The demand for more efficient and environmentally benign energy systems as alternatives to fossil fuels to reduce the level of pollutions has continued to increase.1–3 This has prompted intense research efforts on polymer electrolyte membrane fuel cells (PEMFCs), as a prospective supply of clean energy for applications such as space shuttles and automobiles.3–5 In PEMFCs, the polymer electrolyte membrane (PEM) serves as a medium for transporting protons from the anode to the cathode to balance the flow of electrons in an external circuit. Given that ion transport across the PEM is rate-determining step in PEMFCs rather than the electron transfer between electrodes, PEMs are considered as a key component in PEMFCs to optimize the performance of these systems.4–6
Among the wide range of existing PEMs, sulfonated polymers have been of particular interest,4–21 which are a class of ionomers having been investigated over the last 30 years for diverse applications of thermoplastics, dispersants, and packing materials.22 The specific ionic interactions in discrete regions of ionomers can yield significant changes in physical properties of polymers such as glass transition temperature (Tg) and water uptake.22–24 One of the most well-established ionomers is perfluorosulfonic acid ionomers, commercialized by DuPont in the late 1960s under the trademark Nafion™. Nafion™ has been utilized as a benchmark polymer in many sulfonated PEMs studies owing to its superior proton conductivity and good chemical/mechanical stability.4–8 Hydrocarbon-based sulfonated polymers have also been extensively studied on account of their low cost in comparison to Nafion™.9–11 The most widely studied materials are sulfonated polyaromatic engineering plastics12–16 and sulfonated polyimides.17–19 Several studies have focused on the optimization of the amount of water absorbed in the PEMs so as to improve the conductivity.14–18
The aforementioned PEMs are often referred to as sulfonated random copolymers owing to the random location of the hydrophilic and hydrophobic moieties.10–14 A simple means of improving the conductivity is to increase the sulfonation level (SL) in the PEMs. Particularly, if the SL reaches the threshold concentration for creating percolated networks of ionic channels, an abrupt improvement in conductivity can be achieved.10–14, 20 However, relatively high SLs are generally required for sulfonated random copolymers to achieve a conductivity comparable to that of Nafion™, and such high SLs lead to excessive swelling of the membranes upon exposure to high humidity (some of these PEMs become brittle when dry), which is a major drawback when applying these copolymers to PEMFCs.10, 14 In this regard, cross-linking the PEMs21, 25 and/or fabrication of composite membranes using inorganic fillers26, 27 are prospective means of increasing the water content to increase proton conductivity without losing the mechanical stability of PEMs.
For the access of durable and efficient PEMs, sulfonated polymers having block and/or graft configuration have received substantial interests lately owing to their ability to self-assemble into periodic microstructures. It has been widely reported that the phase-separated hydrophobic phases can provide effective mechanical support while the well-arranged nanometer-scale hydrophilic domains facilitate ion transport by means of confinement effects (at the same ion exchange capacity (IEC) value, ions confined within microphase-separated hydrophilic phases are in close proximity, compared to their random analogues).8, 10, 28, 29 Examples of such polymers include sulfonated poly(styrene-b-isobutylene-b-styrene),30, 31 sulfonated poly(styrene-b-methylbutylene),28, 32 sulfonated poly(ether ether ketone)-b-poly(ether sulfone),33, 34 and sulfonated poly([vinylidene difluoride-chlorotrifluoro ethylene]-g-styrene).19, 35, 36
Note in passing that although much of the research on the sulfonated polymers indicated that morphology is important in determining transport properties, the achievement of well-defined structures with long-range order is lacking and it has been the subject of the current studies.32, 37 In addition, as it has become clear that the transport properties can be improved by controlling the self-assembled morphologies of sulfonated block/graft copolymers, the need for understanding thermodynamic properties of sulfonated PEMs has increased.30, 32, 37
Very recently, as part of an effort to enhance the performance of PEMFCs, studies on sulfonated PEMs entered a new phase, aiming at PEMFC operation under anhydrous conditions. This tide was triggered by the requirement for high operation temperatures exceeding 120 °C in order to minimize CO poisoning at the surface of the Pt catalyst and to improve the transport efficiency in PEMFC systems.7, 38 This inevitably requires a dry atmosphere, with consequent reduction of the conductivity of most sulfonated PEMs by a few orders of magnitude, owing to the large reduction in the water content within the membranes.38, 39 The key challenge has thus been placed on the discovery of new protic solvents that exhibit non-volatile and non-disruptive characteristics as water substitutes.
