Acid-Group-Content-Dependent Proton Conductivity Mechanisms at

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Acid-Group-Content-Dependent Proton Conductivity Mechanisms at the Interlayer of Poly(N-dodecylacrylamideco-acrylic acid) Copolymer Multilayer Nanosheet Films Takuma Sato, Mayu Tsukamoto, Shunsuke Yamamoto, Masaya Mitsuishi, Tokuji Miyashita, Shusaku Nagano, and Jun Matsui Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03160 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Acid-Group-Content-Dependent Proton Conductivity Mechanisms at the Interlayer of Poly(N-dodecylacrylamide-co-acrylic acid) Copolymer Multilayer Nanosheet Films Takuma Sato,† Mayu Tsukamoto, † Shunsuke Yamamoto,‡ Masaya Mitsuishi, ‡ Tokuji Miyashita, ‡ Shusaku Nagano,§ and Jun Matsui⊥* †

Graduate School of Science and Engineering, Yamagata University, 1-4-12 Kojirakawamachi, Yamagata 990-8560, Japan

‡

Institute for Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

§

Nagoya University Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan

⊥

Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560,

Japan *Author to whom correspondence should be addressed; e-mail: [email protected]

ACS Paragon Plus Environment

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Abstract The effect of the content of acid groups on the proton conductivity at the interlayer of polymer-nanosheet assemblies was investigated. For that purpose, amphiphilic poly(Ndodecylacrylamide-co-acrylic acid) copolymers [p(DDA/AA)] with varying contents of AA were synthesized by free radical polymerization. Surface pressure (π)–area (A) isotherms of these copolymers indicated that stable polymer monolayers are formed at the air/water interface for AA mole fraction (n) ≤ 0.49. In all cases, a uniform dispersion of the AA groups in the polymer monolayer was observed. Subsequently, polymer monolayers were transferred onto solid substrates using the Langmuir-Blodgett (LB) technique. X-ray diffraction (XRD) analyses of the multilayer films showed strong Bragg diffraction peaks, suggesting a highly uniform lamellar structure for the multilayer films. The proton conductivity of the multilayer films parallel to the direction of the layer planes were measured by impedance spectroscopy, which revealed that the conductivity increased with increasing values of n. Activation energies for proton conduction of ~0.3 eV and 0.42 eV were observed for n ≥ 0.32 and n = 0.07, respectively. Interestingly, the proton conductivity of a multilayer film with n = 0.19 did not follow the Arrhenius equation. These results were interpreted in terms of the average distance between the AA groups (lAA), and it was concluded that for n ≥ 0.32, an advanced 2D hydrogen bonding network was formed, while for n = 0.07, lAA is too long to form such hydrogen bonding networks. The lAA for n = 0.19 is intermediate to these extremes, resulting in the formation of hydrogen bonding networks at low temperatures, and disruption of these networks at high temperatures due to thermally induced motion. These results indicate that a high proton conductivity with low activation energy can be achieved, even under weakly acidic conditions, by arranging the acid groups at an optimal distance.


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Introduction Proton-conductive polymer electrolytes have attracted much attention due to their potential applications in fuel cells, sensors, and as model systems for proton transfer in biological processes.1-3 Perfluorosulfonic acid polymers such as Nafion have been widely used as polymer electrolyte membranes due to their high proton conductivity.4 The high proton conductivity in Nafion membranes has been attributed to nanosized hydrophilic ion channels.5,6 Therefore, accomplishing high proton conductivities by controlling the hierarchical structure of polymer electrolytes has become an actively investigated area of research.7 For example, Elabd et al., Kawakami et al., and Tanaka have prepared nanofibers of polymer electrolytes using electrospinning methods to produce one-dimensional (1D) proton-conductive nanochannels, and the proton conductivity of these nanofibers is more than one order of magnitude higher than that of the bulk material.8-10 Several groups have reported an enhanced proton conductivity in the two-dimensional (2D) nanospace in lamellae-structured films.11 Such films were prepared by the self-assembly of block polymers12,13 and liquid crystals,14 as well as by layer-by-layer (LbL) deposition15-17 and Langmuir-Blodgett (LB) techniques.18,19 Recently, we have reported an application of a hydrophilic interlayer in polymer nanosheet multilayer films as 2D proton-conductive nanochannels.18-20 One of the film were constructed by continuous deposition of poly(Ndodecylacrylamide-co-acrylic acid) [p(DDA/AA): Figure 1] monolayer using the LB technique.20 Thus, the multilayer forms showed a well-defined and uniform lamellar structure, which is composed of hydrophobic layers consisting of dodecyl side chains and hydrophilic layers consisting of amide and AA groups. The hydrophilic interlayers act as 2D proton-conductive nanochannels, which results in a high proton conductivity (5.0 × 10-2 S/cm at 60 ˚C under 98% RH) that is comparable to that of Nafion films. However, the mechanistic details that underpin this high conductivity remain to be elucidated.


