Understanding Anion, Water, and Methanol Transport in a

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Understanding Anion, Water, and Methanol Transport in A Polyethylene-b-poly(vinylbenzyl trimethyl ammonium) Copolymer Anion Exchange Membrane for Electrochemical Applications Himanshu N Sarode, Yuan Yang, Andrew R. Motz, Yifan Li, Daniel Michael Knauss, Soenke Seifert, and Andrew M Herring J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09205 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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The Journal of Physical Chemistry

Understanding Anion, Water, and Methanol Transport in A Polyethylene-b-poly(vinylbenzyl trimethyl ammonium) Copolymer Anion Exchange Membrane for Electrochemical Applications Himanshu N. Sarode, a Yuan Yang,b Andrew R. Motz, a Yifan Li,b Daniel M Knauss,b Soenke Seifert,c and Andrew M. Herring a* a

b

Department of Chemical and Biological Engineering and

Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado, 80401, United States of America

c

X-ray Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, United States of America

ACS Paragon Plus Environment

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ABSTRACT Here we report the anion and water transport properties of an anion exchange membrane (AEM) comprising of a block copolymer of polyethylene and poly(vinylbenzyl trimethyl ammonium) (PE-b-PVBTMA) with an ion exchange capacity (IEC) of 1.08 meq/gm. The conductivity varied little between the anions, CO32-, HCO3-, and F- with a Ea = ~20 kJ/mol and a maximum fluoride conductivity of 34 mS/cm at 90°C and 95% relative humidity. Brconductivity showed a transition in conductivity at 60°C. Pulse Gradient Stimulated Spin Echo Nuclear Magnetic Resonance (PGSE-NMR) experiments showed that water diffusion in these AEMs is heterogeneous and affected by the anion present and is fastest in the presence of F-. We report methanol self-diffusion in this membrane and observed that it is lower than in Nafion® 117, because of the lower water uptake. This paper reports the first measurements of 13C labeled bicarbonate self-diffusion in an AEM using PGSE NMR and shows that it is significantly slower than F- self-diffusion. Back-calculation of the bicarbonate conductivity using the Nernst-Einstein equation gave a significantly lower value than the measured value, implying that bicarbonate transport involves OH- in the transport mechanism. Fourier transform infrared spectroscopy, PGSE NMR, and small angle X-ray Scattering (SAXS) indicates presence of different types of waters present in the membrane, at different length scales. The SAXS indicates that there is a water rich region within the hydrophilic domains of the polymer, which has a temperatures dependence in intensity at 95%RH.


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1. INTRODUCTION There has been a resurgence of interest in anion exchange membranes (AEMs) over the last decade for energy conversion uses and they are now being proposed in many applications for electrochemical energy conversion devices such as fuel cells,1 electrolyzers,1 redox flow batteries,2 microbial fuel cells,3 sensors, and electrochemical reactors.1 Depending on various operating conditions of the applications, AEM materials have to perform under a range of highlow humidity as well as in liquid water, alkaline conditions, and a range of (25 to >80°C) temperatures.

Although the benzyl trimethyl ammonium cation, may not have the ultimate

stability for the more extreme operating conditions,4, 5 it is certainly stable enough for study and suitable for wetter, lower temperature operation. However, the homopolymer polyvinyl benzyl trimethyl ammonium (PVBTMA) is water soluble and does not have any significant mechanical strength, because there are too many cations evenly distributed along the polymer backbone as shown in Figure 1. One approach to make a water-stable material and to impart mechanical strength to the membrane is to synthesize a block copolymer with a suitable hydrophobic block giving a morphology that can range from spherical, cylindrical or lamellar on the microscopic scale.6, 7 A detailed study of the transport properties of various ions, water, and organic species such as methanol, that could be potential fuels or products for electrochemical energy conversion, in these materials is important as the community seeks to design improved materials based on new physical insight. In all these devices we need fast ion transport, the ability to control water transport, and good barrier properties against fuels to avoid cross-over. For example, in the case of AEM fuel cells, water is produced at the anode and consumed at the cathode, and for system simplicity water diffusion should be fast so that it can be transported back from the anode to the


