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[Je-Deok Kim](https://orcid.org/0000-0003-4301-1044), Akihiro Ohira, [Hidenobu Nakao](https://orcid.org/0000-0002-4014-9366)

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[Chemically Crosslinked Sulfonated Polyphenylsulfone (CSPPSU) Membranes for PEM Fuel Cells](https://mdr.nims.go.jp/datasets/8b832924-3e3d-44a7-8d6c-fd3bc17432a0)

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membranesArticleChemically Crosslinked SulfonatedPolyphenylsulfone (CSPPSU) Membranes for PEMFuel CellsJe-Deok Kim 1,*, Akihiro Ohira 2 and Hidenobu Nakao 31 Hydrogen Production Materials Group, Center for Green Research on Energy and Environmental Materials,National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2 Energy Storage Technology Group, Research Institute for Energy Conservation, National Institute ofAdvanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan;a-oohira@aist.go.jp3 Hydrogen Materials Engineering Group, Research Institute for Energy Conservation, National Institute ofAdvanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan;nakao.hidenobu@nims.go.jp* Correspondence: kim.jedeok@nims.go.jpReceived: 22 January 2020; Accepted: 14 February 2020; Published: 18 February 2020�����������������Abstract: Sulfonated polyphenylsulfone (SPPSU) with a high ion exchange capacity (IEC) wassynthesized using commercially available polyphenylsulfone (PPSU), and a large-area (16 × 18 cm2)crosslinked sulfonated polyphenylsulfone (CSPPSU) membrane was prepared. In addition, wedeveloped an activation process in which the membrane was treated with alkaline and acidicsolutions to remove sulfur dioxide (SO2), which forms as a byproduct during heat treatment. CSPPSUmembranes obtained using this activation method had high thermal, mechanical and chemicalstabilities. In I-ViR free studies for fuel cell evaluation, high performances similar to those using Nafionwere obtained. In addition, from the hydrogen (H2) gas crossover characteristics, the durability ismuch better than that of a Nafion212 membrane. In the studies evaluating the long-term stabilities byusing a constant current method, a stability of 4000 h was obtained for the first time. These resultsindicate that the CSPPSU membrane obtained by using our activation method is promising as apolymer electrolyte membrane.Keywords: PPSU; High IEC, CSPPSU; activation; PEMFCs1. IntroductionA low-carbon society in which safe, highly efficient, renewable and sustainable energy sources areused to sustain economic growth, environmental protection and energy security has become important.Proton exchange membrane fuel cells (PEMFCs) are among the most promising electrochemical devicesfor a low-carbon society and highly efficient power generation. Although PEMFCs using perfluorosulfonicacid polymer membranes have been commercialized, to increase their performances, more developmentand new material components, such as catalyst electrodes and polymer membranes, are required.Although it is important to find non-platinum catalyst materials for use as the catalyst electrode [1–3],non-fluorine proton exchange membranes are also necessary. The most commonly used proton exchangemembranes for PEMFCs are poly(perfluorosulfonic acid) (PFSA) copolymers such as Nafion, whichhave high hydrolytic and oxidative stability and excellent proton conductivities [4,5]. However, theglass-transition temperatures (Tg), mechanical stabilities, and gas permeabilities must be improved.Moreover, perfluorinated polymers have high production costs and environmental incompatibilities.The drawbacks of perfluorinated membranes have prompted research into alternative membranes.Membranes 2020, 10, 31; doi:10.3390/membranes10020031 www.mdpi.com/journal/membraneshttp://www.mdpi.com/journal/membraneshttp://www.mdpi.comhttp://dx.doi.org/10.3390/membranes10020031http://www.mdpi.com/journal/membraneshttps://www.mdpi.com/2077-0375/10/2/31?type=check_update&version=2Membranes 2020, 10, 31 2 of 14For example, several aromatic polymer ionomer membranes, such as polybenzimidazole (PBI) [6,7],sulfonated polyphenylene oxide (SPPO) [8], sulfonated polyimide [9–11], sulfonated polyethersulfone(SPES) [12,13], sulfonated polyphenylene (SPP) [14], sulfonated polyphenylene sulfone (SPPS) [15],sulfonated polysulfone (SPSU) [16–21], sulfonated polyetheretherketone (SPEEK) [22–33], and sulfonatedpolyphenylsulfone (SPPSU) [34–49], are being actively investigated. The rigid molecular chain structuresof aromatic polymers provide high thermal stabilities. However, their conductivities are generally lowerthan those of PFSA also because the structures can make it difficult to form proton conducting channels.However, increasing the ion exchange capacity (IEC) of the aromatic polymer to increase the conductivityresults in weaker mechanical and chemical properties of the polymer. In other words, there is a trade-offrelationship between the increase in conductivity and the mechanical and chemical properties. Thus,further development of hydrocarbon polymers without fluorinated moieties is necessary.Polyphenylsulfone (PPSU), which has an excellent thermal stability, high chemical resistanceand low cost, has been studied widely. The sulfonation of PPSU to balance the IEC values andmembrane stability is a crucial focus. High IEC values are desirable for good proton conductivities butresult in excess swelling, which causes mechanical and chemical instabilities. These mechanical andchemical properties are big obstacles for applying SPPSUs in PEMFCs. We are developing a crosslinkedsulfonated polyphenylsulfone (CSPPSU) membrane having good thermal and chemical propertieswith good proton conductivities [34,36,37]. Organic solvent-free CSPPSU membranes have beenreported [34,37]. Moreover, an SPPSU with a high sulfonation degree from bis(4-fluorophenyl)sulfone(FPS) monomer has been used to prepare a CSPPSU membrane [36]. The membranes have high protonconductivities (>0.1 S/cm at 90% RH, 80 ◦C) [34,36,37]. However, the cell performances are not stableover a long period. We thought that a byproduct such as sulfur dioxide (SO2) produced during thecrosslinking process remaining in the membrane was the cause. In this paper, we report an activationtreatment for the CSPPSU membrane to achieve good thermal, chemical, and long-term stabilities.The nanostructures, conductivities, cell performances, and thermal, mechanical and long-term stabilitiesof the CSPPSU membranes after our activation treatment were investigated for the first time andcompared with