Water Electrolysis Using a Porous IrO2/Ti/IrO2 Catalyst Electrode and Nafion Membranes at Elevated Temperatures

Porous IrO2/Ti/IrO2 catalyst electrodes were obtained by coating IrO2 on both sides of three types of porous Ti powder sheets (sample 1, sample 2, and sample 3) using different surface treatment methods, and a hydrogen evolution catalyst electrode was obtained by coating Pt/C on carbon gas diffusion layers. A Nafion115 membrane was used as an electrolyte for the membrane electrode 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 were investigated. The sheet resistance of sample 1 was higher than those of samples 2 and 3, although during water electrolysis, a high current density was observed due to the nanostructure of the IrO2 catalyst. In addition, the structural stabilities of Nafion and Aquivion membranes up to 150 °C were investigated by using small angle X-ray scattering (SAXS). The polymer structures of Nafion and Aquivion membranes were stable up to 80 °C, whereas the crystalline domains grew significantly above 120 °C. In other words, the initial polymer structure did not recover after the sample was heated above the glass transition temperature.


Introduction
Global warming is driving societies that burn fossil fuels for energy, which produces huge amounts of CO 2 , towards renewable and sustainable energies. In particular, the use of renewable energy, such as solar and wind power, is drawing attention because it does not emit CO 2 . However, the generation of electricity from photovoltaic cells and wind power generators is variable, and large-capacity energy storage systems are required for efficient use. Energy storage methods include physical energy storage (flywheel, compressed-air energy storage (CAES), pumping) and chemical energy storage (redox flow battery (RFB), sodium-sulfur battery (NAS), Li-ion battery (LIB), methane, hydrogen (H 2 )). In order to supply stable renewable energy, systems with high energy densities and conversion efficiencies are required. Hydrogen has a high energy density, is a CO 2 -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 industrial byproducts and biomass, using water electrolysis methods, etc. Among these methods, hydrogen production by water electrolysis has been attracting attention in recent years for use in renewable energy and mobile systems [1][2][3]. Polymer electrolyte membrane water 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 noble metal catalyst electrode are used, has a higher hydrogen production cost (4.1-8.6 €/kg H 2 ) The porous IrO 2 /Ti/IrO 2 catalyst electrode is called a porous IrO 2 catalyst electrode. As the electrode for hydrogen evolution, a Pt/C catalyst electrode (EIWA Co., Ltd., Tokyo, Japan) obtained by applying 0.3 mg/cm 2 of Pt on carbon GDL (Sigracet ® 25BC of SGL Group Co. Ltd., Japan) was used. MEAs were obtained by hot pressing at 130 • C and 9.8 kN for 20 min.

Experimental Setup and Test Procedure
To evaluate water electrolysis, a single cell consisting of an Al end plate of 8.8 × 8.8 cm and a carbon current collector plate of 6 × 6 cm with a channel of 2.2 × 2.2 cm was used. The homemade evaluation system consisted of a part that pumped water to the anode side, an oven that controlled the temperature of the cells, and a part that controlled electrochemical equipment with a personal computer. The MEA produced by hot pressing was assembled into a single cell and then set in the evaluation system. The structure of the single cell MEA was basically the same as in previous references [3,16,26]. The water on the anode side was heated in the range of 80-100 • C using an oil bath. A water circulator was used on the anode side to supply water at a rate of 2.0 mL/min using a pump. Water was not supplied to the cathode side. The pressure on both sides of the cell was set to atmospheric pressure. The cell temperatures used in this study were 80, 100, 120, and 150 • C. Regarding the cell temperature, since the cell is placed in the oven, the set temperature of the oven was considered as the cell temperature. On the anode side, water at 80 • C was supplied when the cell was at the same temperature, and water at about 100 • C was supplied when the cell was in the range of 100-150 • C. For electrochemical measurements, the current-voltage and EIS characteristics were determined using a 1280C electrochemical test system (Solatron analytical, Japan) with a 20A booster (Tyoyo Co., Japan). The applied voltage was in the range of 1.4-1.8 V, and the current was measured while sweeping the voltage at a rate of 10 mV/s. The data were measured 2-3 times under these conditions, and values with stable current-voltage characteristics were used. EIS was measured at 1.5 V and in a frequency range of 1 Hz-20 kHz. In addition, the current characteristics over time at a cell temperature of 120 • C and 1.7 V were determined.

