Concentration Cells using Ion Exchange Membranes

When one surface of an ion exchange membrane contacts a concentrated solution and the other a dilute one, a membrane potential is generated as explained in Chapter 2.4. For example, about 100 mV (unit pair)-1 of membrane potential is observed in seawater concentration to produce sodium chloride. Usage of the membrane potential as a battery has been studied in detail.281 Especially, detailed studies and analysis have been made by researchers in Israel due to the existence of salt lakes such as the Dead Sea.282 It was calculated that 1.08 X 106 m3/s of river water flows in seawater, which corresponds to about 2.43 X 1012 W of electricity if it is generated by a concentration cell using ion exchange membranes. This electric power is calculated to be larger than the electric power consumption of the world.283 

The basic structure of the battery is the same as the electrodialyzer a plurality of a pair of cation and anion exchange membranes is alternately installed to form the concentrated and dilute compartment between electrodes at both ends. Then the concentrated and dilute solutions flow into each compartment and electric power based on the membrane potential is taken out from the electrodes. Various ion exchange membranes have been examined to calculate the energy conversion efficiency.284 A maximum power would be 0.33 Wm-2/pair when 0.57moll-1 solution (concentrated stream) and 0.026 mol l-1 solution (the dilute stream) are fed into a electrodialyzer with 30 pairs of cation and anion exchange membrane (effective membrane area 232 cm2).283 Also, it is calculated to be 0.6 W m-2/pair of electric power in an ideal scale-up based on experimental data when 30 gl-1 and 3 gl-1 solutions flow into the concentrated and dilute compartments.285 

---