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cathode, at the same time the material should be a barrier to methanol. Methanol is a key fuel for direct methanol fuel cells (DMFCs) with very high energy density (6.1 kWh kg-1)1 but suffers from fuel crossover in PEMs and this could be reduced by switching to an AEM. Also it has been known that AEM fuel cells (AEMFC) running on methanol as fuel or air as oxidizer, tend to form bicarbonates and carbonates by reacting with OH- in the membrane which reduce its performance. HCO3- has a strong negative effect on fuel cell performance,8-10 the hydration radius of HCO3- is larger than OH- (ca. 4 vs. 3 Å)11 which is found to reduce the conductivity and reduce the DMFC performance. Considering the case of CO32- which has 2 charges on it, despite its larger size, there will be less effect on AEM conductivity.12 Polyethylene has been a commodity polymer for decades, because of its chemically inert nature, beneficial mechanical, and film forming properties. Polyethylene based anion exchange materials have been fabricated in the past where porous polyethylene was used as the substrate for ion conducting polymers13, 14 which reduced the water uptake of the membrane significantly. Radiation grafting vinyl benzyl groups to polyethylene backbones showed decent stability and ionic conductivity at low grafting densities and showed low water and methanol uptake.15 Crosslinked polyethylene membranes have been reported with very high (> 100mS/cm) conductivity and high thermal stability along with low water swelling where crosslinks might have helped to reduce swelling in the membrane.16 Various synthetic routes have been used to achieve cation functionalization and good mechanical properties for polyethylene based AEMs.17,

18

Many

researchers have used polyethylene for synthesizing mechanically stable and solvent processable AEMs in past with quaternary ammonium cations,15,

18

but reports of polyethylene as a

hydrophobic block in AEMs are relatively less frequent.17


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We have been studying a group of phase separated AEMs comprised of a block copolymer of polyethylene and poly(vinylbenzyl trimethyl ammonium bromide) (PE-b-PVBTMA).19,

20

The

initial materials synthesized had poor mechanical properties so we made a polymer with a high MW and a large polyethylene block with adequate IEC. This material had adequate ionic conductivity, and improved mechanical properties, but these were highly dependent on the water content in the film.15 In this report we use the AEM as a model system to try to elucidate the ionic and water transport mechanisms and relate them to chemistry and morphology. We studied the effect of the anion type on water uptake, conductivity, and the chemistry of the AEM by FTIR. Pulse gradient stimulated spin echo nuclear magnetic resonance (PGSE-NMR) has been used previously to study water and alcohol transport in AEMs.21-24 In this work we took advantage of multinuclear 1H,

19

F, and

13

C PGSE NMR experiments to understand water,

fluoride, methanol, and bicarbonate transport in this AEM. By studying the self-diffusion of these species using PGSE NMR, new physical insights into the ion transport mechanisms are elucidated. We also measured the small angle x-ray scattering (SAXS) as a function of temperature to understand morphological changes occurring in the membrane.


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Figure 1 : Schematic of PE-b-PVBTMA[Br-] showing clustered water, reducing membrane swelling drastically 2. Experimental 2.1 Materials and membrane fabrication The polyethylene-b-poly(vinylbenzyl) bromide polymer was synthesized as described previously and had a Mw of 70 kg/mol and a ratio of polyethylene to polyvinylbenzylbromide of ca. 70:30.19 A 10% wt/wt solution of the polymer was made in xylenes by heating to 90°C. The solution was drop cast on a glass plate held at 80°C and vacuum dried at 80°C overnight to remove the xylenes solvent. The films were peeled off from the glass plate and soaked in 25% aq. trimethyl amine solution for 48 h for quaternization of the ammonium group. 13C labeled sodium bicarbonate was purchased from Cambridge Isotopes Labs. 2.2 Ion Exchange Procedure In general, the membranes were exchanged to the corresponding ionic forms by soaking in a solution of the sodium or potassium salt of the chosen anion for 24 h. The membranes were thoroughly washed in DI water before study. Membranes were soaked in 2.5M benzyl trimethyl ammonium fluoride solution for 3 days under a pressure of 500 psig for exchanging to the fluoride form (Parr Instruments, high pressure compact reactor). A modified technique had to be used to convert the membrane to was found that mixture of

13

13

C labeled bicarbonate form. From our early experiments it

13

C labeled bicarbonate readily exchanges with CO2 from air and we get a

C and

12

C bi/carbonates. To overcome this issue, 1M NaH13CO3 solution was

prepared in the glove box using UHP N2 degassed DI water. The membrane was then soaked and rinsed and a 10mm NMR tube was prepared in the glove box to avoid any 12CO2 contamination from atmospheric air.


