Abstract
Electrochemical cells containing two electrodes dipped in an ionic solution are widely used as charge accumulators, either with polarizable (supercapacitor) or nonpolarizable (battery) electrodes. Recent applications include desalination ("capacitive deionization") and energy extraction from salinity differences ("capacitive mixing"). In this Letter, we analyze a general relation between the variation of the electric potential as a function of the concentration and the salt adsorption. This relation comes from the evaluation of the electrical and mechanical energy exchange along a reversible cycle, which involves salt adsorption and release by the electrodes. The obtained relation thus describes a connection between capacitive deionization and capacitive mixing. We check this relation with experimental data already reported in the literature, and moreover by some classical physical models for electrodes, including polarizable and nonpolarizable electrodes. The generality of the relation makes it very useful in the study of the properties of the electric double layer.
Highlights
Electrochemical cells containing two electrodes dipped in an ionic solution are widely used as charge accumulators, either with polarizable or nonpolarizable electrodes
The origin of salt adsorption is not trivial, and resides in the process of charge storage into the electric double layers (EDL) which form at the interface between the carbon and the solution [11]
Schematic representation of the quantities of the EDL that are connected by Eq (4), which is shown in a simplified version
Summary
Electrochemical cells containing two electrodes dipped in an ionic solution are widely used as charge accumulators, either with polarizable (supercapacitor) or nonpolarizable (battery) electrodes. An important difference between CDI and capmix is that they are based on two different physical properties, namely, the salt adsorption during the charge and the dependence of the cell voltage on the salt concentration, respectively.
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