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[Je-Deok Kim](https://orcid.org/0000-0003-4301-1044), [Satoshi Matsushita](https://orcid.org/0000-0001-7824-4732), [Kenji Tamura](https://orcid.org/0000-0001-6578-0923)

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[Crosslinked Sulfonated Polyphenylsulfone-Vinylon (CSPPSU-vinylon) Membranes for PEM Fuel Cells from SPPSU and Polyvinyl Alcohol (PVA)](https://mdr.nims.go.jp/datasets/f743df87-f624-4a47-a930-67013b081447)

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Crosslinked Sulfonated Polyphenylsulfone-Vinylon (CSPPSU-vinylon) Membranes for PEM Fuel Cells from SPPSU and Polyvinyl Alcohol (PVA)polymersArticleCrosslinked Sulfonated Polyphenylsulfone-Vinylon(CSPPSU-vinylon) Membranes for PEM Fuel Cellsfrom SPPSU and Polyvinyl Alcohol (PVA)Je-Deok Kim 1,2,3,*, Satoshi Matsushita 1 and Kenji Tamura 31 Polymer Electrolyte Fuel Cell Group, Global Research Center for Environmental and Energy Based onNanomaterials Science (GREEN),Tsukuba Ibaraki 305-0044, Japan; satoshi.matsushita@agc.com2 Hydrogen Production Materials Group, Center for Green Research on Energy and Environmental Materials,Tsukuba Ibaraki 305-0044, Japan3 Functional Clay Materials Group, Research Center for Functional Materials, National Institute for MaterialsScience (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; Tamura.Kenji@nims.go.jp* Correspondence: kim.jedeok@nims.go.jp; Tel.: +81-29-860-4764; Fax: +81-29-860-4984Received: 3 June 2020; Accepted: 14 June 2020; Published: 16 June 2020�����������������Abstract: A crosslinked sulfonated polyphenylsulfone (CSPPSU) polymer and polyvinyl alcohol (PVA)were thermally crosslinked; then, a CSPPSU-vinylon membrane was synthesized using a formalizationreaction. Its use as an electrolyte membrane for fuel cells was investigated. PVA was synthesizedfrom polyvinyl acetate (PVAc), using a saponification reaction. The CSPPSU-vinylon membranewas synthesized by the addition of PVA (5 wt%, 10 wt%, 20 wt%), and its chemical, mechanical,conductivity, and fuel cell properties were studied. The conductivity of the CSPPSU-10vinylonmembrane is higher than that of the CSPPSU membrane, and a conductivity of 66 mS/cm was obtainedat 120 ◦C and 90% RH (relative humidity). From a fuel cell evaluation at 80 ◦C, the CSPPSU-10vinylonmembrane has a higher current density than CSPPSU and Nafion212 membranes, in both high (100%RH) and low humidification (60% RH). By using a CSPPSU-vinylon membrane instead of a CSPPSUmembrane, the conductivity and fuel cell performance improved.Keywords: PPSU; SPPSU; PVA; CSPPSU-vinylon; PEMFCs1. IntroductionIn order to realize a low-carbon society that includes highly efficient energy systems which makeeffective use of renewable energy, economically sustainable growth, environmental protection, andenergy security are required. Energy conversion/storage devices—such as fuel cells, water electrolysis,secondary batteries, and solar cells—are core technologies for building a low-carbon society. In orderto produce these devices safely and with high performances, their constituent materials must havehigh performances. Polymer electrolyte membranes for proton exchange in fuel cells are requiredfor polymer electrolyte membrane fuel cells (PEMFCs), and polymer electrolyte membrane waterelectrolysis (PEMWE), and Nafion membranes mainly using perfluorosulfonic acid (PFSA) ion exchangeresins, are often used. Nafion membranes have high proton conductivities and excellent chemicalstabilities, and have been used in both mobile and stationary fuel cells [1,2]. However, the performanceof these fuel cells suffers from the deterioration of their mechanical properties, due to thinning ofthe electrolyte membrane. In addition, higher operating temperatures are required to improve boththe glass transition temperature (Tg) and the proton conductivity at high temperatures. At the sametime, research on hydrocarbon-based electrolytes, instead of fluorine-based electrolytes, has beenconducted. Non-fluorine-based electrolytes are low cost materials, have high Tg values, and havebeen studied for many years. However, their performances and chemical stabilities, which are lowerPolymers 2020, 12, 1354; doi:10.3390/polym12061354 www.mdpi.com/journal/polymershttp://www.mdpi.com/journal/polymershttp://www.mdpi.comhttp://www.mdpi.com/2073-4360/12/6/1354?type=check_update&version=1http://dx.doi.org/10.3390/polym12061354http://www.mdpi.com/journal/polymersPolymers 2020, 12, 1354 2 of 13than those of fluorine-based ones, have been the biggest obstacles towards their practical use. Thus,materials for proton exchange electrolytes with higher performances are still needed.The drawbacks of PFSA membranes have prompted research into alternative membranes.Various aromatic polymer ionomer membranes are being actively investigated. Sulfonatedpolyphenylsulfone (SPPSU) [3–20], sulfonated polyetheretherketone (SPEEK) [21–32], sulfonatedpolysulfone (SPSU) [33–38], sulfonated polyphenylene sulfone (SPPS) [39], sulfonated polyphenylene(SPP) [40], sulfonated polyethersulfone (SPES) [41,42], sulfonated polyimide (SPI) [43–45], sulfonatedpolyphenylene oxide (SPPO) [46], and polybenzimidazole (PBI) [47,48] are attracting special