J. Phys. Chem. B 2008, 112, 10535–10543
10535
Infrared Spectroscopic and DFT Vibrational Mode Study of Perfluoro(2-ethoxyethane) Sulfonic Acid (PES), a Model Nafion Side-Chain Molecule David S. Warren* and A. James McQuillan Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed: March 2, 2008; ReVised Manuscript ReceiVed: June 23, 2008
We have used attenuated total reflection infrared (ATR-IR) spectroscopy to study the model compound perfluoro(2-ethoxyethane) sulfonic acid (PES) and the spectral changes induced by humidity variations to improve understanding of the IR spectrum of Nafion. This work was supported by density functional theory (DFT) calculations of the PES molecule and ion complexes to confirm assignments and determine local modes that contributed to specific absorptions in the IR spectrum. The work illustrates the difficulties of interpreting the spectrum of Nafion with several mixed modes being present. However, the loss of degeneracy in the -SO3- asymmetric stretching mode is clearly observed in difference spectra, and the use of DFT calculations provides an insight into changes induced by the variation in humidity. Introduction Perfluorinated sulfonic acid ionomers have attracted much interest since they first appeared in the 1960s. Initial interest was in their ability to act as permselective membranes for use in the chloralkali industry, but more recently, the focus has been on their development as membranes for proton -exchange membrane fuels cells.1,2 While a variety of polymer membranes including nonfluorinated materials have been developed for fuel cell use, the perfluorosulfonic acid ionomer Nafion has emerged as the benchmark material against which all others are judged. The use of Nafion in fuel cells is the result of its excellent ionic conductivity and electrochemical/chemical resistance. Nafion (Figure 1) consists of a hydrophobic polymer backbone (essentially polytetrafluoroethene) with hydrophilic fluorocarbon side chains containing two ether linkages and a terminating sulfonic acid group. Nafion is generally referred to by its equivalent weight (EW), which is the mass in grams of dry Nafion per mole of sulfonic acid group when the material is in its acid form. Nafion with an EW of 1100 g mol-1, the most commonly used version of the substance, has a structure with 12-13 backbone CF2 groups between each side chain and is available as cast or extruded membranes or as an aqueous or alcoholic dispersion. Tuning fuel cell membranes for optimum performance requires a deep understanding of their chemical behavior and microstructure, and there is much published research aimed at improving the current state of knowledge of Nafion. For example, it is known that the state of hydration of a fuel cell membrane plays an important role in the efficiency of the fuel cell, and much work has focused on microstructural and chemical changes induced by humidity variation.3-8 The detailed mechanism of the proton exchange has yet to be fully elucidated. It has been reported that protons appear to migrate through Nafion as hydronium ions, although there is a component due to a hopping mechanism.9-13 As a result, the efficiency of the proton transfer is also related to the state of hydration of the membrane.9,10,14 In efforts to improve the efficiency and high-temperature performance of Nafion membranes, there has been considerable work carried out on composite membranes * To whom correspondence should be addressed. E-mail: dwarren@ chemistry.otago.ac.nz. Phone +64 3 4794102. Fax +64 3 479 4906.
Figure 1. Comparison of (a) the Nafion side chain and (b) perfluoro(2ethoxyethane) sulfonic acid (PES).
incorporating a wide range of materials such as clay,15 metal ions,16,17 alkyl ammonium salts,18,19 surfactants,20 and metal oxides.21 Despite this work, issues still remain regarding the interactions between cations and polar groups from the Nafion side chains. Given the importance of their role in the properties of Nafion, it can be seen that an improved understanding of the interactions that take place within the ionic regions of Nafion membranes may benefit the development of Nafion-based materials. The ionic domains of Nafion contain the sulfonic-acidterminated pendant side chains. They can be considered as the centers of activity within the Nafion membrane in a working environment. The chemistry of these domains in an environment containing water is dominated by interactions involving the polar sulfonic acid terminal groups.2 Although the majority of the literature data, for both membranes and suspensions, is based on various scattering