Capacitive deionization of NaCl solutions at non-steady-state conditions: Inversion functionality of the carbon electrodes

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.