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[Jedeok Kim](https://orcid.org/0000-0003-4301-1044), Akihiro Ohira

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[Crosslinked Sulfonated Polyphenylsulfone (CSPPSU) Membranes for Elevated-Temperature PEM Water Electrolysis](https://mdr.nims.go.jp/datasets/32b1a137-0d5a-4046-bf9e-7ada030f2970)

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Crosslinked Sulfonated Polyphenylsulfone (CSPPSU) Membranes for Elevated-Temperature PEM Water ElectrolysismembranesArticleCrosslinked Sulfonated Polyphenylsulfone (CSPPSU)Membranes for Elevated-Temperature PEM Water ElectrolysisJedeok Kim 1,* and Akihiro Ohira 2�����������������Citation: Kim, J.; Ohira, A.Crosslinked SulfonatedPolyphenylsulfone (CSPPSU)Membranes for Elevated-TemperaturePEM Water Electrolysis. Membranes2021, 11, 861. https://doi.org/10.3390/membranes11110861Academic Editors: Simona Salerno,Francesca Macedonio,Philippe Knauth and RamatoAshu TufaReceived: 7 October 2021Accepted: 5 November 2021Published: 8 November 2021Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2021 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).1 Research Center for Functional Materials, Functional Clay Materials Group,National Institute for Materials Science (NIMS), 1-1 Namiki, Ibaraki, Tsukuba 305-0044, Japan2 Energy Storage Technology Group, Research Institute for Energy Conservation,National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Ibaraki,Tsukuba 305-8565, Japan; a-oohira@aist.go.jp* Correspondence: Kim.jedeok@nims.go.jp; Tel.: +81-29-860-4764; Fax: +81-29-860-4667Abstract: In order to reduce the burden on the environment, there is a need to develop non-fluorinatedelectrolyte membranes as alternatives to fluorinated electrolyte membranes, and water electrolysisusing hydrocarbon-based electrolyte membranes has been studied in recent years. In this paper, forthe first time, we report elevated-temperature water electrolysis properties of crosslinked sulfonatedpolyphenylsulfone (CSPPSU) membranes prepared by sulfonation and crosslinking of hydrocarbon-based PPSU engineering plastics. The sulfone groups of the CSPPSU membrane in water were stableat 85 ◦C (3600 h) and 150 ◦C (2184 h). In addition, the polymer structure of the CSPPSU membranewas stable during small-angle X-ray scattering (SAXS) measurements from room temperature to180 ◦C. A current density of 456 mA/cm2 was obtained at 150 ◦C and 1.8 V in water electrolysis usingthe CSPPSU membrane and IrO2/Ti as the catalytic electrode for oxygen evolution. The stability ofthe CSPPSU membrane at elevated temperatures with time was evaluated. There were some issues inthe assembly of the CSPPSU membrane and the catalytic electrode. However, the CSPPSU membranehas the potential to be used as an electrolyte membrane for elevated-temperature water electrolysis.Keywords: engineering plastic PPSU; hydrocarbon electrolytes; CSPPSU membrane; PEM waterelectrolysis; elevated temperature1. IntroductionThe abnormal phenomena caused by global climate change require the construction ofa social system that can provide a stable supply of energy (energy security) while reducingCO2 emissions to zero or below zero through carbon neutrality on a global scale. Thedevelopment of renewable energy utilization technologies that do not emit CO2 and usesustainable energy sources, such as solar, wind, and biomass, is underway. Researchon energy storage, conversion, and utilization technologies, such as batteries [1–3], solarcells [4], fuel cells [5], and water electrolysis [6] is being actively conducted to efficientlyutilize these energies. Moreover, hydrogen, which has a high energy density, can beproduced from renewable energy sources, fossil fuel reforming, industrial process by-products, biomass, water electrolysis, etc., and is suitable for energy storage systems [7–9].Hydrogen production by water electrolysis has been actively researched and devel-oped due to the effective utilization of renewable energy systems and the expansion ofits use in mobile systems. There are three types of water electrolysis: alkaline water elec-trolysis (AWE), solid oxide water electrolysis (SOWE), and polymer electrolyte membranewater electrolysis (PEMWE). Research and development on each type of water electrolysishave been conducted to achieve larger sizes, lower cost, higher performances, and higherefficiencies [6,10–13]. For PEMWE, fluorinated electrolytes are used as the proton exchangepolymer electrolyte membranes. Platinum, which is a catalyst for the hydrogen genera-tion reaction, is used as the cathode catalyst electrode, and iridium (iridium oxide) andMembranes 2021, 11, 861. https://doi.org/10.3390/membranes11110861 https://www.mdpi.com/journal/membraneshttps://www.mdpi.com/journal/membraneshttps://www.mdpi.comhttps://orcid.org/0000-0003-4301-1044https://doi.org/10.3390/membranes11110861https://doi.org/10.3390/membranes11110861https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.3390/membranes11110861https://www.mdpi.com/journal/membraneshttps://www.mdpi.com/article/10.3390/membranes11110861?type=check_update&version=1Membranes 2021, 11, 861 2 of 13ruthenium (ruthenium oxide), which have excellent oxygen evolution activity, are mainlyused as the anode catalyst electrode [13–35]. The aim is to reduce the cost and improvethe performance. Fluorinated electrolytes (PFSA) [36], such as Nafion [6,12,13,15,33,37],Aquivion [6,24,33,38], and radiation-grafted ethylene tetrafluoroethylene (ETFE) mem-branes [39,40], have been reported to exhibit high proton conductivities, high