Far-infrared studies on Nafion and perfluoroimide acid (PFIA) and their alkali salts

Accepted Manuscript Title: Far-infrared Studies on Nafion and Perfluoroimide Acid (PFIA) and their Alkali Salts Author: A.D.O. Bawagan S.J. Hamrock M. Schaberg I. Yousef E. Ritter U. Schade PII: DOI: Reference:

S0924-2031(14)00098-8 http://dx.doi.org/doi:10.1016/j.vibspec.2014.05.010 VIBSPE 2356

To appear in:

VIBSPE

Received date: Revised date: Accepted date:

3-3-2014 23-5-2014 29-5-2014

Please cite this article as: A.D.O. Bawagan, S.J. Hamrock, M. Schaberg, I. Yousef, E. Ritter, U. Schade, Far-infrared Studies on Nafion and Perfluoroimide Acid (PFIA) and their Alkali Salts, Vibrational Spectroscopy (2014), http://dx.doi.org/10.1016/j.vibspec.2014.05.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Far-infrared Studies on Nafion and Perfluoroimide Acid (PFIA) and their Alkali Salts A.D.O. Bawagana*, S.J. Hamrockb , M. Schabergb, I. Yousefc, E. Ritterd and U. Schadee

b

3M Fuel Cell Components Group, 3M Center, St Paul, MN 55144, USA c

Humboldt-Universität zu Berlin, Experimentelle Biophysik, 10115 Berlin, Germany Helmholtz-Zentrum Berlin, BESSY II, 12489 Berlin, Germany

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e

A.D.O. Bawagan deceased during the revision of the manuscript.

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*

SESAME, Allan 19252, Jordan

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Department of Chemistry, Carleton University, Ottawa, ON K1S5B6, Canada

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a

polymer

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Keywords: Nafion, perfluoroimide acid, far-infrared, terahertz, alkali salt, force constant,

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Abstract

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Corresponding Author: [email protected]

The terahertz/far-infrared spectra (< 300 cm-1) of perfluorinated sulfonic acid (Nafion NR211 polymer) and perfluoroimide acid (PFIA polymer) and their alkali (M+) salts have been analyzed and the results are presented. Pronounced features in the spectra of these ionomers that correlate systematically with the corresponding cation mass are reported and from their spectral position the force constants are derived. The average vibrational force constants for Nafion/M+ and PFIA/M+ are found to be 54 ± 7 N/m and 39 ± 4 N/m, respectively. Such terahertz/far-infrared signatures probe the detailed structure of the Nafion/M+ and PFIA/M+ ionic clusters and, in turn, provide benchmarks for elucidating the ionomer “water channels” or water molecules located in the ionomer-water interface upon hydration. Qualitative trends in the vibrational energies of Nafion and PFIA can be explained by consideration of electronic and/or structural (ionic domain-size) effects. 1 Page 1 of 20

1. Introduction Increased

interest

in

Nafion™

(registered

trademark

of

DuPont

for

its

perfluorosulfonated acid products) [1, 2, 3] stem from its use as the proton exchange

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membrane (PEM) in fuel cells. Here, high proton conductivity and good mechanical-chemical stability are essential membrane parameters for the fuel cell performance, which Nafion can provide for operating temperatures below 80 °C. However, there is a technological demand for

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novel PEMs which operate at higher temperatures and drier conditions with improved

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mechanical stability and proton conductivity [4]. New multi acid side-chain ionomers, such as 3M perfluoroimide acid (PFIA) [5, 6, 7], could meet this challenge.

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The molecular structure of the Dupont Nafion polymer consists of a tetrafluoroethylene (PTFE) backbone with perfluorinated pendant chains that terminate with sulfonate groups

M

(SO3 -M+) where M+ = H+ in the acid form or M+ = Li+, Na+, K+, Rb+, Cs+ in its alkali salt form. Likewise, the 3M PFIA polymer consists of a PTFE backbone and pendant chains terminated with sulfonate groups. However, the 3M-PFIA polymer offers an additional exchangeable-H in the

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< Figure 1 >

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(see Figure 1).