techniques, such as small-angle X-ray scattering (SAXS)22 and small-angle neutron scattering (SANS),23 there have been several studies that have used the sensitivity of Fourier transform infrared (FTIR) spectroscopy to monitor structural and chemical changes that occur in the ionic domains as the water content or the type of exchange cation is varied. These FTIR studies have used a variety of sampling techniques including KBr discs,4 transmission,3,24-26 reflectance,27 aswellasattenuatedtotalreflection(ATR-IR).4,25,28-30 Most of the reported work has been carried out using precast membranes (typically 180 µm thick), for which the penetration depth (∼1-2 µm) of the ATR-IR evanescent wave is seen as
10.1021/jp801838n CCC: $40.75 2008 American Chemical Society Published on Web 08/06/2008
10536 J. Phys. Chem. B, Vol. 112, No. 34, 2008 a limitation to its use in the analysis of such membranes. In such cases, the technique is considered to be surface specific, and data collected are not representative of the bulk. However, with the preparation of micron thin films from Nafion solutions, this is no longer a limitation to in situ ATR-IR studies, although such films are more amorphous than commercial membranes. The IR spectrum of Nafion is dominated by the absorptions of the CF2 groups in the backbone chain, which mask some of the absorptions of the sulfonate and ether groups of the side chain (see Figure 1a). Both the CF2 and ether groups have C2Vlike symmetry, which gives rise to symmetric and antisymmetric stretch modes. In the Nafion IR spectrum, the respective CF2 stretch modes are observed as strong broad bands with peaks close to 1160 and 1230 cm-1; the ether symmetric COC stretch absorptions are found at around 980 cm-1, but the ether antisymmetric COC stretch mode absorptions are obscured by the strong broad CF2 stretch mode absorptions. 3,25,30,31 The free sulfonate group -SO3- has C3V symmetry, which gives rise to symmetric (A1) and antisymmetric (E) S-O stretching modes. In the Nafion IR spectrum, the symmetric S-O stretch mode is observed at around 1060 cm-1, but the corresponding antisymmetric mode absorption is obscured by the CF2 absorptions. Thus, there is still considerable uncertainty surrounding the wavenumbers of the antisymmetric ether and sulfonate group absorptions.2,3,25,32 Further complications in making these assignments are due to the possible splitting from loss of degeneracy of the antisymmetric (E) S-O stretch mode4,25,28,30,33 and coupling between the CF3 and -SO3- groups which share C3V symmetry.34-37 The uncertainties associated with assigning absorptions in the IR spectrum of Nafion may be clarified by studying perfluoro(2ethoxyethane) sulfonic acid (PES). This relatively simple molecule has a strong similarity to the sulfonic acid end of the Nafion side chain (Figure 1b). In the absence of backbone absorptions, assigning the IR absorptions of PES should be relatively simple. While the microstructure of the pendant side chains within the ionic domains of a Nafion membrane are not expected to be the same as that of PES, the IR spectrum is expected to be similar enough to be relevant to the interpretation of Nafion spectra. Literature data indicates that, due to their similar C3V symmetry and mass, coupling occurs between -SO3- and -CF3 groups38,39 in molecules such as perfluoromethane sulfonic acid (triflic acid). Such coupling results in IR absorptions containing contributions from both groups, complicating spectral assignments. For this reason, DFT calculations were also carried out (at the B3LYP 6-311+(d,p) level) on both the PES molecule and its anion to assist in the assignment of vibrational modes. These calculations show that at least part of the difficulty encountered by previous groups in interpreting IR spectra of perfluorinated sulfonate species may be in assigning spectral bands in terms of localized vibrations. Many of the significant absorptions in the IR spectrum of PES (and by inference Nafion) contain contributions from vibrational modes involving more than one functional group. In this paper, we report the IR spectrum of perfluoro(2ethoxyethane) sulfonic acid and its anion. The assignments, made through comparison with the spectrum of Nafion and other fluorinated sulfonic acids, are supported by DFT calculations of the energy-minimized structures of the molecule and its anion. The behavior of thin films of PES exposed to varying humidities closely resembles that of Nafion and allows a direct comparison to be made between the two molecules, confirming that PES is a suitable model for the Nafion side chain.
