Research Article www.acsami.org
Novel Composite Proton Exchange Membrane with Connected LongRange Ionic Nanochannels Constructed via Exfoliated Nafion−Boron Nitride Nanocomposite Wei Jia, Beibei Tang, and Peiyi Wu* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, People’s Republic of China S Supporting Information *
ABSTRACT: Nafion−boron nitride (NBN) nanocomposites with a Nafion-functionalized periphery are prepared via a convenient and ecofriendly Nafion-assisted water-phase exfoliation method. Nafion and the boron nitride nanosheet present strong interactions in the NBN nanocomposite. Then the NBN nanocomposites were blended with Nafion to prepare NBN Nafion composite proton exchange membranes (PEMs). NBN nanocomposites show good dispersibility and have a noticeable impact on the aggregation structure of the Nafion matrix. Connected longrange ionic nanochannels containing exaggerated (−SO3−)n ionic clusters are constructed during the membrane-forming process via the hydrophilic and H-bonding interactions between NBN nanocomposites and Nafion matrix. The addition of NBN nanocomposites with sulfonic groups also provides additional proton transportation spots and enhances the water uptake of the composite PEMs. The proton conductivity of the NBN Nafion composite PEMs is significantly increased under various conditions relative to that of recast Nafion. At 80 °C−95% relative humidity, the proton conductivity of 0.5 NBN Nafion is 0.33 S·cm−1, 6 times that of recast Nafion under the same conditions. KEYWORDS: proton exchange membrane, Nafion−boron nitride nanocomposite, long-range ionic nanochannels, Nafion, water-phase exfoliation
1. INTRODUCTION The proton exchange membrane (PEM), one of the core parts of the direct methanol fuel cell (DMFC), has aroused considerable research interest during the past few decades for clean emission, high energy density, abundant fuel source, low starting temperature, and etc.1−3 PEMs generally are made of polyelectrolytes and used for proton conduction and methanol impedance so that the performance of PEMs is closely related to the cell performance and efficiency.4−8 Nafion, commercialized by DuPont, is the most widely applied PEM material for its various chemical and physical advantages.9−12 Nafion is bestowed with an amphiphilic nature, possessing a hydrophobic polytetrafluoroethylene (PTFE) backbone and short perfluoroether side chains terminated with hydrophilic superacidic sulfonic acid groups.6,13 This unique chemical structure of Nafion leads to the formation of interpenetrating nanophase separation consisting of crystalline regions for membrane stability and amorphous regions containing (−SO3−)n ionic clusters for proton transportation during the membraneforming process. However, Nafion is still not an ideal PEM material because the proton conductivity of Nafion suffers from a dramatic decline under high-temperature and low-humidity conditions, due to insufficient connectivity of ionic clusters as © 2017 American Chemical Society
well as poor water retention ability under such harsh conditions.14 Besides, the methanol crossover in the Nafion matrix also needs to be controlled to maintain cell efficiency.5,15−17 Unfortunately, because protons and methanol molecules transport through the same channels, the methanol permeation phenomenon is probably also enhanced when attempts are made to increase the proton conductivity by making ionic clusters more continuous. Therefore, it is very challenging to overcome the trade-off effect between the proton conductivity and the methanol resistance and comprehensively promote the performance of Nafion-based PEM. Two-dimensional (2D) layered material nanosheets including graphene, graphene oxide (GO), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and boron nitride nanosheets (BNNS), etc., have attracted intensive research interests for their unique properties such as high specific surface area, rich physics, and stimulation responsibility since the discovery of free-standing graphene by Geim in 2004.18−23 In the field of PEM for DMFC, 2D layered Received: January 17, 2017 Accepted: April 17, 2017 Published: April 17, 2017 14791
DOI: 10.1021/acsami.7b00858 ACS Appl. Mater. Interfaces 2017, 9, 14791−14800
Research Article
ACS Applied Materials & Interfaces
Continuous long-range ionic nanochannels are consequently constructed in the composite PEMs. Besides, the addition of NBN nanocomposites could also enhance the water uptake of the composite PEMs, especially under high-temperature and low-humidity conditions. On the basis of these advantages, the proton conductivity of the NBN Nafion composite PEMs is significantly increased at various conditions relative to that of recast Nafion. Meanwhile, the methanol permeability of the NBN Nafion composite membranes slightly increases despite the successful construction of connected long-range ionic nanochannels. This could be attributed to the barrier effect of NBN nanocomposites to make the methanol crossover channels more tortuous.