those of Nafion212 membranes.2. Experimental2.1. MaterialsPPSU (Solvay Radel R-5000 NT) (Mn = 26,000; Mw = 50,000; Mw/Mn = 1.9) was provided by SolvaySpecialty Polymers Japan K.K. (glass transition temperature (Tg) = 220 ◦C). A DuPontTM Nafion® PFSAmembrane (NR-212) was purchased from DuPont (USA). The chemical compounds were purchasedfrom commercially available sources and were used as received. Hydrogen peroxide (H2O2), sodiumchloride (NaCl), sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were purchased from NacalaiTesque, Inc, Japan. Iron (II) chloride tetrahydrate (FeCl2·4H2O) was purchased from Wako Pure ChemicalIndustries, Ltd., Japan. A dialysis tubing cellulose membrane (molecular weight cut-off; MWCO = 14,000)and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich Co., Ltd. Deionized (DI) H2O wasobtained using a PURELAB® Option-R 7 ELGA LabWater at 15 Mohm cm and 25 ◦C.2.2. Synthesis and Characterization of SPPSUSPPSU was synthesized using an electrophilic aromatic substitution reaction of PPSU withH2SO4. The synthesis and properties of SPPSU have been described in detail in a previous paper [34].The SPPSU polymers had IEC values of 3.68 meq/g, and the degree of sulfonation (D.S.) values were2.3, which are close to the theoretical IEC value of 3.57 for D.S. = 2.0. The yield was 76%.2.3. Preparation of Chemically Crosslinked SPPSU (CSPPSU) MembranesSPPSU (5 g) was dissolved in DMSO 20 mL with stirring at 60 ◦C to prepare a 20 wt% solution.The solution was casted at a speed of 4.0 mm/min with an applicator (blade) (Tester Sangyo CO.,Membranes 2020, 10, 31 3 of 14Ltd., Japan; 15 cm, gap = 0.5 mm) on a glass plate (27 × 30 cm2) heated at 80 ◦C by an automaticfilm-coating apparatus (KIPAE, KP-3000VH), and then dried for 24 h at 80 ◦C. Next, the film coated onthe glass plate was moved to an oven and annealed in air at 120 ◦C (24 h), 160 ◦C (24 h), and 180 ◦C(24 h). After that, the membrane was peeled off the glass plate with water and activated to removethe remaining sulfur dioxide (SO2), which is a byproduct from the annealing process, and to cure thenanostructure of the CSPPSU membrane. Activation was performed using the following procedure:heating in 0.5 M NaOH at 80 ◦C overnight, boiling in DI H2O for 2 h, heating at 1M H2SO4 at 80 ◦C for2 h, and boiling in DI H2O for 2 h. Finally, the CSPPSU membranes were dried at room temperaturebefore using. The CSPPSU membranes were very flexible and light brown.2.4. Pretreatment of Nafion212 MembranesNafion212 membranes were prepared for a comparison with the CSPPSU membranes. Beforeusing, the Nafion212 membranes were pretreated as follows: they were boiled in DI H2O for 2 h,heated in 1M H2O2 at 80 ◦C for 2 h, followed by heating in 1M H2SO4 at 80 ◦C for 2 h, and then boiledin DI H2O for 2 h. The membranes were kept in DI H2O until use.2.5. IEC, D.S., Water-Uptake (W.U.), λ, and Crosslink Rates (Dcrosslink)The ion exchange capacity (IEC) was defined as milliequivalents of sulfonic groups per gram ofdried sample. A piece of membrane was soaked in 20 mL of a 2 M NaCl solution and equilibratedfor more than 24 h to replace the protons with sodium ions. The solution was then titrated with a0.01 M NaOH solution. The IEC was calculated using the following equation: IEC (meq/g) = cv/Wdry,where c (mmol/L) is the concentration of the standardized NaOH aqueous solution used for titration(0.01mol/L), v (L) is the volume of the standardized NaOH aqueous solution used for titration, andWdry (g) is the mass of the dry membrane. The degree of sulfonation (D.S.) of the membranes wascalculated using the following equation: D.S. (Sulfonic acid group/repeating unit; R.U.) = [IEC/1000 ×Fw (R.U.)]/[1 − (IEC/1000 × Fw (SO3))], where Fw (R.U.) = 400.45 and Fw (SO3) = 80.06.The water-uptake (W.U.) of the membranes at room temperature was calculated using thefollowing: W.U. (%) = [(Wwet −Wdry)/Wdry] × 100, where Wwet is the mass of the wet membrane.The membranes were cut into 10 mm × 10 mm squares and dried for >24 h at 80 ◦C in a dry oven.The membranes were immersed in boiling DI H2O for >1 h before the measurements.The hydration number (λ) for the membranes was calculated using the following: λ ([H2O]/[SO3H])= [1000(Wwet −Wdry)]/18 WdryIEC.The degree of crosslinking (crosslink rate, Dcrosslink) in the membranes was calculated using thefollowing: Dcrosslink (%) = [(IECbefore annealing − IECafter annealing)/IECbefore annealing] × 100.2.6. Oxidative Stability (Fenton’s Test)The oxidative stabilities of the membranes were evaluated by immersing a small piece of sampleinto Fenton’s reagent [3 wt% H2O2 and 2 ppm Fe(II) (added as FeCl2·4H2O)] at 80 ◦C for 1 h whilestirring. The samples were dried at 60 ◦C in a vacuum oven before the measurements. The membraneswere washed with DI H2O repeatedly and dried in a vacuum oven at 60 ◦C overnight after thereaction. The oxidative stabilities were determined as follows: 100[(mass of residual membrane afterthe test)/(initial mass of membrane)].2.7. Thermal BehaviorThe thermal and mass properties of the membranes were investigated using thermogravimetricand mass analysis with a Discovery TGA/MS (TA instruments). The samples were heated from roomtemperature (RT) to 600 ◦C at 20 ◦C/min in an He atmosphere.The dynamic elastic modulus and dissipation (tan δ) of the membranes were measured by DMAQ800 (TA Instruments) at 1 Hz in the temperature range of 20–200 ◦C at a heating rate of 5 ◦C/min.Membranes 2020, 10, 31 4 of 142.8. Mechanical BehaviorThe stress–strain tests on the membranes were performed using a Tension Test Machine (Shimazu,EZ-S) at room temperature with a constant crosshead speed of 5 mm/min. The samples were cutusing the super dumbbell cutter SDMP-1000 (Dumbbell Co., Japan). The membrane thicknesses weremeasured using a Mitutoyo 547-401 ABSOLUTE Digimatic Thickness Gauge, Japan.2.9. Membrane NanostructureSmall angle X-ray scattering (SAXS) measurements were performed in the beamline BL15A2 of thePhoton Factory in KEK (Tsukuba, Japan). The X-ray beam was monochromatized to 1.2 Å with a curvedmonochromator (triangular Ge (111) crystal with the asymmetric angle of 8.0 degrees). The membranesamples were put on the stage of a rotational autochanger, and the measurements were performedon the membranes under wet conditions, for which a small amount of water was added dropwise.Scattered photons were detected with a two-dimensional semiconductor detector (PILATUS3 1M,981 × 1043 pixels, pixel size: 0.172 mm × 0.172 mm, Dectris, Switzerland) with a camera distance of170 cm, and the signals