Warren and McQuillan Materials and Methods (a) ATR- IR Spectroscopy. Spectra of perfluoro(2-ethoxyethane) sulfonic acid (PES, Matrix Scientific, 97%) as-received were taken by depositing 8 µL of the sample onto a 3 mm diameter diamond-faced 3 reflection ZnSe prism (DuraSamplIR, ASI SensIR Technologies). Aqueous solutions (usually 5 × 10-4 g L-1) were used to prepare thin films by depositing 8 µL of the solution and evaporating excess water under a water pump vacuum (∼40 mbar). All solutions were made by adding volumes of the as-received PES to deionized water (Millipore, Milli-Q, resistivity 18 MΩ cm). Infrared spectra were recorded on a Digilab FTS4000 spectrometer at 4 cm-1 and averaged over 64 scans using Win IR Pro software version 3.4. The use of a flow cell40 allowed the films to be exposed to nitrogen gas at various humidities. The humidity was controlled by mixing two nitrogen (BOC, oxygen free, >99.99%) flows, one passing through a Dreschler bottle containing silica gel and the other bubbled though a Dreschler bottle containing deionized water. The resultant relative humidity (RH) in the mixed gas flow was measured to (1% using a Vaisala HMP45A humidity probe. Spectra were recorded when no further spectral changes were observed, that is, when the film had reached equilibrium. Two types of absorbance spectra are reported in this work; first are “absolute” spectra of a compound where the spectrum is referenced against the bare prism. These spectra provide information regarding all IR-active vibrational modes. Second is the “difference” spectra, where a condition, such as RH, is changed. The spectrum taken after the change of condition is referenced to that of the spectrum before the change occurred. Difference spectra provide information about the response of vibrational modes to changes in conditions. (b) Computation of Vibrational Spectra. The IR spectra of both the undissociated PES molecule and it is anion were calculated with Gaussian 0341 using hybrid density functional theory (DFT) with the B3LYP method and utilizing the 6-311G+(d,p) basis set for all atoms. All calculations refer to isolated molecules (e.g., gas phase), and solvent influences have not been taken into account, apart from application of a solvent function in the calculations method. This function applies a constant external dielectric to the molecule based on the chosen solvent, that is, water. The structures were first optimized to an energy minimum before the spectra were calculated. To aid with spectral assignments, additional cations (Na+ and H3O+) were also included in the PES ion calculations. The data from the Gaussian output file were converted into a comma delimited (.csv) file using Microsoft Excel 2003. These files were then exported to DigiLab Resolutions Pro (version 4.0.0.03) software in order to display the calculated spectrum. The vibrational modes contributing to IR absorptions were analyzed using the animation mode of GaussView 3.0.9,42 and assignments were made on the basis of these visualizations. Results and Discussion (a) IR spectrum of PES. The spectrum of the as-received perfluoro(2-ethoxyethane) sulfonic acid (PES) is shown in Figure 2a, and principal absorptions are summarized in Table 1. The spectrum is typical of an undissociated sulfonic acid (RSO2OH)3,43 with a broad peak at 1407 cm-1 (asymmetric SO2 stretching mode) and a more intense absorption at 913 cm-1 (S-O stretching mode). There are also intense absorptions at 1220, 1192, 1125, and 1098 cm-1. A broad, weak peak at 3300-2500
PES, A Model Nafion Side-Chain Molecule
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Figure 3. The IR spectrum of PES above 1500 cm-1. (a) “As-received” PES, (b and c) during solvent evaporation, (d) at 3% RH, (e) at 11% RH, and (f) at 20% RH. The background to all spectra is the bare prism. The spectra are offset on the absorbance scale for clarity.
Figure 2. The spectra of (a) “as-received” PES and (b) PES after solvent evaporation. The background to both spectra is the bare ATRIR prism. Spectrum (b) is offset on the absorbance scale.
TABLE 1: Summary of IR Peak Wavenumber/cm-1 for PES and Assignments PES “as received”
PES PES PES film film “solvent aqueous evaporated”a solution “aqueous”b
assignment
1712 1407 1325
1232 1220 1192
1406 v weak 1324 1289
1217 1192 1155
1125 1140 1134 shoulder 1098 1098 1069 1060 1005 971 971 913 804 804 790 790
SO2 asym C-C 1325 1237
1326 1255 1239 1230
CF3 asym SO3 asym
1200 1155
1196 1159
SO2 sym CF3 sym CF2 SO3- asym
1142
1145sh
COC asymm
1101 1069 1060
1098 1065
975
975
CF2 SO3- sym str SO3- sym str S-OH str C-O-C sym str S-OH C-S str CF3 def
a Prepared from the “as-received” solution. b Prepared from dilute aqueous solution.
cm-1 (not shown) was also observed from the OH stretching mode of the sulfonic acid. Exposure of the “as-received” PES to the atmosphere results in rapid dissociation of the sulfonic acid group as a result of the adsorption of water vapor. As this process occurs, spectrum (a) of Figure 2 changes to spectrum (b). The absorptions due to undissociated -SO3H at 1407 and 913 cm-1 decrease in intensity, and that of the symmetric -SO3- stretch of the RSO3species emerges at 1060 cm-1. Changes also take place within the 1250-1100 cm-1 region where the asymmetric stretching mode(s) of -SO3- are known to absorb.28,30,38,44,45 The absorption of water by the film during hydrolysis can be observed in the new band at ∼1712 cm-1 due to the presence of hydronium H2n+1On+ ions.3,5 The loss of the features at 1407 and 913 cm-1 (Figure 4) upon the increase in the water content of the sample
Figure 4. The IR spectrum of PES below 1500 cm-1. (a) “As-received” PES, (b and c) during solvent evaporation, (d) at 3% RH, (e) at 11% RH, and (f) at 20% RH. The background to all spectra is the bare prism. The spectra are offset on the absorbance scale for clarity.
