# Fileset

[2024_Decal_membranes-14-00173.pdf](https://mdr.nims.go.jp/filesets/76b4d547-e99b-45ac-8ab2-70987a5aa9bf/download)

## Creator

[Je-Deok Kim](https://orcid.org/0000-0003-4301-1044)

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[High-Temperature Water Electrolysis Properties of Membrane Electrode Assemblies with Nafion and Crosslinked Sulfonated Polyphenylsulfone Membranes by Using a Decal Method](https://mdr.nims.go.jp/datasets/73eaa75f-5b6a-4789-bb57-68d756f52d19)

## Fulltext

High-Temperature Water Electrolysis Properties of Membrane Electrode Assemblies with Nafion and Crosslinked Sulfonated Polyphenylsulfone Membranes by Using a Decal MethodCitation: Kim, J.-D.High-Temperature Water ElectrolysisProperties of Membrane ElectrodeAssemblies with Nafion andCrosslinked SulfonatedPolyphenylsulfone Membranes byUsing a Decal Method. Membranes2024, 14, 173. https://doi.org/10.3390/membranes14080173Academic Editor: Jin-Soo ParkReceived: 2 July 2024Revised: 30 July 2024Accepted: 7 August 2024Published: 8 August 2024Copyright: © 2024 by the author.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/).membranesArticleHigh-Temperature Water Electrolysis Properties of MembraneElectrode Assemblies with Nafion and Crosslinked SulfonatedPolyphenylsulfone Membranes by Using a Decal MethodJe-Deok KimEnvironmental Circulation Composite Materials Group, Functional Materials Field, Research Center forElectronic and Optical Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044,Japan; kim.jedeok@nims.go.jp; Tel.: +81-29-860-4764; Fax: +81-29-860-4984Abstract: To improve the stability of high-temperature water electrolysis, I prepared membraneelectrode assemblies (MEAs) using a decal method and investigated their water electrolysis properties.Nafion 115 and crosslinked sulfonated polyphenylsulfone (CSPPSU) membranes were used. IrO2was used as the oxygen evolution reaction (OER) catalyst, and Pt/C was used as the hydrogenevolution reaction (HER) catalyst. The conductivity of the CSPPSU membrane at 80 ◦C and 90% RH(relative humidity) is about four times lower than that of the Nafion 115 membrane. Single-cell waterelectrolysis was performed while measuring the current density and performing electrochemicalimpedance spectroscopy (EIS) at cell temperatures from 80 to 150 ◦C and the stability of the currentdensity over time at 120 ◦C and 1.7 V. The current density of water electrolysis using Nafion 115 andCSPPSU membranes at 150 ◦C and 2 V was 1.2 A/cm2 for both. The current density of the waterelectrolysis using the CSPPSU membrane at 120 ◦C and 1.7 V was stable for 40 h. The decal methodimproved the contact between the CSPPSU membrane and the catalyst electrode, and a stable currentdensity was obtained.Keywords: high temperature; polymer electrolyte water electrolysis; nafion membrane; SPPSUpolymer; CSPPSU membrane; decal method; CCM1. IntroductionAbnormal phenomena caused by global climate changes are leading to an economicsociety that can provide a stable energy supply while reducing CO2 emissions on a globalscale. It is hoped that a sustainable energy society will be built through storage and con-version technology using naturally derived energy, such as sunlight, wind, and biomass.Hydrogen is suitable as an energy storage and transport carrier, and it can be producedfrom renewable energy, biomass, water electrolysis, etc., has a high energy weight density,and is suitable for long-term energy storage systems. The most significant advantage of hy-drogen is that it emits zero CO2 when burned and can significantly contribute to achievingcarbon neutrality. The New Energy and Industrial Technology Development Organization(NEDO) predicts that the hydrogen market will reach 160 trillion yen by 2050 [1]. In recentyears, research on the production of green hydrogen by water electrolysis [2], photolysis [3],biomass conversion [4], and thermochemical methods [5] has become active. Among thesegreen hydrogen production methods, water electrolysis systems could absorb the surpluselectricity and output fluctuations from renewable energy sources, enable the further expan-sion of renewable energy use, and contribute to the realization of carbon neutrality. Waterelectrolysis includes polymer electrolyte membrane water electrolysis (PEMWE) [2], anionexchange membrane water electrolysis (AEMWE) [6], alkaline solution water electrolysis(AWE) [7], and solid oxide water electrolysis [8], which aim to reduce costs, increase dura-bility, and improve efficiency. Polymer ion exchange electrolyte membranes are used inwater electrolysis (H+, OH−) [2,6], fuel cells (H+, OH−) [9,10], batteries [11,12], redox flowMembranes 2024, 14, 173. https://doi.org/10.3390/membranes14080173 https://www.mdpi.com/journal/membraneshttps://doi.org/10.3390/membranes14080173https://doi.org/10.3390/membranes14080173https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/membraneshttps://www.mdpi.comhttps://orcid.org/0000-0003-4301-1044https://doi.org/10.3390/membranes14080173https://www.mdpi.com/journal/membraneshttps://www.mdpi.com/article/10.3390/membranes14080173?type=check_update&version=1Membranes 2024, 14, 173 2 of 12batteries (RFBs) (H+, OH−) [13–15], and solar cells [16] and are required to have improvedconductivity, improved mechanical and chemical properties, and lower cost.PEMWE uses a proton exchange polymer electrolyte membrane as the electrolytemembrane, platinum, which is a hydrogen evolution reaction (HER) catalyst, as the cath-ode catalyst, and iridium (iridium oxide) and ruthenium (ruthenium oxide), which areoxygen evolution reaction (OER) catalysts, as the anode catalyst electrode [2]. Fluorine-based polymers, non-fluorine-based polymers, and composite polymers are being studiedas proton-exchange polymer electrolyte membranes, and efforts are being made to findelectrolyte membranes that can withstand the high temperatures of electrolytic systems [17].PEMWE’s high-temperature operation (100–200 ◦C) should significantly improve the cat-alytic activity and the electrolyte membrane conductivity and reduce membrane–electrodeinterfacial resistance due to kinetic and thermodynamic advantages. Therefore, the over-voltage of the entire water electrolysis cell should be significantly reduced, improving