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In biological systems, protons are used to produce energy. For example, the gradient of the proton concentration across a bilayer membrane generated by proton pumps produces the proton motive force (PMF) for the synthesis of ATP.21 Although the detailed mechanism for the proton flux at the membrane still remains a matter of debate, several groups have suggested that the pumped protons efficiently diffuse at the membrane surface to maintain a large PMF for the synthesis of ATP.22-27 Consequently, the interest in proton conductivity at membrane surfaces is high. Several groups have applied a lipid monolayer as a model system to study proton dynamics on bilayer membrane surfaces. The diffusion behavior of protons at the lipid monolayer surface has been studied using e.g. fluorescence,28-30 redox,31,32 or ion probes.28,33 However, direct measurements of the proton conduction at the lipid monolayer have so far failed to provide unified results due to the difficulties associated with the measurement systems.34-36 Polymer nanosheet multilayer films are usually prepared by deposition of polymer monolayers using the LB technique, whereby the hydrophilic interlayers resemble a bilayer membrane surface. In other words, such polymer nanosheet multilayer films can be used as a model to study proton conduction at the membrane surface. In this study, we studied the effect of the AA mole fraction (n) on the proton conduction in the 2D nanospace in order to elucidate the origin and mechanism underlying the high conductivity at the interlayer of the polymer nanosheet multilayer films. For that purpose, we prepared p(DDA/AA) with different AA contents by free radical copolymerization. The multilayer structures of p(DDA/AA) were studied by surface pressure (π)-area (A) isotherms and X-ray diffraction analyses. The proton conductivity levels of these p(DDA/AA) multilayer films via the 2D conductive nanochannels were studied by impedance measurements. The proton conductivity increased with increasing content of AA groups, whereas the activation energy (Ea) for the proton conduction did not show a linear correlation with the AA content. Ea values of ~0.3 eV and 0.45 eV were determined for n ≥


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0.32 and n = 0.07, respectively. Interestingly, the proton conduction of a polymer multilayer film constructed from p(DDA/AA) with n = 0.19 did not follow an Arrhenius-type behavior. The proton conductivity initially increased with increasing temperature up to 50 ˚C, before decreasing at 60 ˚C. The unique dependence of Ea on the AA content was interpreted in terms of the formation of an advanced 2D hydrogen-bonding network.

Experimental section. DDA was purchased from TCI and recrystallized from chloroform/hexane prior to use. AA was purchased from TCI and used without further purification. Random p(DDA/AA) copolymers were synthesized by free radical copolymerization of DDA with AA in toluene (total monomer concentration: 0.2 M) at 60 ˚C using 2,2’-azobis-(isobutyronitrile) as the thermal initiator (1% relative to the total monomer concentration). The obtained copolymers were isolated and purified by repeated precipitation from toluene into an excess of acetonitrile (at least three cycles). The mole fractions of AA in the copolymers (n) were determined by elemental analysis. Molecular weights were determined by gel permeation chromatography using polystyrene standards, and are summarized in Table 1. The π−A isotherm measurements and the deposition of the p(DDA/AA) monolayers were carried out using a computer-controlled Langmuir trough (FSD-50 and 51; USI). Distilled and deionized water (>17.5 MΩ cm; Smart2Pure, Thermo Scientific) was used as the subphase, and the copolymer monolayers were compressed at 15.0 cm2/min. All copolymer monolayers were transferred onto solid substrates by a vertical deposition method using a dipping speed of 10 mm/min and a surface pressure of 40 mN/m at 20 °C. The glass and silicon substrates on which the monolayers were deposited were cleaned by treatment with a UV−O3 (SSP16-110, SEN Lights Corp.) apparatus. Subsequently, the surfaces of these substrates were made hydrophobic by immersion into a chloroform solution that contained ~1 × 10−6 M


