Abstract
Ion exchange membranes (IEMs) have consolidated applications in energy conversion and storage systems, like fuel cells and battery separators. Moreover, in the perspective to address the global need for non-carbon-based and renewable energies, salinity-gradient power (SGP) harvesting by reverse electrodialysis (RED) is attracting significant interest in recent years. In particular, brine solutions produced in desalination plants can be used as concentrated streams in a SGP-RED stack, providing a smart solution to the problem of brine disposal. Although Nafion is probably the most prominent commercial cation exchange membrane for electrochemical applications, no study has investigated yet its potential in RED. In this work, Nafion 117 and Nafion 115 membranes were tested for NaCl and NaCl + MgCl2 solutions, in order to measure the gross power density extracted under high salinity gradient and to evaluate the effect of Mg2+ (the most abundant divalent cation in natural feeds) on the efficiency in energy conversion. Moreover, performance of commercial CMX (Neosepta) and Fuji-CEM 80050 (Fujifilm) cation exchange membranes, already widely applied for RED applications, were used as a benchmark for Nafion membranes. In addition, complementary characterization (i.e., electrochemical impedance and membrane potential test) was carried out on the membranes with the aim to evaluate the predominance of electrochemical properties in different aqueous solutions. In all tests, Nafion 117 exhibited superior performance when 0.5/4.0 M NaCl fed through 500 µm-thick compartments at a linear velocity 1.5 cm·s−1. However, the gross power density of 1.38 W·m−2 detected in the case of pure NaCl solutions decreased to 1.08 W·m−2 in the presence of magnesium chloride. In particular, the presence of magnesium resulted in a drastic effect on the electrochemical properties of Fuji-CEM-80050, while the impact on other membranes investigated was less severe.
Highlights
The increasing demand for water and energy requires sustainable and environmentally friendly solutions
For the brine solution coming from seawater desalination is currently considered as a waste, example, the brine solution coming from seawater desalination is currently considered as a thanks to its high salinity it can be as a valuable source for waste, thanks to its high salinity it exploited can be exploited as a valuable source
Since water uptake increases with increasing ion exchange capacity (IEC), the concentration of fixed charged groups attached to the polymeric matrix decreases, reducing permselectivity
Summary
The increasing demand for water and energy requires sustainable and environmentally friendly solutions. The old-fashioned linear approach (“take, make and dispose”) gives way to the circular economy approach in which any waste is potentially considered as a valuable source for another process. In this regard, reverse electrodialysis (RED) is a promising electromembrane-based process. For the brine solution coming from seawater desalination is currently considered as a waste, example, the brine solution coming from seawater desalination is currently considered as a thanks to its high salinity it can be as a valuable source for REDfor [2,3,4].
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