Experimental and Theoretical Infrared Spectroscopic Study on

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Experimental and Theoretical Infrared Spectroscopic Study on Hydrated Nafion Membrane Raman K. Singh, Keiji Kunimatsu, Kenji Miyatake, and Takao Tsuneda* Fuel Cell Nanomaterials Centre, University of Yamanashi, Kofu 400-0021, Japan S Supporting Information *

ABSTRACT: A collaborative experimental and theoretical study on the dependence of the infrared (IR) spectrum of hydrated Nafion electrolyte membrane on the hydration number is investigated in great detail. Experimental time-resolved attenuated total reflection Fourier transform IR spectroscopic results show that Nafion membrane has a unique IR peak intensity dependence on the hydration number. Calculated IR spectra indicate that this unique IR peak intensity dependence is correctly reproduced not for the singly hydrated Nafion but for the doubly hydrated Nafion. This result strongly supports the relay mechanism of the proton conductance, in which protonated water clusters are relayed by the side chains through the doubly hydrated sulfonic acid groups under lowhumidity conditions.

1. INTRODUCTION Nowadays, fuel cells attract much attention as the nextgeneration energy conversion system because they become popular for fuel cell vehicles developed by major automakers. The greatest hindrance to the progress of fuel cells is its high cost. Besides the electrode catalysts usually including precious platinum, the electrolyte membranes, which are usually proton exchange membranes in fuel cell vehicles, share a significant portion for the cost of fuel cells. The most popular proton exchange membrane is the Nafion membrane developed by Du Pont (Figure 1)1 despite its relatively higher price. For proton

environmental compatibility. To circumvent these problems, various electrolyte membranes have been developed to substitute Nafion. Recently, hydrocarbon-series membranes containing benzene rings have emerged as the promising substitutes. These membranes, however, have not yet replaced Nafion due to their low proton conductivities at low relative humidity. So, the quest for root causes of the high proton conductivity of Nafion membrane at low humidity is still on. The proton conductivity of proton exchange membranes has so far been vigorously investigated both experimentally and theoretically. It is established that proton conductance in water proceeds by the Grotthuss mechanism,2 which goes through the H5O2+ complex, i.e., [H2O···H···OH2]+ complex.3,4 As another proton conductance mechanism, there is the vehicle mechanism,5 in which protonated water molecules diffuse. However, this mechanism has been disregarded because the Grotthuss mechanism is overwhelmingly advantageous due to the much lower activation energy than that of the vehicle mechanism in acidic solution.6 For the proton conductance in electrolyte membranes, many theoretical studies have been reported especially for the proton conductivity in Nafion. Most theoretical studies have used density functional theory (DFT) calculations of small single-unit membrane models in which one sulfonic acid group is hydrated to discuss the proton dissociation ability.7−12 Paddison and Elliot determined the doubly hydrated structures of Nafion, in which two sulfonic acid groups are simultaneously hydrated, and suggested that the main chain structure should be considered to discuss the proton conductivity of Nafion.13 For the proton conductance in

Figure 1. Calculated models of singly (a) and doubly hydrated (b) Nafion membrane where d12 is the distance between the sulfur atoms of two sulfonic acid groups.

exchange membranes, high proton conductivity, low gas permeability, high chemical and mechanical durability, and high environmental compatibility are required. Nafion membrane is excellent especially for high proton conductivity under low-humidity conditions. However, the disadvantages of this membrane include high cost, high gas permeability, and low © XXXX American Chemical Society