thesystem’s efficiency [18–21]. However, an electrolyte membrane that can withstand high-temperature operation has not yet been put into practical use. Therefore, there are manyunknowns, such as membrane properties and catalyst properties during high-temperatureoperation.It has been reported that water electrolysis performance varies depending on the MEAmanufacturing method [22–26]. There are two manufacturing methods for PEMWE-MEAs.One is the CCM method. In the CCM method, the catalyst layers are either directly coatedon the membrane surface or transferred to the membrane surface by a decal process [22–26].An alternative to the CCM method is the PTE method [18,19,24,25]. The PTE method facesthe issue of high resistance when assembling the membrane and electrode [18,19]. In theCCM method, the interfacial resistance can be reduced by directly coating the surface ofthe membrane or by transferring it using the decal method, which allows for good contactbetween the membrane and the catalyst. Therefore, it has been reported that using theCCM method leads to a better water electrolysis performance than the PTE method [24].In our previous paper [18,19], we used an IrO2 catalyst (7.5 mg/cm2) as a poroustransport electrode (PTE) attached to a porous Ti electrode by electroplating the OERelectrode. In addition, the HER electrode used a Pt/C (pt: 0.3 mg/cm2) catalyst coated witha gas diffusion layer (GDL). Nafion 115 and CSPPSU membranes were used as electrolytemembranes. In a high-temperature water electrolysis evaluation of a single cell, the currentdensity at constant voltage decreased rapidly, and stability over time was an issue. Themain reason is the connection between the membrane and the electrode. The PTE was hardand did not stick to the CSPPSU electrolyte membrane when the hot-press method wasused. Therefore, it is thought that the stability of the current density over time at a constantvoltage could not be obtained.In this paper, a decal method, which is a catalyst-coated membrane (CCM) method,was used instead of the IrO2/Ti PTE. An IrO2 catalyst or Pt/C slurry was prepared on aTeflon sheet. These were transferred to a Nafion 115 membrane or a CSPPSU electrolytemembrane using a hot press to prepare membrane electrode assemblies (MEAs). The waterelectrolysis properties, such as current density and stability over time, of the MEAs using adecal method improved.2. Experimental SectionThe electrolyte membranes used were Nafion 115 (ChemoursTM, Wilmington, DE,USA) and CSPPSU. PPSU (Solvay Radel R-5000 NT) (Mw = 50,000) was provided bySolvay Specialty Polymers Japan K.K. (glass transition temperature (Tg) = 220 ◦C). Sodiumhydroxide (NaOH), Sulfuric acid (H2SO4), and sodium chloride (NaCl) were purchasedfrom Nacalai Tesque, Inc, Japan. The treatment method for the Nafion 115 membrane hasbeen described in a previous paper [18]. In addition, the synthesis of SPPSU membranesand the preparation of CSPPSU membranes were performed using the same method asused in previous papers [19,27]. Activation treatment was performed using the samemethod. Briefly, the crosslinking step was performed using heat treatment at 120 (24 h),Membranes 2024, 14, 173 3 of 12160 (24 h), and 180 ◦C (24 h) in this order. After cross-linking, the membranes were treatedwith 0.5 M NaOH (85–87 ◦C, 7 h), boiling water (2 h), 1 M H2SO4 (80 ◦C, 2 h), and boilingwater (2 h) and finally dried at room temperature.The ion exchange capacity (IEC) was defined as the milli-equivalent value of sulfonicacid groups per gram of dry sample. A portion of the membrane was immersed in 20 mLof 2 M NaCl solution and equilibrated for over 24 h to replace protons with sodium ions.This solution was then titrated with 0.01 M NaOH solution. The IEC value was calculatedusing the following formula:IEC (meq/g) = CV/Wdrywhere C (mmol/L) is the concentration of the standardized NaOH aqueous solution used inthe titration (0.01 mol/L), V (L) is the volume of the standardized NaOH aqueous solutionused in the titration, and Wdry (g) is the mass of the dry membrane.The degree of sulfonation (D.S.) of the SPPSU polymer was calculated using thefollowing formula:D.S. (sulfonic acid group/repeat unit; R.U.) = [IEC/(1000 × Fw (R.U.))]/[1 − (IEC/(1000 × Fw (SO3)))]where Fw (R.U.) = 400.45, Fw (SO3) = 80.06.The water uptake (W.U.) of the membrane was calculated using the following formula.W.U. (%) = [(Wwet − Wdry) × 100]/WdryThe mass of the dry membrane (Wdry) was obtained by placing it in a drying ovenat 80 ◦C for 24 h. The mass of the membrane containing water (Wwet) was obtained bysoaking it in boiling water for 1 h, immediately removing it, eliminating the surface water,and measuring the mass.The degree of crosslinking of the membrane was calculated using the followingmethod.Dcrosslink (%) = [(IECbefore annealing − IECafter annealing) × 100]/IECbefore annealingλ of the membrane was calculated using the following formula.λ ([H2O]/[SO3H]) = [1000(Wwet − Wdry)]/18WdryIEC = (10 × W.U.)/(18 × IEC)A stress–strain test on the membrane was performed at room temperature using atensile testing machine (Shimadzu Co., Ltd., EZ-S, Kyoto, Japan). The sample was cutusing a Super Dumbbell Cutter SDMP-100 (manufactured by Dumbbell Co., Ltd., Saitama,Japan).The conductivity of the CSPPSU membrane was determined by measuring the impedanceusing an MTS740 membrane test system (MTS, Scribner Associates, Inc., Southern Pines,NC, USA) and the four-probe method in a frequency range of 1 Hz to 1 MHz and a peak-to-peak voltage of 10 mV. The electrode used was a carbon paper electrode (electrodearea = 0.9 cm2, Scribner Associates, Inc., Southern Pines, NC, USA) exclusively for theMTS740 device.IrO2 (1 mg/cm2) and Pt/C (Pt, 1 mg/cm2) catalysts on PTFE sheets were purchasedfrom Chemix Co. Ltd., Kanagawa, Japan. Nafion was used as the catalyst ionomer. Thearea of the electrode was 1 cm2.The MEAs were made by using a decal method and hot pressing (Model A-010D,FC-R&D Company, Kanagawa, Japan) at 130 ◦C for the Nafion 115 membrane and 165 ◦Cfor the CSPPSU membrane at 9.8 kN for 10 min.An 8.8 cm × 8.8 cm SUS316L end plate was used in the water electrolysis evaluation cell(Ulimeng Eng Co., Ltd., Chungbuk, Korea). In addition, the separator plate (6 cm × 6 cm)Membranes 2024, 14, 173 