interest.We are developing a crosslinked sulfonated polyphenylsulfone (CSPPSU) membrane usingthe sulfonation of the polyphenylsulfone (PPSU) polymer, which has an excellent thermal stability, highchemical resistance, and is low cost [3,5,6,19,20]. According to a fuel cell evaluation using the CSPPSUmembrane, the membrane could be used for 4000 h [19]. However, compared to fluorine-basedelectrolyte membranes, hydrocarbon-based CSPPSU membranes are still insufficient in terms of highperformances and high durabilities in fuel cells. It is necessary to further improve the performanceand durability of SPPSU membranes. In this study, we prepared a CSPPSU-vinylon membrane, whichhas a higher performance than both CSPPSU and Nafion212 membranes. Vinylon was obtained usinga formalization reaction with polyvinyl alcohol (PVA), and PVA was obtained by the saponification ofpolyvinyl acetate (PVAc). Crosslinked SPPSU-vinylon membranes from the SPPSU and PVA, wereobtained using thermal crosslinking and a formalization reaction. The vinylon was somewhat stable,even in a high-hydration environment. First, we prepared a crosslinked SPPSU-vinylon membraneusing thermal crosslinking and the vinylonization of a SPPSU-PVA composite, and these methodsappeared promising for the thinning of other polymer electrolyte membranes.2. Experimental2.1. MaterialsPolyvinyl acetate (PVAc, (C4H6O2)n, Mw = 100,000) was purchased from Sigma-AldrichCorporation (St. Louis, MO, USA). A DuPontTM Nafion212 membrane (NR-212) was purchasedfrom DuPont (USA). Methanol (CH3OH), formaldehyde (CH2O, 37%), sodium chloride (NaCl),sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were purchased from Nacalai Tesque, Inc.Polyphenylsulfone (Solvay Radel R-5000 NT) (Mn = 26,000; Mw = 50,000; Mw/Mn = 1.9) was providedby Solvay Specialty Polymers Japan K.K. (glass transition temperature (Tg) = 220 ◦C, (Tokyo, Japan).A dialysis tubing cellulose membrane, which has a molecular weight cut-off (MWCO) of 14,000, anddimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich Co., Ltd. Deionized (DI) water wasobtained using a PURELAB® Option-R 7 ELGA LabWater, at 15 Mohm cm and 25 ◦C. Sodium sulfate(Na2SO4) and Iron (II) chloride tetrahydrate (FeCl2·4H2O) were purchased from Fujifilm Wako PureChemical Corporation (Osaka, Japan).2.2. Synthesis of SPPSU and PVA, and Preparation of CSPPSU-vinylon MembranesThe synthesis and properties of SPPSU (Mw ≈ 150,000) have been described in detail in previousreports [3,19]. PVA was synthesized using the following method. PVAc (1 g) was dissolved in a flaskwith methanol (50 mL). Then, a 40% NaOH solution was added, and the mixture was allowed to reactat 40 ◦C for 10 min (saponification). The reaction mixture was washed with methanol 4 times andfiltered. To remove the remaining solvent, the product was dried at 80 ◦C for 24 h, and PVA (0.5 g,Mw ≈ 50,000) was obtained. Crosslinked SPPSU-vinylon membranes were obtained from SPPSU andPVA using the following method. A glass vial was charged with SPPSU (0.5 g) and DMSO (20 mL), anddissolved. PVA (0.025 g, 0.05 g, 0.1 g) and DMSO (4 mL) were put into another glass vial and dissolved.Then, the PVA-DMSO solution was added to the SPPSU-DMSO solution, and the mixture was stirredfor 1 h. The SPPSU-PVA-DMSO solution was transferred to a glass container, dried for 24 h at 80 ◦C,and then annealed in air at 120 ◦C (24 h), 160 ◦C (24 h), and 180 ◦C (24 h). Next, the vinylon fromPolymers 2020, 12, 1354 3 of 13the PVA was prepared using a formalization solution (H2O:H2SO4:Na2SO4:CH2O = 1.00:0.21:0.20:0.06in mass ratio) reaction, for 2 h at 60 ◦C. Activation was performed using the following procedure:heating in 0.5 M NaOH at 80 ◦C overnight, washing in DI H2O, heating at 1M H2SO4 at 80 ◦C for 2 h,and washing in DI H2O for 2 h. Finally, the crosslinked SPPSU-vinylon membranes were dried atroom temperature before use. The crosslinked SPPSU-vinylon membranes were very flexible and darkbrown. The classification of the crosslinked SPPSU-vinylon membranes is shown in Table 1.Table 1. Classification of crosslinked sulfonated polyphenylsulfone-vinylon(CSPPSU-vinylon) membranes.Varied Parameter Variable Parameter Membrane ClassificationPVA loading (wt%)0 CSPPSU5 CSPPSU-5vinylon10 CSPPSU-10vinylon20 CSPPSU-20vinylon2.3. Iron-Exchange Chromotography (IEC), D.S. (Degree of Sulfonation), Water-Uptake (W.U.), λ, andCrosslink Rates (Dcrosslink)The IEC values were determined using the following equation: IEC (meq/g) = cv/Wdry, wherec (mmol/L) is the concentration of standardized NaOH aq. used for titration (0.01mol/L), v (L) isthe volume of standardized NaOH aq. used for titration, and Wdry (g) is the mass of the dry membrane.The water-uptake (W.U.) of the membranes at room temperature was calculated using the following:W.U. (%) = [(Wwet – Wdry)/Wdry] × 100, where Wwet is the mass of the wet membrane. The hydrationnumber (λ) for the membranes was determined using the following: λ ([H2O]/[SO3H]) = [1000(Wwet– Wdry)]/18WdryIEC. The degree of crosslinking (crosslink rate, Dcrosslink) in the membranes wasdetermined using the following: Dcrosslink (%) = [(IECbefore annealing – IECafter annealing)/IECbefore annealing]× 100 [19].2.4. Oxidative Stability (Fenton’s Test)The oxidative