(Figure 3) confirm that they are due to the SO2 asymmetric and S-OH stretching modes, respectively, of the undissociated RSO2OH moiety. It is reported that liquid trifluoromethanesulfonic acid has an IR absorption at 1134 cm-1 which has been assigned to the SO-H bending mode.46 Therefore, the absorption lost at 1125 cm-1 when PES is hydrated is taken as being due to the SO-H bending mode. At the same time as the 1125 cm-1 absorption decreases, another at 1134 cm-1 increases, which appears to be matched by another such increase at ∼1289 cm-1, indicating that this pair of absorptions is associated with the -SO3- asymmetric mode. Gruger et al.4 have similarly reported for Nafion a -SO3- asymmetric stretching mode at 1135 cm-1 and a mixed CF/SO3- stretching mode at 1204-1235 cm-1, while Falk assigns an absorption at 1210 cm-1 to the degenerate asymmetric stretching mode of the sulfonate group in the sodium-exchanged form of Nafion.30 There is an absorption at 1005 cm-1 in spectrum (a) of Figure 2 that is absent after evaporation of the solvent (spectrum (b)), and from the similar behavior of this peak to that of those at 913 and 1407 cm-1, it is assigned to the S-OH stretching mode, which has been shown in DFT calculations of trifluorosulfonic acid to occur in this region.38 In their IR work on the dehydration of Nafion membranes, Buzzoni et al.3 reported a shoulder at ∼1000 cm-1 that decreases in intensity as a completely dehydrated Nafion 117 film is rehydrated. However, they assigned this absorption to a combination mode that includes the COC symmetric stretching mode. Earlier work by Pacansky et al.47 with perfluoronated ethers showed that such combination modes occur at ∼1080 cm-1 in both the calculated (scaled) and matrix isolation spectra of such compounds. This assignment is discussed further below. The absorption at 971 cm-1 in Figure 2a and b occurs at the same wavenumber as the absorption
10538 J. Phys. Chem. B, Vol. 112, No. 34, 2008
Warren and McQuillan
Figure 5. The IR spectra of (a) 2 × 10-2 mol L-1 PES solution and (b) PES thin film prepared from a 5 × 10-4 mol L-1 aqueous solution. The background to (a) is a bare prism, and (b) is water on the bare prism.
widely attributed in Nafion IR and Raman spectra as arising from the symmetric C-O-C stretching mode.4,5,19,27,29,30,48 By a process of elimination, the remaining absorptions at 1324, 1192, 1125, and 1098 cm-1 can be assigned to various CF stretch modes. More precise assignments of these CF modes are discussed below after consideration of the calculated IR spectra of PES and its anion. Figures 3 and 4 show different regions of the IR spectrum of the PES at different states of hydration. There have been many studies of the IR spectrum of water associated with Nafion membranes,3-7,24,28,30,49 and the spectral changes observed here (Figures 3 and 4) match the literature data closely. Figure 3 gives the spectral region where OH stretching and bending modes are observed. The as-received PES spectrum has a broad peak at ∼2882 cm-1 that is assigned to the SO-H stretch.3 Initial evaporation of the solvent in air leads to absorption of water (Figure 3b and c) and ionization of the -SO3OH group, as evidenced by the loss of the absorption at 2882 cm-1 and the appearance of absorptions in the region of ∼3400-3000 cm-1 (water OH stretching modes) as well as that at 1712 cm-1. Upon increasing the amount of water in the ambient, the spectrum changes further until it finally resembles that of pure trifluorosulfonic acid3 (Figure 3d-f). The region associated with the bending mode of water shows an absorption at 1712 cm-1 with a shoulder at 1640 cm-1 that increases in intensity with increasing water content. Buzzoni et al.3 used a model of solvated H5O2+ ions to assign peaks in the 1800-1650 cm-1 region in the Nafion spectrum to OH groups in, and closely associated with, the H5O2+ ion. Absorptions below this wavenumber were assigned to water in a solvation sphere around the H5O2+ ion. The spectral features seen in Figure 3 as water is adsorbed are very similar to those observed by Buzzoni et al.3 when dehydrated Nafion self-supporting films were slowly rehydrated in a gas flow. As seen in Figure 4, the PES components of the IR spectrum change little during the same sequence of events, giving the spectra in Figure 3. After the initial changes as the solvent evaporates (Figure 4a and b) and the humidity of the ambient increases (Figure 4c-f), the spectrum is essentially unchanged from that in Figure 2b discussed above. (b) IR Spectrum of a PES Thin Film Deposited from Aqueous Solution. The spectrum of a 2 × 10-2 g L-1 aqueous solution of PES is shown in Figure 5a, and absorptions due to PES are summarized in Table 1. As would be expected in an aqueous solution spectrum, the absorptions are less clearly
Figure 6. The IR spectrum of a PES film in the OH stretching region at various humidities; (a) 96, (b) 80, (c) 50, (d) 30, and (e) 7% RH. The background to all spectra is the bare prism.
