## 3 Results and Discussion  
### PEMFC Stack Degradation  
The durability experiments of the self-designed dynamic load cycle were conducted for more than 1000 h. Two 10-cell stacks were used, each evaluated under different temperature conditions. One experiment was conducted under 65 \({}^{\circ}\)C conditions, and another under 80 \({}^{\circ}\)C conditions. Other operating conditions, such as flow rate, humidification, and pressure, were kept identical. Figure 3 compares the I-V curves measured under identical conditions before and after the durability test. The beginning of test (BOT) performance of both stacks was similar. However, when comparing the end of test (EOT) performance, a greater performance degradation was observed under high-temperature conditions (80 \({}^{\circ}\)C). Furthermore, it was confirmed that the higher the current density, the greater the performance degradation. Figure 4 illustrates the variability in cell voltages observed at different positions within the fuel-cell stacks, highlighting the influence of the load cycle protocol and operating conditions. Figure 4a shows the maximum power output (1.19 A/cm\({}^{2}\)), whereas Figure 4b shows the minimum power output (0.02 A/cm\({}^{2}\)). At the BOT and a current density of 1.19 A/cm\({}^{2}\), the average cell voltage for the 10-cell stack was 0.697 V at 65 \({}^{\circ}\)C and 0.674 V at 80 \({}^{\circ}\)C. By the EOT, these values decreased by 6.46% to 0.652 V at 65 \({}^{\circ}\)C, and by 11.15% to 0.599 V at 80 \({}^{\circ}\)C (Figure 4a). At a lower current density of 0.02 A/cm\({}^{2}\), the average  
Figure 3: The I-V curve reflects the voltage decrease due to the durability test, comparing BOT and EOT performance at 65 \({}^{\circ}\)C (**a**) and at 80 \({}^{\circ}\)C (**b**). BOT voltages were 0.926 V at 65 \({}^{\circ}\)C and 0.920 V at 80 \({}^{\circ}\)C. The corresponding EOT voltages showed reductions of 3.15% to 0.897 V at 65 \({}^{\circ}\)C and by 3.13% to 0.891 V at 80 \({}^{\circ}\)C (Figure 4b). While minimal temperature-dependent voltage reduction was observed at the minimum power output (0.02 A/cm\({}^{2}\)), the impact of elevated temperature on voltage degradation was more pronounced at the maximum power output (1.19 A/cm\({}^{2}\)). This suggests that analyzing degradation at the maximum power output provides a more sensitive and relevant assessment of PEMFC stack reliability. At maximum power output (1.19 A/cm\({}^{2}\)), the standard deviation of cell voltages at BOT was 6.659 mV at 65 \({}^{\circ}\)C and 2.848 mV at 80 \({}^{\circ}\)C. Similar standard deviations were observed at EOT (5.746 mV at 65 \({}^{\circ}\)C and 7.542 mV at 80 \({}^{\circ}\)C). These findings indicate negligible position-dependent voltage deviation within the 10-cell PEMFC stack. The consistent standard deviations throughout the stack's lifetime, well within acceptable limits, suggest that degradation occurs uniformly across all cells, independent of their position within the stack [8]. Figure 5 depicts the cell-voltage degradation paths for the PEMFC operated at 1.19 A/cm\({}^{2}\) under 65 and 80 \({}^{\circ}\)C conditions. The conditions are classified by temperature using the same color, and the relative voltage degradation path of each cell in the 10-cell stack is represented with individual lines. Voltage degradation was measured relative to each cell's initial voltage, with data sampled at 50-h intervals over the 1000 h durability evaluation period. As expected, the voltage drop was more significant at the higher operating temperature of 80 \({}^{\circ}\)C. The degradation profiles exhibit a characteristic two-stage pattern: an initial phase of rapid voltage decline followed by a slower, more gradual decrease. This pattern aligns with previous observations in PEMFC degradation studies. To represent the degradation paths of fuel cells, four popular degradation models are employed: linear, exponential, power, and bi-exponential models.