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form of a sulfonyl imide group along the pendant chain prior to the terminal sulfonate group

Although there have been significant publications on the mid-IR (MIR) spectra of Nafion and its salts (see, ie. [8], and citations therein), comparatively sparse experimental terahertz (THz) or far-infrared (FIR) data of Nafion are available in the literature. So far, the only data on FIR vibration energies of the cation-motion for some Nafion alkali salts are about 35 years old and are quoted in a paper of Mattera and Risen [9]. However the original source is an unpublished Office of Naval Research Technical Report 79-01 of S.L. Peluso, A.T. Tsatsas and W.M. Risen (1979). The actual spectra were eventually published in a 1986 ACS book [10] without the experimental details. As will be shown in the present work, some of the FIR spectra in previously published spectra [10] for Nafion/M+ can contain spectral interference features at ~ < 120 cm-1. FIR data for Nafion/Li+ (spectra and cation-peak-location) and for Nafion/H+

2 Page 2 of 20

(cation-peak-location) are missing in the literature. Up to date, no published FIR data or spectra exist for PFIA. The present work aims to contribute in filling this void. We investigated the FIR cation-motion vibrations below 300 cm-1 in Nafion and in PFIA

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by means of Fourier Transform infrared (FTIR) spectroscopy. Etalon effects (interferences fringes) in the spectra of the polymer films were corrected in the present study using calibration PTFE spectra measured under same conditions. We calculated the effective force constant for

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the forces acting between the ionomer anion site and the counter-cations. The present results

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for Nafion and PFIA and their alkali salts may encourage further development of more accurate force field models as well as more accurate molecular dynamics simulations and quantum

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mechanical calculations [11, 12, 13, 14]. 2. Experimental procedure

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2.1. Materials

Nafion NR211 perfluoro-sulfonate membranes with an equivalent weight (EW, grams

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polymer/moles acidic hydrogen) of 1100 g/equivalent and a nominal thickness of 25 µm were

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obtained from the manufacturer (Ion Power GmbH) in acid form (Nafion/H+). Perfluoroimide acid (PFIA) membranes (25 µm thick, 625 g/equivalent) were provided by the 3M Fuel Cell

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Components Group. Teflon (PTFE) calibration polymers (several thicknesses, including 25 µm thick) were obtained from Goodfellow. All Nafion NR211 and PFIA membranes were pretreated according to Zawodzinski et al. [15, 16]. The pre-treatment procedure involves 4 steps: boiling in 3% H2O2, boiling in deionized H2O, boiling in 0.5 M H2SO4 and finally boiling in deionized H2O with each step for about 1 hour duration. The pre-treated membranes were soaked in a sealed glass container with deionized water of very high purity (>18 Mcm, Millipore) at room temperature. High purity alkali salts were obtained from Carl Roth GmbH (LiCl 99%, NaCl 99.5%, KCl 99%, RbCl 99% and CsCl 99.999%). Only glassware and other implements that have been previously treated with piranha solution (equal amounts of 96% H2SO4 and 30% H2O2) [17] were used during the preparation of all solutions and all spectral measurements.

3 Page 3 of 20

To obtain the Nafion NR211 and PFIA membranes in the alkali salt-forms, the pre-treated membranes were soaked in the corresponding 1.0 M alkali halide solution for at least 24 hours at room temperature.

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The physico-chemical properties of Nafion and PFIA are strongly dependent on its water content as revealed in previous MIR studies [18, 19, 20]. The control of the relative humidity (RH) and thus, the average number of H2O molecules per SO3-group (w) is important. Without

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losing generality, the present FTIR spectral study is focused only on a very low humidity state

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(relative humidity RH < 3%, w ≈ 0). The dry samples were prepared directly from the previously soaked Nafion and PFIA pre-treated membranes. The humidity of the samples was directly

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monitored (during the drying procedure and before and during the measurement) by the water absorption band in the OH stretching region of H2O around 3500 cm-1 which allows precise estimation of residual humidity in the film [21]. Before drying the membranes, surface water

M

was removed by carefully pressing the membranes between lens papers (Motic). Drying was conducted in vacuum (<2 mbar) at 30 °C for at least 2 hours, until no further change in water

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2.2. Spectral measurements

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content could be observed spectroscopically.

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All spectra shown in this study were obtained with FTIR vacuum spectrometers at a temperature of about 30 °C. Spectra in the region between 10 and 300 cm-1 were measured with a Bruker Vertex 80/v instrument equipped with appropriate optical beam splitters and detectors (DTGS detector and liquid He-cooled 4-K Si-bolometer) using a mercury arc lamp as a radiation source. In order to obtain higher sensitivity, spectra below 30 cm-1 were acquired using a synchrotron light source (HZB, BESSY II, IRIS beamline [22]) operating at low-alpha mode and a Bruker IFS 66v spectrometer. The low-alpha mode is necessary to generate stable coherent synchrotron radiation (CSR) in the very far IR which is at least 100 times more intense than usual incoherent synchrotron sources [23]. The Bruker IFS 66v spectrometer at the IRIS beamline was operated with a 50-µm Mylar beamsplitter and a liquid He-cooled 1.6-K Si-bolometer. The dry sample spectra were measured in transmission mode. Spectral resolution was 1 cm-1. At least 128 scans were co-added both for the sample and for the reference (no sample in place) 4 Page 4 of 20