material nanosheets are widely used to reinforce the polyelectrolyte-based PEMs and could effectively reduce the methanol permeability of the composite PEMs because of their methanol impedance ability.9,24−26 However, the addition of 2D nanosheets would probably block the proton conduction channels, leading to a decrease in proton conductivity. Therefore, 2D nanosheets are usually functionalized with sulfonic groups to construct more proton conduction channels in the membrane matrix.27,28 Moreover, sulfonated 2D nanosheets could adjust the size of the ionic clusters in the membrane matrix by changing the osmotic pressure and construct long-range ionic nanochannels via the hydrophilic interaction and H-bonding interaction with the sulfonic groups of polyelectrolyte during the membrane-forming process.13,29−31 Compared with graphene, GO, TMDs, and LDHs, boron nitride (BN) nanosheets possess two intrinsic advantages, electrical insulation and desirable stability under acid and oxidation−reduction environment, which make BN nanosheets more adaptable for practical application in DMFCs.32−35 Nevertheless, there are few oxygen-containing functional groups on the surface of BN nanosheets, which makes the sulfonation modification very challenging and confines its dispersibility in polyelectrolyte membrane matrix as well as the influence on the nanostructure of the membrane. Liquid-phase mechanical exfoliation has been reported as a convenient method to obtain noncovalently functionalized BN nanosheets with controllable size.36 It is important to match the surface energy of BN nanosheets and the solvent in liquidphase mechanical exfoliation so that an amphiphilic surfactant is usually required to stabilize the exfoliated BN nanosheets from reaggregation in the solvent.37 The noncovalent functionalization of BN nanosheets could not only improve its dispersibility in membrane matrix but also maintain the inherent advantages of BN nanosheets.38 Oh et al. used 1-pyrenesulfonic acid (PSA) to assist the ultrasonication exfoliation of BN in N,Ndimethylformamide (DMF).39 Then the PSA-functionalized BNNFs were blended with sulfonated poly(ether ether ketone) (sPEEK) to prepare composite PEMs for the polymer electrolyte membrane fuel cell (PEMFC) application, which shows the enhanced mechanical stability and proton conductivity.40 In comparison to small molecular surfactants, surface active amphiphilic polymers benefitted from the much higher degree of functionality have stronger interactions with BN nanosheets and within polymer chains.41,42 This endows the polymer-functionalized BN nanosheets with greater potential for further modification and practical applications. Herein, Nafion was used to assist the water-phase ultrasonication exfoliation of BN. The amphiphilic nature of Nafion not only helps to match the surface energy between BN and water but also promotes the formation of Nafion-modified BN 2D nanocomposites (NBN) nanocomposites via hydrophilic− hydrophobic interaction and H-bonding interaction. Then the NBN nanocomposites were blended with Nafion to prepare the NBN Nafion composite PEMs. NBN nanocomposites show good dispersibility in the Nafion matrix and have a unique influence on the aggregation structure of Nafion matrix. NBN nanocomposites have abundant sulfonic groups on the surface, which exaggerates the size of ionic clusters in the membrane matrix by changing the internal osmotic pressure of the clusters.29 Furthermore, the ionic clusters would form along the interfaces between NBN and Nafion matrix promoted by the hydrophilic and H-bonding interactions during the membraneforming process to reach the thermodynamically stable state.