stabilities of the membranes were evaluated by immersing a small piece ofthe membrane into Fenton’s reagent [3 wt% H2O2 and 2 ppm Fe(II) (added as FeCl2·4H2O)], at ~80 ◦Cfor 1 h while stirring. The samples were dried at 80 ◦C before the measurements. The membranes wererepeatedly washed with DI H2O, and dried at 80 ◦C overnight following the reaction. The oxidativestabilities were determined as follows: [(mass of residual membrane after the test)/(initial mass ofmembrane)] × 100.2.5. Chemical Structure of the SamplesFourier-transform infrared (FTIR) absorption spectra of the samples were obtained on a ThermoScientific Nicolet 6700 spectrometer, in an attenuated total reflection (ATR) mode.2.6. Mechanical and Thermal Behavior of the SamplesStress-strain tests on the membranes were accomplished using a Tension Test Machine (Shimazu,EZ-S) at room temperature [19]. The thermal and mass properties of the membranes were investigatedusing thermogravimetric and mass analyses with a Thermoplus TG8120 TG-DTA/H (Rigaku Co. Ltd.,Japan). The samples were heated from 60 to 800 ◦C at 5 ◦C/min in air, after keeping them for 1 h at60 ◦C.Polymers 2020, 12, 1354 4 of 132.7. Conductivity and Single Cell Measurements of the SamplesThe proton conductivities of the membranes were evaluated using a four-point probe impedancespectroscopy. For the membrane electrode assembly (MEA), the thickness of the membranes wasapproximately 50 µm, and a Pt/C/ionomer (ionomer/carbon = 1) catalyst electrode (EIWA corporation)containing 0.3 mg/cm2 of Pt on a GDL electrode (Sigracet® GDL 25BC of SGL Group Co. Ltd., Japan),was used. The effective electrode area of the single cell was 4 cm2. The MEA was acquired by loadinga membrane between the anode and cathode, and hot-pressing at 130 ◦C and ~9.8 kN for 20 min.A single cell performance was measured in relation to the amount of hydrogen (H2) and oxygen (O2)at the anode and cathode, respectively, at 80 ◦C, 100% and 60% RH (relative humidity), and ambientpressure. The gas flow rate of hydrogen and oxygen were 50 cc/min and 100 cc/min, respectively.Linear sweep voltammetry (LSV) was evaluated in the potential range of 0.02–0.5 V at 2 mV/s [19].3. Results and Discussion3.1. CSPPSU-vinylon MembranesThermally crosslinked membranes of SPPSU polymers have been reported in previousresearch [5,19]. Crosslinking occurs between the sulfone groups of SPPSU under a thermal environment.The same phenomenon occurs in composite membranes of SPPSU polymers and PVA polymers. Inaddition, crosslinking occurs between the sulfone groups of SPPSU and the hydroxy groups of PVAupon heat treatment. A crosslinked SPPSU-vinylon membrane could be obtained using a formalizationreaction with PVA (Figure 1).Polymers 2018, 10, x FOR PEER REVIEW  4 of 13  by loading a membrane between the anode and cathode, and hot-pressing at 130 °C and ~9.8 kN for 20 min. A single cell performance was measured in relation to the amount of hydrogen (H2) and oxygen (O2) at the anode and cathode, respectively, at 80 °C, 100% and 60% RH (relative humidity) , and ambient pressure. The gas flow rate of hydrogen and oxygen were 50 cc/min and 100 cc/min, respectively. Linear sweep voltammetry (LSV) was evaluated in the potential range of 0.02–0.5 V at 2 mV/s.  3. Results and Discussion 3.1. CSPPSU-vinylon Membranes Thermally crosslinked membranes of SPPSU polymers have been reported in previous research [5,19]. Crosslinking occurs between the sulfone groups of SPPSU under a thermal environment. The same phenomenon occurs in composite membranes of SPPSU polymers and PVA polymers. In addition, crosslinking occurs between the sulfone groups of SPPSU and the hydroxy groups of PVA upon heat treatment. A crosslinked SPPSU-vinylon membrane could be obtained using a formalization reaction with PVA (Figure 1).  Figure 1. Schematic diagram of CSPPSU-vinylon membrane. PVA was synthesized by hydrolyzing PVAc (saponification). The peak for the carbonyl groups (C=O) of the PVAc appeared at 1728 cm–1 in the IR spectrum (Figure 2a). For PVA synthesized using the hydrolysis of PVAc, the peak due to the carbonyl group disappeared, and new peaks for the OH groups appeared at 3272 cm–1, 1655 cm–1, and 1324 cm–1 (Figure 2b). These results indicate that PVA can be obtained by hydrolyzing PVAc. The IR spectra of the CSPPSU-vinylon membranes did not change significantly with the amount of PVA added and had similar characteristics (Figure 2c–e). The peaks for both SPPSU and PVA appeared in the spectra. The crosslinking of SPPSU has been reported in more detail in a previous paper [3,5]. In the IR spectra, it was not clear whether the sulfone bridge of SPPSU and the hydroxy group of PVA were crosslinked using hydrolysis to form a sulfone bridge (-SO2-). Moreover, it was difficult to determine whether PVA had been changed to vinylon. However, we can assume that crosslinking and vinylon formation progressed, as the appearance of the obtained membranes were very uniform and flexible. Figure 2 and Table 2 show the FTIR spectra and summarize the assignments of the peaks, respectively. Figure 1. Schematic diagram of CSPPSU-vinylon membrane.PVA was synthesized by hydrolyzing PVAc (saponification). The peak for the carbonyl groups(C=O) of the PVAc