Faradaic Reactions in Water Desalination by Batch-Mode Capacitive Deionization

TLDR

Faradaic reactions in batch‑mode CDI can cause pH swings and electrode degradation, but the generated H₂O₂ offers potential for disinfection or contaminant degradation if activated, e.g., by UV. The authors developed a kinetic model that accurately predicts steady‑state H₂O₂ concentrations across CDI operating conditions. Higher charging voltages in batch‑mode CDI amplify both ion electrosorption and Faradaic reactions, producing substantial H₂O₂ and causing pronounced pH swings.

Abstract

Non-Faradaic (ion electrosorption) and Faradaic (oxidation–reduction) effects in a batch-mode capacitive deionization (CDI) system were investigated, with results showing that both effects were enhanced with an increase in charging voltage (0.5–1.5 V). Significant concentrations of hydrogen peroxide (H2O2) were observed with the generation of H2O2 initiated by cathodic reduction of O2 with subsequent consumption occurring as a result of cathodic reduction of H2O2. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concentration of H2O2 generated over a range of CDI operating conditions. Significant pH fluctuations were observed at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H2O2 could provide the means of inducing disinfection or trace contaminant degradation provided H2O2 could be effectively activated to more powerful oxidants (by, for example, ultraviolet irradiation).