Cross-Linked Sulfonated Poly(arylene ether sulfone) - ACS Publications

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Cross-Linked Sulfonated Poly(arylene ether sulfone) Containing a Flexible and Hydrophobic Bishydroxy Perfluoropolyether Cross-Linker for High-Performance Proton Exchange Membrane Kihyun Kim, Pilwon Heo, Wonchan Hwang, Ji-Hoon Baik, Yung-Eun Sung, and Jong-Chan Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05139 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Cross-Linked Sulfonated Poly(arylene ether sulfone) Containing a Flexible and Hydrophobic Bishydroxy Perfluoropolyether Cross-Linker for High-Performance Proton Exchange Membrane

Kihyun Kim†, Pilwon Heo‡, Wonchan Hwang†, Ji-Hoon Baik†, Yung-Eun Sung† and JongChan Lee†*

†

Department of Chemical and Biological Engineering, Seoul National University, 599

Gwanak–ro, Gwanak–gu, Seoul 151–744, Republic of Korea ‡

Cell Development Group, Automotive & ESS Business Division, Samsung SDI Co. Ltd.

150-20, Gongse-ro, Giheung-gu, Yongin-si, Gyeonggi-do 446-577, Republic of Korea

Keywords: proton exchange membrane, bishydroxy perfluoropolyether, sulfonated poly(arylene ether sulfone), cross-linking, phase separation, hydration-dehydration cycling

ⓢ Supporting Information 1

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ACS Applied Materials & Interfaces 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

Abstract Here we show a simple and effective cross-linking method to prepare high performance cross-linked sulfonated poly(arylene ether sulfone) (C-SPAES) membrane using bishydroxy perfluoropolyether (PFPE) as a cross-linker for fuel cell applications. C-SPAES membrane shows much improved physicochemical stability due to the cross-linked structure and reasonably high proton conductivity than non-crosslinked SPAES membrane due to the incorporation of flexible PFPE and the effective phase-separated morphology between the hydrocarbon and perfluorinated moieties forming well-connected networks. Under intermediate-temperature and low humidity conditions (90 °C, 50% RH, and 150 kPa), the membrane electrode assembly employing C-SPAES membrane reveals an outstanding cell performance (1.17 W cm-2 at 0.65 V) ascribed to its reasonably high proton conductivity and enhanced interfacial compatibility between the perfluorinated moieties in electrode and CSPAES membrane. Furthermore, a hydration-dehydration cycling test result at 90 °C reveals that C-SPAES membrane has notable durability against rigorous operating conditions.

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Polymer electrolyte membranes (PEMs) operating at intermediate-temperature (80-120 °C) and low relative humidity (RH) conditions have been intensively studied for the use of polymer electrolyte membrane fuel cells (PEMFCs) in automobile transportation.1 Since the currently used radiators in automobiles are constructed to be operated at an intermediatetemperature, the PEMs need to be applicable to the same temperature range. However, the optimum operating temperature for the commonly used perfluorinated PEMs, such as Nafion®, is lower, 60-80 °C.2-3 A series of studies have been performed on the development of alternative PEMs using sulfonated hydrocarbon polymers (SHPs) for the application at intermediate-temperature due to their high thermomechanical property, low cost, and structural diversity.4 However, high electrochemical performances (i.e., proton conductivity and cell performance) of SHP membranes can only be achieved when SHPs have a sufficiently high degree of sulfonation (DS), while the SHP membranes having high DS do not have sufficiently high physical and chemical stabilities to exhibit desirable cell performance for a lengthy operation. The physicochemical stability of SHPs having high DS has been improved by cross-linking technology without the deterioration of proton conductivity,5-6 while the cell performances of membrane electrode assemblies (MEAs) employing cross-linked PEMs were lower than those with the corresponding linear PEMs due to 1) the brittle characteristics of the cross-linked membranes resulting in low chain flexibility,7 and 2) the poor interfacial compatibility of the hydrocarbon based cross-linked membranes with the fluorinated polymer based electrode materials containing fluorinated polymer binder.5 In this study, a cross-linked sulfonated poly(arylene ether sulfone) (C-SPAES) membrane showing highly improved physicochemical stability as well as cell performance is prepared 3



by simple and effective one-step process using a mixture of modified SPAES having chloromethyl side groups and bishydroxy perfluoropolyether (PFPE) consisting of a fluorinated backbone and two hydroxy end groups as a cross-linker. The flexible PFPE crosslinker does not significantly reduce the flexibility of the linear SPAES in the cross-linked structure, while the fluorinated groups in PFPE can effectively increase the interfacial compatibility between C-SPAES membrane and the electrode surfaces containing fluorinated polymers. This results in much improved cell performance and long-term durability of MEA employing C-SPAES membrane compared to those employing the SPAES membrane. SPAES was synthesized via polycondensation under a nitrogen atmosphere, as described in Supporting Information. The chemical structure and composition of SPAES was proven by 1

