Effect of Superacidic Side Chain Structures on High Conductivity

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Effect of Superacidic Side Chain Structures on High Conductivity Aromatic Polymer Fuel Cell Membranes Ying Chang,†,‡ Angela D. Mohanty,† Sarah B. Smedley,§ Khaldoon Abu-Hakmeh,∥ Young Hun Lee,∥ Joel E. Morgan,⊥ Michael A. Hickner,*,§ Seung Soon Jang,*,∥ Chang Y. Ryu,† and Chulsung Bae*,†,‡ †

Department of Chemistry and Chemical Biology, New York State Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States ‡ Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, Nevada 89154-4003, United States § Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Dr., Atlanta, Georgia 30332-0245, United States ⊥ Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *

ABSTRACT: Proton-conducting superacidic polymer membranes with different fluoroalkyl sulfonate pendants attached to aromatic polymer backbones were synthesized via C−H functionalization and Suzuki coupling reactions. Variation in the chemical structures of the pendant acidic sulfonate moieties and their effects on membrane properties including water uptake, ion exchange capacity, morphology, and proton conductivity were systemically investigated. Membranes containing the short −OCF2SO3H pendant (PSU-S5) showed a smaller hydrophilic domain size and lower proton conductivity than those containing the longer pendants −OCF2CF2SO3H (PSU-S1) and −SCF2CF2SO3H (PSU-S4), likely due to the short chain’s less favorable aggregation and lower acidity. Polymer electrolyte membranes with unique branched fluoroalkyl sulfonate pendants (PSU-S6) gave larger ionic domain sizes, more uniform hydrophilic channels, and higher proton conductivity than samples with analogous linear pendant chains (PSU-S1), indicating that branched sulfonate structures may be a key future direction in the field of fuel cell membrane.

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INTRODUCTION Increasing concerns about the environmental impact of our overdependence on fossil fuels have motivated research on alternative clean energy technologies. Proton exchange membrane (PEM) fuel cells, which are composed of a cathode, an anode, and a PEM, generate electricity cleanly via electrochemical reactions of hydrogen and oxygen to yield water and heat as the only byproducts.1−5 The development of perfluorosulfonic acid ionomers, such as Nafion, has greatly contributed to fuel cell technologies, and these materials are still widely used as the benchmark membrane in fuel cells. Because of its perfluorinated structure and superacidic pendant side chain, Nafion possesses high proton conductivity as well as good chemical stability. However, Nafion is still not an ideal PEM material, and its drawbacks (e.g., high cost, low operation temperature, and high methanol crossover) require development of alternative PEMs for successful adoption of fuel cells as reliable and inexpensive energy conversion devices.6−9 Over the past decades, extensive efforts have been devoted to the development of hydrocarbon-based PEMs, and many aryl and alkyl sulfonated polymers have been described.10−16 In general, © XXXX American Chemical Society

these sulfonated aromatic polymer PEMs with high ion exchange capacity and high conductivity swell excessively under high hydration conditions and give much lower proton conductivity than Nafion when the relative humidity (RH) or water content of the membrane is reduced. PEMs with good proton conductivity at high temperature (above 100 °C) and low RH can bring many advantages to the fuel cell system, such as fast electrode reaction kinetics, good tolerance toward carbon monoxide and other fuel and air impurities, and simplified water management strategies.8,13,17−19 To achieve highly conductive materials under low hydration conditions, creation of well-connected hydrophilic channels within the membrane through architectural controls of polymer morphology has been pursued over the past decade. For example, several sulfonated multiblock copolymers20−22 and graft polymers23−26 show significantly higher proton conductivities at low RH conditions than conventional randomly sulfonated Received: August 5, 2015 Revised: September 16, 2015

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DOI: 10.1021/acs.macromol.5b01739 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules polymers due to the facilitated proton transport within the hydrophilic channels. Recently, we have suggested strong acidity-driven enhancement of proton conductivity in random copolymers as an alternative approach to block and graft copolymer aromatic PEMs. To investigate the acidity effect of different tethered sulfonate groups in randomly functionalized aromatic copolymers, we synthesized polystyrenes and polysulfones functionalized with fluoroalkyl sulfonate, aryl sulfonate, and alkyl sulfonate pendants and compared their PEM properties. Among them, fluoroalkyl sulfonated polymers demonstrated significantly higher proton conductivity especially at low RH compared to those having less acidic sulfonate groups.27,28 We observed that the fluoroalkyl sulfonated groups of superacidic polymers had greater proton dissociation and weaker hydrogen bonding of the water around the electron-deficient sulfonate group,29 both of which lead to greater proton conductivity in these materials. Morphological differences among these polymers functionalized with the different sulfonate groups were found to be insignificant compared to the well-developed ionic channel structures of sulfonated block copolymer PEMs, supporting the conclusions that proton conductivity enhancement of superacid random copolymer PEMs is not morphology-driven but acidity-driven. Similar improvements in performance of superacidic PEMs have been reported by other groups using different synthetic methods.30−35 In these reports that corroborate our work, the reported random copolymers also displayed proton conductivities on par with the morphology-driven increase of block and graft PEMs. While there are still different opinions on the ideal chemical structure of aromatic PEMs, we believe superacidic fluoroalkyl sulfonate groups are superior side chain structures for PEMs because these groups can promote proton conductivity at low RH without the need to create block structures or increase ion exchange capacity (IEC), which would complicate the synthesis, induce more water absorption, and sacrifice the membrane’s mechanical properties. Additionally, our synthetic method for preparation of fluoroalkyl sulfonate polymers via C−H bond functionalization and Suzuki cross-coupling reactions is scalable and can be performed on a variety of commercially available aromatic polymers. To date, however, there are very few examples of hydrocarbon or aromatic-based superacidic polymers with different side chain structures. In order to fill this important gap in our knowledge of PEM materials, we have now synthesized polysulfones functionalized with different fluoroalkyl sulfonated pendants and systematically studied the effects of these structures on proton conductivity, water uptake properties, and morphology. Furthermore, FTIR study of the interaction of sulfonate side chains with water and computational simulation of the superacidic PEMs were performed to gain a better understanding of the sulfonate side chain structure−PEM property relationships.

