Polyelectrolyte Composite Membranes Containing Electrospun Ion

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Interface-Rich Materials and Assemblies

Polyelectrolyte composite membranes containing electrospun ionexchange nanofibers: effect of nanofiber surface charge on ionic transport Fumiyasu Seino, Yuichi Konosu, Minoru Ashizawa, Yuriko Kakihana, Mitsuru Higa, and Hidetoshi Matsumoto Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02747 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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Polyelectrolyte Composite Membranes Containing Electrospun Ion-Exchange Nanofibers: Effect of Nanofiber Surface Charge on Ionic Transport Fumiyasu Seino, † Yuichi Konosu, † Minoru Ashizawa, † Yuriko Kakihana, ‡ Mitsuru Higa, ‡ and Hidetoshi Matsumoto†,*

†Department

of Materials Science and Engineering, Tokyo Institute of Technology, Mail Box S8-27,

2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan

‡Division

of Applied Fine Chemistry, Graduate School of Sciences and Technology for Innovation,

Yamaguchi University, and Blue Energy Center for SGE Technology (BEST), 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan *Address correspondence to [email protected]

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Abstract Poly(vinyl alcohol) (PVA)-based ion-exchange nanofibers (IEX-NFs) and their composite polyelectrolyte membranes were prepared and characterized. The PVA-based NFs are well dispersed and form a 3-dimensional network structure in the polymer matrix, Nafion. All the prepared membranes show a similar ion-exchange capacity of ~1.0 mmol g-1. The ionic conductivities through the PVA-b-PSS-NF/Nafion composite membranes are superior to that of the Nafion membrane, but the conductivity through the PVA-NF/Nafion composite membrane is half that of the Nafion membrane. Our electrokinetic measurements clearly indicate that a high density of ion-exchange groups on the NF surface result in a continuous ionic transport path in the polymer matrix. In addition, the mechanical strength of all the NF composite membranes is improved compared with that of the membrane without NF.


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Introduction Polyelectrolyte membranes or ion-exchange membranes have attracted much attention due to their potential applications in energy and environmental fields, including fuel cells, secondary batteries, and separation and purification processes1-3. In particular, polyelectrolyte membranes such as perfluorocarbon ionomers and hydrocarbon ion-exchange membranes have been commercialized and industrially used due to their high ionic conductivity and chemical stability, but further improvements in the ionic conductivity through the membranes is strongly required4. To address this issue, the formation of continuous nanoscale ionic transport pathways in a polymer matrix is essential. The use of nanostructured materials is a promising approach to improve the ionic conductivity through polyelectrolyte composites due to the increased interfacial area of the ion-permeable region (e.g., amorphous region) between nanomaterials and polymers5. In particular, one-dimensional (1D) nanomaterials can increase ionic transport pathways in a polymer matrix compared with that in 0D nanomaterials (e.g., nanoparticles)6. Among 1D nanomaterials, electrospun nanofibers (NFs) have attracted much attention due to their simplicity and versatility. Electrospinning is an electrohydrodynamic process for forming continuous thin fibers and can be used for the one-step formation of fibrous nonwoven webs7. Such nanofiber networks enable the construction of a continuous transport pathway in a polymer matrix8. There are many reports on the preparation and characterization of NF composite polyelectrolyte membranes9. It 3 ACS Paragon Plus Environment

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has been reported that the 3D nanofiber networks improve the electrochemical properties of the composites and mechanically reinforce the polymer matrix10. Several researchers pointed out that the NF surface is a key factor to construct an efficient ion conduction pathway11, but the effect of the NF surface charge on ionic transport has not been elucidated in detail. In the present study, we prepared polyelectrolyte composite membranes containing ion-exchange nanofibers (IEX-NFs) and investigated the additive effect of NFs on their properties. Herein, a commonly-used perfluorocarbon ionomer, Nafion12, was used as a polymer matrix and poly(vinyl alcohol)-based NFs, poly(vinyl alcohol-b-styrene sulfonic acid) (PVA-b-PSS)13 NFs were used as IEX-NFs. In addition, we evaluated the surface charge density of the NFs using electrokinetic measurements14 and investigated the influence of NF surface charge on the properties of ionic transport through the composite membranes.