4 of 12has a 2 cm × 2 cm serpentine channel. A Ti/Pt separator was used on the anode side, anda carbon separator was used on the cathode side. Photographs of a single cell, the anodeside, and the cathode side are shown in Figure 1. No porous transport layer (PTL) wasused on the anode side, and carbon cloth GDL (EIWA Corporation, Tokyo, Japan) was usedon the cathode side. The water on the anode and cathode sides was heated (80 ◦C) using anoil bath and supplied to the cell at a rate of 2.0 mL/min via a pump. Water was suppliedby circulation, and the outlet pressures on the anode and cathode sides were atmosphericpressure. The single cell was placed in a dry oven (DX301, Yamato Scientific CO., Ltd.,Tokyo, Japan), and the oven temperature was used as the temperature of the single cell.Membranes 2024, 14, x 4 of 12   An 8.8 cm × 8.8 cm SUS316L end plate was used in the water electrolysis evaluation cell (Ulimeng Eng Co., Ltd., Chungbuk, Korea). In addition, the separator plate (6 cm × 6 cm) has a 2 cm × 2 cm serpentine channel. A Ti/Pt separator was used on the anode side, and a carbon separator was used on the cathode side. Photographs of a single cell, the anode side, and the cathode side are shown in Figure 1. No porous transport layer (PTL) was used on the anode side, and carbon cloth GDL (EIWA Corporation, Tokyo, Japan) was used on the cathode side. The water on the anode and cathode sides was heated (80 °C) using an oil bath and supplied to the cell at a rate of 2.0 mL/min via a pump. Water was supplied by circulation, and the outlet pressures on the anode and cathode sides were atmospheric pressure. The single cell was placed in a dry oven (DX301, Yamato Scientific CO., Ltd., Tokyo, Japan), and the oven temperature was used as the temperature of the single cell.  Figure 1. Photographs of a single cell, the anode side, and the cathode side; (a) SUS316L end plate, (b) Pt/Ti separator plate, and (c) carbon separator plate. For water electrolysis, electrochemical measurements were performed at cell temper-atures of 80, 100, 120, and 150 °C. For electrochemical measurements, current–voltage and EIS characteristics were investigated using a 1280C electrochemical test system (Solartron Analytical, Farnborough, UK) with a 20 A booster (Toyo corporation, Tokyo, Japan). The applied voltage was 1.4–2.0 V, and the current was measured while sweeping at 10 mV/s. The data were measured 2–3 times under these conditions, and the values with stable cur-rent-voltage characteristics were used. EIS was measured at 1.5 V in the frequency range of 1 Hz–20 kHz. Moreover, we evaluated the current characteristics over time at a cell temperature of 120 °C and 1.7 V. 3. Results The properties of the Nafion 115 and CSPPSU membranes are summarized in Table 1. The IEC of the CSPPSU membrane is approximately 2, which is two times higher than that of Nafion 115. The WU of the CSPPSU membrane is equivalent to Nafion 115, but the λ value is half. The conductivity is about four times lower than Nafion 115. The tensile strength of the CSPPSU membrane is higher than Nafion 115, and the tensile elongation is lower than Nafion 115. The elastic modulus of the CSPPSU membrane is about five times larger than that of Nafion 115. The low λ and conductivities of the CSPPSU mem-branes were attributed to the rigidity of the CSPPSU polymer structure. We are investi-gating ways to improve the conductivity by introducing more sulfonic acid groups per unit of PPSU and plan to report on this in the future. The hydration stability of the CSPPSU membrane showed no decrease in IEC and conductivity even after 2184 h of au-toclaving at 150 °C [19]. However, the hydration stability of the Nafion 115 membrane changed significantly after 2184 h of autoclaving at 150 °C. Specifically, the conductivity of the Nafion 115 membrane decreased by 41% at 80 °C and 90% RH. Figure 1. Photographs of a single cell, the anode side, and the cathode side; (a) SUS316L end plate,(b) Pt/Ti separator plate, and (c) carbon separator plate.For water electrolysis, electrochemical measurements were performed at cell tempera-tures of 80, 100, 120, and 150 ◦C. For electrochemical measurements, current–voltage andEIS characteristics were investigated using a 1280C electrochemical test system (SolartronAnalytical, Farnborough, UK) with a 20 A booster (Toyo corporation, Tokyo, Japan). Theapplied voltage was 1.4–2.0 V, and the current was measured while sweeping at 10 mV/s.The data were measured 2–3 times under these conditions, and the values with stablecurrent-voltage characteristics were used. EIS was measured at 1.5 V in the frequency rangeof 1 Hz–20 kHz. Moreover, we evaluated the current characteristics over time at a celltemperature of 120 ◦C and 1.7 V.3. ResultsThe properties of the Nafion 115 and CSPPSU membranes are summarized in Table 1.The IEC of the CSPPSU membrane is approximately 2, which is two times higher thanthat of Nafion 115. The WU of the CSPPSU membrane is equivalent to Nafion 115, but theλ value is half. The conductivity is about four times lower than Nafion 115. The tensilestrength of the CSPPSU membrane is higher than Nafion 115, and the tensile elongation islower than Nafion 115. The elastic modulus of the CSPPSU membrane is about five timeslarger than that of Nafion 115. The low λ and conductivities of the CSPPSU membraneswere attributed to the rigidity of the CSPPSU polymer structure. We are investigating waysto improve the conductivity by introducing more sulfonic acid groups per unit of PPSUand plan to report on this in the future. The hydration stability of the CSPPSU membraneshowed no decrease in IEC and conductivity even after 2184 h of autoclaving at 150 ◦C [19].However, the hydration stability of the Nafion 115 membrane changed significantly after2184 h of autoclaving at 150 ◦C. Specifically, the conductivity of the Nafion 115 membranedecreased by 41% at 80 ◦C and 90% RH.Membranes 2024, 14, 173 5 of 12Table 1. The Nafion 115 and CSPPSU membranes.Nafion 115 CSPPSUIEC (meq/g), 25 ◦C ∼1.0 1.8Crosslink rate (%), 25 ◦C - 50Water uptake (%), 100 ◦C 38 37l, 25 ◦C 21.1 11.4Elongation strength (MPa), 25 ◦C 31 53Elongation stain (%), 25 ◦C 200 33Flexural modulus (MPa), 25 ◦C 196 986Conductivity, 80 ◦C, 90%RH, mS/cm 47 12The