appeared at 1728 cm−1 in the IR spectrum (Figure 2a). For PVA synthesized usingthe hydrolysis of PVAc, the peak due to the carbonyl group disappeared, and new peaks for the OHgroups appeared at 3272 cm−1, 1655 cm−1, and 1324 cm−1 (Figure 2b). These results indicate thatPVA can be obtained by hydrolyzing PVAc. The IR spectra of the CSPPSU-vinylon membranes didnot change significantly with the amount of PVA added and had similar characteristics (Figure 2c–e).The peaks for both SPPSU and PVA appeared in the spectra. The crosslinking of SPPSU has beenreported in more detail in a previous paper [3,5]. In the IR spectra, it was not clear whether the sulfonebridge of SPPSU and the hydroxy group of PVA were crosslinked using hydrolysis to form a sulfonebridge (-SO2-). Moreover, it was difficult to determine whether PVA had been changed to vinylon.However, we can assume that crosslinking and vinylon formation progressed, as the appearance ofthe obtained membranes were very uniform and flexible. Figure 2 and Table 2 show the FTIR spectraand summarize the assignments of the peaks, respectively.Polymers 2020, 12, 1354 5 of 13Polymers 2018, 10, x FOR PEER REVIEW  5 of 13   Figure 2. Fourier transform infrared (FTIR) properties of (a) polyvinyl acetate (PVAc), (b) syn. polyvinyl alcohol (PVA), (c) CSPPSU-5vinylon, (d) CSPPSU-10vinylon, and (e) CSPPSU-20vinylon membranes with different amounts of PVA. Table 2. Summary of assignments of the FTIR spectra of PVAc, syn. PVA, and CSPPSU-vinylon membranes. Polymer PVAc PVA CSPPSU-vinylon vH-O-H (cm–1)  3272 3421 Aromatic vC-H (cm–1)   3093, 3074, 3038 Aliphatic vC-H (cm–1) 2974, 2928, 2852, 1431, 1367 2932, 2898, 1424 2920, 2846 -O-H (cm–1)  1655 1714, 1665 Aromatic vC=C (cm–1)   1584, 1469 vC=O (cm–1) 1728   -O-H (cm–1)  1324 1324 vas, C-O-C (cm–1)   1232 Aliphatic vC-O (cm–1) 1220, 1116, 1016, 939 1568, 1083  vs, O=S=O (cm–1)   1142 3.2. Thermal and Mechanical Properties of the CSPPSU-vinylon Membranes The thermal (Figure 3) and mechanical properties (Figure 4) of the CSPPSU-vinylon membranes, prepared by varying the amount of PVA added, were investigated. The CSPPSU-vinylon membrane exhibited a lower desorption of the sulfone groups and had a lower decomposition temperature of the polymer backbone than the CSPPSU membrane (Figure 3, Table 3). The thermal behavior of the CSPPSU-vinylon membrane on the amount of PVA added was similar. As for the weight reduction ratio of water due to water vaporization, the CSPPSU-vinylon membrane had a higher water content than the CSPPSU membrane (Table 3). In the TGA (thermal gravimetric analysis) curves for the CSPPSU sample, residuals (inorganic substances) appeared after 600 °C. However, every CSPPSU-Figure 2. Fourier transform infrared (FTIR) properties of (a) polyvinyl acetate (PVAc), (b) syn. polyvinylalcohol (PVA), (c) CSPPSU-5vinylon, (d) CSPPSU-10vinylon, and (e) CSPPSU-20vinylon membraneswith different amounts of PVA.Table 2. Summary of assignments of the FTIR spectra of PVAc, syn. PVA, andCSPPSU-vinylon membranes.Polymer PVAc PVA CSPPSU-vinylonvH-O-H (cm−1) 3272 3421AromaticvC-H (cm−1) 3093, 3074, 3038AliphaticvC-H (cm−1)2974, 2928, 2852, 1431,1367 2932, 2898, 1424 2920, 2846δs, H-O-H (cm−1) 1655 1714, 1665AromaticvC=C (cm−1) 1584, 1469vC=O (cm−1) 1728δs, C-O-H (cm−1) 1324 1324vas, C-O-C (cm−1) 1232AliphaticvC-O (cm−1) 1220, 1116, 1016, 939 1568, 1083vs, O=S=O (cm−1) 11423.2. Thermal and Mechanical Properties of the CSPPSU-vinylon MembranesThe thermal (Figure 3) and mechanical properties (Figure 4) of the CSPPSU-vinylon membranes,prepared by varying the amount of PVA added, were investigated. The CSPPSU-vinylon membraneexhibited a lower desorption of the sulfone groups and had a lower decomposition temperatureof the polymer backbone than the CSPPSU membrane (Figure 3, Table 3). The thermal behaviorof the CSPPSU-vinylon membrane on the amount of PVA added was similar. As for the weightreduction ratio of water due to water vaporization, the CSPPSU-vinylon membrane had a higherwater content than the CSPPSU membrane (Table 3). In the TGA (thermal gravimetric analysis)curves for the CSPPSU sample, residuals (inorganic substances) appeared after 600 ◦C. However,every CSPPSU-vinylon sample burned at 600 ◦C. This suggests that the SPPSU and the vinylon werecrosslinked into one polymer.Polymers 2020, 12, 1354 6 of 13Polymers 2018, 10, x FOR PEER REVIEW  6 of 13  vinylon sample burned at 600 °C. This suggests that the SPPSU and the vinylon were crosslinked into one polymer.  