The MEA for the AEM-URFC was prepared using the catalyst-coated membrane (CCM) method reported previously by us [20,21]. An FAA-3-50 membrane (Fumatech Co., Germany) was used as the AEM, which was pretreated by immersion in a 1.0 M KOH solution (Samchun Chemical Co., Republic of Korea) for 30 min and washed with Deionized water for 30 min. The catalyst layer of all the electrodes was prepared from slurry containing ionomer and spray-coated on both sides of the AEM. The catalyst slurry was prepared by mixing the catalyst powder with a solvent. The solvent included an FAA-3-Br ionomer (Fumatech Co., Germany), deionized (D.I.) water, and isopropanol. The ink was uniformly dispersed using a homogenizer (VCX130; Sonics & Materials, Inc., USA). Pt/C (40 wt%; Alfa Aesar Co., USA) was used as the catalyst in the hydrogen electrode. The Pt loading on the hydrogen electrode was 0.2 mg cm⁻², and the ionomer content of the hydrogen electrode was 30 wt%. Different oxygen electrode designs (i.e., the S and D electrodes) were employed using Pt black (Johnson Matthey Co., UK) as the ORR catalyst and IrO₂ black (Alfa Aesar Co., USA) as the OER catalyst. The S electrode was prepared by spraying a mixture of Pt and IrO₂ onto the AEM. Two types of D electrode were prepared: dual-A (DA) and dual-B (DB). A Pt inner layer and an IrO₂ outer layer were fabricated for the DA electrode, whereas an IrO₂ inner layer and a Pt outer layer were used for the DB electrode, with Pt and IrO₂ catalyst loadings of 0.25, 0.5, and 1.0 mg cm⁻² used. Stainless-steel-based (SUS-based) and carbon-based paper were used as gas-diffusion layers (GDLs) for the oxygen and hydrogen electrodes, respectively.

The morphologies of the AEM-URFC oxygen electrodes were characterized using field-emission scanning electron microscopy (FE-SEM; SUPRA 55VP, Carl Zeiss, Germany), with energy dispersive X-ray spectroscopy (EDS) used to analyze their compositions. The hydrophilicity/hydrophobicity of the catalyst layer in each oxygen electrode was measured using a contact angle analyzer (Phoenix-MT; Surface Electro Optics Co., Republic of Korea).

Each AEM-URFC was single-cell tested using an active area of 5 cm². The performance of each AEM-URFC operated in both WE and FC modes was evaluated at a cell temperature of 60°C. A 1.0 M KOH solution was fed to both electrodes at a flow rate of 1 mL min⁻¹ in WE mode, which is a value widely used in AEM WE experiments [22]. WE performance was examined using the voltage-sweep method in the 1.35–2.15 V applied-voltage range. Fully humidified hydrogen (800 mL min⁻¹) and oxygen (1000 mL min⁻¹) were supplied to the anode and cathode, respectively, in FC mode [20]. FC performance was evaluated using the current-sweep method from the open-circuit voltage (OCV) to 0.3 V. Round-trip efficiency was calculated by dividing the WE voltage by the FC voltage at a current density of 20 mA cm⁻². The resistance of each AEM-URFC was characterized by electrochemical impedance spectroscopy (EIS, Zennium, Zhaner Elekrik Co., USA) at a current density of 20 mA cm⁻². Cyclic testing was conducted at a constant current density of 20 mA cm⁻² for 50 h (FC mode for 25 h and WE mode for 25 h).

Three different oxygen electrodes were investigated to examine the effect of the dual-type electrode on AEM-URFC performance. Fig. 1 displays schematic representations of the three AEM-URFC designs. The first electrode is a single-type oxygen electrode (S) with a single catalyst layer that includes well-dispersed ORR and OER catalysts that are evenly distributed throughout the S electrode. The other electrodes are dual-type electrodes, in which separate ORR and OER layers exist in different orders; one (DA) has the ORR catalyst as the inner layer and the OER catalyst as the outer layer, with the other (DB) having the OER catalyst as the inner layer and the ORR catalyst as the outer layer. Fig. 2 shows FE-SEM and EDS images of the three oxygen electrodes (S, DA, and DB). Fig. 2a shows that Pt and IrO₂ nanoparticles are well dispersed in the S electrode. In contrast, both the DA and DB electrodes have two separate layers consisting of Pt and IrO₂, as shown in Fig. 2b,c. The Pt catalyst is present in the inner layer, which is closer to the membrane than to the GDL in the DA electrode (Fig. 2b). In contrast, the DB electrode has two separate layers, with the Pt catalyst as the outer layer, close to the GDL (Fig. 2c). The EDS images also reveal that the oxygen electrode designs are well-developed. EDS showed well-distributed pink dots (Pt) and blue dots (Ir), which implies that the Pt and IrO₂ catalysts are evenly mixed (Fig. 2d). As shown in Fig. 2e, pink dots exist close to the membrane and blue dots are observed far from the membrane, which indicates that the DA electrode comprises Pt and IrO₂ as the inner and outer layers, respectively. The reverse order was observed for the DB electrode, with blue and pink dots observed close to and far from the membrane, respectively, confirming that the DB electrode consists of inner IrO₂ and outer Pt layers.