Scheme 1. Synthesis of Superacidic Fluoroalkyl Sulfonated PSUs

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RESULTS AND DISCUSSION A. Synthesis of Sulfonated Polymers. Synthesis of superacidic polysulfones (PSU-Sn) with different structures is summarized in Scheme 1. The iridium-catalyzed C−H borylation of polysulfone (PSU) was conducted to incorporate a pinacolboronic ester (Bpin) group onto the aromatic rings of the polymer.27,28,36−38 Distinctive NMR resonances of the Bpin group ranging from 1.0 to 1.27 ppm appeared after the C−H borylation (Figure 1). The degree of borylation (190 mol %; an

Figure 1. 1H NMR spectra of PSU-Bpin and SU-Sn-Ar.

average of 1.9 Bpin group per repeating unit) was calculated based on the integral ratio of Bpin group and the B


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methods, IECNMR values would be better reflective of the final polymer structure because IECtitr values are sensitive to remaining water content of hygroscopic membranes in the dried state. Titration may also not be able to have good access to the buried sulfonate groups within the hydrophobic domains. Figure 2 shows the humidity-dependent water uptake values and hydration numbers (i.e., the number of water molecules per

isopropylidene group of polymer backbone (1.58−1.70 ppm) in the 1H NMR spectrum. The borylated polymer (PSU-Bpin) was cross-coupled with fluoroalkyl sulfonated aryl bromides (S1, S4, S5, and S6) in the presence of Pd(PPh3)4 and K2CO3 in a mixture of THF/H2O (20:1, v/v). The Suzuki cross-coupling reactions proceeded cleanly, giving excellent conversions to the corresponding protected sulfonate polymers (PSU-Sn-Ar). The Bpin group peaks disappeared completely from the 1H NMR spectrum, and a new methyl group peak from the protected sulfonates appeared at 1.75 ppm (Figure 1). The integral ratio of the methyl group on the side chain and the isopropylidene group of the polymer backbone matched well with the molar concentration of the precursor polymer (i.e., 190 mol %), indicating quantitative conversion of the boryl group in the polymer. The 3,5-dimethylphenol protecting group of the sulfonates was cleaved by treatment with sodium hydroxide, and the resulting sodium sulfonate polymers precipitated from the solutions due to a dramatic solubility change. The precipitated polymers PSU-Sn-Na (n = 1, 4, 5, and 6) were purified by dissolving in methanol and precipitating into water. Note that PSU-S2 and PSU-S3, which contain aryl sulfonate (−C6H4− SO3H) and alkyl sulfonate (−CH2CH2CH2−SO3H), respectively, were reported in our previous paper28 and are not included in this study because they do not have a fluoroalkyl sulfonated pendant group. After removal of the sulfonate protecting group, the 19F NMR resonances of CF2SO3Na were shifted to higher field by 4−6 ppm (Figure S1). All sodium sulfonated polymers showed good solubility in DMSO from which dark transparent, flexible membranes could be prepared. The membranes in sodium sulfonated form were acidified in 1 M H2SO4 to give their acid form. B. Properties of Fluoroalkyl Sulfonated Polymers. Ion Exchange Capacity, Water Uptake, and Proton Conductivity. Table 1 summarizes the membrane properties of the fluoroalkyl

Figure 2. Water uptake (a) and hydration number (b) of fluoroalkyl sulfonated PSUs versus relative humidity at 30 °C.

Table 1. Properties of Fluoroalkyl Sulfonated PSUs and Comparison with Nafion

mol SO3H). As shown in Table 1, the water uptake values of the PSU membranes generally follow the trend of IEC values except for PSU-S5. Although PSU-S5 and PSU-S6 have comparable IECNMR, the water uptake and hydration number of the former were significantly smaller in comparison to the latter. Although all PSU-Sn of Table 1 have superacidic fluoroalkyl sulfonated side chains, PSU-S6 had consistently higher water uptake and hydration numbers than other sulfonated PSU membranes. In-plane proton conductivities of the sulfonated PSU membranes and Nafion 112 were measured as a function of RH at 100 °C, and the data are shown in Figure 3. Among sulfonated PSU membranes, PSU-S 6 gave the highest conductivity over a wide range of humidity and showed values even higher than Nafion at 50% RH or above. Although PSU-S5 has higher IECNMR and greater water uptake than PSU-S1 (2.19 vs 1.97 mequiv/g and 29 vs 24% for IECNMR and water uptake, respectively), this sample had lower proton conductivity than PSU-S1. Noticeably, PSU-S5 gave significantly lower conductivity than all other PSU membranes over the entire RH range. This inferior performance might be due to the presence of the shorter fluoroalkyl chain (−OCF2−) which has weaker electron-withdrawing ability and, as a result, led to a lower degree of dissociation of the sulfonate group. Thus, we can

