Abstract
Reverse electrodialysis (RED) is a method to produce electricity from the reversible mixing of two salt solutions with different concentrations. RED was first employed for energy generation using sea and river water. New fields of application are energy storage and heat-to-power conversion. In energy storage applications, a stack operates in ED mode during charge and in RED mode during discharge. In a heat-to-power system, the RED stack produces electricity and the outgoing solutions are returned to their original concentrations in a heat-driven regenerator. In both new applications, the salt solutions are circulated and there is a free choice of the combination of salt and membranes for optimal performance. However, classical polymer-based membranes have some disadvantages: they are less suited for operation at higher temperatures, have reduced permselectivity at higher concentrations, and are rather permeable to water, causing an imbalance of the feed waters. We developed a new concept of pseudo-membrane (PM): a metal sheet (sometimes covered with an insoluble salt) on which opposite electrochemical reactions occur at each side of the metal surface. Because a PM is dissolving at one side and growing at the other side during operation, the current should be inverted periodically. We tested a zinc sheet as a pseudo cation exchange membrane for Zn2+ ions and a silver chloride–covered silver plate as a pseudo anion exchange membrane for Cl− ions in three steps. First, a stack was built with Ag/AgCl membranes in combination with normal cation exchange membranes and operated with NaCl solutions. The next stack was based on Zn membranes together with normal anion exchange membranes. This stack was fed with ZnCl2 solutions. Finally, we tested a stack with zinc and Ag/AgCl pseudo-membranes with a ZnCl2 solution. The latter RED system worked; however, after standing for one night, the stack did not function and appeared to be damaged by redox reactions. This failure was the basis for general considerations about the possibilities of ED and RED hybrid stacks, consisting of a combination of classical and pseudo ion exchange membranes. Finally, we consider the possibility of using intercalation electrodes as a pseudo-membrane.
Highlights
Reverse electrodialysis (RED) is one of the methods to convert the potential energy fromAppl
We developed a new concept of pseudo-membrane (PM): a metal sheet on which opposite electrochemical reactions occur at each side of the metal surface
Ofisthe stack equipped with pseudo are mostly sulfonate groups membranes (AEMs) (Ag/AgCl. From the densities (AgCl)) and pseudo
Summary
From1.natural sources [1,2]; (ii) production of H2 [3,4]; (iii) energy storage: the stack is used in the electrodialysis (ED). There is no free choice of the salts and the RED electrolyte; important parameters areforthe total power cycle efficiency, charge and discharge powerwith density, stack and membranes are designed optimal density and/or efficiency using feed water energy density the electrolyte, price ofstorage the electrolyte, and aspects concerning health, safety, and the natural ionofcomposition. 3)3+) permselectivity in the anion exchange membranes (AEMs) These membranes have someand drawbacks: theatpermselectivity at higher salt concentrations, andpolymeric they facilitate water transport deteriorate high temperatures. The generated voltage over each pseudo-membrane is, in principle, the same as over a conventional ion exchange membrane This can be seen as follows: a zinc rod in a solution of H mol/L ZnCl2 gets a potential EH with respect to the solution:. Water transport through the Zn and Ag/AgCl pseudo-membranes is unthinkable These inorganic structures are very resistant to high temperatures. We will distinguish two classes of pCEMs: pure metals and intercalation compounds
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