defined than those in Figure 2a and b with peaks at 1326, 1237, 1201, 1155, 1142, 1101, 1069, 1060, 985, and 975 cm-1. In addition, the large asymmetric absorption at 1237 cm-1 indicates an underlying absorption in the region 1250-1280 cm-1. In Figure 5, spectrum (b) is that of a film formed by drying an aqueous PES solution under a water pump vacuum. The spectrum has a broad absorption in the 1200-1280 cm-1 region with at least two shoulders. This feature is assigned to the asymmetric stretching mode of the -SO3- group.3,4,25,28,30,43 The wavenumber of this band in the spectrum of other fluorinated sulfonates is known to be dependent upon the cation species and the geometry of the complex.33,44 The bandwidth of the feature seen here may be due to a variation in the environment/ microstructure of the PES anions within the film.27 There is little evidence of bulk water or hydronium ions (∼1700 cm-1 region, not shown), but it is presumed that the hydronium ions are present as hydrated species of the form H2n+1On+ (n > 2), as has been proposed to exist in Nafion thin films in similar states of hydration.7,16,18,50 When the film was exposed to decreasing relative humidities in the range from 96 to 7%, the spectrum underwent significant changes in the region associated with the OH stretching modes of water. Figure 6 shows “absolute” spectra recorded at five RHs from 96 to 7%, and there is a striking spectral resemblance, both overall and at a given humidity, to spectra for dehydration stages of a Nafion membrane reported by Laporta et al.5 At RHs of 96 and 80%, broad bands typical of bulk water can be observed at ∼3470 and 3241 cm-1. At RHs of 50 and 30%, sharper, weaker bands appear at 3503 and 3200 cm-1, with a weak shoulder at 3083 cm-1. Laporta et al.5 refer to similar
PES, A Model Nafion Side-Chain Molecule
Figure 7. The difference spectra of the PES thin film at different humidities; (a) 80, (b) 50, (c) 30, and (d) 7% RH. All spectra have the film at 96% RH as the background.
J. Phys. Chem. B, Vol. 112, No. 34, 2008 10539
Figure 9. The optimized structure of the PES- anion associated with H3O+ .
Figure 8. The optimized structure of the gas-phase PES molecule.
absorptions as unspecified overtones, but it would seem to be more realistic to assign them to OH stretching modes of water closely adjacent to PES sulfonate groups in a more localized environment than that in Figure 6a and b. Spectrum (e) at 7% RH shows three distinct OH stretch absorptions at 3083, 3232, and 3474 cm-1 that have been assigned3 as arising from the hydronium ion, water that is hydrogen bonded to the hydronium ion in an inner hydration sphere, and water that is loosely hydrogen bonded to the hydronium ion in an outer hydration sphere, respectively. As previously described, the “absolute” spectrum of PES shows little variation when the humidity is changed. However, difference spectra often reveal details not readily seen in “absolute” spectra and show a greater sensitivity to small spectral changes. Thus, Figure 7 shows the difference spectra of the PES vibrational modes as the RH is varied from 96 to 7%. The difference spectra are all referenced to the spectrum of the PES film at 96% RH. As the RH is changed from 96 to 80% (Figure 7a) prominent absorption losses occur at 1279, 1134, 1059, and 975 cm-1 and appear to be closely linked with corresponding absorption increases at 1241, 1230, 1169, 1159, 1105, 1068, and 987 cm-1. The negative absorptions match well with the -SO3- asymmetric stretching modes (1279 and 1134 cm-1), the symmetric stretching mode (1060 cm-1), and the COC symmetric stretching mode (975 cm-1). The splitting in the nondegenerate -SO3- asymmetric stretching modes is ∼145 cm-1, similar to values seen by Bernson et al.51 when reporting trifloromethanesulfonate coordinated to a variety of rare earth metal ions as well as values from DFT calculations discussed below. In addition, Heitner-Wirguin,25 McComb,35 and Boumizane et al.44 all reported an apparent shift in absorption bands due to -SO3- asymmetric stretching modes when different
Figure 10. The optimized structure of PES- anion associated with the Na+ ion.
