Characterization of Heterogeneous Solvent Diffusion Environments in

Jan 30, 2014 - In the PFG NMR diffusion experiment, in the limit δ̅ ≪ Δ (eq 3), the ..... For the AEM membranes described in this manuscript, the...
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Characterization of Heterogeneous Solvent Diffusion Environments in Anion Exchange Membranes Todd M. Alam†,* and Michael R. Hibbs‡ †

Department of Electronic, Optical and Nanostructured Materials and ‡Department of Materials, Devices and Energy Technologies, Sandia National Laboratories, Albuquerque, New Mexico 87123, United States S Supporting Information *

ABSTRACT: 1H high resolution magic angle spinning (HRMAS) NMR spectroscopy was used to characterize the solvent environments in a series of poly(phenylene)- and poly(phenylene alkylene)-based anion exchange membranes (AEMs). Multiple water and methanol environments were resolved in the membranes under HRMAS NMR. This allowed the self-diffusion rate constants to be evaluated for each different solvent environment as a function of the membrane identity, ion exchange capacity, water content, and sample temperature. These ionomers have been designed to function as binders within the catalyst layers of direct methanol fuel cells. In such applications, it is desirable to maximize the diffusion of the fuel (methanol) as well as the solvated ions to increase power output. To that end, the flexibilities of the backbone and the cationic side chains have been varied with the expectation that greater polymer mobility will lead to improved permeability. For the two types of AEMs investigated, it was observed that the methanol self-diffusion rates were preferentially reduced with respect to the water diffusion rates. It was also shown that the water diffusion rates within the AEMs were the largest at high water concentration, as observed in membranes containing the hexamethylene chain spacer in both the polymer backbone and the trimethylammonium (TMA+) cation-containing side chains.

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INTRODUCTION Fuel cells are a promising power source technology that impacts both energy issues and environmental concerns. There are a variety of fuel cell types employing different fuels and electrolyte substances that can be tailored for a range of uses including automotive transportation, military applications and portable electronics.1 Direct methanol fuel cells (DMFCs) are an example of technology being explored for alternative power source uses.2−6 DMFCs (Scheme 1) provide several advantages including a readily handled liquid fuel (versus a gas fuel such as hydrogen), and the ability to operate at low and intermediate temperatures (48 h prior to the NMR analysis. The membranes were not allowed to dry at any point during the preparation. Synthesis of PPC6 Polymer. The polymerization was based on the procedure of Fujimoto et al. with the exchange of 1,9-decadiyne for diethynylbenzene. 26 1,4-bis(2,4,5-triphenylcyclopentadienone)benzene (2.999 g, 4.341 mmol), 1,9-decadiyne (0.583 g, 4.341 mmol), and diphenyl ether (47 mL) were charged to a flask under argon. The mixture was frozen in a dry ice/acetone bath and was freeze−thaw degassed (2 times) before heating at 160 °C for 24 h. The reaction was cooled to 100 °C, and toluene (40 mL) was added to thin the solution before cooling to room temperature. The solution was poured into excess acetone, and the precipitate was dried, dissolved in methylene chloride (40 mL), and reprecipitated in acetone. The resulting solid was dried under vacuum at 180 °C to yield a tan solid (2.07 g, 62%).

terized. One of the synthetic goals of including additional alkane chain spacers both within the polymer backbone and on the trimethylammonium cation (TMA+) functional groups was to increase the degree of water adsorption and increase the solvent diffusion rates. By using these 1H HRMAS PFG NMR techniques, the self-diffusion rate for water and methanol as a function of ion exchange capacity (IEC), water uptake, and temperature were investigated.

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EXPERIMENTAL SECTION

Animated Tetramethyl Poly(phenylene) (ATMPP). The Diels− Alder synthesis of the ATMPP polymer membranes was previously described in detail by Hibbs et al.25 For simplicity in Scheme 2, ATMPP is depicted with precisely two ammonium groups per repeat unit. In fact, the ATMPP samples listed in Table 1 have average values of 1.4 to 2.3 ammonium groups per repeat unit. The final hydroxyl form of the ATMPP membranes were obtained by soaking the 1075