Figure 3. (a) TG (thermal gravimetric) and (b) DTA (differential thermal analysis) results of (i) CSPPSU-5vinylon, (ii) CSPPSU-10vinylon, (iii) CSPPSU-20vinylon, and (iv) CSPPSU membranes. Table 3. Summary of temperature ranges and mass losses observed for each step in the TG-DTA curves for CSPPSU-vinylon membranes. Sample name Evaporation of H2O interacting with –SO3H or -OH group Desubstitution of –SO3H group  Thermal decomposition of polymer backbone oC)  (°C) (%) Peak of exothermic (°C) CSPPSU 61–210 4.5 210–453 19.1 548 CSPPSU-5vinylon 61–212 4.9 212–403 17.5 532 CSPPSU-10vinylon 61–197 5.0 197–389 19.5 534 CSPPSU-20vinylon 61–197 5.0 197–400 19.8 528 On the other hand, the dependence of the CSPPSU-vinylon membrane on the amount of added PVA, was noticeable in the evaluation of its mechanical properties (Figure 4, Table 4). The CSPPSU-10vinylon membranes obtained by adding 10 wt% PVA to SPPSU had higher tensile strengths than the other crosslinked membranes. However, the tensile elongation increased with an increase in the amount of added PVA, and the tensile strength and tensile elongation of the CSPPSU-5vinylon membrane containing 5 wt% PVA, was low in comparison to the other membranes. The flexural modulus of the membrane decreased with an increase in the amount of PVA. The tensile elongation characteristics of the CSPPSU-vinylon membrane were smaller than those of the CSPPSU membranes. The favorable tensile elongation of the CSPPSU-10vinylon membrane may be due to its better homogeneity in comparison to the other membranes. Figure 3. (a) TG (thermal gravimetric) and (b) DTA (differential thermal analysis) results of (i)CSPPSU-5vinylon, (ii) CSPPSU-10vinylon, (iii) CSPPSU-20vinylon, and (iv) CSPPSU membranes.Polymers 2018, 10, x FOR PEER REVIEW  7 of 13   Figure 4. Stress-strain properties of (a) CSPPSU-5vinylon, (b) CSPPSU-10vinylon, (c) CSPPSU-20vinylon, and (d) CSPPSU membranes. Table 4. Mechanical properties of CSPPSU-vinylon membranes.  CSPPSU CSPPSU -5vinylon CSPPSU -10vinylon CSPPSU -20vinylon Tensile strength (MPa) 48 27 45 40 Tensile elongation (%) (break) 74 26 48 55 Flexural modulus (MPa)* 757 781 759 548 *stress/strain. 3.3. Proton Conductivities of the CSPPSU-vinylon Membranes Polymer electrolyte membranes for high-performance fuel cells require high proton conductivities of >0.01 S/cm, from low to high temperatures and high to low humidification [1]. The conductivity of the CSPPSU-vinylon membranes due to the difference in the amount of PVA added—which is the average value of the error bars of the data obtained after three measurements with varying humidity, at cell temperatures of 40 and 120 °C—is shown in Figure 5. Table 5 shows the physicochemical properties of the CSPPSU-vinylon membranes, depending on the amount of PVA added. The IEC value of the SPPSU polymer was 3.8 meq/g, and the IEC value of the SPPSU-PVA composite polymer was assumed to be equivalent to the IEC value of the SPPSU polymer. Then, the crosslinking degree (Dcrosslink) of the CSPPSU-vinylon membranes was calculated. Moreover, the chemical stability of the membrane was determined using Fenton's reagent (3 wt% H2O2 + 2 ppm Fe (II), 80 oC, 1h). Since vinylon has a high chemical resistance, we thought that the chemical stability of CSPPSU [19] would be improved by incorporating vinylon. However, as shown in Table 5, there was little improvement. With Fenton's reagent, the CSPPSU membrane was radically attacked from the edge, but the CSPPSU-10vinylon membrane was attacked from the inside of the membrane, generating a hole. It is thought that the SPPSU part is selectively vulnerable to attack. On the other hand, the conductivity of the CSPPSU-vinylon electrolyte membrane increased with increases in the temperature and humidity. The conductivity of the CSPPSU-vinylon membrane was higher than that of the CSPPSU membrane. In particular, the CSPPSU-10vinylon membrane had a higher conductivity than the other membranes. The diffusion of protons in the electrolyte membrane depended on the concentration and proton mobility of the sulfonic acid groups (-SO3H) in the electrolyte membrane, and became faster as the temperature and humidity were increased. In Figure 4. Stress-strain properties of (a) CSPPSU-5vinylon, (b) CSPPSU-10vinylon, (c)CSPPSU-20vinylon, and (d) CSPPSU membranes.Table 3. Summary of temperature ranges and mass losses observed for each step in the TG-DTA curvesfor CSPPSU-vinylon membranes.SampleNameEvaporation of H2OInteracting with –SO3Hor -OH GroupDesubstitution of –SO3HGroupThermalDecomposition ofPolymer Backbone∆T (◦C) ∆Wt. Loss(%) ∆T (◦C) ∆Wt. Loss(%) Peak of Exothermic (◦C)CSPPSU 61–210 4.5 210–453 19.1 548CSPPSU-5vinylon 61–212 4.9 212–403 17.5 532CSPPSU-10vinylon 61–197 5.0 197–389 19.5 534CSPPSU-20vinylon 61–197 5.0 197–400 19.8 528On the other hand, the dependence of the CSPPSU-vinylon membrane on the amount of added PVA,was noticeable in the evaluation of its mechanical properties (Figure 4, Table 4). The CSPPSU-10vinylonmembranes obtained by adding 10 wt% PVA to SPPSU had higher tensile strengths than the otherPolymers 2020, 12, 1354 7 of 13crosslinked membranes. However, the tensile elongation increased with an increase in the amountof added PVA, and the tensile strength and tensile elongation of the CSPPSU-5vinylon membranecontaining 5 wt% PVA, was low in comparison to the other membranes. The flexural modulus ofthe membrane decreased with an increase in the amount of PVA. The tensile elongation characteristicsof the CSPPSU-vinylon membrane were smaller than those of the CSPPSU membranes. The favorabletensile elongation of the CSPPSU-10vinylon membrane may be due to its better homogeneity incomparison to the other membranes.Table 4. Mechanical properties of CSPPSU-vinylon membranes.CSPPSU CSPPSU-5vinylonCSPPSU-10vinylonCSPPSU-20vinylonTensile strength (MPa) 48 27 45 40Tensile elongation (%) (break) 74 26 48 55Flexural modulus (MPa) * 757 781 759 548* ∆stress/∆strain.3.3. Proton