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2.3 Ion Exchange Capacity Measurement The ion exchange capacity (IEC) is the number of cations (here quaternary ammonium cations) in the membrane per gram of the polymer. IEC measurement was performed using Mohr titration. The membrane was vacuum dried and its dry weight was measured. The dried membrane was soaked in 1M KBr for at least 48 h to make sure all ions are bromide. Then the membrane was soaked in 1M NaNO3 for 48 h in 30 ml solution. 3 aliquots of this solution were titrated with AgNO3 with K2CrO3 as an indicator. End point of this titration is indicated by presence of red colored particles in the solution. 2.4 Ionic Conductivity Measurements In plane ionic Conductivity was measured by electrochemical impedance spectroscopy using a 4 platinum electrode setup. The following equation was used for calculations,

σ =

l R.w.t

(1)

where l is the length of the membrane, R is the resistance of the membrane, w is the width of the membrane and t is the thickness of the membrane. Impedance spectra were obtained over a wide range of frequencies, from 0.5 MHz to 1 Hz using a 16 channel VMP3 potentiostat (Bio Logic Scientific Instruments). Samples were equilibrated in a Test Equity oven (Model 1007H) at a given temperature and humidity. 2.5 PGSE NMR Measurements: Self-diffusion coefficients of fluoride, methanol and water were determined via PGSE NMR. The diffusion coefficients were determined by fitting the measured data to the Stejskal -Tanner equation25


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 S   δ   = exp  − γ 2 G 2δ 2  ∆ − 3    S0 

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(2)

  D   

where S0 is the signal amplitude, γ is the gyromagnetic ratio, G is the gradient strength, δ is the length of gradient pulse (1 ms) and ∆ is the time between pulses. The experiments were carried out using a Bruker AVANCE III NMR spectrometer and 400 MHz (1H frequency) wide bore Magnex magnet. 19F (376.02 MHz) and 1H (400 MHz) diffusion measurements were made using a 5 mm Bruker single-axis DIFF60L Z-diffusion probe while 13

C diffusion measurements were carried out with a 10mm NMR tube and a 10mm

13

C coil

(100.48 MHz) without proton decoupling. The 90° pulse length was on the order of 5 µs. The range of gradient strength was 0-1000 G/cm, which was incremented in 16 steps. The maximum value of the gradient was chosen such that the signal decays completely. The Bruker TopSpin software package was used to control the spectrometer and to analyze the data. The NMR tube was maintained at saturated conditions of humidity by the water at the bottom. A schematic of a typical NMR tube can be found in the supplementary information (Figure S1) of our previous work.26 Membranes were soaked in water and excess surface water was wiped off with Kimwipes before rolling and inserting in a NMR tube. For methanol diffusion study, membranes were soaked in 2M 13C labelled methanol (13CH3OH) and excess was wiped off and then it was rolled in a NMR tube. 2M methanol was kept at the bottom of the NMR tube to maintain saturated humidified conditions in the NMR tube. 2.6 Small Angle X-ray scattering (SAXS) SAXS was performed using a custom built temperature and humidity controlled oven, its details are described in the earlier work.27 The experiments were performed at beamline 12-ID-C


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at Advanced Photon Source at the Argonne National Laboratory, Argonne, IL, USA. The x-ray energy was fixed at 12keV. The SAXS spectra were collected as temperature was increased from 40°C to 90°C. Temperature was increased in 10°C increments, the humidity chamber was maintained at 95% RH during the whole duration of the experiment. 2.7 Fourier Transform Infrared Spectroscopy (FTIR) FTIR was measured using a Thermo Nicolet Nexus™ 470 FT-IR instrument with a liquid nitrogen cooled MCT detector in attenuated total reflectance mode with 256 scans. The spectra were collected as a function of RH and anion at 60°C. The RH was increased from 0 to 90% RH in 10% increments, and equilibrated for 45 mins at each humidity. A detailed experimental setup can be found in the earlier work.28

3. Results and Discussion 3.1 Membrane properties Solvent casting the bromide form of the polymer from solution, followed by quaternization with trimethyl amine, led to transparent and thin (20-35 µm) AEMs, as shown in Figure 2, which also gives a general chemical structure of the polymer. The titrated IEC of the membrane was found to be 1.08 meq/gm. For comparison purposes this same material was studied previously by us with a slightly higher IEC 1.21 meq/gm and a film thickness of 12 ± 3 µm, the IEC being slightly higher due to the higher quarternization efficiency of a thinner film.20 We can infer some of the properties of the material from the original study using different block lengths and similar IECs.19