Among the range of potential candidates as new protic solvents, ionic liquids (ILs) composed of heterocyclic diazoles and counterions are particularly promising, taking into account their unique physicochemical properties such as negligible vapor pressure and high ionic conductivity.40–42 While various combinations of polymer matrices (sulfonated polymers, polybenzimidazole, poly(vinylidene fluoride), poly(methyl methacrylate), and so on) and ILs have been examined in the drive towards high temperature fuel cells, it is noteworthy that sulfonated polymers remain the popular choice of materials for achieving high anhydrous conductivity because of the efficient ion pairing behavior of the sulfonic acid groups with ILs.43–45 Analogous to the case of hydrated PEMs, diverse observations of morphology-dependent conductivities have motivated the research on the anhydrous transport properties of self-assembled IL-integrated PEMs, especially by employing sulfonated block/graft polymers.44–47
In this article, we describe the thermodynamics and transport properties of sulfonated polymers under hydrated and anhydrous conditions. This article is organized as follows. The first section presents an overview of proton transport in hydrated sulfonated PEMs and then describes strategies that can be used to access improved transport properties with the use of sulfonated block copolymers having well-defined morphologies. In subsequent sections, recent studies on ILs incorporated sulfonated PEMs geared towards anhydrous ion transport in high temperature fuel cells are expounded. The morphology-transport relationship in IL-integrated PEMs is discussed to highlight promising avenues for the rational design of new PEMs.
ION TRANSPORT IN SULFONATED POLYMERS UNDER HYDRATED CONDITIONS
Factors Governing Proton Transport in Sulfonated PEMs
Over the past decade, a variety of avenues have been explored for the development of efficient sulfonated PEMs for fuel cell applications. In Table 1, we summarize chemical structures of representative sulfonated polymers studied to date. The traditional sulfonated PEMs include Nafion™,3–7 sulfonated polyaromatics,12–16, 48–51 and sulfonated polyimide derivatives.17–19 Commercially available Nafion™ membranes have been the benchmark in the sulfonated PEM studies owing to their high conductivity (on order of 0.1 S/cm) under fully hydrated conditions, despite their low IEC value of 0.9 mmol/g.4–6
Table 1. Molecular Structures of Representative Sulfonated Polymers Studied to Date
Proton transport in aforementioned sulfonated polymers is commonly understood through a combination of vehicular and hopping motions of the excess protons in the presence of water medium.52, 53 Both experimental and simulation results have suggested that the proton hopping motion increases faster than the vehicular motion as hydration levels increase since the proton diffusion coefficient is much greater than diffusion coefficient of the surrounding water medium.10, 52 At low hydration levels, most of the water molecules and the hydronium ions (H3O+) are strongly bound to the −SO3H groups and the proton transport is strongly coupled to and impeded by the structure of the membrane.10
In this respect, the most common route to achieving high conductivity from sulfonated polymers is the solvation of ionic domains with water molecules to facilitate proton dissociation from the −SO3H groups. In the presence of moisture, particularly, the −SO3H groups absorb atmospheric water molecules to yield the segregation of the 2∼6 nm sized ionic domains in which the water molecules are surrounded by −SO3H surfaces.39, 51, 54, 55 This reduces the unfavorable contact between water and the hydrophobic polymer chains. If the water-rich domains create percolating networks, that is, water channels, proton conduction then occurs through the shuttling of hydrogen bonds between water molecules (Grotthuss mechanism) and/or by the diffusion of protons (vehicular mechanism) along the water channels.39, 56, 57
Systematic studies were carried out to underpin the factors governing proton transport in hydrated sulfonated PEMs and following deductions have been elucidated, as schematically illustrated in Figure 1: (1) the IEC values of PEMs are known to impact the conductivity by affecting the size and connectivity of the water channels. Membranes possessing high IEC values generally exhibit enhanced conductivity on account of the presence of well-connected water channels, as confirmed by scanning transmission electron microscopy (STEM) experiments.15, 50 High proton conductivity, surpassing that of Nafion™ has been reported with the use of highly sulfonated PEMs, although the optimization of conductivity and mechanical properties of those PEMs under humidified conditions has been the subject of intensive contemplation.14, 20, 32 (2) In addition to the IEC values, the local concentration of sulfonic acid groups within the hydrophilic phases is found to play a crucial role in determining the proton conductivity. This can be rationalized as a consequence of different proximities of the sulfonic acid groups, where the closely located ionic moieties are beneficial for obtaining high conductivities.15, 33, 36, 58 Accordingly, efforts to locate the acid groups in close proximity via synthetic control or by morphological optimization have been widely described.34, 59 (3) In sulfonated polymers, there is an underlying source of polydispersity, originating from the dissimilar spatial distributions of the sulfonic acid groups. This is due to the randomness of the majority of sulfonation reactions. Chains with acid groups in different locations may behave differently because of the extremely large strong segregation between sulfonated and non-sulfonated monomers, as reflected by the magnitude of the Flory-Huggins interaction parameter (χ).37, 60, 61 Based on this consideration, the precise positioning of acid groups has recently been attempted, and the resultant PEMs having controlled ionic sequencing revealed well-defined ionic aggregates with a higher degree of organization in comparison to the conventional systems having a statistical distribution of acid groups.16, 62–64
Although such issues in diverse sulfonated PEMs have clearly been elucidated, no consensus has thus far been reached concerning the role of the phase separation between the hydrophilic phases and hydrophobic domains in determining the membrane transport properties. The issue of phase separation should be fundamentally important in unraveling the structure-property relationship of sulfonated PEMs; however, the ill-defined, process-dependent, heterogeneous structures of many sulfonated PEMs have hampered the acquisition of quantitative information on them. For example, although scattering experiments on sulfonated PEMs have attempted to determine how the water channels are connected,51, 54, 55 there is no general consensus on the exact nature of water channels located within hydrophilic phases. In order to achieve a breakthrough for enhancing the performance of PEMFCs, the development of new PEMs designed on the basis of both molecular characteristics and structural aspects is required.