2. EXPERIMENTAL SECTION 2.1. Materials. Bulk hexagonal boron nitride (BN, 1−2 μm) was bought from Aladdin Co. (China). Nafion solution (perfluorinated resin solution, 5 wt %) was obtained from Sigma-Aldrich. H2SO4, H3PO4, H2O2 and DMF were purchased from Sinopharm Chemical Reagent Co. All reagents were directly used without purification. 2.2. Simultaneous Exfoliation and Functionalization of BN. Two hundred milligrams of bulk BN powder was mixed with 40 mL of 5 mg/mL Nafion aqueous solution. The obtained aqueous dispersion was then ultrasonicated for 12 h. The NBN nanocomposites were collected by centrifugation at 3000 rpm for 20 min and then washed with deionized water three times to remove the excess Nafion. The obtained NBN nanocomposites were dried at 80 °C for characterizations. Similarly, 200 mg of BN powder was also exfoliated in 40 mL of Nafion solution with concentrations of 2.5 and 7.5 mg/mL. The 5 mg/mL Nafion aqueous solution was dried under the same conditions as for the control experiment. 2.3. Preparation of NBN Nafion Composite PEMs. The preparation method is close to that in our previous paper.13 Typically, 0.5 mL of 1.2 mg/mL NBN aqueous dispersion was first added into 4 mL of 5 wt % as-received Nafion solution. The acquired mixed aqueous dispersion was ultrasonicated for 30 min and then exchanged with 3 mL of DMF via rotary evaporation. The DMF dispersion was then cast in a homemade glass mold in a vacuum oven to remove the solvent. The as-obtained composite PEM was further treated with 3 wt % H2O2 and 1 M H2SO4 solution and named 0.3-NBN Nafion, where 0.3 stands for the weight content of NBN in the Nafion matrix. Similarly, 0.5-NBN Nafion and 1-NBN Nafion composite PEMs were prepared by adding 0.8 and 1.7 mL of NBN aqueous dispersion into the as-received Nafion solution, respectively. Besides, recast Nafion was prepared via a similar procedure without adding NBN nanocomposites for comparison. 2.4. Characterizations. High-resolution transmission electron microscopy (HRTEM) images of NBN nanocomposites were obtained using a JEM2011 (JEOL) at 200 kV. Atomic force microscopy (AFM) images of NBN nanocomposites and NBN Nafion composite PEMs were acquired on a Multimode 8 (Bruker) in tapping mode and QNM mode, respectively. An Ultra 55 (Zeiss) was used to obtain the field-emission scanning electron microscopy (FESEM) images. The X-ray diffraction (XRD) characterizations were conducted on D8 ADVANCE and DAVINCI.DESIGN (Bruker) with Cu Kα radiation. The Fourier transform infrared (FTIR) spectra were collected with a spectral resolution of 4 cm−1 on a NEXUS 6700 (ThermoFisher) with 32 scans. The thermogravimetric analysis (TGA) experiments were conducted on a TGA 1 (Mettler Toledo) in N2 atmosphere at a heating rate of 20 °C min−1. An RBD upgraded PHI-5000C ESCA system (PerkinElmer) was adopted to collect X-ray photoelectron spectroscopy (XPS) spectra with Mg Kα radiation. Zeta potential values were achieved on a Zetasizer Nano (Malvern). Water uptake (WU) of the PEMs at 20 °C in water and under 100 °C−40% relative humidity (RH) conditions was measured according to a method described in our previous paper.13 The WU values are calculated from the weight of the wet membranes (Wwet) and the dry membranes (Wdry) according to eq 1: 14792
DOI: 10.1021/acsami.7b00858 ACS Appl. Mater. Interfaces 2017, 9, 14791−14800
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Figure 1. HRTEM images (a−c) and AFM image (d) of NBN nanocomposite. The inset in panel b is the selected area electron diffraction pattern of the NBN nanocomposite. The inset in panel d is the height profile of the NBN nanocomposite.
Figure 2. XRD patterns (a) of bulk BN and NBN nanocomposites; FTIR spectra (b), TGA curves (c), and DTG curves (c1) of Nafion and NBN nanocomposites; XPS spectrum (d) of the NBN nanocomposite; (d1) high-resolution B 1s XPS spectrum and peak-split results of NBN nanocomposite. WU (%) =
Wwet − Wdry Wdry
where σ is the proton conductivity tested at 40 °C−40% RH and P is the methanol permeability tested at 40 °C.