IEC (mequiv/g) sulfonated polymera

NMRb

titrc

water uptaked (%)

λe

PSU-S1 PSU-S4 PSU-S5 PSU-S6 Nafion 112

1.97 1.91 2.19 2.13

1.83 1.96 1.99 2.23 0.86

24 29 29 37 15

6.8 8.5 7.3 9.6 9.7

a

All sulfonated PSUs contain average of 1.9 sulfonic acid groups per repeating unit (190 mol %). bIEC calculated from polymer structure based on the integral ratio of 1H NMR spectrum of 3,5dimethylphenol protected sulfonated PSU. cIEC measured by titration. d Water uptake (%) = (Wwet − Wdry)/Wdry, where Wdry and Wwet are the weights of dried and wet membranes, respectively. Water uptake of wet membrane was measured at 30 °C and 98% RH. eHydration number (i.e., molar ratio of water molecules per sulfonate group) at 98% RH.

sulfonated PSUs. The NMR-based IEC values (IECNMR) were estimated from the 1H NMR spectra using the integral ratio of the methyl pendant groups of PSU-Sn-Ar and the isopropylidene group of the polymer backbone. IECs were also measured from titration (IECtitr), and they matched well with the IECNMR values (1.9−2.2 mequiv/g). Between the two IEC measurement C


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shorter pendant chains of PSU-Sn compared to those of Nafion. Compared to PSU-S1, PSU-S6 with branched pendant chains showed larger hydrophilic domains while PSU-S5 with a short tethered chains had smaller hydrophilic domains in the TEM images. This trend might be ascribed to the shorter side chains of the S5-polymer and its less favorable aggregation behavior of the hydrophilic sulfonate head groups. However, it is difficult to draw definite conclusions about overall morphological structure from the TEM images because they give only a small representation of the morphology of a limited sample area of the membrane. To complement the localized morphology study of TEM, the average nanoscale morphology in bulk of the sulfonated PSU membranes was studied using SAXS. Because the X-ray beam of our in-house SAXS is about 0.4 mm in diameter (defined by the second pinhole from the rotating anode), these measurements reflect averaged morphology of the membranes over a relatively large area as compared to the TEM measurements. Figure 5 shows the SAXS profile for PSU-S1, -S4, -S5, and -S6 in

Figure 3. Proton conductivity of sulfonated PSUs versus relative humidity at 100 °C.

conclude that at least two CF2 groups are needed to obtain the desired superacidic functionality of the sulfonate, which has been demonstrated computationally by Yeh et al.39 PSU-S1 and PSU-S4 have almost the same IECNMR (1.97 vs 1.91 mequiv/g); however, the latter polymer absorbed more water and had a higher proton conductivity than the former. The only difference in chemical structure between the two polymers is the linkage to the fluoroalkyl group: thioether (−SCF2CF2−) for PSU-S4 versus ether (−OCF2CF2−) for PSU-S1. Because sulfur atom is larger and has a higher polarizability than oxygen atom, it is possible that PSU-S4 with a thioether linkage might absorb more water molecules than PSU-S1 with an ether linkage, giving enhanced proton conductivity. Morphology. The morphology of sulfonated PSU membranes was studied by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). For the TEM characterization, membranes were stained with lead acetate.40 Therefore, the dark and light areas in the images represent the hydrophilic and hydrophobic domains, respectively (Figure 4). All fluoroalkyl sulfonated PSU membranes showed distinct phase separation. The hydrophilic domains range from 1 to 3 nm, which are smaller than those of Nafion 112 (3 to 5 nm). This difference may be due to a combination of less hydrophobic and more rigid backbone structure and

Figure 5. SAXS profiles for PSU-S1 (red), PSU-S4 (green), PSU-S5 (orange), PSU-S6 (blue), and unfunctionalized PSU (purple) in sodium salt form. Data in the inset are plotted on a logarithmic scale.

their sodium salt form. Both H+ and Na+ forms of the membranes were analyzed (see the Supporting Information, Figures S62−S65), but the membranes in Na+ form showed a more pronounced pattern of phase separation (i.e., peaks in Na+ form were narrower than those in H+ form) and gave higher intensity peaks owing to the greater electron density of sodium compared to hydrogen. SAXS data allows the quantitative comparison of the overall interdomain spacing distance based on the position of the correlation peak, q*. As shown in Figure 5, unfunctionalized PSU did not show any sign of phase separation in the q-regions from 0.5 to 3.4 nm−1, while broad-yet-distinct interdomain correlation peaks were present in all functionalized PSU membranes. These phase-separated ionic domains were likely to be formed by the self-assembly of fluoroalkyl sulfonated pendant groups along the hydrocarbon PSU backbone chain in the bulk membranes. As indicated by the solid arrows in the inset of Figure 5, PSU-S6 exhibited a distinct peak maximum at a lower q value than any of the other PSU membranes. This lower q* value reflects a larger distance between the ionic domains, as calculated from the peak maxima using the equation d = 2πq−1 (listed in Table 2). The larger domain spacing for the