Received: May 12, 2016 Revised: August 5, 2016

A

DOI: 10.1021/acs.macromol.6b00999 Macromolecules XXXX, XXX, XXX−XXX

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frequency.51 On the basis of the DFT vibrational frequency calculation with a single-unit Nafion model, Kendrick et al.52 recently interpreted that the shift of these peaks comes from the replacement of the sulfonate site in the C3v group modes to sulfonic acid sites in the C1 group modes by the hydration. Though the high sensitivity of the peaks at 1157 and 1231 cm−1, which are assigned as C−F stretching modes, has been attributed to the effect of the −CF moieties in the side chains on the hydration of −SO3− groups, Warren and McQuillan interpreted that this comes from the mixing of the −SO3− stretching mode in the C−F stretching modes.43 As mentioned above, the IR absorption peaks of Nafion have been assigned in many studies. Note, however, that Nafion membrane is exposed to different humidity conditions in operating fuel cells. The IR spectrum analysis under the hydration/dehydration conditions is therefore required to investigate the proton conductivity of Nafion in further detail. In this study, we investigate the hydration state of Nafion membrane by comparing the time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of Nafion NRE211 membrane observed during its dehydration53 and the theoretical IR spectra of Nafion for various hydration numbers. Focusing on the unique variation of the symmetric and asymmetric stretching modes of the sulfonic acid groups in the experimental IR spectra, we explore the basis of this unique behavior by comparing with the calculated IR spectra of the state-of-the-art long-range corrected (LC) DFT54,55 that we have developed. Consequently, we make clear the reason for the high proton conductivity of Nafion at low humidity.

Nafion membrane, molecular dynamics (MD) simulations have also been performed.14−18 Choe et al.17 carried out firstprinciples MD simulations of the proton conductance in the channel of Nafion and suggested that the electro-osmotic coefficient is proportional to water uptake. Note, however, that this calculation assumes that a hydrogen bond network is always constructed in between the sulfonic acid groups to proceed the Grotthuss mechanism. This calculation cannot explain the low-humidity proton conductance because experiments have suggested that there are regions where no hydrogen bond network is constructed in between the sulfonic acid groups under low-humidity conditions.19 We recently showed that the high proton conductivity of Nafion at low humidity is attributable to the high dissociation ability of protonated water clusters.20 That is, we proposed the relay mechanism, in which the proton conductance at low humidity proceeds through the transfer of not only protons but also protonated water clusters analogous to the vehicle mechanism. In this relay mechanism, the side chains of Nafion are advantageous to construct the doubly hydrated sulfonic acid groups (Figure 1). This may significantly contribute to the high proton conductivity of Nafion at low humidity. Many studies have been carried out on the variation of the vibrational states of the Nafion membrane during its hydration using infrared (IR)21−36 and near-IR37 transmission, attenuated total reflection IR (ATR-IR),38−45 and IR reflection− adsorption spectroscopy (IRRAS).46,47 These studies mainly focus on the states of water molecules in the channels of the Nafion membrane because the hydration dominantly changes the vibrational spectrum peaks of water molecules. Consequently, these studies have succeeded to reveal the vibrational states, hydrogen bond network structures, and distributions of water molecules in the channels of Nafion. Note, however, that water molecules in the vicinity of the sulfonic acid groups are supposed to make a major contribution to the proton conductance in Nafion due to the strong bonds of protonated water clusters to the sulfonic acid groups,20 while the IR analyses of hydrated ionomer membrane suggest that weakly hydrogen-bonded water molecules form ion channels to cause an increase in the proton conductivity.25 Revealing the hydration effect on the vibrational states of the sulfonic acid groups is therefore useful to make clear the cause for the high proton conductance of Nafion. The IR spectrum peaks of the sulfonic acid groups have also been thoroughly investigated. For the sulfonic acid groups of dry Nafion, traditional IR band analyses have assigned main peaks around 1057, 1320, 1435, and 3200 cm−1 as −SO3− symmetric stretching, −SO3− asymmetric + SO symmetric stretching, SO asymmetric stretching, and OH stretching modes, respectively.48 Chu et al.46 and Greso et al.49 suggested that C−O−C vibrational mode also contributes to the peak at 1057 cm−1. Okamoto50 performed the vibrational frequency calculations of the side chain of Nafion in vacuo and confirmed the contribution of the C−O−C vibrational mode. Combining the DFT vibrational frequency calculation using the single-unit model of dehydrated Nafion with IR spectrum analysis, Warren and McQuillan proposed that the peaks at 972 and 1057 cm−1 are the group modes of the C−O−C bending and −SO3− stretching modes.43 The IR spectrum analysis of hydrated Nafion membrane has recently been performed to explore its proton conductivity. It is known that the hydration increases the peak intensities around 1157 and 1231 cm−1, decreases the peak intensity around 1305 cm−1, and shifts the peak around 1057 cm−1 to slightly lower

