Modification of the surface of activated carbon electrodes for capacitive mixing energy extraction from salinity differences

• CAPMIX is a technique for producing energy from solutions of different salinities.
• The produced voltage depends on adsorption of charged molecules by the electrodes.
• Two electrodes with adsorbed molecules of opposite charges give a voltage of 90 mV.
• At low levels of charge the electrodes follow the Gouy–Chapman–Stern theory.
• An unexpected saturation of the voltage is observed for high levels of charge.

The “capacitive mixing” (CAPMIX) is one of the techniques aimed at the extraction of energy from the salinity difference between sea and rivers. It is based on the rise of the voltage between two electrodes, taking place when the salt concentration of the solution in which they are dipped is changed. We study the rise of the potential of activated carbon electrodes in NaCl solutions, as a function of their charging state. We evaluate the effect of the modification of the materials obtained by adsorption of charged molecules. We observe a displacement of the potential at which the potential rise vanishes, as predicted by the electric double layer theories. Moreover, we observe a saturation of the potential rise at high charging states, to a value that is nearly independent of the analyzed material. This saturation represents the most relevant element that determines the performances of the CAPMIX cell under study; we attribute it to a kinetic effect.

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