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Synthesis of BrKC6PPC6. PPC6 (1.40 g, 1.82 mmol) was dissolved in dichloromethane (88 mL) in a flask under argon. The flask was chilled in an ice/water bath and 6-bromohexanoyl chloride (1.55 g, 7.28 mmol) was added. Aluminum chloride (0.971 g, 7.28 mmol) was added to the flask and the mixture was allowed to stir for 2 h. The bath was removed, and the reaction was allowed to warm to room temperature over 2 h while stirring. The solution was poured into a beaker containing 200 mL deionized water and the beaker was heated to 60 °C to evaporate the organic solvent. After cooling to room temperature the mixture was filtered and the solid was blended with ethanol in a Waring blender. The mixture was filtered and the solid was dried at room temperature under vacuum to yield BrKC6PPC6 as an off-white solid (2.19 g, 81%). Synthesis of BrC6PPC6. To a solution of BrKC6PPC6 (2.13 g, 1.69 mmol) in dichloroethane (100 mL) was added trifluoroacetic acid (25 mL) and triethylsilane (1.25 mL, 7.83 mmol). The solution was heated to reflux for 24 h, then cooled to room temperature and poured into a beaker containing KOH (17 g) dissolved in water (150 mL). The beaker was heated to 80 °C to evaporate the organic solvent. After cooling to room temperature, the mixture was filtered and the solid was blended with ethanol in a Waring blender. The mixture was filtered and the solid was dried at room temperature under vacuum. The solid was dissolved in methylene chloride (30 mL), reprecipitated in ethanol, blended with more ethanol, and dried at room temperature under vacuum to yield BrC6PPC6 as an off-white solid (2.03 g). Synthesis of TMAC6PPC6. To a solution of BrC6PPC6 (1.20 g) in N,N-dimethylacetamide (27 mL) was added trimethylamine (3.8 mL of a 33 wt % solution in ethanol), and the solution was allowed to stir at room temperature for 18 h. The solution was filtered through a syringe filter onto a square glass casting plate with 5.0 in. sides. The dish was held in a vacuum oven at room temperature for 4 h and then at 50 °C for 18 h. The resulting membrane was then immersed in 0.5 M HBr for 2 h and then in deionized water for at least 24 h to yield a TMAC6PPC6 membrane in its bromide counterion form. The membrane was then soaked in 1 M NaOH for 48 h to exchange the bromide ions for hydroxide ions, followed by equilibration in distilled water. Finally, the tetramethyl aminated hexylpolyphenylene−hexane (TMAC6PPC6) copolymer membranes were immersed in 1 N MeOH for at least 24 h prior to NMR analysis. The membranes were not allowed to dry at any point during the preparation. The overall TMAC6PPC6 membrane synthesis is shown in Scheme 3. For simplicity in Schemes 2 and 3, TMAC6PPC6 is depicted with precisely two ammonium groups per repeat unit. In fact, the TMAC6PPC6 samples listed in Table 1 have average values of 2.5 to 3.4 ammonium groups per repeat unit. Membrane Characterization. The water solvent uptakes for the ATMPP and TMAC6PPC6 membranes were obtained from differences in membranes mass following immersion in water overnight. The membranes were wiped with a Kimwipe to remove excess water and quickly weighed (Wwet). The dry membrane weight (Wdry) was obtained following drying the membrane at 25 °C over P2O5 for 24 h. Only the water uptake was determined for these membranes with the solvent (%) uptake being defined as

water (%) =

Wwet − Wdry Wdry

solution, lightly patted with a Kimwipe to remove excess solvent, then quickly packed and sealed into a 4 mm Kel-F 30 μL HRMAS rotor insert (Bruker Biospin). The NMR data was collected on a Bruker Avance-III 600 MHz spectrometer using a 4 mm single magic-angle gradient triple resonance 1H/13C/31P HRMAS probe operating at 600.13 MHz for 1H. All of the 1H HRMAS NMR experiments were performed at 4 kHz spinning speed with a ± 0.1 K temperature regulation. The sample temperature under HRMAS conditions (TMAS) were determined using the methanol thermometer method of van Geet27 corrected for differences in spectrometer observe frequencies via TMAS = 435.5 − [1.193|Δν|(60/600.13)] − [29.3(0.01|Δν|)2 (60/600.1)2 ]