Figure 4. TEM images of Nafion 112 and superacidic sulfonated PSU membranes. D


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artificial density contrast as follows. The local density variables are ϕjA and ϕjB, where ϕjA is equal to 1 if the site j is occupied by a hydrophilic entity such as water or sulfonate group and equal to 0 otherwise and ϕjB is equal to 1 if the site is occupied by hydrophobic entities such as the polysulfone backbone or equal to zero otherwise. The quantity S(q) is spherically averaged as follows:

Table 2. Interdomain Spacing (d) of Sulfonated PSU Membranes polymera

q*b (1/Å)

dc (nm)

PSU-S1 PSU-S4 PSU-S5 PSU-S6 polysulfone

0.24 0.24 0.24 0.22

2.61 2.61 2.61 2.80

S(q) =

a c

+

Sulfonated polymer in Na form. Calculated from q* using d = 2πq−1.

b

|q|

Value at peak maximum.

(2)

Figure 6. Structure factor profiles calculated from PSU-S1 (red), PSUS4 (brown), PSU-S5 (green), and PSU-S6 (blue) with experimental water uptake. The interdomain spacings calculated from qmax are 2.4, 2.5, 2.3, and 3.5 nm for PSU-S1, PSU-S4, PSU-S5, and PSU-S6, respectively.

nm for PSU-S1, PSU-S4, PSU-S5, and PSU-S6, respectively. These simulated d values are slightly different from those experimentally obtained by the SAXS experiments (Table 2); however, both data present a consistent conclusion: PSU-S1, PSU-S4, and PSU-S5 have similar d values, and PSU-S6 has a significantly larger d value than those of the monosulfonated PSUs. Considering the sulfonate group in PSU-S6 has the same structure as that of PSU-S1 except for branching, the higher proton conductivity and the larger d spacing of PSU-S6 suggest that the branched sulfonate side chain can generate better developed nanophase segregation compared to the monosulfonated polymer membranes in this study as indicated in both our experimental data and the simulations. Pair Correlation Analysis. The proton dissociation from sulfonate group can be interpreted as a measure of the acid strength in hydrated polymer membrane. Thus, in order to investigate the extent of proton dissociation from each sulfonate group, we calculated the pair correlations of sulfonate-hydronium pair, ρgS−O(hydronium). The definition of pair correlation function, gA−B(r), was explained in our previous publication for sulfonated polystyrene membranes.27 The ρgS−O(hydronium) data in Figure 7 showed that PSU-S1 and PSU-S4 have a similar sulfonate−hydronium correlation while PSU-S5 and PSU-S6 have the most and the least correlation, respectively. Thus, it appears that although all sulfonated polymers in this study have superacidic fluoroalkyl sulfonate side chains, there are subtle differences in acidity among them,

S(q) = ⟨∑ ∑ exp(iq·rij)(ξ iξ j − ⟨ξ⟩2 )⟩/L3 rj

|q|

with q = (2π/L)n, where n = 1, 2, 3, ... denotes that for a given n a spherical shell is taken as n − 1/2 ≤ qL/2π ≤ n + 1/2. The simulated structure factor profiles in Figure 6 show that the calculated interdomain spacings (d) are 2.4, 2.5, 2.3, and 3.5

PSU-S6 sample presumably resulted from its bulky and branched fluoroalky sulfonate group. The larger d-spacing in PSU-S6 (2.80 nm) correlates well with the larger size and branched structure of the S6 relative to S1, S4, and S5. Furthermore, because PSU-S6 has two sulfonate groups per repeat unit instead of just one, it could create greater localized charge density and larger domains with greater d-spacing, resulting in stronger SAXS scattering contrast upon phase separation. Most importantly, these morphology data suggest that PSU-S6 has more uniform ionic channel size and a distinct phase separation of hydrophilic domains since its scattering peak is narrower and more intense than the other samples. According to the SAXS data in Table 2, all other functionalized polymers (i.e., PSU-S1, -S4, and -S5) have a shorter and almost identical interdomain spacing of 2.61 nm. This is probably because they all have broader peaks with lower intensity in the SAXS profile. Although the morphology structures of PSU-S1, -S4, and -S5 from TEM analysis are slightly different each other, they all could be characterized by a wide distribution of different sizes of interdomain spacing between ionic channels. Such a wide distribution of domain sizes is likely to result in the development of “bottleneck” regions in the ionic channels, leading to lower conductivity compared to that of PSU-S6. C. Computational Simulation Study of Superacid Polysulfones. For better understanding of experimentally observed differences in proton conductivity and morphology with respect to the structures of tethered sulfonate groups, we investigated the nanoscale structures of PSU-S1, PSU-S4, PSUS5, and PSU-S6 using a full-atomistic molecular dynamics (MD) simulation method, which was employed in our previous studies of sulfonated polymers and other polymers.27,28,41−46 To simplify description of the effects of sulfonate group concentration in the simulations, a polysulfone backbone with 200 mol % sulfonation degree was selected. The details of materials and simulation parameters are listed in Table S1. The water contents of the simulated polymer system were set to be the same as the experimental water uptakes of Table 1 to mimic the experimental conditions of hydrated membranes. Structure Factor Profile Analysis. To quantitatively analyze the effect of sulfonate headgroup on the nanophase-segregated morphology of the PEM materials, we calculated the structure factor, S(q), as used in previous studies of hydrated polymer membranes.38−40 S(q) is defined as ri