TABLE 2: Summary of the Bond Lengths in the Energy-Optimized Structures bond C1-F13 C1-F14 C2-F11 C2-F12 C4-F9 C4-F10 C5-F6 C5-F7 C5-F8 C1-C2 C4-C5 C2-O C4-O S15-O16 S15-O17 S15-O18 C1-S15 Xa-O17 Xa-O18 Xa-F7 a
bond length /pm -
PES
PES
PES-H3O+
PES-Na+
134 134 134 134 134 134 133 134 133 156 156 156 156 144 145 163 192
136 136 134 137 135 136 135 133 134 156 156 139 136 147 147 148 192
134 134 135 135 135 135 134 134 133 156 156 138 138 144 146 159 190 231 101 348
135 135 134 135 135 134 132 137 133 156 156 140 137 145 150 150 190 227 227 241
Where X ) H3O+ or Na+.
metal ions were complexed to different sulfonate compounds. Blanchard and Nuzzo18 reported changes in Nafion difference spectra of films loaded with different counterions. Their data18 show spectral changes occurring in the region of 1300-1200 cm-1 and at ∼1150 cm-1, and they assign the former to the
10540 J. Phys. Chem. B, Vol. 112, No. 34, 2008 TABLE 3: Calculated Infrared Spectrum of PES and Assignments absorption wavenumber/cm-1 637 719 772 802 817 952 1083 1095 1121 1127 1155 1168 1177 1186 1202 1212 1224 1296 1373 1390
intensity/km mol-1
assignment
6.55 37.51 145.82 61.27 8.56 85.58 77.91 223.52 127.25 383.27 8.83 55.03 128.36 102.29 869.07 224.41 347.47 169.07 22.78 263.30
νsCF2 νsCF2 S-OH str νsCOC + νsCF2 νsCOC + νsCF2 νC-S νsCF2 + νasCOC νsCF2 + νasCOC SO-H bend SO-H bend + νC-O νasCF2 νsSO2 νasCF2 νasCF3 + νasCF2 νasCF2 CF2 + νsCOC νasCF3 νC1-C2 νC3-C4 νasSO2
TABLE 4: Calculated Infrared Spectrum of PES-/Na+ and Assignments absorption wavenumber/cm-1 620 632 635 714 789 816 929 1000 1054 1084 1107 1133 1145 1156 1162 1187 1210 1246 1284 1296 1372
intensity/km mol-1
assignment
8.26 76.94 8.59 43.60 12.67 11.71 128.15 25.67 130.11 452.18 101.83 21.88 126.18 27.17 332.90 862.62 243.83 387.85 323.24 177.00 17.67
νsCF2 νsCF2 νsCF3 νsCF3 νsCOC + νsCF2 νsCOC νC-S + νsSO3 νC-S + νsSO3 νasCOC + νasCF3 νasCOC + νasCF3 νasSO3 νsCF2 νasCF2 + νasCF3 νasCF2 + νasCF3 νasCF2 + νasCOC νasCF2 + νasCF3 νsCF2 + νasCOC νasCF3 νasSO3 νC-C (C1-C2) νC-C (C3-C4)
-SO3 asymmetric stretching mode and the latter as variations in the -CF2 stretching modes induced by structural rearrangements. In a similar manner, Korzeniewski et al. 49 reported spectral changes in the Nafion IR difference spectrum during hydration at 1275-1249 and 1163 cm-1. They also assigned the former to the -SO3- asymmetric stretching mode and the latter to -CF2 stretching vibrations but admitted that it was difficult to explain the reason for the change to the -CF2 absorption. The PES spectral changes resulting from humidity variations as described above are similar to those observed for Nafion,3,4 which shows absorptions at 1410 and 910 cm-1 that start to disappear as the water content of the membrane increases. Likewise, Nafion and PES show comparable changes in the region of 1000-1100 cm-1 where a new absorption upon hydration is assigned to the -SO3- symmetric stretching mode.3,4,25,28,30 As seen above for the PES spectrum, changes within the 1100-1300 cm-1 region of the Nafion spectrum are
Warren and McQuillan TABLE 5: Calculated Infrared Spectrum of PES-/H3O+ and Assignments absorption wavenumber/cm-1 632 635 720 793 816 824 844 967 1081 1099 1134 1141 1159 1167 1184 1204 1212 1215 1299 1322 1373 1421
intensity/km mol-1
assignment
12.64 18.58 57.19 42.99 3.28 48.08 282.73 74.85 75.77 104.93 551.05 22.67 43.29 170.83 277.16 612.08 304.16 363.32 185.88 165.74 9.19 188.66
νsCF2 νsCF2 νsCF2 + νsCOC νsCF2 + νsCOC νsCOC νsS-OH + δOH νsS-OH + δOH νsCS νsCF3 + νsCF2 νsCF2 νasCOC νsSO2 + others νsSO2 + others νasCF2 νasCF2 + νasCF3 νasCF3 νasCF3 νasCF3 + νsCF2 + νsCOC νC-C (C1-C2) νasSO2 + δOH νC-C (C3-C4) δSO-H
unclear, and bands are difficult to assign unless difference spectra are examined. For example, studies such as those carried out by Falk30 using hydrated membranes exchanged with different counterions and Gruger et al.4 using membranes over a range of hydration states reported little variation in the “absolute” spectrum of Nafion. However, others18,49 have observed significant changes in the Nafion (H+-exchanged) difference spectrum over a similar spread of hydration states. Gruger et al.4 also showed increased absorption at 1249 cm-1, near to the absorption at 1225 cm-1 present in the dry film, as well as shifts, with hydration changes, in the wavenumber of the symmetric -SO3- stretch at 1060 cm-1 and COC symmetric stretch at 970 cm-1. Cable et al.29 assigned an absorption at 965 cm-1 as being due to the COC group nearest the sulfonic acid group in the Nafion side chain, while an absorption at higher wavenumber (980 cm-1) was assigned to the COC group nearest the backbone. They also observed very similar spectral shifts to those seen here and proposed that the COC group nearest the sulfonic acid group is involved in the hydrophilic domains of the membrane. The