× 100% (1)
3. RESULTS AND DISCUSSION
The proton conductivity (σ) under different conditions and the methanol permeability (P) at 40 °C were measured according to the same method as previously reported.43 The longevity test was conducted at 100 °C−40% RH. The 0.5-NBN Nafion composite membrane was dipped in 40 wt % H3PO4 solution for 4 h before the stability test. To evaluate the overall performance of the PEMs, the membrane selectivity (S) at 40 °C was calculated by using eq 2 S=
σ P
3.1. Characterization of NBN Nanocomposite. HRTEM and AFM were used to investigate the morphology and structure of the NBN nanocomposite (Figure 1). As shown in Figure 1a,d, bulk BN powder was exfoliated to thin nanosheets with a size of ca. 50 nm and a thickness near 8 nm. The HRTEM image and the selected area electron diffraction pattern presented in Figure 1b clearly reveal the typical hexagonal lattice structure of the exfoliated BN nanosheets.44
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2d1), which shows the bonding configurations of B and O.51,53 Similarly, N−O bonds could be found in N 1s high-resolution XPS spectrum as well (Figure S4). These findings demonstrate that the oxygen-containing functional groups are introduced onto the surface of the BN nanosheet during the water-phase ultrasonication exfoliation process, which would reinforce the hydrophilic and H-bonding interactions between Nafion and the BN nanosheet in NBN. From the above results, it could be inferred that the superhydrophobic PTFE backbone of Nafion physically adsorbs onto the surface of the exfoliated BN nanosheets via the hydrophobic interaction to decrease the surface energy in the aqueous dispersion. The completely ionized sulfonic groups on the end of side chains create a high surface charge around the BN nanosheets to stabilize the nanosheets from reaggreation via electrostatic repulsion. Simultaneously, oxygen-containing functional groups are introduced onto the surface of the nanosheets during the water-phase ultrasonication exfoliation process, which further reinforces the H-bonding interactions between the two components. This process is schematically illustrated in Figure 3.
The lattice spacing of the exfoliated BN nanosheets is ca. 0.25 nm, which is equal to the (100) lattice constant of BN.45 According to the XRD patterns shown in Figure 2a, only a weak characteristic 2θ peak at 27°, corresponding to the (002) planes of the hexagonal BN, remains in the XRD pattern of NBN after the exfoliation. The XRD patterns further statistically prove the effectiveness of Nafion in assisting the exfoliation of BN in water.46 Besides, BN powder could also be exfoliated in the Nafion solution with concentrations of 2.5 and 7.5 mg/mL (Figure S1). The size of the exfoliated BN nanosheets is influenced by the Nafion concentration, which could be ascribed to the differences in self-aggregation phenomenon of Nafion and viscosity of the dispersion at various Nafion concentrations.47,48 The components of the NBN nanocomposites are further analyzed. A broad peak from 12 to 20° could also be observed in the XRD pattern of NBN, which could be ascribed to the characteristic peak of Nafion.49 This result indicates the existence of Nafion in the NBN nanocomposite, which is also supported by the results of FTIR, XPS, and TGA characterizations. Four of Nafion’s characteristic peaks appear in the spectral range from 1300 to 950 cm−1 in the FTIR spectrum of NBN, and the peaks corresponding to F 1s, O 1s, C 1s, and S 2p could be identified in the XPS spectrum (Figure 2b,d; Figure S2).13,50 Besides, NBN presents degradation features similar to those of Nafion in the DTG curves, where the two major degradation peaks at 360 and 411 °C could be attributed to the degradation of sulfonic groups and backbones of the adsorbed Nafion, respectively (Figure 2c1).13 The content of Nafion in the NBN nanocomposite is ca. 15 wt % according to the weight loss of NBN in the TGA curve because BN undergoes little weight loss under such conditions (Figure 2c).51 In the HRTEM image of Figure 1c, an amorphous Nafion modification layer with a thickness of ∼3.4 nm could be clearly observed outside the crystalline BN nanosheets (Figure 1c). Integrating the above characterization results, it could be concluded that NBN nanocomposites with a Nafion-functionalized periphery containing abundant superacidic sulfonic groups could be conveniently obtained via ecofriendly waterphase exfoliation. Furthermore, Nafion displays strong noncovalently physical hydrophilic−hydrophobic interactions with BN nanosheets in the NBN nanocomposites, which provides a basis to understand the mechanism of the stabilization and functionalization abilities of Nafion during the water-phase exfoliation process.52 The backbone degradation temperature