∑ S(q)/ ∑ 1

(1)

where the angular bracket denotes a thermal statistical average, ξi represents a local density contrast, (ϕjA − ϕjB), q is the scattering vector, and rij is the vector between the sites i and j. While SAXS and SANS experiments rely on scattering length density contrast, the structure factor is calculated from an E


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PSU-S4 contains a thioether linkage on the side chain and yields O−D peak stretching frequency of 2552 cm−1, suggesting that it has a weaker hydration interaction with water compared to PSU-S1 which contains an ether linkage and gave an O−D peak stretching frequency at 2549 cm−1. This subtle difference in the linkage causes the two side chains to behave differently when hydrated. When the two membranes are hydrated with 10 wt % water, the peak position of O−D stretch for the S4polymer is blue-shifted significantly compared to the S1polymer, indicating that the thioether sulfonate groups are forming longer and weaker hydrogen bond with the surrounding water molecules. Interestingly, PSU-S5 yielded the lowest O−D peak position at 2540 cm−1, suggesting that eliminating one of the CF2 units from the S1-polymer strongly reduces the acidity. This decrease in acidity has been demonstrated computationally by firstprinciples studies of the electron affinity of sulfonate groups with various amounts of fluorination.39 As the amount of fluorine substitution increased, the proton affinity decreased. Because perfluoroalkylated unit is a strong electron-withdrawing group, the proton of perfluoroalkyl sulfonate group can be easily dissociated. Additionally, once the proton is dissociated, the perfluoroalkyl substituents help to stabilize the resulting negative charge on the sulfonate, allowing them to behave as a weak conjugate base. The O−D stretch peak can be deconvoluted to extract the percentages of waters in each microenvironment. Figure 8 displays the fitting of each membrane when hydrated with 10 and 20 wt % water. Each hydrated spectrum was obtained using the dry polymers’ spectra as a reference. With this correction some of the initially bound water that cannot be abstracted from the materials will not be accounted for in the FTIR analysis, resulting in lambda values that are slightly lower than expected. This process also eliminated peaks inherent to the polymer appearing in the 2700−2400 cm−1 range, such as substituted benzene overtones.47,48 Although these peaks have very weak intensity, they appear when the sample is thick and thus need to be removed to achieve adequate O−D signal. The resulting spectrum gives the pure O−D stretching peak from water adsorbed in the polymer film. This peak was fitted using a three-state model where we consider water exists in three different microenvironments. The first peak with the lowest frequency represents bulk-like water, which has a characteristic peak frequency of 2509 cm−1 and a full width at half-maximum (fwhm) of 170 cm−1. This microenvironment represents water that is experiencing hydrogen bonds with other water molecules. The second peak at a slightly higher frequency represents water in the intermediate state where perturbations in the hydrogen bonding network are still experienced by water molecules, but they do not directly interact with the sulfonate group.7,49,50 The position for this peak was determined by fitting the lowest hydrated sample (30% RH) and was fixed for the remainder of the fitting. The fwhm was allowed to vary. The third peak represents water that forms hydrogen bonds directly to the sulfonate group and this microenvironment is termed headgroup water. The frequency of this peak is substantially blueshifted because the hydrogen bond between water and sulfonate is weak, resulting in a weak O−D bond. The peak position and fwhm of this peak were determined by fitting the lowest hydrated samples (30% RH) for each sample, and then the peak position and the fwhm were held constant for the remainder of the fitting at each hydration. Once fit, the

Figure 7. Calculated pair correlation functions of sulfonate− hydronium ion in PSU-S1 (red), PSU-S4 (brown), PSU-S5 (green), and PSU-S6 (blue) with experimental water uptake.

and the order of the acid strength of side chains is PSU-S6 > PSU-S1 ≈ PSU-S4 > PSU-S5. The short side chain of PSU-S5 seems to impart the lower acidity and cause smaller interdomain spacing compared to other sulfonated PSUs in this study. Overall, the simulation results of the structure factors and pair correlations for these materials confirm that subtle acidity differences in this series of superacidic PSU-S n membranes are the origin of different interactions with water and nanophase segregated morphology structures. D. Sulfonate Side Chain Effect Study with FTIR. The extent of hydrogen bonding of dilute HOD in H2O, which can be measured by FTIR spectroscopy, offers valuable information on the local solvation environment of the sulfonate group in PEMs. The O−D stretching frequency is an indicator of hydrogen bond strength. As the acid strength of the pendant sulfonate group of a PEM becomes enhanced through the addition of electron-withdrawing moieties, the lower charge density of the anion weakens the water−sulfonate hydrogen bond, resulting in a blue-shifted O−D stretching frequency compared to that of less acidic sulfonated PEM with a higher anion charge density.29 Thus, by analyzing the peak position of the O−D stretching frequency and the hydrogen bond strength between pendant sulfonate groups and water, we were able to estimate the acidity difference among the side chains of four sulfonated PSUs. The O−D peak positions for all samples hydrated at 10 and 20 wt % water are summarized in Table 3. When hydrated with Table 3. Peak Position of O−D Stretch of Superacidic Polysulfones When Hydrated at 10 and 20 wt % Water

a

polymer

νO−D (10% H2O)a

νO−D (20% H2O)a

PSU-S1 PSU-S4 PSU-S5 PSU-S6

2547 2560 2548 2566

2549 2552 2540 2572

Wavenumber in cm−1.