chemicalstabilities, and water electrolysis properties. Non-fluorinated hydrocarbon electrolyte mem-branes, such as sulfonated poly(ether ether ketone) (SPEEK) composite [41,42], polyethy-lene oxide grafted polyether sulfone (PES-g-PEO) [43], sulfonated polysulfone (SPSf) [44],sulfonated poly(phenylene sulfone) (sPPS) [45], and sulfonated poly(arylene ether sul-fone) [46], have been reported. Hydrocarbon-based electrolyte membranes with high glasstransition temperatures are being investigated as alternatives to fluorinated electrolytemembranes because they are expected to operate at elevated temperatures and have theadvantages of low cost and low environmental impact due to the absence of fluorine. Atthe same time, higher operating temperatures (100–200 ◦C) are thought to be one way ofincreasing the efficiency of PEMWE. Moreover, the higher temperatures should improvethe overvoltage of the entire cell due to the kinetic and thermodynamic advantages of thecatalytic electrodes in water splitting, which improve the catalytic electrode activity [6,12],the conductivity of the electrolyte membrane [33], and the interfacial resistance betweenthe electrolyte membrane and the catalytic electrode. Currently, water electrolysis charac-teristics at temperatures above 100 ◦C have been reported, but in those experiments, mostlyfluorinated electrolyte membranes, especially Nafion membranes [33–35,47–52], are used.However, the application of fluorinated electrolyte membranes at temperatures above100 ◦C is difficult for practical use because of the degradation of the thermal, mechanical,and chemical properties [33,53,54]. In other words, the durability of these membranes islower at higher temperatures. There is a need to develop polymer electrolyte membraneswith thermal, mechanical, and chemical stabilities at temperatures above 100 ◦C [5,19,55].We have been studying the sulfonation and crosslinking of PPSU, an industrial engi-neering plastic, as an electrolyte membrane for fuel cells [56–63]. The conductivities andmechanical and chemical properties of the crosslinked SPPSU membranes have been foundto be highly dependent on the degree of sulfonation, heat treatment, activation treatment,and additives. There is a trade-off between the improvement of the proton conductivity byhighly sulfonating polymers and the improvement of physical and chemical stabilities, andwe are trying to improve the performance of CSPPSU membranes to suit the application. Inthis study, we investigated the stabilities of the sulfone groups in water and of the polymerstructure and the MEA properties of the CSPPSU membranes at elevated temperatures inelevated temperature water electrolysis.2. Experimental2.1. MaterialsPPSU (Radel R-5000 NT; Mn = 26,000, Mw = 50,000, Mw/Mn = 1.9, glass transitiontemperature (Tg) = 220 ◦C), an engineering plastic, was provided by Solvay SpecialtyPolymers Japan K.K. Sulfuric acid (H2SO4, 95%), sodium hydroxide (NaOH, 97%), andsodium chloride (NaCl, 99.5%) were purchased from Nacalai Tesque, Inc, Kyoto, Japan,and dimethyl sulfoxide (DMSO, ≥99.5%) was purchased from Sigma-Aldrich Co., Ltd. MO,USA. Deionized (DI) water (15 Mohm-cm) was produced by using a Purelab Option-R 7/15ELGA LabWater apparatus (Veolia Water Solutions & Technologies (VWS) Ltd., England,UK). Dialysis membranes (Sigma-Aldrich Co., Ltd., St. Louis, MO, USA; MWCO = 14,000)were utilized to remove excess acid from the sulfonated PPSU polymer.2.2. Sulfonation of PPSUH2SO4 (95%, 2 L) was placed in a large glass container and heated at 60 ◦C in an oilbath (Advantec, Tokyo, Japan; TBX243RA). Dried PPSU (70.12 g) was added to the H2SO4solution, and sulfonation was allowed to occur at 60 ◦C for 2 days. After sulfonation,the product was precipitated in ice water and collected by filtration. After filtration, theMembranes 2021, 11, 861 3 of 13product was washed until a pH of 7 using a dialysis membrane. After that, the SPPSUpolymer was filtered again to remove impurities and dried by using a freeze dryer (ASONE Corporation, Osaka, Japan; FDV-12AS).2.3. Preparation of CSPPSU MembranesA 20 wt% SPPSU solution was prepared in DMSO solvent. A glass plate (27 cm × 30 cm)was placed on a coater (KIPAE Co., Gyeonggi, Korea; KP-3000VH), and an applicator(Tester Sangyo Co., Ltd., Tokyo, Japan) was set. When the temperature of the coater was80 ◦C, a 20 wt% SPPSU solution was poured onto the glass plate, and coating was carriedout by moving the applicator at a speed of 4.0 mm/min. After evaporation of DMSO at80 ◦C for 1 day, the SPPSU-coated glass plate was transferred to a convection oven (Yamatoscientific Co., Ltd., Tokyo, Japan; DX302) for cross-linking. The SPPSU-coated glass platewas heated at 120 ◦C, 160 ◦C, and 180 ◦C, respectively, for 1 day each. The obtained CSPPSUmembranes were about 20 cm in length, 16 cm in width and 0.07–0.130 mm in thickness.2.4. Activation Treatment of CSPPSU MembranesThe CSPPSU membranes were removed from the glass plates and subjected to activa-tion treatment to remove impurities and to improve the conduction path in the membranes.The activation involved placing the membranes in boiling water for 2 h and heatingovernight at 80 ◦C in a 0.5 M NaOH solution. Then they were placed in boiling water for2 h, in a 1 M H2SO4 solution at 80 ◦C for 2 h, and then in boiling water for 2 h. Finally, theCSPPSU membranes were dried at room temperature and used for evaluation.2.5. Molecular Weight (Mw), Ion Exchange Capacity (IEC), Degree of Sulfonation (D.S.), WaterUptake (W.U.)Gel permeation chromatography (GPC) was performed at 60 ◦C on a Tosoh