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Keywords:

  • charge transport;
  • diffusion;
  • hydrogen bonding;
  • ionomers;
  • morphology

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

Water-mediated ion conduction enables high conductivity in hydrated polymer membranes commonly used in electrochemical devices. Understanding the coupling of the absorbed water with the polymer matrix and the dynamics of water inside the polymer network across the full range of length scales in the membrane is important for unraveling the structure–property relationships in these materials. By considering the water behavior in ion-containing polymers, next-generation fuel cell membranes are being designed that exceed the conductivity of the state-of-the-art materials and have optimized conductivity and permeability that may be useful in other types of devices such as redox flow batteries. Water–polymer associations can be exploited to tune the transport and mechanical property tradeoffs in these polymers. Measurements of water motion provide important criteria for assessing the factors that control the performance of these types of materials. This review article discusses current understanding of water behavior in ion-containing polymers and the relationship between water motion and ion and molecular transport. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

Polymer membranes are important functional components of low-temperature fuel cells and electrolyzers. The scientific and engineering push for alternative energy conversion and storage technologies has intensified efforts to develop new ion-containing membranes that display high conductivity (10−1 S cm−1 or greater) with low or no water absorption. However, water is an excellent medium for fast ion conduction; thus, the most successful approaches to new materials that display ionic conductivities high enough for application in devices have water uptakes of usually 10 wt % or greater. The water behavior in ion-conducting membranes underpins their transport properties, especially in tradeoff relationships between ion conductivity and molecular or ion permeability and permselectivity. In many cases, if higher conductivity is desired, more water must be absorbed into the membrane which promotes larger molecular and ion diffusion coefficients.

In these types of materials, the functionalized polymer introduces excess ions into the water phase in the membrane. The concentration of ions in the membrane contributes to the conductivity (σi) by:

  • equation image(1)

where ci is the concentration of ions that contribute to the conductivity, zi is their charge, and Di is the diffusion coefficient of the ions that give the conductivity response. For proton exchange membranes (PEMs), which are single ion conductors, the proton concentration in the membrane can be expressed as cH+. In most cases, the activity of the protons or the concentration of free protons in the membrane is not known;1 therefore, the analytical proton concentration in the membrane, as determined by titration of the sulfonate groups, is usually sufficient as an approximation to cH+ to estimate the effective proton diffusion coefficient. For a membrane with a proton conductivity of 10−1 S cm−1 and a proton concentration of about 1 M, the effective proton diffusion coefficient is on the order of 3 × 10−5 cm2 s−1, which can be compared to a proton diffusion coefficient of 10−4 cm2 s−1 in liquid water. The diffusion or mobility of protons in these materials is not quite as high as in bulk water, but most measurements of the water behavior and ion and small molecule diffusion coefficients are indicative of liquid-like dynamics.2

The polymer, in essence, provides a framework for the absorbed water. Thus, understanding the water sorption properties of these materials and studying the water–polymer interactions provides basic mechanistic information on the behavior of the membranes. The fundamental studies of water motion in these types of polymers help to rationalize the transport properties of materials with new polymer compositions or ionic domain structures. This understanding can then be used to boost the performance of next-generation materials for application in devices. Instead of designing and optimizing materials toward a nebulous goal of “higher conductivity,” considering the interplay between polymer chemical structure, adsorbed water, and ion transport may give researchers in this area an additional tool for predictive design of these systems.

The archetypal ion-containing polymer used as a PEM in fuel cells and other types of electrochemical devices is NAFION® (a product of DuPont) and its related structures, which are generally classified as poly(perfluorosulfonic acid)s (PFSAs). These perfluorinated polymers have tethered sulfonic acid groups that render the materials hydrophilic and increase the proton concentration in the membrane to achieve high conductivity. Much research has been devoted to exploring the details of the arrangement of hydrophilic and hydrophobic components of PFSAs in the solid state, particularly in the context of deciphering their proton conduction properties. This ionic domain morphology is thought to give rise to NAFION®'s high ionic conductivity, high water permeability, and other interesting properties. The nanophase morphological structure in fuel cell membranes is indeed an important factor in their performance as the ionic nanophases provide the “pipes” through which water and ions move. However, the connectivity and size of these ionic domains does not fully explain the ion conduction behavior of these materials.

Many of the new classes of ion-containing polymers designed to supplant PFSAs are based on aromatic backbones. There are distinct differences in the ionic domain structure of sulfonated poly(aromatic) membranes compared to PFSAs. Foremost, the ability of the ions to cluster in the stiffer-chained poly(aromatic) membranes is poor which leads to smaller ionic domains than what are observed in PFSAs, Figure 1. This domain structure and the chemical properties of the polymer have a direct influence on the absorbed water mobility as evidenced by the self and Fickian diffusion coefficients of water and the deuterium T1 relaxation times measured as a function of hydration for each type of material.3, 5, 6 There have been many studies of NAFION®'s ionic domain structure,7–10 and comparisons to aromatic polymers have revealed some of the structural origins of the differences in conductivity between these two classes of ion-conducting polymers.11 However, there is no direct ionic domain structure to conductivity relationship when materials with different polymer backbones are considered. Interesting insights to explain the differences in transport properties between PFSAs and aromatic polymers have arisen from exploring how the water rotational and translational motion, or in short, the water binding and dynamics, in the membrane influence the material's transport properties. Different conceptual frameworks have been used to describe the water–polymer associations in a range of hydrophilic polymers. Terms such as “state of water” or “free, loosely bound, and tightly bound” help to describe the distribution of physical and chemical environments that water experiences in a hydrophilic polymer network. In practice, an observable property of the water such as its T1 relaxation time, self-diffusion coefficient, or vibrational frequency of a characteristic mode is needed to quantify how water is held within the polymer and in many cases the water inside a polymer membrane shows different signatures than bulk water.

