sensing atomic force microscopy (CSAFM) is a technique that can be used to map microscopically the surface morphology and the local conductivity of a sample simultaneously.29-39 It has been demonstrated that given the radius of the tip used in imaging, the mapping of local conductance contains information related to conductance of the individual channel and the density of the proton channel.35 The experimental samples used in this work are Nafion NR212 membranes hot pressed onto carbon cloth gas-diffusive electrode substrates to form half membrane-electrode assemblies (MEA).40 The Nafion MEA samples were used as received. The MEA was installed in the sample stage of an AFM; the bottom electrode was connected to the current sensing loop, which connects to the probe tip of the AFM. In the measurements, a

10.1021/jp108163a  2010 American Chemical Society Published on Web 10/28/2010

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Figure 1. Illustration of a current sensing AFM in the study of an ion exchange membrane. The lines in the membrane represent ionic channels. Catalyst layer on the tip and the catalyst on carbon cloth (black) are marked as yellow objects. The red lines indicate the ionic current flowing in the ionic channel network of the membrane.

platinum-coated AFM tip is brought in contact with the exposure surface of the membrane in the MEA; the tip/MEA system (tip/ platinum/membrane/platinum-carbon cloth) forms a miniature fuel cell with the tip function as a nanoscaled electrode. The tip/MEA is placed in an environment with controlled temper-

Kwon et al. ature and relative humidity inside an environmental chamber in the AFM system.41,42 A bias voltage for initiating electrolysis of water is applied to the tip to produce hydrogen that subsequently gets oxidized at the tip forming proton. The bias voltage applied to the AFM tip throughout this work was 1.5 V. As the tip scans across the surface of the membrane, it passes over areas making contacts with ionic channels exposed on the surface. The proton generated by the catalyst at the tip-membrane contacts transports through the proton channel network across the MEA, and a current is detected in CSAFM. The passageway of the current across the proton channel network in the membrane can be quite complicated. But if each proton channel branches out or merges with others over a distance much shorter than the thickness of the membrane (according to the recent proposed parallel proton channel model, the length of each channel bundles is about 100 nm,26 while the thickness of Nafion membranes is typically in a range of 50-170 µm), the proton current flowing from the tip-membrane contact would spread out over a large portion of the network before reaching the opposite electrode, as shown in Figure 1. Then the proton

Figure 2. Topography and accompanied current sensing images obtained on a Nafion NR212 membrane after it was annealed at different temperatures for a period indicated on the images. Topographic images: (a), (d), (g). Current sensing images: (b), (e), (h). (c), (f), and (i) are the line profiles in topography (red dotted line) and current sensing images.

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Figure 3. Topography and accompanied current sensing images obtained together on a Nafion NR212 membrane after being annealed at 120 °C. Topographic images: (a), (d), (g). Current sensing images: (b), (e), (h). (c), (f), and (i) are the line profiles in topography (red dotted line) and current sensing images.

conductance across the proton channel network is more or less a constant; the proton current variation detected by the CSAFM reflects mainly the contract variation between the tip and proton conducting channels. Hence, a current sensing image essentially maps the contact probability of the tip and ionic clusters on the membrane surface, which connects to the proton channel network in the membrane.35 Several different Nafion NR212 membranes were annealed over a temperature range from 80 to 120 °C. In performing current sensing imaging, the MEA was placed in a CSAFM sample stage that was housed inside an environmental chamber in which the relative humidity was precisely monitored and controlled through a water vapor and dry nitrogen blowing system. The pristine membranes were imaged with CSAFM to obtain surface morphology and proton conductance images. After that, the probe tip was retreated from the membrane surface, and the MEA was heated on the sample stage to an elevated temperature under a relative humidity of 50% for a predesigned period. After the heating, the MEA was slowly cooled to room temperature, and the CSAFM imaging was conducted again on the membrane. The cooling process typically took several hours; after reaching room temperature the MEA

was left for several hours under the same relative humidity before imaging was conducted, to ensure the membrane was in equilibrium with the environment. This heating-cooling-imaging process was repeated until the measurements on the aging process are completed. Topographic and current sensing images were obtained simultaneously at different stages of the annealing. Figure 2 depicts typical topography and current sensing images on a Nafion NR212 membrane after it was annealed at different temperatures for certain periods. Changes occur to the membranes annealed at temperature of 80 °C or below are very slow; we do not see significant changes to the membranes annealed for about 5 days through CSAFM imaging. However, significant changes can be observed when the membranes are annealed at a temperature above 100 °C for a few hours. Figure 3 plots the topographic and current sensing images of a Nafion NR212 membrane after being annealed at 120 °C for different time periods. The topographic images in Figures 2 and 3 show that annealing has almost no effect on the membrane’s surface morphology. In current sensing images, the bright areas correspond to conductive regions in the membrane. These regions reflect those in which there are ionic channels exposed on the

