CHARACTERIZATION OF NAFION PROTON EXCHANGE MEMBRANE FILMS USING WIDE-ANGLE X-RAY DIFFRACTION

Transcription

1 CHARACTERIZATION OF NAFION PROTON EXCHANGE MEMBRANE FILMS USING WIDE-ANGLE X-RAY DIFFRACTION 128 T.N. Blanton 1 and R. Koestner 2 1 International Centre for Diffraction Data, Newtown Square, PA 19073, USA 2 General Motors Corporation, Pontiac, MI 48340, USA ABSTRACT Nafion is a perfluorosulfonate proton exchange membrane polymer that is used in hydrogenoxygen fuel cells. Pores in a Nafion membrane allow for the movement of cations, however the membrane does not conduct anions or electrons. Depending on the manufacturing process, the cation conductivity of a Nafion membrane can be modified to meet a specified application requirement. In this study thin membrane Nafion type films (10 25 microns) were analyzed using wide-angle XRD. Data were collected in ambient air conditions, as well as variable relative humidity. With careful sample preparation and a properly aligned diffractometer, scattering from ionic clusters, amorphous component, and crystalline component was observed. Raw data XRD patterns have been collected and will be submitted for inclusion in the Powder Diffraction File TM database. These data entries will be particularly useful in cases where Nafion is present with other components that give rise to a multiphase XRD pattern. INTRODUCTION A fuel cell is an electrochemical reactor that converts a fuel into electricity by a chemical reaction of the fuel with an oxidant (usually oxygen) resulting in the generation of electricity and heat (Srinivasan, 2006). It is this process for the generation of electricity that is the focus of many research programs, looking for ways to generate clean energy (U.S. Department of Energy, 2013). Of particular interest are fuel cells that use hydrogen as a fuel source to produce electricity by an electrochemical reaction of hydrogen with oxygen, with the resulting byproduct being water. Though both batteries and fuel cells can generate electricity from an electrochemical reaction, the fuel cell uses an external supply of chemical energy and can run indefinitely as long as it is supplied, for example, with hydrogen and oxygen. The first report of a hydrogen fuel cell was from William R. Grove (1838). Combining an iron sheet, unglazed porous porcelain plates, in a solution of copper sulfate and dilute acid, Grove showed For 100 years, fuel cells were primarily studied as part of research programs. The beginning of commercial development of fuel cells was in 1939, when Francis T. Bacon developed a fuel cell that used an alkaline rather than acid electrolyte (Behling, 2013). Twenty years later he demonstrated the first multi kilowatt (6 kw) fuel cell comprised of 40 cells that was used to power a forklift, welding equipment and a circular saw. United Technologies eloped an alkaline fuel cell that was used by NASA (National Aeronautics and Space Administration) from Today there are many types of fuel cells being studied for commercial applications. These include proton exchange membrane, phosphoric acid, solid oxide, direct methanol, molten carbonate along with continued development of alkaline fuel cells. In general these fuel cells

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3 work in a similar manner and have similar components including a ing of a stack of a number of individual cells. Each cell has two electrodes, a cathode (positive) and an anode (negative), an electrolyte (solid or liquid) and a catalyst. There is particular interest in proton exchange membrane, PEM, fuel cells for transportation, particularly automotive applications. PEM fuel cells use hydrogen as the fuel source carrier, have an electrolyte that is a proton conducting polymer membrane, and have a low operating temperature ( C), ideal for the automobile industry. However, PEM fuel cells have poor cogeneration (heat) capability compared to fuel cells that operate at higher temperatures. Figure 1 shows a schematic of a PEM fuel cell. 129 Figure 1. PEM type fuel cell. (A) Hydrogen (H 2 ) fuel flows through field flow plates to the anode while oxygen (O 2 in air) flows through field flow plates to the cathode. (B) The anode has a catalyst (platinum) that splits hydrogen into protons and electrons, H 2 2H + + 2e -. (C) The PEM membrane allows H + to pass through the membrane. Electrons travel along an external circuit to the cathode, resulting in an electrical current. (D) At the cathode, protons, electrons, and oxygen combine resulting in water as a byproduct 2H + + 1/2O 2 + 2e - H 2 O. Proton exchange membranes (PEM) based on Nafion (Grot, 1974) are a critical component of hydrogen-oxygen fuel cells. Nafion (a registered trademark of E.I. DuPont de Nemours and Co.) is a perfluorosulfonate polymer that consists of a linear backbone of fluorocarbon chains (similar to polytetrafluoroethylene, PTFE) and ethyl ether pendant groups. Processing the sulfonyl