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octadecyltrichlorosilane. Interdigitated array (IDA) electrodes were fabricated by consecutive thermal deposition of Ti (2 nm) and Au (20 nm) using a metal mask onto a hydrophobic glass substrate. The IDA electrodes exhibited two sets of comb-type Au arrays. Each array contained eight electrode elements (width: 0.5 mm; length: 8.0 mm; separation between elements: 0.2 mm). X-ray diffraction measurements were carried out using a Rigaku Smartlab diffractometer (Rigaku Corporation) with a Cu-Kα X-ray source (λ = 0.1542 nm) and a scintillation counter detector. The measurements were carried out by the symmetrical reflection geometry (θ-2θ) method. The data collection time was 3 s per step at 0.02˚ intervals. Impedance measurements were carried out using a 1260 Impedance/Gain-Phase Analyzer with a 1296 Dielectric Interface system (Solartron Analytical) under humidity- and temperature-controlled conditions in a chamber (SH-221; Espec Corp).

Figure 1

Chemical structure of p(DDA/AA).

Table 1 Characterization of p(DDA/AA). Feed ratio DDA:AA

n

Mn ×10-4

Mw/Mn (PDI)

9:1

0.07

1.9

2.1

8:2

0.19

2.3

3.0

7:3

0.32

1.6

1.9

6:4

0.40

1.4

3.2

5:5

0.49

0.87

2.6

4:6

0.59

1.0

3.3

2:8

0.78

0.52

3.1


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Results and discussion. Properties of the p(DDA/AA) monolayers at the air/water interface. The synthesis of p(DDA/AA) copolymers with different AA ratios (n) was accomplished by simply changing the feed ratio of DDA and AA (Table 1). The properties of the p(DDA/AA) monolayers were studied by depositing the copolymer solutions onto an air/water interface. The copolymers with n ≥ 0.59 were soluble in water and the corresponding π-A isotherms exhibited a negligible rise of the surface pressure. On the other hand, the copolymers with n ≤ 0.49 showed a steep rise in surface pressure and relatively high collapse pressure, indicating that the copolymers formed a densely packed polymer monolayer at the air/water interface (Figure 2a). The average limiting surface area (Aav) of the copolymer monolayers was determined by extrapolating the linear section of the steep increase owing to the condensed state in the π-A isotherm to zero surface pressure. The plots of Aav as a function of n showed good linearity (Figure 2b). This result suggest that the AA units were not aggregated and do not form domains in the copolymer monolayers.37,38 Indeed, the molecular area of the DDA monomer units determined by interception of the linear plot at n = 0 was 0.29 nm2/molecule, which is closed to the reported value (0.28 nm2/molecule).39 A molecular area of AA in the copolymer of 0.03 nm2/molecule was calculated by extrapolating the linear plot to n = 1. This value is more than four times smaller than that calculated using the Corey-Pauling-Koltun (CPK) space-filling model (0.14 nm2/molecule), indicating that the AA units were located below the water surface.20


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(a)

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(b)

Figure 2 (a) π-A isotherms for the p(DDA/AA) copolymers at 20 ˚C. (b) Average limiting surface area (Aav) as a function of the mole fraction of AA (n).