Conductivities of the CSPPSU-vinylon MembranesPolymer electrolyte membranes for high-performance fuel cells require high proton conductivitiesof >0.01 S/cm, from low to high temperatures and high to low humidification [1]. The conductivityof the CSPPSU-vinylon membranes due to the difference in the amount of PVA added—which isthe average value of the error bars of the data obtained after three measurements with varying humidity,at cell temperatures of 40 and 120 ◦C—is shown in Figure 5. Table 5 shows the physicochemicalproperties of the CSPPSU-vinylon membranes, depending on the amount of PVA added. The IECvalue of the SPPSU polymer was 3.8 meq/g, and the IEC value of the SPPSU-PVA composite polymerwas assumed to be equivalent to the IEC value of the SPPSU polymer. Then, the crosslinking degree(Dcrosslink) of the CSPPSU-vinylon membranes was calculated. Moreover, the chemical stability ofthe membrane was determined using Fenton’s reagent (3 wt% H2O2 + 2 ppm Fe (II), 80 ◦C, 1 h).Since vinylon has a high chemical resistance, we thought that the chemical stability of CSPPSU [19]would be improved by incorporating vinylon. However, as shown in Table 5, there was littleimprovement. With Fenton’s reagent, the CSPPSU membrane was radically attacked from the edge,but the CSPPSU-10vinylon membrane was attacked from the inside of the membrane, generatinga hole. It is thought that the SPPSU part is selectively vulnerable to attack.Polymers 2018, 10, x FOR PEER REVIEW  8 of 13  addition, the nanostructure (conduction path) of the electrolyte membrane was greatly affected. The homogeneity of the CSPPSU-10vinylon membrane was better than that of the other membranes (Figure 4). As shown in Table 5, changes in the IEC values of the CSPPSU-vinylon membrane due to the difference in the amount of PVA added, was small and slightly higher than those of the CSPPSU membrane. However, the water content and the number of water molecules per sulfonic acid group () of the CSPPSU-10vinylon membrane, were higher than those of the other crosslinked membranes. These differences contributed to the high proton conductivity of the CSPPSU-10vinylon membrane. The degree of crosslinking of the CSPPSU-vinylon membrane was 42%–45%. It is possible that the hydroxy groups (-OH) of vinylon in the CSPPSU-vinylon membrane contributed to the proton transfer. From the above, it is clear that the conduction mechanism of the SPPSU-vinylon composite membrane is very complicated.  Figure 5. Proton conductivities of the CSPPSU-vinylon membranes vs. the relative humidity at (a) 40 and (b) 120 °C. Table 5. Physicochemical and conductivity properties of the CSPPSU-vinylon membranes. Sample name CSPPSU CSPPSU -5vinylon CSPPSU -10vinylon CSPPSU -20vinylon IEC (meq/g) 2 2.2 2.1 2.1 W.U. (%) 43 38 66 36  11.9 9.6 17.5 9.5 Dcrosslink (%) 47.3 42.1 44.7 44.7 Roxidation (%) 91–99 _a 81–99 _a Conductivity (mS/cm) 40 °C 20% RH 0.43 0.31 0.52 0.57 90% RH 7.23 10.12 15.23 12.43 80 °C 20% RH 0.7 0.7 0.9 1.1 90% RH 18 36 56 55 120 °C 20% RH 0.6 1.3 2.2 1.0 90% RH 22 34 66 65 _a: disassembly. 3.4. Fuel Cell Properties using CSPPSU-vinylon Membranes The performance of fuel cells depends not only on the ionic conductivity of the electrolyte membrane, but also on the interfaces between the electrode layers (catalyst, carbon, ionomer) and between the membrane and the electrode layer [45,49]. Here, the electrode layer and the MEA (membrane electrode assembly) were placed under the same conditions, and only the electrolyte membrane was different. In addition, the measurement conditions for obtaining the current-voltage (I-V) characteristics were the same. The I-V characteristics were evaluated at a cell temperature of 80 °C, and humidities of 100% RH and 60% RH. Figure 6 shows I-Vir free and I-iR loss characteristics, Figure 5. Proton conductivities of the CSPPSU-vinylon membranes vs. the relative humidity at (a) 40and (b) 120 ◦C.Polymers 2020, 12, 1354 8 of 13Table 5. Physicochemical and conductivity properties of the CSPPSU-vinylon membranes.Sample Name CSPPSU CSPPSU-5vinylonCSPPSU-10vinylonCSPPSU-20vinylonIEC (meq/g) 2 2.2 2.1 2.1W.U. (%) 43 38 66 36λ 11.9 9.6 17.5 9.5Dcrosslink (%) 47.3 42.1 44.7 44.7Roxidation (%) 91–99 _a 81–99 _aConductivity(mS/cm)40 ◦C20% RH 0.43 0.31 0.52 0.5790% RH 7.23 10.12 15.23 12.4380 ◦C20% RH 0.7 0.7 0.9 1.190% RH 18 36 56 55120 ◦C20% RH 0.6 1.3 2.2 1.090% RH 22 34 66 65_a: disassembly.On the other hand, the conductivity of the CSPPSU-vinylon electrolyte membrane increasedwith increases in the temperature and humidity. The conductivity of the CSPPSU-vinylon membranewas higher than that of the CSPPSU membrane. In particular, the CSPPSU-10vinylon membranehad a higher conductivity than the other membranes. The diffusion of protons in the electrolytemembrane depended on the concentration and proton mobility of the sulfonic acid groups (-SO3H)in the electrolyte membrane, and became faster as the temperature and humidity were increased.In addition, the nanostructure (conduction path) of the electrolyte membrane was greatly affected.The homogeneity of the CSPPSU-10vinylon membrane was better than that of the other membranes(Figure 4). As shown in Table 5, changes in the IEC values of the CSPPSU-vinylon