Nafion: Humidified Stress Relaxation and Surface Wetting Introduction Results and Discussion Fig. 1:

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1 Nafion: Humidified Stress Relaxation and Surface Wetting Shannon Klaus Princeton University PCCM REU 2007 Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are a promising low-emission energy technology, but they lack full optimization due to several challenges. One difficulty lies in the dynamic physical and mechanical properties of the Nafion membrane that are dependant on varying hydration levels. These changes within the polymer membrane can be better characterized by two primary studies: humidified stress-relaxation tests to quantify membrane swelling and surface wetting tests to measure the effect of microphase separation. Comprehension of these phenomena can increase PEMFC efficiency and help create feasible energy conversion. Results and Discussion Humidified stress relaxation For humidified stress relaxation experiments, saturated salt solutions with various relative humidities (RHs), as listed in Table 1 (appendix), maintained polymer samples at a range of hydration levels. 1,2 Desiccant and water were also used to create dry and 100 % RH environments, respectively. Nafion 115 and Nylon 6-6 (both thick) were tested at each humidity level with applications of 3% and 12% engineering strain, defined as ΔL ε = (1) L 0 where ΔL is the final change in sample length and L 0 is the original sample length. Each material was pre-equilibrated to a given relative humidity, cut into a strip (W 0.25, L=2 ) for 3% strain samples or a dog bone (gauge measurements: W=0.176, L=0.876 ) for 12% strain samples and placed into a Ziploc baggie containing 10mL saturated salt solution, where it was allowed to equilibrate approximately twenty-four hours. After this time, an Instron (Model 1122) strained each sample at a rate of 10 per minute to the predetermined strain rate. Stress was measured over a minimum of 10,000 seconds as F σ = (2) A 0 where F is the force exerted by the polymer and A 0 is the original cross-sectional area of the sample. Nafion and Nylon runs were performed at least in triplicate and duplicate, respectively. Due to slight variance in the strain level applied to each sample, the modulus: σ E = (3) ε was used to normalize all stress values. Figure 1 shows representative modulus vs. time for Nafion and Nylon at 3% strain at all tested relative humidities. Modulus increased in the first fraction of a second while strain was initially applied and then decayed as relaxation occurred. For all humidities, the modulus for Nylon is higher than that of Nafion. Fig. 1: Nafion/Nylon moduli vs. time at 3% strain, 23ºC 1

2 12% strain, 23ºC Fig.3: Cubic fits for Nafion log [modulus] vs. log [time] at 3% strain, 23ºC Fig. 4: Linear fit of Nafion log [modulus] vs. log [time] at 12% strain, 23ºC An overwhelming trend of increased RH resulting in decreased modulus is apparent for both polymers; however, the Nafion modulus curves begin to converge over time, while Nylon modulus values maintain fixed rates of relaxation. 12% strain stress-relaxation runs, displayed in Figure 2, exhibit the same relationship between RH and modulus values as well as Nafion modulus convergence. However, stress decay follows a linear trend at Fig. 2: Nafion/Nylon moduli vs. time at 12% strain on a log-log scale, while 3% strain modulus values appear to decay non-linearly. This tendency is better depicted in Figure 3, where the log of both modulus and time axes is taken and a cubic equation is fitted to each RH curve. As all R 2 values exceed 0.999, these equations appear to accurately characterize observed stressrelaxation behavior. The linear trend displayed for Nafion 12% strain modulus values is further explored in Figure 4. The log of modulus and time was plotted for each RH, and applied linear fits resulted in R 2 values near unity. Slopes from this relation become less negative with increasing RH, which can be more directly compared in Figure 5. The log-modulus versus log-time slope becomes exponentially flatter with increasing relative humidity, suggesting that water acts as a stabilizer within the strained membrane due to its interaction with sulfonic acid groups. 2

3 Fig. 5: Nafion log-log linear slope vs. humidity at 12% strain, 23ºC Fig. 6: Contact angle vs. distance in DI water, r=0.075 /min. Surface wetting: Wilhelmy plate method The Wilhelmy plate method was used to determine the effect of microphase separation on the Nafion-water contact angle, measured as F + Fb θ = (4) γ where F is the total force measured by microbalance, F b is the buoyant force of the water on the tile, and γ is the surface tension of the water. However, Nafion poses several problems during surface wetting, including conformation changes which previously have not been addressed in experiment. 3 To minimize these effects, a 1μm film of Nafion (Liquion LQ-1105) was airbrushed onto an Alumina tile ( ) and then annealed for one hour at 160 C. To obtain microphase separation, tiles were reheated to 160ºC between slides of glass or blocks of Teflon for at least one hour to exceed the theoretical glass transition temperature of 110 C. 4 Glass and Teflon pretreated tiles as well as standards of Nylon, Alumina, and nontreated Nafion-coated Alumina were submerged and removed from deionized water at a rate of per minute, a standard immersion rate, 5 with microbalance readings taken every twenty seconds. The resulting contact angles can be seen in Figure 6, with advancing and receding contact angles for both Nylon and Alumina displaying relative similarity. Nafion coated tiles, meanwhile, show a contrasting trend: contact angle slope change over time is a result of changing rate of water absorption and is apparent for all Nafion samples. The receding contact angles become undefined due to water-uptake within the samples, which affects total empirical force. Also, the different tile pretreatments exhibit minimal difference in contact angle, which shows that hypothesized microphase separation is either a negligible influence or occurs instantaneously after hydration for accurate quantification. Surface Wetting: Microscope time-lapse videos To more directly determine contact angle as well as Nafion evaporation and diffusion effects, drop tests were recorded with a microscope time-lapse video (Intel Play). Representative still-frame images in Figure 7 illustrate representative sample behavior. After the Nafion surface is placed in 3