water electrolysis properties of the Nafion 115 and CSPPSU membranes wereinvestigated by fabricating MEAs using a decal method (Figure 2). Figure 2a,b show thecurrent density-voltage (I–V) characteristics at cell temperatures of 80–150 ◦C using MEAswith the Nafion 115 and CSPPSU membranes, respectively. The current density increased,and the voltage decreased with an increase in the cell temperature. The maximum currentdensity of the MEAs with the Nafion 115 and CSPPSU membranes was 1.2 A/cm2 at acell temperature of 150 ◦C and a voltage of 2 V. The voltage and current density valuesat each temperature are summarized in Table 2. The same trends have been reported atlower electrolysis voltages, higher current density, and lower cell resistance due to higheroperating temperatures [21,28,29]. From these results, the high-temperature operation ofwater electrolysis effectively reduces the overvoltage of the entire cell, and high currentdensities are obtained at low voltage.Membranes 2024, 14, x 5 of 12   Table 1. The Nafion 115 and CSPPSU membranes.  Nafion 115 CSPPSU IEC (meq/g), 25 °C ∼1.0 1.8 Crosslink rate (%), 25 °C - 50 Water uptake (%), 100 °C 38 37 l, 25 °C 21.1 11.4 Elongation strength (MPa), 25 °C 31 53 Elongation stain (%), 25 °C 200 33 Flexural modulus (MPa), 25 °C 196 986 Conductivity, 80 °C, 90%RH, mS/cm 47 12 The water electrolysis properties of the Nafion 115 and CSPPSU membranes were investigated by fabricating MEAs using a decal method (Figure 2). Figure 2a,b show the current density-voltage (I–V) characteristics at cell temperatures of 80–150 °C using MEAs with the Nafion 115 and CSPPSU membranes, respectively. The current density increased, and the voltage decreased with an increase in the cell temperature. The maximum current density of the MEAs with the Nafion 115 and CSPPSU membranes was 1.2 A/cm2 at a cell temperature of 150 °C and a voltage of 2 V. The voltage and current density values at each temperature are summarized in Table 2. The same trends have been reported at lower electrolysis voltages, higher current density, and lower cell resistance due to higher oper-ating temperatures [21,28,29]. From these results, the high-temperature operation of water electrolysis effectively reduces the overvoltage of the entire cell, and high current densities are obtained at low voltage.  Figure 2. Polarization curves: (a) Nafion 115 and (b) CSPPSU membranes at different operation tem-peratures. Table 2. Current–voltage properties at different operating temperatures using Nafion 115 and CSPPSU membranes. Cell Temperature (°C) Nafion 115 CSPPSU mA/cm2 at 1.8 V mA/cm2 at 2 V mA/cm2 at 1.8 V mA/cm2 at 2 V 80  659 960 403 734 100  701 1008 455 828 120  758 1068 537 959 150 836 1155 688 1161 Figure 3 shows the results of analyzing the IV characteristics of the Nafion 115 mem-brane (Figure 2a). Figure 3a shows the I–V characteristics of the HFR-free membrane, Fig-ure 3b shows the high-frequency-resistance (HFR) and current density characteristics, and Figure 2. Polarization curves: (a) Nafion 115 and (b) CSPPSU membranes at different operationtemperatures.Table 2. Current–voltage properties at different operating temperatures using Nafion 115 and CSPPSUmembranes.Cell Temperature(◦C)Nafion 115 CSPPSUmA/cm2 at 1.8 V mA/cm2 at 2 V mA/cm2 at 1.8 V mA/cm2 at 2 V80 659 960 403 734100 701 1008 455 828120 758 1068 537 959150 836 1155 688 1161Membranes 2024, 14, 173 6 of 12Figure 3 shows the results of analyzing the IV characteristics of the Nafion 115 mem-brane (Figure 2a). Figure 3a shows the I–V characteristics of the HFR-free membrane,Figure 3b shows the high-frequency-resistance (HFR) and current density characteristics,and Figure 3c,d show the current density and I–V characteristics of HFR free on a log scale.HFR decreased with an increase in the cell temperature (Figure 3b). In addition, due tothe low current density on the log scale and the I–V characteristics of the HFR-free cell,the HFR-free voltage decreased as the cell temperature increased (Figure 3c). It is thoughtthat the higher operating temperature improves the catalyst activity, lowers the reactionovervoltage, and reduces the HFR free voltage [22]. On the other hand, the HFR-freevoltage on a log scale of 960 mA/cm2 decreased from 1.73 V at a cell temperature of 80 ◦Cto 1.65 V at 150 ◦C. The decrease in the HFR-free voltage at a high current density due to anincrease in operating temperature is thought to be due to a decrease in the mass transportloss [22].Membranes 2024, 14, x 6 of 12   Figure 3c,d show the current density and I–V characteristics of HFR free on a log scale. HFR decreased with an increase in the cell temperature (Figure 3b). In addition, due to the low current density on the log scale and the I–V characteristics of the HFR-free cell, the HFR-free voltage decreased as the cell temperature increased (Figure 3c). It is thought that the higher operating temperature improves the catalyst activity, lowers the reaction overvoltage, and reduces the HFR free voltage [22]. On the other hand, the HFR-free volt-age on a log scale of 960 mA/cm2 decreased from 1.73 V at a cell temperature of 80 °C to 1.65 V at 150 °C. The decrease in the HFR-free voltage at a high current density due to an increase in operating temperature is thought to be due to a decrease in the mass transport loss [22].  