## 3 Results
### Comparison and Analysis  
In this section, a comparison is presented for the PEMFC models with straight flow channels, optimized trapezoidal baffles, and rectangular baffles (hereinafter, straight flow channel model, optimized trapezoidal baffle model, and rectangular baffle model). Figure 7 shows the gas velocity distribution contour along the oxygen flow in the three models at 0.4 V. As can be seen from the figure, the gas velocity in the straight flow channel model remained almost constant. The flow velocity of the optimized trapezoidal and rectangular baffle models varied periodically according to their geometry. Figure 8 is a detailed vector diagram of the velocity in the three models at the same distance to the inlet at 0.4 V. In the straight flow channel, most of the velocity vectors point in the axis direction and are thus parallel with the gas diffusion layer. The reaction of the PEMFC is hindered by the inevitable uneven oxygen concentration along the channel. In the optimized trapezoidal baffle model and rectangular baffle model, baffles disturb the uniform oxygen flow, creating turbulence adjacent to the baffles that directs more oxygen towards the gas diffusion layer. Moreover, the throttling effect of the baffles on the airflow increases the flow rate below the baffles, forming a local low-pressure area, which is also conducive to droplet removal on the gas diffusion layer and liquid discharging in the flow channel. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Results** & **Values** & **Unit** \\ \hline \(L_{f}\) & 3.0344 & mm \\ \(H\) & 0.3999 & mm \\ Predicted net power density & 2.6357 & W/cm\({}^{2}\) \\ Calculated net power density & 2.6294 & W/cm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of the single-objective optimization. Figure 6: Schematic diagram of single-objective optimization. Also shown in Figure 8, there is visible boundary layer separation and vortex generation upstream and downstream of the rectangular baffle--especially downstream, where significant backflow zones are formed, which will significantly increase the required pump power. Figure 9 shows the pressure distribution contour along the oxygen flow in the three models at 0.4 V. In all three models, the pressure decreases gradually along the flow direction. The pressure drop is drastic along the flow direction in the rectangular baffle model, while the pressure drop of the optimized trapezoidal baffle model is moderate. Figure 10 is the oxygen concentration at the cathode GDL/CL interface in the three models at 0.4 V. The distribution of the oxygen concentration in all three models is generally similar and peaks at the inlet and gradually decreases along the channel. In the rectangular  
Figure 8: Vector diagram of velocity of air inflow channels. Figure 7: Velocity distribution of air inflow channels. (**a**) The straight flow channel model; (**b**) the rectangular baffle model; (**c**) the optimized trapezoidal baffle model. Figure 9: Pressure distribution of air inflow channels. (**a**) The straight flow channel model; (**b**) the rectangular baffle model; (**c**) the optimized trapezoidal baffle model. baffle model, the oxygen concentration at the position of the baffles is significantly higher than in the adjacent area, while the adjacent area still has a higher concentration than the straight flow channel model. This improved the oxygen concentration along the whole channel and especially improved the reaction in the latter part of the channel. In the optimized trapezoidal baffle model, the oxygen distribution was similar to that of the rectangular baffle model, with a less significant increase around the baffle, which resulted in better oxygen uniformity. The oxygen concentration in Figure 10 is statistically given in Figure 11. The straight flow channel had the lowest concentration of the three, while the other two models had periodically increased oxygen concentrations along the flow.