4 contact with a water drop, it immediately begins to swell locally. The membrane continues this swelling response, as in Figure 7.a, until a maximum swelled state is reached. However, after final evaporation of surface water, the membrane collapses back to its original, pre-wetted state. a b c d e f Fig. 7: Still-frame surface wetting images from microscope using (a) 100mL water on Teflon (0.005 thick) and Nafion 115 (b) 30mL water on Nafion 115 and Nafion 112 (c) 30mL on untreated Nafion 115, Teflon pretreated Nafion 115, glass pretreated Nafion 115 (d) 30mL water on Nafion 115, 3% composite TiO2 Nafion 115, and 10% composite TiO2 Nafion 115 (e) 50mL ethanol on Nafion 115 and 50uL water on Nafion 115 (f) 50mL octane on Nafion 115 and Teflon (0.005 thick) Several variations on this study were examined. A thicker membrane resulted in increased swelling action, depicted in Figure 7.b, which evidenced the importance of diffusion above surface tension. As a swelling gradient was created throughout the thicker membrane, it buckled upward until fully evaporation. Figure 7.c qualitatively verifies glass and Teflon pretreatment results established by the Wilhelmy plate method; minimal difference is seen among sample behaviors. Composite membranes of 3% and 10% TiO 2 were also studied for contact angle and swelling variance. Nafion 115 and Nafion 3% TiO 2 composite (0.005 thick) have previously displayed similar contact angles, 6 which concur with the visual results in Figure 7.d. Finally, results with ethanol and octane drops are presented in Figures 7.e and 7.f., respectively. The ethanol drop diffused at a much faster rate across the membrane, and the image shows the semi-permanent swelling state remaining after evaporation. The faster disappearance of ethanol is due to a faster rate of evaporation as well as diffusion, as the compound interacts with both hydrophilic and hydrophobic regions of Nafion, allowing it to more easily permeate and swell the membrane. Conversely, octane is highly non-polar and shows similar reaction on both Nafion and Teflon surfaces. These revealing differences show that the sulfonic acid groups are responsible for the swelling behavior exhibited. Conclusions Nafion humidified stress-relaxation tests demonstrated increasing relative humidity results in decreasing modulus values for both 3% and 12% strain. Furthermore, while the 12% strain modulus values relaxed at a linear rate on a log-log scale, the 3% strain modulus values displayed a cubic trend of modulus decay. These results are theorized to be due to the hydration of sulfonic acid groups, which creates stability within the strained membrane. The Wilhelmy plate method and microscope time-lapse videos determined various Nafion surface wetting properties. Minimal difference between the glass and Teflon pre-treated Nafion-water 4

5 contact angles was exhibited by the Wilhelmy plate method. This provides evidence that microphase separation is either negligible or re-conforms immediately under hydrated conditions, resulting inaccurate quantification of contact angle. Microscope time-lapse videos demonstrate a qualitative change in Nafion under various conditions of thickness, liquid drop size, and liquid drop material. Although effects of microphase separation were also not exhibited by this method of surface wetting, the importance the Nafion sulfonic acid groups on surface swelling was very apparent. References 1. Greenspan, L. (1976). Humidity Fixed Points of Binary Saturated Aqueous Solutions. Journal of Research of the Natl. Bureau of Stds. 81A: Wexler A. S. (1995). Constant humidity solutions. CRC Handbook of Chemistry and Physics, 73 rd edition. Boca Raton: CRC Press. 3. Zawodzinski, T.A., Gottesfeld, S., Shoichet, S., and McCarthy, T.J. (1992). The contact angle between water and the surface of perfluorosulphonic acid membranes. Journal of Applied Electrochemistry. 23: Corti, H.R., Nores-Pondal, F, and Buera, M.P. (2006). Low temperature thermal properties of Nafion 117 membranes in water and methanol-water mixtures. Journal of Power Sources. 161: Berg, J.C. (1993). Wettability. New York: Marcel Dekker, Inc. 6. Adjemian et al. (2006). Function and Characterization of Metal Oxide-Nafion Composite Membranes for Elevated Temperature H2O2 PEM Fuel Cells. Chemistry of Materials.. 18: Appendix Table 1: RH values of saturated salt solutions 1,2 Salt % RH Lithium Chloride 11 Magnesium Chloride 33 Sodium Bromide 58 Sodium Chloride 75 Potassium Chloride 85 Copper Sulfate 95 Acknowledgements: Thank you to Dr. Jay Benziger, Sharon Goswami, Barclay Satterfield and Paul Majsztrik for their contributions to this project. Also, the support of the NSF REU program and PCCM is greatly appreciated. 5