Figure 3. Electrochemical performance analysis of Nafion 115 membrane (Figure 2a): (a) Polariza-tion curves of the HFR-free cell; (b) HFR vs. current density; (c) HFR-free polarization data at low current densities, plotted on a logarithmic scale and (d) at current densities between 800 and 1200 mA/cm2. Figure 4 shows the results of analyzing the I–V characteristics of the CSPPSU mem-brane (Figure 2b). Figure 4a shows the I–V characteristics of the HFR-free cell, Figure 4b shows the HFR and current density characteristics, and Figure 4c and d show the current density and I–V characteristics of HFR free on a log scale. The characteristics of HFR free voltage and current density due to the increase in cell temperature of water electrolysis using the MEA with the CSPPSU membrane showed the same trend as when using the Nafion 115 membrane, as shown in Figure 3. Although the conductivity of the CSPPSU membrane was about four times lower than that of the Nafion 115 membrane, a similar current density was obtained from high-temperature water electrolysis at 150 °C (Table 1,2). Thus, the CSPPSU membrane with high glass transition temperature [27] can be used as an electrolyte membrane for high-temperature water electrolysis. Figure 3. Electrochemical performance analysis of Nafion 115 membrane (Figure 2a): (a) Polarizationcurves of the HFR-free cell; (b) HFR vs. current density; (c) HFR-free polarization data at low currentdensities, plotted on a logarithmic scale and (d) at current densities between 800 and 1200 mA/cm2.Figure 4 shows the results of analyzing the I–V characteristics of the CSPPSU mem-brane (Figure 2b). Figure 4a shows the I–V characteristics of the HFR-free cell, Figure 4bshows the HFR and current density characteristics, and Figure 4c and d show the currentdensity and I–V characteristics of HFR free on a log scale. The characteristics of HFR freevoltage and current density due to the increase in cell temperature of water electrolysis us-ing the MEA with the CSPPSU membrane showed the same trend as when using the Nafion115 membrane, as shown in Figure 3. Although the conductivity of the CSPPSU membraneMembranes 2024, 14, 173 7 of 12was about four times lower than that of the Nafion 115 membrane, a similar current densitywas obtained from high-temperature water electrolysis at 150 ◦C (Tables 1 and 2). Thus, theCSPPSU membrane with high glass transition temperature [27] can be used as an electrolytemembrane for high-temperature water electrolysis.Membranes 2024, 14, x 7 of 12    Figure 4. Electrochemical performance analysis of CSPPSU membrane (Figure 2b): (a) Polarization curves of HFR-free cell; (b) HFR over current density; (c) HFR-free polarization data at low current densities, plotted on a logarithmic scale and (d) at current densities between 500 and 1200 mA/cm2. After I–V measurements at each temperature using a water electrolysis cell (Figure 2), EIS was performed. Figure 5a,b show Nyquist plots depending on the temperature after I–V measurements at 2 V on the cells containing the Nafion 115 and CSPPSU mem-branes. Figure 5c shows the model equivalent circuit used to analyze the Nyquist plot. In the Nyquist plot for the cells using Nafion 115 and CSPPSU membranes, the impedance decreased as the cell temperature increased. These results are the I–V characteristics under high-temperature operation, and they are consistent with the results when the voltage decreased and the current density increased as the cell temperature increased (Figure 2). In other words, high-temperature operation lowered the cell overvoltage and mem-brane resistance, resulting in a high current density at low voltage. The results obtained by fitting the Nyquist plot using a model equivalent circuit are summarized in Table 3. It is difficult to fit using the membrane resistance (Rs), charge transfer resistance (Rct), dou-ble layer capacitance (Cdl), and constant phase element (Cpe) [18–20,23,30], which are commonly used in equivalent circuits. Therefore, fitting was performed using the equiva-lent circuit shown in Figure 5c. The resistance (Rs) of the cells with the Nafion 115 and CSPPSU membranes decreased as the cell temperature increased. In addition, R1 and R2 decreased as the cell temperature increased. A more detailed study is required regarding the components of R1 and R2. In this study, R1 was the interfacial charge transfer re-sistance, and R2 was the resistance due to water (water vapor) and bubbles (oxygen) gen-erated at the membrane and electrode interface or between the IrO2 catalyst layer and the ionomer. Since the R2 and C2 components were not present when a porous IrO2 electrode catalyst was used [18,19], they are the EIS characteristics of an MEA cell fabricated using the decal method. Figure 4. Electrochemical performance analysis of CSPPSU membrane (Figure 2b): (a) Polarizationcurves of HFR-free cell; (b) HFR over current density; (c) HFR-free polarization data at low currentdensities, plotted on a logarithmic scale and (d) at current densities between 500 and 1200 mA/cm2.After I–V measurements at each temperature using a water electrolysis cell (Figure 2),EIS was performed. Figure 5a,b show Nyquist plots depending on the temperature afterI–V measurements at 2 V on the cells containing the Nafion 115 and CSPPSU membranes.Figure 5c shows the model equivalent circuit used to analyze the Nyquist plot. In theNyquist plot for the cells using Nafion 115 and CSPPSU membranes, the impedancedecreased as the cell temperature increased. These results are the I–V characteristics underhigh-temperature operation, and they are consistent with the results when the voltagedecreased and the current density increased as the cell temperature increased (Figure 2).In other words, high-temperature operation lowered the cell overvoltage and mem-brane resistance, resulting in a high current density at low voltage. The results obtainedby fitting the Nyquist plot using a model equivalent circuit are summarized in Table 3.It is difficult to fit using the membrane resistance (Rs), charge transfer resistance (Rct),double layer capacitance (Cdl), and constant phase element (Cpe) [18–20,23,30], whichare commonly used in equivalent circuits. Therefore, fitting was performed using theequivalent circuit shown in Figure 5c. The resistance (Rs) of the cells with the Nafion 115and CSPPSU membranes decreased as the cell temperature increased. In addition, R1 andR2 decreased as the cell temperature increased. A more detailed study is required regardingthe components of R1 and R2. In this study, R1 was the interfacial charge transfer resistance,and R2 was the resistance due to water (water vapor) and bubbles (oxygen) generated atthe membrane and electrode interface or between the IrO2 catalyst layer and the ionomer.Membranes 2024, 14, 173 8 of 12Since the R2 and C2 components were not present when a porous IrO2 electrode catalystwas used [18,19], they are the EIS characteristics of an MEA cell fabricated using the decalmethod.Membranes 2024, 14, x 8 of 12    Figure 5. Nyquist plots measured at different operation temperatures of cells with (a) Nafion 115 and (b) CSPPSU membranes; (c) equivalent circuit used to fit the EIS data. Table 3. Parameters obtained from the Nyquist plot (Figure 4a,b) fitted to the equipment circuit shown in Figure 4c.  