where Δν is the chemical shift separation in Hz between the methyl and hydroxyl protons in methanol. The 1D 1H HRMAS NMR spectra utilized a single pulse Bloch decay, 16 scan averages and a long 60 s recycle delay to ensure complete spin−lattice relaxation. The 1H NMR chemical shifts were referenced to an external secondary standard of neat water, δ = +4.8 ppm at 298 K with respect to the chemical shift of TMS, δ = 0 ppm. The frictional heating due to spinning at 4 kHz is relatively small, but is explicitly corrected for in the temperature calibration (eq 3) during the variable temperature studies. The 2D NOESY NMR exchange experiments were performed using a standard phase sensitive 3 pulse sequence (no gradients), a rotor synchronized t1 increment, 1024 t1 points using 8 scan averages, and a 5 s recycle delay. A range of mixing times (τmix) between 0 and 900 ms were employed. The 1H HRMAS PFG diffusion experiments used a stimulated echo (STE) sequence with bipolar gradient pulses and a spoil gradient pulse, a fixed gradient pulse length of δ̅ = 1 ms, and 16 gradient steps between 1.14 to 54.2 G/cm. The gradient strength was calibrated using the self-diffusion coefficient (D) of water at 298 K (2.3 × 10−9 m2/s), with a interpulse delay of Δ = 50 ms, with the echo decay E(q,Δ) fit using the standard Stejskal−Tanner formula28

⎡ ⎛ E(q , Δ) δ ̅ ⎞⎤ = exp⎢ − q2D⎜Δ − ⎟⎥ ⎝ E(0, Δ) 3 ⎠⎦ ⎣ −1

where D is the self-diffusion coefficient (m s ), Δ is the diffusion period, and q = γδ̅g/2π which is the product of γ the gyromagnetic ratio of the observed nuclei, g the gradient strength, and δ̅ the length of the gradient pulse. For each AEM sample, experiments using eight different Δ delay values ranging between 20 and 500 ms were performed with these results being analyzed in a slightly different manner. From the PFG echo-decays the mean-squared displacements ⟨zM2⟩ (see eqs 8 and 9, below) were evaluated for each resolved resonance at each sample temperature. To check consistency the diffusion rates obtained from the mean-squared displacement experiments (eq 9) were compared to the D obtained eq 4. Diffusion Analysis. In the PFG NMR diffusion experiment, in the limit δ̅ ≪ Δ (eq 3), the mean-squared displacement (⟨R2⟩) of the spins is given by the echo-signal attenuation for a given interpulse delay Δ, is given by

× 100

n(H 2O) water (%) × 10 = IEC × M w (H 2O) n(N(CH3)3+ )

(4) 2

(1)

⟨R2(Δ)⟩ = − 6 ln[E(q , Δ)/E(0, Δ)]/q2

The ion exchange capacities were determined from the backtitration procedure previously described,20 and is defined as IEC = mequiv.OH−/Wdry. The lambda (λH2O), which represents the number of water molecules per cation site in the membrane, was calculated from the water uptake

λ H2O =

(3)

(5)

For isotropic, unrestricted diffusion a Gaussian distribution describes the displacement in terms of the D and the mean square displacement ⟨R2⟩ = 6Dt

(6)

Deviations from a Gaussian distribution appear when local geometrical constraints impact the diffusion process. For example, the soft topological constraints of the polymer matrix may restrict solvent diffusion leading to “anomalous” diffusion expressed using a power law29−31

(2)

where Mw(H2O) is the molecular weight of water (18.01 g/mol) and the IEC of the membrane is given in (mmol/g). 1 H HRMAS NMR Characterization. Small pieces of the ATMPP and TMPC6PPC6 membranes were removed from a 1 N MeOH

⟨R2⟩ = 6Dα t α 1076

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Figure 1. 1H HRMAS NMR spectra for ATMPP and TMAC6PPC6 membranes with increasing IEC in 1 N MeOH at 298 K. The value of α allows different regimes of anomalous transport behavior to be distinguished, with α = 1 being the free Gaussian limit and the two-dimensional percolation cluster limit being given by α ∼0.732 In the present HRMAS NMR PFG study, the field gradient is along the magic angle (a unidirectional field gradient) such that one component of ⟨R2⟩ is probed, and is denoted as ⟨zM2⟩ where M designates this as displacement in the magic angle spinning (MAS) rotor frame, with the frame’s Z axis pointing along the spinning axis. The molecular displacement (eqs 5−7) measured during the HRMAS PFG experiments then becomes

■

⟨zM 2(Δ)⟩ = − 2 ln[E(q , Δ)/E(0, Δ)]/q2

(8)