were accumulated for 30 s. The image was processed by using the SAnglerprogram. The image was calibrated with the diffraction image of silver behenate and then averagedover azimuthal angle to obtain a one-dimensional profile with the subtraction of SAXS profile of air.2.10. Conductivity MeasurementsThe proton conductivities of the membranes were determined using four-point probe impedancespectroscopy. Through-plane proton conductivities of the membranes using the MTS cell head weremeasured as a function of the relative humidity (RH, 20–90%) in the temperature range of 40–120 ◦C,using a 740 membrane test system (MTS, Scribner Associates, Inc.) with a phase sensitive multimeter(model PSM1735, Newtons4th Ltd.) combined with an impedance analysis interface. A frequency rangeof 1 Hz–1 MHz and a peak-to-peak voltage of 10 mV were used during the impedance measurements.The samples were equilibrated in a temperature and humidity chamber at specified temperaturesand relative humidities (RHs) for 30 min before the measurements. The chamber was pressurized at130 kPa for the measurements in the temperature range of 100–120 ◦C.2.11. Membrane, Gas Diffusion Layer, and Catalyst Electrode Information for MEAThe thickness of the membranes was about 50 µm. A Pt/C/ionomer (ionomer/carbon = 1) catalystelectrode containing 0.3 mg/cm2 of Pt on a GDL electrode (Sigaracet® GDL 25BC of SGL Group Co.Ltd., Japan) was purchased from EIWA Corporation.2.12. Preparation of Membrane Electrode Assembly (MEA)The effective electrode area of the single cell was 4 cm2. A hot press machine (MODEL A-010D,FC-R&D Company, Sagamihara, Japan) was used. The MEA was prepared on Cu plates. Polyimidefilms were used between the Cu plates to facilitate peeling of the MEA. The MEA was obtained byinserting a membrane between the anode and cathode and hot-pressing at 130 ◦C at ~9.8 kN for 20 min.2.13. Single Fuel-Cell Performance and DurabilityA single cell (active area: 20 × 20 mm2) was purchased from Ulimeng Eng Co., Ltd., Korea.The fuel cell system was evaluated using AutoPEM of Toyo Corporation, Japan. The current-voltage(I-V) performance was measured in relation to the amount of hydrogen and oxygen at the anodeand cathode, respectively, at 80 ◦C, 100% and 60% RH, and ambient pressure. The gas utilizations atthe anode and cathode were 56% and 14%, respectively. The I-V characteristics of a single cell wereobtained by activating at 1 A for 20 h, increasing the current from 0.02 to 6 A, while maintaininga constant current value for 5 min, and sweeping 5 times. Next, the cell was purged for 10 h withnitrogen gas, and cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performedMembranes 2020, 10, 31 5 of 14in the potential range of 0.02–0.9 V at a scan rate of 50 mV/s and 0.02–0.5 V at 2 mV/s, respectively.During the measurements, 100 mL/min humidified nitrogen and 100 mL/min hydrogen were fed tothe working electrode and counter electrode, respectively. The effective catalytic area (ECA) fromthe results (oxidation of adsorbed H) of the CV was calculated using the following equation: ECA([cm2/g] = (charge density, µC/cm2)/[210 (µC/cm2) × electrode loading, (g/cm2)]. A long-term testusing a constant current method was performed at 80 ◦C and 100% RH using a PEMTest8900 fromToyo Corporation.3. Results and Discussion3.1. Activation of the CSPPSU MembranesFigure 1 shows an image (16 × 18 cm2) (Figure 1a) and the chemical structure (Figure 1b) of thechemically crosslinked SPPSU (CSPPSU) membranes. CSPPSU membranes were prepared by usinga heat treatment method as previously reported [34,36,37]. Crosslinking between the sulfo groups(–SO3H) by heat treatment can produce undesirable byproducts, such as sulfur dioxide (SO2), whichdo not completely evaporate from the membrane. The remaining SO2 was found to decrease theoriginal membrane characteristics obtained from the MEA process, causing bad fuel cell performances.We removed SO2 by activating the membranes with an alkaline solution. In addition, it was foundthat the mechanical and chemical properties and the durabilities of the membranes were dramaticallystabilized after our activation process.Membranes 2020, 10, x FOR PEER REVIEW 5 of 15  anode and cathode were 56% and 14%, respectively. The I-V characteristics of a single cell were obtained by activating at 1 A for 20 h, increasing the current from 0.02 to 6 A, while maintaining a constant current value for 5 min, and sweeping 5 times. Next, the cell was purged for 10 h with nitrogen gas, and cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed in the potential range of 0.02–0.9 V at a scan rate of 50 mV/s and 0.02–0.5 V at 2 mV/s, respectively. During the measurements, 100 mL/min humidified nitrogen and 100 mL/min hydrogen were fed to the working electrode and counter electrode, respectively. The effective catalytic area (ECA) from the results (oxidation of adsorbed H) of the CV was calculated using the following equation: ECA ([cm2/g] = (charge density, µC/cm2)/[210 (µC/cm2) × electrode loading, (g/cm2)]. A long-term test using a constant current method was performed at 80 °C and 100% RH using a PEMTest8900 from Toyo Corporation. 3. Results and Discussion 3.1. Activation of the CSPPSU Membranes Figure 1 shows an image (16 × 18 cm2) (Figure 1a) and the chemical structure (Figure 1b) of the chemically crosslinked SPPSU (CSPPSU) membranes. CSPPSU membranes were prepared by using a heat treatment method as previously reported [34,36,37]. Crosslinking between the sulfo groups (–SO3H) by heat treatment can produce undesirable byproducts, such as sulfur dioxide (SO2), which do not completely evaporate from the membrane. The remaining SO2 was found to decrease the original membrane characteristics obtained from the MEA process, causing bad fuel cell performances. We removed SO2 by activating the membranes with an alkaline solution. In addition, it was found that the mechanical and chemical properties and the durabilities of the membranes were dramatically stabilized after our activation process.  