of Nafion in the NBN nanocomposites decreased significantly from 528 to 467 °C, indicating the strong hydrophobic interactions between Nafion’s hydrophobic backbones and the BN nanosheets (Figure 2c1). The desulfonation temperature of NBN (360 °C) also shows a 17 °C decrease compared to that of Nafion (377 °C) (Figure 2c1). The zeta potential of the NBN aqueous dispersion is −54 mV (Figure S3), indicating the stronger electrostatic repulsion and closer distances among sulfonic groups in NBN. Consistently, the two characteristic bands ascribed to the stretching vibrations of backbones and sulfonic groups of Nafion (1240 and 1157 cm−1) show clear peak shifts compared to those of Nafion (1200 and 1144 cm−1) in the FTIR spectra. It supplementarily proves the strong interactions between Nafion and BN nanosheets as well as the varied environment of the sulfonic groups in NBN. Interestingly, a small shoulder peak at higher binding energy (190.3 eV) could be identified in the B 1s high-resolution XPS spectrum (Figure
Figure 3. Schematic illustration of the preparation process of NBN nanocomposites via Nafion-assisted water-phase ultrasonication exfoliation.
3.2. Characterization of the PEMs. The NBN nanocomposites were blended with Nafion to prepare NBN Nafion composite PEMs. The surface and the cross-sectional morphology of the composite PEMs was observed by FESEM (Figure 4). Benefitting from the peripheral modification of Nafion and the electrostatic repulsion among the nanocomposites, NBN nanocomposites show good dispersibility in Nafion matrix without macrophase separation and appear as single-layer membranes (Figure 4).54 Nevertheless, the abundant sulfonic groups on the periphery of NBN result in an inevitable surface energy difference between the Nafion matrix and the NBN nanocomposites, which may lead to a slight aggregation phenomenon in the 1-NBN Nafion membrane due to the excessive NBN content (Figure 4d1). Nafion consists of a linear hydrophobic backbone with short, perfluoroether side chains terminated by hydrophilic sulfonic groups. The unique chemical structure of Nafion leads to the nanophase separation and the formation of interpenetrating hydrophilic and hydrophobic sections in the membrane matrix.55 The diblock morphologies shown in the cross14794
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Figure 4. Surface and cross-sectional FESEM images of recast Nafion, 0.3-NBN Nafion, 0.5-NBN Nafion, and 1-NBN Nafion PEMs. The insets in panels c1, d1, and d2 are magnified FESEM images.
Figure 5. XRD patterns (a), DTG curves (b), FTIR spectrum (c), and water uptake (d) at 20 °C in water and 100 °C−40% RH conditions of recast Nafion, 0.3-NBN Nafion, 0.5-NBN Nafion, and 1-NBN Nafion PEMs.
XRD, TGA, and AFM were used to further investigate the aggregation structure of the NBN Nafion composite PEMs (Figures 5 and 6; Figure S5). The peaks of the NBN Nafion PEMs around 17° shift to higher degrees compared to that of recast Nafion, which demonstrates that the addition of NBN nanocomposites elevates the crystalline degree of the composite membranes.49 The thermostability of the backbones of Nafion in the composite PEMs is improved correspondingly according to the DTG results of the composite membranes (Figure 5b). The desulfonation temperatures of the NBN Nafion composite PEMs are increased compared to that of the recast Nafion, which is probably due to the stronger H-bonding interactions among the sulfonic groups in the Nafion matrix and NBN nanocomposites.49 The slight red-shift of the symmetric stretching vibration peak of sulfonic groups near 1057 cm−1 in the FTIR spectrum supplementarily verifies this inference (Figure 5c).57 Besides, the decreased side-chain
sectional morphology of the membrane could partially reflect the microscopic nanophase separation of the Nafion matrix. As shown in the cross-sectional FESEM images of Nafion-based PEMs, recast Nafion possesses a rather homogeneous crosssectional morphology, whereas the diblock microstructures in the NBN Nafion composite PEMs become more apparent with increasing NBN content (Figure 4a2−d2).56 Increasing rippled structures could be observed in 0.3-NBN Nafion and 0.5-NBN Nafion (Figure 4b2,c2), which could be partially ascribed to the stronger interaction between NBN and Nafion matrix with the increased NBN content. However, granulated structures could be found in 1-NBN Nafion (Figure 4d2), which might be due to the aggregation of excessive NBN nanocomposites in 1-NBN Nafion. These results indicate that the incorporated NBN nanocomposites have a unique influence on the aggregation structure of the Nafion matrix. 14795
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Figure 6. LogDMTModulos AFM images of Rrecast Nafion, 0.3-NBN Nafion, 0.5-NBN Nafion, and 1-NBN Nafion PEMs.
