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

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[Water Electrolysis Using a Porous IrO2/Ti/IrO2 Catalyst Electrode and Nafion Membranes at Elevated Temperatures](https://mdr.nims.go.jp/datasets/66c1543d-5866-4917-b6b3-a65fd1c62c0c)

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Water Electrolysis Using a Porous IrO2/Ti/IrO2 Catalyst Electrode and Nafion Membranes at Elevated TemperaturesmembranesArticleWater Electrolysis Using a Porous IrO2/Ti/IrO2 CatalystElectrode and Nafion Membranes at Elevated TemperaturesJe-Deok Kim 1,* and Akihiro Ohira 2�����������������Citation: Kim, J.-D.; Ohira, A. WaterElectrolysis Using a PorousIrO2/Ti/IrO2 Catalyst Electrode andNafion Membranes at ElevatedTemperatures. Membranes 2021, 11,330. https://doi.org/10.3390/membranes11050330Academic Editor: Orlando CoronellReceived: 27 March 2021Accepted: 28 April 2021Published: 30 April 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 MaterialsScience (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2 Energy Storage Technology Group, Research Institute for Energy Conservation, National Institute ofAdvanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan;a-oohira@aist.go.jp* Correspondence: kim.jedeok@nims.go.jp; Tel.: +81-29-860-4764; Fax: +81-29-860-4984Abstract: Porous IrO2/Ti/IrO2 catalyst electrodes were obtained by coating IrO2 on both sides ofthree types of porous Ti powder sheets (sample 1, sample 2, and sample 3) using different surfacetreatment methods, and a hydrogen evolution catalyst electrode was obtained by coating Pt/C oncarbon gas diffusion layers. A Nafion115 membrane was used as an electrolyte for the membraneelectrode assemblies (MEA). Water electrolysis was investigated at cell temperatures up to 150 ◦C,and the electrical characteristics of the three types of porous IrO2/Ti/IrO2 catalyst electrodes wereinvestigated. The sheet resistance of sample 1 was higher than those of samples 2 and 3, althoughduring water electrolysis, a high current density was observed due to the nanostructure of the IrO2catalyst. In addition, the structural stabilities of Nafion and Aquivion membranes up to 150 ◦C wereinvestigated by using small angle X-ray scattering (SAXS). The polymer structures of Nafion andAquivion membranes were stable up to 80 ◦C, whereas the crystalline domains grew significantlyabove 120 ◦C. In other words, the initial polymer structure did not recover after the sample washeated above the glass transition temperature.Keywords: polymer electrolyte; nafion membrane; porous IrO2/Ti/IrO2 catalyst electrode; waterelectrolysis; elevated temperature1. IntroductionGlobal warming is driving societies that burn fossil fuels for energy, which produceshuge amounts of CO2, towards renewable and sustainable energies. In particular, the use ofrenewable energy, such as solar and wind power, is drawing attention because it does notemit CO2. However, the generation of electricity from photovoltaic cells and wind powergenerators is variable, and large-capacity energy storage systems are required for efficientuse. Energy storage methods include physical energy storage (flywheel, compressed-airenergy storage (CAES), pumping) and chemical energy storage (redox flow battery (RFB),sodium-sulfur battery (NAS), Li-ion battery (LIB), methane, hydrogen (H2)). In orderto supply stable renewable energy, systems with high energy densities and conversionefficiencies are required. Hydrogen has a high energy density, is a CO2-free energy source,and is suitable for energy storage systems (gas, metal hydride, liquid) [1,2].Hydrogen can be produced by using fossil fuel reforming, processing industrialbyproducts and biomass, using water electrolysis methods, etc. Among these methods,hydrogen production by water electrolysis has been attracting attention in recent yearsfor use in renewable energy and mobile systems [1–3]. Polymer electrolyte membranewater electrolysis (PEMWE, cell temperature: 50–80 ◦C), alkaline water electrolysis (AWE,cell temperature: 60–80 ◦C), and solid oxide water electrolysis (SOEC, cell temperature:>600 ◦C) have been reported. PEMWE, in which a proton exchange membrane and a noblemetal catalyst electrode are used, has a higher hydrogen production cost (4.1–8.6 €/kg H2)Membranes 2021, 11, 330. https://doi.org/10.3390/membranes11050330 https://www.mdpi.com/journal/membraneshttps://www.mdpi.com/journal/membraneshttps://www.mdpi.comhttps://orcid.org/0000-0003-4301-1044https://orcid.org/0000-0002-5748-4830https://doi.org/10.3390/membranes11050330https://doi.org/10.3390/membranes11050330https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.3390/membranes11050330https://www.mdpi.com/journal/membraneshttps://www.mdpi.com/article/10.3390/membranes11050330?type=check_update&version=1Membranes 2021, 11, 330 2 of 13than AWE (3.2–5.0 €/kg H2) does. However, the low power and high purity characteristicsof PEMWE should improve its efficiency [1–6]. More than half of the cost of PEMWE isthe stack cost [3,4]. However, the catalyst electrode and electrolyte membrane must havelower costs and higher performances. Iridium oxide and ruthenium oxide, which havehigh corrosion resistances and excellent catalytic activities, are mainly used as catalystsfor oxygen evolution reactions, and platinum is mainly used as a catalyst for hydrogenevolution reactions [6–16]. Nafion and Aquivion membranes, which are perfluorosulfonicacid (PFSA) membranes with high proton conductivities, high chemical stabilities, andhigh mechanical strengths, are often used as electrolyte membranes [16–21].At the same time, to increase the efficiency of PEMWE, the operating temperature(100–200 ◦C) must be increased. Electrolysis at elevated temperatures has kinetic andthermodynamic advantages for catalytic electrodes in water splitting [3,19]. The potentialfor electrolyzing water is about 1.23 V (237 kJ/mol) (25 ◦C, 1 bar), but if there is noexternal heat source, 1.48 V (286 kJ/mol) is required [3]. The reaction rate using anIrO2 catalytic electrode increases by more than a factor of four when the temperature israised from 80 to 120 ◦C, and the Nernst voltage drops by 22 mV [5]. Furthermore, if theelectrolyte membrane is thermally and chemically stable, the resistance decreases with anincrease in the temperature in humidified states due to the Arrhenius equation, meaningthe conductivity should increase. For the entire cell, the overvoltage is reduced due toimprovements in the activity of the catalyst electrode, the conductivity of the electrolytemembrane, and the reduction of the interfacial resistance between the electrolyte membraneand the catalyst electrode. Thus, efficiency improves.At elevated temperatures (>100 ◦C), for PEMWE, Nafion [22–26], Nafion/SiO2 [22,27],Aquivion [28], and Nafion/PBI [23] membranes are used as electrolyte membranes. IrO2 isnormally used as the catalyst electrode for oxygen evolution, and Pt/C is normally usedas the hydrogen evolution catalyst electrode. Oxygen evolution catalyst electrodes areprepared by using a slurry of IrO2 and an ionomer [22,23,27], coating IrO2 on Ti felt [24],coating IrO2 on a membrane [25], or electrodepositing IrO2 on carbon paper [26]. Inaddition, Ti, which is stable even in highly oxidizing environments, is used for the carbongas diffusion layer (GDL) and current collector on the oxygen evolution side [4,25].In this study, as the anode electrode, we used porous IrO2/Ti/IrO2 catalyst electrodes(hereafter referred to as porous IrO2 catalyst electrodes) in which the IrO2 catalyst wascoated on both sides of three types of porous Ti powder sheets with different surfacetreatment conditions. It was speculated that the three-dimensionally coated IrO2 catalystelectrode would be a useful method for improving catalytic performance and systemconstruction for water electrolysis. As the cathode electrode, we used a Pt/C catalystelectrode coated on carbon GDL. A Nafion115 membrane was sandwiched between theporous IrO2 catalyst electrode and the Pt/C carbon electrode, and membrane electrodeassemblies (MEAs) were prepared by using a hot press method. Using a single cell and ahomemade evaluation device for elevated temperature water electrolysis, current–voltagemeasurements and electrochemical impedance spectroscopy (EIS) were performed atatmospheric pressure and cell temperatures in the range of 80–150 ◦C. In addition, thestructural stabilities of the Nafion and Aquivion membranes at elevated temperatureswere evaluated by monitoring the changes over time from room temperature to 150 ◦Cand then back to room temperature using SAXS. To the best of our knowledge, we arethe first to measure variable-temperature SAXS in situ up to 150 ◦C using Nafion andAquivion membranes.2. Experimental2.1. ElectrodesThree types of porous IrO2/Ti/IrO2 catalyst electrodes prepared by coating IrO2(7.5 mg/cm2) on both sides of Ti powder porous sheets (thickness = 0.5 mm, porosity = 57–61%)under different surface treatment conditions were obtained from Gunma, Japan Calit Co.,Ltd. (IrO2 was considered to be three-dimensionally coated on the porous Ti powder sheet.Membranes 2021, 11, 330 3 of 13The porous IrO2/Ti/IrO2 catalyst electrode is called a porous IrO2 catalyst electrode. Asthe electrode for hydrogen evolution, a Pt/C catalyst electrode (EIWA Co., Ltd., Tokyo,Japan) obtained by applying 0.3 mg/cm2 of Pt on carbon GDL (Sigracet® 25BC of SGLGroup Co. Ltd., Japan) was used. MEAs were obtained by hot pressing at 130 ◦C and9.8 kN for 20 min.2.2. MembranesNafion (equivalent weight, EW = 1000 g/mol) and Aquivion (equivalent weight,EW = 790 g/mol) electrolyte membranes were purchased from EIWA Co. Ltd. The elec-trolyte membranes were treated with boiling water (2 h), 1 M H2O2 (80 ◦C, 2 h), 1 M H2SO4(80 ◦C, 2 h), and boiling water (2 h) before use.2.3. Experimental Setup and Test ProcedureTo evaluate water electrolysis, a single cell consisting of an Al end plate of 8.8 × 8.8 cmand a carbon current collector plate of 6 × 6 cm with a channel of 2.2 × 2.2 cm was used. Thehomemade evaluation system consisted of a part that pumped water to the anode side, anoven that controlled the temperature of the cells, and a part that controlled electrochemicalequipment with a personal computer. The MEA produced by hot pressing was assembledinto a single cell and then set in the evaluation system. The structure of the single cellMEA was basically the same as in previous references [3,16,26]. The water on the anodeside was heated in the range of 80–100 ◦C using an oil bath. A water circulator was usedon the anode side to supply water at a rate of 2.0 mL/min using a pump. Water was notsupplied to the cathode side. The pressure on both sides of the cell was set to atmosphericpressure. The cell temperatures used in this study were 80, 100, 120, and 150 ◦C. Regardingthe cell temperature, since the cell