Review pubs.acs.org/CR
Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability Dong Won Shin,†,‡ Michael D. Guiver,§,∥,† and Young Moo Lee*,† †
Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 04763, Republic of Korea Fuel Cell Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea § State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China ∥ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ‡
ABSTRACT: A fundamental understanding of polymer microstructure is important in order to design novel polymer electrolyte membranes (PEMs) with excellent electrochemical performance and stabilities. Hydrocarbon-based polymers have distinct microstructure according to their chemical structure. The ionic clusters and/or channels play a critical role in PEMs, affecting ion conductivity and water transport, especially at medium temperature and low relative humidity (RH). In addition, physical properties such as water uptake and dimensional swelling behavior depend strongly on polymer morphology. Over the past few decades, much research has focused on the synthetic development and microstructural characterization of hydrocarbon-based PEM materials. Furthermore, blends, composites, pressing, shear field, electrical field, surface modification, and cross-linking have also been shown to be effective approaches to obtain/maintain well-defined PEM microstructure. This review summarizes recent work on developments in advanced PEMs with various chemical structures and architecture and the resulting polymer microstructures and morphologies that arise for potential application in fuel cell, lithium ion battery, redox flow battery, actuators, and electrodialysis.
CONTENTS 1. Introduction 2. Relationship between Structure and Morphology/Properties of Proton Exchange Membranes for Fuel Cells 2.1. Random Copolymers 2.2. Block Copolymers 2.3. Graft Copolymers 2.4. Densely Sulfonated Copolymers 3. Morphology and Electrochemical Properties of Tuned Sulfonated Polymers by Physical/Chemical Methods 3.1. Physical Approaches 3.2. Chemical Cross-Linking 4. Morphology and Electrochemical Properties of Anion Exchange Membranes for Fuel Cells 5. Importance of Morphology on Other Electrochemical Applications 5.1. Lithium Ion Battery 5.2. Redox Flow Battery 5.3. Actuator 5.4. Electrodialysis 6. Conclusions and Future Prospects Author Information Corresponding Author ORCID Notes
© 2017 American Chemical Society
Biographies Acknowledgments References
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1. INTRODUCTION Polymer electrolyte membranes (PEMs) have been utilized in various applications including fuel cells, redox flow batteries, and electrodialysis.1−4 Polymer electrolyte membrane fuel cells (PEMFCs) are energy devices which generate electricity efficiently from the electrochemical reaction of hydrogen or
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Figure 1. General chemical structure of perfluorosulfonic acid polymers (PFSAs). Received: August 29, 2016 Published: March 3, 2017 4759
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medium-temperature (70%) than Nafion. This is due to facilitated transport of proton ions through the interconnected hydrophilic ionic nanochannels. In addition, these triblock copolymers showed anisotropic dimensional water swelling, while Nafion exhibited isotropic swelling behavior. Dimensional swelling in the through-plane direction is much more preferable than inplane swelling, because in-plane swelling leads to delamination of the catalyst and gas diffusion layer from the membrane. In general, polymers lose their mechanical strength after hydrophilic modification because of increased water absorption. Chain entanglement of hydrophilic polymer decreases due to the presence of additional bulky functional groups. An ABA triblock copolymer composed of external hydrophobic A oligomers and internal hydrophilic B oligomer may overcome this drawback. External hydrophobic A blocks support mechanical stability, whereas internal hydrophilic B blocks conduct ion transport leading to both excellent mechanical robustness and ion conductivity. Fan et al. reported similar ABCBA pentablock copolymers comprised of tert-butylstyrene (A) and hydrogenated isoprene (B) external hydrophobic blocks and partially sulfonated polystyrene (C) internal hydrophilic block.41 Although the modulus of pentablock copolymer slightly decreased after functionalization and significantly dropped above the Tg of polystyrene (∼102 °C), the polymer still retains reasonable mechanical properties because of A and B external hydrophobic blocks, as shown in Figure 11a. The mechanical property and phase-separation behavior of the polymer showed a positive correlation with the 4765