degradation temperature (460−470 °C) of the NBN Nafion composite PEMs indicates that more flexible side chains are achieved after the addition of NBN.49 The morphology of the nanophase separation and the (−SO3−)n ionic clusters of the Nafion matrix was observed with AFM in QNM mode (Figure 6). In the LogDMTModulus images, the brightness is positively related to the modulus. Hence, the dark area corresponds to the amorphous region containing (−SO3−)n ionic clusters in the membrane matrix because the modulus of the amorphous region is lower than that of the crystalline region. The larger (−SO3−)n ionic clusters in the NBN Nafion composite PEMs could be observed. The dimensions of the ionic clusters are regulated by the equilibrium between the internal osmotic pressure of the clusters and the counteracting elasticity of the matrix.29 The NBN nanocomposites with abundant sulfonic groups could change the osmotic pressure of the ionic clusters so that larger ionic clusters form in the membrane matrix. Interestingly, the ionic clusters become more continuous in 0.3-NBN Nafion, and some long-range ionic nanochannels could be observed (partially marked by white dashed lines for demonstration) (Figure 6b).13 The long-range ionic nanochannels become more clear and connected when the NBN content is increased to 0.5 wt % as presented in Figure 6c. This phenomenon could also be observed in the HRTEM image of 0.5-NBN Nafion (Figure S6). The crystalline area in the HRTEM image is the NBN nanocomposite, whereas the amorphous area is the Nafion matrix. Connected long-range ionic (−SO3)n− nanochannels (the dark region in the image because S is heavier than C) could be observed, which is consistent with the AFM images.58,59 However, the crystalline region in the Nafion matrix is very vulnerable to radiation damage even under low-
intensity illumination conditions so that the crystalline regions of Nafion could be hardly observed in the HRTEM image.59 The operating mechanism of NBN nanocomposites on the aggregation structure of Nafion matrix could be inferred from the various structure characterization results. The abundant sulfonic groups in NBN have hydrophilic and H-bonding interactions with the ionic clusters of the Nafion matrix. These interactions induce the ionic clusters to gather along the interfaces between the two components during the membraneforming process to reach a thermodynamically stable state. The long-range ionic nanochannels are constructed as a result in the composite membranes, as schematically illustrated in Figure 7.30 Meanwhile, the rearrangement of the ionic clusters triggers the adjustment of the side chains of Nafion and finally has an influence on the crystalline behavior as well as the aggregation structure of the Nafion matrix. However, the number of longrange ionic nanochannels is reduced when the NBN content increases to 1 wt %. This is probably due to the weaker influence of NBN on the nanophase separation structure of
Figure 7. Schematic illustration of the formation of long-range ionic nanochannels induced by NBN nanocomposites in Nafion matrix. 14796
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Figure 8. Temperature-dependent (at 40% RH, a) and humidity-dependent (at 80 °C, b) proton conductivities of recast Nafion, 0.3-NBN Nafion, 0.5-NBN Nafion, and 1-NBN Nafion PEMs.