is placed in the oven, the set temperature of the oven wasconsidered as the cell temperature. On the anode side, water at 80 ◦C was supplied whenthe cell was at the same temperature, and water at about 100 ◦C was supplied when the cellwas in the range of 100–150 ◦C. For electrochemical measurements, the current–voltage andEIS characteristics were determined using a 1280C electrochemical test system (Solatronanalytical, Japan) with a 20A booster (Tyoyo Co., Japan). The applied voltage was inthe range of 1.4–1.8 V, and the current was measured while sweeping the voltage at arate of 10 mV/s. The data were measured 2–3 times under these conditions, and valueswith stable current–voltage characteristics were used. EIS was measured at 1.5 V and in afrequency range of 1 Hz–20 kHz. In addition, the current characteristics over time at a celltemperature of 120 ◦C and 1.7 V were determined.2.4. Electrical Characteristics of the IrO2 ElectrodesThe electrical characteristics of the porous IrO2 catalyst electrode were measuredby using a four-probe method with the PSP electrode Loresta-GX (Nittoseiko Analytech,Yamato, Japan).2.5. Surface Characteristics of the IrO2 ElectrodesThe surface characteristics of the porous IrO2 catalyst electrode were observed byusing a field emission scanning electron microscope (FE-SEM, JSM-6700F, JEOL, Japan).2.6. SAXSThe stabilities of Nafion and Aquivion membranes at elevated temperatures weremeasured by using SAXS (beamline BL15A2 of the Photon Factory in KEK, Tsukuba, Japan).Details of the SAXS device setup conditions can be found in a previous report [29]. Thetemperature was raised to 30, 80, 120, and 150 ◦C at a rate of 10 ◦C/min, and then the cellwas cooled from 150 to 30 ◦C. The measurements were done after waiting for 10–15 min atthe target temperature.Membranes 2021, 11, 330 4 of 133. Results and Discussion3.1. Characteristics of the Porous IrO2 Catalyst ElectrodesVarious methods have been reported to improve the assembly of the oxygen evolutioncatalyst IrO2 and the membrane [22–28]. However, PFSA ionomers [17], which have lowglass transition temperatures, and carbon [8], which is unstable at high overvoltages, arenot suitable for the oxygen evolution side of elevated temperature water electrolysis cells.On the other hand, methods wherein the catalyst is directly coated on the membrane andon porous Ti GDLs are considered to be promising for improving the durability of thecell. Table 1 shows the electrical characteristics of the three types of porous IrO2 catalystelectrodes measured by using a four-probe method. The electrical measurements of thethree samples are expressed as surface resistance (sheet resistance, ohm/sq.), volumeresistivity (ohm·cm), and conductivity (S/cm). There were no significant differencesin the volume resistances and conductivities of the three electrodes. The resistance ofbulk IrO2 was 5 × 10–5 ohm·cm [30], and the resistances of the porous IrO2 catalystelectrodes (3–5 × 10–4 ohm·cm) were an order of magnitude higher than that of bulk IrO2.On the other hand, the surface resistance value of sample 1 was larger than those ofsamples 2 and 3. The differences in the surface resistances were thought to be related tothe surface structures of the IrO2 coated on the porous Ti powder sheets, and therefore,SEM was performed (Figure 1). In low-magnification images (Figure 1a,c,e), IrO2 particlesof about 10 mm were observed with large Ti powder particles below them. In addition,the surface morphologies of samples 1 and 2 appeared more uniform than that of sample3. Furthermore, the differences in the surface structures were clear at high magnification.Sample 1 had a uniform morphology (Figure 1b) with a nanostructure, sample 2 appearedas an undefined mass (Figure 1d), and sample 3 had a morphology with severe unevenness(Figure 1f). The differences in the IrO2 morphologies are thought to be related to the surfacetreatment conditions for the Ti powder porous sheets before coating with IrO2.Table 1. Electrical properties of the IrO2 catalyst electrodes coated on porous Ti power sheets.SampleSheet Resistance Volume Resistivity Conductivity(ohm/sq.) (ohm·cm) (S/cm)1 1.057 × 10–2 5.180 × 10–4 1.931 × 10–32 6.295 × 10–3 3.116 × 10–4 3.209 × 10–33 7.181 × 10–3 3.562 × 10–4 2.807 × 10–3Membranes 2021, 11, x FOR PEER REVIEW 4 of 14   3. Results and Discussion 3.1. Characteristics of the Porous IrO2 Catalyst Electrodes Various methods have been reported to improve the assembly of the oxygen evolu-tion catalyst IrO2 and the membrane [22–28]. However, PFSA ionomers [17], which have low glass transition temperatures, and carbon [8], which is unstable at high overvoltages, are not suitable for the oxygen evolution side of elevated temperature water electrolysis cells. On the other hand, methods wherein the catalyst is directly coated on the membrane and on porous Ti GDLs are considered to be promising for improving the durability of the cell. Table 1 shows the electrical characteristics of the three types of porous IrO2 cata-lyst electrodes measured by using a four-probe method. The electrical measurements of the three samples are expressed as surface resistance (sheet resistance, ohm/sq.), volume resistivity (ohm·cm), and conductivity (S/cm). There were no significant differences in the volume resistances and conductivities of the three electrodes. The resistance of bulk IrO2 was 5 × 10–5 ohm·cm [30], and the resistances of the porous