Phase Behavior and Thermodynamics of Sulfonated Polymers for Designing Less Tortuous Ion Conduction Pathways
The appending of hydrophobic chains to sulfonated homopolymers in a controlled manner to yield sulfonated block or graft copolymers was initially attempted to improve the mechanical stability of PEMs under humidified conditions.20, 30, 47 The uniqueness of these materials can be attributed to the fact that they can self-assemble into ordered periodic microstructures,29–32, 37, 65 which offers the potential of gaining an extremely valuable insight into the effects of nanoscale morphology on proton transport. In these systems, the structural aspects, such as the types of self-assembled structures, the width of the hydrophilic phases, and the orientation of the water channels located within the hydrophilic domains become highly relevant parameters for understanding the transport properties of PEMs.
A range of early studies demonstrated that the use of self-assembled sulfonated block/graft copolymers could enhance the proton conductivity relative to random analogues with comparable IEC values.8, 20, 30, 33, 58, 65 This discovery stimulated several efforts of investigating the transport behavior of a range of sulfonated PEMs possessing nanoscale morphologies, and significantly improved transport properties were unanimously reported in a number of articles.8, 10, 32, 66 This observation has been rationalized in terms of the enhanced local concentration of −SO3H groups within the water channels of the nanostructured PEMs owing to confinement. It has also been anticipated that the phase-separated microstructures with low compositional fluctuation should provide less tortuous pathways for ion transport.
Although the experimental results effectively demonstrated that the creation of a phase-separated morphology in sulfonated PEMs is an effective way to improve proton conductivity, most studies did not include control of the self-assembled nanostructures. In other words, these investigations were restricted to simple comparison of the conductivity of polymers with some degree of order (block/graft) versus ill-defined counterparts (random). The difficulty arises from the poorly characterized morphologies of sulfonated block/graft copolymers as a function of the SL, the composition of hydrophilic phases, and the molecular weight of the copolymers.10 In fact, the self-assembled morphologies were reported for a wide variety of sulfonated block/graft copolymers, based on X-ray scattering and TEM experiments, but the level of long-range order achieved is lacking.30, 35, 36 In theory, the tendency towards formation of ordered structures is expected with an increasing SL due to the higher degree of segregation between the sulfonated block and non-sulfonated chains,60, 61 if the effects of coulombic interactions among sulfonic acid groups on morphology can be ignored. However, increasing the SL of the polymers occasionally results in disruption of the ordered morphologies in the sulfonated PEMs.20, 30, 35 Although this observation has been attributed to the aggregation of ionic clusters at high SL values, it has contributed to the lack of fundamental understanding of the structure-property relationship of sulfonated PEMs.
In 2007, the first systematic study on the tunable morphologies of sulfonated PEMs was performed by varying SLs and molecular weights of a set of poly(styrenesulfonate-b-methylbutylene) (PSS-PMB) diblock copolymers.32 A wide variety of ordered morphologies of lamellar (LAM), gyroid, hexagonally perforated lamellae (HPL), and hexagonally packed cylindrical (HEX) phases were observed, which were successfully mapped on to a conventional phase diagram of non-ionic block copolymer/selective solvent mixtures.67 Notably, increasing the SLs in a set of PSS-PMB copolymers was found to effectively stabilize the ordered phases,37 which is markedly different from the results obtained with other sulfonated block/graft copolymers, as mentioned above. Figure 2 shows the molecular structure of PSS-PMB copolymers and their well-defined morphologies. The effect of increasing the SLs on the self-assembled morphologies of the PSS-PMB copolymers, that is, the increase in segregation strength, was elucidated based on small angle X-ray scattering (SAXS) profiles, and the formation of various nanostructures from a set of PSS-PMB copolymers was confirmed by TEM. Judicious control of the morphologies of sulfonated PEMs is crucial for quantitative exploration of the morphology-transport, which is in sharp contrast to the results given in most of the previous studies.