⟨zM 2⟩ = 2Dα t α

(9)

chemical shifts and diffusion rates are assumed to be approximately equal to that of water since the mole fraction of MeOH and hydronium cations are relatively low for these solutions. In the ATMPP and TMAC6PPC6 membranes for the IEC levels investigated, the use of HRMAS NMR allowed the spectral resolution and identification of the polymer membrane-associated water (A-H2O) and methanol (A-MeOH) environments along with the “free” or more mobile water (FH2O) and methanol (F-MeOH) environments as previously described.24 The observation of different solvent environments is not surprising given that two water species have previously been observed by NMR for PVDF−PSSA membranes where the new environment shifted downfield (higher ppm) with decreasing water content and increasing methanol concentration.33 In that case, the new water environment was argued to result from water species in fast exchange with the sulfonic acid groups within the membrane, and is similar to the argument of water/methanol associating with the trimethylammonium cation presented in this manuscript. Two types of water environments have also been detected for different cation forms of Nafion34 and in sulfonated poly(ether sulfone) membranes35 based on 1H NMR relaxation experiments. The 1H HRMAS NMR chemical shift trends for the two types of AEM membranes are different. In the ATMPP membranes, the chemical shifts of the “free” environments (FH2O and F-MeOH) do not change significantly with variation of the IEC or water content (Table 1) and differ from the pure 1 N MeOH solution only at low IEC and low λH2O values. The slow diffusion rates for these “free” environments (see later discussion) in comparison to the bulk 1 N MeOH solutions reveal that these “free” solvent environments do have some interaction with the polymer membrane. The 1H NMR chemical shifts of the associated water (A-H2O) and methanol (A-MeOH) environments were observed at lower δ values (upfield) from the F-H2O and F-MeOH resonances. The extent of this chemical shift difference (Δδ) between the free and

RESULTS AND DISCUSSION The IECs, water (%), and λH2O values for the different AEM membranes are given in Table 1. In the remainder of the paper the IEC values are used to identify the different membrane samples with a given polymer type (e.g., ATMPP, IEC = 1.79). As predicted the water uptake is higher for increasing IEC in both the ATMPP and TMAC6 PPC 6 membranes. The TMAC6PPC6 membranes reveal between a 2 and 3 times increase in λH2O, over the ATMPP system, as was one of the intentions of introducing the hexamethylene spacers in both the backbone and cationic side chains (Scheme 2). The 1H HRMAS NMR spectra for the ATMPP and TMAC6PPC6 membranes swollen in 1 N MeOH are shown in Figure 1, with the results given in Table 1. The resonances for water (δ ∼ +4.8 ppm) and the methyl group protons in MeOH (δ ∼ +3.3 ppm) are clearly observed. It is important to note that the observed chemical shifts (and measured diffusion rates) for the water environments contain contributions from methanol hydroxyl protons and free hydronium cations that are in rapid exchange with water producing dynamically averaged chemical shifts (and diffusion constants). The measured 1077

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Figure 2. 2D 1H HRMAS NOESY exchange spectrum for ATMPP (IEC = 1.48) at 298 K. The normalized cross peak buildup between (a) the associated water and free water environments, (red dashed line) and (b) the associated water and associated methanol environments (green dashed lines) as a function of temperature.

associated chemical shifts varies between −0.1 and −0.3 ppm depending on the IEC value, and become smaller with increasing λH2O or water content. At high water concentrations the Δδ between the A-H2O and F-H2O environments was only −0.04 ppm, but are still clearly resolved under HRMAS. It has been shown by 2D 1H NOESY NMR correlation experiments that the A-H2O environment corresponds to water species in close contact with the trimethylammonium cation.24 The magnitude of λH2O would suggest that the environment does not represent just the first hydration layer around the cation, but more likely reflects the rapid exchange of several solvent hydration layers around the cation or cation cluster. The AH2O and F-H2O environments must have some spatial separation as rapid chemical shift exchange between these species is not occurring (see additional discussion below). In the ATMPP AEMs the observed Δδ show a linear variation with λH2O (Figure S1, Supporting Information), which is also consistent with the model of a local hydration layer around the ammonium cation. The 1H NMR chemical shift temperature dependence for the A- and F-H2O environments was also evaluated (Figure 2S, Supporting Information). The F-MeOH 1 H chemical shift revealed no temperature variation, and provided an internal reference (very similar to using MeOH solutions for temperature calibration in solution NMR). The water environments have a decreased (upfield) chemical shift with increasing temperature, which results from the disruption of the local hydrogen bonding environment. For the F-H2O the