20 wt % water, PSU-S6 gave an O−D peak stretch position of 2572 cm−1 indicating that it has the most acidic sulfonate group among all samples tested. The larger blue-shift of PSU-S6 in comparison to other PSU membranes can be ascribed to its unique side chain architecture with two perfluorosulfonate groups. As the strongest acid side chain of PSU-S6 experiences the weakest hydrogen bond with surrounding water molecules, the resulting more loosely bound water in the membrane affords the highest transport of the aqueous proton. F


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Figure 8. O−D stretch deconvolution of water adsorbed into the superacidic PSU members shown at both 10 and 20 wt % water uptake.

spectrum must be corrected for non-Condon effects which are amplified by strong hydrogen bonding environments.51 NonCondon effects cause an increase of the transition dipole strength with decreasing frequency, resulting in peaks that must be corrected to accurately extract population values.29 Based on data of Figure 8, the corrected population values of water molecules in microenvironment interacting with

sulfonated group (i.e., headgroup water) are given in Table 4, which reveals that PSU-S6 and PSU-S4 have a significantly less amount of water bound to the sulfonate headgroup compared to PSU-S1. The least acidic side chain of PSU-S5 appears to have the greatest interaction with the headgroup water. These data clearly explain the proton conductivity trend observed from a series of superacidic polymers in Figure 3. As we G


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Macromolecules

3,5-dimethylphenyl 2-(4-bromophenyl)-sulfionyl-1,1,2,2-tetrafluoroethanesulfonate (S7), and compound S6 are given in the Supporting Information. Methods. 1H, 19F, and 13C NMR spectra were obtained using a Varian NMR spectrometer (400 MHz for 1H, 376 MHz for 19F, and 100 MHz for 13C) at room temperature, and chemical shifts were referenced to TMS (1H and 13C) and CFCl3 (19F). FTIR spectra were recorded on a Shimadzu IR Prestige-21. All the other instruments and testing methods are given in the Supporting Information. Synthesis of Borylated Polysulfone (PSU-Bpin). 10.0 g (22.6 mmol) of PSU, 6.32 g (24.9 mmol, 1.1 equiv) of B2pin2, 251 mg (0.370 mmol, 1.1 × 0.015 equiv) of [IrCl(COD)]2, 200 mg (0.75 mmol, 1.5 × 0.03 equiv) of dtbpy, and THF (50 mL) were added in sequence to a flask under nitrogen. The mixture was refluxed at 80 °C for 12 h, cooled to room temperature, and diluted with chloroform. The solution was filtered through a short plug of silica gel, concentrated, and precipitated into hexane. The obtained polymer was redissolved in chloroform, filtered through a plug of silica gel, concentrated, and precipitated one more time to hexane. The polymer was obtained as a white fibrous solid (14.6 g). 1H NMR (benzene-d6): δ 8.88−9.10, 7.67−8.20, 6.41−7.00 (multiple ArH from polymer backbone), 1.30−1.46 (CH3 of isopropylidene), 0.78−1.08 (CH3 from Bpin). Synthesis of PSU-S1-Ar. 2.00 g of PSU-Bpin (5.54 mmol of Bpin), 2.29 g of K2CO3 (16.6 mmol, 3 equiv), 192 mg of Pd(PPh3)4 (0.17 mmol, 3 mol %), and 80 mL of THF were added to a flask under nitrogen. After addition of 5.07 g of S1 (11.1 mmol, 2 equiv) and 4 mL of water, the flask was placed in an oil bath preheated to 80 °C. The reaction mixture was stirred at 80 °C for 12 h, cooled to room temperature, diluted with chloroform, filtered through a short plug of silica gel, concentrated, and precipitated into methanol. The recovered polymer was purified one more time by dissolving in chloroform, filtering through silica gel, and precipitating into methanol. The polymer was obtained as a white fibrous solid (3.21 g). 1H NMR (benzene-d6): δ 6.40−8.30 (multiple ArH), 1.80 (CH3 from protecting aryl group), 1.30−1.60 (CH3 from isopropylidene). 19F NMR (benzene-d6): δ −80.4 (2F, −OCF2−), −112.1 (2F, −CF2SO3−). Synthesis of PSU-S4-Ar. The same procedure as above with 1.00 g of PSU-Bpin and compound S2 produced 1.54 g of PSU-S4-Ar as a white fibrous solid. 1H NMR (benzene-d6): δ 6.40−8.26 (multiple ArH), 1.80 (CH3 from protecting aryl group), 1.30−1.60 (CH3 from isopropylidene). 19F NMR (benzene-d6): δ −84.8 (2F, −SCF2−), −106.8 (2F, −CF2SO3−). Synthesis of PSU-S5-Ar. The same procedure as above with 0.46 g of PSU-Bpin and compound S5 produced 0.54 g of PSU-S5-Ar as a white fibrous solid. 1H NMR (benzene-d6): δ 6.20−8.10 (multiple ArH), 1.65 (CH3 from protecting aryl group), 1.30−1.60 (CH3 from isopropylidene). 19F NMR (benzene-d6): δ −71.8 (−OCF2SO3−). Synthesis of PSU-S6-Ar. The same procedure as above with 0.60 g of PSU-Bpin and compound S6 produced 1.91 g of PSU-S6-Ar as a white fibrous solid. 1H NMR (benzene-d6): δ 6.37−8.32 (multiple ArH), 1.73−1.90 (CH3 from protecting aryl group), 1.30−1.50 (CH3 from isopropylidene). 19F NMR (benzene-d6): δ −59.0 (CF3), −80.7 (−OCF2−), −112.2 (−CF2SO3−). Synthesis of PSU-S1-Na. PSU-S1-Ar (1.60 g, 2.62 mmol of S1 sulfonate group) was dissolved in dioxane (60 mL), and NaOH (0.52 g, 13.1 mmol, 5 equiv) and water (1.2 mL) were added. The resulting mixture was stirred at 80 °C for 4 h, and the polymer was precipitated as gray sticky solid. The upper solution was poured out and the residual precipitate was dissolved in methanol, filtered through a pad of silica gel, concentrated, and precipitated into water. After being dried, the polymer was purified one more time by dissolving in methanol, filtering through a plug of silica gel, and precipitation into water. The polymer was obtained as a gray solid after drying at 80 °C overnight (1.29 g). 1H NMR (DMSO-d6): δ 6.40−8.20 (multiple ArH), 1.50−1.80 (CH3 from isopropylidene). 19F NMR (DMSO-d6): δ −80.5 (2F, −OCF2−), −116.2 (2F, −CF2SO3−). Synthesis of PSU-S4-Na. The same procedure above with PSU-S4Ar (1.45 g, 2.31 mmol of S4 sulfonate group) gave 1.11 g of gray solid. 1 H NMR (DMSO-d6): δ 6.35−8.20 (multiple ArH), 1.50−1.80 (CH3