HLC-8220GPC equipped with a Shodex GPC LF-804 column using N,N-dimethylformamide (DMF)as the eluent. Ion exchange capacity (IEC) was defined as the millimeter equivalent ofsulfonic acid groups per gram of dry sample. A portion of the membrane was soaked in20 mL of 2 M NaCl solution and equilibrated for at least 24 h to replace the protons withsodium ions. The solution was then titrated with 0.01 M NaOH solution. The IEC valuewas calculated using the following equation: IEC (meq/g) = CV/Wdry, where C (mmol/L)is the concentration of the standardized NaOH solution used in the titration (0.01 mol/L),V (L) is the volume of the standardized NaOH solution used in the titration, and Wdry(g) is the mass of the dry membrane. The degree of sulfonation (D.S.) of SPPSU polymerwas calculated by using the following equation: D.S. (sulfonic acid groups/repeat units;R.U.) = [IEC/1000 × Fw(R.U.)]/[1 − (IEC/1000 × Fw(SO3))], where Fw (R.U.) = 400.45and Fw (SO3) = 80.06. The water content (W.U.) of the membrane was calculated as:W.U. (%) = [(Wwet − Wdry) × 100]/Wdry. Wdry was obtained after placing the membranein a dry oven at 80 ◦C for at least 24 h. Wwet was obtained after placing the dried membranein boiling water for 1 h, immediately removing the water from the surface of the membrane,and then measuring the mass.2.6. Small Angle X-ray Scattering (SAXS) Measurements of CSPPSU MembranesThe stabilities of the polymer structure of CSPPSU membranes were measured byusing SAXS (beamline BL15A2 of the Photon Factory in KEK, Tsukuba, Japan [64]) andmeasuring the change with an increase in the temperature from room temperature to 180 ◦Cand a decrease from 180 ◦C to room temperature. The X-ray beam was monochromatized to1.2 Å. The membrane samples were placed on a temperature-controlled stage using 10002L(Linkam Scientific) to perform the measurements. The scattered photons were detected byusing a two-dimensional semiconductor detector (PILATUS3 2M, W1475 × H1679 pixels,Dectris, Switzerland) at a camera distance of 170 cm, and the signal was accumulated for30 s. The X-ray scattering images were averaged for each polar angle to obtain the radialdistribution of the intensities of the scattered X-rays [62]. For SAXS measurements, thetemperature was increased to 30, 80, 120, 150, and 180 ◦C at a rate of 10 ◦C/min withMembranes 2021, 11, 861 4 of 13natural cooling from 180 to 30 ◦C. At each temperature, the samples were stabilized for10–15 min before the measurements.2.7. Water Stability Measurements on the CSPPSU MembranesThe stabilities of the sulfone groups of the CSPPSU membranes were evaluated bycomparing the changes in their W.U., IEC, and conductivities after 3600 h (150 days) inwater at 85 ◦C and 2184 h (91 days) in an autoclave at 150 ◦C. The autoclave vessel usedwas an HU-50 from Sanai-Kagaku (Nagoya, Japan).2.8. Conductivity Measurements on the CSPPSU MembranesThe conductivities of the CSPPSU membranes were measured by using impedancemeasurements on an MTS740 membrane test system (MTS, Scribner Associates, Inc. NC,USA) in the frequency range of 1 Hz–1 MHz with a peak-to-peak voltage of 10 mV using afour-probe method. The electrode was a carbon paper electrode (area = 0.9 cm2) speciallydesigned for the MTS740 system. The samples were equilibrated at the given temperaturesand relative humidities (RH) for 30 min before measurement. At the same time, theconductivities of CSPPSU membranes were determined using fabricated MEAs and a waterelectrolysis evaluation system (Figure 1). The anode and cathode electrodes (area = 4 cm2)were Pt/C catalyst electrodes (EIWA co. ltd., Tokyo, Japan), which were prepared by coating0.3 mg/cm2 of Pt (20 wt% Pt/C, JM) on carbon GDL (SGL 25BC, Sigracet® Meitingen,Germany). The MEA was obtained by hot pressing (Model A-010D, FC-R&D companyKanagawa, Japan) for 20 min at 130 ◦C and 9.8 kN. A water supply unit and a pump wereinstalled on both sides of the anode and cathode, and water was supplied to the cell ata rate of 2.0 mL/min. The electrochemical impedance spectroscopy (EIS) characteristics(AC: 10 mV, frequency: 1 Hz to 20 kHz) were evaluated at cell temperatures in the range of80–150 ◦C. The water was supplied to both sides using an oil bath (EOS-200RD, AS ONECorporation Osaka, Japan) at 80 ◦C without pressurization.Membranes 2021, 11, x FOR PEER REVIEW 5 of 13    Figure 1. Schematic of PEM water electrolysis system. 2.9. Water Electrolysis Measurements A CSPPSU membrane was sandwiched between an IrO2/Ti catalytic electrode for ox-ygen evolution and a Pt/C catalytic electrode for hydrogen generation, and the MEA was fabricated using a hot press method to evaluate the high-temperature water electrolysis characteristics. Figure 1 shows a schematic diagram of the water electrolysis evaluation system, which we developed. 2.9.1. CSPPSU Membranes for Membrane Electrode Assembly (MEA) The size of the CSPPSU membranes for water electrolysis evaluation was 6 cm × 6 cm, and the thicknesses were in the range of 0.07–0.08 mm. 2.9.2. Catalyst Electrodes for MEA The catalytic electrode for oxygen evolution on the anode side (sample 1) was pro-vided by Japan Carlit Co. Sample 1 had good catalytic electrode properties in the previous paper [33]. The cathode electrode for hydrogen generation was a Pt/C catalyst electrode (EIWA co. ltd. Tokyo, Japan), which was prepared by coating 0.3 mg/cm2 of Pt (20 wt% Pt/C, JM) on a carbon GDL (SGL 25BC, Sigracet® Meitingen, Germany). The MEA was obtained by hot pressing (Model A-010D, FC-R&D company Kanagawa, Japan) at 130 °C and 9.8 kN for 20 min. The electrode areas were in the range of 4.00–4.84 cm2. 