resulting in a reproducibility of better than 1%. Spectra of the two spectral regions from the two spectrometers were merged in order to get the complete FIR spectrum of the polymer between 10 and 300 cm-1. The high precision and reproducibility of the spectral measurements of the dry samples were obtained by careful, simultaneous monitoring of well-known H2O MIR

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absorbances in the OH stretching region of H2O around 3500 cm-1 [18, 20].

In addition, measurements were performed in the MIR spectral region and also on wet

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samples (100% RH, w ≈ 12), which are beyond of the scope of this report. However, our MIR

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measurements for Nafion and PFIA in acid form (M+ = H+) and under dry and wet conditions reveal the same spectral peaks and intensities in the region between 1500 and 700 cm-1 as

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observed previously by Danilczuk et al. [6]. 2.3. Data analysis

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The interpretation of transmission spectra is often complicated by interference fringes. Such fringes are caused by the coherent superposition of light from the two parallel film

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surfaces and can lead to an erroneous quantitative analysis of the chemical study. Especially in

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the far infrared and for thin films such fringe features can be very similar to the absorption band under investigation. In order to remove such interference fringes from the spectra of our

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samples we calculated the absorbance spectrum of a model film of same thickness and subtracted it from the experimental absorbance. For the model film a refractive index of n = 1.43 (no extinction) was used which we derived from the optical thickness, d·n, of a series of transmission measurements of PTFE films of different known thicknesses, d, and which corresponds well with previously published far-infrared data below 40 cm-1 for PTFE [24]. < Figure 2 >

Figure 2 shows the interference correction for two 25-µm thick films, PTFE and Nafion/Li+, respectively. While both the uncorrected spectra indicate a possible absorption band at about 80 cm-1 this feature completely disappeared in the corrected spectrum of PTFE and for the Nafion/Li+ sample the low-energy wing of a possible broad absorption band appeared clearly after correction. Measurements taken at the IRIS beamline with coherent synchrotron 5 Page 5 of 20

radiation confirmed that the absorbance of all PTFE, Nafion and PFIA film systems smoothly vanish after correction as very low wavenumbers are approached. This behavior at very low energies which can be explained by a non-absorbing dielectric film confirms that the apparent peaks below 80 cm-1 in the uncorrected spectra are, in fact, spectral interferences. Within the

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accuracy of our measurements no additional PTFE bands below 200 cm-1 are observed but which may appear at lower temperatures [25].

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This correction method works well for removing interference fringes from spectral

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regions with weaker absorption bands. However, special care has to be taken in the spectral region with stronger absorption bands since in this case the film does not show interference

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fringes [26] and band shape and strength may be altered by this correction. 3. Results and discussion

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3.1. Far-infrared spectra of Nafion and PFIA and their salts in the region 30 - 300 cm-1 The far infrared spectra for the Nafion and PFIA alkali salts investigated after

d

interference correction are shown in Figures 3 and 4, respectively. All spectra are characterized

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by a broad cation-motion vibration superimposed onto vibrational modes of the PTFE backbone. For all spectra, in common, the broad cation-related feature shifts to higher energies with

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decreasing cation (M+) mass which have been discussed previously for Nafion [9, 10] and other ionomer [27, 28, 29]. < Figure 3 > < Figure 4 >

The broad cation vibrational bands (20 - 100 cm-1 FWHM) along with the strong 202-cm-1 PTFE band [30] and other PTFE backbone-related absorption bands at higher energies were fitted by sum of Gaussian curves (black dashed curves in Figures 3 and 4) for precise estimation of cation vibrational energies. The backbone vibrational bands are subtracted when necessary (resulting spectra are shown as black solid curves) and the maximum of the remaining spectra was taken as the cation vibrational energy. This method works well for the heavier cations and the uncertainty based on our curve fits is estimated to be ± 0.5 cm-1. The estimated uncertainty 6 Page 6 of 20

for H+ and Li+ of ± 10 cm-1 is much higher due spectral artifacts which could not be removed by the method above. The derived FIR vibrational energies for Nafion/M+ are tabulated in Table 1 along with older data of Peruso et al. (1979) for the four cations Na+, K+, Rb+ and Cs+, which are investigated are reported for the first time and are presented in Table 1.