The hydrophilicities and hydrophobicities of the catalyst layers of the S, DA, and DB electrodes were evaluated using a contact angle analyzer. Fig. 3 displays contact angles and schematic diagrams of the IrO₂ + Pt layer in the S electrode and the IrO₂ and Pt layers in the DA and DB electrodes. The IrO₂ + Pt layer exhibited a contact angle of approximately 108°, consistent with its hydrophobicity [23,24]. The Pt and IrO₂ layers exhibited contact angles of approximately 123° and 10°, respectively, which indicates that the Pt layer is hydrophobic, whereas the IrO₂ layer is hydrophilic [23,24]. The hydrophilicity/hydrophobicity of each of the three electrodes was predicted based on the hydrophilicity/hydrophobicity of each layer. The entire S electrode layer is hydrophobic, whereas the DA and DB electrodes exhibit distinct hydrophilic and hydrophobic characteristics, respectively. DA consists of a hydrophobic inner layer and a hydrophilic outer layer; in contrast, DB comprises a hydrophilic inner layer and a hydrophobic outer layer.

Based on FE-SEM and EDS data, we confirmed that the dual-type electrode is well-formed with two distinct layers: an IrO₂ layer and a Pt layer. Additionally, contact angle data indicated that each layer possesses separate hydrophilic/hydrophobic properties. Therefore, it can be anticipated that the dual-type electrode, consisting of these two layers, will exhibit distinct catalytic activities for each layer.

Fig. 4 shows the cell performance of AEM-URFCs with the three different oxygen electrodes (S, DA, and DB) operating in WE and FC modes. In WE mode (Fig. 4a), DA exhibited a slightly better WE performance than S and DB in the low-current-density region (< 300 mA cm⁻²). In addition, the DA electrode performed better than the S and DB electrodes in the high-current-density region, as shown in the inset of Fig. 4a. This difference in performance is attributable to different charge-transfer resistances. The ohmic and charge-transfer resistances were determined from the Nyquist plots in Fig. 4b. the ohmic resistances of the three samples are similar, whereas their charge-transfer resistances are different. Their similar ohmic resistances are ascribable to the WE operating conditions. All IrO₂ catalysts are immersed in a 1.0 M KOH solution in WE mode. Because the 1.0 M KOH solution serves as a liquid electrolyte for transferring hydroxide ions to the IrO₂ layer [25], ion transfer through the immersed IrO₂ catalysts is not affected by the electrode design. In other words, the ohmic resistances of the three electrodes are similar. In contrast, the charge-transfer resistance of DA, determined from the diameter of the semicircle in the Nyquist plot, was the smallest among the three AEM-URFCs. Notably, only the OER catalyst in the oxygen electrode affects WE performance. Consequently, the position of the OER catalyst is important for determining performance. The IrO₂ catalysts on the three oxygen electrodes are located differently. The IrO₂ layer is located on the GDL side of the DA electrode, while half of the IrO₂ catalyst is in contact with the GDL in the case of the S electrode, and the IrO₂ catalyst in the DB electrode is closer to the membrane than the GDL. Since 1.0 M KOH solution was supplied to the oxygen electrode, applying IrO₂ as the outer layer resulted in superior WE performance than that of the inner layer. These results imply that oxygen electrode designs in which OER catalysts are closer to the GDL than to the membrane (i.e., the DA electrode) show the lowest charge-transfer resistances. The DA electrode exhibited the best performance in WE mode among the three electrodes.

