Proton Transport in Hierarchical-Structured Nafion Membranes: A

Jul 21, 2017 - It is known that hierarchical structure plays a key role in many unique material properties such as self-cleaning effect of lotus leave...
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Proton Transport in Hierarchical-Structured Nafion Membranes: A NMR Study Hengrui Yang,†,‡ Jin Zhang,§,∥ Jingliang Li,† San Ping Jiang,§ Maria Forsyth,†,‡ and Haijin Zhu*,†,‡ †

Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia ARC Centre of Excellence for Electromaterials Science, Deakin University, 221 Burwood Highway, Burwood, VIC 3125, Australia § Fuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia ∥ Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Space and Environment, Beihang University, Beijing, 100191 China ‡

S Supporting Information *

ABSTRACT: It is known that hierarchical structure plays a key role in many unique material properties such as self-cleaning effect of lotus leaves and the antifogging property of the compound eyes of mosquitoes. This study reports a series of highly ordered mesoporous Nafion membranes with unique hierarchical structural features at the nanometer scale. Using NMR, we show for the first time that, at low RH conditions, the proton in the ionic domains migrates via a surface diffusion mechanism and exhibits approximately 2 orders of magnitude faster transport than that in the nanopores, whereas the nanopores play a role of reservoir and maintain water and thereby conductivity at higher temperature and lower humidities. Thereby creating hierarchical nanoscale structures is a feasible and promising strategy to develop PEMs that would enable efficient electrochemical performance in devices such as fuel cells, even in the absence of high humidity and at elevated temperatures. uel cell and redox flow battery technologies are becoming increasingly important as clean and sustainable alternatives to the traditional fossil fuel.1 As a vital part of fuel cell devices, the electrolyte membrane is generally required to achieve higher ionic conductivity while maintaining or improving other properties such as lowering the permeability.2 Vast majority of current membranes for commercial polymer electrolyte membrane fuel cells (PEMFCs) are based on perfluorosulfonic acid and its derivatives.1−3 The most prominent and successful polymer electrolyte membrane (PEM) product to date is Nafion (DuPont).4 Because of its technological importance, the microstructure and morphology of Nafion has been a hot research topic for decades since its initial development in the mid-1960s.4 It is well accepted that hydrogen bonds exist among the sulfonic acid end groups and form continuous hydrophilic domains or ionic clusters that allow proton transport. The hydrophilic domains allow for fast proton transport, whereas the hydrophobic domains provide morphological and mechanical stability to the matrix. The ion clustering model in Nafion was first proposed by Gierke etc. in 1981.5 In this model, the clusters are approximately spherical in shape and are interconnected by narrow channels in order to explain the percolation of protons. This model has been evolved over a few decades, and now it has been well accepted that there is a continuous morphological reorganization of Nafion with hydration.6 Gebel et al. studied the microphase structure evolution of the Nafion films with different water contents and found that, with a water volume fraction greater than 75%, the spherical interconnected ionic clusters form a colloidal dispersion of rod-like polymer particles.7 Noto et al. system-

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© XXXX American Chemical Society

atically investigated the proton transport mechanisms in various multiphase proton membranes, and found that the long-range proton migration occurs trough proton exchange processes between “delocalization bodies” that are facilitated by the dynamics of the host polymer.8−14 Despite its great success in industrial applications, an essential problem associated with these perfluorosulfonic acid based membranes is that all these membranes can only be operated at high relative humidities (RH) and low temperatures, as proton transport requires an aqueous medium to maintain a high conductivity. Recent studies have shown that this high RH sensitivity is related to the randomly ordered nanostructure consisting of hydrophobic perfluorinated main chains surrounded by hydrophilic ionic domains.15−17 More recently, a series of mesoporous Nafion membranes with welldefined and ordered pore arrays in the range of 2−50 nm were synthesized by Jiang et al. using a self-assembly method.15,18 It was anticipated that the nanoscale pore and channel structures could provide the unique material properties of capillary condensation effect19 and, thereby, exhibit a superior water retention ability and a higher proton conductivity under reduced RH conditions. Although previous studies have shown that the presence of ordered mesoporous structure in Nafion membrane enhances proton conductivity under reduced humidity conditions,15,20 the underlying molecular-level mechReceived: June 19, 2017 Accepted: July 21, 2017 Published: July 21, 2017 3624