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Table 2. Properties of Sulfonated Block Copolymers
sample HFB (10−10)
architecture block [Figure 8]
DFBP (10−10) R-SPSN B-SPSN PPO-PAES-PPO PBC-0.0 PBC-1.0 PBC-1.5 PBC-2.0 SPAE-PI (5−5) SPAE-PI (20−20) SS-ES SS-FS 5k-5k 7k-7k 17k-12k 25k-16k 11k-19k 11k-14k 11k-10k 8k-17k 8k-11k 8k-8k random 11k-11k 81k-81k
random block [Figure 9] block [Figure 10] block [Figure 11]
size of hydrophilic domain [nm]
IEC [mequiv g−1]
connected channel connected channel
5
connected channel connected channel
morphology
connected channel connected channel connected channel block connected [Figure 12] channel connected channel block connected [Figure 13] channel connected channel block connected [Figure 14a] channel connected channel connected channel connected channel block connected [Figure 14b] channel connected channel connected channel connected channel connected channel connected channel random isolated cluster block isolated [Figure 14c] cluster connected channel
WU [%]
water uptake measurement conditions [°C-% RH]
proton conductivity [mS cm−1]
proton conductivity measurement conditions [°C-% RH]
ref
1.38
68
25−100
0.8
80−30
50
8
1.28
60
1.52 1.58
88 39
30−100
1 2
80−30
51
15 10
1.28
62
25−100
10
80−30
52
30−100
41
80−30
53
120−50
54
80−30
56
80−30
57
50−30
58
4
27 26
1.00
2
27
1.50
44
29
2.00
99
5
1.65
59
7
1.22
57
15
1.59
30
15
1.60
50
10
1.31
20
12
1.30
22
0.6
12
1.32
42
0.8
12
1.29
44
1
10
1.02
40
10
1.27
55
10
1.67
82
10
1.07
25
10
1.27
39
2
10
1.56
46
3
5
1.64
33
10
1.66
30
1.71
partially fluorinated hydrophobic repeat unit magnified contrast between hydrophilic and hydrophobic domains. For this reason, the SU-FS membrane exhibited much higher proton conductivity than SU-ES membrane despite having similar IEC values, as listed in Table 2. Moreover, the SU-FS block copolymer showed a somewhat similar proton and water diffusion coefficient as Nafion due to the developed hydrophilic domains. The block length of the hydrophilic and hydrophobic repeating unit is a critical factor affecting the morphology formation behavior.55 Figure 14 shows chemical structures of two representative block copolymers. McGrath and co-workers synthesized SPAE-based block copolymers (14a) with different
25−100
0.9 28
30−100
0.2
25−100
1
50−90
0.06 0.08
43
0.15
hydrophilic and hydrophobic block lengths.56 While all block copolymers had similar IEC values, their proton conductivity increased with increasing block length because the connectivity of hydrophilic domains was improved by increasing the block length, as shown in Table 2 and Figure 15a. Jannasch’s group demonstrated the effect of hydrophobic block length on phaseseparated microstructure (see chemical structure for 14b and Figure 15b).57 The block copolymers had similar hydrophilic domain sizes due to the same hydrophilic block length. However, the size of the hydrophobic domains gradually increased relative to the hydrophilic chain length. Consequently, the short hydrophobic block increased the proton conductivity by increasing the IEC value and decreasing the 4766
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increased with increasing block length, though defects may remain above certain block lengths despite the formation of more uniformly oriented domains, as represented in Figure 16.59 Thus, the control of block length is important to form well-defined morphology in block copolymers. 2.3. Graft Copolymers
