## 3 Results
### Comparison and Analysis
The concentration variations in the rectangular baffle model included higher and sharper oxygen concentration increases near the baffles, which improved the chemical reaction at the GDL at the cost of sharper pressure loss, as shown in Figure 9. The optimized trapezoidal baffle model had a relatively gradual oxygen concentration increase, which improved oxygen uniformity along the flow channel. Figure 12 shows the current flux density at the cathode GDL/CL interface in the three models. In all three models, the current density at the oxygen flow channel outlet was relatively low, since the oxygen concentration was low at the channel outlet due to consumption along the channel. The hydrogen concentration was also low at its flow channel outlet, which in turn resulted in the low current density at the oxygen flow channel inlet. The current flux density near the ribs of bipolar plates was greater than that at the  
Figure 11: Oxygen concentration at the cathode GDL/CL interface around the axis of symmetry. Figure 10: Oxygen concentration at the cathode GDL/CL interface. (**a**) The straight flow channel model; (**b**) the rectangular baffle model; (**c**) the optimized trapezoidal baffle model. flow channel. This is because electrochemical reactions require a complete circuit, and more negative charges accumulate near the bipolar plates than in the flow channel, making it easier to react with protons that pass through the proton exchange membrane. The central area with sufficient concentrations of both oxygen and hydrogen had a higher current flux density. The current density of the rectangular baffle model was the highest, with significantly higher current density near the baffles, caused by the high oxygen concentration. The current density of the optimized trapezoidal baffle model was lower than that of the rectangular baffle model, while compensating with better uniformity. Figure 13 shows the H2O concentration at the cathode GDL/CL interface in the three models. The concentration of water molecules in the straight flow channel model gradually increased along the flow direction, since water generated in the electrochemical reaction gradually accumulated downstream. The other two models had a generally similar distribution; the major difference between the two and the straight flow channel was the higher H2O concentration downstream of the baffles. The reason for this concentration was the high oxygen concentration and current flux density downstream of the baffles, which resulted in increased water molecule generation. It was also observed that the optimized trapezoidal baffle model had better uniformity in its H2O concentration compared with the rectangular baffle model. The H2O concentration in Figure 13 is statistically given in Figure 14. In the straight flow channel, the H2O concentration in the first half was visibly higher than in the other two models, while visibly lower in the second half. In the straight flow channel model, the water discharging process was limited by the smooth oxygen flow, while in the other two models, the baffle structure improved the process, so the molar concentration of water in the first half of the straight flow channel was slightly higher than that of the other two  
Figure 6:0 Current flux density at the cathode GDL/CL interface. (**a**) The straight flow channel model; (**b**) the rectangular baffle model; (**c**) the optimized trapezoidal baffle model. Figure 6:1 H2O concentration at the cathode GDL/CL interface. (**a**) The straight flow channel model; (**b**) the rectangular baffle model; (**c**) the optimized trapezoidal baffle model. models. This caused the difference in the first half. In the two models with baffles, on one hand, due to the existence of the baffles, the oxygen concentration in the flow channel was higher on average, which resulted in a larger amount of water generation; on the other hand, the baffle structure improved the water discharging process within the flow channel, which in turn discharged water from upstream to downstream.