4 fluoride (-SO 2 F) with hot NaOH results in a sulfonate salt (-SO 3 Na). Proton exchange converts the salt to the acid form (-SO 3 H) referred to as Nafion PFSA (Figure 2), perfluorosulfonic acid/poly(tetrafluorethylene) copolymer. The salt and acid forms are both hydrophilic. Protons from the sulfonic acid groups can move from one acid site to another. 130 Figure 2. Nafion PFSA, C 2 F 4 C 7 HF 13 O 5 S, structural formula. Pores in a Nafion PFSA membrane allow for the movement of cations (i.e. H + ) but the membrane does not conduct anions or electrons. Depending on the manufacturing process, the cation conductivity of a Nafion membrane can be modified to meet a specified application requirement. A number of models have been proposed to define the microstructure of Nafion (Mauritz and Moore, 2004). The microstructure is still being debated, with some indication that the morphology can change as the Nafion membrane is exposed to varying levels of hydration. What is consistent in these models is that Nafion is comprised of ionic clusters, an amorphous component, and can have a crystalline component. X-ray diffraction (XRD) results (Starkweather, 1982) for data collected from an oriented fiber of the sulfonyl fluoride form of Nafion (equivalent weight of 1100) indicate the unit cell of the crystalline component is hexagonal, similar to PTFE with a=5.85 Å and c=2.59 Å. Both small-angle and wide-angle X- ray scattering techniques have been used extensively for analysis of Nafion films. However, there are no reference XRD patterns for Nafion in the Powder Diffraction File (PDF TM is a registered trademark of ICDD) that analysts can use for phase identification or whole pattern fitting analysis. In this study thin membrane Nafion PFSA films (10 25 microns) were analyzed using wideangle XRD. Data were collected in ambient air conditions, as well as variable relative humidity. With careful sample preparation and a properly aligned diffractometer, scattering from the ionic clusters, amorphous component, and crystalline component was observed. Raw data XRD patterns have been collected and submitted for inclusion in the PDF database. These data entries will be particularly useful in cases where Nafion films are present with other components that give rise to a multiphase XRD pattern. EXPERIMENTAL Nafion PSFA DuPont D2020 solution, 20% Nafion PSFA (equivalent weight 1000) in n-propyl alcohol, was cast on a FEP (fluorinated ethylene propylene)/kapton support layer and dried under an IR lamp. The dried Nafion films (10 15 micron thickness) were removed from the FEP/Kapton and

5 placed onto zero-background quartz plates for XRD analysis. Some dried films were annealed at elevated temperature in N 2 atmosphere, then cooled to ambient temperature before removal from the support. 131 DuPont NR211 as received extruded Nafion PFSA (equivalent weight 1100) films (25 micron thickness) on a FEP/Kapton support layer was removed from the FEP/Kapton and placed onto zero-background quartz plates for XRD analysis. Since the molecular weight of Nafion is uncertain, it is represented as an equivalent weight defined as the weight of Nafion per sulfonyl group (Mauritz and Moore, 2004) where EW ~ 100x (100 for PTFE, 446 for perfluorovinyl ether with terminating sulfonyl fluoride). For Nafion PFSA where OH has replaced F, EW ~ 100x The value listed for a Nafion sample EW is rounded to the nearest hundred. For DuPont D2020 EW=1000 (x=6) and DuPont NR211 EW=1100 (x=7). X-ray Diffraction XRD data were collected in reflection mode geometry using a Rigaku D2000 Bragg-Brentano (B-B) diffractometer equipped with a Cu rotating anode, diffracted beam graphite monochromator tuned to Cu K radiation, and scintillation detector. The temperature during data collection was 23 C. Samples were analyzed in ambient air (40% relative humidity, RH) or with the sample in a humidity chamber mounted on the diffractometer for in situ humidity XRD measurements at a specified % RH (Blanton, 2005). The % RH was measured using a VWR digital hygrometer. RESULTS AND DISCUSSION Previous Nafion studies can be combined to help define four specific regions observed in smallangle and wide-angle X-ray diffraction (scattering) patterns of Nafion PFSA films analyzed in this study. (Fujimura et al., 1981; Gierke et al., 1981; Starkweather, 1982, Mauritz and Moore, 2004). 1. A small-angle peak < (Cu K ) associated with crystalline domains, possibly from lamellae platelets Usually observed when using a small-angle scattering camera and only when the percent crystallinity is several percent 2. A low angle peak (Cu K ) associated with ionic domains (clusters) Can be observed using a B-B diffractometer 3. A broad amorphous peak (Cu K ) Observed in all samples using a B-B diffractometer 4. A wide-angle peak at ~ (Cu K ), (100) peak based on a hexagonal unit cell Usually observed using a B-B diffractometer when the percent crystallinity is several percent The (101) peak is difficult to resolve from an amorphous peak at (Cu K )