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Figure 2 : (a) Image of the drop cast PE-b-PVBTMA[Br-] membrane showing transparent, , (b) Chemical structure of PE-b-PVBTMA polymer with 70% PE and 30% PVBC 3.2 Ionic conductivity Figure 3 shows the in-plane conductivity of the PE-b-PVBTMA AEM under humidified conditions (95%RH) from 30°C-90°C. The native bromide form of the membrane shows conductivity in the range of 1-20 mS/cm from 30 to 90°C, which is typical for the bromide form of many AEMs.29-31 Recent improvements in our conductivity measurements allowed us to reevaluate the bromide conductivity of these membranes.20 We now find that bromide conductivity doesn’t follow an Arrhenius behavior, but shows a transition at 60°C, and has an unexpectedly low value at 30°C. The fluoride, carbonate, and bicarbonate forms of the membrane show more or less similar conductivity (6-34 mS/cm) in the range of 30-90°C. These values are somewhat impressive considering that the film only has an IEC of 1.08 meq/g. It was found that fluoride, bicarbonate, and carbonate ionic conductivity does follow an Arrhenius behavior, above 40°C with increasing temperature with similar Ea of ~20kJ/mol. At 50°C the conductivity of fluoride ions is about 1.75 times higher than bromide. It can be seen from Figure 3 that the conductivity of all the ions is not linear on the Arrhenius plot at lower temperatures, possibly suggesting different ion transport mechanisms at lower temperatures or lower water


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uptake at lower temperatures. The addition of a polyethylene block to the polymer makes the membrane mechanically robust even at elevated temperatures up to 90°C compared to PVBTMA homopolymer which gels when in contact with liquid water. Fluoride conductivity was found to be highest at 34 mS/cm at 90°C and 95% RH for this AEM compared to other halides, presumably because of its small ionic size, previous researchers have also reported similar behavior for conductivity in membranes.32

Temperature (°C) 97 Ionic Conductivity (S/cm)

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84

72

60

49

39

30

3.2

3.3

95% RH

0.010

0.001 2.7

Bicarbonate Fluoride Carbonate Bromide

2.8

2.9

3.0 3.1 -1 1000/T (K )

Figure 3: Conductivity of PE-b-PVBTMA membrane in bicarbonate, fluoride, carbonate and bromide form We also attempted to measure hydroxide conductivity by converting the film to its hydroxide form in a CO2 free glove box, but we were not able to get reasonable conductivity under humidified CO2 free conditions, suggesting rapid degradation of the cationic moieties. From our experience on previous membranes and this membrane we have found that membranes which are


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synthesized with bromination chemistry tend to be less stable in hydroxide environments and lead to degradation in presence of hydroxide ions. This is because of the electron withdrawing nature of bromine which causes polymer degradation which has been observed in our lab and also published previously by Arges et. al.33 Table 1: Activation energies for PE-b-PVBTMA Anion

Ea, ion (kJ/mol)

Ea, water (kJ/mol)n

Fluoride

20.9 ± 0.3c, 20.8n

7.6

Bromide

n/a

12.2

Carbonate

19.4 ± 0.9c

Bicarbonate

20.4 ± 1.6c, 23.8n

c

10.5

from conductivity, nfrom diffusion NMR

3.3. Water Uptake Water uptake for the membrane was measured with fluoride, bicarbonate, carbonate, and bromide, as the counterion Figure 4. At 95%RH it was found that the λ (no. of water molecules per quaternary cation) for the membrane was lowest when the counter ion was bromide λ = 7.9 ± 0.3, which increased to 11.3 for the smaller fluoride anion with the bicarbonate form of the membrane λ = 10.4, and carbonate with λ = 9.5. Compared to commercial proton exchange membranes such as Nafion® 117

34

(λ= 14 @ 30°C) our reported values of λ are lower and

membrane swelling is much less. The membrane strength and dimensional stability is important from a practical fuel cell point of view and it is advantageous to have lower water uptake, which can result in higher dimensional stability and higher mechanical strength. The promising mechanical properties of this polymer material have been studied in detail previously.20


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Water Uptake @ 60°C

Water Uptake (%)