DOI: 10.1021/acs.jpclett.7b01557 J. Phys. Chem. Lett. 2017, 8, 3624−3629

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Figure 1. (a) Schematic illustration of the geometries of 2D-H, 3D-FC, and 3D-CB structures of the ordered mesoporous Nafion. (b) Water uptake and (c) proton conductivity of the synthesized highly ordered mesoporous Nafion films measured under 80% RH and 100% RH conditions at room temperature. All the water uptake and conductivity values are an average of five measurements on the same sample. The data of pristine Nafion membrane are also shown as a reference for comparison.

mesoporous Nafion membranes: mesopores and ionic channel. Protons in the mesopores shows two-orders of magnitude lower diffusion than that in the ionic channels of the Nafion bulk membranes, and thus the conductivity of Nafion membranes are literally dominated by the proton transport in the ionic channels. Figure 1c showed that the mesoporous membranes exhibit superior proton conductivity at low humidity conditions. Increased proton conductivity of PEMs is generally related to either a higher dissociable proton density or a faster proton migration. To understand which factor is dominant, 1H NMR experiments were performed, and the spectra are shown in Figure 2. The NMR chemical shift and spectral line width can provide useful information on the microstructures and ion dynamics at the molecular level.22−24 Interestingly, all the mesoporous membranes clearly show two peaks with chemical shifts in the range of 4−4.5 and 8−10 ppm, respectively, suggesting two distinctly different proton environments. The 1 H chemical shift of pure H2O is about 4.5 ppm, as shown in the bottom of Figure 2. It is well-known that the NMR chemical shift of H2O is very sensitive to the acidity of the solution. It shifts to lower field in acidic solution due to the faster chemical exchange with acidic protons. The right-side peaks have chemical shifts similar to water, suggesting a neutral proton environment, thus can be attributed to the bulk water trapped in the ordered mesoporous channels. Whereas the leftside peaks are of chemical shifts of 8−10 ppm, which is similar to that of the pristine Nafion membrane, suggesting a more acidic environment. It is reasonable to assign this peak to the protons in the hydrophilic ionic domains of the Nafion bulk phase,25,26 which consists of sulfonic acid end groups. This assignment is supported by recent studies on the phase structure of Nafion membranes using a combination of solidstate NMR and SAXS techniques, which revealed a characteristic ionomer peak arising from long parallel hydrophilic nanochannels.25,26 These channels are surrounded by polymer side branches with hydrophilic sulfonic acid end groups, forming inverted-micelle cylinders and allow for locally directed and collective proton transport. For simplicity, in the following discussion the two peaks at 4−4.5 and 8−10 ppm will be referred as the mesopores and ionic channel protons, respectively.

anism is still unknown. In the present work, we investigate the paths and rates of proton transport in various mesoporous Nafion membranes of different structural symmetry using pulsed field gradient (PFG) NMR, with the aim of achieving a fundamental understanding of the relationship between the nanoscale structure and the proton transport behavior. As we show, the hierarchical nanostructure plays an important role in maintaining high proton conductivity particularly under reduced RH conditions. Mesoporous Nafion with different structure symmetries including 2D hexagonal (2D-H), 3D face-centered cubic (3DFC), and 3D cubic-bicountinous (3D-CB) was synthesized following procedures described elsewhere.15,21 The space groups of these mesoporous Nafion membranes have been confirmed by SAXS (see Figure S1 in the Supporting Information), and the microporous structure symmetries are shown in Figure 1a. The pore sizes, as quantified in the previous work by using N2 adsorption isotherm experiments, are 5.3, 3.8, and 3.8 nm for the 2D-H, 3D-FC, and 3D-CB Nafion samples, respectively.15 Figure 1b,c shows the water uptake and the proton conductivities of the mesoporous Nafion membranes measured at 80% and 100% RH conditions. At 100% RH, all the mesoporous membranes exhibit water uptake ratios significantly higher than that of the pristine Nafion (casted from 5 wt % aliphatic alcohol and water suspension, Sigma-Aldrich), which can be explained by the nanosized pores that can absorb and retain water. At a relatively low RH of 80% (see Figure 1b), the water uptake drops to 24, 36, and 30 wt % for 2D-H, 3D-FC, and 3D-CB, receptively, whereas still significantly higher than the pristine Nafion of 9.5 wt %. In general, proton conductivity of Nafion membranes are closely related to the water content. Higher water content usually leads to higher proton conductivity. At 80% RH, the proton conductivities for the 2D-H, 3D-FC, and 3D-CB membranes are 1.6 × 10−2, 2.0 × 10−2, and 1.8 × 10−2 S·cm−1, which are apparently higher than that of the pristine Nafion membrane (1.1 × 10−2 S·cm−1), attributing to the higher water content. At 100% RH, however, the pristine Nafion shows the highest proton conductivity despite of the lowest water content. This is related to the hierarchical nanostructure of membranes. We will show in the later discussion that there are two different proton environments at different length scales presented in the 3625