Phase behavior of PSS-PMB block copolymers was further studied by varying temperature for concrete underpinning of thermodynamics of sulfonated PEMs. Various thermo-reversible order-order and order-disorder phase transitions were seen for samples with relatively low molecular weights and/or low SLs while the highly sulfonated polymers exhibited temperature-insensitive well-defined morphologies.37 Theories such as the random phase approximation, the Flory-Huggins theory, and the self-consistent field theory have been used to estimate χ parameters in PSS-PMB diblock copolymers. Extremely large χ values of about 6 between the sulfonated and nonsulfonated chains were determined,37 which is consistent with the results reported in other studies.60, 61 This suggests that the utilization of sulfonated block copolymers should give rise to well-defined and SL-sensitive ordered structures, as illustrated by our results, if the extensive ionic aggregation behavior can be ruled out.
With the increased fundamental understanding of the thermodynamic properties of sulfonated block copolymers, various methods of controlling the conductivity on the nanometer scale have been attempted. Initial examinations of the effects of the size of the hydrophilic phases on the conductivity were executed using PSS-PMB copolymers of various molecular weights while keeping all other parameters such as the volume fraction of PSS phases, IEC value, and the local concentration of sulfonic acid groups.28, 32 This enabled determination of the microdomain size effects exclusively given that other factors were ruled out.
The preparation of a set of symmetric PSS-PMB copolymers with dissimilar PSS molecular weights ranging from 1,500 to 61,600 g/mol yielded well-defined PSS domain thicknesses in range 2.5 to 39 nm. Interestingly, a discontinuous change in the proton conductivities of the PEMs was noted when the width of the hydrophilic PSS phases was reduced from 6 to 5 nm; that is, proton transport in the PSS-PMB copolymers was facilitated by the decrease in the PSS domain size.32 The most intriguing observation is that the PSS-PMB copolymers comprising narrow hydrophilic PSS domain widths below 6 nm demonstrated effective water retention capacity at high temperatures up to 90 °C, as evidenced by the significant enhancement in the proton conductivity of the PSS-PMB membranes with increasing temperature. Such anti-drying behavior became effective even under relatively dry conditions of relative humidity (RH) = 50%. Simple calculation suggests that the small channels within the membranes are beneficial for retention of water molecules with the aid of capillary action. Figure 3 shows the effects of microdomain size on the conductivity, as well as variations in the vapor pressure of water confined within the nanometer-sized hydrophilic channels.
The results are noteworthy since dehydration is typically observed for most sulfonated PEMs with exposure to high temperatures.38, 39, 43 There are, in fact, very few studies that deal with methods of resolving the dehydration issues.68 In this regard, manipulation of the morphologies of sulfonated block copolymers on the nanometer scale can be considered as a new prospect for controlling the ion transport properties of the membranes for efficient function under diverse operation conditions.
Effect of Hierarchical Morphologies of Hydrated Sulfonated Polymers on Proton Conductivity
To pinpoint the origin of the effects of microdomain size on the proton transport of PSS-PMB copolymers, the morphologies of hydrated PSS-PMB copolymers were examined over a wide range of temperatures and humidities using in situ small angle neutron scattering (in situ SANS) experiments, equipped with an environmental sample chamber.28, 69, 70 Several studies of traditional sulfonated PEMs have shown that nominally hydrophilic phases contain periodically arranged water-rich domains with characteristic lengths in the range of 2 to 6 nm.39, 54, 55 There is some degree of similarity in the behavior of the PSS-PMB block copolymers where 3 to 5 nm sized water-rich domains were detected in the hydrated states.28
Notably, a strong correlation between the SL and the water-rich domain spacing was observed, that is, increasing the SL from 20 to 42 mol % results in a decrease in the water-rich domain spacing from 4.2 to 3.0 nm. This is within reason given that the average distance between the −SO3H groups decreases with increasing SL. It was also demonstrated that the evolution of water-rich domains (substructure) results in improved ordering of the self-assembled superstructure because of the favored segregation between the hydrated hydrophilic PSS phases and the hydrophobic PMB domains. This is in sharp contrast to the results given in literatures, where disordering of both the superstructure and substructure was observed as the amount of water uptake increased.35, 71
The most intriguing observation is that the substructure ceases to exist over the entire evaluated range of temperature and humidity when the width of the nominally hydrophilic PSS phases is decreased to 6 nm. In contrast, if the width of the PSS domain is larger than 6 nm, a scattering peak indicative of 3 to 5 nm sized water-rich domains becomes apparent. Representative in situ SANS profiles are shown in Figure 4. It should be noted here that once the PSS width is larger than the 6 nm cut-off, no significant influence on the nature of the water-rich domains was detected with a further increase in the PSS width up to the evaluation limit of 39 nm.