good dispersibility in the membrane matrix, which is the precondition of the desirable interaction between NBN and Nafion. Second, connected long-range ionic nanochannels containing larger ionic clusters are constructed in the composite membrane via the hydrophilic and H-bonding interactions during the membrane-forming process. This effectively reduces the energy barrier and facilitates the fast transportation of protons in the membrane and increases the proton conductivity under low water content conditions. The proton conductivity of NBN membrane at 100 °C−40% RH is 0.008 S·cm−1, which is even lower than that of recast Nafion under the same condition. Therefore, the significant increase of proton conductivity of 0.5-NBN Nafion is mainly attributed to the construction of the connected long-range ionic nanochannels in the membrane matrix. Third, the water retention ability of the composite membrane is enhanced by the addition of NBN nanocomposites, especially at high-temperature and low-humidity conditions (Figure 5d). Because water molecules are the major carriers in proton conduction, the higher water content in the membrane also contributes to the increased proton conductivity.57 Similarly, 0.3-NBN Nafion and 1-NBN Nafion also display increased proton conductivity under various conditions. Compared with our previous CNT/GONR Nafion composite PEMs also possessing long-range ionic nanochannels, the NBN Nafion composite PEMs show prior proton conduction performance.13 The 0.5 wt % CNT/GONR Nafion composite PEM displays a 3-fold increase under 100 °C−40% RH condition versus the recast Nafion, whereas the 0.5 wt % NBN Nafion composite PEM presents a 30-fold increase under the same condition.13 This could be attributed to the better interactions between NBN and Nafion matrix because of the peripheral modification of Nafion on NBN nanocomposites as well as the more connected long-range ionic nanochannels in the membrane matrix. Nevertheless, disadvantages such as slight aggregation of excessive NBN, weaker interactions between NBN and Nafion matrix, lower high-temperature water uptake, and less connected long-range ionic nanochannels of 0.3-NBN Nafion and 1-NBN Nafion lead to inferior proton conductivity compared to that of 0.5-NBN Nafion. The methanol molecules permeate the membrane through the ionic channels by electroosmotic drag and concentration gradient.61,62 In other words, the methanol molecules share the same transportation channels with protons so that there is a trade-off effect between the proton conductivity and the methanol resistance.12,63 However, the methanol permeability of the NBN Nafion composite membranes increases only
Nafion matrix caused by the inevitable aggregation of excess NBN nanocomposites (Figure 6d). As presented in Figure 8, the proton conductivity of the NBN Nafion composite PEMs is significantly increased compared to that of recast Nafion under various conditions. 0.5-NBN Nafion displays typical proton conduction properties and the highest proton conductivity among the composite PEMs under various conditions (Figure 8; Figure S7). The proton conductivity of 0.5-NBN Nafion reaches 0.067 S·cm−1 at 100 °C−40% RH, 1 order of magnitude higher than that of recast Nafion. At 80 °C−95% RH, the proton conductivity of 0.5-NBN Nafion is 0.33 S·cm−1, 6 times that of recast Nafion under the same conditions. According to the linear fitting of the Arrhenius plots for proton conductivity in Figure S8, the apparent activation energy of 0.5-NBN Nafion is 0.25 eV, which is 40% lower than that of the recast Nafion (0.41 eV). This indicates the addition of the NBN nanocomposites efficiently decreases the energy barrier and facilitates proton transportation in the composite membranes. The slight deviation might result from the adjustment of complex proton transportation mechanisms and the inorganic phase volume at different water uptake conditions of the membranes.57,60 Besides, the good insulation and stability also make NBN a more competitive nanofiller for the modification of Nafion composite PEMs. After the 720 min test at 100 °C and 40% RH, the proton conductivity of 0.5-NBN Nafion decreases by 50%, whereas that of the recast Nafion decreases by 70%. The proton conductivity of 0.5-NBN Nafion is still much higher than that of the recast Nafion (Figure S9). These results demonstrate the longevity of the increased proton conductivity of the NBN Nafion composite PEMs. After being dipped in 40 wt % phosphoric acid solution for 4 h, the 0.5-NBN Nafion still displays high proton conductivity, which demonstrates the stability of the 0.5-NBN Nafion composite membrane in a strong acid environment (Figure S10). Furthermore, with the help of the adsorbed phosphoric acid in the membrane, the proton conductivity of H3PO4-processed 0.5-NBN Nafion decreases only by 9% after a 720 min test at 100 °C−40% RH, whereas H3PO4-processed recast Nafion displays a 40% decrease. This reveals the promising potential of NBN Nafion composite to enhance long-term proton conductivity via H3PO4 processing under high-temperature and low-humidity conditions, benefitting from the good stability of NBN nanocomposites in the acidic and high-temperature environment. The impressive increase in the proton conductivity of 0.5-NBN Nafion could be attributed to three main reasons. First, benefitting from the modification of Nafion, NBN has 14797