Nafion 115 CSPPSU Cell Temperature (°C) Cell Temperature (°C) 80 100 120 150 80 100 120 150 Rs (ohm) 0.27 0.26 0.24 0.22 0.37 0.33 0.29 0.24 R1 (ohm) 0.1 0.08 0.07 0.06 0.19 0.16 0.14 0.12 C1 (mF) 0.474 0.481 0.417 0.412 0.447 0.354 0.378 0.422 R2 (ohm) 0.36 0.31 0.25 0.18 1.1 0.97 0.76 0.52 C2 (mF) 90.8 78.0 71.0 68.8 40.9 36.3 34.3 34.7 Figure 6 shows the time dependence of the current density for water electrolysis con-ducted five times at a cell temperature of 120 °C and a voltage of 1.7 V. The temporal stability of the cell using the Nafion 115 membrane decreased by approximately 8.5% (0.47 A/cm2 in the first 7 h; 0.43 A/cm2 in the fifth 7 h) from the first to the fifth repeated meas-urement. On the other hand, regarding the temporal stability of the cell using the CSPPSU membrane, the current density did not decrease even after repeated measurements from the first to the fifth time. From the EIS properties before and after the time dependence of the Nafion 115 and CSPPSU membranes, the resistance of the Nafion 115 membrane is not significantly different before and after, but there is a difference in the resistance of the CSPPSU membrane, which may be due to the thinner CSPPSU membrane. Figure 5. Nyquist plots measured at different operation temperatures of cells with (a) Nafion 115 and(b) CSPPSU membranes; (c) equivalent circuit used to fit the EIS data.Table 3. Parameters obtained from the Nyquist plot (Figure 4a,b) fitted to the equipment circuitshown in Figure 4c.Nafion 115 CSPPSUCell Temperature (◦C) Cell Temperature (◦C)80 100 120 150 80 100 120 150Rs (ohm) 0.27 0.26 0.24 0.22 0.37 0.33 0.29 0.24R1 (ohm) 0.1 0.08 0.07 0.06 0.19 0.16 0.14 0.12C1 (mF) 0.474 0.481 0.417 0.412 0.447 0.354 0.378 0.422R2 (ohm) 0.36 0.31 0.25 0.18 1.1 0.97 0.76 0.52C2 (mF) 90.8 78.0 71.0 68.8 40.9 36.3 34.3 34.7Figure 6 shows the time dependence of the current density for water electrolysisconducted five times at a cell temperature of 120 ◦C and a voltage of 1.7 V. The temporalstability of the cell using the Nafion 115 membrane decreased by approximately 8.5%(0.47 A/cm2 in the first 7 h; 0.43 A/cm2 in the fifth 7 h) from the first to the fifth repeatedmeasurement. On the other hand, regarding the temporal stability of the cell using theCSPPSU membrane, the current density did not decrease even after repeated measurementsfrom the first to the fifth time. From the EIS properties before and after the time dependenceof the Nafion 115 and CSPPSU membranes, the resistance of the Nafion 115 membrane isnot significantly different before and after, but there is a difference in the resistance of theCSPPSU membrane, which may be due to the thinner CSPPSU membrane.Membranes 2024, 14, 173 9 of 12Membranes 2024, 14, x 9 of 12    Figure 6. Time dependence of the single cell with (a) Nafion 115 and (b) CSPPSU membranes at 120 °C and 1.7 V. Figure 7 shows a comparison of the time dependence of a single cell of an MEA with a Nafion 115 or a CSPPSU membrane made using the decal method and a porous IrO2 electrode (PTE) at 120 °C and 1.7 V. During the measurement of the water electrolysis properties of the MEA with an electrode (PTE) [18,19] made by coating IrO2 on the surface of porous Ti by electroplating, the current density decreased with time. In particular, the decrease in current density over time was much greater for the MEA with the CSPPSU membrane than with the Nafion 115 membrane. On the other hand, the stability of the cells with the CSPPSU and Nafion 115 membranes made using the decal method during water electrolysis was largely improved. Table 4 summarizes this research and literature evaluating water electrolysis stability at temperatures above 100 °C. The stability of the cell with the CSPPSU membrane made using the decal method during water electrolysis was found to be more durable than those reported in other literature, and it is expected to be used as a high-temperature electrolyte membrane.  Figure 7. Comparison of the time dependence of a single cell of an MEA with a Nafion 115 or a CSPPSU membrane made using a decal method and a porous IrO2 electrode at 120 °C and 1.7 V [18,19]. Figure 6. Time dependence of the single cell with (a) Nafion 115 and (b) CSPPSU membranes at120 ◦C and 1.7 V.Figure 7 shows a comparison of the time dependence of a single cell of an MEA witha Nafion 115 or a CSPPSU membrane made using the decal method and a porous IrO2electrode (PTE) at 120 ◦C and 1.7 V. During the measurement of the water electrolysisproperties of the MEA with an electrode (PTE) [18,19] made by coating IrO2 on the surfaceof porous Ti by electroplating, the current density decreased with time. In particular, thedecrease in current density over time was much greater for the MEA with the CSPPSUmembrane than with the Nafion 115 membrane. On the other hand, the stability of thecells with the CSPPSU and Nafion 115 membranes made using the decal method duringwater electrolysis was largely improved. Table 4 summarizes this research and literatureevaluating water electrolysis stability at temperatures above 100 ◦C. The stability of the cellwith the CSPPSU membrane made using the decal method during water electrolysis wasfound to be more durable than those reported in other literature, and it is expected to beused as a high-temperature electrolyte membrane.Membranes 2024, 14, x 9 of 12    Figure 6. Time dependence of the single cell with (a) Nafion 115 and (b) CSPPSU membranes at 120 °C and 1.7 V. Figure 7 shows a comparison of the time dependence of a single cell of an MEA with a Nafion 115 or a CSPPSU membrane made using the decal method and a porous IrO2 electrode (PTE) at 120 °C and 1.7 V. During the measurement of the water electrolysis properties of the MEA with an electrode (PTE) [18,19] made by coating IrO2 on the surface of porous Ti by electroplating, the current density decreased with time. In particular, the decrease in current density over time was much greater for the MEA with the CSPPSU membrane than with the Nafion 115 membrane. On the other hand, the stability of the cells with the CSPPSU and Nafion 115 membranes made using the decal method during water electrolysis was largely improved. Table 4 summarizes this research and literature evaluating water electrolysis stability at temperatures above 100 °C. The stability of the cell with the CSPPSU membrane made using the decal method during water electrolysis was found to be more durable than those reported in other literature, and it is expected to be used as a high-temperature electrolyte membrane.  