The absence of segregated water-rich domains for the PEMs possessing narrow PSS hydrophilic phases can be attributed to confinement effects, that is, the formation of periodically arranged 3 to 5 nm water-rich substructures within the 6 nm wide PSS superstructure is highly unfavorable. The disappearance of the water-rich domain substructures appeared to be intimately associated with the improved conductivities of the PSS-PMB copolymers with narrow PSS domains below 6 nm. In other words, the homogeneous PSS phases derived from the finite size effects are responsible for the rapid proton transport and increased water retention (see Fig. 3), signaling that the transport properties of sulfonated PEMs can be improved by manipulating the nature of the water-rich domain substructures.
The validity of our approach was substantiated by similar observations in the literature reports for sulfonated graft copolymers having different grafting chain lengths. It has been found that higher proton conductivity can be achieved if the size of the substructure is reduced or the graft chains are more homogeneously distributed as these factors allowed the membranes to retain more water under low humidity and high temperature conditions.36, 72 It is thus clear that a range of parameters linked to the morphology should be carefully considered for the development of efficient PEMs.
Nanostructured Sulfonated PEMs for Routing Proton Conduction Pathway
Beyond the issues of the microdomain size and the water-rich substructure, we turn the focus to two other issues connected to the morphology effects, the first of which is the impact of various types of self-assembled morphologies on the proton transport rate in the PEMs with variation of the curvatures of the ionic domains. Recently, we performed an in-depth study of the ionic domain shape-dependent proton mobility of micellar solutions of nearly symmetric PSS-PMB copolymers where the SL value was fixed at about 50 mol %.59 It was found that concentrated solutions (ionic gels) of the PSS-PMB copolymers exhibited either a body-centered cubic (BCC) morphology or HEX structure, depending on the concentration of the copolymers. In both cases, the hydrophobic PMB chains were localized in the core phases whereas the shell phases were composed of hydrophilic PSS chains.
As summarized in Figure 5, the proton mobility in solutions was largely governed by the kinds of micellar morphologies, where the mobility obtained with the cylindrical morphologies was 3 to 10 times faster than the value acquired from spherical ones. These results were attributed to the close proximity of the ionic moieties on the cylindrical curvature, which appears to positively affect the proton mobility by decreasing the tortuosity of ion conduction. Note that, analogus to the observation for PSS-PMB membranes, decreasing the width of the PSS domains was favorable for proton transport in ionic gels of PSS-PMB copolymers. A plot of the proton mobility versus micellar radius on the log-log scale revealed a unique scaling relationship between these factors, which is a clear indicator that the strategy of controlling both the size and shape of ordered nanostructures is a promising route for augmenting the transport properties of sulfonated PEMs.
The second point of consideration is that for most reported sulfonated PEM studies, the samples were typified by randomly oriented conducting domains if no external field was applied. In contrast, in-plane orientation of microdomains was readily attainable for certain sulfonated block/graft PEMs by simple solvent casting,30, 73 and the ratio of in-plane conductivity to through-plane conductivity (hereafter referred to as anisotropy in conductivity) was largely influenced by the kinds of casting solvents. Typically, the well-aligned samples exhibit a few orders of magnitude reduction in the through-plane conductivity relative to the in-plane conductivity.30, 74, 75 However, given that high conductivity along the membrane thickness direction is most relevant for practical applications, research has focused on the alignment of microdomains using different external stimuli in order to achieve the anticipated gain along the desired direction.74, 76 Nevertheless, only a limited number of studies have provided conclusive, quantitative experimental results to validate the efficacy of alignment in enhancing the transport properties. Particularly, few studies have established methods of obtaining improved conductivity along the through-plane direction.