6 An X-ray diffraction pattern for a Nafion PFSA sample annealed 2hr at 140 C in N 2 is shown in Figure 3, identifying ionomer cluster, amorphous and crystalline regions. 132 Figure 3. X-ray diffraction pattern for 10 micron D2020 DuPont Nafion PFSA film, annealed 2 hr. at 140 C in N 2 (data collection at 23 C, 40% RH). In Figure 4, XRD patterns for D2020 Nafion PFSA before and after annealing are shown. In both diffraction patterns, low-angle peaks due to ionomer clusters are present. However the unannealed sample diffraction pattern data show evidence of well resolved first and second order low-angle scattering peaks. The annealing of Nafion PFSA at 110 C results in some water loss. Figure 4. X-ray diffraction patterns for 15 micron D2020 DuPont Nafion PSFA films (A) unannealed, and (B) annealed 16hr at 110 C in N 2. (data collection at 23 C, 40% RH).

7 Though both samples were equilibrated at room temperature and 40% RH ambient air before XRD analysis, recovery of the water loss that was due to annealing did not return the sample to its original film state. Though ionomer clusters remain, they are not as well defined after annealing indicating further hydration is required. In contrast crystallinity was observed to improve after annealing, as the (100) diffraction peak is evident in Figure 4B. Fujimura et al. (1981) describe a method to determine relative crystallinity where the XRD pattern is analyzed in the range of 10 - this selected range diffraction pattern decomposed into two peak profiles. An amorphous component with a peak at ~16 and a crystalline component with a peak at ~17.5. In the present study, the data collection range was increased to 8-22 for improved background determination, and unlike the Fujimura method where the peak widths were fixed, the fitting of the peak profiles was not constrained. An example of a relative crystallinity measurement for the annealed D2020 DuPont Nafion PSFA film sample of Figure 4B is shown in Figure Figure 5. Profile fitting (Pearson VII function) of the selected range X-ray diffraction pattern for 15 micron D2020 DuPont Nafion PSFA annealed 16hr at 110 C in N 2. (data collection at 23 C, 40% RH). Based on peak area the relative crystallinity is 18% for this annealed sample. Though this approach does not provide absolute percent crystallinity, it does allow for a ranking of sample crystallinity for multiple samples, which is useful in assessing the effect of processing on Nafion microstructure. Comparison of XRD results for the two types of Nafion PFSA evaluated in this study can be found in Figure 6 (both unannealed). While D2020 (Figure 6A) was cast from solution, NR211 (Figure 6B) was extruded. Extrusion, which occurs at elevated temperature, results in a loss of order in the ionomer clusters based on the decrease in XRD peak intensity for the low-angle ionomer peaks when compared to the solution cast Nafion PSFA. Similar to what was observed in Figure 4B for the annealed solution cast D2020 DuPont Nafion PFSA film, the extruded

8 NR211 DuPont Nafion PFSA (Figure 6B) is more crystalline than the unannealed D2020 DuPont Nafion PFSA film. Though NR211 DuPont Nafion PFSA was not subjected to a defined annealing process the extrusion of the film provided the same effect. Using the method described previously, the relative crystallinity of NR211 DuPont Nafion PFSA was found to be 33%. 134 Figure 6. X-ray diffraction patterns for (A) 15 micron D2020 DuPont Nafion PFSA film (solution cast), and (B) 25 micron NR211 DuPont Nafion PFSA film (extruded), (data collection at 23 C, 40% RH). In addition to the effect of thermal extrusion enhancing Nafion PFSA crystallinity, Gierke et al. (1981) and Fujimura (1981) were able to show based on XRD and DSC results that an increase in Nafion equivalent weight will result in an increase in crystallinity. The EW of NR211 is 1100, for D2020 EW is To further illustrate the effect of water loss on Nafion PFSA, in situ humidity XRD studies were carried out. An unannealed D2020 DuPont Nafion film was placed in a humidity chamber and equilibrated for 30 minutes at a specified relative humidity. An XRD pattern was then collected, with the sample still at the specified %RH. This measurement was carried out at 0, then 40, and finally 100% RH. The XRD results are shown in Figure 7. Figure 7. In situ variable humidity XRD results for 15 micron D2020 DuPont Nafion PFSA film, data collected at 23 C and (A) 0% RH, (B) 40% RH, (C) 100% RH. The low-angle region of the diffraction pattern is shown in the inset.