EXPERIMENTAL SECTION Materials. Poly(vinyl alcohol)s (PVAs) with average molecular weights of 88,000 (PVA2k) and 154,000 (PVA3.5k) were purchased from Wako, Japan. Block copolymers, poly(vinyl alcohol-b-styrene sulfonic acid)s (PVA-b-PSS), PVA-b-PSS4, and PVA-b-PSS10 (each sulfonic acid group content 4 mol% and 10 mol%), were prepared by the same procedure described elsewhere15. Hydrochloric acid (HCl, extra-pure grade) was purchased from Kanto Chemical, Japan. 1-Propanol (NPA, extra-pure grade), ethanol (EtOH, extra-pure grade), potassium chloride (KCl, extra-pure grade), and pyridine (Py, 4 ACS Paragon Plus Environment

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extra-pure grade) were purchased from Wako, Japan. A 25% glutaraldehyde (GA) solution and a 20 wt% Nafion dispersion (DE2020 CS type) were purchased from Wako, Japan. These reagents were used without further purification. Ultrapure water was prepared using a water purification system (Milli-Q Advantage, Millipore, USA) and then used for the preparation of the aqueous solutions. Electrospinning. PVA-b-PSS and PVA3.5k (80:20 w/w) were dissolved in water at a total concentration of 7 wt.%. PVA2k was dissolved in a mixture of water and Py (99:1 w/w) at a total concentration of 5 wt.%. These solutions were stirred at 95 °C for 1 day. Thereafter, the solutions were cooled to room temperature and used for electrospinning. The electrospinning device was the same as that used in our previous study16. The spinning solution was contained in a syringe with a stainless-steel nozzle (0.31 mm inner diameter). The nozzle was connected to a high-voltage DC power supply (HJPQ-30P1, Matsusada Precision, Japan). A constant volume flow rate of 0.1 mL/min was maintained via a syringe-type infusion pump (MCP-III, Minato Concept, Japan). An aluminum (Al) plate was used as a collector. The applied voltage and the nozzle-to-collector distance were 16 kV, and 150 mm, respectively. All electrospinning was carried out at approximately 25 ºC and 30-40 RH%. After deposition, as-spun NF webs with a thickness of approximately 30 m were obtained. Japanese calligraphy paper was used to cover the Al plate to facilitate the removal of the as-spun webs. To improve the water resistance of the webs, the as-spun NFs were cross-linked in a vapor from a 25% GA/DMSO solution and 35% HCl/DMSO solution at 25 ºC for 48 h.


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Membrane preparation. NF composite membranes were prepared by the casting method. A 20 wt% Nafion dispersion was cast on PVA NF webs. A 10 wt% Nafion dispersion diluted with the mixed solvent, H2O/NPA/EtOH (=49.4:39.6:1 in weight) was cast on the PVA-b-PSS NF webs. The NF content of the composite membranes was fixed at approximately 15 wt%. For comparison, an as-cast Nafion membrane without NF webs was also prepared. The membrane thickness was determined by a height gauge (digimatic indicator ID-C112AXB, Mitutoyo, Japan). The measurements were carried out at least 5 times for each membrane. All data were reproducible within 0.5 m. Characterization of the NFs. The morphologies of the as-spun and crosslinked NF webs were observed using a scanning electron microscope (SEM, JCM-5700, JEOL, Japan) operated at 5 kV. All samples were sputter-coated with Pt. The Fourier transform (FT-IR) spectra of the as-spun and cross-linked NF webs were measured using an FTIR spectrometer (FT/IR-6300, JASCO) with an infrared microscope (IRT-3000, JASCO) in reflectance mode. Potentiometric titration measurements were performed using a potentiometric titrator (888Titrando, Metrohm). First, the crosslinked NF webs were immersed in a 1 mol/L HCl solution for 24 h to ensure that the counterions were exchanged with H+. After sufficiently washing the webs in ultrapure water, the webs were soaked in a 1 mol/L KCl solution for 3 h to elute the H+ ions. Then the webs were titrated by adding a 0.01 mol/L KOH solution to obtain the titration curve. The amount of the fixed charge groups (𝑁𝑋) in the webs is equal to the titer of KOH. The ion-exchange capacity (IEC) was determined by the following equations17:


