## 4 Results and Discussion  
### Polarization Curves with 10 Single Cells  
Figure 5 presents the polarization curves with 10 cells for an operating temperature of 70 \({}^{\circ}\)C, a gas inlet pressure of 0.1 MPa and an elevated anode relative humidity of 40%, 55%, 70%, 85%, and 100%. Tables 2-6 show the dynamic performance at different relative humidity about the voltage drop. It can be found that the voltage was in the range of 0.551 V\(-\)0.973 V, while the current density was 0\(-\)360 mA/cm\({}^{2}\). Moreover, the maximum voltage appeared on single cells in different locations in the stack, because of different current densities. Around 0 current density, the maximum voltage appeared at C4 or C5, which was in the middle of stack. Oppositely, for a maximum current density of 360 mA/cm\({}^{2}\), the maximum voltage almost focused on a single cell of the gas inlet inside, due to a lower flow rate and the accumulation of gas. [PERSON] et al. [33] also came to similar conclusions. They revealed that the cell performance on both sides of the stack was slightly better than that of the center cell. Additionally, the voltage differences between the single cells increased as the current density increased, because this was related to the increase of the reaction rate. The water flooding phenomenon did not occur because the experimental maximum current density almost did not reach the concentration polarization area. Figure 4: Test schematic. Relative humidity and other parameters are controlled and recorded by a personal computer (PC) and instances of pressure drop are taken manually. Figure 5: \(Cont\). As shown in Figure 5a and Table 2, the maximum voltage difference was 0.108 V between C1 and C10 at a current density of 360 mA/cm\({}^{2}\). The maximum voltage differences generated in Figure 5b-e were between two cells, which were C1 and C10, the same as Figure 5a, which respectively were 0.044 V, 0.065 V, 0.056 V, and 0.05 V at a current density of 360 mA/cm\({}^{2}\). It was obvious that the voltage differences between the cells gradually decreased as the relative humidity increased, resulting from the increase of ionic conductivity and resistance reduction [26]. It also could be observed that C10 voltage was the lowest, and its performance was the poorest for different current densities and relative humidity decreased. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline
**Current Density (mA/cm\({}^{2}\))** & **CV1** & **CV2** & **CV3** & **CV4** & **CV5** & **CV6** & **CV7** & **CV8** & **CV9** & **CV10** & **Voltage** \\ \hline
0.012 & 0.978 & 0.982 & 0.992 & 0.999 & 0.987 & 0.990 & 0.984 & 0.992 & 0.987 & 0.961 & 9.840 \\
18.494 & 0.865 & 0.869 & 0.864 & 0.860 & 0.867 & 0.866 & 0.858 & 0.867 & 0.863 & 0.849 & 8.613 \\
54.861 & 0.814 & 0.822 & 0.817 & 0.804 & 0.822 & 0.818 & 0.809 & 0.820 & 0.819 & 0.804 & 8.109 \\
145.783 & 0.754 & 0.760 & 0.750 & 0.731 & 0.761 & 0.745 & 0.741 & 0.755 & 0.753 & 0.733 & 7.459 \\
218.502 & 0.723 & 0.724 & 0.709 & 0.680 & 0.717 & 0.697 & 0.691 & 0.710 & 0.703 & 0.683 & 7.010 \\
291.222 & 0.689 & 0.687 & 0.672 & 0.639 & 0.674 & 0.655 & 0.639 & 0.666 & 0.643 & 0.617 & 6.561 \\
363.595 & 0.659 & 0.654 & 0.639 & 0.601 & 0.652 & 0.637 & 0.606 & 0.631 & 0.611 & 0.551 & 6.158 \\ \hline Voltage drop (V) & 0.319 & 0.328 & 0.353 & 0.398 & 0.335 & 0.353 & 0.378 & 0.361 & 0.376 & 0.410 & 3.683 \\ \hline \end{tabular}
\end{table} 
Table 2: Dynamic performance at different relative humidities (40%). Figure 5: Polarization curves with 10 single cells with different relative humidity. The inlet pressure is 0.1 MPa. Working and ambient temperatures are 70 \({}^{\circ}\)C and 25 \({}^{\circ}\)C, respectively. **(a)** RH: 40%; **(b)** RH: 55%; **(c)** RH: 70%; **(d)** RH: 85%; **(e)** RH: 100%. these conditions of working temperature, 70 \({}^{\circ}\)C, and different anode relative humidities. It could be found that the voltage and power density changing tendencies had a good agreement.