2. EXPERIMENTAL AND THEORETICAL METHODS The ATR-FTIR observation of Nafion NRE211 membrane has been performed using the ATR cell, which is originally designed to carry out the in situ measurement of the IR spectrum variation at the Pt/Nafion interface during the hydration/dehydration cycle (see ref 53 for the details of the cell and the method). In the ATR-FTIR measurements, a Digilab FTS6000 spectrometer equipped with a broadband mercury cadmium telluride detector cooled by liquid nitrogen was used at room temperature. The ATR cell was designed based on a carbon separator for fuel cells with interdigitated-type gas flow fields. A PTFE mesh and NRE211 membrane were placed on the gas flow fields, and a Ge ATR prism of 20 mm × 25 mm in a hemicylindrical shape was placed on the membrane and fixed. The prism gives an angle of incidence of 70°, for which we have ca. 0.47 μm as the penetration depth of infrared radiation at 1000 cm−1 in the ATR measurements. The membrane hydration/dehydration was controlled by switching dry N2 gas to wet one through the gas flow fields. The relative humidity of the gas at the inlet to the cell was monitored by a humidity sensor. It was confirmed that the humidity changed between almost 0 and 100% in a few minutes upon membrane hydration by the present apparatus. The ATR measurements were conducted using unpolarized IR radiation with the resolution of 4 cm−1. The spectrum results are displayed in absorbance units, defined as −log(I/I0), where I and I0 indicate the spectral intensities in the sample and background states, respectively. The curve-fitting analysis of the ATR-FTIR spectra was also performed using GRAMS/AI Version 8 software to show the detailed behavior of each band component. All theoretical calculations have been done by the generalized Kohn−Sham method56 using long-range corrected54,55 Becke 1988 exchange57 + Lee−Yang−Parr correlation58 (LC-BLYP) functional with the only parameter μ = 0.47, which is the optimum value for ground state calculations, in conjunction with cc-pVDZ basis set59,60 in the gas phase without imposing any symmetry restriction. In the geometry optimizations of the hydrated Nafion structures, we constructed the initial structures to maximize the number of the hydrogen bonds. In order to investigate high-humidity conditions, the B


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mode.53 The presence of this peak is found by calculating the difference of the spectra between the dehydrated and fully hydrated states. This peak is, however, unclear even in the difference of the spectra. The curve-fitting analysis is required to figure out the peaks of the spectra. Figure 3 displays the curve-fitting analysis of the ATR-FTIR spectrum of Nafion NRE211 membrane after the 10 min

solvent effects employing the conductor-like polarizable continuum model (CPCM)61 and the self-consistent reaction field (SCRF) method were also examined. The doubly hydrated structures have been optimized with fixing only the distance between the sulfur atoms of two sulfonic acid groups at the averaged distance reported in a classical molecular dynamics simulation study of hydrated Nafion as shown in Figure 1b.15 For the doubly hydrated structures, we have examined two initial structures for the geometry optimizations: One initial structure has a proton attached to one of the sulfonic acid groups while another one has a proton attached to the hydration water cluster. In the list of the IR peaks, we extracted only the peaks providing higher peak intensities than 3.0 × 10−38 esu2 cm2. The Gaussian 09 suite of program62 has been used to perform all the LCBLYP calculations. All the optimized structures have been checked to give positive and real frequencies. The vibrational modes contributing to IR spectra and their assignments were analyzed using GaussView 5.0.8.63 The IR spectra are plotted with 4 cm−1 IR peak full width at half-height.

3. RESULTS AND DISCUSSION 3.1. Experimental ATR-FTIR Spectra of Nafion under Dehydration. Let us first discuss the dependence of the experimental FTIR spectra of Nafion on humidity. As mentioned in section 1, the spectrum peaks at 1157 and 1231 cm−1 have been assigned to the symmetric and asymmetric C−F stretching modes mixing with the asymmetric −SO3− stretching mode, respectively, for Nafion at full hydration, while a satellite band at 1057 cm−1 assigned to the symmetric −SO3− stretching mode of the sulfonic acid groups attached to the pendant side chain of the membrane. We therefore focus on these peaks to explore the proton conductivity of Nafion at low humidity. Figure 2 illustrates the ATR-FTIR spectra of Nafion NRE211 membrane during dehydration. As shown in the figure, the