H NMR spectroscopy (Figure S1). The degree of sulfonation measured by the 1H NMR

spectrum was 63 mol%. The number- and weight-average molecular weights calculated from the gel permeation chromatography were 219,000 and 386,000, respectively. SPAES containing chloromethyl side group (SPAES-Cl) was prepared by chloromethylation of SPAES as described in the Supporting Information.5 The chemical structure and degree of chloromethylation of SPAES-Cl were also proven by 1H NMR spectroscopy (Figure S1). 5 The content of chloromethyl group in SPAES obtained by comparing the integrals of the signals at 7.96 ppm and at 4.42 ppm was found to be 8.0 mol% per repeating unit. Cross-linked SPAES (C-SPAES) membrane was simply fabricated by one step casting and thermal treatment process using a mixture of SPAES-Cl, trimethylamine (TEA), and PFPE as a cross-linker in N,N-dimethylacetamide (DMAc). The cross-linked structure is formed by the nucleophilic substitution reaction between the hydroxy groups in PFPE and chloromethyl groups in SPAES-Cl upon heating, where TEA increases the reaction rate (Figure 1).8-9 A 4


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model reaction of PFPE with benzyl chloride was carried out as a separate experiment under the same conditions as that used for the membrane preparation to confirm the cross-linking reaction (Figure S2), because the chemical structures of the insoluble and infusible crosslinked polymers could not be easily identified by the common chemical analysis method such as NMR spectroscopy.

Figure 1 Chemical structures of SPAES-Cl, PFPE and preparation method to C-SPAES membrane, wherein the photo-image of C-SPAES membrane is included.

The cross-linked structure of the C-SPAES membrane was further evaluated by performing a solubility test (Table S1). The SPAES and C-PAES membranes were soaked in various solvents at 30 and 80 °C for 1 h. The SPAES membrane is soluble in polar aprotic solvents such as DMAc, dimethly sulfoxide, and N-methly-2-pyrrolidone, while the C-SPAES 5



membrane is insoluble in these aprotic solvents at 30 °C, although it is partially soluble at 80 °C. This result indicates that the C-SPAES membrane has a lightly cross-linked structure that is not soluble in polar aprotic solvents at room temperature, although some part of the membrane can be dissolved at high temperatures. The mechanical properties described in Figure S3 can support the lightly cross-liked structure of the C-SPAES membrane, showing the small decrease and the small increase of elongation at break and tensile strength values, respectively. The gel fraction indicating the content of fully cross-linked portion in the crosslinked polymers is 27.8 wt.% (Figure 2(a)). 10 The clear image of the membranes after the gel fraction test is shown in Figure 3(c). Fenton test was performed to estimate the oxidative stability of the membranes.11 After immersing the membranes in the Fenton’s reagent (3 wt.% H2O2 having 16 ppm Fe2+) at 80 °C, the oxidative stability was estimated by measuring the times when the membranes broke into pieces (τ1) and dissolved completely (τ2) (Figure 2(b)). The τ values of the C-SPAES membrane were found to be much larger than those of the SPAES membrane, indicating the significantly increased oxidative stability of the C-SPAES membrane by the combined effect of the cross-linked structure and the introduction of extremely chemically stable PFPE.12

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Figure 2 (a) Residual weights of SPAES and C-SPAES membranes after the gel fraction test in DMAC at 80 °C for 24 h. The photo image shows the DMAc solutions after the gel fraction test. (b) Oxidative stability of SPAES and C-SPAES membranes using a Fenton's reagent (3 wt% H2O2 aqueous solution having 16 ppm Fe2+) at 80 °C.