Structures of the p(DDA/AA) multilayer films Thirty copolymer monolayers were transferred onto Si substrates. The transfer ratios for all copolymer monolayers were 1.0 for the downward and upward strokes, resulting in Y-type LB films which adopt a hydrophilic head-to-head and hydrophobic tail-to-tail arrangement as shown in Figure 3b.40 The p(DDA/AA) layer structures were studied by XRD measurements (Figure 3a). Second-(n ≥ 0.32) and third-order (n = 0.07 and 0.19) Bragg diffraction peaks were clearly observed, indicating that the p(DDA/AA) monolayers formed a highly ordered lamellar film. The d value for the first diffraction peak was assigned to the bilayer spacing of the multilayer film. Hydrophilic co-monomers in multilayer films have been located in the hydrophilic head-to-head region, resulting in an increased bilayer thickness.20,41 For low contents of AA (n = 0.07), the Bragg peak is an overlap between the d values of the DDA bilayer (dDDA) and the DDA+AA bilayer (dAA’) regions (Figure 3b; left). Therefore, the observed d values should represent an average of these periodicities. On the other hand, for high contents of AA (n = 0.49), the diffraction arises predominantly from the periodicity of


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the AA region (Figure 3b; right). Therefore, the monolayer thickness gradually increases with increasing content of AA (Table 2). Indeed, the full width at half maximum (FWHM) of the first-order Bragg peak for the p(DDA/AA) multilayer film with an AA content of n = 0.49 (FWHM = 0.213 nm-1) is lower than that of n = 0.07 (FWHM = 0.341 nm-1), which indicates that the layer periodicity of the former exhibits higher uniformity than the latter (Figure 3b).

(a)

(b)

COOH COOH COOH

COOH

interlayer proton conductive nanochannel

COOH COOH COOH

dDDA

dAA'

dAA COOH

COOH COOH COOH COOH COOH COOH

Figure 3 (a) XRD patterns for multilayer films of p(DDA/AA) (30 layers) deposited onto hydrophilic Si substrates. (b) Schematic representation of the lamellar structures for low (n = 0.07; left) and high (n = 0.49; right) contents of AA.

We have reported that polymer nanosheet assemblies consisting of p(DDA/AA) (n = 0.44) multilayers show a high proton conductivity in parallel direction to the layers, which is comparable to that in Nafion.20 Furthermore, it is necessary to anneal the polymer nanosheet assemblies under humid conditions in order to obtain high proton conductivity levels. The annealing induces multilayer rearrangement to form highly proton-conducive 2D nanochannels through the hydrophilic interlayer region. Therefore, we studied the effects of annealing p(DDA/AA) multilayer films on their film structure. Thirty layers of p(DDA/AA) with varying contents of AA were deposited onto Si substrates, and the thus obtained


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multilayer films were annealed at 60 ˚C (98% RH) for ten days. Figure 4 shows the XRD spectra of the p(DDA/AA) multilayer films after annealing. All multilayer films exhibited strong Bragg and higher-order peaks, indicating that uniform lamellar structures were maintained in the p(DDA/AA) multilayer films after annealing and the conformation of the main chains was retained. On the other hand, the d values decreased (Table 2), which has previously been attributed to a tilting of dodecyl side chains by changing their conformation from all-trans zigzag to disordered conformation.20

Figure 4 XRD patterns for p(DDA/AA) multilayer films after annealing at 60 ˚C (98% RH) for ten days.

Table 2 Properties of the p(DDA/AA) nanosheet multilayer films.

n

d (nm) before annealing

d (nm) after annealing

Ea (eV)

lAA (Å)

0.07

3.49

3.11

0.47

22

0.19

3.63

3.14

-

13

0.32

3.75

3.15

0.30

8.9

0.40

3.80

3.24

0.31

7.4

0.49

3.88

3.39

0.30

6.4


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Proton conductivity of the p(DDA/AA) multilayer films The proton conductivity of the p(DDA/AA) multilayer films in parallel to the direction of the layer planes was studied using IDA electrodes. Ten layers of p(DDA/AA) nanosheets were deposited onto IDA electrodes and annealed at 60 ˚C (98% RH) for several tens of hours in order to obtain saturated conductivity (Figure S1). Figure 5 shows the saturated conductivity values for nanosheets of p(DDA/AA) multilayer films with varying contents of AA at 60 ˚C (98% RH). The conductivity increases with increasing content of AA, as the number of proton carriers increases with the AA concentration. The highest conductivity in the polymer multilayer nanosheets (0.059 S cm-1) was obtained for n = 0.49. This value is one of the highest values hitherto reported for polymer electrolytes that use weak acids as a proton source, and it is even comparable to that of Nafion film.20

Figure 5 Proton conductivity of p(DDA/AA) multilayer nanosheets in parallel to the lamellar layer planes. Conductivities were measured at 60 ˚C (98% RH).

