## 4 Results and Discussion
### Test of the Ag/AgCl Pseudo-Membrane
#### 4.1.2 RED Test  
Feed solutions were 0.1 M NaCl and 2 M NaCl. Two tests were performed, a maximum power point tracking (MPPT) and a constant current (CC) test.  
The stack was tested in RED mode with maximum power point tracking for 5 min. Rest time was 10 min, flow rate H and L were both 80 mL/min, and five runs were performed.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{\(\boldsymbol{y=ax+b}\)} \\ \hline
**No.** & \(\boldsymbol{a}\) & \(\boldsymbol{b}\) & \(\boldsymbol{\mathrm{R^{2}}}\) \\ \hline
1 & 0.119 & 0.282 & 0.990 \\
2 & 0.062 & 1.545 & 0.987 \\
3 & 0.064 & 1.516 & 0.990 \\
4 & 0.097 & 0.731 & 0.951 \\ mean & 0.086 & 1.018 & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Properties of the anodizing regression lines from Figure 3.  
Figure 3: Anodizing potential of four silver plates; regression lines of 2 and 3 coincide.  
Figure 4 shows the results of the RED tests. After each run, the power production increased. Amazing is the relatively short duration of power production. This may be ascribed to suboptimal operation of one or more Ag/AgCl pAEMs.  
The stack was tested in RED mode with a constant current of 80 mA (8 A/m\({}^{2}\)) for 10 min (Figure 5). Rest time was 10 min, H and L flow rates were both 80 mL/min. Again there was an improvement in the power. The working time was doubled with respect to the MPPT experiment. Before each run, the OCV (Open Circuit Voltage) was measured (Table 7).  
Discussion of RED Tests  
The expected OCV of the stack was estimated to be 647 mV based on a permselectivity of 0.99 for the CMX [28] and 1.00 for the Ag/AgCl pseudo-membrane, and an activity coefficient of 0.6 for the 0.5 M and 0.7 for the 2 M NaCl solutions. The measured OCV (Tables 7 and 8) was substantially lower, and we ascribe it to the extremely high membrane resistance in combination with parasitic currents [29].  
\begin{table}
\begin{tabular}{c c} \hline
**Run** & **OCV (V)** \\ \hline
1 & 0.528 \\
2 & 0.412 \\
3 & 0.569 \\
4 & 0.431 \\
5 & 0.567 \\ mean & 0.503 \\ \hline \end{tabular}
\end{table}
Table 7: OCV (Open Circuit Voltage) of the stack before each MPPT run.  
Figure 4: Results of maximum power point tracking (MPPT) test of the RED stack equipped with normal CEMs (CMX) and pseudo AEMs (Ag/AgCl).  
Figure 5: Results of constant current of the RED stack equipped with normal CEMs (CMX) and pseudo AEMs (Ag/AgCl).  
With an OCV of 81 mV per membrane, we expected a voltage of 40.5 mV per membrane at the stack under the condition of maximal power. The mean area resistance of the Ag/AgCl and CMX membranes was \((0.0438+0.0003)/2=0.0221\)\(\Omega\)m\({}^{2}\); the estimated current density would be 1.64 A/m (if we ignore the much lower resistance in the electrode and feed water compartments), and this would result in an external power density of 66 mW/m\({}^{2}\).  
The voltage collapsed after about 150 s in the MPPT experiment and also very quickly in the CC experiment. That is inexplicable, because the anodizing was done with 59 A/m\({}^{2}\) for 30 min (delivering an electrical charge of 106,200 C/m\({}^{2}\)) and RED was performed with a current of about 0.1 A for 150 s (i.e., 15 C/m\({}^{2}\)).  
A possible explanation could be that an inhomogeneous AgCl layer was formed during the anodizing process. If there were parts in the surface with a thinner layer, these parts would have lower resistance and would dissolve preferentially during the RED process [30].