IrO2 catalyst electrodes (3–5 × 10–4 ohm·cm) were an order of magnitude higher than that of bulk IrO2. On the other hand, the surface resistance value of sample 1 was larger than those of samples 2 and 3. The differences in the surface resistances were thought to be related to the surface structures of the IrO2 coated on the porous Ti powder sheets, and therefore, SEM was performed (Figure 1). In low-magnification images (Figure 1a,c,e), IrO2 particles of about 10 mm were observed with large Ti powder particles below them. In addition, the surface morpholo-gies of samples 1 and 2 appeared more uniform than that of sample 3. Furthermore, the differences in the surface structures were clear at high magnification. Sample 1 had a uni-form morphology (Figure 1b) with a nanostructure, sample 2 appeared as an undefined mass (Figure 1d), and sample 3 had a morphology with severe unevenness (Figure 1f). The differences in the IrO2 morphologies are thought to be related to the surface treatment conditions for the Ti powder porous sheets before coating with IrO2.  Figure 1. Cont.Membranes 2021, 11, 330 5 of 13Membranes 2021, 11, x FOR PEER REVIEW 5 of 14     Figure 1. SEM images of the IrO2 catalyst electrodes coated on porous Ti power sheets: (a,b) sample 1, (c,d) sample 2, and (e,f) sample 3. Table 1. Electrical properties of the IrO2 catalyst electrodes coated on porous Ti power sheets. Sample Sheet Resistance Volume Resistivity Conductivity (ohm/sq.) (ohm·cm) (S/cm) 1 1.057 × 10–2 5.180 × 10–4 1.931 × 10–3 2 6.295 × 10–3 3.116 × 10–4 3.209 × 10–3 3 7.181 × 10–3 3.562 × 10–4 2.807 × 10–3 3.2. Elevated Temperature Water Electrolysis The characteristics of elevated temperature water electrolysis were determined by using MEAs with a Nafion115 membrane, a Pt/C catalyst electrode, and the three porous IrO2 catalyst electrodes (Figures 2 and 3). On the anode side, water at 80 °C was supplied to the cell at the same temperature, and water at about 100 °C was supplied to the cell in the range of 100–150 °C. Water liquid and vapor were both present when a cell was over 100 °C. The current–voltage characteristics depended on the porous IrO2 catalyst electrode (Figure 2). The current density increased with an increase in the cell temperature, and the magnitude of the increase followed the order sample 1 >sample 2 >sample 3 (Table 2). According to the values in Table 2, the voltage ranges were 10–80 mV for Sample 1 (1.57 V  1.55 V  1.54 V  1.46 V), 20–70 mV for Sample 2 (1.59 V  1.57 V  1.52 V  1.45 V), and 20–60 mV for Sample 3 (1.61 V  1.58 V  1.56 V  1.50 V). This is due to im-Figure 1. SEM images of the IrO2 catalyst electrodes coated on porous Ti power sheets: (a,b) sample 1, (c,d) sample 2, and(e,f) sample 3.3.2. Elevated Temperature Water ElectrolysisThe characteristics of elevated temperature water electrolysis were determined byusing MEAs with a Nafion115 membrane, a Pt/C catalyst electrode, and the three porousIrO2 catalyst electrodes (Figures 2 and 3). On the anode side, water at 80 ◦C was suppliedto the cell at the same temperature, and water at about 100 ◦C was supplied to the cellin the range of 100–150 ◦C. Water liquid and vapor were both present when a cell wasover 100 ◦C. The current–voltage characteristics depended on the porous IrO2 catalystelectrode (Figure 2). The current density increased with an increase in the cell temperature,and the magnitude of the increase followed the order sample 1 >sample 2 >sample 3(Table 2). According to the values in Table 2, the voltage ranges were 10–80 mV for Sample1 (1.57 V →1.55 V →1.54 V →1.46 V), 20–70 mV for Sample 2 (1.59 V →1.57 V →1.52 V→1.45 V), and 20–60 mV for Sample 3 (1.61 V →1.58 V →1.56 V →1.50 V). This is dueto improvements in the catalyst electrode activity [21,22,28] and the conductivity of theelectrolyte membrane and the decrease in the interfacial resistance (Figure 3 and Table 3)by increasing the operating temperature. In some reports, water is supplied to both sidesof the cell [25,27], whereas in others, it is supplied to only the anode side [22,24,26]. At thesame time, in some cases, steam, instead of liquid water, is supplied to the anode side [28].Regarding the pressure, some cells are operated at atmospheric pressure [23,28], whereasothers are operated under pressure [22,24–27]. The characteristics strongly depend on theevaluation method, how water is supplied to the cell, and the application of pressure toboth sides, and these factors affect the efficiency and safety. Supplying steam has beenMembranes 2021, 11, 330 6 of 13reported by Bjerrum et al. to be good [23,28], but it is necessary to suppress the increase inthe resistance because the membrane becomes dry [28]. Operating at temperatures of about100 ◦C where water liquid and vapor are both present, as in this study, is also consideredto be good. However, the EIS characteristics at 150 ◦C (Figure 3d) show that the resistanceand an electric double layer capacitance are higher even on the hydrogen evolution sidebecause the membrane becomes dry.Membranes 2021, 11, x FOR PEER REVIEW 6 of 14   provements in the catalyst electrode activity [21,22,28] and the conductivity of the electro-lyte membrane and the decrease in the interfacial resistance (Figure 3 and Table 3) by increasing the operating temperature. In some reports, water is supplied to both sides of the cell [25,27], whereas in others, it is supplied to only the anode side [22,24,26]. At