The performance of the three AEM-URFCs in FC mode were evaluated and compared to demonstrate the effect of electrode design on FC performance. Fig. 4c shows polarization curves for the three AEM-URFCs (S, DA, and DB) in FC mode. Significantly greater differences in FC performance were observed among the three AEM-URFCs than were observed in WE mode. The ORR occurs at triple-phase boundaries in FC mode, where ions, electrons, and reactants meet, which is a significant difference. As mentioned earlier, the 1.0 M KOH feed solution serves as a liquid electrolyte in WE mode; therefore, performance depends little on the position of the IrO₂ layer. However, in FC mode, the fed reactants do not serve as an electrolyte; therefore, performance depends significantly on the position of the Pt layer. FC performance displayed a similar trend to WE performance, namely: DA > S > DB. In particular, the DB electrode exhibited significantly lower FC performance than the S and DA electrodes; this difference is attributable to the ohmic and charge-transfer resistances (Fig. 4d). Ohmic resistance followed the order: DB > S > DA. The DA electrode exhibited the lowest ohmic resistance, which indicates that an electrode design in which the inner Pt layer contacts the membrane is important for achieving high FC performance. The size of the triple-phase boundary increases as the contact area between the Pt layer and membrane increases, leading to higher performance. In contrast, the DB electrode, in which the IrO₂ inner layer exists between the membrane and the Pt outer layer, showed the largest ohmic resistance, which is ascribable to the sandwiched IrO₂ layer hindering ion-transfer between the membrane and the Pt layer.

In addition, charge-transfer resistance is significantly affected by the design of the oxygen electrode. Two overlapping semi-circles were observed in the Nyquist plots at 20 mA cm⁻². As same components except the oxygen electrode were employed, the second semi-circle corresponds to charge-transfer resistance in the FC cathode. The charge-transfer resistance was observed to decrease in the following order: DB > S > DA. The three electrodes contain different ratios of ORR catalyst layer in contact with the membrane. A larger ORR catalyst layer area in contact with the membrane results in higher FC performance. Because the entire Pt layer in the DA electrode contacts the membrane, the DA electrode exhibited the best FC performance among the three oxygen electrodes. Therefore, the electrode design in which the ORR catalyst exists in the inner layer with the OER catalyst in the outer layer exhibited the best WE and FC performance, thereby achieving high round-trip efficiency.

How the ORR- and OER-catalyst loadings affect cell performance was investigated to determine the optimal loadings in the DA electrode. Firstly, three AEM-URFCs with different Pt loadings (0.25, 0.5, and 1.0 mg cm⁻², herein referred to as DA-Pt-0.25, DA-Pt-0.5, and DA-Pt-1.0, respectively) were evaluated. The loading of IrO₂ loading was fixed at 0.5 mg cm^–2. Fig. 5a shows the WE performance of DA-Pt-0.25, DA-Pt-0.5, and DA-Pt-1.0, which was observed to improve with decreasing Pt loading. In WE mode, the inner Pt layer acts as a barrier that inhibits the OER. The Pt layer becomes thinner with decreasing Pt loading; consequently, DA-Pt-0.25 exhibited the highest WE performance owing to low ohmic resistance (Fig. 5b). A different trend was observed in FC mode. Fig. 5c shows polarization curves for DA-Pt-0.25, DA-Pt-0.5, and DA-Pt-1.0 acquired in FC mode. Pt loading is the main factor that affects FC performance because the ORR occurs in FC mode. Poor FC performance was observed at a Pt loading of 0.25 mg cm⁻², which is ascribable to two reasons: 1) insufficient Pt loading and 2) insufficient water supply to the thin Pt layer due to the hydrophilicity of IrO₂. The ORR degrades and the membrane is not properly hydrated as the thin Pt layer (where the ORR occurs) dries out. To date, the low ionic conductivity of the AEM has been pointed out as a major factor that affects performance [26–28]. The ionic conductivity of the membrane is very low when the water content in the cell is low. Unlike a PEMFC, an AEMFC requires adequate water content at the oxygen electrode because water is consumed during the ORR for anion transfer. To date, the low ionic conductivity of the AEM has been pointed out as a major factor that affects performance [26–28]. The ionic conductivity of the membrane is very low when the water content in the cell is low. Unlike a PEMFC, an AEMFC requires adequate water content at the oxygen electrode because water is consumed during the ORR for anion transfer. It is well-established that water naturally transports due to its wettability properties [29–32]. When liquid water adheres to a surface, it spontaneously moves from hydrophobic areas to hydrophilic areas, forming water clusters. As the thickness of the hydrophobic layer increases, the length of the hydrophobic channel that water must pass through also increases, making it more difficult for water to penetrate the layer [33]. Similarly, when using the same amount of hydrophilic IrO₂ (0.5 mg_Ir cm-²), the distribution of water within the electrode can vary depending on the amount of the hydrophobic Pt layer. DA-Pt-0.25, with lower Pt loading compared to other electrodes, has a shorter hydrophobic channel length, making it easier for water to migrate into the hydrophilic IrO₂ layer. This can lead to the cathode layer drying out, which in turn results in poor ORR activity, and high charge-transfer resistance (Fig. 5d). membrane dehydration occurs, leading to a reduction in membrane conductivity and a catastrophic increase in ohmic resistance.