Acid-bearing graft copolymers, which structurally mimic PFSAs, are synthesized by attaching hydrophilic aromatic or aliphatic side chains to the hydrophobic main chain or vice versa. Similar to block copolymers, graft copolymers are composed of different homosequences of ion-conducting and nonconducting units, which facilitate the formation of phase-separated morphology. The morphology of graft copolymers can be controlled by varying the length and periodicity of the side chains. The side-chain length and rigidity affect the size of hydrophilic ionic clusters or nanochannels, whereas the frequency of side chains over the polymer main-chain length influences the number of ionic domains per unit volume, allowing graft copolymers to be tailored for the desired electrochemical performances. We reported the synthesis of SPAE-based copolymer with sulfonated diphenyl ketone (SK) and sulfonated PPO (SPPO) side chain, as shown in Figure 17.60,61 These comb-shaped copolymers containing sulfonic acid groups in the side chain showed well-defined phase-separated hydrophilic and hydrophobic domains, which resulted from the hydrophobic SPAE main chain and the hydrophilic SK and SPPO side chain. In particular, the hydrophilic domain of this comb-shaped copolymer consisted of ionic clusters, interconnected with ionic channel-like morphology similar to PFSAs. The SPAE-SK membrane had isolated ionic clusters at the 50−60 nm scale, and the SPAE-SPPO membranes represented connected ionic channel at the 3−20 nm scale. The continuous hydrophilic domains facilitated proton transport leading to high proton conductivity over a wide range of relative humidity conditions. In addition, the proton conductivity was stably maintained due to the nature of the chemical structure with pendent sulfophenyl groups. As a result, the SPAE-SPPO showed excellent proton conductivity (20 mS cm−1) at 90 °C and 30% RH. Jannasch and co-workers reported polysulfone-based random copolymers containing disulfonaphthoxybenzoyl (PSU-dsnb) or trisulfopyrenoxybenzoyl (PSU-tspb) side chains.62 Small angle X-ray scattering (SAXS) is an effective tool for estimating the morphology by calculating the interdomain distance from q values of scattering maxima and observing scattering profiles. Figure 18 shows the chemical structures of PSU random copolymers containing sulfonic acid groups on the main chain or side chain. The random copolymer having two sulfonic acid groups on the main chain (PSU-ds) exhibited a broader SAXS profile than those of PSU-dsnb and PSU-tspb. In addition, the scattering maxima of PSI-ds appeared at higher q range than those of PSU-dsnb and PSU-tspb. These data indicated less phase-separated morphology and a wider distribution of ionic domains for PSU-ds. The short flexible side chain enhanced the mobility and hydrophilicity of sulfonic acid groups by separating from hydrophobic main chain leading to welldefined morphology. In addition, PSU-tspb membranes showed much sharper peaks as the DS was increased. This implies that the DS also affects the size and uniformity of ionic clusters. PSU-dsnb and PSU-tspb membranes showed excellent proton
Figure 10. Chemical structure of sulfonated poly(phenylene oxide) triblock copolymer (IEC = 1.28 mequiv g−1) and its (a) AFM and (b) TEM images. Adapted with permission from ref 52. Copyright 2012 Royal Society of Chemistry.
Figure 11. Chemical structure of ABCBA pentablock copolymers and their (a) viscoelastic properties and (b) SAXS patterns. Adapted with permission from ref 41. Copyright 2014 Elsevier.
volume fraction of the hydrophobic domain. Block length also influenced defects in the phase-separated domains as well as their size and connectivity. Kins et al. investigated the relationship between the phase-separation behavior and the block length of imide-based block copolymers (see the chemical structure for 14c and Figure 15c).58 A block copolymer having a molecular weight of 11 kg mol−1 for each hydrophilic and hydrophobic oligomer showed highly clustered hydrophilic domains compared to random copolymers despite having the same IEC value (1.66 mequiv g−1). The hydrophilic domain size of the block copolymer incorporating 81 kg mol−1 oligomer molecular weight showed the most significantly improved phase-separation behavior and ion conductivity among the polymers tested. The domain size and connectivity 4767
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Figure 12. Chemical structure of sulfonated poly(arylene ether) (SPAE) hydrophilic oligomer and polyimide (PI) hydrophobic oligomer and morphology images of SPAE-PI block copolymers (IEC = 1.22−1.65 mequiv g−1). Adapted with permission from ref 53. Copyright 2007 Springer.
