3-1. OCV change during AST
Figure 2 illustrates the OCV changes during the AST process. The voltage is increased in the middle of the OCV graph in the AST process because of the point at which activation was performed to evaluate the degree of MEA degradation, which is a result of the MEA performance being restored by the activation effect. In the OCV graph, the upper line (upper limit line, ULL) represents the OCV when dry and the lower line (lower limit line, ILL) is humidified. In the DOE AST, up to 216 h, the OCV gradually decreased while the variation ranges of the ULL and ILL was consistently maintained. Subsequently, the variation ranges gradually increased, and at 408 h (19,581 cycles), the ILL decreased to 0.70 V, thereby terminating the durability evaluation. SCNU AST exhibited an initial OCV of 1.0 V or more by supplying oxygen to the cathode to shorten the evaluation time, and the OCV decreased by approximately 0.15 V from the initial value at 72 h of OCV holding, which is an electrochemical durability evaluation method. Because OCV/wet-dry and OCV holding were repeated for 12 h each, the overall OCV decreased uniformly; however, the ILL decreased to 0.40 V at 127 h (310 cycles), and the durability evaluation was completed. In addition, it can be noted that significant degradation of the polymer membrane occurred as evidenced by the voltage of the ILL at the termination point of the durability evaluation.
The durability evaluation ended at 408 h for the DOE AST and 127 h for the SCNU AST, reducing the evaluation time by approximately 1/3. Evidently, the decrease in OCV was more significant during mechanical degradation than electrochemical degradation, thus indicating that the influence of mechanical durability on the polymer membrane was more significant than its electrochemical durability.
3-2. Hydrogen permeability and short resistance change during AST
Figure 3 illustrates the DOE LSV and HCCD changes during the AST process. In the DOE AST, HCCD decreased from an initial value of 1.084 mA/cm2 to 0.978 mA/cm2 at 360 h. However, it increased significantly to 39.68 mA/cm2 at 408 h, exceeding the durability evaluation termination criterion of 15 mA/cm2, thereby terminating the durability evaluation. In the SCNU AST, there were negligible changes up to 120 h; however, at 127 h, HCCD increased abruptly to 23.05 mA/cm2, thereby terminating the durability evaluation. Therefore, at the termination point of the durability evaluation, it was verified that the MEA degradation rate in SCNU AST was accelerated by more than three times compared to DOE AST, thus resulting in a shortened evaluation time. In addition, a correlation was observed between the rapid decrease in OCV and the significant increase in HCCD during the AST process.
Figure 4 illustrates the NEDO LSV and SR changes during the AST process. Therefore, reaching below the DOE durability evaluation termination criterion of 1 kΩcm2, the evaluation was also concluded based on the SR criterion. As the polymer membrane undergoes degradation, resulting in a thinning of the membrane thickness and a reduction in the distance between the electrodes, it can be observed that the SR decreases, leading to shorting [19]. In addition, the change in SR exhibited the same trend as the change in HCCD, cross-confirming that the polymer membrane degradation rate of SCNU AST was accelerated compared to that of DOE AST.
3-3. ECSA and resistance change during AST
Figure 5 illustrates the CV and ECSA change during the AST process. At the termination point of the durability evaluation, the ECSA reduction rates for DOE AST and SCNU AST were 96% and 30%, respectively, illustrating a significantly larger difference of over three times. This is attributed to the degradation of the electrode catalyst due to the voltage changes during wet and dry conditions in the AST process [11]. DOE AST had 19 581 cycles at 408 h, while SCNU AST had 310 cycles at 127 h, thus indicating that SCNU AST, which distinguished the evaluation method, proceeded with significantly fewer voltage change cycles. Therefore, it was confirmed that the impact of electrode degradation was reduced in the SCNU AST supplemented with the number of voltage changes in which electrode degradation occurred.
Figure 6 presents the impedance, HFR, and CTR change during the AST process. The HFR increased by 20.5% in DOE AST and 54.6% in SCNU AST at the termination point of the durability evaluation owing to the progression of polymer membrane degradation. This result indicates that SCNU AST with increased drying time was more affected by polymer membrane degradation. CTR increased by 19.7% in DOE AST and 27.9% in SCNU AST, thus indicating simultaneous degradation in both the polymer membrane and electrode. In addition, DOE AST exhibited a similar increase rate of HFR and CTR, while SCNU AST exhibited an increase rate of HFR approximately twice that of CTR, confirming that polymer membrane degradation occurred more than electrode catalyst degradation.
3-4. Change in polymer membrane and electrode thickness after AST.
Figure 7 compares the thickness alterations of the polymer membrane and the electrode layer via the SEM cross-sectional analysis after the AST process. MEA comprises an anode electrode in the upper layer, a polymer membrane in the middle layer, and a cathode electrode in the lower layer.
In the SEM cross-sectional image, the thickness of the cathode electrode layer decreased by 15.4% for DOE AST (6.6 µm) and 1.3% for SCNU AST (7.7 µm) from the initial 7.8 µm, thus confirming that the electrode deterioration in DOE AST was significant. As voltage variations are repeated, Pt dissolves and is discharged into the polymer membrane and cathode effluent, thereby reducing the electrode thickness [20]. Because the number of voltage changes under wet-dry conditions in the AST process is higher for DOE AST than for SCNU AST, it can be observed that significant electrode deterioration occurred.
The polymer membrane thickness decreased by 13.1% in DOE AST (23.9 µm) and by 18.2% in SCNU AST (22.5 µm) from the initial 27.5 µm. Hence, the polymer membrane degradation occurred more significantly in SCNU AST. Consequently, the drying time in SCNU AST increases, polymer membrane degradation occurs more in SCNU AST than in DOE AST due to the formation of low humidification conditions in which polymer membrane degradation is accelerated [6]. Therefore, SCNU AST confirmed that polymer membrane degradation occurred more than electrode catalyst degradation.