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Figure 1. Backbone chain schematics, transmission electron micrographs, ion concentrations, and water self-diffusion coefficients for PFSA and sulfonated aromatic PEMs.3, 4

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The preceding discussion of morphology and water dynamics in water-absorbing, ion-containing polymers is decidedly separate from polymer-mediated ion transport, for example, in Li-ion battery polymer electrolytes, where the ion motion is coupled tightly to the polymer segmental dynamics.12 Water as the medium of ion transport leads to thermally activated transport where the temperature-dependent conductivity can be described by an Arrhenius-type activation energy. For polymer-mediated ion conductors, the conductivity is often described by a Vogel-Fulcher-Tammann dependence on temperature. In many cases, the conductivity of polymer-based ion conductors can be scaled by the Tg of the material, which collapses data from polymers with different Tgs onto a characteristic curve.13 Deviations from Tg scaling in polymer-mediated ion conductors can be due to increased free ion content,14 phase separation and morphology,15 or increased density of ion donors or acceptors.16 Similar observations have been made for the temperature dependence of ionic liquids where the long-range molecular dynamics controls the temperature-dependent conductivity response.17–19

The purpose of this review is to highlight the important insights and tools that have facilitated our understanding of ion transport in water-absorbing membrane and further the discussion of water–polymer interactions in materials that rely on the presence of water for high transport rates. The relationship between water mobility and proton conductivity will be discussed. Different length-scale measurements for water mobility will be described and sections are included on the effect that water has on molecular transport properties and mechanical properties. Finally, recommendations are given for how to take the fundamental knowledge developed for proton transport in fuel cell membranes and extend these concepts to new types of ion-containing membranes and applications.

WATER MOTION AND PROTON CONDUCTIVITY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

Seminal studies on PEM fuel cell (PEMFC) materials have revealed the basic phenomena that are important to consider when dealing with proton conductivity in water-absorbing systems. The diffusion coefficient of protons, calculated from the Nernst–Einstein relationship and measured conductivity values, is greater than the self-diffusion coefficient of water, determined by pulsed-field gradient nuclear magnetic resonance (PFG-NMR) self-diffusion coefficient measurements, at high levels of hydration (Fig. 2).20, 21 When minimally hydrated, less than three to four water molecules per excess proton, the water and protons move in concert as evidenced by their similar diffusion coefficients. However, when sufficient hydration is present, the proton diffusion coefficient is much greater than diffusion coefficient of the surrounding water medium and proton “hopping” is evident. This high hydration case in a NAFION® membrane is similar to what is observed in bulk liquid water where the excess proton diffusion coefficient exceeds the self-diffusion coefficient of water by factor of 5.11

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Figure 2. Proton (▪) and water (□) diffusion coefficients as a function of hydration number. Adapted from Ref.20.

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As water is added to a membrane, the signatures of water rotational and translational mobility increase. Measurements of the 2H Tmath image and the self-diffusion coefficient of water by PFG-NMR11, 20 show that water is more mobile for higher water to acid group ratios. The water:acid molar ratio is termed the hydration number (λ or wo). It is tempting to invoke hopping-type transport when the rotational dynamics of water are slow. However, the above experimental measurements demonstrate that fast water rotation is required to achieve hopping. Otherwise, at low hydration numbers, the hydronium or H2n+1On (hydronium plus excess waters) must diffuse through the water matrix and hopping is minimized.

Water plays a special role in the context of proton conduction because of its amphotericity. The dual role of water as a proton donor (as hydronium) and proton acceptor coupled with its fast rotational dynamics imparts high ionic conductivities to water-absorbing polymers as long as the materials remain hydrated and there exists sufficient acid concentration to create a number of excess protons. Other amphoteric molecules such as phosphoric acid23 and triazole/imidazole16, 24, 25 conduct protons with a hopping-type mechanism, but their larger molecular structures and thus lower rotational dynamics and higher viscosities lead to significant depressions in their proton conductivity compared to water. The fast dynamics of proton transport in water is contrasted with the slower molecular dynamics of the polymer molecules themselves. Paddison and Zawodzinski26 studied the side chain of NAFION® and concluded that although the sulfonate group can freely rotate, the rest of the side chain is rather stiff. Also, the perfluoroether side chain of NAFION® is hydrophobic and likely exists in a coiled configuration in the hydrated polymer. Thus, the flexibility and molecular motion of the side chain likely plays a minor role in the proton conduction process. However, the topology and flexibility of the ion-containing polymer determines the ability of the ionic nanophase to self-assemble into ordered domains. The difficulty in deconvoluting the morphology is in part due to the subtle changes in morphology that can occur with even minor variations in polymer chemical structure.