the same time, in some cases, steam, instead of liquid water, is supplied to the anode side [28]. Regarding the pressure, some cells are operated at atmospheric pressure [23,28], whereas others are operated under pressure [22,24–27]. The characteristics strongly depend on the evaluation method, how water is supplied to the cell, and the application of pressure to both sides, and these factors affect the efficiency and safety. Supplying steam has been reported by Bjerrum et al. to be good [23,28], but it is necessary to suppress the increase in the resistance because the membrane becomes dry [28]. Operating at temperatures of about 100 °C where water liquid and vapor are both present, as in this study, is also con-sidered to be good. However, the EIS characteristics at 150 °C (Figure 3d) show that the resistance and an electric double layer capacitance are higher even on the hydrogen evo-lution side because the membrane becomes dry.  Figure 2. Polarization curves at different operation temperatures using the three IrO2 catalyst electrodes (samples 1–3): (a) 80, (b) 100, (c) 120, and (d) 150 °C. Table 2. Current–voltage properties at different operating temperatures using different IrO2 cata-lyst electrodes. Temp (°C) Sample Voltage (V) at 50 mA/cm2 Current Density (mA/cm2) at 1.8 V 80 1 1.57 689 2 1.59 580 3 1.61 446 Figure 2. Polarization curves at different operation temperatures using the three IrO2 catalystelectrodes (samples 1–3): (a) 80, (b) 100, (c) 120, and (d) 150 ◦C.Table 2. Current–voltage properties at different operating temperatures using different IrO2catalyst electrodes.Temp (◦C) Sample Voltage (V) at 50 mA/cm2 Current Density (mA/cm2) at 1.8 V801 1.57 6892 1.59 5803 1.61 4461001 1.55 8562 1.57 7703 1.58 6031201 1.54 9062 1.52 10103 1.56 6931501 1.46 13192 1.45 12293 1.50 948Membranes 2021, 11, 330 7 of 13Membranes 2021, 11, x FOR PEER REVIEW 7 of 14   100 1 1.55 856 2 1.57 770 3 1.58 603 120 1 1.54 906 2 1.52 1010 3 1.56 693 150 1 1.46 1319 2 1.45 1229 3 1.50 948 The EIS plot in Figure 3 was fitted using the electric circuit model in Figure 4a, af-fording the resistance (Rs), interfacial charge transfer resistance (Rct), and constant phase element (CPE) of the electrolyte membranes. The results are summarized in Table 3. In addition, Figure 4b–d shows the temperature dependences of Rs, Rct, and CPE, respec-tively. The Rs and Rct values of the electrolyte membrane decreased with an increase in the cell temperature. On the other hand, the double layer capacitance (CPE-T) clearly de-creased with an increase in the cell temperature up to 120 °C and then only slightly in-creased up to 150 °C. Since water at about 100 °C was supplied to the anode side without pressurization, the electrolyte membrane may have dried at that cell temperature. In ad-dition, it is thought that water liquid and vapor and oxygen bubbles were present at the interface between the membrane and the porous IrO2 catalyst electrode, and that the mem-brane dried as the temperature increased. Therefore, CPE-T tended to be higher at a cell temperature of 150 °C. The characteristics of sample 1 are the best among the electrodes used. This fact indicates that the nanostructure of the IrO2 catalyst (Figure 1b) is very ef-fective [6–8,10,11,21].  Figure 3. EIS properties at different operating temperatures for samples 1–3: (a) 80, (b) 100, (c) 120, (d) 150 °C. Figure 3. EIS properties at different operating temperatures for samples 1–3: (a) 80, (b) 100, (c) 120,(d) 150 ◦C.Table 3. Parameters obtained from EIS data (Figure 3) fitted to the equivalent circuit shown inFigure 4a.Temp. (◦C) Sample Rs (ohm) Rct (ohm) CPE-T (F) CPE-p801 0.028 1.64 0.045 0.802 0.031 2.10 0.024 0.803 0.035 2.77 0.020 0.821001 0.027 0.58 0.040 0.852 0.030 0.93 0.018 0.853 0.033 1.25 0.020 0.831201 0.026 0.30 0.033 0.872 0.029 0.27 0.015 0.903 0.032 0.65 0.017 0.861501 0.024 0.08 0.038 0.882 0.024 0.10 0.028 0.873 0.024 0.19 0.020 0.87The EIS plot in Figure 3 was fitted using the electric circuit model in Figure 4a,affording the resistance (Rs), interfacial charge transfer resistance (Rct), and constant phaseelement (CPE) of the electrolyte membranes. The results are summarized in Table 3. Inaddition, Figure 4b–d shows the temperature dependences of Rs, Rct, and CPE, respectively.The Rs and Rct values of the electrolyte membrane decreased with an increase in the celltemperature. On the other hand, the double layer capacitance (CPE-T) clearly decreasedMembranes 2021, 11, 330 8 of 13with an increase in the cell temperature up to 120 ◦C and then only slightly increased up to150 ◦C. Since water at about 100 ◦C was supplied to the anode side without pressurization,the electrolyte membrane may have dried at that cell temperature. In addition, it is thoughtthat water liquid and vapor and oxygen bubbles were present at the interface between themembrane and the porous IrO2 catalyst electrode, and that the membrane dried as thetemperature increased. Therefore, CPE-T tended to be higher at a cell temperature of 150 ◦C.The characteristics of sample 1 are the best among the electrodes used. This fact indicatesthat the nanostructure of the IrO2 catalyst (Figure 1b) is very effective [6–8,10,11,21].Membranes 2021, 11, x FOR PEER REVIEW 8 of 14    Figure 4. (a) The equivalent circuit used to fit the EIS data. (b) Ohmic resistance (Rs), (c) charge transfer resistance (Rct), (d) double layer capacitance (CPE-T) vs. temperature. Table 