spectral shifts with variations of the degree of hydration in the above literature are very similar to those reported in this work, with the exception that in the present work, PES shows no loss peak at 1249 cm-1. This would suggest that the 1249 cm-1 absorption observed by Korzeniewski et al.49 involves the COC group present in the Nafion side chain furthest from the sulfonate group (Figure 1) but absent from the PES molecule. Spectral affects such as those seen in Figure 7 and reported by Korzeniewski et al.49 and Blanchard and Nuzzo18 have the appearance of absorption peak first derivative spectra and appear to correspond to significant wavenumber shifts resulting from humidity changes. Difference spectra are much more sensitive to spectral band shifts than absolute spectra, and peak fitting to simulate the spectra of Nafion exchanged with various metal ions has shown that the actual shifts are much smaller than they may appear to be.35 This data indicates the value of using difference spectra when looking at small spectral changes in absolute spectra. However, care should be taken when using difference spectra to assess the magnitude of the actual shift in absorption wavenumber.
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J. Phys. Chem. B, Vol. 112, No. 34, 2008 10541
Figure 11. The calculated spectra of (a) the PES molecule, (b) PES-/H3O+, (c) the PES- anion, and (d) PES-/Na+.
The observed peak wavenumbers for PES “as received”, PES film “solvent evaporated”, PES aqueous solution, and PES film “aqueous” are summarized in Table 1. (c) Calculated Energy Minimum Structures of the PES Molecule and PES- Anion. Given the rather complicated PES spectra reported above, it was felt that DFT calculations should be used to help with the assignments of observed spectral bands. To this end, the energy minimum (fully optimized) structures were calculated for the undissociated PES molecule (Figure 8) and the PES anion as well as those for two different ion pairs, namely, the PES anion combined with (a) H3O+ (Figure 9) or (b) Na+ (Figure 10). A summary of the calculated bond lengths is given in Table 2. The calculated absorptions for the PES molecule and PES-H3O+ and PES-Na+ species as well as assignments indicating the major contributing local mode are shown in Tables 3-5. The minimum-energy model of the undissociated PES molecule used for the calculation of vibrational modes is shown in Figure 8. From Table 2, there is evidence in the structure that intramolecular hydrogen bonding occurs between the OH group and one of the fluorine atoms in the -CF3 group (H · · · F distance of 279 pm and longer CF bond length) as well as the oxygen in the ether group (H · · · O distance of 272 pm) and one oxygen attached to the sulfur (H · · · O distance of 249 pm and longer S–O bond). The minimum-energy structure for the PES- anion is not shown. However, the -SO3- group is often described as having C3V symmetry,34,36,37 and the calculated structure confirms this as a suitable description for this group in PES with calculated S-O bond lengths of 147.3 (×2) and 147.6 pm (×1); all OSO bond angles are 115°. When a hydronium ion was included with the PES- anion and the structure reoptimized, H+ was seen to be associated with both the sulfonate group (H-O distance is 100 pm) and the water molecule (H to O (water) distance is 165 pm). The latter distance is the same as the bond length of the OH bond in the undissociated water molecule (see Table 2), indicating a strong hydrogen bonding interaction with the H2O molecule. When the H3O+ is replaced by a Na+ ion, the optimized structure shows the Na+ to be located in a similar position to
the water molecule (Figure 10) in the PES-H3O+ structure. There is a change in the S-O bond lengths/strengths (Table 2), with one bond directly opposite the Na+ ion shortening to 145.1 pm and the remaining two bonds lengthening to 149.6 and 149.9 pm. Furthermore, the three Na+ · · · O distances of 403.7, 227.0, and 226.8 pm indicate that the Na+ is “bonded” to two of the sulfonate oxygen atoms, resulting in a loss of symmetry. These observations are in line with those of Gejji et al.,33 who calculated the structure of the lithium triflate ion pair and found bidentate coordination to be energetically favorable over the monodentate form. (d) Calculated Spectrum of the PES Molecule and PESAnion. The calculated spectrum of the undissociated PES molecule is shown in Figure 11a. The spectrum shows a good resemblance to the “as-received” PES spectrum in Figure 4b. The wavenumber of the calculated absorptions and their assignments based on a GaussView visualization of the vibrational modes are shown in Table 3. It should be recognized here and throughout the discussion of the calculated spectra that in a system such as that studied, there is much mixing of the vibrational modes