Figure 7. Comparison of the time dependence of a single cell of an MEA with a Nafion 115 or a CSPPSU membrane made using a decal method and a porous IrO2 electrode at 120 °C and 1.7 V [18,19]. Figure 7. Comparison of the time dependence of a single cell of an MEA with a Nafion 115 ora CSPPSU membrane made using a decal method and a porous IrO2 electrode at 120 ◦C and1.7 V [18,19].Membranes 2024, 14, 173 10 of 12Table 4. Degradation experiments from the literature involving high temperature (>90 ◦C) operation.Ref. Temp. (◦C) Pressure(Bar)Opera. Cond. Operating Time(h)MembraneCatalyst (Loading)PTL or PTE orCCMGDL Degradation RateAnode(mg/cm2)Cathode(mg/cm2)This study 120 1 1.7 V,0.58 A/cm2 37 Nafion 115 IrO2 (1) Pt (1) CCM (Decal),Pt backingCarbon cloth(0.291 mm) −4.1 mA/cm2/hThis study 120 1 1.7 V,0.42 A/cm2 40 CSPPSU IrO2 (1) Pt (1) CCM (Decal),Pt backingCarbon cloth(0.291 mm) 0 mA/cm2/h[18] 120 1 1.7 V,0.31 A/cm2 8 Nafion 115 IrO2 (7.5) Pt (0.3)PTE: Ti powderporous sheets(plating) (500 mm)Carbon fiberpaper (0.235 mm) −18.9 mA/cm2/h[19] 120 1 1.7 V,0.03 A/cm2 8 CSPPSU IrO2 (7.5) Pt (0.3)PTE: Ti powderporous sheets(plating) (500 mm)Carbon fiberpaper (0.235 mm)Separation ofmembrane andelectrode[31] 120 3 1.5 V,0.4 A/cm2 300 CompositeSiO2-Nafion IrO2 (5) Pt (0.8) CCM, Ti backing Carbon cloth −0.39 mA/cm2/h[32] 130 1 1.9 V,0.35 A/cm2 ~1200 PA-dopedAquivion IrO2 (0.7) Pt (0.7)CCM (Decal)Ta-coated stainlesssteel feltNon-wovencarbon cloth +0.09 mV/h[33] 150 5 1.7 V,1.2 A/cm2 160 Nafion 117 IrO2 (0.8) Pt (0.5) PTE: Ti felt Pt-based GDE −2.5 mA/cm2/h[34] 120 2.5 1.72 V,1.25 A/cm2300 (e-dep) or150 (spray) Nafion 212 IrO2 (0.4) Pt (0.4) PTE: Ti-based Carbon fiberpaper (0.325 mm)−1.5 mA/cm2/h−3.93 mA/cm2/hMembranes 2024, 14, 173 11 of 124. ConclusionsThe decal method was applied to obtain the time at which the CSPPSU membrane wasstable in high-temperature water electrolysis. It was found that the stability of the CSPPSUand the Nafion 115 membranes was greatly improved by adopting the decal methodcompared to the PTE method used in the previous paper. And also, these results indicatethat the CSPPSU membrane can be used as a high-temperature electrolyte membrane andthat the decal method is effective for preparing MEAs. On the other hand, when the cellwas disassembled after 40 h of stability evaluation, the CSPPSU membrane and the catalyston the anode side were found to have peeled off. It became clear that an ionomer that iscompatible with the CSPPSU membrane is necessary to ensure the long-term stability ofwater electrolysis using the CSPPSU membrane.Funding: Part of this research was supported by the Toyota Mobility Fund’s “Innovative ResearchGrant for Building a Hydrogen Society”.Institutional Review Board Statement: Not applicable.Data Availability Statement: The data presented in this study are contained within the article, furtherinquiries can be directed to the corresponding author.Conflicts of Interest: The authors declare no conflict of interest.References1. NEDO Hydrogen Energy White Paper; NEDO: Tokyo, Japan, 2015.2. Carmo, M.; Fritz, D.L.; Mergel, J.; Stolten, D. A comprehensive review on PEM water electrolysis. Int. J. Hydrog. Energy 2013, 38,4901–4934. [CrossRef]3. Nandal, V.; Shoji, R.; Matsuzaki, H.; Furube, A.; Lin, L.; Hisatomi, T.; Kaneko, M.; Yamashita, K.; Domen, K.; Seki, K. Unveilingcharge dynamics of visible light absorbing oxysulfide for efficient overall water splitting. Nat. Commun. 2021, 12, 7055–7062.[CrossRef] [PubMed]4. Jara-Cobos, L.; Abril-Gonzalez, M.; Pinos-Velez, V. Production of hydrogen from lignocellulosic biomass: A review of technologies.Catalysts 2023, 13, 766. [CrossRef]5. Ozcan, H.; El-Emam, R.S.; Horri, B.A. Thermochemical looping technologies for clean hydrogen production—Current status andrecent advances. J. Clean. Prod. 2023, 382, 135295–135308. [CrossRef]6. Li, D.; Motz, A.R.; Bae, C.; Fujimoto, C.; Yang, G.; Zhang, F.-Y.; Ayers, K.E.; Kim, Y.S. Durability of anion exchange membranewater electrolyzers. Energy Environ. Sci. 2021, 14, 3393–3419. [CrossRef]7. Zhou, H.; Yu, F.; Zhu, Q.; Sun, J.; Qin, F.; Yu, L.; Bao, J.; Yu, Y.; Chen, S.; Ren, Z. Water splitting by electrolysis at high currentdensities under 1.6 volts. Energy Environ. Sci. 2018, 11, 2858–2864. [CrossRef]8. Vostakola, M.F.; Ozcan, H.; El-Emam, R.S.; Horri, B.A. Recent advances in high-temperature steam electrolysis with solid oxideelectrolyzes for green hydrogen production. Energies 2023, 16, 3327. [CrossRef]9. Jiao, K.; Xuan, J.; Du, Q.; Bao, Z.; Xie, B.; Wang, B.; Zhao, Y.; Fan, L.; Wang, H.; Hou, Z.; et al. Designing the next generation ofproton-exchange membrane fuel cells. Nature 2021, 595, 361–369. [CrossRef] [PubMed]10. Arges, C.G.; Wang, L.; Parrondo, J.; Ramani, V. Best practices for investigating anion exchange membrane suitability for alkalineelectrochemical devices: Case study using quaternary ammonium poly(2,6-dimethyl 1,4-phenylene)oxide anion exchangemembranes. J. Electrochem. Soc. 2013, 160, F1258–F1274. [CrossRef]11. Amici, J.; Torchio, C.; Versaci, D.; Dessantis, D.; Marchisio, A.; Caldera, F.; Bella, F.; Francia, C.; Bodoardo, S. Nanosponge-basedcomposite gel polymer electrolyte for safer Li-O2 batteries. Polymers 2021, 13, 1625. [CrossRef]12. Piana, G.; Ricciardi, M.; Bella, F.; Cucciniello, R.; Proto, A.; Gerbaldi, C. Poly(glycidyl ether)s recycling from industrial waste andfeasibility study of reuse as electrolytes in sodium-based batteries. Chem. Eng. J. 2020, 382, 122934–122940. [CrossRef]13. Ohira, A.; Sakata, W.; Ishida, E.; Mitsuru, T.; Sato, Y. Application of high-performance hydrocarbon-type sulfonated polyethersul-fone for vanadium redox-flow battery. Int. J. Energy Res. 2021, 45, 19405–19412. [CrossRef]14. Zhang, B.; Zhao, M.; Liu, Q.; Zhang, X.; Fu, Y.; Zhang, E.; Wang, G.; Zhang, Z.; Zhang, S. Advanced anion exchange membraneswith selective swelling-induced ion transport channels for vanadium flow battery application. J. Membr. Sci. 2022, 642, 119985–119992. [CrossRef]15. Thiam, B.G.; Vaudreuil, S. Review—Recent membranes for Vanadium redox flow batteries. J. Electrochem. Soc. 2021, 168,070553–070573. [CrossRef]16. de Haro, J.C.; Tatsi, E.; Fagiolari, L.; Bonomo, M.; Barolo, C.; Turri, S.; Bella, F.; Griffini, G. Lignin-based polymer electrolytemembranes for sustainable aqueous dye-sensitized solar cells. ACS Sustain. Chem. Eng. 2021, 9, 8550–8560. [CrossRef] [PubMed]17. Corti, H.R. Polymer electrolytes for low and high temperature PEM electroyzers. Curr. Opin. Electrochem. 