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IEC = 𝑤𝑑𝑟𝑦

　　　　　　　　　　　　 (1)

where wdry is the NF web weight in the dry state. The NF webs were dried in a vacuum oven at 100 °C for 6 h, and the wdry was determined. The measurements were carried out 3 times for each membrane, and the mean value (±standard deviation) is indicated. The streaming potential measurements were performed using an electrokinetic analyzer (SurPASS, Anton Paar GmbH, Austria) equipped with an adjustable gap cell14. A pair of NF webs with an area of 10 × 10 mm2 were placed in the measuring cell. The streaming potential was detected by Ag/AgCl electrodes with an average gap height of 110 μm. A background electrolyte of 10 mM KCl solution was used, and the pH was adjusted in the range of 2–11 with 0.1 M HCl and 0.1 M KOH. All measurements were performed at approximately 25 °C. The measurements were carried out 4 times for each membrane, and the mean value (±standard deviation) is indicated. The apparent surface charge density of the NFs was calculated from the mean value of the zeta potentials at pH 5.6 using the Gouy-Chapman equation18:

S 

where

k

2kT  z e  sinh    z e  2kT 

　

(2)

is the Boltzmann constant, T is the temperature,  is the reciprocal of the electrical double

layer thickness, z  is the valence of the counterion, and e is the Coulombic charge. 7 ACS Paragon Plus Environment

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Characterization of the NF composite membranes. The surface and cross-sectional morphologies of the NF composite membranes were observed using a scanning electron microscope (SEM, JCM-5700, JEOL, Japan) operated at 5 kV. All samples were sputter coated with Pt. The potentiometric titration measurements were performed using the same procedure used for the NF webs. The membrane weight in the equilibrium swollen state (𝑤𝑤𝑒𝑡) was measured. Subsequently, the membranes were dried in a vacuum oven at 100 °C for 6 h, and the membrane weight in the dry state (𝑤𝑑𝑟𝑦) was determined. The water content (𝑤𝑤) of the membranes and the volumetric charge density (𝐶𝑋) were defined by the following equation17:

𝑤𝑤 =

𝑤𝑤𝑒𝑡 ― 𝑤𝑑𝑟𝑦 𝑤𝑑𝑟𝑦

× 100%　(3)

𝜌𝑁𝑥

𝐶𝑥 = 𝑤𝑤𝑒𝑡 ― 𝑤𝑑𝑟𝑦　 (4)

where ρ is the density of water at 25 °C ( 0.99704×103 g L-1). The swelling ratio was calculated from the change of the dimension of the membranes. Swelling ratio was calculated by dimension differences between hydrated and dry composite membranes19. Lwet and Ldry are the average dimensions of the membranes in the equilibrium swollen state and in the dry state, respectively. Lwet = (Lwet1 × Lwet2) 1/2, Ldry = (Ldry1 × Ldry2)

1/2.

Lwet1, Lwet2 and Ldry1, Ldry2 are the length and width of the swollen and dry


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membranes, respectively. All data of the water content and swelling ratio were reproducible within 0.5 %.

𝑆𝑤𝑒𝑙𝑙𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 =

𝐿𝑤𝑒𝑡 ― 𝐿𝑑𝑟𝑦 𝐿𝑑𝑟𝑦

× 100% (5)

The through-plane ionic conductivities of all the NF composite membranes were determined by an impedance analyzer (SI1287, Solartron) with measurement cells containing two platinum black electrodes, and 0.1 mol/L HCl and 0.1 mol/L KCl were used for the measurements. All measurements were carried out at 25 °C in the frequency range of 100 Hz to 1 MHz. The measurements were carried out at least 3 times for each membrane, and the mean value (±standard deviation) is indicated. Tensile tests for the composite membranes were performed using a tensile tester (STA-1150, A&D). Test samples were cut with a width of 5 mm and length of 15 mm. Three samples were measured for each membrane and the mean value (±standard deviation) is indicated.

RESULTS AND DISCUSSION Characterization of the NFs. 　 To improve the water resistance of the NFs containing hydrophilic polymers, all the as-spun NFs were crosslinked with glutaraldehyde (see Figure S1). Figure 1 shows the surface SEM images of the crosslinked NFs. The average diameter of the NFs is approximately 250 nm. After the crosslinked NFs were immersed in water for 2 days, their dimensions were still 9 ACS Paragon Plus Environment

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maintained (see Figure S2). Figure 2 shows the pH dependence of the zeta potential determined from the streaming potential measurements. The zeta potentials of the PVA-b-PSS-NFs decreased drastically in the lower pH region. A pH dependence is commonly observed on strong acid surfaces and is due to the sulfonic acid groups on the nanofiber surface21. In contrast, the PVA-NFs without sulfonic acid groups showed a different behavior. The physicochemical properties of the crosslinked NFs are summarized in Table 1. The IEC and surface charge density of the PVA-b-PSS-NFs are higher than those of the PVA-NF. The IEC of PVA-b-PSS10-NF is larger than that of PVA-b-PSS4-NF, but the both surface charge densities showed a similar value. This might rely on migration of the PVA-block to the air-polymer interface during spinning, driven by a reduction in surface energy20. We successfully prepared NFs with similar average diameters and different charge properties and used them for the preparation of NF-composite polyelectrolyte membranes.