Figure 1. (a) Image of a crosslinked sulfonated polyphenylsulfone (CSPPSU) membrane on a glass plate and (b) chemical structure of a CSPPSU. Figure 2 shows the thermal stabilities of synthesized SPPSU, CSPPSU after heat treatment at 180 °C, and CSPPSU membranes activated without and with an NaOH solution. The thermal stabilities of the CSPPSU membranes after heat treatment at 180 °C were better than those of SPPSU. There was no significant difference in the thermal stabilities of the CSPPSU membranes after the heat treatment at 180 °C and the CSPPSU membrane activated without NaOH solution. On the other hand, the stability of the CSPPSU membranes was much higher after activation treatment with NaOH. Mass analysis indicated that the stability of the CSPPSU membrane was due to the removal of the SO2 Figure 1. (a) Image of a crosslinked sulfonated polyphenylsulfone (CSPPSU) membrane on a glassplate and (b) chemical structure of a CSPPSU.Figure 2 shows the thermal stabilities of synthesized SPPSU, CSPPSU after heat treatment at180 ◦C, and CSPPSU membranes activated without and with an NaOH solution. The thermal stabilitiesof the CSPPSU membranes after heat treatment at 180 ◦C were better than those of SPPSU. There wasno significant difference in the thermal stabilities of the CSPPSU membranes after the heat treatment at180 ◦C and the CSPPSU membrane activated without NaOH solution. On the other hand, the stabilityof the CSPPSU membranes was much higher after activation treatment with NaOH. Mass analysisindicated that the stability of the CSPPSU membrane was due to the removal of the SO2 trapped in themembrane. From these results, it is suggested that activation with an alkaline solution is necessarywhen crosslinked membranes are obtained using DMSO as a solvent.Membranes 2020, 10, 31 6 of 14Membranes 2020, 10, x FOR PEER REVIEW 6 of 15  trapped in the membrane. From these results, it is suggested that activation with an alkaline solution is necessary when crosslinked membranes are obtained using DMSO as a solvent.  Figure 2. Thermal stabilities in a helium gas flow at a heating rate of 20 °C/min of (a) synthesized sulfonated polyphenylsulfone (SPPSU), (b) CSPPSU after the heat treatment at 180 °C, and CSPPSU membranes activated (c) without and (d) with NaOH solution process. 3.2. Mechanical Properties of the CSPPSU Membranes Figure 3 shows the dynamic elastic modulus and Tan δ properties of CSPPSU and Nafion212 membranes with respect to temperature. The dynamic elastic modulus of CSPPSU membranes was constant up to 200 °C, whereas that of the Nafion212 membrane decreased due to changes in the molecular structure with an increase in the temperature. From Tan δ, the glass transition temperature of the Nafion212 membrane was around 100 °C, and that of the CSPPSU membrane was >200 °C. This result suggests that the CSPPSU membrane is suitable for application as a high-temperature electrolyte membrane over 100 °C.  Figure 3. Dynamic elastic modulus and Tan δ of (a) Nafion212 and (b) CSPPSU membranes at 1 Hz. Figure 2. Thermal stabilities in a helium gas flow at a heating rate of 20 ◦C/min of (a) synthesizedsulfonated polyphenylsulfone (SPPSU), (b) CSPPSU after the heat treatment at 180 ◦C, and CSPPSUmembranes activated (c) without and (d) with NaOH solution process.3.2. Mechanical Properties of the CSPPSU MembranesFigure 3 shows the dynamic elastic modulus and Tan δ properties of CSPPSU and Nafion212membranes with respect to temperature. The dynamic elastic modulus of CSPPSU membranes wasconstant up to 200 ◦C, whereas that of the Nafion212 membrane decreased due to changes in themolecular structure with an increase in the temperature. From Tan δ, the glass transition temperatureof the Nafion212 membrane was around 100 ◦C, and that of the CSPPSU membrane was >200 ◦C. Thisresult suggests that the CSPPSU membrane is suitable for application as a high-temperature electrolytemembrane over 100 ◦C.Membranes 2020, 10, x FOR PEER REVIEW 6 of 15  trapped in the membrane. From these results, it is suggested that activation with an alkaline solution is necessary when crosslinked membranes are obtained using DMSO as a solvent.  Figure 2. Thermal stabilities in a helium gas flow at a heating rate of 20 °C/min of (a) synthesized sulfonated polyphenylsulfone (SPPSU), (b) CSPPSU after the heat treatment at 180 °C, and CSPPSU membranes activated (c) without and (d) with NaOH solution process. 3.2. Mechanical Properties of the CSPPSU Membranes Figure 3 shows the dynamic elastic modulus and Tan δ properties of CSPPSU and Nafion212 membranes with respect to temperature. The dynamic elastic modulus of CSPPSU membranes was constant up to 200 °C, whereas that of the Nafion212 membrane decreased due to changes in the molecular structure with an increase in the temperature. From Tan δ, the glass transition temperature of the Nafion212 membrane was around 100 °C, and that of the CSPPSU membrane was >200 °C. This result suggests that the CSPPSU membrane is suitable for application as a high-temperature electrolyte membrane over 100 °C.  Figure 3. Dynamic elastic modulus and Tan δ of (a) Nafion212 and (b) CSPPSU membranes at 1 Hz. Figure 3. Dynamic elastic modulus and Tan δ of (a) Nafion212 and (b) CSPPSU membranes at 1 Hz.Figure 4 shows the tensile strengths and elongation results for the CSPPSU and Nafion212membranes at room temperature. The Nafion212 membrane had a low tensile strength but veryhigh tensile elongation. The tensile strength of the CSPPSU membranes was higher than that ofMembranes 2020, 10, 31 7 of 14the Nafion212, but the tensile elongation was lower than that of the Nafion212. On the other hand,the tensile strength and elongation of the CSPPSU membranes were similar or better than thoseof other hydrocarbon-based polymer electrolyte membranes [26,28,34,37,50]. However, an increasein the tensile elongation (high flexibility) is required to obtain good contact, which can reduce thecontact resistance and proton transfer in the membrane at the interface between the membrane and theelectrode. The mechanical and thermal properties of PPSU, CSPPSU and Nafion212 membranes aresummarized in Table 1.Membranes 2020, 10, x FOR PEER REVIEW 7 of 15  Figure 4 shows the tensile strengths and elongation results for the CSPPSU and Nafion212 membranes at room temperature. The Nafion212 membrane had a low tensile strength but very high tensile elongation. The tensile strength of the CSPPSU membranes was higher than that of the Nafion212, but the tensile elongation was lower than that of the Nafion212. On the other hand, the tensile strength and elongation of the CSPPSU membranes were similar or better than those of other hydrocarbon-based polymer electrolyte membranes [26,28,34,37,50]. However, an increase in the tensile elongation (high flexibility) is required to obtain good contact, which can reduce the contact resistance and proton transfer in the membrane at the interface between the membrane and the electrode. The mechanical and thermal properties of PPSU, CSPPSU and Nafion212 membranes are summarized in Table 1.  