2022, 36, 101109–101116.[CrossRef]https://doi.org/10.1016/j.ijhydene.2013.01.151https://doi.org/10.1038/s41467-021-27199-3https://www.ncbi.nlm.nih.gov/pubmed/34876590https://doi.org/10.3390/catal13040766https://doi.org/10.1016/j.jclepro.2022.135295https://doi.org/10.1039/D0EE04086Jhttps://doi.org/10.1039/C8EE00927Ahttps://doi.org/10.3390/en16083327https://doi.org/10.1038/s41586-021-03482-7https://www.ncbi.nlm.nih.gov/pubmed/34262215https://doi.org/10.1149/2.049311jeshttps://doi.org/10.3390/polym13101625https://doi.org/10.1016/j.cej.2019.122934https://doi.org/10.1002/er.7031https://doi.org/10.1016/j.memsci.2021.119985https://doi.org/10.1149/1945-7111/ac163chttps://doi.org/10.1021/acssuschemeng.1c01882https://www.ncbi.nlm.nih.gov/pubmed/34239783https://doi.org/10.1016/j.coelec.2022.101109Membranes 2024, 14, 173 12 of 1218. Kim, J.D.; Ohira, A. Water electrolysis using a porous IrO2/Ti/IrO2 catalyst electrode and Nafion membranes at elevatedtemperatures. Membranes 2021, 11, 330. [CrossRef] [PubMed]19. Kim, J.; Ohira, A. Crosslinked sulfonated polyphenylsulfone (CSPPSU) membranes for elevated-temperature PEM waterelectrolysis. Membranes 2021, 11, 861. [CrossRef]20. Bonanno, M.; Müller, K.; Bensmann, B.; Hanke-Rauschenbach, R.; Peach, R.; Thiele, S. Evaluation of the efficiency of an elevatedtemperature proton exchange membrane water electrolysis system. J. Electrochem. Soc. 2021, 168, 094504–094517. [CrossRef]21. Bonanno, M.; Müller, K.; Bensmann, B.; Hanke-Rauschenbach, R.; Aili, D.; Franken, T.; Chromik, A.; Peach, R.; Freiberg, A.T.S.;Thiele, S. Review and prospects of PEM water electrolysis at elevated temperature operation. Adv. Mater. Technol. 2024, 9,2300281–2300305. [CrossRef]22. Holzapfel, P.; Bühler, M.; Van Pham, C.; Hegge, F.; Böhm, T.; McLaughlin, D.; Breitwieser, M.; Thiele, S. Directly coated membraneelectrode assemblies for proton exchange membrane water electrolysis. Electrochem. Commun. 2020, 110, 106640–106644.[CrossRef]23. Mandal, M.; Valls, A.; Gangnus, N.; Secanell, M. Analysis of inkjet printed catalyst coated membranes for polymer electrolyteelectrolyzers. J. Electrochem. Soc. 2018, 165, F543–F552. [CrossRef]24. Buhler, M.; Holzapfel, P.; McLaughlin, D.; Thiele, S. From catalyst coated membranes to porous transport electrode basedconfigurations in PEM water electrolyzers. J. Electrochem. Soc. 2019, 166, F1070–F1078. [CrossRef]25. Liu, C.; Carmo, M.; Bender, G.; Everwand, A.; Lickert, T.; Young, J.L.; Smolinka, T.; Stolten, D.; Lehnert, W. Performanceenhancement of PEM electrolyzers through iridium-coated titanium porous transport layers. Electrochem. Commun. 2018, 97,96–99. [CrossRef]26. Bernt, M.; Gasteiger, H.A. Influence of ionomer content in IrO2/TiO2 electrodes on PEM water electrolyzer performance. J.Electrochem. Soc. 2016, 163, F3179–F3189. [CrossRef]27. Kim, J.-D.; Ohira, A.; Nakao, H. Chemically crosslinked sulfonated polyphenylsulfone (CSPPSU) membranes for PEM fuel cells.Membranes 2020, 10, 31. [CrossRef] [PubMed]28. Garbe, S.; Futter, J.; Schmidt, T.J.; Gubler, L. Insight into elevated temperature and thin membrane application for high efficiencyin polymer electrolyte water electrolysis. Electrochim. Acta 2021, 377, 138046–138057. [CrossRef]29. Li, H.; Huang, S.; Guan, C.; Wang, H.; Nakajima, H.; Ito, K.; Wang, Y. Experimental optimization of the Nafion® ionomer contentin the catalyst layer for polymer electrolyte membrane water electrolysis at high temperatures. Front. Energy Res. 2023, 11,1313451–1313459. [CrossRef]30. Xu, W.; Scott, K.; Basu, S. Performance of a high temperature polymer electrolyte membrane water electrolyser. J. Power Sources2011, 196, 8918–8924. [CrossRef]31. Antonucci, V.; Di Blasi, A.; Baglio, V.; Ornelas, R.; Matteucci, F.; Ledesma-Garcia, J.; Arriaga, L.; Aricò, A. High temperatureoperation of a composite membrane-based solid polymer electrolyte water electrolyser. Electrochim. Acta 2008, 53, 7350–7356.[CrossRef]32. Xu, J.; Aili, D.; Li, Q.; Christensen, E.; Jensen, J.O.; Zhang, W.; Hansen, M.K.; Liu, G.; Wang, X.; Bjerrum, N.J. Oxygen evolutioncatalysts on supports with a 3-D ordered array structure and intrinsic proton conductivity for proton exchange membrane steamelectrolysis. Energy Environ. Sci. 2014, 7, 820–830. [CrossRef]33. Mališ, J.; Mazúr, P.; Paidar, M.; Bystron, T.; Bouzek, K. Nafion117 stability under conditions of PEM water electrolysis at elevatedtemperature and pressure. Int. J. Hydraul. Eng. 2016, 41, 2177–2188.34. Choe, S.; Lee, B.-S.; Cho, M.K.; Kim, H.-J.; Henkensmeier, D.; Yoo, S.J.; Kim, J.Y.; Lee, S.Y.; Park, H.S. Electrodeposited IrO2/Tielectrodes as durable and cost-effective anodes in high-temperature polymer-membrane-electrolyte water electrolyzers. Appl.Catal. B Environ. Energy 2018, 226, 289–294. [CrossRef]Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.3390/membranes11050330https://www.ncbi.nlm.nih.gov/pubmed/33946195https://doi.org/10.3390/membranes11110861https://doi.org/10.1149/1945-7111/ac2188https://doi.org/10.1002/admt.202300281https://doi.org/10.1016/j.elecom.2019.106640https://doi.org/10.1149/2.1101807jeshttps://doi.org/10.1149/2.0581914jeshttps://doi.org/10.1016/j.elecom.2018.10.021https://doi.org/10.1149/2.0231611jeshttps://doi.org/10.3390/membranes10020031https://www.ncbi.nlm.nih.gov/pubmed/32085526https://doi.org/10.1016/j.electacta.2021.138046https://doi.org/10.3389/fenrg.2023.1313451https://doi.org/10.1016/j.jpowsour.2010.12.039https://doi.org/10.1016/j.electacta.2008.04.009https://doi.org/10.1039/c3ee41438hhttps://doi.org/10.1016/j.apcatb.2017.12.037 Introduction  Experimental Section  Results  Conclusions  References