Table 4. Number of Water Molecules Interacting with the Sulfonate Head Group (λbound) of the Four Different Polymers Hydrated at 10 and 20 wt % Water polymer

λbound (10% H2O)

λbound (20% H2O)

PSU-S1 PSU-S4 PSU-S5 PSU-S6

1.33 0.33 1.61 0.37

2.15 0.46 3.50 0.66

demonstrated in previous reports using pair correlation theory and FTIR peak deconvolution method, the more acidic the sulfonate headgroup, the weaker the hydrogen bond between the sulfonated group and surrounding water, the lower percentage of bound water molecules.27−29 The weaker hydrogen bond formation between the side chains of PSU-S4 and PSU-S6 and water makes them better proton conducting polymers than PSU-S1.

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CONCLUSIONS We have developed a series of superacidic polymers containing different fluoroalkyl sulfonate groups and systematically investigated the structural influence of sulfonic acid pendant (e.g., side chain length, linear vs branched structure, ether vs thioether linkage) on PEM properties (e.g., water uptake, IEC, proton conductivity, and morphology). The membrane containing the short −OCF2SO3H pendant showed smaller hydrophilic domain size and lower proton conductivity than the membrane containing −OCF2CF2SO3H because of the less favorable aggregation of sulfonate groups and lower acidity of the shorter pendant chain. The polymer membrane with −SCF2CF2SO3H pendant chains (PSU-S4) absorbed more water and showed enhanced proton conductivity compared to the polymer with −OCF2CF2SO3H chains (PSU-S1), possibly due to higher polarizability of sulfur than oxygen. In contrast to unfunctionalized polysulfone, all sulfonated polymers exhibited ordering peaks from the aggregation of ionic domains. Among them, the polymer with a branched sulfonate side chain structure (PSU-S6) produced larger interdomain size and more distinct phase separation behavior compared to the linear fluoroalkyl sulfonated polymers. These results suggest that not only the superacidity of fluoroalkyl sulfonic acid but also the topology of sulfonate groups (e.g., linear vs branched side chains) can play a significant role in determining proton conduction in a fuel cell membrane. This present work significantly broadens the scope of aromatic-based superacidic polymers as alternative PEMs, and the structure−property study of sulfonic acid pendants can be an important guide for the future PEM and ionomer development for fuel cell technology.