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proton and the water molecules. In general higher acidic proton concentration leads to downfield shift. Assuming the sulfonic acid group density is the same in ionic domain for all the Nafion membranes, less water content means higher acidic proton concentration. It is also noted that the line widths of both peaks are obviously different, indicating different molecular mobility of water in ionic domains and mesopores. The mesopore protons has significantly broader line width compared to that of the ionic domains, suggesting a slower proton mobility. This is in a good agreement with the PFGNMR results in Table 1 that show that protons in the mesopores has approximately 2 orders of magnitude slower diffusion than that in the ionic domains. A schematic illustration of the microphase structure of the mesoporous Nafion membranes is shown in Figure 3a. The larger channels (diameter of ∼5 nm) are highly ordered with various structural symmetry (2D-H as an example), and the walls of the channels are composed of Nafion material, which exhibits an ionic channel structure at a smaller length-scale (diameter of ∼1 nm). This unique hierarchical structure can provide the material with superior electrochemical performance at higher temperatures and reduced RH conditions. The larger mesopores can store and release water under extreme conditions, and the ionic channels can facilitate proton transport. The proton transport mechanism through Nafion membranes bulk phase has been described as a combination of surface diffusion, structural (Grotthus) diffusion, and vehicular diffusion.28,29 Surface diffusion occurs in a two-dimensional surface close to the walls of the ionic channel, that is, in a layer of around 1 nm from the wall.28 In Nafion membranes, the major energy barrier for surface diffusion is related to the proton hopping in between negatively charged fixed sulfonic ions and the positively charged hydronium ions. In contrast, structural diffusion in Nafion is primarily accomplished via continuous breakage and formation of hydrogen bonds in between hydronium ions through bulk aqueous phase. It happens only in the bulk water phase in the central region of the ionic channel when the membrane is fully hydrated. Vehicular diffusion considers hydronium ion as a diffusing entity in a continuum of water. In a fully hydrated Nafion, the Grotthus diffusion is the fastest among all the three possible mechanisms, as has been confirmed experimentally,30,31 and has also been supported by MD simulations.32 At 25 °C, the Grotthus diffusion coefficient is around 7 × 10−9 m2/s. Surface diffusion exhibits a much lower (2 orders of magnitude) diffusivity of 1.01 × 10−11 m2/s,21 and the vehicle diffusion coefficient is 1.71 × 10−9 m2/s,28 which is very close to, yet slightly lower than, that of water 2.3 × 10−9 m2/s.33 The possibility to measure the chemical shift resolved diffusion coefficients using PFG-NMR helps to clearly identify the contribution of different ion/molecule species and conducting mechanisms in the material. Table 1 shows the diffusion coefficients of protons located in ionic channels and mesopores of the 80% RH samples obtained by PFG-NMR at 60 °C. All the ionic channel protons exhibit diffusion coefficients in the order of 10−11 m2/s, significantly lower than Grotthus and vehicle diffusion of bulk water, whereas slightly higher than the surface diffusion yet still within the same order of magnitude. This result strongly suggests that in the ionic channel surface transport mechanism dominates the acidic proton transfer at low RH conditions, as the other two diffusion mechanisms would results in much faster diffusion

Figure 2. 1H NMR spectra of the mesoporous Nafion membranes. The small sharp peak on the right shoulder of the spectrum of pristine Nafion is probably attributed to the tiny amount of water on the surface of the membrane which was squashed out during the rotor packing.

Figure 2 shows that the resonance of pristine Nafion is shifted to the lower field compared to the mesoporous Nafion membranes. This is because of the less water content in the ionic domain of pristine Nafion (see Table 2). As we know, the Table 1. Diffusion Coefficients of the Protons in Mesopores and Ionic Channels of the Membranes Measured at 60 °Ca sample

2D-H (m2/s)

3D-FC (m2/s)

3D-CB (m2/s)

Nafion (m2/s)

ionic channel mesopore

3.8 × 10−11 1.5 × 10−13

5.9 × 10−11 7.4 × 10−13

3.4 × 10−11 3.3 × 10−13

1.7 × 10−11 NA

a The diffusion coefficients of the mesopore protons was not measurable at lower temperature due to the extremely short T2, which is probably caused by the fast surface relaxation. The samples were stored at room temperature and 80% RH conditions for over 48 h before sealed in a 4 mm solid-state NMR tube for PFG-NMR measurements.