chemical shift temperature variation was nearly identical for the different IEC values (Figure S2, Supporting Information). The chemical shift temperature variation in the A-H2O environment is marginally lower (suggesting a smaller disruption of local hydrogen bonding with temperature), but approaches the behavior of F-H2O with increasing IEC and λH2O. The 1H NMR chemical shift trends for 1 N MeOH swollen TMAC6PPC6 membranes were not as pronounced. The AMeOH resonance was still shifted upfield shifted from the FMeOH environment, but with Δδ < −0.1 ppm. The A-H2O and F-H2O environments in the TMAC6PPC6 membranes show even smaller chemical shift differences (−0.03 ppm), with the A-H2O species being downfield (higher δ). The differences in the 1H HRMAS NMR spectra for the IEC = 2.27 and 2.60 membranes, even though they have similar λH2O, are presently not fully understood. The small Δδ values observed in the TMAC6PPC6 membranes again demonstrates the power of HRMAS in resolving different polymer solvent environments. The water uptake and λH2O values in the TMAC6PPC6 membranes are 2−3 times larger than in the ATMPP membranes, and the reduced Δδ likely reflects the increased water content and averaging of the water/methanol associated with the trimethylammonium cations. The 1H NMR chemical shift temperature variations in the TMAC6PPC6 membranes were not evaluated. The ability to resolve different water and methanol environments in the 1H HRMAS NMR spectra demonstrate 1078

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Figure 3. Echo decays for the 1H HRMAS NMR PFG diffusion experiments in the ATMPP (IEC = 1.79) membrane at 298 K as a function of the diffusion time Δ.

normalized peak integrations were evaluated as a function of mixing time using the empirical function.

that on the NMR chemical shift time scale rapid exchange between the associated and free environments is not occurring. For example, in the ATMPP (IEC = 2.35) membrane the Δδ was 0.04 ppm (Δν ∼ 24 Hz) between A-H2O and F-H2O, requiring the exchange rate (k > πΔν/√2) be slower than ∼53 s−1. The Δδ for the MeOH species was ∼0.28 ppm (Δν ∼ 168 Hz), putting an upper limit of that exchange rate at ∼373 s−1. These slow exchange rates argue that the different resolved solvent environments are not spatially adjacent (such as within the same hydration layer) where rapid exchange is expected to occur. The exchange between these different solvent environments was further explored using 2D 1H NOESY HRMAS NMR experiments. An example is shown in Figure 2 for the ATMPP (IEC = 1.48) membrane (τmix = 200 ms, 298 K) were numerous cross-peaks were observed and result from either dipolar interactions or chemical exchange. These nondiagonal cross-peaks include correlations between the free water and free methanol and the associated water and associated methanol environments (green dashed lines), the free and associated water environments (red dashed lines), and the free and associated methanol environments (blue dashed lines). No cross-peaks were observed between the free water and associated methanol, or the associated water and the free methanol. The normalized NOESY cross-peak buildup rates as a function of mixing time at different temperatures were evaluated attempting to distinguish between NOE throughspace dipolar correlations and exchange process. The

ICP(τmix) = 1 − exp[−στmix] IDiag(τDiag )

(10)

The NOE buildup rates (σ) are predicted to decrease at higher temperature due to motional averaging of the dipolar interaction, while for exchange (transport) processes the buildup rates were predicted to increase with temperature. Finally, if the magnetization exchange process is dominated by spin-diffusion with the polymer membrane σ is predicted to be less sensitive to temperature changes. The normalized NOESY cross-peak buildups between the A-H2O and F-H2O are shown in Figure 2a and reveal a slow and very similar behavior as a function of temperature (σ ∼ 1 s). The σ values suggest that magnetization transfer for these correlations were dominated by spin diffusion effects within the membrane. The cross-peak buildup between the A-H2O and A-MeOH becomes smaller with increasing temperatures suggesting a NOE driven magnetization exchange process consistent with these water and methanol species being in close spatial contact with each other. The diffusion (transport) properties for the different solvent environments present in the AEM were also investigated using 1 H HRMAS NMR PFG diffusion experiments. The meansquared displacements (⟨ΔzM2⟩) were evaluated for each 1079

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Figure 4. Mean-squared displacement as a function of diffusion time for the different solvent environments at 298 K, (●) F-H2O, (blue ●) FMeOH, (red ●) A-H2O, (green ●) A-MeOH. The mixed colored symbols for the IEC = 2.27 TMAC6PPC6 membrane represent averages over Fand A- environments.