3. Parameters obtained from EIS data (Figure 3) fitted to the equivalent circuit shown in Figure 4a. Temp. (°C) Sample Rs (ohm) Rct (ohm) CPE-T (F) CPE-p 80 1 0.028 1.64 0.045 0.80 2 0.031 2.10 0.024 0.80 3 0.035 2.77 0.020 0.82 100 1 0.027 0.58 0.040 0.85 2 0.030 0.93 0.018 0.85 3 0.033 1.25 0.020 0.83 120 1 0.026 0.30 0.033 0.87 2 0.029 0.27 0.015 0.90 3 0.032 0.65 0.017 0.86 150 1 0.024 0.08 0.038 0.88 2 0.024 0.10 0.028 0.87 3 0.024 0.19 0.020 0.87 In order to evaluate the time dependence of elevated temperature water electrolysis, an MEA cell containing sample 1 was used. Figure 5a,b shows the characteristics of the current density over time and EIS, respectively, measured three times for 8 h at a cell tem-perature of 120 °C and a cell voltage of 1.7 V. The current density decreased with time over the three runs. In addition, the resistance of the membrane and Rct increased with time over the three runs. From the EIS results, the decrease in the current density over time was due to the decrease in membrane conductivity and the increase in Rct. It is thought that water liquid and vapor and oxygen were present on the anode side and that Figure 4. (a) The equivalent circuit used to fit the EIS data. (b) Ohmic resistance (Rs), (c) charge transfer resistance (Rct),(d) double layer capacitance (CPE-T) vs. temperature.In order to evaluate the time dependence of elevated temperature water electrolysis,an MEA cell containing sample 1 was used. Figure 5a,b shows the characteristics of thecurrent density over time and EIS, respectively, measured three times for 8 h at a celltemperature of 120 ◦C and a cell voltage of 1.7 V. The current density decreased with timeover the three runs. In addition, the resistance of the membrane and Rct increased withtime over the three runs. From the EIS results, the decrease in the current density over timewas due to the decrease in membrane conductivity and the increase in Rct. It is thought thatwater liquid and vapor and oxygen were present on the anode side and that the membranebecame dry and bubble layers formed in the microscopic part, resulting in a decrease inthe current density and an increase in the cell resistance [16,31,32]. The performance andstability of water electrolysis largely depends on the interface between the membrane andthe catalyst electrode [20,21,24,33]. In order to improve the performance and durability ofwater electrolysis, an ionomer can be added to the IrO2 catalyst electrode or coated on anelectrolyte membrane [4,14,15,20,21,33]. However, it has been reported that a porous IrO2catalyst electrode sheet affords similar results to ours [24]. The durability should increaseif the interface between the porous IrO2 catalyst electrode and the membrane and the masstransfer of water liquid and vapor and oxygen are improved. On the other hand, from aMembranes 2021, 11, 330 9 of 13study investigating the deterioration of the electrolyte membrane in fuel cells and waterelectrolysis [34], the deterioration rate of the membrane in water electrolysis is faster thanit is in fuel cells, and at 150 ◦C, deterioration progresses 15 times faster than it does at 80 ◦C.Therefore, deterioration of the electrolyte membrane must be considered.Membranes 2021, 11, x FOR PEER REVIEW 9 of 14   the membrane became dry and bubble layers formed in the microscopic part, resulting in a decrease in the current density and an increase in the cell resistance [16,31,32]. The per-formance and stability of water electrolysis largely depends on the interface between the membrane and the catalyst electrode [20,21,24,33]. In order to improve the performance and durability of water electrolysis, an ionomer can be added to the IrO2 catalyst electrode or coated on an electrolyte membrane [4,14,15,20,21,33]. However, it has been reported that a porous IrO2 catalyst electrode sheet affords similar results to ours [24]. The durabil-ity should increase if the interface between the porous IrO2 catalyst electrode and the membrane and the mass transfer of water liquid and vapor and oxygen are improved. On the other hand, from a study investigating the deterioration of the electrolyte membrane in fuel cells and water electrolysis [34], the deterioration rate of the membrane in water electrolysis is faster than it is in fuel cells, and at 150 °C, deterioration progresses 15 times faster than it does at 80 °C. Therefore, deterioration of the electrolyte membrane must be considered.  Figure 5. Time dependence of the single cell using sample 1 at 120 °C and 1.7 V: (a) current density vs. time and (b) EIS properties. 3.3. SAXS Characteristics of PFSA Membranes Nafion and Aquivion membranes with high chemical stabilities are used in a wide range of applications, such as fuel cells and water electrolysis. Moreover, they have been used in the study of high temperature water electrolysis above 100 °C, as in this study and previous reports. However, the glass transition temperatures of the fluorine-based elec-trolyte membranes (H+ form) are in the range of 90–120 °C [17], and their applications above 100 °C have stability problems [35,36]. Alberti, Casciola, Narducci, et al. are devel-oping electrolyte membranes that are stable above 100 °C by using heat treatment of so-lutions containing Nafion and Aquivion membranes [36–40]. On the other hand, the con-ductivity measured by using atomic force microscopy (AFM) from 90 to 120 °C for up to 150 h using