Aliphatic side chains, which are more flexible than aromatic ones, are also an effective structure for developing phase separation. Sulfobutyl aliphatic side chains were introduced onto SPAE-based main-chain polymer to induce microphase separation in random copolymer.63−65 Figure 19 represents the chemical structure of aliphatic side-chain-induced SPAE random copolymer (S2-SPAE). This random copolymer with clusters of sulfonic acid groups tethered on flexible side chains exhibited well-developed hydrophilic−hydrophobic-separated microstructures, as shown in Figure 19. Aliphatic side-chain graft copolymers showed much smaller hydrophilic ionic clusters compared with aromatic side-chain-grafted copolymer due to the smaller molecular volume of aliphatic side chain. The S2-SPAE graft copolymer showed high proton conductivity (183 mS cm−1) due to well-defined hydrophilic ionic clusters despite having a relatively low IEC value (1.60 mequiv g−1). The acidity of side chains in the polymer also affects the morphology and properties. Bae and co-workers synthesized polysulfone random copolymers functionalized with various strongly acidic pendant fluorosulfonic acid groups, as shown in Figure 20.66 All side-chain copolymers had narrower hydrophilic domains (1−3 nm) than Nafion (3−5 nm), resulting in lower conductivity at low relative humidity conditions (100 °C) and low-RH ( BF4 > PF6, and the temperature dependence also followed the Tg of the composite membranes (TFSI > BF4 > PF6). As a result, their actuator performance at room temperature also follows the order of ion conductivity. 5.4. Electrodialysis
In electrodialysis, ion selectivity is also important as well as ion conductivity. Most research has overlooked the importance of morphology on electrodialysis performance. Recently, we reported the effect of membrane surface morphology on electrodialysis performance in terms of permselectivity using plasma-treated membranes.45 Plasma-treated membranes showed unusual ion transport behavior for other electrochemical applications (e.g., reverse electrodialysis) (see Figures 44 and 77). They had improved ion selectivity of monovalent ions, such as sodium ions (Na+) and chloride ions (Cl−), while their membrane resistance was constant. Moreover, membrane surface nanocracks limited co-ion transport, resulting in an enhancement in permselectivity. Consequently, modification of both bulk morphology control and surface morphology is effective in facilitating ion transport because ions should be adsorbed on the membrane surface first to transport through to the other side of the membrane.
6. CONCLUSIONS AND FUTURE PROSPECTS Polymer chemical structure and architecture as well as posttreatment are important avenues for the investigation and development of advanced PEMs with microphase separation. The morphology of sulfonated random copolymers depends on the appropriate selection of monomer structure to generate hydrophilic ionic clusters. Much research has been devoted to inducing microphase separation in sulfonated random copolymers by controlling the size of hydrophobic moieties or introducing spacer groups and the number and proximity of hydrophilic ion-conducting groups, which influenced the distance between hydrophilic domains. In addition, the morphology of sulfonated polymers appeared to be affected by acidification conditions. Compared with random copolymers, block copolymers induce more ordered microphase-separated morphology because the different characteristics and length of each block induced phase separation. The chemical structure of hydrophobic sequences is one of the critical factors leading to phase separation. Fluorine atoms, which have an extremely hydrophobic characteristic, magnify the contrast between hydrophilic and hydrophobic domains, resulting in well-ordered phaseseparated morphology. Furthermore, oligomer block lengths
Figure 54. Chemical structures and (a) SAXS patterns of imidazolium incorporated poly(styrene- b-acrylate) block copolymers. Adapted with permission from ref 137. Copyright 2013 American Chemical Society. (b) SAXS profiles of phosphonium-incorporated poly(styrene-b-acrylate) block copolymers. Adapted with permission from ref 138. Copyright 2014 American Chemical Society.