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membrane surface. In the pristine membranes, the bright areas (Figures 2b and 3b) have sizes much larger than the sizes of ionic channels or clusters determined using other techniques.22-26 Repeat efforts have been made by using shaper tips and varying imaging conditions, but the current sensing images remain about the same. This leads us to speculate that ionic channels are mostly clumped together in the pristine membranes used in this work. The patterns of the bright regions in the current sensing images change drastically with annealing. After the membranes were annealed for more than 10 h, the bright areas evolve into chainlike filament structures (see, e.g., Figures 2e,h and 3e,h); the width of the bright filaments is about 10-20 nm, comparable to those reported in a similar study that are attributed to proton conductance from individual ionic channels.29 If the same treatment used in a recent work by K. A. Friedrich et al. (Electrochimica Acta 2009, 55, 423-429) is applied to the current sensing images obtained in this work, images with well separated bright spots of sizes on nanometer scales can also be obtained. Since the bright filaments correspond to where high currents are detected, these features are related to the proton conducting channels in the membranes. Some detailed features about these filament features can be more clearly seen in the line scan profiles extracted from the current sensing images. Some typical profiles are plotted in Figures 2c,f,i and 3c,f,i. For comparison purposes, the corresponding topographic line scan profiles are also plotted. Clearly, there is almost no correlation between the topographic and current sensing profiles. The features shown in the current profiles in the current sensing images are much more rugged, displaying a series of peaks and valleys. The roughness and magnitude of the proton current from the valley to the peak increase from the pristine membrane, reaching a maximum when the entire membrane surface shows a filament-like network structure; after that, the roughness in the current image decreases as the annealing continues, but the filament feature persists. These results indicate that annealing of Nafion NR212 membranes at temperatures above 100 °C causes significant change to the proton channels and the ionic activity on the membrane surface. A possible explanation is that ionic channels near the surface of the membrane become aligned in parallel to the membrane surface due to annealing.43 Continued annealing at such elevated temperatures causes the conductance of individual channels to decrease, which is probably due to changes of chemical composition in the membrane. By integrating the current measured at each pixel over the entire current sensing image and multiplying a geometric factor that is related to the size of the scan area and the thickness of the membrane, we can calculate the conductivity of the membrane. Figure 4 plots the conductivity normalized by the conductivity of the pristine membrane as a function of annealing time. The conductivity initially rises with annealing followed by a gradual decay. Clearly, as the conducting area changes from clusters to strips due to annealing, the overall proton conductance increases. The changes in the configuration of the proton channel network in the membranes above 100 °C can be explained in terms of a rapid increase of molecular mobility as the glass transition temperature of the membrane is approached. The glass transition temperature of Nafion membranes varies in the range 100-150 °C, depending on the water uptake by the membrane.44 As the temperature is close to this temperature range, the mobility of molecules should increase rapidly, making the proton channels easier to rearrange to form a stable state. Despite the

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Figure 4. Conductivity changes of the Nafion membranes as a function of annealing time for annealing at different temperatures: (a) 90 °C; (b) 100 °C; (c) 110 °C; (d) 120 °C. Lines are the guides to the eye.

fact that the parallel proton channel model can explain the X-ray and neutron scattering data of Nafion membranes and provides a unified view of their ionic structures,26 it is not clear whether such a configuration, presumably formed in the fabrication of the membranes, is a stable one. The effective interaction between

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Figure 5. Proposed mechanism to explain the observed changes in conductivity of Nafion membranes. (a) In pristine membranes, proton channels are predominantly orientated in the direction perpendicular to the membrane surface, as suggested in the recently proposed parallel proton channel model.26 After annealing at a temperature above 100 °C, proton channels near the membrane surface become parallel to the surface. The parallel channels near the surface increase the probability of contacting probe tip as well as the connectivity with the rest of the proton channel network.

inverted micelles with the same radius polarization is shortrange repulsion plus long-range van der Waals attraction.43,45 As the temperature becomes close to the glass transition of the membrane, the inverse micelle cylindrical channels may start to diffuse and rearrange themselves to form a stable state. For those channels in a position near the membrane surface, the interaction between the inverse micelle and the membrane surface should be attractive due to image charges created by the interface, which may cause some of the channels lying predominantly parallel to the surface, as illustrated in Figure 5. These channels in parallel to the membrane surface might be open on one side forming a groove on the surface, which should increase the contact probability of the channels and the probe tip, and get more channels connected in parallel across the membrane, thus increasing the conductance between the tip and the opposite electrode, as shown in Figure 5b. Thus, as proton channels near the membrane surface change from the configuration in which the channels are predominantly perpendicular to the membrane surface to a configuration where they are predominantly parallel to the membrane surface, the proton conductance is expected to rise, as observed in this study. The CSAFM images show that as the annealing continues, the configuration of the proton channel network remains about the same (see Figure 3e,h), but the conductivity along individual channels gradually decreases. Eventually, the membrane’s conductivity drops to near zero. The time constant for the conductance decay depends sensitively on the annealing temperature, as demonstrated in Figure 4. The mechanism for such decay is not clear. A recent study proposed that the proton conductance decay in an aging process is associated with disappearance of sulfonate acid groups in the polymer backbone of the membrane.7 As the glass transition of the membrane is approached, the molecules’ mobility in the region increases; sulfonic acid groups may encounter and react with each other more easily, forming sulfonic anhydrides.7 The results from this study suggest that decay of the proton conductivity in Nafion membranes accelerates as the glass transition is approached, and the decay is associated with the deterioration of the conductance along individual channels. The rearrangement of