2.9.3. Single Cell and Water Electrolysis System The single cell consisted of an 8.8 cm × 8.8 cm Al end plate and a 6 cm × 6 cm carbon current collector with 2.2 cm × 2.2 cm channels. The evaluation system that we designed (Figure 1) consisted of a pump to supply water to the anode, an oven to control the tem-perature of the cells and cables, and a PC to control the electrochemical devices. The key point of the system is the use of an oven to maintain the stability of the cell temperature. The entire cell is placed in the oven, which makes the system reproducible while minimiz-ing errors due to temperature.   Figure 1. Schematic of PEM water electrolysis system.2.9. Water Electrolysis MeasurementsA CSPPSU membrane was sandwiched between an IrO2/Ti catalytic electrode foroxygen evolution and a Pt/C catalytic electrode for hydrogen generation, and the MEAwas fabricated using a hot press method to evaluate the high-temperature water electrolysisMembranes 2021, 11, 861 5 of 13characteristics. Figure 1 shows a schematic diagram of the water electrolysis evaluationsystem, which we developed.2.9.1. CSPPSU Membranes for Membrane Electrode Assembly (MEA)The size of the CSPPSU membranes for water electrolysis evaluation was 6 cm × 6 cm,and the thicknesses were in the range of 0.07–0.08 mm.2.9.2. Catalyst Electrodes for MEAThe catalytic electrode for oxygen evolution on the anode side (sample 1) was providedby Japan Carlit Co. Sample 1 had good catalytic electrode properties in the previouspaper [33]. The cathode electrode for hydrogen generation was a Pt/C catalyst electrode(EIWA co. ltd. Tokyo, Japan), which was prepared by coating 0.3 mg/cm2 of Pt (20 wt%Pt/C, JM) on a carbon GDL (SGL 25BC, Sigracet® Meitingen, Germany). The MEA wasobtained by hot pressing (Model A-010D, FC-R&D company Kanagawa, Japan) at 130 ◦Cand 9.8 kN for 20 min. The electrode areas were in the range of 4.00–4.84 cm2.2.9.3. Single Cell and Water Electrolysis SystemThe single cell consisted of an 8.8 cm × 8.8 cm Al end plate and a 6 cm × 6 cmcarbon current collector with 2.2 cm × 2.2 cm channels. The evaluation system that wedesigned (Figure 1) consisted of a pump to supply water to the anode, an oven to controlthe temperature of the cells and cables, and a PC to control the electrochemical devices.The key point of the system is the use of an oven to maintain the stability of the celltemperature. The entire cell is placed in the oven, which makes the system reproduciblewhile minimizing errors due to temperature.2.9.4. Water Electrolysis MeasurementsThe MEAs fabricated by hot pressing were assembled into single cells and then setinto the system for evaluation. The procedures for the measurements have been describedin detail in a previous paper [33].3. Results and Discussions3.1. Properties of the SPPSU PolymerThe yield of the synthesized SPPSU polymer was about 82%, the ion exchange capacityof the SPPSU polymer was 3.68 meq/g, and the degree of sulfonation was about 2. Theweight-averaged molecular weight (Mw) of the SPPSU polymer was 134,472, the numberaverage molecular weight (Mn) was 76,750, and Mw/Mn = 1.75. The structural propertiesof the SPPSU polymers determined by using 1H NMR spectroscopy agree with those inour previous papers [59,62].3.2. Stability of the Sulfone Groups of the CSPPSU MembranesThe thermal and mechanical properties, conductivities, and fuel cell properties of theCSPPSU membranes are reported in our previous paper [62]. For water electrolysis, it isimportant to investigate the stabilities of the membranes under wet conditions because themembranes are always in a wet state. The stabilities of the CSPPSU membranes under wetconditions were investigated in water at 85 ◦C for 3600 h (150 days) and in an autoclave at150 ◦C for 2184 h (91 days). Table 1 shows the IEC, W.U., and conductivity results of theCSPPSU membranes before and after the stability measurements in water. The IEC valueof the CSPPSU membrane treated in water was slightly higher than that of the membranebefore the water test. On the other hand, the IEC value of the CSPPSU membrane after thesubmersion test at 150 ◦C (about 0.5 MPa) was similar to that after the submersion test at85 ◦C. The increase in the IEC value after submersion may be due to the cleavage of theweak part of the sulfone cross-linking in the CSPPSU polymer, resulting in an increasein the number of sulfone groups, which may have caused the increase in the W.U. andconductivity values. However, since the IEC values at 85 ◦C and 150 ◦C were similar andMembranes 2021, 11, 861 6 of 13the appearance of the CSPPSU membrane before and after the submersion tests was thesame, it is unlikely that the sulfone groups of the CSPPSU membrane degraded significantlyunder the test conditions.Table 1. Properties of the CSPPSU membranes before and after water stability tests: IEC, W.U., andconductivity.CSPPSUMembraneIEC(meq/g)W.U.