Figure 3. Fitted peaks of the ATR-FTIR spectrum of NRE211 membrane observed after 10 min dehydration. Conditions for the analysis are given in the text. All peaks are fitted by the combination of 80% Gaussian and 20% Lorenzian functions. For Pk3 and Pk5, the line widths are fixed at 53.6 and 58.6 cm−1, respectively, which are the optimum values for the full hydration, with no restriction for their peak positions.

dehydration for the 1000−1400 cm−1 region. We found that the spectrum is decomposed into seven peaks, Pk1 through Pk7, as shown in the figure. The figure shows that Pk2, Pk4, Pk5, and Pk7 provide relatively large variations during dehydration in contrast to Pk1, Pk3, and Pk6 giving small variations. As shown later, the present most reliable theoretical result assign these peaks as follows: Pk1 to the C−C stretching mode, Pk2 to the −SO3− asymmetric stretching + CF2 and CF3 symmetric stretching mode, Pk3 to the C−F stretching + C− O−C symmetric stretching mode, Pk4 to the −SO 3 − asymmetric stretching + CF2 and CF3 symmetric stretching mode, Pk5 to the CF3 symmetric stretching + C−O−C twisting mode, Pk6 to the C−C−C scissoring mode, and Pk7 to the −SO3− symmetric stretching + C−O−C symmetric stretching mode. These assignments are supposed to be consistent with the peak intensity behaviors upon dehydration because the peaks undergoing large variations mostly include the −SO3− stretching modes and the peaks undergoing small variations are assigned to the vibrations of the main and side chains. Let us explore the peak intensity variations of the −SO3− stretching modes of the sulfonic acid groups. Figure 4 illustrates the variations of the asymmetric and symmetric −SO3− stretching modes, νas(SO) and νs(SO), respectively, which are determined using the above-mentioned curve-fitting analysis during the dehydration of Nafion NRE211 membrane. The figure clearly shows the increase of the νas(SO) peak intensity during dehydration, whereas the νs(SO) peak slightly shifts from 1058 to 1063 cm−1 with keeping the peak intensity almost constant upon dehydration. This is contrasting to the corresponding peak intensity variation of sulfonated poly(arylene ether sulfone) (SPE) aromatic hydrocarbon membrane,64 which shows that both the νas(SO) and νs(SO) peak intensities decrease during dehydration (Figure S1). Since Nafion membrane is known to be superior to SPE membrane for the proton conductivity at low humidity, this suggests that

Figure 2. ATR-FTIR spectra of Nafion NRE211 membrane for hydrated state and dehydrated stated after 10 min dehydration by dry N2 gas and spectral difference between the dehydrated and hydrated states. Note that an expanded scale is applied for the spectral difference.

intensities of the peaks at 1055 and 1210 cm−1 apparently increase upon dehydration. The largest peak intensity difference is given at 1206 cm−1, which has been assigned to the asymmetric C−F stretching vibrational modes mixing with the asymmetric −SO3− stretching mode. This peak increases its intensity upon dehydration. The bipolar shaped difference is also found in the 1055−1066 cm−1 region of the IR spectra. This is interpreted as the shift of the symmetric −SO3− stretching mode of the sulfonic acid groups upon dehydration. Note that there is a peak overlapping with the peaks of the C− F stretching modes in the region of 1250−1350 cm−1. This peak has been assigned as an asymmetric −SO3− stretching C