In many applications, ions must move through a 10–150-μm-thick membrane, which is at least three orders of magnitude greater than the 1–10-nm-size ionic domains observed for many systems. Clearly, ionic conduction across a membrane is a multilength-scale problem from the atomic-scale hydrogen bonding dynamics between water molecules, to the ionic domains that may contain thousands of water molecules, to the micron length scales that the protons must traverse. Thus, a variety of techniques are required to probe the water motion over a range of length and time scales to more completely understand the role that water plays in mediating ion transport in these materials.

Vibrational spectroscopy has been used to probe the hydrogen bond dynamics of water bound in NAFION®.27, 28 In Falk's work, the D[BOND]O stretch in hyrogen-oxygen-deuterium (HOD) was used to quantify the hydrogen bonding environment of water in Na-form NAFION®.29 This vibrational signature is convenient as the D[BOND]O stretch appears in a relatively featureless region of the mid-IR spectrum at about 2509 cm−1, and there are no complications with interpretation of the D[BOND]O stretch as compared to H[BOND]O due to lack of Fermi resonance of D[BOND]O bonds. Moilanen et al.30 observed a high frequency shoulder for D[BOND]O in NAFION® at a frequency of ∼2708 cm−1. The water associated with this stretch was interpreted to be in strong contact with the hydrophobic portion of the NAFION® membrane. The main D[BOND]O peak frequency was dependent on the hydration of the membrane and shifted from 2590 cm−1 at a λ = 1 to 2553 cm−1 at a λ = 9 as result of the relative populations of water changing as the membrane became more hydrated. This type of measurement yields interesting information on the distribution of water environments, but a vibrational frequency is difficult to correlate to rotational or translational diffusion coefficients, which may be more relevant to transport processes. Similar insights into NAFION®–water interactions have been gained from fluorescence spectroscopy and UV–vis adsorption of fluorescent probe dyes.31–33

Ultrafast spectroscopy has been used to make measurements of the multiple hydrogen bond environments of water in NAFION® membranes.30, 34 The vibrational lifetimes in NAFION® membranes showed two components with values between 2 and 11 ps for hydration numbers between 1 and 7.5, which compares to a vibrational lifetime of HOD in bulk water of 1.7 ps with a single exponential decay. Interestingly, the vibrational lifetime experiments show that the characteristic relaxation times of fast and slow relaxing water both change as a function of hydration. This result indicates that the character of each type of water or the population of environments occupied by water changes as the membrane is hydrated. This assertion is supported by poor single exponential or two component fits (bulk water and λ = 1 water) to the relaxation data and shows that the distribution of water across multiple hydrogen bonding environments changes with hydration. These changes in the water–polymer interactions could be due to the character of the water–NAFION® contacts at the hydrophilic/hydrophobic interface evolving as a function of hydration.

NMR relaxation measurements such as T1 probe the local rotational mobility of water within a sample. 2H T1 values between 5 and 200 ms have been measured in a range of PEMs imbibed with 2H2O, which correlate to water rotational rate constants on the order of 107 to 1010 s−1, similar to values found in supercooled liquid water.35, 36 These values compare to an 2H T1 of 400 ms for ambient pressure liquid 2H2O at 292 K with a rotational rate constant of 2.8 × 1011 s−1.372H T1 measurements show a strong correlation to the hydration number and conductivity for NAFION®.22 Lee et al.5 showed that sulfonated aromatic polymers showed lower 2H T1 values than those observed in PFSAs at equivalent hydration numbers. For instance at a λ = 5 for NAFION® 117 the 2H T1 was 80 ms and for a sulfonated Radel sample at λ = 5 the 2H T1 was 20 ms which resulted in depressed conductivity for sulfonated Radel. These authors hypothesized that the ionic domain structure had a significant influence on the observed results.

PFG-NMR measurements have been used extensively to measure water self-diffusion on the 50 ms and longer time scales. In a 1D experiment, the length scale over which the diffusion coefficient is measured is given by:

  • equation image(2)

Typical water self-diffusion coefficients in PEMs are on the order of 1 × 10−10 to 1 × 10−8 cm2 s−1, which gives diffusion lengths of 1–10 μm. This size scale is clearly larger than the nanoscale features of the ionic domain morphology in PFSAs or the ionic domains in a block copolymer-based PEMs; however, good correspondence between chemical structure and the water self-diffusion coefficient has been observed.

Roy et al.38, 39 reported that block copolymers display higher water self-diffusion coefficients than their random analogs (Fig. 3). In random systems, the self-diffusion coefficient measured by PFG-NMR declined steadily with increasing diffusion time. At long diffusion times, the self-diffusion coefficients in NAFION® and block copolymers were not a function of diffusion length. The high conductivity in block copolymers is thought to be due to their higher concentration of ionic groups in the functionalized domains and the connectivity of the phases. However, the block copolymer architecture also appears to have an influence on the water behavior, which is intimately tied to their proton conduction properties.

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Figure 3. Water self-diffusion coefficient for NAFION® (▵), random poly(aromatic) (□), and block poly(aromatic) (▪) PEMs. Adapted from Ref.39.