Figure 4. Stress–strain properties of (a) Nafion212 (t = 0.050 mm) and (b) CSPPSU (t = 0.066 mm) membranes at room temperature. Table 1. Mechanical and thermal properties of PPSU, CSPPSU and Nafion212 membranes. Polymer Membrane PPSU (Solvay) * CSPPSU Nafion212 Tensile modulus (MPa) 2340  2000  ~500  Tensile strength (MPa) 69.6 48 16 at 150% Tensile elongation (%) (break) 60–120 74 >150 Flexural modulus (MPa) 2410  757  155  Glass transition (°C) 220  >200  ~100  Decomposition (°C) 540  ~300  ~300  * From Solvay PPSU R-5000_Datasheet. 3.3. Nanostructures of the CSPPSU Membranes The nanostructures of wet CSPPSU membranes were characterized by using small angle X-ray scattering (SAXS) (Figure 5) [4]. An intensity maximum was observed at q = 1.6 nm–1 (q = 4πsinθ/λ). The average spacing, L, of the scattering objects was calculated from the peak position by using the Bragg equation (L = 2π/θ) to be 3.9 nm. This value was lower than the L value for Nafion (5 nm) [4,51,52]. In addition, the peak intensity of the CSPPSU membrane was lower than that of Nafion. In general, in a Nafion membrane, there is good separation between hydrophilic and hydrophobic phases and it has a good ion conduction path in a humidified state. On the other hand, it is known that hydrocarbon-based membranes, such as SPEEK, have a longer distance between sulfone groups and less continuity of the conduction paths than Nafion membranes do [4]. From these results, we believe that the CSPPSU membranes have good conduction paths and that the IEC is high (Table 2). Figure 4. Stress–strain properties of (a) Nafion212 (t = 0.050 mm) and (b) CSPPSU (t = 0.066 mm)membranes at room temperature.Table 1. Mechanical and thermal properties of PPSU, CSPPSU and Nafion212 membranes.Polymer Membrane PPSU (Solvay) * CSPPSU Nafion212Tensile modulus (MPa) 2340 2000 ~500Tensile strength (MPa) 69.6 48 16 at 150%Tensile elongation (%) (break) 60–120 74 >150Flexural modulus (MPa) 2410 757 155Glass transition (◦C) 220 >200 ~100Decomposition (◦C) 540 ~300 ~300* From Solvay PPSU R-5000_Datasheet.3.3. Nanostructures of the CSPPSU MembranesThe nanostructures of wet CSPPSU membranes were characterized by using small angle X-rayscattering (SAXS) (Figure 5) [4]. An intensity maximum was observed at q = 1.6 nm−1 (q = 4πsinθ/λ).The average spacing, L, of the scattering objects was calculated from the peak position by using theBragg equation (L = 2π/θ) to be 3.9 nm. This value was lower than the L value for Nafion (5 nm) [4,51,52].In addition, the peak intensity of the CSPPSU membrane was lower than that of Nafion. In general, in aNafion membrane, there is good separation between hydrophilic and hydrophobic phases and it has agood ion conduction path in a humidified state. On the other hand, it is known that hydrocarbon-basedmembranes, such as SPEEK, have a longer distance between sulfone groups and less continuity ofthe conduction paths than Nafion membranes do [4]. From these results, we believe that the CSPPSUmembranes have good conduction paths and that the IEC is high (Table 2). However, the volumedensity of the ion clusters in the nanostructures is low, and the SAXS peak intensity is low.Membranes 2020, 10, 31 8 of 14Membranes 2020, 10, x FOR PEER REVIEW 8 of 15  However, the volume density of the ion clusters in the nanostructures is low, and the SAXS peak intensity is low.  Figure 5. Small angle X-ray scattering (SAXS) profile of a wet-CSPPSU membrane at room temperature. 3.4. Proton Conductivities of the CSPPSU Membranes Since SPPSU was crosslinked by using heat treatment, it was difficult to precisely control the crosslinking in the nanostructure. However, using this method, stable and macroscopically reproducible CSPPSU membranes were obtained. Figure 6 shows the proton conductivities of CSPPSU membranes obtained from the average value of the conductivities of four different membranes in relation to the temperature and relative humidity. These conductivities were lower than those obtained in our previous studies [34,36,37] due to different synthetic and activation conditions. The conductivities of the CSPPSU membranes were lower than those of the Nafion212 membranes [5,37,52] and were lower by one order of magnitude under low relative humidification conditions (Table 2). The reasons for the difference include the following: the ion cluster size of the CSPPSU membrane is small, the distance between the sulfone groups due to the backbone structure of PPSU is long, and the number of continuous conduction paths is small, as described previously. The design of CSPPSU membranes with high conductivities and high mechanical stabilities is required.  Figure 6. Proton conductivities of the CSPPSU membranes vs. the relative humidity at the specified temperatures. Figure 5. Small angle X-ray scattering (SAXS) profile of a wet-CSPPSU membrane at room temperature.3.4. Proton Conductivities of the CSPPSU MembranesSince SPPSU was crosslinked by using heat treatment, it was difficult to precisely control thecrosslinking in the nanostructure. However, using this method, stable and macroscopically reproducibleCSPPSU membranes were obtained. Figure 6 shows the proton conductivities of CSPPSU membranesobtained from the average value of the conductivities of four different membranes in relation tothe temperature and relative humidity. These conductivities were lower than those obtained in ourprevious studies [34,36,37] due to different synthetic and activation conditions. The conductivitiesof the CSPPSU membranes were lower than those of the Nafion212 membranes [5,37,52] and werelower by one order of magnitude under low relative humidification conditions (Table 2). The reasonsfor the difference include the following: the ion cluster size of the CSPPSU membrane is small, thedistance between the sulfone groups due to the backbone structure of PPSU is long, and the number ofcontinuous conduction paths is small, as described previously. The design of CSPPSU membraneswith high conductivities and high mechanical stabilities is required.Membranes 2020, 10, x FOR PEER REVIEW 8 of 15  However, the volume density of the ion clusters in the nanostructures is low, and the SAXS peak intensity is low.  Figure 5. Small angle X-ray scattering (SAXS) profile of a wet-CSPPSU membrane at room temperature. 