cr

< Table 1 >

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in good agreement with our results. The vibrational energies for all the alkali salts of PFIA

The systematic trend in the FIR vibrational energies as a function of alkali cation mass for

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both the ionomers results from the potential field created by the cluster of counter-anions and residual H2O [9, 10, 27, 28, 29, 31, 32]. In addition, the broad spectral width simply indicates the

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wide range of possible anion/M+ and residual H2O configurations. 3.2. Vibrational force constants for Nafion/M+ and PFIA/M+

M

Figure 5 shows the energies of the Nafion/M+ and the PFIA/M+ vibrations as a function of the inverse square root of the cation mass. Previous studies by Risen and co-workers [9, 10, 28,

d

29] have shown that the calculated force field parameters are consistent with experiment. Thus,

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the vibrational energy, ~; , scales with the inverse square root of the reduced mass as expected from the classical harmonic model, where µ is the cation-polymer reduced mass, c is the speed

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of light and k is the force constant:

1 k ~  2c µ

with

1 1 1   . µ µM µ polymer 7 Page 7 of 20

To first order, µ is simply the cation mass, µM, since it holds µM ≪ µpolymer, the polymer

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mass. Other assumptions of the reduced mass depend on the force field model (from an infinitely heavy counter-anion to a finite set of anions) and do not significantly depart from the

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straight lines shown in Figure 5. The trend in the FIR vibrational energies energies of PFIA/M+ parallels the results for Nafion/M+ thus indicating the general validity of the classical harmonic

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model at least for medium to heavy alkali cations. It was found that the vibrational energies observed for PFIA are lower relative to Nafion for the same counter-ion. Hence the

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experimental energies are caused by the cation-anion site vibrations, where the cations move under the influence of the coulombic (electrostatic) field of the protogenic group [e.g., 9]. The

M

case for M+ = H+ and Li+ for both the ionomers under study (Figures 3 and 4, respectively) departs from this classical relation because the relatively small masses of the counter-cations lead to vibrational energies which are very sensitive to the local environment and also quantum

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< Figure 5 >

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the heavier alkali-ions.

d

nuclear effects . Furthermore, the FIR peak widths are even broader (50 - 100 cm-1) compared to

The lower vibrational energies observed for PFIA relative to Nafion for the same counterion can be explained qualitatively following similar observations by Mattera and Risen [9] regarding the FIR energies of alkali-cation containing ionomers of polyethylene-methacrylic acid (PEMA), polystyrene sulfonic acid (PSSA) and perfluorocarbon-sulfonic acid (PFSA or Nafion). The trend of decreasing FIR energies along the series: PEMA > PSSA > Nafion > PFIA, results from electronic and/or structural variations among the different ionomers. Comparing PSSA and Nafion, the electron withdrawing effect of the PTFE-backbone in Nafion results in a lower Nafion/M+ vibrational energy. Comparing Nafion and PFIA, the electron withdrawing effect of the additional sulfonic groups in PFIA results in a lower PFIA/M+ vibrational energy. Another important point is the structural factor or the ionic-domain sizes of the respective ionomers which are currently under intense investigation and intense debate. While simple models such 8 Page 8 of 20

as “chelate-like bonding” [33] and “double-cation bonding” [34] can also be advanced to rationalize the lower FIR energies in PFIA compared to Nafion, any further conclusions beyond the qualitative observations of Mattera and Risen [9] will have to await more sophisticated theoretical models that explicitly include detailed cation/anion- environment and water

ip t

molecule interactions.

Based on the linear relationship shown in Figure 5, the average FIR vibrational force

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constants for Nafion/M+ and PFIA/M+ are found to be 54 ± 7 N/m and 39 ± 4 N/m, respectively.

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The PFIA force constant is significantly smaller than the Nafion force constant. Within the statistical uncertainties, the calculated force constants are largely invariant to the force field

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model-dependent choice of the reduced mass, m, at least, for M+ = Na+, K+, Rb+ and Cs+. It would therefore be very interesting if theoretical quantum and/or classical molecular dynamics models could reproduce these experimentally derived force constants and corresponding vibrational

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energies. It is also noteworthy that similar FIR investigations of zeolite/M+ mixtures provided key spectral information to the specific absorption sites of cations in zeolites especially when

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4. Conclusions

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combined with accurate molecular dynamics calculations [35, 36].