Electrical Characteristics of the IrO 2 Electrodes
The electrical characteristics of the porous IrO 2 catalyst electrode were measured by using a four-probe method with the PSP electrode Loresta-GX (Nittoseiko Analytech, Yamato, Japan).

Surface Characteristics of the IrO 2 Electrodes
The surface characteristics of the porous IrO 2 catalyst electrode were observed by using a field emission scanning electron microscope (FE-SEM, JSM-6700F, JEOL, Japan).

SAXS
The stabilities of Nafion and Aquivion membranes at elevated temperatures were measured 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]. The temperature was raised to 30, 80, 120, and 150 • C at a rate of 10 • C/min, and then the cell was cooled from 150 to 30 • C. The measurements were done after waiting for 10-15 min at the target temperature.

Characteristics of the Porous IrO 2 Catalyst Electrodes
Various methods have been reported to improve the assembly of the oxygen evolution catalyst IrO 2 and the membrane [22][23][24][25][26][27][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 IrO 2 catalyst 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 IrO 2 was 5 × 10 -5 ohm·cm [30], and the resistances of the porous IrO 2 catalyst electrodes (3-5 × 10 -4 ohm·cm) were an order of magnitude higher than that of bulk IrO 2 . 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 IrO 2 coated on the porous Ti powder sheets, and therefore, SEM was performed ( Figure 1). In low-magnification images (Figure 1a,c,e), IrO 2 particles of 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 sample 3. 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 appeared as an undefined mass (Figure 1d), and sample 3 had a morphology with severe unevenness (Figure 1f). The differences in the IrO 2 morphologies are thought to be related to the surface treatment conditions for the Ti powder porous sheets before coating with IrO 2 .

Characteristics of the Porous IrO2 Catalyst Electrodes
Various methods have been reported to improve the assembly of the oxygen evolution catalyst IrO2 and the membrane [22][23][24][25][26][27][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 catalyst 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 morphologies 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 uniform 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.

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-

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 IrO 2 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 IrO 2 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 , and 20-60 mV for Sample 3 (1.61 V →1.58 V →1.56 V →1.50 V). This is due to improvements in the catalyst electrode activity [21,22,28] and the conductivity of the electrolyte 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][25][26][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  [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 considered 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 evolution side because the membrane becomes dry.
the cell [25,27], whereas in others, it is supplied to only the anode side [22,24,26]. same time, in some cases, steam, instead of liquid water, is supplied to the anode sid Regarding the pressure, some cells are operated at atmospheric pressure [23,28], w others are operated under pressure [22,[24][25][26][27]. The characteristics strongly depend evaluation method, how water is supplied to the cell, and the application of press both sides, and these factors affect the efficiency and safety. Supplying steam ha reported by Bjerrum et al. to be good [23,28], but it is necessary to suppress the in in the resistance because the membrane becomes dry [28]. Operating at temperatu about 100 °C where water liquid and vapor are both present, as in this study, is als sidered to be good. However, the EIS characteristics at 150 °C (Figure 3d) show th resistance and an electric double layer capacitance are higher even on the hydroge lution side because the membrane becomes dry.    dition, it is thought that water liquid and vapor and oxygen bubbles were presen interface between the membrane and the porous IrO2 catalyst electrode, and that th brane dried as the temperature increased. Therefore, CPE-T tended to be higher a temperature of 150 °C. The characteristics of sample 1 are the best among the ele used. This fact indicates that the nanostructure of the IrO2 catalyst (Figure 1b) is v fective [6][7][8]10,11,21].    The 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 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, respectively. 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 decreased with an increase in the cell temperature up to 120 • C and then only slightly increased 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 addition, it is thought that water liquid and vapor and oxygen bubbles were present at the interface between the membrane and the porous IrO 2 catalyst electrode, and that the membrane 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 IrO 2 catalyst (Figure 1b) is very effective [6][7][8]10,11,21].  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 temperature of 120 °C and a cell voltage of 1.7 V. The current density decreased with time 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 temperature 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 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 performance 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 IrO 2 catalyst electrode or coated on an electrolyte membrane [4,14,15,20,21,33]. However, it has been reported that a porous IrO 2 catalyst electrode sheet affords similar results to ours [24]. The durability should increase if the interface between the porous IrO 2 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. membrane and the catalyst electrode [20,21,24,33]. In order to improve the performa and durability of water electrolysis, an ionomer can be added to the IrO2 catalyst electro or coated on an electrolyte membrane [4,14,15,20,21,33]. However, it has been repor that a porous IrO2 catalyst electrode sheet affords similar results to ours [24]. The dura ity should increase if the interface between the porous IrO2 catalyst electrode and membrane and the mass transfer of water liquid and vapor and oxygen are improved. the other hand, from a study investigating the deterioration of the electrolyte membr in fuel cells and water electrolysis [34], the deterioration rate of the membrane in wa electrolysis is faster than it is in fuel cells, and at 150 °C, deterioration progresses 15 tim faster than it does at 80 °C. Therefore, deterioration of the electrolyte membrane must considered.