Table 2. Water Content in the Ionic Channel of the Nafion Membranesa sample

2D-H (%)

total water uptake, Ctotal (wt 24 ± 1 %) 47 ± 4 ionic channel water percentage,a Pionic (mol %) 11.3 ± ionic channel water uptake, 1.1 Pionic (wt %)

3D-FC (%)

3D-CB (%)

Nafion (%)

36 ± 2

30 ± 1

9.5 ± 0.4

35 ± 3

40 ± 3

100

12.6 ± 1.3

12.0 ± 1.0

9.5 ± 0.4

a Obtained by deconvolution of the 1H NMR spectra shown in Figure 2.

proton chemical shift of water is highly dependent on the acidity due to the fast proton exchange between the acidic 3626

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Figure 3. (a) Hierachical structure of the mesoporous Nafion membranes.6,27 (b) Arrhenius plot of the diffusion coefficients of the protons located in the ionic channel. The solid lines represent the best fit to the experimental data (dots). The samples were stored at room temperature and 80% RH atmosphere conditions for over 48 h, and then sealed in a 4 mm solid-state NMR rotor for PFG-NMR measurements.

Therefore, the observed proton diffusion coefficient is a population weighted value between the surface and bulk protons:23,35,36

coefficients. This is understandable in that at low (80%) RH condition, there is an insufficient amount of water to hydrate the ionic channels and provide the bulk water environment required for structural and vehicle diffusion. The water molecules are restricted on the surfaces of the ionic channels, and are forced to migrate within a two-dimensional layer parallel to the wall. This argument is supported by the previous MD simulation work by Datta et al., who showed that surface diffusion is the most efficient proton transport mechanism in Nafion at low water content.28 To the best of our knowledge, this is the first direct experimental evidence of a surface diffusion for Nafion membranes under a reduced RH condition. Comparing the diffusion coefficients of the protons in the mesopores and the ionic channels, one can find that the former show much lower (approximately 2 orders of magnitude) diffusion coefficients than the latter. The PFG-NMR attenuation experiments in Figure S2, as well as the twodimensional diffusion ordered spectra (DOSY) presentation of the same data in Figure S3 clearly support this result. The slower proton transport in the mesopores as compared to the ionic channel can be understood in terms of restricted diffusion within the closed mesopores. As mentioned in the previous discussion, the fast proton transport in the ionic domain is achieved most efficiently via surface diffusion mechanism at low RH conditions, and this argument is corroborated by other studies that show different charge migration pathways through the bulk of the hydrophilic domains of Nafion membranes,10,12 and particularly the long-range proton migration processes take place by proton exchange between the different “delocalization bodies”, which is modulated by dipole local fluctuation and segmental motion of polymer chains.8,34 It is also interesting to note that the diffusion coefficients of the ionic channel protons in the mesoporous Nafion are systematically faster than that of the pristine Nafion, and are varied significantly across different mesoporous structures. This seems strange because all these membranes are of exactly the same chemical composition, and are expected to give similar ionic channel structure and thus similar proton surface diffusion behavior. The differences may be explained by the proton exchange between the surface and the bulk H2O in the ionic channel. Depending on the hydration level of the ionic channel, there may be bulk water phase where the molecules are not directly bonded to the channel walls and can transport efficiently via structural and vehicle diffusion mechanisms. These bulk water molecules can exchange protons very fast with the surface water molecules and the sulfonic acid groups.

Dobs =

nsurf Dsurf + nbulk D bulk nsurf + nbulk

(1)

where n and D are the population and diffusion coefficient of the corresponding proton pieces. The surface proton diffusion coefficients Dsurf for all the Nafion membranes equilibrated at 80% RH and room temperature should be identical because of the essentially the same chemical composition. As discussed before, the theoretical surface diffusion coefficient of protons in Nafion is 1.01 × 10−11 m2/s according to the MD simulation by Choi et al.21 Therefore, the observed diffusion coefficient Dobs is solely dependent on the quantity of the surface water nsurf and bulk water nbulk, which is further determined by the mesoporous structures of the membrane. It is possible to estimate the actual water uptake in the ionic channel using the following relationship:

