Abstract
Reverse electrodialysis of saline solutions (RED) is a renewable energy technology with large potential. Key components in the system are the ion exchange membranes. This work evaluates the efficiency of commercially available anion and cation selective membranes for RED. Their efficiency is often described with the apparent transport number for the ion in question, and this number depends on the salt concentration. It is known since long that the water transference coefficient contributes to the apparent transport number, but much of the recent literature on RED takes the permselectivity as defined by the apparent transport number as a direct measure of the membrane selectivity. The purpose of this paper is to clarify that situation with new data on Fumasep membranes. Concentration cell potentials were measured for anion- and cation exchange membranes from Fumasep (FAD and FKS, respectively, Fumatech, Germany) in the temperature range 12−45°C in salt solutions relevant for reverse electrodialysis. The results show that the anion exchange membrane is a perfectly selective ion exchange membrane, and has a water transference coefficient of −6±1. The cation selective membrane has a cation transport number of 0.93 and a water transference coefficient of 8± 7. We suggest that the developers of membranes should pay more attention to the water transference coefficient. To enhance the performance of ion selective membranes and RED, it is beneficial to have water transference coefficients as close to zero as possible.
Highlights
The Gibbs energy of mixing of seawater and freshwater can be converted into electrical energy in several ways [1,2,3], including in reverse electrodialysis (RED) [4,5,6]
The permselectivity as defined by the apparent transport number has been used to characterize the selectivity of ion exchange membranes
The explicit formulation uses the ion transport number, and the water transference coefficient. This is useful in the evaluation of membranes relevant for reverse electrodialysis, where it is important to understand the difference between ion and water transport properties
Summary
The Gibbs energy of mixing of seawater and freshwater can be converted into electrical energy in several ways [1,2,3], including in reverse electrodialysis (RED) [4,5,6]. Electrodialysis is a practical and most important technology for the treatment of unusable water; it is in general used for demineralization of solutions in a wide variety of industrial fluids encountered in the food industry [13,14,15], and in pharmaceutical industries [16] It is important for several reasons to have a precise understanding of the coupled transport processes that take place in the IEMs, in particular of how the ion and water transports interact. This work evaluates interaction in commercially available anion- and cation exchange membranes, for application in reverse electrodialysis In this context, their permselectivity is described with the apparent transport number, defined as the ratio between the measurable electromotive force (Emf) over the membrane and the ideal electric potential, which is the maximum achievable value [17]. When the apparent transport number is below unity, it has been understood that the IEM is not perfectly selective to one ion, but allows more ions to pass
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