(%)Conductivity (mS/cm), 80 ◦C40% RH 90% RHBefore treatment 1.64 32.5 1.1 12.085 ◦C, 3600 h 1.72 43.5 1.3 14.0150 ◦C, 2184 h 1.71 41.9 1.0 12.73.3. Elevated-Temperature Stabilities of the CSPPSU Membranes Using SAXSFluorinated electrolyte membranes are useful in elevated-temperature water elec-trolysis due to their high proton conductivities and their ability to form good interfaceswith catalytic electrodes. However, the glass transition temperatures of fluorinated elec-trolyte membranes (proton form) have been reported to be in the range of 90–120 ◦C, andapplications above the glass transition temperature have durability issues [36,65]. Fromstability measurements involving Nafion and Aquivion membranes at elevated temper-atures performed by using SAXS, the polymer structures are unstable above 100 ◦C [33].On the other hand, hydrocarbon polymers with high glass transition temperatures, suchas sulfonated poly(ether ether ketone) (SPEEK) [41,42], sulfonated polysulfone (SPSf) [44],sulfonated poly(phenyl sulfone) [45,62], sulfonated poly(arylene ether sulfone) [46,58],poly(benzimidazole) [50], etc., show great promise as electrolyte membranes for waterelectrolysis above 100 ◦C [5,12,13]. The glass transition temperatures of CSPPSU mem-branes are about 200 ◦C [62], making them suitable as electrolyte membranes for waterelectrolysis above 100 ◦C. We first investigated the stabilities of CSPPSU membranes upto 180 ◦C using temperature-controlled SAXS (Figure 2). The polymer structures of theCSPPSU membranes did not change from room temperature to 180 ◦C. In addition, thepolymer structures after spontaneous cooling from 180 ◦C to room temperature were thesame as those at room temperature before the heating was applied. These results suggestthat the polymer structures of the CSPPSU membranes are thermally stable up to 180 ◦C.3.4. Conductivity Properties of the CSPPSU Membranes Using Water Electrolysis SystemMEAs were fabricated using CSPPSU membranes and Pt/C catalytic electrodes, andelectrochemical impedance spectroscopy (EIS) measurements were performed using awater electrolysis system with water supplied to both sides of the anode and cathode(Figure 3). As can be seen from the inset in Figure 3, the resistances of the CSPPSUmembranes decreased with an increase in the cell temperature. The conductivities of theCSPPSU membranes with these resistances were 26 mS/cm at 80 ◦C and 30 mS/cm at150 ◦C (Table 2), which are higher than that at 80 ◦C and 90% RH (12 mS/cm) using theconductivity evaluation system. This may be attributed to the difference in the relativehumidity. The activation energy (Ea) of the conductivities using the water electrolysissystem was determined to be 0.025 eV (2.45 kJ/mol), which is smaller than the activationenergy of 0.128 eV (12.41 kJ/mol) [66] for the Nafion membrane with a good conductionpath. These results suggest that the conduction path of the CSPPSU membranes is saturated(percolated) above 100 ◦C even using this water electrolysis system without pressurizationon both sides of the anode and cathode.Membranes 2021, 11, 861 7 of 13Membranes 2021, 11, x FOR PEER REVIEW 7 of 13   poly(benzimidazole) [50], etc., show great promise as electrolyte membranes for water electrolysis above 100 °C [5,12,13]. The glass transition temperatures of CSPPSU mem-branes are about 200 °C [62], making them suitable as electrolyte membranes for water electrolysis above 100 °C. We first investigated the stabilities of CSPPSU membranes up to 180 °C using temperature-controlled SAXS (Figure 2). The polymer structures of the CSPPSU membranes did not change from room temperature to 180 °C. In addition, the polymer structures after spontaneous cooling from 180 °C to room temperature were the same as those at room temperature before the heating was applied. These results suggest that the polymer structures of the CSPPSU membranes are thermally stable up to 180 °C.  Figure 2. SAXS profiles of CSPPSU membranes at various temperatures. 3.4. Conductivity Properties of the CSPPSU Membranes Using Water Electrolysis System MEAs were fabricated using CSPPSU membranes and Pt/C catalytic electrodes, and electrochemical impedance spectroscopy (EIS) measurements were performed using a water electrolysis system with water supplied to both sides of the anode and cathode (Fig-ure 3). As can be seen from the inset in Figure 3, the resistances of the CSPPSU membranes decreased with an increase in the cell temperature. The conductivities of the CSPPSU membranes with these resistances were 26 mS/cm at 80 °C and 30 mS/cm at 150 °C (Table 2), which are higher than that at 80 °C and 90% RH (12 mS/cm) using the conductivity evaluation system. This may be attributed to the difference in the relative humidity. The activation energy (Ea) of the conductivities using the water electrolysis system was deter-mined to be 0.025 eV (2.45 kJ/mol), which is smaller than the activation energy of 0.128 eV (12.41 kJ/mol) [66] for the Nafion membrane with a good conduction path. These results suggest that the conduction path of the CSPPSU membranes is saturated (percolated) above 100 °C even using this water electrolysis system without pressurization on both sides of the anode and cathode. Figure 2. SAXS profiles of CSPPSU membranes at various temperatures.Membranes 2021, 11, x FOR PEER REVIEW 8 of 13    Figure 3. EIS properties at different cell temperatures with the water supplied at a rate of 2.0 mL/min at the temperature of 80 °C to both anode and cathode sides using PEMWE system. Table 2. Parameters obtained from EIS data (Figure 3).  