due to the similar masses and symmetry of the various components of the molecule.38,39 Thus, the listed assignments are of the dominant vibrational mode(s) that contribute to any given absorption. The calculated spectrum of the PES molecule shows a good visual correlation with the experimental spectrum. In addition, it shows a good match with the DFT-calculated spectra of trifluoromethanesulfonic acid.38 For these calculations, Fernandez et al.38 derived scaling factors from experimental data for a series of molecules (CF3SO2X, X ) F, OH, NH2, CH3) and pointed out that larger variations between calculations and experimental values for vibrational modes occurred when comparing gas-phase calculations with condensed-phase experimental data due to the intermolecular attractions that occur in the latter. The calculated spectra of PES- ions in different environments (PES-, PES-/Na+, and PES-/H3O+) are shown in Figure 11b-d. The mode assignments of the calculated frequencies are based on the GaussView visualization as shown in Tables 4 and 5 for the PES-/Na+ and PES-/H3O+ systems. The loss of -SO3- C3V symmetry upon coordination with the sodium
10542 J. Phys. Chem. B, Vol. 112, No. 34, 2008 ion results in a large split (∼180 cm-1 in Table 4) between the degenerate asymmetric -SO3- stretching absorptions. A similar, but slightly smaller, splitting of ∼110 cm-1 has been observed with the asymmetric stretching mode of trifloromethanesulfonate when coordinated to a variety of rare earth metal ions.51 We now return to the absorption at 1005 cm-1 referred to in part (a). As already mentioned above, Buzzoni et al.3 noticed that upon hydration, the Nafion IR spectrum lost an absorption at ∼1000 cm-1, which was replaced by another one at 971 cm-1. This absorption was assigned as a mixed mode with contributions from CF and COC stretching modes,3 and the loss of intensity upon hydration was assigned to perturbation of the COC bond by an intramolecular hydrogen bond from the hydronium ion associated with the sulfonate group. However, the DFT calculations on the PES-/ H3O+ system reported in this work indicate that the interatomic distances involved when a hydronium ion associates with the sulfonic acid group are too large to contribute to such a perturbation. It is acknowledged that the system described here does not explicitly include solvation water around the hydronium ion, and it is possible that such water would interact with the COC group. However, from the calculated PES bond lengths (Table 3), it seems likely that changes in the bond length of the COC group are the result of interactions involving counterions and the terminal CF3 group. Nafion has a similarly positioned CF3 group in its side chain, and from the work reported here, it seems that this group may play a role in the binding of counterions in Nafion membranes. When Gejji et al.39 calculated the vibrational frequencies of triflic acid, they observed that the C-S stretching mode contributed to several different normal modes. It can be seen that while it is possible to associate the C-S stretch with a single absorption in the spectrum of the PES-H3O+ complex, in the spectrum of PES-Na+ it is a major contributor to absorptions at both 929 and 1000 cm-1. Thus, it seems more likely that the absorption loss/gain at 1000/971 cm-1 is due to a change in the C-S stretching mode resulting from the conversion of an undissociated sulfonic acid to a hydrated sulfonate group, rather than the reported assignment to the symmetric stretching mode of the COC group. Conclusions The IR spectrum of PES and subsequent changes under a range of humidities indicates that PES is a good vibrational spectroscopic model for the Nafion side chain. The use of PES also shows that the -SO3- asymmetric stretching mode is not degenerate in the presence of water and results in two absorptions that are clearly seen in difference spectra. The use of difference spectra with their enhanced sensitivity to small changes in spectra offers the possibility of a tool to monitor the sensitivity of the Nafion sulfonate groups to changes in the ionic domains of the polymer. DFT vibrational mode calculations on PES show that there is a mixed contribution to certain IR absorptions from different functional group vibrations, especially the COC, CF2, and C-S groups. This mixing complicates spectral assignment based on an interpretive model centered around isolated local modes. The combined use of a model compound and DFT calculations has improved the understanding of these mixed modes, allowing a clearer interpretation of spectral changes in difference spectra. This combination provides a powerful tool in the interpretation of complex spectral changes such as those seen in the PES spectrum and hence in the ionic domains of Nafion.
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