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ACS Applied Materials & Interfaces Table 1. Transport Properties of the PEMs at 40 °C methanol permeability (cm2·s−1) recast Nafion 0.3-NBN Nafion 0.5-NBN Nafion 1-NBN Nafion
1.8 4.7 4.1 2.7
× × × ×
−8
10 10−8 10−8 10−8
membrane selectivity (S·s·cm−3)
error 0.2 0.2 0.2 0.2
× × × ×
modestly compared to that of the recast Nafion and decreases with the incremental NBN content (Table 1). The methanol permeability of 1-NBN Nafion is 50% higher than that of recast Nafion. This could be attributed to the barrier effect of the NBN nanocomposites with the unique 2D structure to make the methanol crossover channels more zigzag and set back the methanol permeation. Consequently, the selectivity at 40 °C of NBN Nafion composite PEMs is enhanced by >1 order of magnitude benefitting from the highly increased proton conductivity at 40 °C. The membrane selectivity of 0.5-NBN reaches 3.5 × 105 S·s·cm−3. Therefore, high-performance NBN Nafion composite PEMs with connected long-range ionic nanochannels are successfully obtained and present high membrane selectivity, which shows strong competitiveness compared to the previously reported works (Table S1).
−8
10 10−8 10−8 10−8
■
1.1 2.7 3.5 1.2
× × × ×
104 105 105 105
error 0.1 0.2 0.2 0.1
× × × ×
104 105 105 105
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00858. Additional AFM images, HRTEM image, XRD patterns, Nyquist plots, FTIR spectrum, zeta potential distribution profile, high-resolution XPS spectra, TGA curves, Arrhenius plots for proton conductivity, longevity tests of proton conductivity, and comparison of transport properties with previously reported work (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*(P.W.) E-mail:
[email protected]. ORCID
Peiyi Wu: 0000-0001-7235-210X
4. CONCLUSION Nafion was used to assist the water-phase ultrasonication exfoliation of BN inspired by the amphiphilic nature of Nafion. NBN nanocomposites with a Nafion-functionalized periphery containing abundant superacidic sulfonic groups are successfully achieved during the exfoliation process via the strong noncovalent hydrophilic−hydrophobic interaction and Hbonding interactions. Then the NBN nanocomposites were blended with Nafion to prepare the NBN Nafion composite PEMs. NBN nanocomposites show good dispersibility in the Nafion matrix because of the outer Nafion modification layer. NBN with abundant sulfonic groups on the surface could change the internal osmotic pressure of the ionic clusters and exaggerate the size of ionic clusters in the membrane matrix. Furthermore, the ionic clusters would gather along the interfaces between NBN and the Nafion matrix promoted by the hydrophilic and H-bonding interactions during the membrane-forming process to reach the thermodynamically stable state. Connected long-range ionic nanochannels are consequently constructed in the composite PEMs. Besides, the addition of NBN nanocomposites also enhances the water uptake of the composite PEMs, especially under hightemperature and low-humidity conditions. Profiting from these advantages, the proton conductivity of the NBN Nafion composite PEMs is significantly increased at various conditions compared to that of recast Nafion. At 80 °C−95% RH, the proton conductivity of 0.5-NBN Nafion is 0.33 S·cm−1, 6 times that of recast Nafion under the same conditions. Meanwhile, the methanol permeability of the NBN Nafion composite membranes increases slightly despite the constructed connected long-range ionic nanochannels, which could be attributed to the barrier effect of NBN nanocomposites to impede methanol molecules and make methanol transportation channels more zigzag. Finally, high-performance NBN Nafion composite PEMs with connected long-range ionic nanochannels are successfully obtained and present high membrane selectivity.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are very grateful for financial support from the National Natural Science Foundation of China (NSFC, No. 21674025). REFERENCES
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