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EXPERIMENTAL SECTION

Materials. Udel polysulfone (PSU) (Mn = 22 000) was purchased from Sigma-Aldrich and used as received. 4,4′-Di-tert-butyl-2,2′dipyridyl (dtbpy), chloro-1,5-cyclooctadiene iridium(I) dimer ([IrCl(COD)]2), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), 4(dimethylamino)pyridine (DMAP), Na2S2O4, and 3,5-dimethylphenol were reagent grade and used without further purification. B2pin2 was from Frontier Scientific Co., and anhydrous tetrahydrofuran (THF) was obtained from Acros Organic. 3,5-Dimethylphenyl 2-(4-bromophenoxy)tetrafluoroethanesulfonate ester (S1) was prepared using our reported procedures.27,28 Synthetic procedures of 3,5-dimethylphenyl 2-(4-bromophenyl)sulfinyl-1,1,2,2-tetrafluoroethanesulfonate (S4), 3,5dimethylphenol (4-bromophenoxy)difluoromethane-sulfonate (S5), H


Article

Macromolecules from isopropylidene). 19F NMR (DMSO-d6): δ −83.3 (2F, −SCF2−), −112.4 (2F, −CF2SO3−). Synthesis of PSU-S5-Na. The same procedure above with PSU-S5Ar (0.53 g, 0.95 mmol of S5 sulfonate group) gave 0.40 g of gray solid. 1 H NMR (DMSO-d6): δ 6.80−8.10 (multiple ArH), 1.40−1.75 (CH3 from isopropylidene). 19F NMR (DMSO-d6): δ −78.5 (−OCF2SO3−). Synthesis of PSU-S6-Na. The same procedure above with PSU-S6Ar (1.00 g, 0.870 mmol of S6 sulfonate group) gave 0.76 g of gray solid. 1H NMR (DMSO-d6): δ 6.76−8.20 (multiple ArH), 1.40−1.80 (CH3 from isopropylidene). 19F NMR (DMSO-d6): δ −57.8 (CF3), −80.8 (−OCF2−), −116.4 (−CF2SO3−). Membrane Preparation. PSU-Sn-Na (0.70 g) was dissolved in 5 mL of DMSO, filtered through a cotton ball, and cast on a clean glass plate. After drying at 50 °C for 1 day with a positive air flow and at 80 °C under vacuum overnight, the membrane was peeled from the glass plate by soaking in deionized water. Acidification was conducted by immersing the membrane in 1 M H2SO4 solution for 3 days at room temperature while the acid solution was changed daily. Flexible dark transparent membranes were obtained. Ion Exchange Capacity (IEC). The calculated IECs of sulfonated PSUs were estimated from the mol % of 3,5-dimethylphenol protected sulfonated polymers of the 1H NMR spectra. The experimental IECs were determined using the following titration method. Membranes were equilibrated in 2 M NaCl solution at room temperature for 3 days before titration. The protons released into the aqueous solution were titrated with 0.025 M NaOH solution using phenolphthalein as an indicator. The experimental IEC values of the sulfonated PSU membranes were calculated according to the equation

grids with carbon film. Transmission electron microscopy image was taken by TECNAI-F30 Supertwin TEM using an accelerating voltage of 300 kV. Small-Angle X-ray Scatterting. SAXS profiles were collected on Bruker Nanostar-U instrument with a turbo (rotating anode) X-ray source. The sample-to-detector distance was 65 cm. Measurements of the dried films were obtained under vacuum at ambient temperature. Typical collection time was 30 min with membrane thickness on the order of 80 μm. The scattering profiles were normalized based on sample thickness and are represented in arbitrary intensity units as a function of the scattering vector (q).

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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.5b01739. Experimental procedures, NMR (1H, 19F, and 13C) spectra, and SAXS profiles of 190-PSU-Sn in both acid and sodium forms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (C.B.). *E-mail [email protected] (M.A.H.). *E-mail [email protected] (S.S.J.).

IEC (mequiv/g) = MNaOH × VNaOH/Wdry

Notes

The authors declare no competing financial interest.

where MNaOH and VNaOH are the molar concentration and volume (mL) of the aqueous NaOH solution used in titration and Wdry (g) is the weight of dry membrane. Water Uptake and Hydration Number. Relative humidity dependent water uptake was measured at 30 °C using a TA Instruments Q5000SA dynamic vapor sorption analyzer. The relative humidity steps and equilibration times were the same those used in the conductivity experiments. Water uptake and hydration number (λ) were calculated according to the equations

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ACKNOWLEDGMENTS C.B. thanks the NSF (CAREER DMR-0747667), Rensselaer Polytechnic Institute (start-up fund), and Nevada Renewable Energy Consortium for their generous support. We acknowledge Frontier Scientific Co. for a gift of B2pin2, Sinocompound Technology Co. for a donation of Ir complex, and Dr. Longzhou Ma of UNLV HRC for his help on running TEM of membranes. A.D.M. also thanks the Slezak Fellowship. M.A.H. acknowledges support from the Office of Naval Research under Grant N00014-10-1-0875.

water uptake (%) = (WRH − Wdry) × 100/Wdry

⎛ WRH − Wdry ⎞⎛ 1000 ⎞ ⎟ λ=⎜ ⎟⎜ ⎝ 18.01 ⎠⎜⎝ Wdry × IEC ⎟⎠

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where WRH is the sample mass at a given RH, Wdry is the dry mass of the sample, and IEC is the IECcal of the sample in milliequivalents of sulfonate group per gram of polymer. Proton Conductivity Measurement. To measure proton conductivity of sulfonated PSU membranes, the membrane in sulfonic acid form was immersed in deionized water for at least 24 h. The proton conductivity was measured using a four-electrode method with a BT-512 membrane conductivity test system (BekkTech LLC). The proton conductivity was measured by changing the relative humidity from 20 to 100% at 100 °C. The proton conductivity was calculated according to the equation

σ (mS/cm) =

ASSOCIATED CONTENT

REFERENCES

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