The same group incorporated ionic liquid as an additional ionic conductor into polystyrene and polyacrylate-based block copolymers containing a zwitterionic group (SB-DMAEA).167 The zwitterionic group improved affinity toward ionic liquids. Pristine block copolymer showed long-range ordered phase separation because of strong electrostatic interactions between zwitterionic groups (see Figure 75). This long-range ordered morphology was maintained after incorporation of ionic liquids up to 22 wt %, while the scattering maxima shifted to lower q values and the long-range ordered peak broadened. The difference in polarity between trifluoromethanesulfonate 4792
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Figure 55. (a) Chemical structure, (b) TEM image, (c) SAXS patterns, and (d) dependence of hydroxide conductivity on RH of PHMA-PS triblock copolymers. Adapted with permission from ref 139. Copyright 2013 American Chemical Society.
quaternary ammonium, guanidine, imidazolium, etc.) into hydrophobic units. Densely clustered functional groups shorten the ion-conducting pathway by self-aggregation, improving the connectivity of hydrophilic domains to a greater extent compared with those of conventional copolymers. Modification methods such as composite, blend, and crosslinking also influence the polymer morphology as well as the chemical architecture. Inorganic fillers can act as a seed for ionic clusters, leading to well-defined morphology. In addition, phase contrast is intensified by blending various homo- or copolymers with different characteristics. Application of pressing, E-field, and shear field into the membrane changed the morphology (connectivity and alignment) of the ionic domain compared with as-cast membrane, which influenced the ionic conductivity. Surface plasma treatment on sulfonated hydrocarbon polymer membranes produced nanocracks while maintaining the bulk morphology, which acted as nanovalves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. These membranes with surface nanocrack coating operated at intermediate temperatures show improved electrochemical performance.45 Polymer chain rearrangement during cross-linking causes morphological transformation. For this reason, cross-linked membranes showed well-defined morphology, whereas non-cross-linked membranes exhibited homogeneous phase images. A well-defined polymer morphology improved the electrochemical performance in various applications like rechargeable secondary battery, redox flow battery, and electroactive polymer (actuator) as well as fuel cell. However, more interconnected hydrophilic domains facilitate liquid transportation like vanadium redox couples and methanol. Therefore, the morphology should be well controlled, especially in liquidbased electrochemical systems, like a redox flow battery and direct methanol fuel cell. The size of the hydrophilic ionic clusters or ionic channels affects the membrane stability and ion conductivity. In particular, the connectivity of hydrophilic domains appears to be the most dominant factor in ion
Figure 56. Chemical structures and morphology images of poly(aryl ether sulfone) (PES)-based grafted copolymers containing pendent quaternary ammonium group (PES-P-OH = 1.50 mequiv g−1 and PESPF-OH = 1.44 mequiv g−1). Adapted with permission from ref 140. Copyright 2010 Elsevier.
influenced the morphological features and interdomain distances. Longer block lengths induced clearer phase separation and increased domain sizes. Pendent or side-chain-grafted copolymers also showed developed hydrophilic domains. In grafted copolymers, the flexibility and chain length of pendent or side chains are important for morphology control. Long aliphatic side chains formed much more connected ionic domains than short aromatic pendent groups because they can easily aggregate each other. Different characteristics between hydrophobic main chain and hydrophilic side chain (or hydrophilic main chain and hydrophobic side chain) induced a well-ordered morphology like block copolymers. Furthermore, the hydrophilic−hydrophobic phase-separation behavior of polymers is enhanced by introducing densely clustered ion-conducting functional groups (e.g., sulfonic acid, 4793
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Figure 58. Chemical structures and AFM images of (a) BPPO-QVBC (IEC = 1.55 mequiv g−1) and (b) QPPO (IEC = 2.09 mequiv g−1) copolymers. Adapted with permission from ref 143. Copyright 2013 Royal Society of Chemistry.