The application of a variety of external fields to orient the self-assembled microdomains was conducted using a lamellar forming PSS-PMB copolymer.74 Shear flow, pressure field, and electric field were chosen as external stimuli and the structural anisotropy was quantified by combining 2D SAXS, in situ birefringence, and TEM. Remarkably, a few orders of magnitude difference in the in-plane and through-plane conductivities was observed depending on the choice of external field. For example, the pressure-aligned sample exhibited dominant parallel orientation of the lamellar grains with a consequently high anisotropy in the conductivity of 75. A 30% enhancement of the absolute in-plane conductivity relative to that obtained from the as-cast samples (isotropic) was achieved following the alignment, which is in good agreement with the theoretical projection. These results validate the feasibility of achieving in-plane orientation of lamellar grains with negligible defects and large grain sizes.
It is noteworthy that the application of an electric field (E-field) and shear flow results in perpendicular orientation of the lamellar grains, which has rarely been achieved with sulfonated PEMs. Comparison of the effect of the flow- and E-fields demonstrated that the alignment obtained from the flow field is significantly better than that gained with the E-field. The relationship between the domain orientation and transport property was monitored by in situ experiments and the optimal enhancement in the through-plane conductivity was 20% (with shear flow) relative to the isotropic sample. This is lower than the theoretical expectation due, in part, to the disparity in the connectivity of the ionic domains as a result of alignment perpendicular to the PEM thickness. Our work thus far suggests that the pursuit of judicious tuning of the conductivity in targeted directions is worthwhile. Figure 6 shows the external field-assisted alignments of the microdomain for the lamellar forming PSS-PMB copolymer, and the resulting anisotropic conductivities of the membranes are also given.
ION TRANSPORT IN SULFONATED POLYMERS UNDER ANHYDROUS CONDITIONS
Recent Strategies to Develop High Temperature Fuel Cells for Operation under Water-Free Conditions
To date, the operating temperature of fuel cells has been constrained below 90 °C, although the need for high temperature operation above 120 °C was recognized early on to advance the performance of PEMFCs. This is to guarantee the long-term stability of platinum catalyst and the enhancement of reaction kinetics.7 The reason high temperature PEMFCs get in troubles is due to excessive drying of the sulfonated PEMs since the high temperature operation is inevitably accompanied by low humidity conditions taking into account the water boiling temperature.7, 38 Until recently, the realization of efficient high temperature PEMFCs appeared to be a distant prospect, with urgent requirement for the development of new PEM systems.
The key challenges for realization of efficient high temperature PEMFCs can be placed on the discovery of new protic solvents that are non-volatile, have high boiling points, and have high ionic conductivities. A large number of early studies utilized hygroscopic additives, that is, H2SO4 and H3PO4, to obtain improved proton conductivity under anhydrous conditions, and traditional PEMs such as polybenzimidazole (PBI), SPAEK, and Nafion™ were chosen as matrix materials.77–79 Although those composite membranes exhibited superior anhydrous conductivity in excess of 0.10 S/cm at high temperature of 200 °C, they also revealed disruptive characteristics resulting in degradation of the electrodes.79 The use of heterocyclic diazoles as new protic solvents was also proposed by Kreuer et al. where imidazole, pyrazole, and benzimidazole were tested as potential substitutes for the water molecules.43, 80 The diazoles were characterized by high crystallinity owing to the dominant intermolecular interaction between the aromatic rings, and thus, reasonable conductivity values were only accessible at the relatively high temperatures exceeding 150 °C using high concentrations of diazoles within sulfonated PEMs.
