Fig. 6(a) shows the impedance curve (Nyquist plot) of the four MEAs before the test at 80°C and 10 mV. Only a low-frequency response arc was detected, indicating that mass transport limitation was negligible in this operation [
22]. Thus, the widely employed equivalent circuit (EC) can be used to fit the impedance data using the Nyquist plot, as shown in
Fig. 6(b) [
16]. The low-frequency response arc is attributed to charge transfer processes at the anode and is represented by the resistor (
RCT, A). The constant phase element (CPE) in parallel to
RCT, A represents the double-layer capacitance of the anode. The high-frequency response arc is attributed to the cathode process and represented by a resistor (
RCT, C) in parallel with the CPE. The total charge transfer resistance (
RCT) is then obtained by adding
RCT, C and
RCT, A. The high-frequency intercept on the real axis,
RΩ, represents the total ohmic resistance of the single-cell, which is the sum of the ohmic resistances of each component (i.e. the membrane, catalyst layer, backing layers, and bipolar plates) and the interfacial contact resistances between them. The diameter of the arc is a measure of the charge transfer resistance of the OER, which is designated as
Rct [
23]. The cell resistance (
RΩ) and the total charge transfer resistance (
RCT) were calculated as shown in the table in
Fig. 6(a) through equivalent circuit simulation of the four MEAs. The IrO
2-1 cell had the largest ohmic resistance and charge transfer resistance, presumably due to insufficient loading of the catalyst. However, the IrO
2-2 cell showed the lowest ohmic resistance when the anode catalyst loading was increased under the same conditions. These results can be attributed to reduced ion and electron resistance in the anode. In addition, for the IrO
2-3 cell, an increase in both the anode and cathode catalyst loading induced a slight increase in the ohmic resistance, but this cell showed the lowest charge transfer resistance. Thus, the total resistance, including the ohmic resistance and the charge transfer, followed the order from highest to lowest: IrO
2-2, IrO
2-3, Ir black, and IrO
2-1. The resistance characteristics of these cells are consistent with the polarization performance evaluation in
Fig. 7.
Figs. 7(a) and (b) show the polarization characteristics of the MEA with variation of the IrO
2 catalyst loading and the change in the over-potential according to the current density, respectively, as compared with those of the counterpart employing the Ir black electrode. For the MEAs composed of Ir-black, IrO
2-1, IrO
2-2, and IrO
2-3 (
Fig. 7(a)), the cell voltage was 1.95, 1.93, 1.90, and 1.88 V, respectively, at a current density of 1 A cm
−2. As shown in
Fig. 7(b), the corresponding over-potentials were 720, 700, 670, and 650 mV, indicating a decrease with increasing catalyst loading. The cell voltages at the current density of 1.5 A cm
−2 for the same MEAs were 2.24, 2.08, 2.05, and 2.05 V, respectively. The over-potential of the same cells decreased to 1101, 850, 820, and 820 mV, respectively. Here, it can be seen that the over-potential depended strongly on the current density. At a current density of 1 A cm
−2, the cell over-potential decreased in proportion to the catalyst loading of IrO
2. Moreover, for the catalytic IrO
2 electrode, the cell voltage decreased even when the current density was increased to 2.0 A cm
−2. Notably, the MEA composed of Ir black was operational at low cell voltage when the current density was less than 1 A cm
−2, and the cell voltage increased rapidly at current densities of 1 A cm
−2 or more. Overall, the polarization characteristics of the MEA with the IrO
2-1 and IrO
2-2 anodes were superior to those of the congener with Ir black, although the IrO
2 cells were fabricated with a lower catalyst loading than the cathode of the Ir black catalyst cell. In particular, for the IrO
2-3 cell, the cell voltage decreased when the current density was increased to about 2 A cm
−2. This is in contrast with the fact that the cell employing the nanoparticle catalyst showed no significant change in voltage even when the catalyst loading was increased [
14]. From the practical application point of view, the stability of the MEA is very important, and many studies on this feature are on-going [
24–
26]. Thus, water electrolysis was conducted with the MEA employing the IrO
2-1 and IrO
2-3 catalyst electrodes for 2 h at atmospheric pressure, 80°C, and 1 A cm
−2.
Fig. 8 shows that the MEAs employing both IrO
2 electrodes exhibited good stability during water electrolysis. The cell voltages of the two electrodes showed almost similar characteristics without any performance degradation over 2 h. However, the voltage of the two electrode cells increased slightly as the operating time increased. From the above results, it was found that the cell voltage decreased in proportion to the increase in the loading of the IrO
2 catalyst, which is a bulk nanoparticle. It is expected that the over-potential of the water electrolysis cell can be further reduced if further studies on the ionomer content based on the IrO
2 catalyst particle size and specific surface area are undertaken.