J. Phys. Chem. B, Vol. 114, No. 46, 2010 14993 proton channels near the membrane surface after the membrane is annealed briefly at an elevated temperature is an interesting phenomenon that could open up a new approach in improving the performance of polymer electrolyte membrane fuel cells. In summary, we have studied changes of the proton channel network in Nafion membranes annealed at elevated temperatures using current sensing atomic force microscopy aimed at understanding the aging process of the membranes. The results indicate that the configuration of the ionic domains on the membrane surface changes from cluster-like to chain-like structure, accompanied by an increase of the proton conductivity of the membrane, in the initial stages of the annealing. As the annealing continues, the chain-like configuration for the proton channels persists but the conductance of the membranes decreases. The time constant of the conductivity decay decreases with annealing temperature. The observed changes can be explained in terms of reorientation of proton channels near the membrane surface from perpendicular to parallel to the surface as the annealing temperature approaches the glass transition of the membranes. Acknowledgment. This work is supported in part by grants from Research Corp., Agilent Foundation, and Kansas City Life Science Inc. This work is also partially supported by US Army Research Office (ARO) under contract (W911NF-10-1-0476). References and Notes (1) For reviews, see: O’Hayre, R.; Colella, W.; Cha, S.-W.; and Fritz B. Prinz, F. G. Fuel Cell Fundamentals, 2nd ed: John Wiley & Sons: New York, NY, U.S., 2009. (2) Srinvasana, S. Fuel Cells: from Fundamentals to Applications, Springer Science and Business, LLC: New York, NY, U.S., 2006. (3) Beuscher, U.; Cleghorn, S. J. C.; Johnson, W. B. Intl. J. Energy Res. 2005, 29, 1103–1112. (4) Borup, R.; Meyer, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, F.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Chem. ReV. 2007, 107, 3904–3951. (5) Perrot, C.; Meyer, G.; Gonon, L.; Gebel, G. Fuel Cell 2006, 1, 10–15. (6) Wu, J.; Yuan, X. Z.; Martin, J. J.; Wang, H.; Zhang, J.; Shen, J.; Wu, S.; Merida, W. J. Power Sources 2008, 184, 104–119. (7) Collette, F. M.; Lorentz, C.; Gebel, G.; Thominette, F. J. Membr. Sci. 2009, 330, 21–29. (8) Knights, S. D.; Colbow, K. M.; St-Pierre, J.; Wilkinson, D. P. J. Power Sources 2004, 127, 127–134. (9) Li, Liang; Xing, Yangchuan. Electrochemical durability of carbon nanotubes at 80 C. J. Power Sources 2008, 178, 75–79. Li, Liang; Xing, Yangchuan. Pt-Ru Nanoparticles Supported on Carbon Nanotubes as Methanol Fuel Cell Catalysts. J. Phys. Chem. 2007, 111, 2803–2808. (10) Mauritz, K.; Moore, R. B. Chem. ReV. 2004, 104, 4535–4586. (11) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. ReV. 2004, 104, 4587–4612. (12) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13, 307–326. (13) Fujimura, M.; Hashimoto, T.; Kawai, H. Macromolecules 1981, 14, 1309–1315. (14) Fujimura, M.; Hashimoto, T.; Kawai, H. Macromolecules 1982, 15, 136–144. (15) Dreyfus, B.; Gebel, G.; Aldebert, P.; Pineri, M.; Escoubes, M.; Thomas, M. J. Phys. (Paris) 1990, 51, 1341–1354. (16) Fujimura, M.; Hashimoto, R.; Kawai, H. Macromolecules 1982, 15, 136–144. (17) Gebel, G.; Moore, R. B. Macromolecules 2000, 33, 4850–4855. (18) Gebel, G.; Lambard, J. Macromolecules 1997, 30, 7914–7920. (19) Rollet, A. L.; Gebel, G.; Simonin, J. P.; Turq, P. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 548. (20) Litt, M. H. Polym. Prepr. 1997, 38, 80–81. (21) Haubold, H. G.; Vad, T.; Jungbluth, H.; Hiller, P. Electrochim. Acta 2001, 46, 1559–1563. (22) Rubatat, L.; Rollet, A. L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050–4055. (23) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci.: Polym. Phys. 1981, 19, 1687–1704.

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