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an important peak at 947 cm−1 corresponding to the symmetric stretching of SO3− group as shown in Table 2. For Nafion with one hydration water molecule, Table 2 supports the abovementioned assignment that the peak around 1057 cm−1 comes from the sum of the S−O−H and C−O−C bending modes, which is the group mode of the SO3− and C−O−C vibrational modes. We expect that the theoretical IR peak assignments may be available to interpret the experimental IR spectra of Nafion membrane at low humidity in more detail. For discussing the humidity dependence of the IR spectrum of Nafion membrane, the peak energies of Nafion are compared for the hydration with one and ten water molecules with the peak assignments in Table 2. The solvent effect was taken into account in the case of ten hydration water molecules. As shown in the table, the peak energies of SO3− asymmetric stretching are significantly affected by the increase of hydration water molecules as expected. For the hydration with ten water molecules, the proton dissociation from the sulfonic acid group leads to the vibration mixing of the SO3− asymmetric stretching with the CF2 and CF3 asymmetric stretching modes and eliminates the peaks corresponding to the S−O−H bending mode. Naturally, the hydration increases the peak intensities of protonated water molecules. As a result, one peak of the SO3− asymmetric stretching modes is buried in the peak of H3O+ outof-plane bending and C−C stretching modes. We should also notice that the hydration of only one water molecule drastically changes the IR peak energies of Nafion. As mentioned above, the hydration of one water molecule increases the peak energy of the SO3− symmetric stretching from 893 to 947 cm−1. Since IR peak intensities are proportional to the response of dipole moment to the corresponding nuclear vibration,62 IR peak assignments based on the fingerprint method are not so reliable in the cases that the positions of protons significantly change as seen in the hydration of Nafion membrane. 3.3. Dependence of IR Spectrum of Hydrated Nafion Membrane on Hydration Number. Based on the high reliability of the present calculation method, let us investigate the IR peak intensities of hydrated sulfonic acid group in Nafion membrane for various hydration number to determine if the present calculations correctly reproduce the humidity dependence of Nafion. Figure 5 illustrates the calculated IR spectra of singly hydrated Nafion membrane for three hydration numbers: 4, 6, and 8. As shown in the figure, the peak intensity of the SO3− symmetric stretching mode for

Figure 4. Peak intensity variations corresponding to the (A) asymmetric and (B) symmetric −SO3− stretching modes, νas(SO) and νs(SO), respectively, of Nafion NRE211 membrane. The numbers attaching the peaks are dehydration time in min. The peaks are determined by the curve-fitting analysis, which is the same as that of Figure 2.

the −SO3− stretching peak intensity variations may be related to the difference in the proton conductivities. 3.2. Theoretical IR Peak Energies and Peak Assignments of Dry and Singly Hydrated Nafion Membrane. Before investigating the hydrations of Nafion membrane, we compare the calculated and traditional IR spectrum peak energies of dry Nafion to assess the reliability of the present calculations. Table 1 displays the calculated peak energies and corresponding peak assignments of nonhydrated Nafion model with those of the experimental IR spectrum. The table intelligibly shows that the computed IR peak energies give a close agreement with the band regions of the experimental IR spectrum. This result justifies the optimized structures of Nafion model and therefore indicates that the calculation method and the membrane model in the present calculations are appropriate to investigate the features of the membranes. On the basis of the calculated peak energies, we suggest theoretical assignments for the IR peaks, which are different from traditional assignments. On the basis of the very accurate calculated peak energies, we consider that these theoretical peak assignments are more reliable than traditional ones, which were determined by the fingerprint method. In the table, we also found that no significant peaks appear around 972−982 cm−1 in calculated peaks, and a peak is given at 893 cm−1 in place of these peaks. As Kendrick et al.52 recently suggested, we also assume that this is due to the inclusion of the sulfonic acid group hydrated with one water molecule because the calculated IR spectrum of Nafion with one hydration water molecule gives

Table 1. Calculated and Traditional Peak Energies and Peak Assignments for the IR Spectrum of Dehydrated Nafion Membranea exptl band regionb (cm−1)

1305 1231

O−H stretching (−SO3H group) SO3− asymmetric stretching SO3− asymmetric stretching + SO symmetric stretching CF3 asymmetric stretching CF2 asymmetric stretching

1157

CF2 asymmetric stretching

1288, 1310 1265, 1269 1212, 1225 1194

1057 972, 982

SO3− symmetric stretching C−O−C symmetric stretching

1134 1036 893

3200 1435 1320

a

calcd peak energy (cm−1)

traditional exptl assignt for dry Nafionb

theor assignt for dehydrated Nafion

3791 1460 1325, 1326,1355, 1359

O−H stretching (−SO3H group) SO3− asymmetric stretching+ C−C stretching C−C symmetric stretching + CF3 asymmetric stretching CF2 and CF3 asymmetric stretching C−O−C asymmetric stretching C−C−C symmetric stretching SO3− asymmetric stretching + C−O−C symmetric stretching S−O−H bending C−O−C bending SO3− symmetric stretching