(b)

(a)

1μm

(c)

1μm

1μm

Figure 1. Surface SEM images of the crosslinked NFs. (a) PVA-b-PSS4, (b) PVA-b-PSS10, and (c) PVA.


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0 -5 -10 zeta potential [mV]

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

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-15 -20 -25 -30 -35 -40 -45

2

4

6

8

10

12

pH Figure 2. The pH dependence of the zeta potentials of the crosslinked NFs. green: PVA; red: PVA-b-PSS4; and blue: PVA-b-PSS10.

Table 1. Physicochemical properties of the crosslinked NFs. Average fiber diameter

IEC

Apparent surface charge density*

[nm]

[mmol g-1]

[µC cm-2]

PVA-b-PSS4-NF

264±37

0.46±0.03

1.0

PVA-b-PSS10-NF

249±25

0.70±0.01

1.0

PVA-NF

260±68

0.14±0.05

0.3

Nanofiber

*Calculated

using eq 2 from the mean value of the zeta potentials at pH 5.6 in 0.1 M KCl at 25 °C.


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Characterization of the NF composite membranes. Figure 3 shows typical cross-sectional SEM images of the NF web, as-cased Nafion membrane, and NF-composite polyelectrolyte membrane. For all the composite membranes, the NFs are well dispersed and form a three-dimensional (3D) network structure in the polymer matrix. The interfiber space (Figure 3a) were filled with a polymer matrix, Nafion (Figure 3c).

(b)

(a)

1μm

(c)

1μm

1μm

Figure 3. Typical cross-sectional SEM images of (a) PVA-b-PSS4-NF web (b) as-casted Nafion, and (c) PVA-b-PSS4-NF/Nafion composite membrane.

The physicochemical properties of the prepared membranes are summarized in Table 2. All the prepared membranes show a similar ion-exchange capacity (IEC) of ~1.0 mmol g-1. The water content of the prepared composited membranes is higher than that of the as-casted Nafion membrane. This would be that the PVA-based NFs in the matrix can retain more water molecules than the Nafion membranes. The FT-IR spectra of the PVA-NF and Nafion membranes are shown in Figure 4. The peak at approximately 3450 cm-1 is assigned to the -OH stretching band, and the peak at 1630 cm-1 is 12 ACS Paragon Plus Environment

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1 assigned to the –OH bending band of water molecules22. The spectra indicate that the PVA-based NFs 2 3 4 with uncrosslinked hydroxyl groups can retain more water molecules in their matrix. However, all the 5 6 7 prepared membranes show a similar swelling ratio of 13-15%. This would be due to the reinforcement 8 9 10 effect of the NFs described later (see Figure 5). 11 12 13 14 Table 2. Physicochemical properties of the prepared NF-composite polyelectrolyte membranes 15 16 17 Ionic conductivity Volumetric 18 Water Swelling 19 IEC Thicknessc charge -1] 20 [mS cm content ratio Membrane density 21 -1] [µm] [mmol g 22 [%] [%] 0.1 M KCla 0.1 M HClb 23 [mmol L-1] 24 25 26 PVA-b-PSS4-NF/ 24±2 63±2 1.02±0.05 44 15 23 2.3 27 Nafion 28 29 PVA-b-PSS10-NF 30 23±1 53±1 1.07±0.01 40 14 23 2.6 /Nafion 31 32 33 PVA-NF/Nafion 10±1 16±1 0.97±0.08 40 13 21 2.3 34 35 Nafion 16±1 24±2 1.05±0.01 32 13 21 3.2 36 37 aMeasured in 0.1 M KCl aqueous solution at 25 °C. 38 39 bMeasured in 0.1 M HCl aqueous solution at 25 °C. 40 cIn the dry state. 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 13 58 59 ACS Paragon Plus Environment 60

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100 80

T [%]

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

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60 40 20 0 4000

3500

3000

2500

2000

1500

1000

500

-1

[cm ] Figure 4. FT-IR spectra of the hydrated PVA-NFs (red line) and as-casted Nafion membrane (black line).