a Nafion membrane decreases due to changes in the conduction path as the membrane morphology changes [41]. In addition, it has been reported that the changes in the conduction properties are irreversible under high humidification conditions (RH = 100%) in the temperature range of 40–180 °C using a Nafion membrane [42]. Heat treat-ment above the glass transition temperatures of the membranes causes structural changes in the ionomer of the electrolyte membrane [42]. We investigated the thermal stabilities of Nafion and Aquivion membranes above their glass transition temperatures using SAXS. The SAXS analysis of Nafion membranes is described in detail in previous literatures [17,43,44]. Figures 6 and 7 show scattering profiles and images using Nafion and Aquivion membranes, respectively. The polymer domain structures of Nafion and Aquivion mem-branes were stable from room temperature to 80 °C, except in the peak change region (q > 1), due to structural changes in the ionic clusters. However, upon raising the temperature from 80 to 150 °C, the crystal domain (q < 1) of the polymer structure grew. In the case of Figure 5. Time dependence of the single cell using sample 1 at 120 ◦C and 1.7 V: (a) current densityvs. time and (b) EIS properties.3.3. SAXS Characteristics of PFSA MembranesNafion and Aquivion membranes with high chemical stabilities are used in a widerange of applications, such as fuel cells and water electrolysis. Moreover, they have beenused in the study of high temperature water electrolysis above 100 ◦C, as in this studyand previous reports. However, the glass transition temperatures of the fluorine-basedelectrolyte membranes (H+ form) are in the range of 90–120 ◦C [17], and their applicationsabove 100 ◦C have stability problems [35,36]. Alberti, Casciola, Narducci, et al. aredeveloping electrolyte membranes that are stable above 100 ◦C by using heat treatmentof solutions containing Nafion and Aquivion membranes [36–40]. On the other hand, theconductivity measured by using atomic force microscopy (AFM) from 90 to 120 ◦C forup to 150 h using a Nafion membrane decreases due to changes in the conduction pathas the membrane morphology changes [41]. In addition, it has been reported that thechanges in the conduction properties are irreversible under high humidification conditions(RH = 100%) in the temperature range of 40–180 ◦C using a Nafion membrane [42]. Heattreatment above the glass transition temperatures of the membranes causes structuralchanges in the ionomer of the electrolyte membrane [42]. We investigated the thermalstabilities of Nafion and Aquivion membranes above their glass transition temperaturesusing SAXS. The SAXS analysis of Nafion membranes is described in detail in previousliteratures [17,43,44]. Figures 6 and 7 show scattering profiles and images using Nafionand Aquivion membranes, respectively. The polymer domain structures of Nafion andAquivion membranes were stable from room temperature to 80 ◦C, except in the peakchange region (q > 1), due to structural changes in the ionic clusters. However, upon raisingthe temperature from 80 to 150 ◦C, the crystal domain (q < 1) of the polymer structure grew.In the case of the Aquivion membrane, the crystal domain became anisotropic. Furthermore,when the temperature was decreased from 150 to 30 ◦C, the polymer did not recover itsoriginal structure. These results indicate that the polymer crystal structures of the Nafionand Aquivion membranes change above 120 ◦C. The SAXS method has been shown tobe useful for investigating the high temperature stability of membranes. Moreover, ourresults are consistent with the literature in which the polymer structure of the electrolytemembranes have been reported to change on the basis of conductivity measurements abovethe glass transition temperature [41,42]. In addition, PFSA membranes with a low glasstransition temperature (<80 ◦C) undergo a fatal breakdown at 120 ◦C due to changes in thecrystal domain of the polymer [35].Membranes 2021, 11, 330 10 of 13Membranes 2021, 11, x FOR PEER REVIEW 10 of 14   the Aquivion membrane, the crystal domain became anisotropic. Furthermore, when the temperature was decreased from 150 to 30 °C, the polymer did not recover its original structure. These results indicate that the polymer crystal structures of the Nafion and Aquivion membranes change above 120 °C. The SAXS method has been shown to be use-ful for investigating the high temperature stability of membranes. Moreover, our results are consistent with the literature in which the polymer structure of the electrolyte mem-branes have been reported to change on the basis of conductivity measurements above the glass transition temperature [41,42]. In addition, PFSA membranes with a low glass transition temperature (<80 °C) undergo a fatal breakdown at 120 °C due to changes in the crystal domain of the polymer [35].  