9 As relative humidity is increased from 0 to 100% RH, the low-angle diffraction peak is observed to increase significantly in intensity. At 40 and 100% RH, a second order low-angle peak is also observed. At 0% RH most of the water is removed from Nafion PFSA, resulting in a reduction of order in the ionomer clusters. This loss of water would also occur in a similar manner for Nafion PEMs that are exposed to elevated temperature conditions, for example in automotive fuel cell applications. In addition to changes in peak intensity the peak position is also observed to change, shifting to (larger d-spacing) with an increase in relative humidity. From Figure 7 the observed ionomer cluster spacings are 26.7 Å at 0% RH, 29.5 Å at 40% RH and 32.3 Å at 100% RH. An increase of 5.6 Å in ionomer spacing going from 0 to 100% RH is an indication that as relative humidity rises so does ionomer cluster water uptake. The exact microstructure of the Nafion ionomer is still a subject of debate as several proposed models exist including spherical inverted-micelle water clusters (Gierke et al., 1981), layered structures (Starkweather, 1982), channel networks (Kruer, 2001), and polymer bundles (Rollet A. L. et al., 2002). Schmidt-Rohr and Chen (2008) have used a structure comprised of long parallel randomly packed water channels surrounded by partially hydrophilic side branches forming invertedmicelle cylinders to explain small-angle X-ray scattering data for Nafion ionomer clusters. However none of these or other proposed models have been accepted as absolute in defining the morphology of the Nafion ionomer cluster. All do, nonetheless, allow for an expansion in the ionomer cluster size with increasing water content in Nafion. Though loss of water would have an adverse effect on Nafion PFSA ionomers and subsequently on proton conduction, the XRD data in Figure 7 do show that Nafion PFSA ionomer cluster order can recover upon exposure to elevated humidity (or water), indicating there is a water content ionomer cluster order hysteresis loop for Nafion films. 135 The wide-angle XRD region of 8- in Figure 7 shows only a small decrease in intensity with increasing relative humidity. This minimal intensity loss can be attributed to an increase in X-ray absorption due to the increase in water content in the sample humidity chamber. From a Nafion PFSA microstructure perspective, changes in humidity did not show an effect on Nafion crystallinity. SUMMARY Wide-angle X-ray diffraction has been used to characterize Nafion PFSA films. Attention to instrument alignment and sample preparation has allowed for collection of XRD data that include low-angle ionomer cluster peaks, and wide-angle amorphous and crystalline peaks. To observe lamellae spacings requires a small-angle scattering instrument. Nafion PFSA films cast from solution were found to have higher ionomer cluster order but less crystallinity than extruded films. Crystallinity can also be enhanced by annealing, however too high of a temperature will result in water loss and a decrease in ionomer cluster order. Recovery of ionomer cluster order is possible with prolonged exposure of Nafion PFSA films to elevated relative humidity. High quality XRD patterns for Nafion PFSA (Figures 4A, 4B, and 6B) have been generated and submitted for inclusion in the ICDD Powder Diffraction File to assist in phase identification and materials characterization, for scientists studying Nafion PEMs.

10 ACKNOWLEDGEMENT X-ray diffraction data were collected at Eastman Kodak Company, Rochester, NY where T.N. Blanton was an employee. 136 REFERENCES Behling, N.H. (2013). Fuel Cells: Current Technology Challenges and Future Research Needs (Elsevier, Great Britain), p Blanton, T.N. and Barnes, C.L. (2005 Quantitative analysis of calcium oxide desiccant conversion to calcium hydroxide using X--ray Anal., 48, Fujimura, M., Hashimoto, T., Kawai, H. (1981-angle scattering studies of perfluorinated ionomer membranes , Gierke, T.D., Munn, G.E., and Wilson, F.C. (1981 perfluorinated membrane products, as determined by wide- and small-angle X- Phys. Ed., 19, Grot, W.G. ( , issued January 8, Grove, W.R. ( (4), Kruer, K. D. ( , Mauritz, K.A. and Moore, R.B. ( , Rollet, A. L., Diat, O., and Gebel, G. (2002 Chem. B 21, Schmidt-Rohr, K. and Chen, Q. (2004-7, Srinivasan, S. (2006). Fuel Cells: From Fundamentals to Applications (Springer, New York), p Starkweather, H.W. (1982 ity in perfluorosulfonic acid ionomers and related 15, U.S. Department of Energy (2013). DOE Hydrogen and Fuel Cells Program 2013 Annual Progress Report (DOE/GO ). Oak Ridge, Tennessee: Office of Scientific and Technical Information.