Compared to that of the Nafion membrane, the ionic conductivities through the PVA-b-PSS-NF/Nafion composite membranes measured in both 0.1 M KCl and 0.1 M HCl are improved. On the other hand, the ionic conductivity through the PVA-NF/Nafion composite membrane shows a lower value than that of the Nafion membrane. The order of ionic conductivities through the membranes measured in both 0.1 M KCl and 0.1 M HCl was PVA-b-PSS4-NF/Nafion ~ PVA-b-PSS10-NF/Nafion > Nafion > PVA-NF/Nafion, It well known that the volumetric charge density, which is used to describe the amount of fixed ionic charge in the membrane per weight of its absorbed water, plays an important role in determining the ionic conductivity of IEX membranes 1, 23. In Table 2, however, the order of the volumetric charge density of the membranes was Nafion > 14 ACS Paragon Plus Environment

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PVA-b-PSS10-NF/Nafion > PVA-NF/Nafion ~ PVA-b-PSS4-NF/Nafion. This order did not agree with that of the ionic conductivity. On the other hand, we can explain the order of the ionic conductivity by using that of the apparent surface charge density of the NFs and polymer matrix from the zeta potentials measurements: the order of apparent surface charge density was PVA-b-PSS4-NF ~ PVA-b-PSS10-NF (1.0 µC cm-2) > Nafion (0.83 µC cm-2) > PVA-NF (0.3 µC cm-2). These findings clearly indicate that a high density of sulfonic acid groups on the NF surface substantially contribute to the ionic transport through the membranes. In the composite membranes, lower-charge-density PVA-NF networks with the submicroscaled interfiber space divide the “cluster network structure”, which consists of sulfonated ionic clusters (~0.4 nm) interconnected with narrow channels (~0.1 nm), in Nafion24 and higher-charge-density PVA-b-PSS-NF networks function as an efficient ion transport path in Nafion. The additive effect of the IEX-NF on the ionic transport properties is more substantial in 0.1 M HCl. The maximum proton conductivity obtained in this work was 63±2 mS cm-1. This value compares with the reported nanocomposite membrane with superior ionic conductivity (64±1 mS cm-1 for the composite membrane containing aligned ion-exchange nanosheets with high-specific surface area, PVA/iron-oxide nanoparticle-deposited sulfonated graphene oxide nanosheet composite membrane)25 This supports that the high-charge-density NFs is also a promising nanofiller for the construction of an efficient ion transport path in the polymer matrix. The additive effects of the NFs on the mechanical properties (i.e., elongation, strength, and Young’s modulus) of the prepared NF-composite polyelectrolyte membranes were obtained from the 15 ACS Paragon Plus Environment

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stress−strain (S−S) curves (Typical S-S curve is shown in Figure S3). These results are shown in Figure 5. The membrane’s elongation decreased from 45% to 20% or less by addition of the NFs in the polymer matrix, Nafion. The tensile strength and Young’s modulus of all the NF composite membranes were enhanced by ~2 times and 2.5 times or more that of the membrane without NFs, respectively. These results clearly indicate that the crosslinked PVA-based NFs reinforce the uncrosslinked polymer matrix, Nafion and the crosslikned NF network also hinders the deformation of the polymer matrix.

50

(a)

40

Elognation [%]

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

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30

20

10

0

PVA-b-PSS4-NF PVA-b-PSS10-NF /Nafion /Nafion

PVA-NF /Nafion

Nafion


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Tensile strength [MPa]

(b) 20

10

0

7


PVA-NF /Nafion

Nafion

PVA-NF /Nafion

Nafion

(c)

6

Young's modulus [MPa]

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

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5 4 3 2 1 0



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Figure 5. Additive effects of the crosslinked PVA-based NFs on (a) elongation, (b) tensile strength, and (c) Young’s modulus of the NF-composite polyelectrolyte membrane. The as-cased Nafion membrane was used as the reference.