The water uptake and hydration number (λ) of the membranes at different RH conditions were measured to compare the water absorption behavior (Figure S4 and Table S2). The water uptake values of both the SPAES and C-SPAES membranes increase with the increase of RH, while the maximum value at 98% RH for C-SPAES membrane (43% with λ= 11.9) is much smaller than that for the SPAES membrane (79% with λ= 17.9), because C-SPAES membrane has a cross-linked structure by the hydrophobic PFPE. The dimensional stability of the membranes was also evaluated by calculating their dimensional changes after immersion in deionized water at 90 °C for 4 h (Table S3). The C-SPAES membrane shows a much improved dimensional stability compared to the SPAES membrane; the volume changes in C-SPAES and SPAES membranes were 95.6% and 335.3%, respectively. These results directly indicate that the cross-linked structure formed by PFPE can effectively suppress the excessive water absorption and increase the dimensional stability of the membrane. The nanophase morphology of the membranes was measured by atomic force microscopy (AFM) in tapping mode. The phase images of the membranes were characterized in ambient atmosphere conditions on 500 × 500 nm2 size scales. The dark images are derived from the soft, hydrophilic, portion containing sulfonic acid and water. While, the light images are derived from the hard, hydrophobic, polymer moieties (Figures 3(a) and 3(b)).13 The CSPAES membrane shows a more distinct separation between hydrophilic-hydrophobic phase 7



with slightly smaller ionic domains than the SPAES membrane, possibly due to the crosslinked structure formed by the hydrophobic perfluorinated cross-linker.14 The smaller size of hydrophilic ionic cluster and the cross-linked networks by the hydrophobic moieties in the CSPAES membrane should lead to lower water uptake and less swelling than the SPAES membrane, as described above. The enlarged AFM images are also shown in Figure S5. Figure 3(c) and Table S4 show the proton conductivities of the membranes at 90 °C under different humidity conditions from 20% to 95% RH. Although most of the cross-linked membranes have quite smaller proton conductivity than the corresponding linear membranes because the cross-linked membranes have the lower chain mobility and smaller size of ionic cluster facilitating the proton transport,15 the C-SPAES membrane has comparable and/or slightly smaller proton conductivity than the corresponding SPAES membrane. The lightly cross-linked structure of C-SPAES formed by flexible and hydrophobic PFPE does not decrease the chain mobility of SPAES much, if any, while it can produce distinctly phaseseparated and well-connected hydrophilic domains that can effectively transport protons in the cross-linked structures even at low RH conditions.16 The differential scanning calorimetry (DSC) results of the membranes equilibrated at 98% RH condition show that the number of freezable and non-freezable water molecules per sulfonic acid group for the C-SPAES membrane are 3.1 and 8.8, respectively, and those for the SPAES membrane are 7.4 and 10.5, respectively (Figure S5). This indicates that although the C-SPAES membrane has less water uptake than the SPAES membrane, the content of non-freezable (strongly-bound) water in the C-SPAES membrane is not significantly less than that of the SPAES membrane. Therefore, the conductivity of the C-SPAES membrane having flexible and hydrophobic PFPE moiety is quite close to that of the linear SPAES membrane. 8


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Figure 3 AFM phase images of (a) SPAES and (b) C-SPAES membranes (500 × 500 nm2). (c) Proton conductivity of SPAES and C-SPAES membranes at 90 °C as a function of relative humidity. Figure 4(a) shows the cell performance of the MEAs employing the C-SPAES and SPAES membranes at the operating conditions of 90 °C and 50% RH under 150 kPa (H2/air). MEAs with membranes having higher proton conductivity normally exhibit better cell performance than those with membranes having lower proton conductivity.2 However, the cell performance of the MEA with the C-SPAES membrane was better than that with the SPAES membrane. For example, the power density values at 0.65 V of the MEAs with the C-SPAES and SPAES membranes are 1.17 and 0.85 W cm-2, respectively. The better cell performance of the MEA with the C-SPAES membrane having lower proton conductivity could be 9



attributed to the improved interfacial compatibility between the membrane and electrode. It has been reported that fluorine moieties on a membrane surfaces can increase the compatibility with the electrodes in the MEAs, because the binder materials used for the electrode layers are perfluoronated ionomers.17-18 The characterization of membrane surface in terms of elemental composition and concentration using a field-emission scanning electron microscope equipped with energy dispersive spectroscopy (FE-SEM/EDS) is shown in Figure S6. As expected, the EDS spectrum of the C-SPAES membrane shows the fluorine peak (5.14 wt%) that is not shown in the SPAES membrane. Therefore, the existence of the fluorine moieties on the surface of C-SPAES membrane can be expected to increase the compatibility between the membrane and electrodes, resulting in better cell performance of the MEA employing the C-SPAES membrane. It is well known that fluorinated polymers are much more compatible with polymers containing fluorine groups than with other hydrocarbon polymers.18-19 Figure 4(b) shows the impedance spectra of the MEAs observed at the current density of 100 mA cm-2. The intercept on the real axis (x-axis) at high frequency denotes the membrane area resistance (Rm) of the cells including the contact resistance, minor ohmic losses of the electrodes and membrane resistance.20 The Rm value estimated from the in-situ electrochemical impedance spectroscopy of the MEA employing the C-SPAES membrane is smaller than that with the SPAES membrane. The results of impedance spectra and EDS spectrum indicate that compatibility with the electrode is increased by the fluorine moieties on the C-SPAES membrane surface, and the contact resistance between the membrane and electrodes is decreased. 17,21 The charge transfer resistance (Rct) identified by the electrode reaction (here, oxygen reduction reaction of cathode) was estimated from the diameter of the 10