12

20

10

15

8 6

λ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 4

5

2

0 0%

20%

40%

60%

80%

0 100%

Humidity Figure 4: Water uptake for the AEM using Dynamic vapor sorption apparatus as a function of anion at 60°C, Fluoride (

), bicarbonate (), carbonate () and bromide ()

3.4 Self-diffusion coefficients In general NMR PFGSE methods are limited to the protons in the samples as most PFGSE probes are tuned to 1H. Whilst this gives much useful information the protons exchange rapidly between water, hydroxide and hydroxyl groups on the NMR time scale and so selfdiffusion coefficients for these species cannot be distinguished. In this study we employed gradient coils tuned to 19F and 13C, which enabled us to further distinguish fluoride, carbonate, or methanol self-diffusion (the latter two using 13C labeled moieties) from 1H self-diffusion in the samples studied. Whereas the lower bromide conductivity can be explained by the anions larger


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size compared to fluoride and lower water uptake compared to the other anions studied, we were motivated to learn why the conductivity of the fluoride, bicarbonate, or carbonate anions were so similar. Additionally by taking advantage of 19F and 13C being NMR active nuclei, and labeling with 13C, we were able to differentiate fluoride and bicarbonate from water diffusion and begin to consider the self-diffusion of molecules such as methanol, which is an important fuel candidate for electrochemical energy conversion as shown in Figures 5 and 6. NMR diffusion experiments reflect the global material average over local transport properties of the polymer membrane.35 Water diffusion studied in this work was fitted to a two parameter Gaussian decay, which gave two values of self-diffusion coefficients corresponding to two different water environments in the membrane. The presence of water heterogeneity and two different types of waters was found previously in a few AEMs.26, 36 Figure 6 shows the self-diffusion of the slow moving water or bound water in the membrane in the different anionic forms. Figure S1 in the supplementary information shows the faster component of the water diffusion. It was found that the fast moving water was about one order of magnitude higher compared to the slow moving water, a little less than for free water, and that in almost all cases there is a decreasing dependency on the self-diffusion coefficient with the diffusion evolution time indicating restriction of this water in the membrane. Hibbs et. al. also suggest that even if there is free water present in the membrane it interacts with polymer chains and its mobility reduces in the pores of the polymer membranes.23 Here we assume that the slower moving water is associated with the cation/anion pair and that the faster moving water exists in larger pores in the film. The error in this data and some anomalous data sets where recorded for this faster moving water, suggesting that it is not so strongly correlated with the ion pairs and so we do not considered it further in the analysis. Figure 6(a) shows the slower water self-diffusion in the PE-b-


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PVBTMA[Br-] membrane, the value of self-diffusion coefficient (D) is 0.19 x 10-5 cm2/sec @ 50 ms and 30°C which is in agreement with earlier results for AEM materials and is an order of magnitude lower than the self-diffusion coefficient of free water.22,

23

The slower water self-

diffusion coefficient was found to be a function of the anion, being highest for the fluoride form of the membrane, followed by the bicarbonate form, and was the least in the bromide from. This shows that bulkier anions hinder the transport of the slower moving water in the membrane. D monotonically decreases with increasing diffusion time, a typical phenomenon that characterizes the restricted diffusion process inside polymer membranes due to the presence of local barriers (structural boundaries) sampled by diffusing molecules.35 The Ea for the fluoride, bicarbonate, carbonate water self-diffusion is shown in Table 1, as are the data gave a linear Arrhenius plot and can be compared to the Ea for ionic conductivity, Table 1. Water diffusion varies from a Ea = 7.6 kJ/mol for fluoride to 12.6 kJ/mol in the bromide form (compared to ~15 kJ/mol in a typical PFSA37). Intriguingly while water moves quite freely in the fluoride exchanged film, the Ea for fluoride transport measured by

19

F PFGSE NMR calculated from the data in Figure 5 (b) and ionic

conductivity is higher (~ 20kJ/mol). We attribute this to fluoride’s extended hydration shell which must hinders ion transport during diffusion and conductivity measurements.24 Table 2 reports the Ea for water when it’s moving in the presence of different anions. The remaining water, not coordinating to the anion has a very low barrier to diffusion, Ea = 7.6 kJ/mol. We used 13C labelled bicarbonate (this being one of the first reports), Figure 5 (c) to compare this anion to its ionic conductivity and water self-diffusion. We observe that bicarbonate selfdiffusion is not affected by changing diffusion time showing that we have already achieved the final restricted self-diffusion coefficient value on the NMR timescale. The average self-diffusion