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Li et al.40 showed that orientation in extruded Nafion® 112 and dispersion cast NAFION (NRE)-212 membranes could be detected through measurements of 2H NMR quadrupole coupling. The residual quadrupole splittings are related to the orientational order parameter where the motion of water in the ionic domains is biased by the orientation of the ionic domains. Biaxial orientation in the plane of the film was observed in the extruded membranes, but the dispersion cast samples had uniaxial orientation in the thickness direction of the sample. Hou et al.41 used both 2H quadrupole splitting and pulsed field gradient diffusion measurements to explore the anisotropy of block copolymer PEMs over different length scales. The 2H splitting measurements probe order on about the 0.4 μm length scale, where diffusion measurements probe larger length scales as discussed above. These results demonstrate that the nature and number of defects and the local ordering of the material over multiple length scales can affect different measurements of water motion. More combined studies of structural order including defect type and density and water and ion mobility will help to clarify the role that defects play in the transport processes in these types of materials.

The PFG-NMR technique rigorously measures the water self-diffusion coefficient (Dself), which is related to the Fickian diffusion coefficient (DFick) through:

  • equation image(3)

where aw is the activity of water, so ∂ln(aw)/∂ln(λ) describes the hydration of the membrane with relative humidity (RH). Fickian diffusion coefficients for water in these types of polymers can be derived from dynamic water sorption experiments or concentration gradient diffusion experiments. As the water motion in sorption or concentration cell experiments is due to a concentration gradient, these measurements probe the Fickian diffusion coefficient and cannot be compared directly to PFG-NMR self-diffusion coefficients unless the water uptake properties are known.

Hallinan and Elabd42 used attenuated total reflection-FTIR sorption measurements to quantify the water uptake properties of NAFION® during small and large step-changes in RH. These authors found that a Fickian diffusion model described most small RH step change experiments, for example, from 43% to 56% RH, but for dry membranes or large steps in RH, for example, from 0% to 22% RH or 0% to 100% RH, the uptake curves were non-Fickian. The authors were able to decrease the mass transfer resistance at the vapor–membrane interface by increasing the velocity of the gas phase surrounding the membrane, which is critical for recovering the correct value of the diffusion coefficients. Water permeation experiments under steady-state concentration gradients can also give insight into the water mobility of membranes as they are exposed to different hydration conditions.43, 44 However, performing these types of experiments for small gradients in water activity is important to maintain consistent hydration throughout the membrane for accurate determination of the diffusion coefficient at a known hydration value.

Siu et al.45 showed that the water associations with the polymer may be beneficial in promoting conductivity under freezing conditions. On freezing PEMs to −40 °C, the conductivity dropped by four orders of magnitude, and the activation energy of proton conduction increased from 0.15 to 0.4 eV. Appreciable conductivity in the frozen samples was thought to stem from the water that was still tightly associated with the acid groups in the samples and did not show any thermal transitions around 0 °C in DSC experiments. Ma et al.46 observed a water self-diffusion coefficient of 4 × 10−11 m2 s−1 at 240 K using PFG-NMR for membranes initially equilibrated at 98% RH. This value compares to a water self-diffusion coefficient of ∼2 × 10−10 m2 s−1 at 273 K. Although this work demonstrates that absorbed water in PFSAs was moving more slowly in the membranes below 0 °C compared to membranes at higher temperatures, there was enough mobility to promote proton transport. Other studies of water mobility at low temperatures showed step changes in the water self-diffusion coefficient at low temperatures that were attributed to freezing events.47, 48 These membranes were probably more hydrated than the samples studied by Ma et al., which lead to some portion of the water in the membrane freezing. These studies of the water mobility at sub-zero temperatures demonstrate that the water–polymer associations observed in PEMs show a wide range of behaviors and have a significant influence on the transport properties, even under conditions where bulk water is frozen.

Dielectric spectroscopy has been used to explore the motion of water in hydrated PEMs. Paddison et al.49 showed that the dielectric constant of NAFION® increased with hydration number and decreased with increasing frequency, similar to that of liquid water. In sulfonated poly(aromatic) samples, the measured dielectric constant was a weaker function of hydration number than what was observed for the PFSA samples.50 Kreuer11 showed that the dielectric constants measured in each case are a reflection of the ionic nanophase morphology of the material. Additionally, the dielectric constant as a function of water volume fraction extrapolated to 64 in NAFION® at a water volume fraction of unity where a value of 20 was observed for the sulfonated poly(aromatic) sample. The dielectric response is reflective of the lower water mobility observed in sulfonated poly(aromatic) samples compared to PFSAs. Lu et al.51 resolved different populations of water in NAFION® membranes using dielectric spectroscopy. Gigahertz relaxations were observed reflecting water with bulk-like properties in addition to a process that had a lower dielectric relaxation frequency response. The dielectric strength of these two processes showed an increase with hydration number indicating that the populations of water molecules giving rise to each process increased with hydration number. A process with kilohertz relaxations was also assigned to the rotational motions of the hydrated sulfonate groups and their first hydration spheres.51, 52 Shifts in the relaxation frequencies to higher values reflected a larger population of faster moving water as the hydration numbers were increased.

There are a variety of methods for understanding the dynamics of water and its distribution of environments when absorbed in ion-containing membranes. PFG-NMR has the advantage of a direct measurement of the water self-diffusion coefficient, but it is difficult to extract distribution information from this type of measurement. Additionally, the length scale over which the measurement is probing the dynamics of water must be considered. Vibrational and dielectric spectroscopy can extract information on the different environments that water experiences in the material, but the length-scale dependence of the measurements, the exchange of water between different microenvironments, and the meaningful parameters for proton transport are still being studied for a range of different PEMs. Comprehensive measurements tying conductivity values and measurements of water motion will further the community's understanding of proton transport in these types of ion-containing polymers and this basic knowledge can be extended to study ion and molecular transport beyond proton transport.

