## 1 Introduction  
Reverse electrodialysis (RED) is one of the methods to convert the potential energy from salinity gradients into electrical energy. The main applications include: (i) primary energy generation from natural sources [1; 2]; (ii) production of H\({}_{2}\)[3; 4]; (iii) energy storage: the stack is used in the electrodialysis (ED) mode to store energy as a salinity gradient, and this energy can be recovered by operating the stack in the RED mode [5; 6]; and (iv) the generating part of a heat-to-power system in combination with a thermally driven regeneration unit [7; 8]. The requirements of the RED stack, the applied membranes, and the used salt are very different for each application. For primary energy generation, there is no free choice of the salts and the RED stack and membranes are designed for optimal power density and/or efficiency using feed water with natural ion composition. In energy storage systems, there is, in principle, a free choice of the electrolyte; important parameters are the total cycle efficiency, charge and discharge power density, energy density of the electrolyte, price of the electrolyte, and aspects concerning health, safety, and the environment. A special point in such systems is the water transport through the membranes as a source of imbalance during long-time operation. In heat-to-power systems, there are two methods of regeneration: distillation of the fluid phase (usually water) or of the salt (ammonium bicarbonate or ammonium carbonate). In the case of water distillation, there is much freedom in choosing an electrolyte and membranes; in contrast to energy storage systems, the amount of electrolyte is restricted and price is therefore less important than in storage systems. Usually in ED and RED, organic membranes are applied polymer-based structures with exchanging fixed charges. The fixed charges are mostly sulfonate groups (-SO\({}_{3}\)\({}^{-}\)) in the cation exchange membranes (CEMs) and quaternary ammonium groups (-N(CH\({}_{3}\))\({}_{3}\)\({}^{+}\)) in the anion exchange membranes (AEMs). These polymeric membranes have some drawbacks: the permselectivity decreases at higher salt concentrations, and they facilitate water transport and deteriorate at high temperatures. In this paper, we launch a new membrane type based on reversible electrochemical reactions on both sides of the so-called pseudo-membrane. The adjective \"pseudo\" indicates that in most cases there is no real permeation of an ion through the membrane: they change ions at one side for ions at the other side. In fact, these pseudo-membranes are literally real exchange membranes, in contrast with normal \"ion exchange membranes\" that are based on ion transport. Figure 1 shows the principle: a RED stack is equipped with Zn pseudo-membranes, acting as cation-exchange membranes for Zn\({}^{2+}\) ions, and Ag/AgCl pseudo-membranes, acting as AEMs for Cl\({}^{-}\); the stack is fed with high- and low-concentration ZnCl\({}_{2}\) solutions. Figure 1: Concept of a reverse electrodialysis (RED) stack equipped with inorganic pseudo-membranes. AEM, anion exchange membrane; CEM, cation exchange membrane. The Zn pseudo-membrane functions as a CEM:  
at the high concentration side: Zn\({}^{2+}\) + 2e \(\rightarrow\) Zn;  
at the low concentration side: Zn \(\rightarrow\) Zn\({}^{2+}\) + 2e. The AgCl/Ag/AgCl pseudo-membrane functions as an AEM:  
at the high concentration side: Ag + Cl\({}^{-}\)\(\rightarrow\) AgCl + e;  
at the low concentration side: AgCl + e \(\rightarrow\) Ag + Cl\({}^{-}\). The reactions on the end electrodes are:  
cathode: Zn\({}^{2+}\) + 2e \(\rightarrow\) Zn;  
anode: Zn \(\rightarrow\) Zn\({}^{2+}\) + 2e. Obviously, a periodical flow switch should be applied; the switch period is especially dependent on the thickness of the AgCl layer. During the switch, the stack is not generating energy and this dead time is relative shorter with a longer switch period. Otherwise, a longer switch period requires a thicker AgCl layer, which introduces more electrical resistance.