Table 2. Diffusion Rates and α Parameter Obtained from 1H HRMAS PFG Diffusion Studies for the Different Water and Methanol Environments in AEM Membranes at 298 K F-H2O

a

A-H2O

F-MeOH

A-MeOH

sample

Dα (m /s)

α

Dα (m /s)

α

Dα (m /s)

α

Dα (m /s)

α

ATMPP (1.48) ATMPP (1.79) ATMPP (2.35) TMAC6PCC6 (2.13) TMAC6PCC6 (2.27) TMAC6PCC6 (2.60)

1.43E−9 1.64E−9 1.37E−9 1.73E−9 ⟨1.31E−9⟩a

0.96 0.96 0.97 1.0 0.97

2.34E−10 4.02E−10 6.75E−10 7.65E−10 − 7.62E−10

0.74 0.95 0.98 0.95 − 0.90

1.08E−9 1.29E−9 9.79E−10 1.17E−9 ⟨7.92E−10⟩a

0.97 1.0 1.0 1.0 1.0

1.38E−10 1.73E−10 2.97E−10 3.72E−10 − 2.74E−10

0.92 1.00 0.94 0.93 − 0.80

2

2

2

2

Average over free and associated environments.

diffusion time Δ value as a function of q2 according to eq 8. This analysis was performed for the different membranes and IEC values for each of the resolved environments. Figure 3 shows the results for the ATMPP (IEC = 1.79) membrane at 298 K. For short Δ values it was always possible to utilize the complete PFG echo decay, while at longer Δ values deviations from simple linear free diffusion behavior were observed (i.e., restricted diffusion) in some samples and temperature. In that case only the initial portion of the echo decay was utilized to

evaluate ⟨ΔzM2⟩. From Figure 3, it is apparent that F-H2O has the largest ⟨ΔzM2⟩ (fastest diffusion rate) and A-MeOH has the smallest ⟨ΔzM2⟩ (slowest diffusion rate). The echo decay data for the remaining samples and temperatures were similar, but data are not shown. In Figure 4, the measured mean-squared displacements as a function of Δ on a log−log scale are shown for the ATMPP and TMAC6PPC6 membranes (all IEC values) at 298 K. The Dα and α values were evaluated from these correlations according 1080

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Figure 5. Temperature dependence of the diffusion rate Dα for the different solvent environments, (●) F-H2O, (green H2O (blue ■) A-MeOH in AEMs at different IEC values.

to eq 8 and are presented in Table 2. This use of multiple Δ values to estimate Dα is a preferred method for these AEMs because a range of different diffusion rates were present due to the multiple resolved solvent environments and temperatures evaluated, such that the general method of optimizing the analysis with a single Δ for determination of D via the Stejskal− Tanner formula28 (eq 3) becomes troublesome due to the rapid echo decay for some signals and incomplete decay for other environments at a single Δ value. By utilizing the data from multiple Δ values the resulting Dα values were more consistent.

▲)

F-MeOH, (red

▼)

A-

The mean-squared displacement analysis was performed for all membranes and IECs at three additional temperatures and show similar trends to those at 298 K shown in Figure 4 (data not shown). For all membranes and temperatures the following trends in the mean-squared displacement ⟨ΔzM2⟩ were observed: F-H2O > F-MeOH > A-H2O > A-MeOH. The larger displacement (faster diffusion) of H2O versus MeOH is expected purely on differences in molecular size, while the larger displacement (faster diffusion rate) for the free versus associated environments allowed assignment of the associated environments. For the majority of the ATMPP and 1081

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TMAC6PPC6 membranes the calculated α > 0.95−1 (Table 2), and are consistent with the diffusion processes being averaged and unrestricted over the length scale of the PFG diffusion experiment. The diffusion length scale is given by lD = (2DαΔ)1/2, and for Dα ∼ 1 × 10−9 m2/s, Δ = 200 ms, corresponds to lD ∼ 20 μm. These PFG NMR experiments therefore explore diffusion on a length scale several orders of magnitude larger than the local cluster or pore size expected in the membranes. The exception to these free diffusion results was observed for the A-H2O environment in the ATMPP (IEC = 1.48), which showed α ∼ 0.74, which approachs the 2D percolating cluster limit (α ∼ 0.7). For this membrane, raising the temperature increased α to ∼0.8 at 308 K, and finally α ∼ 1.0 at 313 K. This membrane has an intermediate hydration level (λH2O = 60) and at room temperature appears to show anomalous diffusion which diminishes at higher temperatures, and could ultimately impact the performance of this membrane. Similar behavior of α parameter for the TMAC6PPC6 (IEC = 2.60) membrane was observed at 298 K, but quickly approached the free Gaussian limit with increasing temperatures. The water and methanol diffusion rates observed in the ATMPP and TMAC6PPC6 membranes are similar to those observed in other AEM membranes, but depend on the actual λH2O and temperatures being analyzed. Addition information was obtained by evaluating ratios of measured diffusion rate, as presented later. The temperature dependencies of Dα as a function of inverse temperature are shown in Figure 5, from which activation energies (Ea) were evaluated (Table 3).