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Despite significant work, the multiple scale structure of PEM membranes is still ambiguous. However, there is general consensus that “ionic clusters” in the form of clusternetwork models [37] and parallel-pore models [38] are responsible for the high proton mobilities. Basically, fuel cells need durable PEM to reliably transport H+ across the anodecathode boundary. But H+ transport is dependent on water concentration, in particular, the structure of the ionic clusters or so-called “water channels” [39] within the Nafion and PFIA polymer membranes.

Our present FIR results are of relevance for the development of highly-durable PEMs used in fuel cells. The results reveal the distinct low energy modes which are sensitive to the detailed structure of ionic clusters in the dry ionomer/M+ and, in turn, provide necessary benchmarks for understanding the postulated water channels upon hydration. For example, the recent experimental work of Di Noto and co-workers [40,41] illustrate the complex interplay 9 Page 9 of 20

between polymer morphology or conformation as a function of polymer equivalent weight and the 3M-membrane water content. The confirmed data for Nafion/M+ =Na+, K+, Rb+ and Cs+ and the new data for Nafion/M+

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= H+, Li+ and for PFIA/M+ =H+, Li+, Na+, K+, Rb+ and Cs+ provide valuable insight into specific cation-anion-residual-solvent configurations especially when combined with accurate molecular dynamics force field calculations in the same spirit as earlier zeolite/M+ systems [35, 36]. That is,

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our THz/ FIR spectra are obtained on the same experimental platforms and under the same

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chemical conditions thus the relative spectral parameters that are extracted for different ionomers can be compared on a semi-quantitative basis. The present results highlight the

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usefulness of FIR spectra in the development of next-generation PEM with improved durability and conductivity [5, 42, 43, 44].

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Acknowledgments

A. Bawagan acknowledges helpful discussions with W.M. Risen (Brown University) and the

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hospitality of colleagues at BESSY II of the Helmholtz-Zentrum Berlin (HZB) during his 2012

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sabbatical. I. Yousef acknowledges the IAEA Fellowship. E. Ritter acknowledges the European

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Research Council (erc) Advanced Grant TUDOR to K.-P. Hofmann.

10 Page 10 of 20

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13 Page 13 of 20

Captions: Figure 1:

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Chemical structure of the two ionomers, Dupont Nafion™ (left-hand side) and 3M PFIA (righthand side). Exchangeable-H groups are shown in red. The PTFE backbone and the pendant

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chains are shown in black and blue, respectively.

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Figure 2:

FIR absorbance spectra for 25-µm thick PTFE and Nafion/Li+ films (blue and green dashed curve,

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respectively) as obtained by FT-IR measurements. The red dashed curve shows the absorbance spectrum of a 25-µm thick model film having the same index of refraction as PTFE. The blue and

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green solid curves show the corrected spectrum for PTFE and Nafion/Li+ films, respectively,

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where the spectrum of the model film has been subtracted. Figure 3:

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FIR spectra of a) Nafion/M+ (M+ = Na+, K+, Rb+, Cs+) and b) Nafion/M+ (M+ = H+, Li+) after removal

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of the interference fringes. Black curves (bold and dashed) have been used to derive the cationmotion energy (see text for details). Figure 4:

FIR spectra of a) PFIA/ M+ (M+ = Na+, K+, Rb+, Cs+) and b) Nafion/M+ (M+ = H+, Li+) after removal of the interference fringes. Black curves (bold and dashed) have been used to derive the cationmotion energy (see text for details). Figure 5: Cation vibration energy as a function of the inverse root of the counter-ion (M+) mass. Solid lines are linear fits to the Nafion/M+ (blue triangle) and PFIA/M+ (red circle) data excluding the lighter ions (H+, Li+). 14 Page 14 of 20

Tables:

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Table 1:

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FIR band position of the cation vibration in dry Nafion and PFIA and their alkali salts.

(cm-1), present work

-

205

-

182

180

158

146

150

132

111

110

95

90

75

1.0

245

Li+

7.0

223

Na+

23.0

190

K+

39.1

Rb+

85.5

M

H+

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Mass (amu)

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(cm-1), literature [9]

Cation

Cs+

132.9

PFIA/M+

d

(cm-1), present work

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Nafion/M+

93

15 Page 15 of 20

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Figure 1

16 Page 16 of 20

ip t cr us an M d te Ac ce p

Figure 1

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te

d

M

an

us

cr

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251658240

Figure 3

18 Page 18 of 20

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te

d

M

an

us

cr

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251658240

Figure 4

19 Page 19 of 20

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Figure 5

20 Page 20 of 20