Exploiting the spontaneous potential of the electrodes used in the capacitive mixing technique for the extraction of energy from salinity difference

Abstract

The ''capacitive mixing'' (CAPMIX) technique is aimed at the extraction of energy from the salinity difference between the sea and rivers. It is based on the voltage rise that takes place at the electrodes when changing the salt concentration of the solution in which the two electrodes are dipped. In this paper, we focus on activated carbon electrodes, produced with various methods and treatments, and we measure their behaviour in CAPMIX experiments. We find that they behave as polarizable electrodes only on time scales of the order of minutes, while on longer time scales they tend to move to a specific ''spontaneous'' potential, likely due to chemical redox reactions. This analysis sheds light on the charge leakage, i.e. the loss of the stored charge due to undesired chemical reactions, which is one of the main hurdles of the CAPMIX technique when performed with activated carbon electrodes. We show that the leakage finds its origin in the tendency of the electrode to move to its spontaneous potential. Our investigation allows us to completely get rid of the leakage and demonstrates the striking result that CAPMIX cycles can be performed without an external power supply.

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

“…As the base potential is progressively displaced from the spontaneous potential, the effect of the self-discharge becomes increasingly evident [32]. The self-discharge becomes particularly relevant below −100 mV and above 600 mV.…”

“…The open-circuit potential of the electrode can thus be changed. Nevertheless, if the electrode is left in open circuit for a sufficiently long time, it slowly comes back to its spontaneous potential [32]: it is the well-known self-discharge, which takes place on a time scale of the order of hours. Indeed, in the literature [34] the term "open-circuit potential" is usually only reserved to the spontaneous potential.…”

“…Unfortunately, the self-discharge of the activated carbon led to the necessity of continuously charging the electrodes, thus losing an amount of power that was comparable with the produced power. For this reason, experimental and theoretical investigations have been carried out [31,32] about the possibility of "chemically" charging the electrodes, so as to produce on their surface an electric double layer without the need of an external power supply, and so without undergoing the self-discharge phenomenon. A similar chemical charging has been already studied in the context of capacitive deionization [33].…”

Predictions of the maximum energy extracted from salinity exchange inside porous electrodes

“…In our case, the typical duration of the cycle was, as mentioned, 9 min, and hence, the maximum power would be around 0.13 mW/m 2 , although this can be improved by decreasing the cycle duration and optimizing the carbon material selection [23,24].…”

“…The second group of techniques, known as capacitive energy extraction based on double layer expansion (CDLE, [14,15,23,24,25,26]), is based on the fact that electrical double layers (EDLs) can accumulate a large amount of charge if the interfacial area is high enough, and that the capacitance of the EDL depends very significantly on the ionic contents of the medium [25]. If a metal/solution interface is externally charged (using a battery, for instance) in the presence of high ionic strength, and discharged in low ionic strength (less capacitance, more potential for given charge), it might be possible to obtain a net amount of energy: the electrodes are charged at our expense in conditions of lower voltage, and discharged (delivering power) at higher voltage.…”