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coefficient for bicarbonate was 0.11 x 10-5 cm2/sec at 30°C which is significantly lower than for fluoride. We attribute this lower self-diffusion of bicarbonate species to its bulkier nature. The Ea for bicarbonate conductivity and self-diffusion was 20.4 and 23.8 kJ/mol respectively. We also calculated the ionic conductivity of the bicarbonate anion using the self-diffusion coefficients obtained from PFG-NMR and showed that the calculated conductivity is lower than the experimentally measured value. The bulkier nature of the bicarbonate ions leads to lower diffusion coefficients but does not affect the ionic conductivity to the same extent. This enhances the argument that ion transport in the bicarbonate form is always mediated by some hydroxide ions38, 39 which can move via the more freely moving protons (water and hydroxide), Ea =10.6 kJ/mol, Table 1, by both Grotthus and vehicular transport mechanisms. The self-diffusion of 2M methanol was found to be 0.2 x 10-5 cm2/sec @ 100 ms and 30°C, which is lower than some of the previously reported work in aminated tetramethyl polyphenylene based AEMs.23 This lower methanol diffusion is a result of lower water uptake of this AEM (~20%) compared to Nafion® 117(~100%). Methanol self-diffusion has an Ea of 24.8 kJ/mol which is in agreement with earlier work with Nafion®40 and sulfonated polyarylene thioethersulfone (SPTES) polymer membranes41 (~24 kJ/mol) indicating similar ion/solvent transport or similar morphological features for this polyethylene membrane. Researchers in the past have exploited 1H PGSE NMR to monitor diffusion of water and methanol as both of these species have different NMR shifts, our attempts to take advantage of this method were unsuccessful because the methanol and water environments were not resolvable and thus we had to move to 13C NMR which limited out ability to monitor the movement of only methanol. Hibbs et. al.23 and Jayakody et. al.41 found that water diffusion is faster than methanol when present in a


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mixture. Here we compared water and methanol diffusion studied separately as shown in Figure 5 (a) and Figure 6 (a) and we can see comparable self-diffusion coefficients. A full analysis of the water self-diffusion was used to understand morphology of the polymer in detail, Mitra’s equation was applied to the self-diffusion data to obtain the V/S ratio of the pores present in the membrane. If the water diffusion scale length is comparable to the domain size, a relatively large fraction of water will move slowly as it collides with the channel boundaries. Large S/V ratio signifies small domains and its reciprocal Rc = V/S, denotes the structural length scale below which water molecules can move relatively freely.35 The average value of the pores is around 2.3 µm, thus we think that this value is a conglomeration of different ionic channels clustering together. Tortuosity is defined as a ratio of D0 and D∞, = ⁄ , where D0 is the unrestricted self-diffusion value obtained from Mitra’s equation and D∞ is the final restricted value of water diffusion at ∆ = 100ms, when diffusants experience all local structural heterogeneities. The average calculated tortuosity was around 1.6, which means that the water molecules do encounter some channel wall boundary while diffusing. The diffusion of fluoride, methanol, and water reach a constant value of around 0.18-0.19 x 105

cm2/sec after encountering all the local structural heterogeneities, while bicarbonate reaches a

steady state value of 0.11 x 10-5 cm2/sec at larger delta values. Fluoride being the smallest of the ions, diffuse faster initially when they don’t encounter any boundaries. Figure 7 shows the decrease of T1 Relaxation (Spin Lattice Relaxation) as a function of increasing temperature for the fluoride in the AEM. T1 relaxation times probe molecular motions on a time scale of ~1/ω0 (2.7 ns), where ω0 is the

19

F Larmer frequency (376.02MHz).35 This loss of energy has to be

stimulated by nearby molecular motions of nearby atoms.42 We observed that as temperature is increased, the T1 value continued to decrease making it not possible to measure the self-diffusion


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coefficients of fluoride ions on longer time-scales, this might be because fluoride ions are losing their energy at a faster rate as we increase the temperature and we so could not measure the diffusion above 40°C

(a)

(b)

(c)

Figure 5: Self diffusion of various species in PE-b-PVBTMA, (a)

13

C labeled methanol (b)

19

F

labeled fluoride (c) and 13C labeled bicarbonate as a function of temperature and diffusion time


