## 2 Pseudo-Membranes, Systems, and Applications
### Pseudo Cation Ion Exchange Membranes (pCEMs)
#### 2.1.1 Pure Metal: Zn for Zn\({}^{2+}\), Ni for Ni\({}^{2+}\), Cu for Cu\({}^{2+}\)  
The metal should be stable in water and not too expensive or harmful. Pseudo-membranes dissolve at one side and grow at the other side. This process is inverted frequently by current reversal; essential for a good operation is a smooth and homogeneous building of the metal layer.  
Zn: The advantages are that zinc (and zinc salt) is rather cheap and has medium toxicity. These are important issues in energy storage systems, where huge quantities of electrolytes are stored. A disadvantage is that zinc has a rather low reduction potential, promoting the reduction of free Ag\({}^{+}\) if in combination with a pAEM (as discussed in Section 2.3).  
Ni: The reduction potential of Ni is much higher than of Zn [12], which may be advantageous with respect to stability. Drawbacks are the high price [9] and the toxicity [10] of nickel salts, which make the application to energy storage systems impossible (Table 1). However, in heat-to-power systems, nickel pCEMs may be considered.  
Cu: Copper can exist in two oxidation states, which causes some complications. Therefore, copper cannot be used in systems containing CuCl\({}_{2}\). The reason is that the poorly soluble copper(I) chloride can be formed via an auto-redox reaction:  
\[\text{Cu}^{2+}+2\text{ e}\rightarrow\text{Cu 0.34 V}\]  
\[\text{CuCl}+\text{e}\leftarrow\text{Cu}+\text{Cl}^{-}\text{ 0.14 V}\]  
resulting in:  
\[\text{Cu}^{2+}+\text{Cu}+2\text{ Cl}^{-}\to 2\text{ CuCl}\]  
1.2 Intercalation Compounds: MnO\({}_{2}\) for Na\({}^{+}\); FePO\({}_{4}\) for Li\({}^{+}\); and Prussian Blue Analogues for Li\({}^{+}\), Na\({}^{+}\), and K\({}^{+}\)  
These materials come from the battery world and contain transition metals with different oxidation states. Foreign ion can settle within the crystal structure, a process called intercalation. In a battery, there is a metallic- or carbon-based current collector where electrons are transferred from or to the redox material. In a pCEM, this metal or carbon/graphite is the substrate for the active redox layers on both sides. An (expensive) option is a platinized metal substrate. Intercalation pCEMs should be loaded on both sides and should be saturated with ions for 50% of the capacity in the start phase.  
MnO\({}_{2}\)/Na\({}_{2}\)Mn\({}_{5}\)O\({}_{10}\): The use of this manganese salt and oxide for a working sodium electrode is described by [PERSON] et al. [13] and [PERSON] et al. [14]:  
\[2\text{ Na}^{+}+5\text{ MnO}_{2}+2\text{ e}\rightarrow\text{Na}_{2}\text{Mn}_{5} \text{O}_{10}\]  
[PERSON] et al. [15] studied the intercalation of the ions Li\({}^{+}\), Na\({}^{+}\), K\({}^{+}\), and Mg\({}^{2+}\) with MnO\({}_{2}\) and observed only minor structural changes in the two-dimensional layered structures.  
LiFePO\({}_{4}\)/FePO\({}_{4}\): This material is used in Li-ion batteries and many properties are known [16; 17]. This mixture can absorb and release Li\({}^{+}\) ions due to the multivalent property of iron:  
\[\text{Li}^{+}+\text{e}+\text{FePO}_{4}\leftrightarrow\text{LiFePO}_{4}\]  
Fe\({}^{\text{III}}\)[Fe\({}^{\text{III}}\)Fe\({}^{\text{II}}\)(CN)\({}_{6}\)]\({}_{3}\): This insoluble salt, called Prussian Blue, contains iron(III) and iron(II) ions; the blue color is ascribed to charge transfer between the ions with different oxidation states. The iron can be substituted by other transition metals. These Prussian Blue analogues (PBAs) have, in principle, the same structure but with vacancies of different dimensions, suitable for absorbing different alkali ions: Li\({}^{+}\), Na\({}^{+}\), and K\({}^{+}\).