Meanwhile, a higher Pt loading was found to improve FC performance in the low-current-density region. In contrast, the FC performance in the high-current-density regions deteriorated as the Pt loading was increased from 0.5 to 1.0 mg cm⁻², which is attributable to the thick catalyst layer generated at high Pt loading, which increases mass-transport resistance and reduces cell performance in the high-current-density region [34,35]. Considering both modes, the optimal Pt loading in the DA electrode was determined to be 0.5 mg cm⁻².

The effect of the OER catalyst loading in the DA electrode on WE and FC performance is shown in Fig. 6a–d. Three types of AEM-URFC MEAs with different IrO₂ loadings (0.25, 0.5, and 1.0 mg cm⁻², referred to as DA-Ir-0.25, DA Ir-0.5, and DA-Ir-1.0, respectively) were investigated. The loading of Pt loading was fixed at 0.5 mg_Pt cm^–2. The OER catalyst loading was found to significantly affect performance in WE mode (in which the OER occurs).

As shown in Fig. 6a, DA-IrO₂-0.5 exhibited the highest WE performance. In contrast, the low IrO₂ loading led to significantly higher ohmic and charge-transfer resistances in DA-IrO₂-0.25 (Fig. 6b), resulting in poor WE performance. In the WE mode, the outer IrO₂ layer serves as the main active site. In this mode, the performance is not significantly influenced by the location of the active site because the reactant is 1.0 M KOH liquid electrolyte. Therefore, the primary factor affecting performance in WE mode is the loading amount of IrO₂. If the IrO₂ loading is too low, there are insufficient active sites, leading to high charge-transfer resistance. Conversely, DA-IrO₂-1.0 exhibited the lowest charge-transfer resistance owing to its high IrO₂ loading. Despite the low chargetransfer resistance of DA-IrO₂-1.0, its high IrO₂ loading (1.0 mg cm⁻²) led to a high ohmic resistance, which is attributable to the thick IrO₂ layer. As a result, an IrO₂ loading of 0.5 mg cm⁻² afforded the highest WE performance owing to low ohmic and charge-transfer resistances. On the other hand, FC performance improved when the IrO₂ loading was decreased from 1.0 to 0.25 mg cm⁻², as shown in Fig. 6c. In FC mode, the IrO₂ outer layer acts as a resistance to ORR. It simply facilitates the transport of reactants and products between the bipolar plate and the Pt inner layer. Therefore, a thin IrO₂ layer is necessary to attain high FC performance, which suggests that DA-IrO₂-0.25 should exhibit the highest FC performance and the lowest ohmic and chargetransfer resistances (Fig. 6d). An IrO₂ loading of 0.5 mg cm⁻² was deduced to deliver high AEM-URFC performance based on the WE and FC performance results.

Fig. 7a displays performance data for the DA-electrode-containing AEM-URFC in both WE and FC modes using the MEA parameters investigated above. In FC mode, voltages of 0.906, 0.795, and 0.706 V were recorded at current densities of 20, 50, and 100 mA cm⁻², respectively. Meanwhile, voltages of 1.46, 1.50, and 1.53 V were recorded in WE mode at current densities of 20, 50, and 100 mA cm⁻², respectively. Round-trip efficiencies 62.0, 53.0, and 46.1% were calculated at 20, 50, and 100 mA cm⁻², respectively, based on the voltages recorded in WE and FC modes. Table 1 compares the round-trip efficiencies of the AEM-URFC developed in this study (DA electrode) with those reported previously. The round-trip efficiencies achieved in this study are comparable to or higher than those reported in the literature [12,17,18,36–38]. These results indicate that the DA electrode is a highly efficient oxygen electrode for AEM-URFCs.

Five testing cycles were used to examine the stability of the DA electrode in both FC and WE modes, the results of which are shown in Fig. 7b,c, which reveals that FC performance was mostly maintained after five cycles (Fig. 7b). In addition, Fig. 7c shows that WE performance was maintained for five cycles. While the AEM-URFCs reported in the literature [18,36] exhibited performance decay over five cycles, the AEM-URFC developed in this study exhibited stable performance over five cycles. Round-trip efficiencies at current densities of 20, 50, and 100 mA cm⁻² were evaluated for cycles 1–5 and are presented in Fig. 7d. The DA-electrode-containing AEM-URFC exhibited similar efficiencies without significant loss, indicating that it is durable during URFC operation. Additionally, Fig. 7e presents chronopotentiometry-based stability-testing results in FC and WE modes at a constant current density of 20 mA cm⁻². The DA-electrode-containing AEM-URFC exhibited stable FC performance after 25 h, and negligible voltage increase was observed at a constant current density of 20 mA cm⁻² in WE mode. Durable AEM-URFC performance was achieved through an electrode design in which separate layers are used and SUS paper is employed as the GDL. Therefore, the DA electrode operates stably in URFC mode.