Conclusions In the present study, we prepared PVA-based IEX-NFs and their composite polyelectrolyte membranes. The PVA-based NFs are well dispersed and form a 3D network structure in the polymer matrix. The ionic conductivities through the PVA-b-PSS-NF/Nafion composite membranes are improved by above 1.5 times and 2.6 times more than that of the Nafion membrane in 0.1 M KCl and HCl, respectively. However, the conductivity through the PVA-NF/Nafion composite membrane is half that of the Nafion membrane. Our electrokinetic measurements clearly indicate that a high density of ion-exchange groups on the NF surface construct an efficient ion transport path in the polymer matrix. In addition, the mechanical strength of all the NF composite membranes was ~2 times greater than that of the membrane without NFs. The crosslinked PVA-based NFs also function as the mechanical reinforcement. These improved properties represent the potential of the PVA-based NF composite membranes as a promising candidate for applications such as electrodialysis, reverse electrodialysis, and capacitive deionization26. We think that the PVA-based NFs are not suitable for fuel cell applications due to the degradation of PVA induced by hydroxyl radicals27. At present, we have not optimized the IEX-NF/polyelectrolyte composite membranes. It is expected that the ionic conductivity can be 18 ACS Paragon Plus Environment

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improved by optimization of some parameters (e.g., surface charge density of the NFs; the surface area of NFs, and the content of the NFs in the matrix). Further studies are now in progress, and the results will be reported.

Acknowledgement. This work was partly supported by JSPS KAKENHI Grant Numbers JP15K05621 and JP16H01796.

References (1) Sata, T. Ion Exchange Membranes: Preparation, Characterization, Modification, and Application; The Royal Society of Chemistry: Cambridge, 2002. (2) Xu, T. Ion Exchange Membranes: State of Their Development and Perspective. J. Memb. Sci. 2005, 263, 1–29. (3) Ran, J.; Wu, L.; He, Y.; Yang, Z.; Wang, Y.; Jiang, C.; Ge, L.; Bakangura, E.; Xu, T. Ion Exchange Membranes: New Developments and Applications. J. Memb. Sci. 2017, 522, 267–291. (4) Bakangura, E.; Wu, L.; Ge, L.; Yang, Z.; Xu, T. Mixed Matrix Proton Exchange Membranes for

Fuel Cells: State of the Art and Perspectives. Prog. Polym. Sci. 2016, 57, 103–152. (5) Fu, K. K.; Gong, Y.; Dai, J.; Gong, A.; Han, X.; Yao, Y.; Wang, C.; Wang, Y.; Chen, Y.; Yan, C.;

Li, Y.; Wachsman, E. D.; Hu, L. Flexible, Solid-State, Ion-Conducting Membrane with 3D Garnet Nanofiber Networks for Lithium batteries. Proc. Natl. Acad. Sci. USA 2016, 113, 7094–7099. 19 ACS Paragon Plus Environment

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Page 20 of 23

(6) Liu, W.; Liu, N.; Sun, J.; Hsu, P.-C.; Li, Y.; Lee, H.-W.; Cui, Y. Ionic Conductivity Enhancement of Polymer Electrolytes with Ceramic Nanowire Fillers. Nano Lett. 2015, 15, 2740–2745. (7) Uematsu, I.; Uchida, K.; Nakagawa, Y.; Matsumoto, H. Direct Observation and Quantitative Analysis of the Fiber Formation Process during Electrospinning by a High-Speed Camera, Ind. Eng. Chem. Res. 2018, 57, 12122-12126. (8) Ma, H.; Burger, C.; Hsiao, B. S.; Chu, B. Highly Permeable Polymer Membranes Containing Directed Channels for Water Purification. ACS Macro Lett. 2012, 1, 723–726. (9) Zhang, S.; Tanioka, A.; Matsumoto, H. Nanofibers as Novel Platform for High-Functional Ion Exchangers. J. Chem. Technol. Biotechnol. 2018, DOI: 10.1002/jctb.5685. (10)

Choi, J.; Lee, K. M.; Wycisk, R.; Pinaturo, P. N.; Mather, P. T. Nanofiber Network

