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In this Article, the consequence of continuous electrochemical oxidation at the positive electrode in an initially symmetrical capacitive deionization (CDI) cell, comprising two identical activated carbon electrodes, is examined and discussed. Extensive and intensive parameters of the CDI cell are defined, and the deviations occurring among them as a result of continuous electrochemical oxidation processes at the positive electrode during prolonged charge–discharge cycling are discussed. A special flow-through CDI cell containing activated carbon fiber (ACF) electrodes was developed for this purpose. Ex situ XPS measurements were conducted to prove the presence of oxidized surface groups on the positive electrode of these cells due to cycling. A surprising phenomenon that looks like an inversion functionality of the carbon electrodes occurring after numerous charge–discharge cycles is observed and explained.

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In this paper we report on attempts to improve the efficiency of electrochemical capacitive deionization (CDI) by understanding the relevant ion adsorption processes. Specially designed three-electrode cells were elaborated to study the relationship between the charge injected and the charge efficiency of the adsorption/desorption of Na+ and Cl− ions onto highly porous, high surface area carbon cloth electrodes. The counter electrodes in these cells were large reversible Ag–AgCl electrodes. Knowing the volume and concentration of the solution and the charge and mass balance of the working and counter electrodes, the degree of cation and anion adsorptions onto the working electrode could be calculated separately as a function of the applied potential. The relevance of these data to the design of a symmetric CDI cell is briefly discussed.