3.4. Proton Conductivities of the CSPPSU Membranes Since SPPSU was crosslinked by using heat treatment, it was difficult to precisely control the crosslinking in the nanostructure. However, using this method, stable and macroscopically reproducible CSPPSU membranes were obtained. Figure 6 shows the proton conductivities of CSPPSU membranes obtained from the average value of the conductivities of four different membranes in relation to the temperature and relative humidity. These conductivities were lower than those obtained in our previous studies [34,36,37] due to different synthetic and activation conditions. The conductivities of the CSPPSU membranes were lower than those of the Nafion212 membranes [5,37,52] and were lower by one order of magnitude under low relative humidification conditions (Table 2). The reasons for the difference include the following: the ion cluster size of the CSPPSU membrane is small, the distance between the sulfone groups due to the backbone structure of PPSU is long, and the number of continuous conduction paths is small, as described previously. The design of CSPPSU membranes with high conductivities and high mechanical stabilities is required.  Figure 6. Proton conductivities of the CSPPSU membranes vs. the relative humidity at the specified temperatures. Figure 6. Proton conductivities of the CSPPSU membranes vs. the relative humidity at the specified temperatures.Membranes 2020, 10, 31 9 of 14Table 2. Physicochemical and conductivity properties of CSPPSU and Nafion212 membranes.Membrane CSPPSU Nafion212IEC (meq/g) ~2 0.9W.U. (%) 43 50λ 12 31Dcrosslink (%) 47.3 -Roxidation (%) 91.3–99.4 100Cluster size (nm) 3.9 5.0Conductivity(mS/cm)80 ◦C20% RH 0.7 7.390% RH 18 80120 ◦C20% RH 0.6 6.690% RH 22 1053.5. IEC, W.U., λ, and Dcrosslink Values of CSPPSU MembranesIn Table 2, the characteristics of the CSPPSU membranes are summarized in comparison withNafion212 membranes. The IEC of the CSPPSU membranes was higher than that of Nafion212, butW.U. and λ were lower than those of Nafion212. This is significant for hydrocarbon-based electrolytemembranes because high IECs tend to cause swelling of the membrane in water and thus deterioratethe stability of the membrane. The CSPPSU membranes had a crosslink rate (Dcrosslink) of 47.3% andhigh mechanical stabilities with a high IEC and reasonable W.U. values. These results indicate that thecrosslinking method is effective for increasing the mechanical and chemical stabilities of hydrocarbonelectrolyte membranes.3.6. Oxidative Stability of the CSPPSU MembranesThe degradation phenomena of the electrolyte membrane in a fuel cell are thought to be causedby the following [47]. One is degradation on the anode side due to H radicals and O2 diffused fromthe cathode side through the electrolyte membrane. The other is degradation on the cathode side,where HO or HO2 radicals from O2 reduction diffused on the cathode side. However, the HO and HO2radicals on the cathode side are the main causes of electrolyte membrane degradation. The oxidativestabilities of the CSPPSU membranes were evaluated by using an ex situ method wherein they weretreated with Fenton’s reagent (3 wt% H2O2 + 2 ppm Fe (II)) at 80 ◦C for 1 h. The CSPPSU membraneshad stabilities in the range of 91.3–99.4% (Table 2). However, the membranes broke when rubbed byhand after the Fenton tests. A method for improving the oxidative stabilities while improving theproton conductivities of the CSPPSU membrane is currently under study and will be reported later.3.7. Fuel Cell Properties Using CSPPSU MembranesEvaluation of a fuel cell is complicated because the membrane, catalyst electrodes containingan ionomer, the interfaces between a membrane and an electrode and between the electrode and theseparator, and gas supply affect the power generation reaction. However, the current-voltage (I-V),cyclic voltammetry (CV), and hydrogen gas crossover characteristics of a single cell were evaluatedunder similar conditions but with different electrolyte membranes. Figures 7 and 8 show the I-ViR freeand CV characteristics of the CSPPSU membranes and the Nafion212 membrane evaluated at a celltemperature of 80 ◦C and 100% and 60% RH, respectively. The voltage differences using the CSPPSUand Nafion212 membranes were 37 mV at 1 A/cm2 and 100% RH and 33 mV at 0.5 A/cm2 and 60% RH.On the other hand, the cell resistances using the CSPPSU and Nafion212 membranes were 73 mohmand 24 mohm at 100% RH and 161 mohm and 42 mohm at 60% RH, respectively (Table 3). The I-ViR freeresults show that the difference in the conductivities of the CSPPSU and Nafion212 membranes waslarge (Table 2), whereas the difference in the fuel cell was not so large. However, on the basis of theresistance value of the entire cell, the difference between the CSPPSU and the Nafion212 membraneswas large, especially when RH was 60%. This means that the interface resistance between the CSPPSUmembrane and the catalyst electrodes was large and that the resistance due to phase separationMembranes 2020, 10, 31 10 of 14between the hydrocarbon-based CSPPSU membrane and the fluorine-based Nafion ionomer was large.Moreover, the CV characteristics show that the catalytic activity on the anode side and the cathode sideof the cell containing the CSPPSU membrane was lower than that of the cell containing the Nafion212membrane (Table 3). It is possible that the catalyst has been poisoned by the CSPPSU membrane.Membranes 2020, 10, x FOR PEER REVIEW 10 of 15  resistance between the CSPPSU membrane and the catalyst electrodes was large and that the resistance due to phase separation between the hydrocarbon-based CSPPSU membrane and the fluorine-based Nafion ionomer was large. Moreover, the CV characteristics show that the catalytic activity on the anode side and the cathode side of the cell containing the CSPPSU membrane was lower than that of the cell containing the Nafion212 membrane (Table 3). It is possible that the catalyst has been poisoned by the CSPPSU membrane.  Figure 7. (a) I-ViR free and (b) CV properties of CSPPSU and Nafion212 membranes at 80 °C and 100% RH.  Figure 8. (a) I-ViR free and (b) CV properties of CSPPSU and Nafion212 membranes at 80 °C and 60% RH. Table 3. I-V and H2 crossover data for single cells using CSPPSU and Nafion212 membranes. Membrane 80 °C, 100% RH 80 °C, 60% RH iR Loss (mohm) at 1 A/cm2 ECA (m2/g) H2 Crossover (mA/cm2) at 0.4 V iR Loss  (mohm) at 1 A/cm2 ECA (m2/g) CSPPSU 73  82 0.085 161 73 Nafion212 24 99 1.24 42 92   Figure 7. (a) I-ViR free and (b) CV properties of CSPPSU and Nafion212 membranes at 80 ◦C and100% RH.Membranes 2020, 10, x FOR PEER REVIEW 10 of 15  resistance between the CSPPSU membrane and the catalyst electrodes was large and that the resistance due to phase separation between the hydrocarbon-based CSPPSU membrane and the fluorine-based Nafion ionomer was large. Moreover, the CV characteristics show that the catalytic activity on the anode side and the cathode side of the cell containing the CSPPSU membrane was lower than that of the cell containing the Nafion212 membrane (Table 3). It is possible that the catalyst has been poisoned by the CSPPSU membrane.  