membrane due tothe difference in the amount of PVA added, was small and slightly higher than those of the CSPPSUmembrane. However, the water content and the number of water molecules per sulfonic acid group(λ) of the CSPPSU-10vinylon membrane, were higher than those of the other crosslinked membranes.These differences contributed to the high proton conductivity of the CSPPSU-10vinylon membrane.The degree of crosslinking of the CSPPSU-vinylon membrane was 42%–45%. It is possible thatthe hydroxy groups (-OH) of vinylon in the CSPPSU-vinylon membrane contributed to the protontransfer. From the above, it is clear that the conduction mechanism of the SPPSU-vinylon compositemembrane is very complicated.3.4. Fuel Cell Properties using CSPPSU-vinylon MembranesThe performance of fuel cells depends not only on the ionic conductivity of the electrolytemembrane, but also on the interfaces between the electrode layers (catalyst, carbon, ionomer) andbetween the membrane and the electrode layer [45,49]. Here, the electrode layer and the MEA(membrane electrode assembly) were placed under the same conditions, and only the electrolytemembrane was different. In addition, the measurement conditions for obtaining the current-voltage(I-V) characteristics were the same. The I-V characteristics were evaluated at a cell temperature of80 ◦C, and humidities of 100% RH and 60% RH. Figure 6 shows I-Vir free and I-iR loss characteristics,evaluated using CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes. The resistance of the singlecell using the CSPPSU-10vinylon membrane was higher than that of the Nafion212 membrane, andlower than that of the CSPPSU membrane (Table 6). This tendency in the level of conductivity isthe same as that using only the electrolyte membrane (Figure 5). Moreover, the I-ViR free characteristicsshowed the same tendency as the resistance characteristics of the unit cell. On the other hand, whenusing the CSPPSU-10vinylon membrane, a current of 1.5 A/cm2 or more was obtained. In the case ofPolymers 2020, 12, 1354 9 of 13Nafion212 and the CSPPSU membranes, the current was less than 1.5 A/cm2 at 100% RH, and lessthan 1 A/cm2 at 60% RH. Under high humidification conditions, when a high current was applied,flooding occurred on the cathode side, and the voltage tended to drop sharply. Moreover, under lowhumidification conditions, the membrane resistance increased due to the drying of the membrane,making it difficult to obtain a high current. However, when the CSPPSU-10vinylon membranewas used, a high current without a sharp drop in voltage was obtained, under both high and lowhumidity conditions. The CSPPSU-vinylon membrane, therefore, seems to have an excellent watertreatment ability. These results suggest that the CSPPSU-vinylon membrane would be suitable for thinmembrane applications.Polymers 2018, 10, x FOR PEER REVIEW  9 of 13  evaluated using CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes. The resistance of the single cell using the CSPPSU-10vinylon membrane was higher than that of the Nafion212 membrane, and lower than that of the CSPPSU membrane (Table 6). This tendency in the level of conductivity is the same as that using only the electrolyte membrane (Figure 5). Moreover, the I-ViR free characteristics showed the same tendency as the resistance characteristics of the unit cell. On the other hand, when using the CSPPSU-10vinylon membrane, a current of 1.5 A/cm2 or more was obtained. In the case of Nafion212 and the CSPPSU membranes, the current was less than 1.5 A/cm2 at 100% RH, and less than 1 A/cm2 at 60% RH. Under high humidification conditions, when a high current was applied, flooding occurred on the cathode side, and the voltage tended to drop sharply. Moreover, under low humidification conditions, the membrane resistance increased due to the drying of the membrane, making it difficult to obtain a high current. However, when the CSPPSU-10vinylon membrane was used, a high current without a sharp drop in voltage was obtained, under both high and low humidity conditions. The CSPPSU-vinylon membrane, therefore, seems to have an excellent water treatment ability. These results suggest that the CSPPSU-vinylon membrane would be suitable for thin membrane applications.  Figure 6. Current-voltage (I-V) properties of CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes: (a) I-ViR free at 80 °C, 100% RH and (b) I-ViR free at 80 °C, 60% RH. Figure 7 shows the hydrogen crossover characteristics of the CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes. The crossover properties of the CSPPSU-10vinylon membrane are five times lower than those of the Nafion212 membrane, and three times higher than those of the CSPPSU membrane (Table 6). We can assume that the crosslinking of SPPSU with vinylon increases the conduction paths (volume fraction) in the nanophase, and hydrogen crossover is higher than that of the CSPPSU membrane.  