TRANSPORT OF WATER AND SMALL MOLECULES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

The water motion in fuel cells membranes is often measured to interpret the proton conductivity of the materials, but the water dynamics also have a pronounced impact on the transport of water and small molecules in fuel cell membranes. Hickner et al.4 correlated the thermal signatures of water absorbed in a series of sulfonated poly(phenylene)s to the transport of methanol (Fig. 4) and glucose through the membranes. For highly sulfonated poly(phenylene)s, the diffusion of methanol and glucose was greater than for NAFION® membranes. The ionic domains as observed by transmission electron microscopy (TEM) were not as well organized in the sulfonated aromatic polymers as they appeared in NAFION®; however, the absorbed water in the samples with high transport rates showed increased heats of fusion in differential scanning calorimetry experiments, which were correlated to increased water mobility. The activation energies for transport also declined with increased heats of fusion of water leading the researchers to conclude that more rapidly diffusing water could be linked to faster molecular diffusion of the solutes. The ionic domain size and connectivity was not sufficient to explain these differences in the transport properties of the membranes and the measurement of a signature of water binding seemed to bridge the understanding of transport across chemically dissimilar materials. However, it must be stressed in this study that the ionic domains were studied in their dry state. TEM or scattering of dry samples is often performed out of convenience, but techniques to measure the size and connectivity of the hydrated domains for a range of polymers are becoming more commonplace.53–56

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Figure 4. Thermal signature of water in the membrane correlates to proton conductivity (○) and methanol permeability (•). Data taken from Ref.4.

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The concept of electrochemical selectivity57 or the permeability-normalized conductivity is useful for quantifying the tradeoff between the different transport properties of interest for a given application. However, this type of figure of merit can have shortcomings, especially for materials that show high selectivity, but may have very low absolute values of conductivity.58 Thus, a minimum conductivity must be first defined for a given application, and then candidate materials can be selected once the primary goal of conductivity is met. Many sulfonated aromatic polymers show increased relative selectivity for proton conduction versus methanol permeation compared to PFSAs. This increase in selectivity can be interpreted both by considering the relative size of the ionic domains in each system and by visualizing how the water is distributed throughout these different sized phases (Fig. 5). It has been shown that the permittivity within an ionic pore changes radially.59–61 The high dielectric constant in the center of the pore gives rise to high rates of water and solute transport. Therefore, larger pores contain water that is less associated with the polymer, and high transport rates result. This situation not only increases proton conductivity but also can significantly increase methanol and water transport.

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Figure 5. Down-pore view of ionic domains in PFSAs and sulfonated poly(aromatic) PEMs.

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Xu et al.62 were able to increase the selectivity of PEMs through tuning the ion content of crosslinked polymers. In this work, the authors were able to connect the increased electrochemical selectivity of their materials with a lower thermal signature of water by calorimetry experiments. For a PEM with an ion exchange capacity (IEC) of 0.97 mequiv g−1 and a water uptake of 19 wt %, a relative selectivity of 22 was obtained while maintaining a conductivity of 0.048 S cm−1. The low water thermal signature and high selectivity was the result of the small ionic domains in these materials and perhaps some added polarity due to the presence of the urethane bonds at the crosslink junctions.

Hickner and Pivovar63 correlated the water self-diffusion coefficient, electro-osmotic drag, relative selectivity, and backbone stiffness for a variety of PEMs. The observation of electro-osmotic drag being correlated with the relative selectivity is interesting because this connection emphasizes the linkage between water motion and the other transport properties of the membrane. Electro-osmotic drag is a function of the number of water molecules transported with each proton across the membrane. For membranes with significantly confined water, the electro-osmotic drag values will be low. However, for membranes with less water–polymer associations, the viscous drag of the proton as it moves through the ionic domains results in more water transport in concert with the proton and thus a higher electro-osmotic drag coefficient. Hickner and Pivovar's correlation demonstrates that more mobile water is tied with lower electrochemical selectivity.

Interpreting the transport properties of PEMs through insights into the mobility and binding of water absorbed in the polymer is a useful method for understanding the tradeoff in properties that are prevalent in these types of materials. In many cases, if an increase in conductivity is desired, the diffusion of methanol and water will increase as well, which is not always favorable for a given application. The permeability of methanol can be suppressed in many PEMs but often at the expense of ionic conductivity. Even though ion conducting and molecular diffusion occurs through the aqueous ionic domains in the material, the transport property tradeoffs can be optimized because the migration of ions depends on the ionic concentration in the sample but the molecular diffusion does not. For instance, Xu et al.64 were able to maintain a high degree of mobile water in NAFION®/sulfonated poly(silsesquioxane) composite samples by increasing the IEC; however, the selectivity was improved even as the water uptake of the materials was increased. This unusual trend in properties was due to maintaining high ion exchange capacities and hence conductivities in the materials due to the inclusion of the sulfonated filler, but the filler also provided some blockage of the ionic domains thus preventing methanol permeability through the material. This type of strategy will be useful going forward in optimizing the transport property tradeoffs in other types of membranes for various applications.

POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

The dynamic motion of the polymer molecules do not make a primary contribution to proton motion under conditions of steady-state water content (except possibly at very low hydration), but during changes in water uptake, polymer relaxation plays a significant role in determining the rate of water swelling or deswelling. Various models have been proposed for how polymer structure at the polymer/vapor interface controls water transport into a membrane.65 The difference in water uptake of a membrane from the vapor or liquid phase with unit activity (saturated vapor or liquid water at the same temperature) has been ascribed to a hydrophobic surface layer that develops in PFSAs when exposed to dry conditions or low activity water vapor. This Schroeder's paradox has been invoked to explain the interfacial resistance to slow water diffusion into the membrane from the vapor phase. Onishi et al.66 provided evidence that observations of Schroeder's paradox are primarily governed by the equilibration time of the sample in the vapor phase and thus emphasized that polymer dynamics, even at extremely long time scales, control, in part, the water uptake of these types of materials.

Relaxation dynamics of the polymers are evident during dynamic water uptake experiments. Quantitative expressions for the relaxation dynamics have been developed by Hallinan and Elabd67 using a diffusion-relaxation model for NAFION® adsorption. In this work, the authors demonstrated that by minimizing the vapor–polymer boundary layer thickness during the water uptake experiments, the sorption curves could be described by a coupled Fickian diffusion process and a polymer relaxation expression that likely occur on similar timescales. For various models, the calculated diffusion coefficients were similar and spanned values of 2 × 10−7 to 9 × 10−7 cm2 s−1 over the full range of water activities. The self-diffusion coefficient of water in ion-containing polymers increases systematically with hydration number (see Fig. 1). The Fickian diffusion coefficients, however, display a maximum at intermediate hydration.5, 68, 69 This difference in the shape of the self-diffusion and Fickian diffusion coefficients is due to the shape of the sorption relationship with water activity (eq 3).

Satterfield et al.70 reported creep rates of dry and hydrated NAFION® membranes over 1000 min. Hydrated samples were observed to creep quickly at short times due to their plasticization by water, and dry samples have slower creep rates, but showed long-time relaxations. Interestingly, the swelling pressures measured during in situ hydration of the membrane showed relaxation on the timescale of the mechanical creep experiments. Results for the swelling properties of the membranes in this work were discussed in the context of the competing energies of solvation of the ionic groups and the energy required to swell the polymer matrix. Interesting effects have been observed for the creep and mechanical properties of NAFION® with low water contents.71–73 Under specific conditions of temperature and water activity, NAFION® can undergo antiplasticization, where the addition of water increases the tensile modulus of the membrane. The mechanism of this antiplasticization is not entirely clear, but it may be due to microstructural changes in the polymer or ionization/bonding interactions of the sulfonic acid groups on small additions of water.

Harrison et al.74 reported that the modulus of a disulfonated poly(arylene ether sulfone) membrane was one order of magnitude higher than that of NAFION® but declined precipitously with hydration. Similar observations have been made by Fujimoto et al.75 for sulfonated poly(phenylene)s where the Young's modulus of NAFION® declined by a factor of four for hydrated samples, but the modulus of the sulfonated poly(phenylene) dropped by a greater percentage due to a larger plasticization effect. Under hydrated conditions, the modulus of sulfonated aromatic polymers is generally greater than that of PFSAs. The difference in mechanical properties between materials narrows under hydrated conditions due to the greater extent of plasticization of aromatic polymers by water. The aromatic polymers are more plasticized because of the greater extent of water–polymer interactions in non-perfluorianted samples as compared to PFSAs.

Kim et al.76 showed that the Kelley–Bueche equation (a variant of the Fox equation) described the decrease in the Tg of NAFION® and disulfonated poly(arylene ether sulfone)s membranes up to a certain water content. For NAFION® the Tg strongly decreased with increasing water content and followed the Kelley–Bueche equation up to a bulk water uptake of 5 wt %. The drop in Tg was more severe for the disulfonated poly(arylene ether sulfone) sample and the Tg depression followed the Kelley–Bueche equation up to nearly 20 wt % water uptake. At higher water contents than these critical values, the Tg of each material departed from the prediction of plasticization and was only weakly influenced by the further addition of water. The authors hypothesized that the critical value of water uptake for each material was controlled by the amount of water–polymer interactions for a given sample. As the disulfonated poly(sulfone) material had small domains and a polarizable backbone, it had more contact between the polymer and water which increased the critical value of water uptake. NAFION® with its perfluorinated backbone and well-defined ion domain structure was not as plasticized by water and instead the water was collected in the ionic nanophase of the material.

An important aspect of a membrane is its ability to withstand mechanical deformation during device assembly or operating stresses.77 Many ion-containing aromatic polymers have higher moduli than what is observed for PFSAs, but the elongation properties of aromatic polymer rarely exceed 30–50% in the wet state, where NAFION®'s elongation to break is about 170% and does not change significantly from wet to dry conditions.75 The poor elongation properties of many PEMs is demonstrated by their inability to withstand wet–dry cycling. In this type of test, the membrane is mechanically constrained, and the RH surrounding the membrane is cycled on the timescale of minutes between wet (usually 80% RH or higher) and dry (usually 20% RH or lower) conditions. Various PFSAs and reinforced PFSAs can approach or exceed thousands or tens of thousands of cycles in this type of test. However, few aromatic polymers have demonstrated good performance under these conditions. The low oxygen permeability of sulfonated aromatic polymers increases their resistance to chemical attack by reactive oxygen species,78 but the deficiencies of aromatic polymers in mechanical properties are a major concern for long lifetime devices. The synthesis of block copolymer PEMs with soft blocks may be one route to materials with greater toughness and elongation properties, but synthesizing these types of materials with chemical moieties that can withstand the electrochemical stresses and reactive species in an operating device is a technical challenge.