Figure 7. (a) I-ViR free and (b) CV properties of CSPPSU and Nafion212 membranes at 80 °C and 100% RH.  Figure 8. (a) I-ViR free and (b) CV properties of CSPPSU and Nafion212 membranes at 80 °C and 60% RH. Table 3. I-V and H2 crossover data for single cells using CSPPSU and Nafion212 membranes. Membrane 80 °C, 100% RH 80 °C, 60% RH iR Loss (mohm) at 1 A/cm2 ECA (m2/g) H2 Crossover (mA/cm2) at 0.4 V iR Loss  (mohm) at 1 A/cm2 ECA (m2/g) CSPPSU 73  82 0.085 161 73 Nafion212 24 99 1.24 42 92   Figure 8. (a) I-ViR free and (b) CV properties of CSPPSU and Nafion212 membranes at 80 ◦C and60% RH.Table 3. I-V and H2 crossover data for single cells using CSPPSU and Nafion212 membranes.Membrane80 ◦C, 100% RH 80 ◦C, 60% RHiR Loss(mohm)at 1 A/cm2ECA(m2/g)H2 Crossover(mA/cm2)at 0.4 ViR Loss(mohm)at 1 A/cm2ECA(m2/g)CSPPSU 73 82 0.085 161 73Nafion212 24 99 1.24 42 923.8. H2 Crossover of the CSPPSU MembranesFigure 9 shows hydrogen gas crossover characteristics for MEA using the CSPPSU and Nafion212membranes obtained using LSV [53]. The current densities of the MEA using the CSPPSU andthe Nafion212 membranes at 0.4 V were 0.08 and 1.24 mA/cm2, respectively (Table 3). From theliterature [53], gas crossover occurs when the current density is >2 mA/cm2, and short circuit occurswhen the current density is >5.5 mA/cm2. The current density of the Nafion212 membrane was ingood agreement with literature values [53]. On the other hand, the current density of the CSPPSUmembranes is 15 times smaller than that of the Nafion212 membrane. The current density of theCSPPSU membranes was lower than those using other hydrocarbon membranes [40], indicating thatthe CSPPSU membrane has high durability against gas crossover.Membranes 2020, 10, 31 11 of 14Membranes 2020, 10, x FOR PEER REVIEW 11 of 15  3.8. H2 Crossover of the CSPPSU Membranes Figure 9 shows hydrogen gas crossover characteristics for MEA using the CSPPSU and Nafion212 membranes obtained using LSV [53]. The current densities of the MEA using the CSPPSU and the Nafion212 membranes at 0.4 V were 0.08 and 1.24 mA/cm2, respectively (Table 3). From the literature [53], gas crossover occurs when the current density is >2 mA/cm2, and short circuit occurs when the current density is >5.5 mA/cm2. The current density of the Nafion212 membrane was in good agreement with literature values [53]. On the other hand, the current density of the CSPPSU membranes is 15 times smaller than that of the Nafion212 membrane. The current density of the CSPPSU membranes was lower than those using other hydrocarbon membranes [40], indicating that the CSPPSU membrane has high durability against gas crossover.  Figure 9. Hydrogen crossover properties of CSPPSU and Nafion212 membranes at 80 °C and 100% RH. 3.9. Durability Using CSPPSU Membranes Figure 10 shows a plot of the voltage vs. time for MEAs using CSPPSU and Nafion212 membranes with an applied current of 1 A at a cell temperature of 80 °C and 100% RH. The MEA cell using the Nafion212 membrane was very stable during the measurement. On the other hand, the MEA cell using the CSPPSU membrane was a little unstable in the initial stage but became stable by 4000 h. However, the voltage gradually decreased after 4000 h. The single cell was separated, and the MEA was inspected. There were no cracks or pinholes in the membrane where no electrode was attached, and the appearance was the same as that before the test. However, the portion between the membrane and the electrode could not be observed because the membrane and the electrode were firmly attached to each other. However, there is a possibility that the voltage may have decreased due to mechanical deterioration, such as a crack or pinhole, between the membrane and the electrode. Moreover, it is conceivable that HO or HO2 radicals formed during the reduction of oxygen on the cathode side may chemically degrade the CSPPSU membrane. There are few reports on the long-term stabilities of fuel cells using hydrocarbon-based electrolyte membranes, and we believe this is the first time that the long-term stability of a cell using a CSPPSU membrane has been evaluated for 4000 h. We are currently conducting more detailed evaluations of MEA cells and developing CSPPSU membranes with long-term stabilities like Nafion membranes. Figure 9. Hydrogen crossover properties of CSPPSU and Nafion212 membranes at 80 ◦C and 100% RH.3.9. Durability Using CSPPSU MembranesFigure 10 shows a plot of the voltage vs. time for MEAs using CSPPSU and Nafion212 membraneswith an applied current of 1 A at a cell temperature of 80 ◦C and 100% RH. The MEA cell using theNafion212 membrane was very stable during the measurement. On the other hand, the MEA cellusing the CSPPSU membrane was a little unstable in the initial stage but became stable by 4000 h.However, the voltage gradually decreased after 4000 h. The single cell was separated, and the MEAwas inspected. There were no cracks or pinholes in the membrane where no electrode was attached,and the appearance was the same as that before the test. However, the portion between the membraneand the electrode could not be observed because the membrane and the electrode were firmly attachedto each other. However, there is a possibility that the voltage may have decreased due to mechanicaldeterioration, such as a crack or pinhole, between the membrane and the electrode. Moreover, it isconceivable that HO or HO2 radicals formed during the reduction of oxygen on the cathode side maychemically degrade the CSPPSU membrane. There are few reports on the long-term stabilities offuel cells using hydrocarbon-based electrolyte membranes, and we believe this is the first time thatthe long-term stability of a cell using a CSPPSU membrane has been evaluated for 4000 h. We arecurrently conducting more detailed evaluations of MEA cells and developing CSPPSU membraneswith long-term stabilities like Nafion membranes.Membranes 2020, 10, x FOR PEER REVIEW 12 of 15   Figure 10. Durability of CSPPSU and Nafion212 membranes at 80 °C, 100% RH, and 1 A. 4. Conclusions High-IEC SPPSU was synthesized using commercially available PPSU, and a large-area crosslinked membrane was produced. In addition, we