18

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(a)

(b)

(c)

Figure 6: Self diffusion of the water protons in different anionic forms of PE-b-PVBTMA (a) bromide (b) bicarbonate (c) fluoride as a function of temperature and diffusion time


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Figure 7 : T1 relaxation of fluoride ions as a function of temperature


20

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3.5 Infrared spectroscopy on different anionic forms of the membrane

Figure 8: FTIR spectra of PE-b-PVBTMA AEM in different anionic forms at 60°C and 50% RH We recorded the FTIR spectra as a function of anion, temperature, and RH, the most significant differences were observed by counter anion. As humidity is increased the water enters the hydrophilic domains in the polymer matrix. In Figure 8 the intensities of v(OH) and δ(H2O) modes which are associated with the water present in the membrane can be seen centered around 3400 cm-1 and 1640 cm-1 respectively. Experiments were carried out from dry to completely humidified conditions showing that the peaks due to water increased in intensity as expected, but there was little change in their position and so we only show data at ~50% relative humidity at 60°C in Figure 8. Table 2 shows the order of v(OH) as a function of anion. The fluoride form of


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the membrane showed the lowest v(OH) = 3378 cm-1, Table 2, which suggests that water is strongly bound to the ionic species in the membrane. A possible explanation for this can be the highly electronegative nature of the fluoride ion. The wavenumber of v(OH) goes on increasing in the order bromide> carbonate> bicarbonate> fluoride as shown in Table 2. This behavior has been previously observed in a model AEM where they have studied the effect of changing anions on v(OH) which was assigned to differences in hydrogen bonding.36 Intensity of these peaks at ~ 3400 cm-1 are directly proportional to the hydration level of each anion in the AEM.

36

We also

observe that the v(OH) envelope in Figure 8 (c) is composed of two distinct bands showing two types of water, one more strongly bound than the other. Fluoride being a strong electronegative ion, water hydrogen bonding is weakest and bromide being the lease electronegative and bulky ion, water hydrogen bonding is the strongest. Previously v(NH) and v(OH) have been assigned in a model AEM at about 3100 cm-1 and 3400 cm-1.36 In this work we don’t have any N-H bonds and thus the lack of peak around 3100cm-1. The peaks centered at 1640 cm-1 are assigned to the bending modes of water in the membrane. Table 2: Wavenumber of maximum peak height for Figure 8 (c) Anionic form of membrane

Maximum peak height wavelength (cm-1)

Fluoride

3378

Bicarbonate

3409

Carbonate

3452

Bromide

3465

3.6 Small Angle X-ray Scattering (SAXS) We further characterized the membrane morphology and swelling by using small angle X-ray scattering (SAXS) as shown in Figure 9. Figure 9 shows the SAXS spectra of the fluoride,


22

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bromide, and bicarbonate form of the membrane under 95% RH conditions from 40°C to 90°C. The main feature of the spectra is a peak, which is in between 0.01 and 0.02 Å-1 and a shoulder between 0.04 and 0.06 Å-1. These features are in agreement with data for the Br- form of the membrane.20 These peaks represent characteistic lengths of the hydrophilic phase of the membrane, and the lower q peak was shown to clearly correlate with the sizes observed in the TEM of the lower IEC AEM,19 which shows a bicontinuous phase separated morphology of nearly circular features, which is confirmed as the Porod slope of all SAXS patterns is ca. 4. The position of the shoulder does not correspond to a higher order peak predicted by any morphology and so indicates a second set of spherical scattering objects in the membrane, which we assign to a water rich region within the hydrophilic domains, Figure 9. This is similar to related polymer studied by us that had lamella morphology and was shown to partition the water under certain conditions.43, 44 Remarkably, with this polymeric material this second peak always seems to be present, suggesting that the hydrophilic domains are always segregated into two regions. The sizes of the two regions are shown in Table 3. The hydrophilic region is smallest for fluoride and the inner water rich region (within the hydrophilic region) is smallest for bicarbonate. Very little change was seen in either peak at 60°C with changing RH.19,

20

This was not initially

surprising to us as these films do not swell with water much and based on other studies we think significant amounts of water are in larger domains in the polymer matrix than can be probed by the q range of the SAXS. However, based on recent work by us that suggests that cations in AEMs may undergo an order-disorder transition due to their large dipoles which induce clustering44 we were motivated to see how the films behaved at high RH as a function of temperature.