Subsequently, we examined the temperature dependence for the proton conductivity of p(DDA/AA) multilayer nanosheets in order to discuss the proton conductivity mechanism at the polymer nanosheet interlayer. Figure 6 shows an Arrhenius-type plot for the proton


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conductivity of p(DDA/AA). The proton conductivity of p(DDA/AA) follows the Arrhenius equation except for n = 0.19; the conductivity for n = 0.19 increased up to 50 ˚C, but decreased at 60 ˚C. It should be mentioned that in all cases the proton conductivity exhibited a negligible temperature hysteresis (Figure S2). The Ea values for proton conduction were calculated from the least-square fit of the slopes except for n = 0.19 (Table 2). The thus obtained values were similar for n ≥ 0.32 (~0.30 eV), whereas they increased for n = 0.07 (0.47 eV). Several groups have reported an optimal acidic group distance for high proton conduction in 2D nanospaces. For example, the ideal center-to-center distance (lcc) of sulfonic acid groups in 2D interfaces has been calculated by Spohr et al. (lcc = 6.0- 8.5 Å) and Eikerling et al. (lcc = 6.5-6.8 Å).42-46 Olivieto et al. have used stearic acid Langmuir films to study the relationship between the lcc of the stearic acid and the proton conduction, and they observed the highest proton conductivity for lcc = 7.0 Å.34 These groups concluded that the high proton conductivity at lcc is a result of the formation of an advanced 2D hydrogenbonding network between the acid groups and water (Figure 7). In these hydrogen-bonding networks, the Grotthuss mechanism facilitates the transport of protons. We have calculated the corresponding center-to-center distance for the AA groups (lAA) in our polymer nanosheets in order to discuss the dependence of the proton conductivity in the 2D nanospace on the AA concentration. Based on the linear dependence of Aav to n (vide supra), it was concluded that the AA groups are not aggregated and uniformly distributed in the monolayer. Accordingly, the lAA should be calculated from Aav using the following equation (see supporting information for detail): lAA = 2

av

×

(1)

The values of lAA for all AA contents are shown in Table 2. The lAA for n = 0.07 is more than 2.6 times higher than the proposed ideal value, which suggests that the advanced 2D hydrogen-bonding network is difficult to form under these circumstances. Consequently,


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protons mainly conduct via a vehicle mechanism, which is consistent with the experimental result of a high Ea value (vide supra). The lAA values for n ≥ 0.32 (6.4-8.9 Å) are close to the optimal distance, and it can thus be concluded that the advanced 2D hydrogen-bonding network was formed at the interlayer of the multilayer film. Indeed, the Ea values for proton conduction (~0.30 eV) are similar to the theoretically calculated value (0.25 eV),43 which experimentally supports that the protons move via the Grotthuss mechanism using the advanced 2D hydrogen-bonding network. The unique temperature dependence of the proton conductivity for n = 0.19 can be explained by taking into account the following considerations: at low temperatures, the advanced 2D hydrogen-bond network forms among water molecules and the carboxylic acid moieties. There, the bonding is relatively weak, as lAA is longer than the optimum distance. At high temperatures, the hydrogen bonding is broken by the thermal motion of the molecules and the protons start to conduct mainly by Vehicle mechanism, which diffusion constant is smaller than Grotthuss mechanism47 and results in a decrease of the proton conductivity. The good agreement of the experimentally obtained lAA and Ea values with the theoretically proposed values indicates that the high proton conductivity in the interlayer of the polymer nanosheet multilayer films arises from the formation of the advanced 2D hydrogen-bonding network.


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Figure 6 Arrhenius-type plots for the proton conductivity of ten layers of p(DDA/AA) nanosheets with different AA content. Black lines represent the least square fits with r2 = 0.99147 (n = 0.07), 0.99246 (n = 0.32), 0.99346 (n = 0.40), and 0.99161 (n = 0.49).

Figure 7 Schematic illustration of the structure of the advanced 2D hydrogen-bonding network proposed by Oliveira et al.34 Left: side view; right: top view; figure adapted with permission from ref 34. Copyright 1998 American Physical Society.