Figure 59. Chemical structure and AFM images of DIm-PPO copolymers (DIm-PPO-0.42 = 2.50 mequiv g−1 and DIm-PPO-0.54 = 2.80 mequiv g−1). Scan size is 500 nm × 500 nm. Adapted with permission from ref 144. Copyright 2013 Royal Society of Chemistry.
Figure 57. Chemical structures of (a) PPO-DMHDA (55 = 1.92 mequiv g−1, 40 = 1.67 mequiv g−1, 30 = 1.48 mequiv g−1, and 20 = 1.08 mequiv g−1) and (b) PPO-TMA (20 = 1.39 mequiv g−1), (c) their SAXS patterns, and (d) stability of ion conductivity. Adapted with permission from ref 141. Copyright 2012 Royal Society of Chemistry.
transport. Block, grafted, and comb-shaped copolymers are the most effective polymer architectures for formation of welldefined morphology by introducing different characteristics in each sequence. Chain mobility and IEC value are the most critical factors affecting the size and connectivity of hydrophilic ionic domains. The size and connectivity of hydrophilic channels can be controlled by changing the chain length onto which ion exchange groups are attached. There appears to be no ideal hydrophilic domain size for high ionic conductivity. However, it should be optimized according to the nature of polymers (i.e., hydrophilic or hydrophobic) to obtain the best combination of ionic conductivity and mechanical robustness because wide hydrophilic domains can improve ionic
Figure 60. (a) Chemical structure and (b) AFM image of alkyl side chain containing PAEK (IEC = 1.75 mequiv g−1). Scan size is 500 nm × 500 nm. Adapted with permission from ref 145. Copyright 2013 Royal Society of Chemistry.
conductivity but may also diminish mechanical properties by absorbing too much water. Well-connected ionic domain morphology decreases the dependence of ion conductivity on temperature and humidity because polymer electrolyte membranes having well-ordered morphology showed excellent ion conductivity and electrochemical performance not only at moderate-temperature (50%) 4794
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Figure 61. Chemical structure, (a) SAXS profile, and (b) chemical stability of PEG-grafted polysulfone copolymers (QA PSf = 1.98 mequiv g−1, QA PSf-g-PEG350 = 1.36 mequiv g−1, and QA PSf-gPEG750 = 1.57 mequiv g−1). Adapted with permission from ref 146. Copyright 2014 Royal Society of Chemistry.
Figure 63. (a) Chemical structures, (b) SAXS profile, and (c) ion conductivity of densely functionalized poly(arylene ether sulfone) random copolymers (c4PAES-zQ = 0.9−2.5 mequiv g−1 and c4PAESzIm = 1.0−2.5 mequiv g−1). Adapted with permission from ref 147. Copyright 2015 Elsevier.
drawing ionic groups weaken the chemical stability of adjacent chemical bonds.170 Consequently, chemical bonds in the hydrated hydrophilic domains are more readily susceptible to attack by nucleophiles like hydroxide ions or radicals. On the basis of this point, an effective way to improve the membrane chemical stability is to design the chemical structure without aryl−ether bonds, which is well known as a weak point in polymer backbones synthesized by condensation reactions. Furthermore, well-ordered morphology can enable polymers to have better chemical and mechanical stabilities by decreasing water uptake, which also reduces the susceptibility to attack by chemical species present in the system.36 On the point of physical degradation of the membranes, excessive swelling is the most critical factors of membrane failure. Membranes having ordered morphology showed anisotropic swelling behavior (through plane > in plane) which can reduce physical degradation of the membrane. Interestingly, connected ionic domains with small interdomain distances (80 °C) and lowRH (