As a promising water substitute, ionic liquid (IL) has recently emerged as new protic solvents.40–42 The non-disruptive and non-volatile characteristics of ILs are certainly fascinating for achieving anhydrous conductivity of sulfonated PEMs. Extensive research by various groups has established that the use of ionic salts, that is, ILs, composed of heterocyclic diazoles and counter-ions, can substantially improve the transport properties of sulfonated PEMs since the degree of ion dissociation, local concentration of ions, and Tg of the membranes can be manipulated.81–84 Such manipulations should also cause a large alteration in the thermodynamic properties of the PEMs, leading to an increasing number of fundamental studies on the thermodynamics of ILs-containing sulfonated PEMs.46
For IL-incorporated sulfonated PEMs, the key factors governing the anhydrous transport properties should be the type of ILs (cations and anions) and the amounts of ILs within the PEMs.44–46, 83, 85, 86 Apart from this aspect, interestingly, the morphology-dependent conductivities have also been reported for a number of ILs-containing sulfonated PEMs, which bear resemblance to the observations for hydrated sulfonated PEMs.44–46, 84 This has been a motivation for utilizing sulfonated block or graft copolymers, and studies of the morphology of the IL-integrated sulfonated block/graft PEMs have been concerned with the optimal design of PEMs to facilitate access to the desired ion transport properties.29, 45, 87, 88
Role of Nanostructure in Enhancing Anhydrous Conductivity of IL-Incorporated Sulfonated Block Copolymers
In an effort to underpin the thermodynamics and ion transport properties of self-assembled sulfonated PEMs composed of ILs, we examined the morphology and anhydrous conductivity of sulfonated block copolymers by incorporating a range of imidazolium-based ILs.44, 45 A series of PSS-PMB block copolymers were prepared as model PEMs to establish a synergetic means of optimizing the conductivity of the PEMs by varying the type of ILs, the amount of incorporated ILs, the types of self-assembled nanostructures, and the size of the ion conducting channel. Strikingly, for the same IL content (wt %), significantly higher ionic conductivities were achieved with IL-incorporated PSS-PMB block copolymers exhibiting well-defined nanostructures than with IL-embedded PSS homopolymers lacking organization, as shown in Figure 7(a). Given that PSS chains serve as ionic phases for the selective incorporation of the ILs, the conduction mechanisms of IL-incorporated PSS-PMB copolymers can be proposed based on a combination of proton transport by the formation and breaking of ionic bonds between sulfonic acid groups and imidazolium cations and the vehicular diffusion of ILs along the PSS domains.46 We thus infer that the non-conductive PMB microdomain excludes the ILs, thereby producing a higher local IL concentration in the conductive PSS phases owing to confinement.
The role of ILs in determining the conductivity of IL-integrated PEMs was explored by first varying the anions in the ILs while keeping the same imidazolium cations.42 This results in different solvation kinetics of ILs, which was expected to impact the ion transport rate in PEMs.45, 85 As anticipated, the solvation dynamics of the ILs influences the transport properties of IL-embedded PEMs where the conductivity values were governed by the intrinsic characteristics (ionic conductivity, diffusion coefficient, and solvation time) of the ILs. However, this correspondence becomes invalid for PEMs of different morphologies. Variation of the counter-anions of the ILs appeared to alter the thermodynamic properties of the PSS-PMB copolymers significantly, and increasing the IL concentration within the samples eventually yields dissimilar self-assembled morphologies such as LAM, HEX, and HPL at the same level of IL loading.45 This variation of the morphologies is proven to significantly impact the ionic conductivity of IL-embedded PEMs, compared to the effects of the intrinsic properties of the ILs. We found that the HPL morphology is advantageous for obtaining the highest conductivity in both the in-plane and through-plane directions owing to its co-continuous nature. In contrast, the in-plane and through-plane conductivities can differ by a few orders of magnitude if the morphology of ILs incorporated PEMs appeared to be LAM. Figure 7(b) illustrates the effects of morphology on the anhydrous conductivity, where the data were obtained from a set of PSS-PMB copolymers comprising different ILs.
Phase Behavior and Conductivity of Sulfonated Block Copolymers Comprising Different Heterocyclic Diazole-Based ILs: The Link between Morphology and Transport
From a practical point of view, the actual application of IL/sulfonated PEMs requires a high proton concentration within the membranes. In this respect, many IL/sulfonated PEM systems studied to date do not seem to be viable since the most widely studied ILs are generally composed of aprotic cations (quaternary 1,3-alkylimidazolium).44, 45, 83–86 We thus further explored the conductivity of IL-incorporated sulfonated PSS-PMB block copolymers by employing different kinds of heterocyclic diazoles in the ILs with focus on three aspects:46 (1) Is imidazole the best cation choice among the various heterocyclic diazoles to obtain the optimal conductivity for IL-integrated sulfonated PEMs? (2) Are Brönstead-type cations (containing many protic sites) beneficial in achieving higher conductivity? (3) Can the effects of morphology on the conductivity be quantitatively determined?
To provide a clear answer to these questions, the cations in the ILs were varied while the anion bis(trifluoromethanesulfonyl)amine, [HTFSI]) was fixed. Evaluation of a range of heterocyclic diazoles with different ring structures and alkyl substitutions revealed that with an increase in the number of protic sites in the diazoles (Brönstead-type), an improvement in the conductivity of the IL-incorporated PEMs could be achieved, as shown in Figure 8(a) (see the data obtained with [2-MIm] and [1-MIm]). For diazoles having the same number of protic sites but with different ring structures, that is, pyrazole and imidazole, the imidazole appeared to be a better proton source for obtaining high conductivity in IL-containing sulfonated PEMs. This is attributed to the restricted rotational motion of the pyrazole molecules, owing to the strong ionic bond with [−SO3H], as confirmed by Ab initio calculation, which limits long-range ion transport.46
It is intriguing to note that regardless of the types of diazoles, the addition of ILs into the PSS-PMB copolymer always results in enhanced segregation between the IL-containing PSS phases and the PMB domains (increase in χ), whereas the relative degree of the enhancement varied with the ring structure and alkyl substituents in the diazoles. This is attributed to the dissimilar strength of the ionic interaction taking place in the PSS phases depending on the kinds of heterocyclic diazoles. This difference in the thermodynamics yields a variety of well-defined morphologies such as LAM, HEX, and gyroid structures for the IL-impregnated PSS-PMB copolymers, depending on the types of diazoles.