Conclusion We have studied the effect of the AA content on the 2D proton conductivity at the interlayer of p(DDA/AA) nanosheet multilayer films. The proton conductivity increases with


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increasing content of AA units, whereas the activation energy (Ea) for proton conduction exhibited a discontinuous dependence on the AA content. The Ea is almost constant for n ≥ 0.32 and increased to 0.47 eV for n = 0.07. On the other hand, proton conduction for n = 0.19 did not follow the Arrhenius equation. These differences were discussed in terms of different proton conduction mechanisms using the center-to-center distance (lAA), which led to the conclusion that the formation of advanced 2D hydrogen-bonding networks is important for high proton-conductivity values and low activation energies in the 2D nanospace. In their entirety, the results presented herein indicate that not only the amount of acid groups, but also the lAA between adjacent acid groups is important to achieve high proton conductivity. In lamellae-structured polymer electrolytes, thinner lamellar spacings may induce higher proton conductivity.48 Therefore, polymer nanosheet multilayer films that form uniform and subnanometer thick 2D proton-conduction nanochannels may represent attractive materials for the preparation of highly proton-conductive films.

ASSOCIATED CONTENT Supporting Information The relationship between conductivity and humidification time, the lnσT change during heating and cooling cycles as well as derivation process of equation (1). These materials are available free of charge on the ACS publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ORCID 0000-0003-4767-4507 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Japan Society for the Promotion of Science (JSPS) in the form of Grants-in-Aid for Scientific Research B (26286010), Exploratory Research (26620201), and the Nanotechnology Platform Program (Molecule and Material Synthesis). This study was also supported by the Research Program "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" within the "Network Joint Research Center for Materials and Devices".


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REFERENCES (1) Colomban, P. Proton Conductors : Solids, Membranes, and Gels--Materials and Devices; Cambridge University Press: Cambridge ; New York, 1992. (2) Kraytsberg, A.; Ein-Eli, Y. Review of Advanced Materials for Proton Exchange Membrane Fuel Cells. Energy Fuels 2014, 28, 7303-7330. (3) Zhang, H.; Shen, P. K. Recent Development of Polymer Electrolyte Membranes for Fuel Cells. Chem. Rev. 2012, 112, 2780-2832. (4) Cui, Z.; Drioli, E.; Lee, Y. M. Recent Progress in Fluoropolymers for Membranes. Prog. Polym. Sci. 2014, 39, 164-198. (5) Kreuer, K.-D.; Portale, G. A Critical Revision of the Nano-Morphology of Proton Conducting Ionomers and Polyelectrolytes for Fuel Cell Applications. Adv. Funct. Mater. 2013, 23, 5390-5397. (6) Schmidt-Rohr, K.; Chen, Q. Parallel Cylindrical Water Nanochannels in Nafion Fuel-Cell Membranes. Nat. Mater. 2008, 7, 75-83. (7) Li, J.; Park, J. K.; Moore, R. B.; Madsen, L. A. Linear Coupling of Alignment with Transport in a Polymer Electrolyte Membrane. Nat. Mater. 2011, 10, 507-511. (8) Sadrjahani, M.; Gharehaghaji, A. A.; Javanbakht, M. Aligned Electrospun Sulfonated Polyetheretherketone Nanofiber Based Proton Exchange Membranes for Fuel Cell Applications. Polym. Eng. Sci. 2017, 57, 789-796. (9) Tamura, T.; Kawakami, H. Aligned Electrospun Nanofiber Composite Membranes for Fuel Cell Electrolytes. Nano Lett. 2010, 10, 1324-1328. (10) Tanaka, M. Development of Ion Conductive Nanofibers for Polymer Electrolyte Fuel Cells. Polym. J. 2016, 48, 51-58. (11) Lwoya, B. S.; Albert, J. N. L. Nanostructured Block Copolymers for Proton Exchange Membrane Fuel Cells. Energy and Environ. Focus 2015, 4, 278-290.


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Table of Contents Graphic


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Acid-Group-Content-Dependent Proton Conductivity Mechanisms at

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