Figure 6. SAXS profiles for Nafion membrane at various temperatures. Figure 6. SAXS profiles for Nafion membrane at various temperatures.3.4. The Relationship between the SAXS Results for Membranes and the MEA Performance ResultsThe SAXS results of the Nafion membrane showed a tendency for phase separation asthe crystallinity increased with increasing temperature. On the other hand, in the waterelectrolysis characteristics of the MEA using a Nafion membrane, the conductivity ofthe membrane increased as the temperature increased (Figure 3). However, in the timedependence at 120 ◦C, the conductivity of the membrane in the MEA decreased withtime (Figure 5). Although it is difficult to directly link the SAXS results with the waterelectrolysis properties, the decrease in the membrane conductivity over time at 120 ◦C maybe produced by changes in the polymer structure. Of course, it is also possible that thesulfone group of the polymer is eliminated. From the above, it is possible to evaluate waterelectrolysis at 100 ◦C or higher using a Nafion membrane at the experimental level, butit is considered that there are major problems in the polymer structure and the stabilityof the sulfone group as an application membrane for practical use. In the future, we planto study the stability of sulfone groups. On the other hand, the SAXS results suggestedthat the Aquivion membrane, which is more stable than the Nafion membrane, also hasproblems in its application as an elevated temperature electrolyte membrane. In otherwords, elevated temperature water electrolysis requires that the electrolyte membraneshave high glass transition temperatures (about 200 ◦C) and high proton conductivities.Membranes 2021, 11, 330 11 of 13Membranes 2021, 11, x FOR PEER REVIEW 11 of 14    Figure 7. SAXS profiles of Aquivion membrane at various temperatures. 3.4. The Relationship between the SAXS Results for Membranes and the MEA Performance Results The SAXS results of the Nafion membrane showed a tendency for phase separation as the crystallinity increased with increasing temperature. On the other hand, in the water electrolysis characteristics of the MEA using a Nafion membrane, the conductivity of the membrane increased as the temperature increased (Figure 3). However, in the time de-pendence at 120 °C, the conductivity of the membrane in the MEA decreased with time (Figure 5). Although it is difficult to directly link the SAXS results with the water electrol-ysis properties, the decrease in the membrane conductivity over time at 120 °C may be produced by changes in the polymer structure. Of course, it is also possible that the sul-fone group of the polymer is eliminated. From the above, it is possible to evaluate water electrolysis at 100 °C or higher using a Nafion membrane at the experimental level, but it is considered that there are major problems in the polymer structure and the stability of the sulfone group as an application membrane for practical use. In the future, we plan to study the stability of sulfone groups. On the other hand, the SAXS results suggested that the Aquivion membrane, which is more stable than the Nafion membrane, also has prob-lems in its application as an elevated temperature electrolyte membrane. In other words, elevated temperature water electrolysis requires that the electrolyte membranes have high glass transition temperatures (about 200 °C) and high proton conductivities. 4. Conclusions Figure 7. SAXS profiles of Aquivion membrane at various temperatures.4. ConclusionsElevated temperature water electrolysis at cell temperatures up to 150 ◦C was evalu-ated using three types of porous IrO2 catalyst electrodes (samples 1–3) and a Nafion115membrane. The surface resistance of sample 1 was higher than those of samples 2 and3, but the water electrolysis characteristics had high current densities attributed to thenanostructure. On the other hand, the electrolytic voltages and current densities differeddepending on the type of porous IrO2 catalyst electrode. However, when the cell tempera-ture was increased, the electrolytic voltage decreased and the current density increased. Inaddition, the structural stabilities of Nafion and Aquivion membranes at high temperatureswere investigated by using SAXS. The polymer structures of the Nafion and Aquivionmembranes were stable up to 80 ◦C, but above 120 ◦C, the crystal domain grew, and thestructures did not return to the initial polymer structure after cooling. Elevated temperaturewater electrolysis showed high performance due to the reduced overpotential using thecell. On the other hand, electrolyte membranes with high glass transition temperatures(about 200 ◦C) and high proton conductivities are required.Author Contributions: Conceptualization, J.-D.K.; methodology, J.-D.K.; validation, J.-D.K.; formalanalysis, J.-D.K.; investigation, J.-D.K.; data curation, J.-D.K., A.O.; writing—Original draft prepara-tion, J.-D.K.; writing—Review and editing, J.-D.K., A.O. Both authors have read and agreed to thepublished version of the manuscript.Funding: This research received no external funding.Membranes 2021, 11, 330 12 of 13Institutional Review Board Statement: Not applicable.Acknowledgments: This work was partially funded by a research grant from the Toyota MobilityFoundation (TMF). Small-angle X-ray scattering (SAXS) measurements were performed at the PhotonFactory (beamline BL15A2) of KEK (proposal no. 2015G502). A.O. thanks Hideaki Takagi andNobutaka Shimizu of KEK-PF for their help in the SAXS measurement.Conflicts of Interest: The authors declare no conflict of interest.References1. Körner, A.; Tam, C.; Bennett, S.; Gagné, J. Technology Roadmap-Hydrogen and Fuel Cells; International Energy Agency (IEA): Paris,France, 2015.2. Blagojević, V.A.; Minić Dejan, G.; Grbović Novaković, J.; Minic Dragica, M. Hydrogen economy: Modern concepts, challengesand perspectives. Hydrog. Energy Chall. Perspect. 2012, 17, 3–28. [CrossRef]3. 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