Theoretical and traditional assignments are inconsistent for the italicized assignments. bReference 48. D


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Table 2. Theoretical Assignments of the Main IR Peaks of Singly Hydrated Nafion Membrane with One Hydration Water Molecule in Vacuo and Ten Hydration Water Molecules in Aquo (CPCM Solvent Model) one hydration water molecule in vacuo peak energy (cm−1)

ten hydration water molecules in aquo peak energy (cm−1)

theor assignt

2793, 3867

O−H stretching (−SO3H group)

1502

O−H stretching + SO3− asymmetric stretching (−SO3H group) + H2O scissoring C−C stretching SO3− asymmetric stretching + O−H stretching (−SO3H group) CF2 and CF3 asymmetric stretching

1417, 1432 1393 1220, 1245, 1262, 1264, 1277, 1307, 1318, 1324, 1350, 1361, 1381 1186 1068 1036 947

SO3− asymmetric stretching + C−O−C symmetric stretching S−O−H bending C−O−C bending (side chain) SO3− symmetric stretching

1691, 1760, 1904, 2592, ... 1447, 1449 1317, 1328 1289, 1294, 1298 1247, 1253, 1271, 1284 1210, 1211, 1234 1194 1052 1004, 1047

theor assignt H2O bending + O−H stretching (H2O) H3O+ out-of-plane bending + C−C stretching CF3 asymmetric stretching SO3− asymmetric stretching + CF2 asymmetric stretching CF2 and CF3 asymmetric stretching SO3− asymmetric stretching + H2O hindered rotation + CF2 and CF3 asymmetric stretching C−C stretching (side chain) H2O hindered rotation SO3− symmetric stretching + C−S stretching

Figure 5. Calculated IR spectra of singly hydrated Nafion for various hydration number.

1087−1095 cm−1 is kept almost constant for the hydration similarly to the experimental IR peak for 1058−1063 cm−1. However, the figure also shows that contrary to the experimental result, the peak intensities of the SO 3 − asymmetric stretching mode for 1215−1225 cm−1 and the SO3− symmetric stretching mode for 1008−1014 cm−1 go down as the hydration number decreases. This indicates that the hydration model of the single Nafion unit is not appropriate to discuss the hydration of Nafion membrane. We therefore examined the IR spectrum calculations of doubly hydrated Nafion membrane based on our previous argument that the low-humidity proton conductance of Nafion proceeds through the relay mechanism,20 in which protonated water clusters are transferred by the doubly hydrated sulfonic acid groups. Figure 6 illustrates the calculated IR spectra of the doubly hydrated Nafion membrane model where the geometries are optimized using the initial structures containing the proton attached to one of the sulfonic acid groups for three hydration numbers: λ = 4, λ = 6, and λ = 8. Similarly to the singly hydrated Nafion, the figure shows that the peak intensities for 1215−1225 cm−1 drop as the hydration number decreases contrary to the experimental result mentioned in section 1.53 This seems to disprove the existence of such doubly

Figure 6. Calculated IR spectra of doubly hydrated Nafion model, which structures are optimized with the initial geometries having the proton attached to one of the sulfonic acid groups, for various hydration numbers.

hydrated Nafion membrane model. Figure 7 displays the calculated IR spectra of the doubly hydrated Nafion membrane model, for which geometry optimizations are performed using E


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Figure 7. Calculated IR spectra of doubly hydrated Nafion model, which structures are optimized with the initial geometries having protonated water clusters attached to the sulfonic acid groups, for various hydration numbers.

