## 2 Pseudo-Membranes, Systems, and Applications
### Systems Based on Pseudo-Membranes  
#### 2.3.1 Combinations of pAEM with pCEM  
Systems with pIEMs can, in principle, be equipped with classical electrode systems or with capacitive electrodes. However, a more simple solution is to construct the end electrodes of the same material as the pCEM, or eventually as the pAEM. We will denote the (unstable) system described in the introduction as Zn-Ag/AgCl-ZnCl\({}_{2}\), indicating that Zn is the end electrode material and also the pCEM, Ag/AgCl is the pAEM, and ZnCl\({}_{2}\) is the electrolyte; the same system with Ag/AgCl end electrodes will be denoted as Ag/AgCl-Zn-ZnCl\({}_{2}\). A special problem is the combination of an Ag/AgCl pAEM with a metal pCEM, e.g., Zn. The desired electrolyte in this case is a ZnCl\({}_{2}\) solution. Although the solubility product is rather low (L\({}_{\mathrm{AgCl}}\) = 1.8 \(\times\) 10\({}^{-10}\)) and the concentration of free silver ions is therefore also very low, it is known that a remarkable amount of AgCl is dissolved by complexation with chloride ions. From the work of [PERSON] [21] we know that silver is present as Ag\({}^{+}\), AgCl, AgCl\({}_{2}\)\({}^{-}\), AgCl\({}_{3}\)\({}^{2-}\), and AgCl\({}_{4}\)\({}^{3-}\) if silver chloride is in contact with a solution of HCl, NaCl, KCl, or NH\({}_{4}\)Cl. Table 4 shows the solubility of AgCl in different concentrations of NaCl and the distributions over the various compounds. It is remarkable that AgCl has much more solubility in KCl solutions. Moreover, the solubility is strongly temperature dependent. If Ag/AgCl pAEMs are combined with classical CEMs, the rather high concentration of silver complexes seems not to introduce technical problems: the most abundant complex ion is double-negative charged and rejected by the CEM. However, a drawback for storage applications is the high price of the applied silver. The combination of a Zn pCEM with an Ag/AgCl pAEM requires a ZnCl\({}_{2}\) electrolyte. The dissolved silver ions introduce an inherent unstable system due to  
\[2\;\text{Ag}^{+}+\text{Zn}\to 2\;\text{Ag}+\text{Zn}^{2+}\]  
We found that this process occurred during one night, resulting in a black coating on the zinc plates and a damaged Ag/AgCl structure, making it impossible to repeat any RED or ED experiment the next day. The system Zn-Ag/AgCl-ZnCl\({}_{2}\) is exceptionally unstable by two effects: the high solubility of silver in the chloride solution and the large difference in reduction potential of silver and zinc. The silver concentration can be decreased by changing Cl for Br or I, because the solubility products of AgBr and AgI are much smaller than those of AgCl [11], (Table 2). However, the conclusion is that all combinations of a metallic pCEM with a silver halide pAEM are always unstable. Within the system Cu-Pb/PbSO\({}_{4}\)-CuSO\({}_{4}\), the Pb electrode is in direct contact with the Cu\({}^{2+}\) ions, which may result in a fast redox reaction, Pb + Cu\({}^{2+}\) + SO\({}_{4}\)\({}^{2-}\)\(\rightarrow\) PbSO\({}_{4}\) + Cu, because copper is more noble than lead. Thus we wonder if changing Cu for Zn gives a more stable system. In the system Zn-Pb/PbSO\({}_{4}\)-ZnSO\({}_{4}\), the Pb\({}^{2+}\) concentration is dictated by the solubility product of PbSO\({}_{4}\) (L\({}_{\text{PbSO}4}\) = 1.6 \(\times\) 10\({}^{-8}\)). With a ZnSO\({}_{4}\) concentration of about 1 M, the resulting Pb\({}^{2+}\) concentration is about 10\({}^{-8}\) M. Nevertheless, such a system is unstable due to the reaction Pb\({}^{2+}\) + Zn \(\rightarrow\) Pb + Zn\({}^{2+}\). The system Ni-Pb/PbSO\({}_{4}\)-NiSO\({}_{4}\) is also unstable for the same reason as the zinc analogue. However, the difference between the reduction potentials of Zn\({}^{2+}\) and Pb\({}^{2+}\) is rather small, therefore we expect that the nickel system has a longer lifespan. Not all combinations are possible, due to electrochemical considerations.