Dα(H2O)/Dα(MeOH) for the associated environments in both the ATMPP and TMAC6PPC6 membranes are shown in Figure 6. For 1 N MeOH this ratio is ∼1.3, revealing that the diffusion

Figure 6. Ratio of water/methanol self-diffusions rate in the membrane associated environment as a function of temperature.

rate of methanol is about 30% slower than the diffusion rate of water. In the AEMs, this ratio varied from 1.6 to 2.8, demonstrating that all of these AEMs selectively reduce the self-diffusion rate of methanol with respect to water. There is some scatter in the Dα(H2O)/Dα(MeOH) depending on the IEC level, but there is no clear improvements in the reduction of methanol transport for the TMAC6PPC6 versus the ATMPP membranes (or vice versa). It does not appear that modification of the polyphenylene backbone with the hexamethylene (C6) spacer made a significant change to the methanol diffusion selectivity. Another ratio used for characterization involves Dα for free (F) and associated (A) environments compared to D of H2O or MeOH in the bulk 1 N MeOH solution, and is shown in Figure 7, parts a (free environments) and b (associated environments). For the free environments this ratio ranges from ∼0.38 to 0.84, with the ratio for H2O always being higher than the ratio in MeOH. The distribution range also increases with increasing temperature (Figure 7a). No distinct trends separate the ATMPP from the TMAC6PPC6 membranes, nor were there any trends as a function of IEC. Figure 7a does show that the diffusion rates for “free” water and “free” methanol environment are about 2 times slower (Dα/D1N ∼ 0.5) than these components in the bulk 1 N solution. This supports the previous argument that the “free” environment is within the membrane and has some interaction with the polymer leading to a reduction (minor) of the self-diffusion rate constant. This solvent−polymer interaction may either involve surface interactions, or reflect H2O/MeOH that reside in pores or channels but are not in close contact with TMA+ cations, or possibly exist in regions with low TMA+ concentrations. The Dα/D1N ratios for the A-H2O and A-MeOH environments (Figure 7b) range from ∼0.05 to ∼0.4, corresponding to diffusion rates about 5 times slower (Dα/D1N ∼ 0.2) than in the bulk 1 N MeOH solution. Though small, there were a couple of trends observed for the associated environments. The Dα/D1N ratios get larger (diffusion rates increase) for increasing IEC in both the ATMPP and TMAC6PPC6 membranes. This reflects

Table 3. Diffusion rate activation energies obtained from 1H HRMAS PFG NMR Ea (kJ/mol) sample (IEC)

F-H2O

A-H2O

F-MeOH

A-MeOH

1 N MeOH ATMPP (1.48) ATMPP (1.79) ATMPP (2.35) TMAC6PCC6 (2.13) TMAC6PCC6 (2.27) TMAC6PCC6 (2.60)

17.5 13.5 20.0 17.9 25.2 −

− 29.6 16.6 19.3 22.3 15.6 25.2

18.2 19.1 17.6 18.2 26.0 −

− 15.9 19.8 19.6 20.6 11.1 25.3

These water and methanol activation energies are very similar to the 20 kJ/mol reported for methanol diffusion in Nafion.12 The activation energies between 25 and 30 kJ/mol compare well to the water diffusion activation energies reported for the Selemion styrene−vinyl benzene copolymer AEM membranes,21 but do not approach the 44 kJ/mol activation energies observed in the Neosepta AEM membrane.21 The lack of other reported activation energies, especially for different solvent environments, highlights an area for future membrane transport characterization. Evaluation of ratios of measured diffusion rate gives additional information about the transport processes. To improve the AEM performance, it is desirable to maintain high hydration levels with rapid water diffusion rates to allow active transport of hydroxide anions. For AEMs being applied as separators they should also demonstrate a reduced methanol diffusion rate to help prevent methanol crossover within the membrane. AEMs developed as catalyst binders should have high diffusion rates for both water and methanol. The ratio 1082