The mechanical stabilization of the membrane is important to preserve the morphology of the water-filled ionic domains that contribute to high transport rates in these types of membranes. Additionally, too much water uptake by the polymer can dilute the concentration of ionic groups and lower the resulting conductivity.79 Kim et al.80 measured the disruption of the ionic domain morphology of NAFION® and disulfonated poly(arylene ether sulfone) proton conducting membranes and correlated these microscopic changes to the overall membrane swelling and proton conductivity. The authors proposed an “upper limit use temperature” that describes the maximum temperature at which the polymer will not over-swell when exposed to liquid water. This concept can be extended to polymers that swell highly at room temperature, an upper swelling parameter, and provides guidelines for the ideal concentration of ions in the membrane in the swollen state. In other work, Kim et al.81 showed that even though the hydrated ion concentration can decline for samples that uptake significant amounts of water, the proton conductivity can continue to increase up to a point even though the ion concentration in the sample decreases. This increase in conductivity, even under declining proton concentration, is due to faster proton transport through the aqueous phase that displays more rapid water rotation and diffusion with the increased water uptake. These observations lead to the conclusion that there is no one descriptor of high conductivity in PEMs, but the ionic domain morphology, water–polymer interactions, and ion concentration must all be weighed when designing a material for a specific use.

Dynamic water uptake has been investigated in submicron NAFION® films. Kongkanand82 ascribed the slow water uptake of nm-thick films to the hydrophobic interface layer that commonly features in Schroeder's paradox arguments to explain the water uptake of thick (>20 μm) films. However, relaxation may also make a significant contribution to the slow uptake dynamics, especially for these thin films with very short diffusion lengths. In the absence of structural data to confirm and quantify the structure and composition of the interface layer, it is difficult to separate interfacial transport resistance and slow uptake dynamics due to polymer relaxation.

Understanding how nanometer-scale thick films of ion-containing polymers absorb water and probing the water self-diffusion, conductivity, and ionic domain morphology in these thin films are important in understanding how these polymers behave in high surface area porous electrodes. In these electrode structures, the ionomer is generally distributed as thin films throughout the electrode layer as a composite with the catalyst particles. The function of many electrochemical devices relies on a delicate balance of ion conductivity, electrical conductivity, reactant and product molecular transport, and exposure of the reactive sites. Currently in fuel cells, the electrode is a complex mixture of components without the appearance of regular order,83 with perhaps just one notable exception that has demonstrated excellent performance but still has shortcomings in terms of its ability to operate under freezing conditions.84 Extending studies of the structure–function relationships of bulk ion-containing membranes to nanometer-scale thin films85, 86 will promote more rational design of polymer composite electrodes and promote new tools and concepts to understand their performance.

CONCLUSIONS AND DIRECTIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

As the use of ion-containing polymers are expanded to new uses and new technologies, specific focus on the role of water in determining these polymers' transport, permselectivity, and mechanical properties will help to accelerate their development. Specifically, development of materials for alkaline fuel cells that require hydroxide (OH) polymeric conductors and redox flow batteries where high proton conductivity and low vanadium ion crossover is desired can benefit from the body of knowledge surrounding proton conducting membranes for fuel cells.

Alkaline membrane fuel cells are important because they have the potential to free fuel cell technology from the cost constraints of precious metal catalysts. Catalyst corrosion may not be as severe in fuel cells with a high internal pH as compared to the more well-known acidic PEMFCs, and the membrane-based architecture affords facile device and system operation. Yan and Hickner87 have outlined the critical relationship between bicarbonate conductivity and water uptake for a series of poly(sulfone)-based anion exchange membranes (AEMs). A strong correlation was observed between the bulk water uptake of a series of AEMs and their bicarbonate conductivity, regardless of IEC. The methanol permeability also scaled proportionately with the bulk water uptake and the activation energies of ion conduction and methanol permeability were inversely related to the amount of water in the membrane. It is possible that the hopping properties of hydroxide will reveal different trends as has been observed with bicarbonate (which does not undergo hopping transport), but rigorous measurements of hydroxide conductivity remain difficult due to rapid adsorption of CO2 in AEMs.

Interestingly, the water self-diffusion in AEMs appears to scale differently with ion content compared to the relationships observed for PEMs (Fig. 6).3, 88 NAFION® has a high water self-diffusion coefficient because of its large ionic domains. The difference in water association with cationic polymers compared to anion polymers may be similar to that measured in cationic and anionic reverse micelles. Dokter et al.89 have shown that water interacts less strongly with ions in CTAB (cetyltrimethylammonium bromide, a cationic quaternary ammonium-based surfactant) reverse micelles compared to water in AOT (sodium bis(2-ethythexyl) sulfosuccinate, an anionic sulfonate-based surfactant) reverse micelles. The difference in water hydrogen bonding in the presence of anions versus cations was invoked to explain the differences in the two systems. Similar physics may be at play in quaternary ammonium-containing AEMs in comparison to sulfonated PEMs, which could drive different relationships in their water uptake and ion conductivity properties.