developed an activation process using alkaline and acidic solutions to remove the remaining sulfur dioxide (SO2) formed as a byproduct during the heat treatment. CSPPSU membranes activated with our method were compared with a Nafion212 membrane. The proton conductivities of the CSPPSU membranes were lower than that of the Nafion212 membrane, but the glass transition temperature and tensile strength were higher. This indicated that the CSPPSU membrane could be used even at high temperatures. From SAXS analysis, the CSPPSU membrane had an ion cluster structure that could act as a proton conduction path. The I-ViR free characteristics of MEA using the CSPPSU membrane were very good and similar to those when a Nafion212 membrane was used. The durability towards hydrogen gas crossover using the CSPPSU membranes was better than that using the Nafion212 membrane. In the long-term evaluation by using a constant current method, a stability of 4000 h was obtained for the first time. On the other hand, from CV, the resistance between the membrane and the electrode and the poisoning of the catalyst were larger than those when the Nafion212 membrane was used. Thus, further analyses and higher performances of MEAs are required. Author Contributions: Conceptualization, J.-D.K.; methodology, J.-D.K.; validation, J.-D.K.; formal analysis, J.-D.K., A.O.; investigation, J.-D.K., A.O., H.N.; data curation, J.-D.K.; writing—original draft preparation, J.-D.K.; writing—review and editing, J.-D.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. References 1. Gasteiger, H.A.; Kocha, S.S.; Sompalli, B.; Wagner, F.T. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B Environ. 2005, 56, 9–35. 2. Higgins, D.C.; Chen, Z. Recent progress in non-precious metal catalysts for PEM fuel cell applications. Can. J. Chem. Eng. 2013, 91, 1881–1895. 3. Sharma, S.; Pollet, B.G. Support materials for PEMFC and DMFC electrocatalysts—A review. J. Power Sources 2012, 208, 96–119. 4. Mauritz, K.A.; Moore, R.B. State of understanding of Nafion. Chem. Rev. 2004, 104, 4535–4585. 5. Yandrasits, M.; Lindell, M.; Schaberg, M.; Kurkowski, M. Increasing fuel cell efficiency by using ultra-low equivalent weight ionomers. Electrochem. Soc. Interface 2017, 26, 49–53. Figure 10. Durability of CSPPSU and Nafion212 membranes at 80 ◦C, 100% RH, and 1 A.Membranes 2020, 10, 31 12 of 144. ConclusionsHigh-IEC SPPSU was synthesized using commercially available PPSU, and a large-area crosslinkedmembrane was produced. In addition, we developed an activation process using alkaline andacidic solutions to remove the remaining sulfur dioxide (SO2) formed as a byproduct during theheat treatment. CSPPSU membranes activated with our method were compared with a Nafion212membrane. The proton conductivities of the CSPPSU membranes were lower than that of the Nafion212membrane, but the glass transition temperature and tensile strength were higher. This indicated thatthe CSPPSU membrane could be used even at high temperatures. From SAXS analysis, the CSPPSUmembrane had an ion cluster structure that could act as a proton conduction path. The I-ViR freecharacteristics of MEA using the CSPPSU membrane were very good and similar to those when aNafion212 membrane was used. The durability towards hydrogen gas crossover using the CSPPSUmembranes was better than that using the Nafion212 membrane. In the long-term evaluation by usinga constant current method, a stability of 4000 h was obtained for the first time. On the other hand,from CV, the resistance between the membrane and the electrode and the poisoning of the catalystwere larger than those when the Nafion212 membrane was used. Thus, further analyses and higherperformances of MEAs are required.Author Contributions: Conceptualization, J.-D.K.; methodology, J.-D.K.; validation, J.-D.K.; formal analysis,J.-D.K., A.O.; investigation, J.-D.K., A.O., H.N.; data curation, J.-D.K.; writing—Original draft preparation, J.-D.K.;writing—Review and editing, J.-D.K. All authors have read and agreed to the published version of the manuscript.Funding: This research received no external funding.Conflicts of Interest: The authors declare no conflict of interest.References1. Gasteiger, H.A.; Kocha, S.S.; Sompalli, B.; Wagner, F.T. Activity benchmarks and requirements for Pt, Pt-alloy,and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B Environ. 2005, 56, 9–35. [CrossRef]2. Higgins, D.C.; Chen, Z. Recent progress in non-precious metal catalysts for PEM fuel cell applications. Can.J. Chem. Eng. 2013, 91, 1881–1895. [CrossRef]3. Sharma, S.; Pollet, B.G. Support materials for PEMFC and DMFC electrocatalysts—A review. 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This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).http://dx.doi.org/10.1016/j.memsci.2017.09.076http://dx.doi.org/10.1002/app.44218http://dx.doi.org/10.1016/j.ijhydene.2016.02.084http://dx.doi.org/10.1016/j.jpowsour.2015.07.001http://dx.doi.org/10.1039/C4PY01177Ehttp://dx.doi.org/10.1016/j.memsci.2013.12.020http://dx.doi.org/10.1016/j.memsci.2013.06.028http://dx.doi.org/10.1039/c2py00611ahttp://dx.doi.org/10.1039/c2jm34372jhttp://dx.doi.org/10.1016/j.jpowsour.2011.08.091http://dx.doi.org/10.1016/j.memsci.2009.02.034http://dx.doi.org/10.1016/j.crci.2008.06.001http://dx.doi.org/10.1002/pat.758http://dx.doi.org/10.1016/j.electacta.2004.09.003http://dx.doi.org/10.1002/app.11264http://dx.doi.org/10.1016/j.ssi.2014.12.012http://dx.doi.org/10.1149/1.3116254http://dx.doi.org/10.1149/08513.0943ecsthttp://creativecommons.org/http://creativecommons.org/licenses/by/4.0/. Introduction  Experimental  Materials  Synthesis and Characterization of SPPSU  Preparation of Chemically Crosslinked SPPSU (CSPPSU) Membranes  Pretreatment of Nafion212 Membranes  IEC, D.S., Water-Uptake (W.U.), , and Crosslink Rates (Dcrosslink)  Oxidative Stability (Fenton’s Test)  Thermal Behavior  Mechanical Behavior  Membrane Nanostructure  Conductivity Measurements  Membrane, Gas Diffusion Layer, and Catalyst Electrode Information for MEA  Preparation of Membrane Electrode Assembly (MEA)  Single Fuel-Cell Performance and Durability  Results and Discussion  Activation of the CSPPSU Membranes  Mechanical Properties of the CSPPSU Membranes  Nanostructures of the CSPPSU Membranes  Proton Conductivities of the CSPPSU Membranes  IEC, W.U., , and Dcrosslink Values of CSPPSU Membranes  Oxidative Stability of the CSPPSU Membranes  Fuel Cell Properties Using CSPPSU Membranes  H2 Crossover of the CSPPSU Membranes  Durability Using CSPPSU Membranes  Conclusions  References