80 °C 100 °C 120 °C 150 °C R intercept (mohm) 125 120 115 109 Conductivity (mS/cm) 26 27 28 30 3.5. Elevated-Temperature Water Electrolysis To determine the characteristics during elevated-temperature water electrolysis, the MEAs were set in single cells and evaluated at cell temperatures in the range of 80–150 °C while water was supplied at a rate of 2.0 mL/min to the anode side only. Figure 4 shows the current density–voltage (Figure 4a) and EIS (Figure 4b) characteristics as a function of cell temperature. The current densities increased with an increase in the cell temperature. A maximum current density of 456 mA/cm2 was obtained at a cell temperature of 150 °C and 1.8 V. From the EIS characteristics, the resistance of the entire cell tended to decrease with an increase in the cell temperature. In order to analyze the EIS characteristics, a model equivalent circuit, as shown in Figure 5, was used, assuming that the overvoltage occurred mostly on the anode side. Using the model equivalent circuit, we fitted the EIS plot in Figure 4b and calculated the resistance (Rs), interfacial charge transfer resistance (Rct), and CPE of the electrolyte membrane, and the results are summarized in Table 3. The resistance (Rs) of the CSPPSU membrane decreased when the cell temperature increased from 80 to 100 °C but did not change above 100 °C. On the other hand, Rct and CPE, which are the interfacial components between the CSPPSU membrane and the catalytic elec-trode, changed dramatically. As the cell temperature during water electrolysis increased, the capacitance of Rct and CPE (CPE-T) decreased. It is thought that the increase in the cell temperature increases the current density and reduces the interfacial impedance be-tween the membrane and the catalytic electrode. On the other hand, using the same water Figure 3. EIS properties at different cell temperatures with the water supplied at a rate of 2.0 mL/minat the temperature of 80 ◦C to both anode and cathode sides using PEMWE system.Membranes 2021, 11, 861 8 of 13Table 2. Parameters obtained from EIS data (Figure 3).80 ◦C 100 ◦C 120 ◦C 150 ◦CR intercept (mohm) 125 120 115 109Conductivity (mS/cm) 26 27 28 303.5. Elevated-Temperature Water ElectrolysisTo determine the characteristics during elevated-temperature water electrolysis, theMEAs were set in single cells and evaluated at cell temperatures in the range of 80–150 ◦Cwhile water was supplied at a rate of 2.0 mL/min to the anode side only. Figure 4 showsthe current density–voltage (Figure 4a) and EIS (Figure 4b) characteristics as a function ofcell temperature. The current densities increased with an increase in the cell temperature.A maximum current density of 456 mA/cm2 was obtained at a cell temperature of 150 ◦Cand 1.8 V.Membranes 2021, 11, x FOR PEER REVIEW 9 of 13   electrolysis system and catalytic electrodes, the current density for the Nafion115 mem-brane was 1319 mA/Cm2 at 150 °C and 1.8 V. The Rs, Rct, CPE-T, and CPE-p values meas-ured using EIS were 0.024 ohm, 0.08 ohm, 0.038 F, and 0.88, respectively [33]. The perfor-mance of the CSPPSU membrane was lower than that of the Nafion membrane. The IrO2/Ti catalytic electrode was found to form a better interface with the fluorinated elec-trolyte membrane than with the CSPPSU membrane. A few studies on water electrolysis using other hydrocarbon-based electrolyte membranes have been reported [44–46]. Klose et al. have used all-hydrocarbon MEAs to supply water to both sides of the anode and cathode and have reported higher performances than those for Nafion membranes at a cell temperature of 80 °C [45]. However, to the best of our knowledge, ours is the first report on water electrolysis using hydrocarbon-based PEMs at temperatures above 100 °C. Elevated-temperature water electrolysis will enable high-performance production of hydrogen by improving the conductivity of the PEM, the catalytic electrode activity, and the interfacial resistance between the PEM and the catalytic electrode. We believe that research on elevated-temperature water electrolysis using hydrocarbon-based electrolyte membranes will become more active in the future.  Figure 4. (a) Polarization curves and (b) EIS properties at different operating temperatures.  Figure 5. The equivalent circuit used to fit the EIS data. Table 3. Parameters obtained from EIS data (Figure 4b) fitted to the equivalent circuit shown in Figure 5. Temperature (°C) Rs (ohm) Rct (ohm) CPE-T (F) CPE-p 80 0.052 8 0.01 0.7 100 0.049 4.5 0.01 0.7 120 0.049 2.8 0.009 0.75 150 0.049 0.9 0.005 0.85   Figure 4. (a) Polarization curves and (b) EIS properties at different operating temperatures.From the EIS characteristics, the resistance of the entire cell tended to decrease withan increase in the cell temperature. In order to analyze the EIS characteristics, a modelequivalent circuit, as shown in Figure 5, was used, assuming that the overvoltage occurredmostly on the anode side. Using the model equivalent circuit, we fitted the EIS plot inFigure 4b and calculated the resistance (Rs), interfacial charge transfer resistance (Rct),and CPE of the electrolyte membrane, and the results are summarized in Table 3. Theresistance (Rs) of the CSPPSU membrane decreased when the cell temperature increasedfrom 80 to 100 ◦C but did not change above 100 ◦C. On the other hand, Rct and CPE,which are the interfacial components between the CSPPSU membrane and the catalyticelectrode, changed dramatically. As the cell temperature during water electrolysis in-creased, the capacitance of Rct and CPE (CPE-T) decreased. It is thought that the increasein the cell temperature increases the current density and reduces the interfacial impedancebetween the membrane and the catalytic electrode. On the other hand, using the samewater electrolysis system and catalytic electrodes, the current density for the Nafion115membrane was 1319 mA/cm2 at 150 ◦C and 1.8 V. The Rs, Rct, CPE-T, and CPE-p valuesmeasured using EIS were 0.024 ohm, 0.08 ohm, 0.038 F, and 0.88, respectively [33]. Theperformance of the CSPPSU membrane was lower than that of the Nafion membrane.The IrO2/Ti catalytic electrode was found to form a better interface with the fluorinatedelectrolyte membrane than with the CSPPSU membrane. A