The ability to control the morphologies of IL-doped PEMs by adjusting the types of ILs allowed us to explore the link between morphology and transport in a quantitative manner. Based on the decoupling of the segmental motion of the polymer chains from the ion transport, we were able to confirm that the morphology indeed exerted an effect on the anhydrous conductivities of the IL-impregnated PSS-PMB copolymers. As illustrated in Figure 8(b), for LAM and HEX-forming systems, normalization of the conductivities relative to those of IL-embedded PSS homopolymers yielded a similar morphology factor of 0.4. In contrast, the gyroid-forming sample exhibited apparently high morphology factor in the range of 0.6 to 0.7. Although these values are a bit lower than the theoretical expectations of 0.67 (LAM) and 1.0 (gyroid),89 we can infer that the higher morphology factor of the gyroid-forming membranes is intimately related to the better connectivity of ionic channels along the co-continuous PSS phases in this structure. Thus, the creation of gyroid morphology in the PEMs should be beneficial for improved transport efficiency.
From the results discussed thus far, we can conclude that the optimization of both ILs and sulfonated polymers based on molecular characteristics as well as morphological features is a promising avenue for enhancing the anhydrous conductivity of IL-integrated sulfonated PEMs. Such membranes can be utilized not only for high temperature fuel cells but also for diverse applications such as electro-active actuators90 and battery electrolytes.91
The ion transport properties of sulfonated PEMs were explored under humidified and anhydrous conditions. First, a wide variety of approaches for enhancing the transport properties of hydrated sulfonated PEMs were elucidated; these include (1) control of the ion concentration within the PEMs either by tailoring the chemical moieties (degree of sulfonation) or judicious structural design (confinement by tethering the hydrophobic chains), (2) the creation of well-defined nanoscale morphologies of the sulfonated PEMs based on fundamental understanding of the thermodynamic properties, allowing the formation of less tortuous ion pathways, (3) control of the size and curvature of microdomains to manipulate the water retention properties and the ion transport rate, and (4) alignment of ordered ionic domains using various external fields to route the ion transport toward targeted directions. Recent research efforts towards developing new PEMs aimed at high temperature fuel cells operating under anhydrous conditions were described and the link between the morphology and the conductivity of sulfonated PEMs comprising heterocyclic diazole-based ILs was investigated, highlighting the important role of the types of self-assembled structures in determining the transport efficiency. The effects of the types of cations and anions in the ILs on the thermodynamics and conductivity of IL-integrated sulfonated PEMs were also discussed. As the control parameters are becoming more diverse for IL-integrated PEMs, further systematic work is required to establish prospective avenues for obtaining desired ion transport properties. This involves the discovery of alternative protic solvents, taking into account the high penalty for proton conduction for diazoles, and the use of new sulfonated polymers with novel molecular architectures to facilitate access to synergetic effects for enhancing the anhydrous transport properties.
This work was financially supported by Basic Science Research Program (Project No. 2012-0001993), by Midcareer Researcher Program (Project No. 2012-0005267), and WCU (World Class University) program (Project No. R31-10059) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology. The authors also acknowledge the Global Frontier R&D program on Center for Multiscale Energy System funded by the NRF under the Ministry of Education, Science and Technology.
Moon Jeong Park obtained her Ph.D. in Chemical Engineering (2006) from the Seoul National University and moved to University of California at Berkeley as a Postdoctoral Fellow. She joined the faculty of Chemistry at the Pohang University of Science and Technology (POSTECH) as an Assistant Professor in 2009 and became an Associate Professor in 2013. Her main research interests are synthesis and characterization of soft materials and determination of charge and ion transport through these materials with a focus on their structure-property relationship.
Sung Yeon Kim received her Bachelor of Science in Chemistry from Ewha Womans University (Korea) in 2009. She is currently a 4th-year Integrated MS/Ph.D. candidate in the Division of Advanced Materials Science at Pohang University of Science and Technology (POSTECH). Her research under the guidance of Prof. Moon Jeong Park is interested in Polymer Electrolyte Membrane on phase behavior and electrochemical properties of ion-containing block copolymers.