Influence of multivalent ions on renewable energy generation in reverse electrodialysis

Renewable energy can be generated when mixing seawater and river water. This energy is captured in reverse electrodialysis (RED) using ion exchange membranes. Although natural sources of seawater and river water are composed of a mixture of monovalent and multivalent ions, laboratory research on RED is generally performed with artificial solutions of sodium chloride. This research demonstrates that the presence of magnesium- and sulphate ions in feed solutions with NaCl has a major effect on the obtained open circuit voltage and power density for three different membrane types. When using a mixture with a molar fraction of 10% MgSO4 and 90% of NaCl in both feed waters, the experimentally obtained power density in steady state decreases from 29% to 50% compared to the case where the feed solutions contain only NaCl as a salt. This effect is among others explained by the transport of Mg2+ and SO42- against their concentration gradient, as is elaborated in a theoretical framework and which is justified by experimental data. Non-stationary cases, where feed water is switched from a NaCl solution to a mixture of NaCl and MgSO4, show that the voltage response time is in the order of tens of minutes up to several hours, due to ion exchange between the membranes and the feed water. The knowledge gained from electrochemical measurements under stationary and non-stationary conditions and a novel technique to monitor the ion transport inside cation exchange membranes can be used to improve the obtained power density in practical applications of RED using natural feed water.