few studies on water electrol-ysis using other hydrocarbon-based electrolyte membranes have been reported [44–46].Klose et al. have used all-hydrocarbon MEAs to supply water to both sides of the anodeand cathode and have reported higher performances than those for Nafion membranesat a cell temperature of 80 ◦C [45]. However, to the best of our knowledge, ours is thefirst report on water electrolysis using hydrocarbon-based PEMs at temperatures aboveMembranes 2021, 11, 861 9 of 13100 ◦C. Elevated-temperature water electrolysis will enable high-performance productionof hydrogen by improving the conductivity of the PEM, the catalytic electrode activity,and the interfacial resistance between the PEM and the catalytic electrode. We believe thatresearch on elevated-temperature water electrolysis using hydrocarbon-based electrolytemembranes will become more active in the future.Membranes 2021, 11, x FOR PEER REVIEW 9 of 13   electrolysis system and catalytic electrodes, the current density for the Nafion115 mem-brane was 1319 mA/Cm2 at 150 °C and 1.8 V. The Rs, Rct, CPE-T, and CPE-p values meas-ured using EIS were 0.024 ohm, 0.08 ohm, 0.038 F, and 0.88, respectively [33]. The perfor-mance of the CSPPSU membrane was lower than that of the Nafion membrane. The IrO2/Ti catalytic electrode was found to form a better interface with the fluorinated elec-trolyte membrane than with the CSPPSU membrane. A few studies on water electrolysis using other hydrocarbon-based electrolyte membranes have been reported [44–46]. Klose et al. have used all-hydrocarbon MEAs to supply water to both sides of the anode and cathode and have reported higher performances than those for Nafion membranes at a cell temperature of 80 °C [45]. However, to the best of our knowledge, ours is the first report on water electrolysis using hydrocarbon-based PEMs at temperatures above 100 °C. Elevated-temperature water electrolysis will enable high-performance production of hydrogen by improving the conductivity of the PEM, the catalytic electrode activity, and the interfacial resistance between the PEM and the catalytic electrode. We believe that research on elevated-temperature water electrolysis using hydrocarbon-based electrolyte membranes will become more active in the future.  Figure 4. (a) Polarization curves and (b) EIS properties at different operating temperatures.  Figure 5. The equivalent circuit used to fit the EIS data. Table 3. Parameters obtained from EIS data (Figure 4b) fitted to the equivalent circuit shown in Figure 5. Temperature (°C) Rs (ohm) Rct (ohm) CPE-T (F) CPE-p 80 0.052 8 0.01 0.7 100 0.049 4.5 0.01 0.7 120 0.049 2.8 0.009 0.75 150 0.049 0.9 0.005 0.85   Figure 5. The equivalent circuit used to fit the EIS data.Table 3. Parameters obtained from EIS data (Figure 4b) fitted to the equivalent circuit shown inFigure 5.Temperature (◦C) Rs (ohm) Rct (ohm) CPE-T (F) CPE-p80 0.052 8 0.01 0.7100 0.049 4.5 0.01 0.7120 0.049 2.8 0.009 0.75150 0.049 0.9 0.005 0.853.6. Time Dependence of Elevated-Temperature Water ElectrolysisThe stability of the MEA cell containing the CSPPSU membrane at elevated tem-peratures was investigated. Figure 6 shows the current density vs. time characteristics(Figure 6a) and EIS characteristics (Figure 6b) for four runs at 120 ◦C and 1.7 V. The largedrop in current density immediately after the start of the measurement means that thecell overvoltage is large (Figure 6a). In addition, the current density decreased with eachrepetition of the measurement from the first to the fourth time. These results are consistentwith the EIS results. The resistivity of the CSPPSU membrane and the impedance at theinterface between the membrane and the catalytic electrode increased with each repetition.After the fourth measurement, the resistance of the CSPPSU membrane and the impedanceat the interface between the CSPPSU membrane and the catalyst electrode were large. Afterevaluating the stability, the cell was disassembled, and it was found that the CSPPSUmembrane and the catalytic electrode had separated. These results suggest that the increasein the impedance in the EIS characteristics is due to the separation between the membraneand the catalytic electrode. There have been reports on the use of ionomers [23], inkjetprinters [28], and coating methods [29,30] to improve the interface between the electrolytemembrane and the catalytic electrode with the goal of high performance and improveddurability during water electrolysis. In the future, the best method for assembling theCSPPSU membrane and the catalytic electrode to improve the current density and stabilitymust be studied.Membranes 2021, 11, 861 10 of 13Membranes 2021, 11, x FOR PEER REVIEW 10 of 13   3.6. Time Dependence of Elevated-Temperature Water Electrolysis The stability of the MEA cell containing the CSPPSU membrane at elevated temper-atures was investigated. Figure 6 shows the current density vs. time characteristics (Figure 6a) and EIS characteristics (Figure 6b) for four runs at 120 °C and 1.7 V. The large drop in current density immediately after the start of the measurement means that the cell over-voltage is large (Figure 6a). In addition, the current density decreased with each repetition of the measurement from the first to the fourth time. These results are consistent with the EIS results. The resistivity of the CSPPSU membrane and the impedance at the interface between the membrane and the catalytic electrode increased with each repetition. After the fourth measurement, the resistance of the CSPPSU membrane and the impedance at the interface between the CSPPSU membrane and the catalyst electrode were large. After evaluating the stability, the cell was disassembled, and it was found that the CSPPSU membrane and the catalytic electrode had separated. These results suggest that the in-crease in the impedance in the EIS characteristics is due to the separation between the membrane and the catalytic electrode. There have been reports on the use of ionomers [23], inkjet printers [28], and coating methods [29,30] to improve the interface between the electrolyte membrane and the catalytic electrode with the goal of high performance and improved durability during water electrolysis. In the future, the best method for assem-bling the CSPPSU membrane and the catalytic electrode to improve the current density and stability must be studied.  