Table 3. Theoretical Assignments of the Main IR Peaks of Doubly Hydrated Nafion Membrane with One and Eight Hydration Water Molecules one hydration water molecule −1

peak energy (cm )

eight hydration water molecules peak energy (cm−1)

theor assignt

3229, 3399 1493, 1691, 1743 1415, 1417, 1439

O−H stretching in H2O O−H stretching in H3O+ C−C stretching

1377, 1405 1331 1318, 1327, 1444

H3O+ wagging SO3− asymmetric stretching + H2O scissoring C−C−C symmetric stretching

1258

C−O−C asymmetric stretching

1249, 1252, 1265, 1268, 1271, 1275, 1280, 1283, 1295, 1298, 1323, 1340, 1343, 1349, 1356, 1384, 1390, 1473

C−F stretching + C−C stretching

3692, 3753 3144, 3166, 3199 3086, 3189, 3290, 3418, 3483, 3551, 3570, 3607, 3611, 3651, 3744 2200 1506, 1643, 1685, 1692, 1695, 1700, 1717, 1728, 1786, 1808 1320, 1328, 1343, 1383, 1434, 1435 1314, 1348 1305, 1307 1236, 1250, 1254, 1260, 1266, 1277, 1292, 1294, 1302 1218, 1221, 1230 1210, 1225

1238, 1242 1220, 1225, 1229

O−C−F asymmetric stretching C−F stretching

1218 1198, 1202 1061

SO3− asymmetric stretching + H3O+ rocking C−C−C symmetric stretching SO3− symmetric stretching

1086, 1145, 1175 1077, 1083

1041, 1053

C−C−O scissoring

1040, 1043

967, 1011, 1077

SO3− symmetric stretching + C−S stretching + H2O wagging H2O wagging

1036

874

1193

1009, 1010

theor assignt H2O stretching C−H stretching (artificial modes) O−H stretching in H2O O−H stretching in H3O+ H2O scissoring C−C stretching SO3− asymmetric stretching + CF2 and CF3 symmetric stretching C−F stretching C−O−C symmetric stretching + C−F stretching C−F stretching SO3− asymmetric stretching + CF2 and CF3 symmetric stretching C−F stretching + C−O−C bending C−C−C scissoring SO3− symmetric stretching + C− O−C symmetric stretching CF3 symmetric stretching + C− O−C twisting H2O rocking SO3− symmetric stretching + C− S stretching

stretching mode increases as the hydration number decreases, similarly to the experimental IR peak intensity. The peak intensity for 1054−1086 cm−1 corresponding to the SO3− symmetric stretching mode is kept almost constant in agreement with the experimental result. Since these results indicate that protons mainly exist in water clusters attached to

the initial structures containing the protonated water clusters attached to the sulfonic acid groups. This model is based on the relay mechanism, in which protons are conducted by relaying protonated water clusters under low-humidity conditions. Surprisingly, the figure shows that the peak intensity for 1210−1243 cm−1 corresponding to the SO3− asymmetric F


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Macromolecules the sulfonic acid groups under the fuel cell operating condition, they strongly support our previous conclusion that the proton conductance of Nafion proceeds by the relay mechanism under low-humidity conditions. 3.4. Theoretical IR Peak Energies and Peak Assignments of Doubly Hydrated Nafion Membrane. Based on the calculated results, it is meaningful to compare the calculated IR spectrum peaks and their assignments of hydrated Nafion membrane with the experimental one. Table 3 summarizes the calculated IR peak energies and corresponding peak assignments of doubly hydrated Nafion model, in which geometries are optimized using the proton-detached initial structures on the basis of the relay mechanism, with one and eight hydration water molecules. As previously suggested in an experimental IR study,50 the table shows that the C−F and C−O−C stretching modes of the side chain are mixed by the hydration in the peaks of the SO3− stretching modes. The increased peak intensity of the SO3− asymmetric stretching mode by dehydration is, however, supposed not to come from the mode mixing but to result from the displacement of the proton from the edge of hydration water cluster to the sulfonic acid groups because IR peak intensities depend on the dipole moment response to the corresponding vibrational mode,65 and therefore the peak intensities of the sulfonic acid group stretching modes are supposed to increase as protons coming close to the sulfonic acid groups. Comparing to the calculated IR spectra of singly hydrated Nafion in Table 2, we found that many peaks corresponding to the vibrational modes of the main chain appear in the IR spectra of the doubly hydrated Nafion. This is because the vibration of the main chain enhances the effect on the displacement of protons in the double hydration. It indicates that the double hydration increases the IR absorption to make the proton conductivity more dependent on temperature. Considering that the relay mechanism proceeds by the dynamics of the side chains, we can expect that the proton conductivity of Nafion membrane has more temperature dependence than those of singly hydrated membranes. This expectation is consistent with the real behavior of Nafion. We therefore conclude that the doubly hydrated nature of Nafion membrane enhances its proton conductivity especially under low-humidity conditions.