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Figure 7. Ratio of Dα versus D1N (1 N MeOH) versus temperature for the (a) F-H2O environment in the ATMPP (red ●, IEC = 1.48), (green ●, IEC = 1.79), (blue ●, IC = 2.35) and TMAC6PPC6 (red ○, IEC = 2.13), (green ○, IEC = 2.27) membranes and the F-MeOH environment in the ATMPP (red ▼, IEC = 1.48), (green ▼, IEC = 1.79), (blue ▼, IC = 235) and TMAC6PPC6 (red ▽, IEC = 2.13), (green ▽, IEC = 2.27) membranes. (b) Dα/D1N ratios for the A-H2O and A-MeOH environments.

and methanol (F-MeOH) and membrane-associated water (AH2O) and methanol (A-MeOH). By coupling HRMAS NMR with PFG experiments the diffusion rates for these four different environments were evaluated. It was demonstrated that both the ATMPP and TMAC6PPC6 preferentially reduced the diffusion rate of MeOH with respect to H2O for the entire range of temperatures investigated. There was not any significant difference between these two membranes with respect to the reduction of the MeOH diffusion rate. The water diffusion rates were reduced by a factor of between 2 and 5 within these membranes in comparison to the bulk 1 N MeOH diffusion, with higher diffusion rates observed for increasing IEC and λH2O values. The TMAC6PPC6 membranes had the highest H2O diffusion rates resulting from changes in the backbone architecture and the increased solvent content. The observation of heterogeneous solvent diffusion in polymers is not unprecedented, and has been argued for different polymer systems including naphthalenic copolyimide,37 poly(ether sulfone),35 styrene/divinylbenzene copolymers,21 and Nafion34,38 membranes. As anticipated, these results show that the diffusion rates are greater within TMAC6PPC6 membranes than they are within ATMPP membranes with similar IEC values. Thus, the performance of a fuel cell using these or other similar membranes as binders of the electrode catalyst should improve by increasing the flexibility of the polymer backbone and/or by increasing the mobility of the tethered cationic groups. This is an important first step in optimizing the electrode design for alkaline fuel cells although other factors such as polymer stability and self-organization of the membrane into ordered domains are key criteria that must also be understood to achieve the desired fuel cell performance. These studies also demonstrate the power of HRMAS PFG NMR to investigate diffusion in complex polymer membrane systems. These results clearly show that the diffusion process in AEM membranes is not the homogeneous process commonly portrayed, but instead is a very heterogeneous process involving different environments and a range of different diffusion rates.

the increasing number of waters per cation (λH2O) inherent with the increasing IEC. Both the A-H2O and A-MeOH environments in the TMAC6PPC6 membranes have higher Dα/D1N ratios (faster diffusion) than the ATMPP membrane, reflecting the addition of the C6 spacer in the backbone or the C6 side chain spacer for the TMA+ cation, and the corresponding higher λH2O values. The observed Dα/D1N ratios do provide evidence that the FH2O and F-MeOH represent environments with significant solvent−polymer interactions, and they do not represent a polymer-segregated solvent domain resulting during MAS. There are concerns about the possibility of the free-solvent environments arising from the penetration of water and methanol into the polymer membrane due to centrifugal forces. A recent study showed that water migrates into Nafion membranes during MAS spinning, probing polymer environments not normally accessible to water penetration.36 This water penetration occurred mainly at high MAS speeds (12 kHz) or extended periods of spinning. For the AEM membranes described in this manuscript, the MAS speeds were always maintained at 4 kHz or below, and there were no observed spectral changes over extended MAS (e.g., weeks), or following elevated temperature studies. In addition, individual water and methanol resonances were resolved even at slower 1 kHz MAS spinning, and for the ATMPP (IEC = 1.48) membrane static PFG NMR diffusion experiments revealed a biexponential decay for the water environment with diffusion rates on the same order of magnitude as those obtained using PFG HRMAS NMR. This does not fully dismiss the effects of MAS centrifugation as the membranes, as the multiple broad overlapping resonances observed for the water or methanol environments in the TMAC6PPC6 membranes (Figure 1) are very similar in appearance to those reported during the Nafion study,36 and warrant further investigations.

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CONCLUSION H HRMAS NMR has been used to characterize a series of ATMPP and TMAC6PPC6 anion exchange membranes with different IEC values, swollen in 1 N MeOH solution. Four distinct solvent environments were observed for all membranes, and were assigned to “free” for rapidly diffusing water (F-H2O) 1

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ASSOCIATED CONTENT

S Supporting Information *

Variation of the 1H NMR chemical shifts as a function of λH2O and temperature for the ATMPP membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(T.M.A.) E-mail: [email protected]. Telephone: 1-505-8441225. Fax: 1-505-844-2974. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Security Administration. This work was entirely funded by Sandia’s LDRD program.

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