Figure 6. Current-voltage (I-V) properties of CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes:(a) I-ViR free at 80 ◦C, 100% RH and (b) I-ViR free at 80 ◦C, 60% RH.Table 6. I-V and H2 crossover data for single cells using the CSPPSU-10vinylon, CSPPSU, andNafion212 membranes.80 ◦C, 100% RH 80 ◦C, 60% RHOCV(V)iR Loss(mohm)@ 1 A/cm2H2 Crossover(mA/cm2)@ 0.4 VOCV(V)iR Loss(mohm)@ 1 A/cm2CSPPSU 1.020 73 0.085 1.018 161CSPPSU-10vinylon 1.010 69 0.245 1.008 119Nafion212 1.005 24 1.24 1.029 42Figure 7 shows the hydrogen crossover characteristics of the CSPPSU-10vinylon, CSPPSU,and Nafion212 membranes. The crossover properties of the CSPPSU-10vinylon membrane arefive times lower than those of the Nafion212 membrane, and three times higher than those ofthe CSPPSU membrane (Table 6). We can assume that the crosslinking of SPPSU with vinylon increasesthe conduction paths (volume fraction) in the nanophase, and hydrogen crossover is higher than thatof the CSPPSU membrane.Polymers 2020, 12, 1354 10 of 13Polymers 2018, 10, x FOR PEER REVIEW  10 of 13   Figure 7. Hydrogen crossover properties of CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes at 80 °C and 100% RH. Table 6. I-V and H2 crossover data for single cells using the CSPPSU-10vinylon, CSPPSU, and Nafion212 membranes.  80 °C, 100% RH 80 °C, 60% RH OCV (V) iR loss (mohm) @ 1 A/cm2 H2 crossover (mA/cm2) @ 0.4 V OCV (V) iR loss (mohm) @ 1 A/cm2 CSPPSU 1.020 73 0.085 1.018 161 CSPPSU-10vinylon 1.010 69 0.245 1.008 119 Nafion212 1.005 24 1.24 1.029 42 4. Conclusions We focused on improving the performance of CSPPSU membranes with hydrocarbon-based SPPSU polymers, as an alternative electrolyte to fluoropolymer electrolytes. To improve the conductivity and I-V performance properties of CSPPSU membranes, SPPSU and PVA were crosslinked, and CSPPSU-vinylon membranes were synthesized by the formalization of PVA, and compared with Nafion212 and CSPPSU membranes. The conductivities of the CSPPSU-vinylon membranes were higher than those of the CSPPSU membrane. From the results of the fuel cell evaluation, higher current densities than those of Nafion212 and CSPPSU membranes were obtained under both high and low humidification conditions. This is due to the effects of vinylon, and it is thought that the CSPPSU-vinylon membrane has excellent water retention under low humidification conditions. Furthermore, the hydrogen gas crossover properties are lower than those of Nafion212. In other words, the CSPPSU-vinylon membrane would be useful for thin membrane applications. Author Contributions: Conceptualization, J.-D.K.; Data curation, J.-D.K., S.M. and K.T.; Formal analysis, J.-D.K.; Writing – original draft, J.-D.K.; Writing – review & editing, J.-D.K. Funding:. This research received no external funding. Acknowledgments: This work was partially supported by the MEXT Program for the Development of Environmental Technology, using Nanotechnology from Ministry of Education, Culture, Sports, Science and Technology, Japan. Conflicts of Interest: The authors declare no conflict of interest. Figure 7. Hydrogen crossover properties of CSPPSU-10vinylon, CSPPSU, and Nafion212 membranesat 80 ◦C and 100% RH.4. ConclusionsWe focused on improving the performance of CSPPSU membranes with hydrocarbon-based SPPSUpolymers, as an alternative electrolyte to fluoropolymer electrolytes. To improve the conductivityand I-V performance properties of CSPPSU membranes, SPPSU and PVA were crosslinked, andCSPPSU-vinylon membranes were synthesized by the formalization of PVA, and compared withNafion212 and CSPPSU membranes. The conductivities of the CSPPSU-vinylon membranes werehigher than those of the CSPPSU membrane. From the results of the fuel cell evaluation, highercurrent densities than those of Nafion212 and CSPPSU membranes were obtained under both highand low humidification conditions. This is due to the effects of vinylon, and it is thought thatthe CSPPSU-vinylon membrane has excellent water retention under low humidification conditions.Furthermore, the hydrogen gas crossover properties are lower than those of Nafion212. In other words,the CSPPSU-vinylon membrane would be useful for thin membrane applications.Author Contributions: Conceptualization, J.-D.K.; Data curation, J.-D.K., S.M. and K.T.; Formal analysis,J.-D.K.; Writing—original draft, J.-D.K.; Writing—review & editing, J.-D.K. All authors have read and agreed tothe published version of the manuscript.Funding: This research received no external funding.Acknowledgments: This work was partially supported by the MEXT Program for the Development ofEnvironmental Technology, using Nanotechnology from Ministry of Education, Culture, Sports, Science andTechnology, Japan.Conflicts of Interest: The authors declare no conflict of interest.References1. Mauritz, K.A.; Moore, R.B. State of Understanding of Nafion. Chem. 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Introduction  Experimental  Materials  Synthesis of SPPSU and PVA, and Preparation of CSPPSU-vinylon Membranes  Iron-Exchange Chromotography (IEC), D.S. (Degree of Sulfonation), Water-Uptake (W.U.), , and Crosslink Rates (Dcrosslink)  Oxidative Stability (Fenton’s Test)  Chemical Structure of the Samples  Mechanical and Thermal Behavior of the Samples  Conductivity and Single Cell Measurements of the Samples  Results and Discussion  CSPPSU-vinylon Membranes  Thermal and Mechanical Properties of the CSPPSU-vinylon Membranes  Proton Conductivities of the CSPPSU-vinylon Membranes  Fuel Cell Properties using CSPPSU-vinylon Membranes  Conclusions  References