SAXS Characteristics of PFSA Membranes
Nafion and Aquivion membranes with high chemical stabilities are used in a w range of applications, such as fuel cells and water electrolysis. Moreover, they have b used in the study of high temperature water electrolysis above 100 °C, as in this study a previous reports. However, the glass transition temperatures of the fluorine-based e trolyte membranes (H + form) are in the range of 90-120 °C [17], and their applicati above 100 °C have stability problems [35,36]. Alberti, Casciola, Narducci, et al. are dev oping electrolyte membranes that are stable above 100 °C by using heat treatment of lutions containing Nafion and Aquivion membranes [36][37][38][39][40]. On the other hand, the c ductivity measured by using atomic force microscopy (AFM) from 90 to 120 °C for up 150 h using a Nafion membrane decreases due to changes in the conduction path as membrane morphology changes [41]. In addition, it has been reported that the change 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 tre ment above the glass transition temperatures of the membranes causes structural chan in the ionomer of the electrolyte membrane [42]. We investigated the thermal stabilitie Nafion and Aquivion membranes above their glass transition temperatures using SA The SAXS analysis of Nafion membranes is described in detail in previous literatu [17,43,44]. Figures 6 and 7 show scattering profiles and images using Nafion and Aquiv membranes, respectively. The polymer domain structures of Nafion and Aquivion me branes were stable from room temperature to 80 °C, except in the peak change region ( 1), due to structural changes in the ionic clusters. However, upon raising the temperat from 80 to 150 °C, the crystal domain (q < 1) of the polymer structure grew. In the cas

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 electrolyte 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 developing electrolyte membranes that are stable above 100 • C by using heat treatment of solutions containing Nafion and Aquivion membranes [36][37][38][39][40]. On the other hand, the conductivity 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 treatment 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 membranes 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 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 useful for investigating the high temperature stability of membranes. Moreover, our results are consistent with the literature in which the polymer structure of the electrolyte membranes 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]. Aquivion membranes change above 120 °C. The SAXS method has been shown to be useful for investigating the high temperature stability of membranes. Moreover, our results are consistent with the literature in which the polymer structure of the electrolyte membranes 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].

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 dependence 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 electrolysis 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 sulfone 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 problems 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.

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 dependence 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 electrolysis 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 sulfone 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 problems 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.

Conclusions
Elevated temperature water electrolysis at cell temperatures up to 150 • C was evaluated using three types of porous IrO 2 catalyst electrodes (samples 1-3) and a Nafion115 membrane. The surface resistance of sample 1 was higher than those of samples 2 and 3, but the water electrolysis characteristics had high current densities attributed to the nanostructure. On the other hand, the electrolytic voltages and current densities differed depending on the type of porous IrO 2 catalyst electrode. However, when the cell temperature was increased, the electrolytic voltage decreased and the current density increased. In addition, the structural stabilities of Nafion and Aquivion membranes at high temperatures were investigated by using SAXS. The polymer structures of the Nafion and Aquivion membranes were stable up to 80 • C, but above 120 • C, the crystal domain grew, and the structures did not return to the initial polymer structure after cooling. Elevated temperature water electrolysis showed high performance due to the reduced overpotential using the cell. On the other hand, electrolyte membranes with high glass transition temperatures (about 200 • C) and high proton conductivities are required.