Observing the IR spectrum variation of Nafion membrane during dehydration, we have found that the peak intensity of the asymmetric stretching mode of the sulfonic acid groups increase during dehydration, whereas that of the symmetric stretching mode is kept almost constant. Since this IR behavior is contrastive to that of SPE membrane, which gives relatively low proton conductivity at low humidity, we suggested that this behavior has a relationship with the high proton conductivity of Nafion at low humidity. To explore the relationship between the IR spectrum behavior and the proton conductivity, we have compared the calculated IR spectra of dehydrated and singly hydrated Nafion membranes to the experimental ones. Comparing the experimental and calculated IR spectra of dry Nafion, we confirmed that the calculated IR peak energies of dehydrated Nafion are very accurate compared to the experimental results of dry Nafion, and therefore the present method is available to explore the IR peak behavior of Nafion in more detail. We then calculated the IR spectra of singly hydrated Nafion for various hydration numbers. By showing that only one hydration water molecule considerably changes the IR peak energies, we raised questions about the IR peak assignments based on the fingerprint method in cases that protons are displaced as seen in the hydration of Nafion. Next, we have explored the dependence of the IR spectrum of Nafion on the hydration number to figure out the humidity dependence of the proton conductance. We first illustrated the IR spectra of singly hydrated Nafion for three hydration numbers. Consequently, we found that the hydration number dependence of the IR spectrum is not consistent with the experimental one. We therefore examined the IR spectra of the doubly hydrated Nafion for two types of structures: one type has the proton attached to one of the sulfonic acid groups, and another has protonated water clusters following the relay mechanism of the proton conductance. As a result, we found that the IR spectrum behavior is consistent with the experimental result only for the latter case. This result strongly backs up the relay mechanism for the proton conductance in Nafion. Finally, we have looked into the IR peak energies and peak assignments of the doubly hydrated Nafion membrane. Though the result shows that the vibrational modes of the side chains are mixed in the peaks of the SO3− stretching modes, we guessed that the humidity dependence of the SO3− stretching mode peaks come from the displacement of protons from the vicinity of the sulfonic acid groups to the edge of hydration water clusters. On the basis of the result that the peak intensities of the side chains are increased in the IR spectra of the doubly hydrated Nafion in comparison with those of the singly hydrated Nafion, we concluded that the doubly hydrated nature of Nafion upgrades the proton conductivity especially at low humidity.

4. CONCLUSIONS In this article, we have first experimentally showed the timeresolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of Nafion membrane during dehydration and then have theoretically analyzed the IR spectra of hydrated and dehydrated Nafion electrolyte membrane to make clear the reason why Nafion gives much different IR peak dependence on the dehydration from that of a hydrocarbon electrolyte membrane. We have carried out the ATR-FTIR analysis using the originally designed ATR cell, which enables us to make the in situ measurement of the IR spectrum variation at the Pt/ Nafion interface during dehydration. In theoretical calculations, we have performed the long-range corrected (LC) density functional theory (DFT) calculations for the IR spectra of the singly hydrated and doubly hydrated Nafion models with various number of hydration water molecules on the basis of our previous conclusion that the proton conductance of Nafion proceeds by the relay mechanism, in which protonated water clusters are transferred by the dynamics of the side chains through the doubly hydrated sulfonic acid groups.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00999. Figures S1−S6 (PDF) G


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AUTHOR INFORMATION

Corresponding Author

*Tel +81-55-254-7139, e-mail [email protected] (T.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Research on Nanotechnology for High Performance Fuel Cells (HiPer-FC) and Superlative, Stable, and Scalable Performance Fuel Cell (“S”Per-FC) projects of the New Energy and Industrial Technology Development Organization (NEDO) of the Japanese Ministry of International Trade and Industry (MITI) and by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) (Grant 23225001).

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