Ion-Exchange Membranes. Macromolecules 2008, 41, 4569–4572. (11) Tanaka, M. Development of Ion Conductive Nanofibers for Polymer Electrolyte Fuel Cells. Polym. J. 2016, 48, 51–58 (12) Matsumoto, H.; Tanioka, A.; Murata, T.; Higa, M.; Horiuchi, K. Effect of proton on potassium ion in countertransport across fine porous charged membranes. J. Phys. Chem. B 1998, 102, 5011–5016. (13) Higa, M.; Nishimura, M.; Kinoshita, K.; Jikihara, A. Characterization of Cation-Exchange Membranes Prepared from Poly(Vinyl Alcohol) and Poly(Vinyl Alcohol-b-Styrene Sulfonic Acid). Int. J. Hydrogen Energy 2012, 37, 6161–6168. (14) Matsumoto, H.; Yako, H.; Minagawa, M.; Tanioka, A. Characterization of Chitosan Nanofiber 20 ACS Paragon Plus Environment

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Langmuir

Fabric by Electrospray Deposition: Electrokinetic and Adsorption Behavior. J. Colloid Interface Sci. 2007, 310, 678–681. (15) Higa, M.; Mizuno, T.; Anno, M. Characterization of Cation-exchange Membranes Prepared from PVA-based Block Copolymers. Bull. Soc. of Sea Water Sci., Jpn. 2016, 70, 324–325. 　 (16) Saito, Y.; Meguro, M.; Ashizawa, M.; Waki, K.; Yuksel, R.; Unalan, H. E.; Matsumoto, H. Manganese Dioxide Nanowires on Carbon Nanofiber Frameworks for Efficient Electrochemical Device Electrodes. RSC Adv. 2017, 7, 12351–12358. (17) Saga, S.; Matsumoto, H.; Saito, K.; Minagawa, M.; Tanioka, A. Polyelectrolyte Membranes Based on Hydrocarbon Polymer Containing Fullerene. J. Power Sources 2008, 176, 16–22. (18) Matsumoto, H.; Koyama, Y.; Tanioka, A. Characterization of Novel Weak Amphoteric Charged Membranes Using ζ-Potential Measurements: Effect of Dipolar Ion Structure. Langmuir 2001, 17, 3375–3381. (19) Zhang, S.; He, G.; Gong, X.; Zhu, X.; Wu, X.; Sun, X.; Zhao, X.; Li. H. Electrospun nanofiber enhanced sulfonated poly (phthalazinone ether sulfone ketone) composite proton exchange membranes. J. Membrane Sci. 2015, 493, 58–65. (20) Hardman, S. J.; Muhamad-Sarih, N.; Riggs, H. J.; Thompson, R. T.; Rigby, J.; Bergius, W. N. A.; Hutchings, L. R. Electrospinning Superhydrophobic Fibers Using Surface Segregating End-Functionalized Polymer Additives. Macromolecules, 2011, 44, 6461-6470. (21) Hunter, R. J. Zeta Potential In Colloid Science: Princples and Applications; Academic Press: 21 ACS Paragon Plus Environment

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London, 1981. (22) Kunimatsu, K.; Bae, B.; Miyatake, K.; Uchida, H.; Watanabe, M. ATR-FTIR Study of Water in Nafion

Membrane

Combined

with

Proton

Conductivity

Measurements

during

Hydration/Dehydration Cycle. J. Phys. Chem. B 2011, 115, 4315–4321. (23) Choi, J.; Yang, S.; Jeong, N.-J.; Kim, H.; Kim, W.-S. Fabrication of an Anion-Exchange Membrane by Pore-Filling Using Catechol-1,4-Diazabicyclo-[2,2,2]octane Coating and Its Application to Reverse Electrodialysis, Langmuir, 2018, 34, 10837–0846 (24) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion, Chem. Rev. 2004, 104, 4536– 4585. (25) Beydaghi, H.; Javanbakht, M. Aligned Nanocomposite Membranes Containing Sulfonated Graphene Oxide with Superior Ionic Conductivity for Direct Methanol Fuel Cell Application, Ind. Eng. Chem. Res., 2015, 54, 7028–7037. (26) Strathmann, H.; Grabowski, A.; Eigenberger, G. Ion-Exchange Membranes in Chemical Process Industry, Ind. Eng. Chem. Res., 2013, 52, 10364–10379. (27) Jayasekara, R.; Harding, I.; Bowater, I.; Lonergan, G. Biodegradability of a Selected Range of Polymers and Polymer Blends and Standard Methods for Assessment of Biodegradation. J. Polym. Environ. 2005, 13, 231–251. 　


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Ion-exchange nanofiber H+ H

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Polymer matrix

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Polyelectrolyte Composite Membranes Containing Electrospun Ion

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