C ionic = C total·Pionic

(2)

where Cionic is the weight percentage of the water in the ionic channels, Ctotal is the total water uptake percentage of the membranes, and Pionic is the percentage of the water in the ionic channel as compared to the overall water taken by the membranes. Pionic can be obtained from the curve deconvolution of the 1H NMR spectra shown in Figure 2. The parameters used for the calculation and the results are shown in Table 2. Clearly, all the mesoporous Nafion membranes shows higher ionic channel water uptake than that of the pristine Nafion. This means that the mesoporous membranes have more bulk water molecules in the ionic channel than the pristine Nafion. This result is consistent the water uptake results shown in Figure 1 and has nicely explained the higher diffusion coefficients of the ionic channel protons in mesoporous membranes. The activation energy Ea for proton diffusion can be considered as an important parameter that can provide useful information about the proton transport mechanism. Ea can be obtained by fitting the temperature-dependent diffusion coefficients curve with the Arrhenius equation:37 D = D0e(−Ea / RT )

(3)

where D (m2/s) is the diffusion coefficient, D0 (m2/s) is the maximum diffusion coefficient at infinite high temperature, Ea 3627

DOI: 10.1021/acs.jpclett.7b01557 J. Phys. Chem. Lett. 2017, 8, 3624−3629

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The Journal of Physical Chemistry Letters (J/mol) is the activation energy for diffusion, T (K) is temperature, and R is the gas constant of 8.31 [J/(K·mol)]. Figure 3b shows the Arrhenius plot of the diffusion coefficients of protons in the ionic channels of the equilibrated at 80% RH condition. The activation energy values, as estimated from the fitting of the experimental data, are 35, 50, 39, and 23 kJ/mol for 2D-H, 3D-FC, 3D-CB, and pristine Nafion samples, respectively. As a benchmark, the activation energy of fully hydrated Nafion 112 is about 15 kJ/mol38,39 and is found to increase rapidly with decreasing RH.39 Interestingly, the activation energy of proton transport within the ionic channels is highly dependent on the structure of the mesopores, and all the mesoporous Nafion membranes show significantly higher activation energy than the pristine Nafion. This may be attributed the tortuosity of the ionic channels, which can affect the long-range proton transport. The presence of mesopores can disrupt the proton conducting pathways along the ionic channel and impede proton transport, thereby increase the activation energy. In summary, we have developed and characterized a novel mesoporous Nafion material with hierarchical nanostructures. 1 H NMR spectra of these membranes shows two well-resolved resonances, suggesting two different water environments: ionic channels and mesopores. Chemical shift resolved PFG-NMR results clearly showed that the ionic channels exhibit approximately 2 orders of magnitude faster proton transport rate than mesopores, and the surface transport is the dominant transport mechanism as compared to the Grotthus and vehicle transport. The unique mesoporous structure, in combination with the ionic channel structure of Nafion, provides the material with superior proton conduction under reduced RH conditions, in that the mesopores can store and release water and thus maintain a high level of hydration to the ionic channels, which are the primary proton transport pathways in the Nafion membrane. The concept of hierarchical structures developed in this work can also be applied in designing other novel anhydrous proton conductors (e.g., phosphoric acid/PBI composite) for high temperature fuel cell applications.

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(ARC) Centre of Excellence for Electromaterials Science. M.F. wishes to thank the ARC for fellowship support under the Australian Laureate Program Funding FL110100013. S.P.J. would like to acknowledge ARC for the support under Discovery Project Scheme DP150102025 and DP150102044. A.R.C. is also acknowledged for funding Deakin Universitys Magnetic Resonance Facility through LIEF Grant LE110100141.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01557. Experimental details, small angle X-ray scattering data, PFG-NMR attenuation spectra, 2D-DOSY NMR spectra, variable-temperature 1H NMR spectra, and the Arrhenius plot of the ionic-channel proton diffusion coefficients of the 100% RH mesoporous membranes (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +61 3 5227 3696. E-mail: [email protected]. ORCID

San Ping Jiang: 0000-0002-7042-2976 Haijin Zhu: 0000-0001-6352-7633 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.Z. thanks the Central Research Grants Scheme (CRGS) from Deakin University and the Australian Research Council 3628

DOI: 10.1021/acs.jpclett.7b01557 J. Phys. Chem. Lett. 2017, 8, 3624−3629

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DOI: 10.1021/acs.jpclett.7b01557 J. Phys. Chem. Lett. 2017, 8, 3624−3629