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single impedance arc in a Nyquist plot.22 The smaller Rct value calculated from the MEA with the C-SPAES membrane (22.2 mΩ cm2) than that with the SPAES-membrane (25.2 mΩ cm2) demonstrates the increase of electrochemical reaction rate at the membrane-electrode interface. The better cell voltages of the MEA employing the C-SPAES membrane in the kinetically controlled current density region can be attributed to the increase in kinetic activity due to the increase in the compatibility of the C-SPAES membrane with the electrodes. To the best of our knowledge, the cell performance of the MEA employing the CSPAES membrane is best or comparable to those of the MEAs with hydrocarbon membranes as well as perfluorinated polymer membranes measured under intermediate-temperature and low humidity conditions, the practical operating conditions for fuel cell vehicles (Figure S7). Such a high cell performance could be achieved by the optimized MEA fabrication method, in addition to the membrane properties. The detailed MEA preparation is provided in the Supporting Information. The potential application of the C-SPAES membrane for various energy conversion devices is further confirmed by the outstanding cell performances of the MEAs with C-SPAES membranes at various operating conditions (Figure S8). Figure 4(c) shows the durability test results of the membranes during the operation of the cells under rigorous conditions. The applied test protocol involves the use of hydrationdehydration cycling at 90 °C with an open circuit voltage (OCV) holding method to simultaneously evaluate the physicochemical degradation.23 The cell with the SPAES membrane shows the OCV drop after 100 cycles and the lower OCV value of 0.7 V is observed after 500 cycles, while that with the C-SPAES membrane proceeds to 1500 cycles with the higher OCV value of 0.85 V due to the effective cross-linked structure formed by PFPE (Figures 4(d) and 4(e)). 11



Figure 4 (a) Cell performances and (b) impedance spectra of MEAs employing SPAES and C-SPAES membranes at 90 °C and 50% RH under 150 kPa conditions. Active area of MEA was 26 cm2 and humidified H2/air was supplied as feed gases during the measurements and Nyquist plot were obtained at current density of 0.1 A cm-2. (c) Hydration-dehydration cycling tests of SPAES and C-SPAES membranes at 90 °C with an OCV holding method. OCV values of (d) SPAES and (e) C-SPAES membranes according to cycling number. One cycle consisted of supplying dry gases for 30 seconds followed by supplying fully humidified gases for 60 seconds.

In conclusion, we designed and prepared a C-SPAES membrane using PFPE as a cross-linker for the first time. Compared to the linear and non-crosslinked SPAES membrane, the CSPAES membrane reveals much better physical and chemical stabilities and effective water utilization behavior, due to the well-defined phase-separated morphology via the incorporation of PFPE as the cross-linker. The proton conductivity of the C-SPAES 12


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membrane is not much smaller than that of the linear SPAES membrane due to the incorporation of flexible PFPE moiety and the effective phase-separated morphology. The cell performance of the MEA employing the C-SPAES membrane is better than that employing the SPAES membrane at various operating conditions, including the practical operating conditions of automotive fuel cells. This result can be ascribed to the fluorine moieties on the C-SPAES membrane surface that improve the interfacial compatibility and decrease the contact resistance with the electrode surface based on fluorinated ionomer. Furthermore, the C-SPAES membrane shows significantly enhanced long-term durability compared to the SPAES membrane under rigorous conditions. We believe that this crosslinking strategy using PFPE as a cross-linker of the hydrocarbon based polymers could be applicable to various PEM materials to meet the requirements of practical PEMFC applications. In addition, the C-SPAES membrane system is versatile for use in various applications as an energy material because its cross-linking density and ion exchange capacity can be easily adjusted to optimize its properties.

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ASSOCIATED CONTENT Details about materials, synthetic protocols and characterization methods were included in Supporting Information. AUTHOR INFORMATION Corresponding Author * Corresponding Author: Tel. +82 2 880 7070; e-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgments This research was supported by the National Research Foundation of Korea funded by the Korean Government (NRF-2015M1A2A2056729).

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Cross-Linked Sulfonated Poly(arylene ether sulfone) - ACS Publications

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