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Figure 6. Water self-diffusion coefficients of NAFION® (▴), sulfonate (▪) and quaternary ammonium (•) functionalized poly(aromatic) membranes. Data taken from Ref.3.

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It is likely that other studies of AEMs have uncovered similar trends in the water uptake.90–92 However, more focus on the basic structure–property relationships of these materials is needed if their conductivity and water uptake properties are to be further optimized. There also exists the interesting potential to study the influence of polymer architecture93, 94 and cation type95, 96 in these types of materials, which will expand the fundamental knowledge surrounding the properties of ion-containing polymer membranes with tethered cations. The hydration/conductivity tradeoff is critical to optimize in AEMs, but hydration may be important for avoiding degradation of the cation as demonstrated by Chempath et al.97 Sulfonated polymers are known to degrade when they are stored in anhydrous conditions or at high temperatures for significant amounts of time in the acidic form. There is much less degradation observed for sulfonated membranes in the sodium or tetrabutyl ammonium neutralized form. The quaternary ammonium functionality of AEMs is less stable than sulfonate groups, so hydration strategies to preserve the integrity of the cation for membranes in hydroxide or bicarbonate (that becomes hydroxide at higher temperatures under low pCO2 conditions) forms are a consideration when designing membranes for a specific use. Storage of these materials in their chloride form appears to mitigate any degradation due to the weak nucleophilicity of the chloride anion.

Redox flow batteries are a viable technology for grid-scale energy storage and can serve as buffering capacity for widespread renewable energy deployment. In vanadium redox flow batteries (VRFBs), the redox reactions that store and discharge electrical energy are balanced by the flow of protons across a membrane separating the anolyte and catholye containing V2+/V3+ and V4+/V5+ reversible redox species. High proton conductivity of the membrane lowers the resistive contributions to the efficiency loss in these types of devices, but vanadium ion diffusion through the membrane must be prevented. Transport of the vanadium redox species across the membrane causes capacity fade during reversible cycling of the battery. The redox activity of the electrolytes can be regenerated, but maintenance of the electrolyte causes downtime of the energy storage device and adds maintenance costs to the overall system. As grid-scale energy storage devices are multi MW-class systems, any additional fixed or operating expenses must be kept to an absolute minimum.

In one study by Kim et al.,98 NAFION®-based cells experienced a capacity loss of 13 mA h per cycle over 40 cycles compared to a 6.4 mA h per cycle decline over an equivalent number of cycles for VRFBs with sulfonated poly(sulfone) RADEL® membranes. The sulfonated poly(sulfone) membrane displayed six times less V4+ ion transport than NAFION® which was the origin of its low capacity fade. So far, there are no studies of how the water binding in ion-conducting membranes influences the vanadium crossover in VRFBs or transport processes in different types of flow batteries, such as iron-chromium or zinc-bromine.99 However, the trends in the comparison of vanadium ion transport in NAFION® membranes and sulfonated aromatic PEMs appears to be similar to what has been observed for methanol transport (Fig. 5). The biggest caveat thus far in applying membranes other than PFSAs to VRFBs appears to be the increased degradation observed in the case of sulfonate poly(sulfone) membranes.100 New, more intrinsically stable polymers and membrane engineering strategies101 will help to alleviate the lifetime problems of non-PFSA materials, but the demand for tens of thousands of hours of cycle life will remain a challenge.

To conclude, there are many facets to the design of new ion-containing membranes for electrochemical and water treatment applications.102 This review article has emphasized the role that water–polymer associations play in controlling the properties of these materials, but the effect of ionic domain morphology and other factors such as charge concentration in these systems influence the water behavior and ultimately couple to the transport properties. There is no single, simple predictor of conductivity and permeability in ion-conductivity membranes, but investigations of the water behavior have opened up areas of fundamental interest and technological importance and provide one set of design criteria for optimizing these materials for a specific purpose.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

M.A.H. thank the Penn State Materials Research Institute, the Penn State Institutes of Energy and the Environment, the U.S. Army Research Office (W911NF-08-1-0282 and W911NF-11-1-0411), the U.S. Office of Naval Research (N00014-08-1-0730 and N00014-10-1-0875), and the U.S. National Science Foundation (CBET-0932740) for support of his group's research in the synthesis, properties, and applications of ion-containing polymers.

REFERENCES AND NOTES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information

Biographical Information

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. WATER MOTION AND PROTON CONDUCTIVITY
  5. TRANSPORT OF WATER AND SMALL MOLECULES
  6. POLYMER RELAXATION DURING UPTAKE AND MECHANICAL PROPERTIES
  7. CONCLUSIONS AND DIRECTIONS
  8. Acknowledgements
  9. REFERENCES AND NOTES
  10. Biographical Information
Thumbnail image of

Michael Hickner

is the Virginia S. and Philip L. Walker, Jr. Faculty Fellow and an Assistant Professor of Materials Science and Engineering at The Pennsylvania State University. Hickner's work has been recognized by Office of Naval Research and Army Research Office Young Investigator Awards and a Presidential Early Career Award in Science and Engineering (PECASE). He has co-authored five US and international patents and over 70 peer-reviewed publications with more than 3200 citations.