Figure 6. Time dependence of the single cell using sample at 120 °C and 1.7 V: (a) current density vs. time and (b) EIS properties (insert is magnified). 4. Conclusions We have been studying the functionalization of PPSU, a hydrocarbon-based engi-neering plastic, as an alternative material to fluorinated electrolyte membranes. The func-tionalization of PPSU by sulfonation and crosslinking was investigated for the first time for use as an electrolyte membrane for elevated-temperature water electrolysis. The sul-fone groups of the CSPPSU membrane were stable at elevated temperatures (150 °C) and high pressures and in water, and the polymer structure was thermally stable up to 180 °C as determined by using SAXS. From the elevated temperature water electrolysis using the CSPPSU membrane and IrO2/Ti electrode, a current density of 456 mA/cm2 was obtained at 150 °C and 1.8 V. The elevated-temperature stability of the CSPPSU membrane with time was evaluated. There were some issues in the assembly of the CSPPSU membrane and the catalytic electrode. On the other hand, the stabilities of the CSPPSU membranes in water and at elevated temperatures, as determined by using SAXS, suggest that these membranes can be used as electrolyte membranes for elevated-temperature water elec-trolysis. Figure 6. Time dependence of the single cell using sample at 120 ◦C and 1.7 V: (a) current density vs. time and (b) EISproperties (insert is magnified).4. ConclusionsWe have been studying the functionalization of PPSU, a hydrocarbon-based engi-neering plastic, as an alternative material to fluorinated electrolyte membranes. Thefunctionalization of PPSU by sulfonation and crosslinking was investigated for the firsttime for use as an electrolyte membrane for elevated-temperature water electrolysis. Thesulfone groups of the CSPPSU membrane were stable at elevated temperatures (150 ◦C) andhigh pressures and in water, and the polymer structure was thermally stable up to 180 ◦Cas determined by using SAXS. From the elevated temperature water electrolysis using theCSPPSU membrane and IrO2/Ti electrode, a current density of 456 mA/cm2 was obtainedat 150 ◦C and 1.8 V. The elevated-temperature stability of the CSPPSU membrane with timewas evaluated. There were some issues in the assembly of the CSPPSU membrane and thecatalytic electrode. On the other hand, the stabilities of the CSPPSU membranes in waterand at elevated temperatures, as determined by using SAXS, suggest that these membranescan be used as electrolyte membranes for elevated-temperature water electrolysis.Author Contributions: Conceptualization, J.K.; methodology, J.K.; validation, J.K.; formal analysis,J.K.; investigation, J.K.; data curation, J.K., A.O.; writing-Original draft preparation, J.K.; writing—review and editing, J.K., A.O. All authors have read and agreed to the published version of themanuscript.Funding: This research received no external funding.Institutional Review Board Statement: Not applicable.Acknowledgments: This research was partially supported by the Toyota Mobility Fund’s InnovativeResearch Grant for Building a Hydrogen Society.Conflicts of Interest: The authors declare no conflict of interest.References1. Zhang, B.; Zhao, M.; Liu, Q.; Zhang, X.; Fu, Y.; Zhang, E.; Wang, G.; Zhang, Z.; Zhang, S. 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High-performance Nafion membrane modified using a 1,2,4-triazole derivative. ECSarXiv 2019,1–15. [CrossRef]http://doi.org/10.1002/app.44218http://doi.org/10.1016/j.ssi.2017.12.033http://doi.org/10.3390/polym12061354http://www.ncbi.nlm.nih.gov/pubmed/32560108http://doi.org/10.3390/membranes10020031http://www.ncbi.nlm.nih.gov/pubmed/32085526http://doi.org/10.1016/j.jece.2021.105641http://doi.org/10.1063/1.4867351http://doi.org/10.1149/osf.io/3wzse Introduction  Experimental  Materials  Sulfonation of PPSU  Preparation of CSPPSU Membranes  Activation Treatment of CSPPSU Membranes  Molecular Weight (Mw), Ion Exchange Capacity (IEC), Degree of Sulfonation (D.S.), Water Uptake (W.U.)  Small Angle X-ray Scattering (SAXS) Measurements of CSPPSU Membranes  Water Stability Measurements on the CSPPSU Membranes  Conductivity Measurements on the CSPPSU Membranes  Water Electrolysis Measurements  CSPPSU Membranes for Membrane Electrode Assembly (MEA)  Catalyst Electrodes for MEA  Single Cell and Water Electrolysis System  Water Electrolysis Measurements  Results and Discussions  Properties of the SPPSU Polymer  Stability of the Sulfone Groups of the CSPPSU Membranes  Elevated-Temperature Stabilities of the CSPPSU Membranes Using SAXS  Conductivity Properties of the CSPPSU Membranes Using Water Electrolysis System  Elevated-Temperature Water Electrolysis  Time Dependence of Elevated-Temperature Water Electrolysis  Conclusions  References