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Table 3. d spacings for the two features in the SAXS patterns. Anion Larger d spacing, nm

Page 24 of 34

Smaller d spacing, nm

F-

40.3

15.0

Br-

46.7

14.9

HCO3-

46.7

11.9


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Figure 9 : SAXS spectra of PE-b-PVBTMA in fluoride (top left) bicarbonate (top right), and bromide (bottom left) form, light to dark 40°C to 90°C at 95% RH (* - Kapton® peak), cartoon of hydrophilic polymer region (bottom right). At 95 % RH the SAXS patterns clearly show a dependence on temperature, although it is not linear, as seen more clearly with the shoulder the intensity increases through 60°C, then shows little or less change to 80°C, and then increases again to 90°C. We see this plateau in the ionic conductivity for the less conducting bromide ion, Figure 3, suggesting bromide conductivity is more effected by morphology. The smaller q, peak of the larger feature, shows less organization with temperature than the shoulder. This lack of response at 60°C may explain why our previous RH dependent studies at this temperature showed little change. Based on PFGSE NMR there are two water domains, one loosely bound, which resides in larger pores of the membrane, and acts as a reservoir, and one more tightly bound in the structured hydrophilic domains of the material. Self-diffusion for the more tightly bound water has a much lower Ea than for the two anions that we can measure, fluoride and bicarbonate, leading us to suggest that the water is predominantly in the smaller water rich domain and that the anions are close to or in the larger outer polymer brush domains. For these anions the conductivity follows an Arrhenius behavior with the same and relatively good conductivity suggesting that the water in the water rich domain may enhance conductivity and decouple anionic conductivity from morphology. As the membrane is heated more water is drawn into the hydrophilic region of the film further solvating the cationic polymer brushes and the anions, the size of these domians does not change, only the amount of water.

4. Conclusions In this work we have studied a block copolymer of polyethylene and poly(vinylbenzyl trimethyl ammonium bromide). We drop cast thin (20-35 µm) membranes which were thermally


25


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Page 26 of 34

and mechanically20 stable as reported earlier. Reevaluation of the bromide conductivity shows that it doesn’t follow Arrhenius behavior, shows a transition at 60°C, and has an unexpectedly low value of conductivity at 30°C. We report that the ionic conductivity is similar for the anions CO32-, HCO3-, and F- with an Ea = ~20 kJ/mol. The fluoride form of the membrane showed a maximum conductivity of 34 mS/cm at 90°C. We studied ion, water and methanol transport in this membrane using PGSE NMR and observed that fluoride ions have the highest self-diffusion coefficient amongst all species observed. The Ea for water transport of the more tightly bound water was much lower than for the anions. We found that the HCO3- conductivity calculated by the Nernst-Einstein equation using the measured self-diffusion coefficients of 13C labeled HCO3is lower than the measured bicarbonate conductivity. This result indicates that there is a possibility of the presence of additional OH- ions which contributes towards the measured HCO3conductivity. The tortuosity of the membrane was found to be 1.6 for fluoride anion transport. We also report self-diffusion of methanol in this membrane from a 2M solution which was slower than the previously reported diffusion in Nafion® membranes. This low self-diffusion of methanol could make the membrane useful for direct methanol fuel cells or for breathable barrier layers, where organic molecule transport should be inhibited. The FTIR shows that the interaction of water in the membrane differs for each cation anion pair. This is the first successful demonstration of the measurement of bicarbonate diffusion in an AEM using

13

C

labeled PGSE NMR. Both PGSE NMR, FTIR, and SAXS showed heterogeneous water distribution on different time scales. The SAXS data suggests that there are smaller water rich domains with the hydrophilic domains of the polymer. Water is distributed between loosely bound large length scale domains, the hydrophilic domains and water rich domains within where


26

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water is free to move. Ion transport for the faster moving ions and the water in the water rich domains gives the indistinguishable conductivity values. ASSOCIATED CONTENT Supporting information contains the first component of the self-diffusion of water. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] (A. M. Herring) ACKNOWLEDGMENT The authors would like to thank the Army Research Office for support of this research under the MURI program, grant W911NF-11-1-0462, and The Colorado School of Mines NMR facility funded by National Science Foundation under an MRI grant CHE-0923537. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. REFERENCES 1.

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TOC Graphic


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Understanding Anion, Water, and Methanol Transport in a

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